B. Observations Are Made at The Eyepiece!
D. Description of User Interface
ObsReduce is an MS Windows program that reduces observations of satellites relative to the background stars into their precise coordinates. Observers identify their reference stars in a simulated binocular or telescope field of view, select them using the mouse, enter the observed geometric and positional data, and the program automatically produces a formatted observation report.
ObsReduce supports three traditional observational geometries: satellite in-line with reference stars, satellite forming right-angled triangle with reference stars, and appulses (satellite passing near a single star).
Observations in the form of images recorded by cameras may be reduced using two geometries: satellite located to the right or left of a pair of stars, or any configuration of a satellite and three stars. The user enters the pixel coordinates of the satellite and reference stars, or the measured lengths between them, obtained independent of ObsReduce.
ObsReduce supports all three reporting formats in common use: IOD, UK and RDE (Russell Eberst's abbreviated form of the UK format.) All R.A. and Dec formats and epochs are supported, as are visual magnitude and optical period reporting.
Several star databases, derived from the Tycho 2 catalogue, are available for ObsReduce. The magnitude 8.5 database, includes nearly 72,000 stars, The magnitude 10.5 database includes more than 540,000 stars. The magnitude 12.0 database includes more than 2 million stars. The complete set of Tycho 2 stars is also available, consisting of more than 2.5 million stars, to about magnitude 13. The latter two databases are available only to positional observers who share their observations, and who have a demonstrated need. The star data bases range in size from 1.0 MB to 30 MB. ObsReduce enables the addition of stars missing from the database, as well as coordinates of planets and asteroids.
The star databases store Epoch 2000.0 coordinates, proper motion and visual magnitude. About 4 percent of the stars in Tycho 2 do not have proper motion data, and though they are Equinox 2000.0, their epoch is c.1991. I hope to eventually provide data from alternative sources for many of those stars. Where available, ObsReduce makes use of proper motion in the reduction of observations, though it rarely materially affects accuracy.
To aid in finding and confirming reference stars used in observations, ObsReduce has the ability to display predicted satellite tracks computed using the SGP4 and SDP4 orbital models. It also displays the difference between the satellite's predicted and observed time and track, and can compensate for Earth's rotation in the interim, to produce accurate plots of satellite tracks.
ObsReduce can be configured to simultaneously write its results to as many as four text files. One file logs only the IOD, UK or RDE formatted observations. Another logs the underlying details of each observation: the position and magnitude of reference stars, the observational geometry, and the orbital elements used to locate the reference stars. The remaining two files are single session versions of these file types.
ObsReduce provides numerous configuration options. For example, default values for site code, time uncertainty, position uncertainty, and the number of significant decimal points to be reported, can all be set via an easily editable initialization file. Many of the default settings may be readily changed on a per observation basis at run time.
ObsReduce accurately plots a satellite's predicted track, and reports cross-track and time differences between prediction and observation - helpful in finding and confirming the reference stars used in an observation.
One pitfall to be avoided is attempting to "improve" observations on the basis of the predicted track or the cross-track and time differences between prediction and observation. Observations are made at the eyepiece, and cannot be improved afterward, by ObsReduce or any other means.
Differences between prediction and observation are to be expected - indeed they are the reason that we observe - to provide the data by which ever changing orbital elements may be updated. Attempting to "correct out" the differences obliterates the very data that we seek to obtain.
So, please, use the satellite tracks to help locate your reference stars, but do not alter your observation to match them!
ObsReduce may be used with or without the orbital elements of the observed object. It also facilitates making observations without having to stop to sketch reference stars.
C.1 Using ObsReduce With Existing Orbital Elements
ObsReduce assists in locating the reference stars used in an observation, by using the object's existing elements to plot its track through a simulation of the observer's FOV (field of view) at the time of the observation.
Since the vast majority of positional observations are made for the purpose of updating existing orbital elements, this feature will be useful most of the time.
For the greatest ease of use, always use the most recent orbital elements available. Sources of orbital elements are listed here. To find your elements file, ObsReduce follows the path and name entered at line 33 of the ObsReduce.ini file.
If the object was nearly on time (i.e. difference between its observed and predicted position was within the radius of the observer's field of view), follow the steps in Section C.1.1.
If the object was significantly early or late (i.e. the difference between its observed and predicted position exceeded the radius of the observer's field of view), but the time difference is accurately known, then follow the steps in Section C.1.1.
If the time difference is not accurately known, but the approximate R.A. and Dec of the reference stars is known, then follow the steps in Section C.1.2, else, if the location of the stars is not known, but you have a good sketch of them and a few surrounding stars in the field of view, then follow the steps in Section C.1.3.
C.1.1 Satellite Passed Nearly On-Time Or Time Difference Known
If the difference between the object's observed and predicted position was within the radius of the observer's FOV (field of view), or the duration by which it was early or late is known, then perform the reduction as follows:
- select observation site
- IOD users select site Status
- enter date and time of observation
- if the object was on-time, enter zero in Satellite Early / Late text box, else enter the amount by which it was early or late
- press Plot Satellite button
- select the displayed reference star(s). ObsReduce displays the stars and the satellite's track in the FOV, oriented as they appeared at the time of observation. The more accurate the orbital elements, the closer the reference star(s) will be to the centre of the FOV.
- select the geometry of the observation and enter the object's position relative the reference star(s)
- if they were observed, enter the object's magnitude (brightness), period of variation and optical code
- press the Reduce button
- briefly review the Obs - Pred information. If the cross-track and/or time difference seem unreasonable, then you should review to make certain that you entered the correct data and selected the correct stars.
- if the observation minus prediction time difference is sufficiently large that your reference stars are noticeably displaced from the centre of the FOV, then you can centre them, and those of any other observations of the same object, by pressing the Use As Early/Late button. This has the effect of instantly updating the FOV display, and copying the prediction time difference into the Satellite Early/Late text box.
- you may save your observation using the Save button near the lower right corner of the program's window. If you wish to discard it, press the Clear Position button.
- if you have additional observations of the same object, then repeat the steps in this section, beginning with the date and time. The last date and time will still be there, so you need only edit it as required.
C.1.2 Reference Star Coordinates Known
If the object was significantly early or late, by an unknown duration, but you know the coordinates of your reference stars, then follow these steps to bring the object and the reference stars into the FOV:
- select observation site
- IOD users select site Status
- enter date and time of observation
- enter zero in Satellite Early / Late text box
- press Plot Satellite button. The satellite will be in the wrong place, but this step is required to load its elements.
- in the FOV Centre frame (at lower right of the FOV), enter the RA and Dec (or the AZ and EL) approximately centred on the reference stars. The satellite's track will disappear, but your reference stars should now be visible in the FOV.
- select the displayed reference star(s)
- select the geometry of the observation and enter the object's position relative the reference star(s)
- if they were observed, enter the object's magnitude (brightness), period of variation and optical code
- press the Reduce button
- briefly review the Obs - Pred information. These differences have been computed relative to the object's orbital elements loaded earlier. The time difference is an accurate measurement of how early or late the object was relative your observation. The position cross-track difference should be fairly small, because the track has been compensated for Earth's rotation corresponding to the time difference.
- now that the difference between the observed and predicted time is known, it can be used to easily find the reference stars of any additional observations of the same object still to be reduced, Simply press the Use As Early/Late button. This has the effect of instantly updating the FOV display, and copying the prediction time difference into the "Satellite Early/Late text box.
- you may save your observation using the Save button near the lower right corner of the program's window. If you wish to discard it, press the Clear Position button.
- to begin reducing another observation of the same object, edit the date and time. Do not change the value in the Satellite Early/Late box. Press Plot Satellite, which will display your new reference stars, and the satellite's track. From here, the reduction proceeds as any other reduction.
C.1.3 Reference Star Coordinates Not Known
If the object was significantly early or late, by an unknown duration, and you do not know the coordinates of your reference stars, but have a sketch of them, then follow these steps to bring the object and the reference stars into the FOV:
- select observation site
- IOD users select site Status
- enter date and time of observation
- enter zero in Satellite Early / Late text box
- press Plot Satellite button. The satellite will be in the wrong place, but this step is required to load its elements.
- in the Move Satellite frame, select the "by changing" option Early/Late, which tells the program to vary the value in the Early/Late box instead of the time in the FOV Centre frame, when you move the satellite.
- in the same frame, use either set of increment/decrement buttons to move the satellite. One set moves the satellite a specified percentage of the FOV diameter; the other moves it a specified number of seconds of time. Moving a percentage of FOV diameter is preferred, because it ensures that stars will not be skipped over when you increment or decrement.
If the satellite was early, repeatedly click the decrement button to plot its track at the observed time, but for progressively earlier times relative prediction.
If the satellite was late, repeatedly click the increment button to plot its track at the observed time, but for progressively later times relative prediction.
With each click, watch the stars in the FOV change; when the stars you sketched come into view, you can proceed with the reduction.
- select the displayed reference star(s)
- select the geometry of the observation and enter the object's position relative the reference star(s)
- if they were observed, enter the object's magnitude (brightness), period of variation and optical code
- press the Reduce button
- briefly review the Obs - Pred information. These differences have been computed relative to the object's orbital elements loaded earlier. The time difference is an accurate measurement of how early or late the object was relative your observation. The position cross-track difference should be fairly small, because the track has been compensated for Earth's rotation corresponding to the time difference.
- now that the difference between the observed and predicted time is known, it can be used to easily find the reference stars of any additional observations of the same object still to be reduced, Simply press the Use As Early/Late button. This has the effect of instantly updating the FOV display, and copying the prediction time difference into the "Satellite Early/Late text box.
- you may save your observation using the Save button near the lower right corner of the program's window. If you wish to discard it, press the Clear Position button.
- to begin reducing another observation of the same object, edit the date and time. Do not change the value in the Satellite Early/Late box. Press Plot Satellite, which will display your new reference stars, and the satellite's track. From here, the reduction proceeds as any other reduction.
C.2 Using ObsReduce Without Orbital Elements
If you do not have orbital elements for an object, or prefer not to use them, ObsReduce can still help reduce your stars, as long as you know their approximate coordinates.
- select observation site
- enter Desig
- IOD users must enter SSN
- IOD users select site Status
- enter date and time of observation
- in the FOV Centre frame (at lower right of the FOV), enter the RA and Dec (or the AZ and EL) approximately centred on the reference stars. Your reference stars should now be visible in the FOV, oriented as they were at the time of observation.
- select the displayed reference star(s)
- select the geometry of the observation and enter the object's position relative the reference star(s)
- if they were observed, enter the object's magnitude (brightness), period of variation and optical code
- press the Reduce button
- you may save your observation using the Save button near the lower right corner of the program's window. If you wish to discard it, press the Clear Position button. If you have additional observations of the same object, then resume these steps at the point where the observed date and time are entered.
When I began making positional observations, and for many years thereafter, I always stopped after each observation and sketched my reference stars. I labelled them as A and B, and drew a line with an arrow to indicate the satellite's path, noting the fraction of its passage between them, for example 70 percent down from star A to B. I also sketched several of the surrounding stars, to aid in locating the star field in an atlas.
This method ensured that I would not lose my observation, but I rarely made more than one per pass, because the satellite had moved on and was difficult to reacquire, especially if it was faint and fast-moving.
Though my sketches were fairly detailed, the star fields were small, therefore difficult to find in a star atlas. Fortunately, I found that for well-predicted objects, or ones running early or late by a known interval, I could accurately estimate the location of my reference stars by computing the predicted location of the satellite at the time of my observation.
This experience taught me that I did not have to sketch my stars, as long as I could remember the fraction of a satellite's passage between stars. As a result, my observing method evolved, so that by July 2000, I only rarely stopped to sketch reference stars.
The sketch-free method relies upon the fact that accurate paths can be predicted for most of the objects we track, as long as we can compensate for Earth's rotation during the interval by which they are early or late.
Of the more than 1100 observations I have made since July 2000, perhaps 5 percent have involved sketches - mainly long-lost objects, new launches, or recently manoeuvred objects, for which predicted paths tend to be inaccurate.
I wrote ObsReduce to facilitate the sketch-free method, as described below.
I use ObsReduce in conjunction with an ephemeris generator to determine where I will intercept each satellite. I use the ephemeris generator to find satellite passes suitable for observing. I use ObsReduce to simulate each pass, looking for the earliest suitable reference star(s) at which point I will intercept the satellite, and begin tracking.
I always try to intercept at a star pair that is likely to produce an accurate observation, i.e. no more than 0.5 deg apart, with the satellite crossing at close to a right angle. Alternatively, I will use a single star that the satellite is predicted to very closely appulse.
I take care to use stars sufficiently bright to be seen in the expected circumstances. I am mindful of local obstacles that could block my view.
This method almost guarantees me at least one useful observation, and it frees me from having to stop and sketch any of the subsequent reference stars, since ObsReduce can always take me to them, based upon the difference in time between observation and prediction of that first position, which is key to making the method work.
For example, if ObsReduce tells me that a satellite was 23.2 s late at my pre-selected intercept point, then I know it was 23.2 s late at all of the subsequent points I observed. So, I hit the "Use As Early/Late" button to instruct ObsReduce to compensate for the 23.2 s lateness. When I enter the time of my next observation, ObsReduce places its reference stars nearly dead-centre in its simulated FOV.
Freeing myself of the need to stop and sketch each point, made it easy to obtain many accurate positions on a single pass, instead of struggling to make one or two.
All I do is to memorize the geometry, for example, while tracking, I see the satellite about to cross the line between a good star pair. As is almost always the case, one star is noticeably higher than the other, as seen in my FOV. I observe the satellite to cross at 70 percent of the distance from the higher star to the lower star. As I hit the stopwatch, I memorize the event as "70 down", and continue tracking. Had the stars been at the same height in the FOV, I would instead remember "70 left" or "70 right", as the case may be. I use similar short forms for appulses.
My powers of memory are not all that great, but I can easily memorize several such descriptions while tracking, and then quickly jot them down in my log immediately after I stop tracking that object. By repeating the descriptions aloud over and over, I can remember half a dozen reliably.
Tape recording the comments is an alternative, which makes it easier to record additional information, such as brightness and period of variation, at the expense of having to sit through the playback later. A voice-activated recorder can help in that regard - only if the voice-activation is reliable!
This section describes ObsReduce's text boxes and other controls, reading from top to bottom, left to right of the program's window.
NOTE: Frequently mentioned in this section is ObsReduce.ini, a text initialization file, installed in the same directory as ObsReduce. It contains 34 entries used to configure the program, which you may edit with a text editor. The file contains explanations of each one of the 34 entries.
Site is the code number identifying your site. Permanent numbers are issued only to the sites of observers who make fairly frequent observations and share them with others.
If you already have a site number, you can cause ObsReduce to load it automatically, by adding it and its coordinates to line 1 of ObsReduce.ini, after the colon. ObsReduce supports the selection from among multiple sites at run-time. For details, please the notes in ObsReduce.ini.
It is perfectly acceptable for a beginning or very infrequent observer to share reports having a zero value for the site number - just be sure to include your site co-ordinates in your report. If you show signs of becoming a fairly frequent reporter of observations, then you are likely to be offered a permanent site number.
Desig denotes International Designation, which is the unique identifier officially assigned to all regularly tracked objects. This information is required by all three supported report formats: IOD, UK and RDE.
Formally, the designation is written as yyyy-nnn$$$, where yyyy is the year of the launch, nnn is the order of the launch that year, and $$$ is an uppercase alphabetic string, denoting the piece from the launch in question, The first object is A, followed by B, etc. The letters I and O are excluded due to their similarity to 1 and 0 respectively.
To save space, the reporting formats use shorter forms of the designation. The format for entering it into ObsReduce is the same for all formats.
You have two entry options.
1. Enter yynnn$$$, where:
yy are the final two digits of the year of launch; leading zeros must be entered
nnn is the order of the launch that year; leading zeros must be entered
$ is a one to three character string denoting the piece from the launch in question
2. Enter yynnnpp, where:
yy are the final two digits of the year of launch; leading zeros must be entered
nnn is the order of the launch that year; leading zeros must be entered
pp is a one to two digit number denoting the piece from the launch in question; leading zeros need not be entered
The second format is convenient because it enables exclusive use of the numeric keypad. When working in IOD format, which reports the piece alphabetically, numeric pp entries will be converted and displayed as their alphabetic equivalents.
Regardless of input format, ObsReduce correctly formats the designation within the IOD, UK and RDE reports.
SSN (aka NORAD number) is a five digit catalogue number issued by U.S. Strategic Command. The field is required by IOD.
UK and RDE format users may enter the SSN, so that it may used to search the 2-line elements file, and/or be included in the log files that record details of each observation. Users of the UK and RDE format who prefer not to see this prompt, can make it disappear by entering n at line 22 of ObsReduce.ini
Status is a drop-down list that is displayed only for IOD users. It is an alphabetic code that reports on the observing conditions and availability of the site for observations.
D.5 Observation Date and Time Text Boxes
The date and time of observations are entered in the second row of input boxes from the top of the program's window. ALWAYS enter the date and time as UTC (aka GMT), NEVER your local date and time.
Use the option buttons at far left to specify the mode of date and time entry: absolute or elapsed.
D.5.1 Absolute Time Mode
Absolute time is as read directly from a clock. In this mode, the date input boxes are labelled "Obs Date", and the time input boxes are labelled "Obs Time UTC."
D.5.2 Elapsed Time Mode
Elapsed time is as read from a stopwatch. This is intended for use by observers who use what is commonly called a multiple split-time stopwatch. Each time an observer times a satellite position, the stopwatch records the elapsed time of the event as a split-time. Typically, such stopwatches record the split-times in memory, for later readout.
In Elapsed time mode, the date input boxes are labelled "Base Date", and there are two sets of time input boxes, labelled "Base Time UTC" and "Elapsed Time". There is also a box for the rate of drift of the stopwatch.
Enter the date and time that the stop watch was started, into the Base Date and Base Time input boxes. Enter the split-time of the observation into the Elapsed Time input box. The program internally computes the absolute date and time of the observation, for use in the observation report.
Also, since changing the date and time of an observation re-sets the date and time of the centre of the field of view, you can temporarily view the computed absolute date and time in the FOV Centre Frame.
In elapsed time mode, the program provides a permanent record of the Base Date, Base Time, Elapsed Time and Drift Rate, by writing them to the details file, as shown in Section D.18.1 Observation Files.
D.5.2.2 Adjust for Stopwatch Drift
If your stopwatch drifts significantly during your observation, then you should enter its rate of drift into the box labelled "Drift s/h". Units are in seconds per hour. Compute the rate of drift as follows:
drift = 3600 * [(e2 - e1) - (t2 - t1)] / (t2 - t1)
where: drift = rate of drift, seconds per hour
t1 = actual UTC #1, hours
e1 = elapsed time at t1, hours
t2 = actual UTC #2, hours
e2 = elapsed time at t2, hours
D.5.3 Switching Between Modes
You may switch between time modes at any time. When you switch from elapsed to absolute mode, the program remembers the Base Date, Base Time. and Drift Rate. Upon your return to elapsed time mode, those values are restored.
D.5.4 Timing Accuracy
Specify the time accuracy in the input text box labelled "Accy". Please note that the default time accuracy may be changed by editing line 5 of ObsReduce.ini.
D.6 Satellite Early (-) Late (+) Test Box
If you know how early or late the satellite was, enter the number of seconds here. If the object was early, enter the value as negative; otherwise, as positive (the plus sign may be omitted).
The Plot Satellite button reads the satellite's orbital elements and plots its predicted track and star background, at the date and time shown in the FOV Centre frame.
The satellite's track is plotted as a solid line, with a filled circle at one end, to indicate the direction of motion.
The colour of the track denote's the satellite's illumination; blue denotes an object fully in eclipse; red denotes an object in the penumbra of Earth's shadow; yellow denotes an object totally outside Earth's shadow, i.e. in sunlight.
D.7.1 Reading Orbital Elements from a File
For efficiency, Plot Satellite reads the orbital elements file only if a new Desig or SSN has been entered since the last time it was pressed, or the time/date stamp of the orbital elements file has changed since the last time it was pressed.
The latter feature enables editing the file or replacing it with another of the same name, without having to shutdown and restart ObsReduce. This simplifies switching among several elsets of the same object.
ObsReduce uses the first elset that matches the entered Desig and/or SSN. To make it skip an elset, edit one of its two lines to insert any character at the start of the line, for example a semicolon. It will no longer look like an elset to ObsReduce, so it will skip past it. In this example, the first elset will be skipped:
;1 24680U 96072A 03160.08334785 .00031614 00000-0 35000-3 0 01 2 24680 97.8269 223.2766 0505000 31.8143 331.2548 14.81313481 01 1 24680U 96072A 03161.09651195 .00031057 00000-0 33552-3 0 01 2 24680 97.8181 224.2537 0506798 28.7815 334.0379 14.81377728 04To quickly compare plots of the same object switching among multiple elsets, open the elements file in a text editor, edit-out the ones to be skipped, save the file, and press Plot Satellite.
When working with elements, it is not necessary to enter both the Desig and the SSN. ObsReduce will find the value of the missing entry in the elset, and automatically fill in its empty text box.
If you have filled in both the Desig and SSN boxes, then upon reading the elements, ObsReduce will report an inconsistency if it finds one. For example, if it finds an elset with the same Desig that you entered, but a different SSN, it will require you to resolve the discrepancy before allowing you to proceed with plotting the satellite's track. This is intended to promote accuracy of the observation reports.
On occasion, you may find yourself in disagreement with the Desig and/or SSN of an elset. Instead of your having to adopt the elset's values, or edit the elset to match yours, you can assign an alias to the elset. For example, here is an estimated pre-launch elset of the object that became Desig 95034A / SSN 23609:
1 72001U 95191.53601852 .00000000 00000-0 00000-0 0 08 2 72001 55.0600 16.1740 0000320 299.9680 127.5380 16.32783000 01If you observed the object, and wish report it as 95034A / 23609, but use the pre-launch elset, insert an alias line into the file, just ahead of the elset, and save the file:
*alias* 95034A 23609 1 72001U 95191.53601852 .00000000 00000-0 00000-0 0 08 2 72001 55.0600 16.1740 0000320 299.9680 127.5380 16.32783000 01Aliases are also useful in cases of disagreement among analysts over the correct designation for an object.
Depending upon the geometry of the observation, one, two or three reference stars may have been used, which must be selected in the simulated FOV. The first star selected is called A, the second, B, the third C.
If the satellite crossed the imaginary line joining two stars, either between them or outside them, then select them as stars A and B.
If the satellite formed a right-angled triangle with two stars, then select them as stars A and B.
If the satellite appulsed (passed very near to) a star, then up to three stars may have been involved in the observation. The appulsed star must be selected first, and it will be denoted as A. If you judged the miss-distance without reference to any other stars, then no additional stars should be selected.
If you judged the miss-distance as the fraction of the distance between the appulsed star, A, and another star in the FOV, then select that star as B.
If you judged the miss-distance as the fraction of the distance between two other stars in the FOV, then select them as stars B and C.
In the case of a camera observation of a satellite located to the right or left of two reference stars, select them as stars A and B.
In the case of a camera observation of a satellite and three reference stars, select them as stars A, B and C.
Moving the mouse cursor over a star in the displayed FOV (field of view) temporarily changes its colour from white to green, and displays its coordinates and magnitude in the box below the FOV.
To select Star A, left-click the mouse when it is over a star. When you move the cursor away, its colour will change to yellow, and a yellow upper case letter A appears below the FOV.
The next star selected becomes Star B, denoted by its change to the colour red, and the appearance of a red upper case letter B in below the FOV.
The third star selected becomes Star C, denoted by its change to the colour cyan, and the appearance of a cyan upper case letter C in below the FOV.
All stars may be cleared at any time by left-clicking anywhere in the FOV where there is no star. Note that this will clear the results of the reduced observation, if any.
D.9 Geometry and Position Frames
Geometry refers to the geometrical pattern formed by the satellite and reference stars at the moment of your observation. Position refers to the satellite's location relative the reference stars.
ObsReduce supports three traditional observational geometries: satellite in-line with reference stars, satellite forming right-angled triangle with reference stars, and appulses (satellite passing near a single star). They are used primarily by observers who rely on their own memory to record observations. Observers who use cameras may also use these geometries when they arise, but are likely to prefer the special geometries described in Section D.9.2 Camera Geometries.
D.9.1.1 Satellite on Line AB
If the satellite crossed the imaginary line joining star A and B, either between or outside the stars, then in the Geometry frame select the "On line AB" option, and in the Position frame, enter the fraction AS/AB, (the distance from Star A to the satellite, expressed as a fraction of the distance from Star A to B) into the box labelled "Frac AS/AB". If the satellite passed between the stars, then this value will be less than 1; if it passed outside them, it will be greater than 1.
D.9.1.2 Right-Angled Triangle
If the satellite formed a right-angled triangle with Stars A and B, A-B-S, then if it passed to the right of Star B, select option "Right of B"; if it passed to the left of Star B, select option "Left of B". In the Position Frame, enter the fraction BS/AB, i.e. the distance from Star B to the satellite, expressed as a fraction of the distance from Star A to B.
If the satellite appulsed (passed very near to) a star, then in the Geometry frame, select the "Appulse of A" option. Two input fields will appear in the Position frame, the first of which is the position-angle, labelled as "Pos Angle"
In ObsReduce, the position-angle is defined as the angle of the radius from the appulsed star to the satellite, at the moment of appulse, measured in units of degrees clockwise from the top of the FOV (field of view).
If you use ObsReduce to plot a satellite's track using its orbital elements, then it automatically computes an accurate estimate of its position angle relative your selected Star A, and enters the value in the "Pos Angle" box.
Frequently, satellites are observed to appulse stars on the side opposite that predicted by the latest orbital elements, in which case ObsReduce's estimate of the position angle will be wrong by 180 deg. To correct that, press the button labelled "+180", located to the right of Pos Angle input box.
If you prefer to use your own value of position angle, enter it into the Pos Angle input box.
If you prefer never to use ObsReduce's predicted position angle, you can turn it off by editing line 35 of the ObsReduce.ini file. Please note that if your ObsReduce.ini file came with a version of ObsReduce earlier than 1.1, then you must run ObsReduce 1.1 once to create the entry at line 35.
The second position data entry field is used either to specify or help compute the miss-distance, i.e. the angular separation between the satellite and the star at the moment of appulse.
If you estimated the miss-distance without reference to any stars, i.e. selected only the appulsed star, A, then the input box below the Pos Angle box will be labelled "Miss Dist", and you should enter your estimate there.
In case of a coincident appulse, i.e. the satellite appeared to pass directly over the star, enter zero as the miss-distance. The co-ordinates of Star A will be reported as the satellite's coordinates, and the position-angle will be ignored.
If you judged the miss-distance as the fraction of the distance between the appulsed star, A, and another star in the FOV, then the input box below the Pos Angle box will be labelled "Frac AS/AB", and you should enter your estimate there.
If you judged the miss-distance as the fraction of the distance between two other stars in the FOV, then the input box will be labelled as "Frac AS/BC"
ObsReduce computes the miss-distance internally, based upon the fractions you input, but has no room to display the result; however, you can peek at the value by placing your mouse cursor over the "Frac AS/AB" or "Frac AS/BC" label beside the input box.
The miss-distance has default units of degrees, but you may change this to arc minutes by entering min in ObsReduce.ini at line 13.
This is your estimate of the accuracy of your positional observation. You may set a default accuracy value in ObsReduce.ini at line 7. The default units are degrees, but you may change this to minutes by entering min in ObsReduce.ini at line 6.
Entering a positional accuracy of zero, instructs ObsReduce to insert its estimate the positional accuracy into an IOD or UK formatted report. It is computed as a fraction of the distance of between star A and B, or in the case of an appulse, as the fraction of the Miss Distance. The fraction used depends upon the geometry.
The defaults are 0.05 when the satellite passes on-line and between two stars, 0.07 when it passes on-line and outside two stars, 0.10 when it forms a right-angled triangle with two stars, and 0.15 in the case of an appulse. In the case of a coincident appulse, i.e. zero miss-distance, the default uncertainty is fixed at 0.005 deg. You may change the defaults, by editing lines 8 through 12 of ObsReduce.ini.
Even if you do not use ObsReduce's estimated positional accuracy within an IOD or UK report, the value will appear in the box labelled "Std. Pos Accy +/-", located near the bottom of the window.
The traditional geometries are ideal for non-camera observing, but they occur too infrequently to be relied upon for use with cameras. ObsReduce supports two special geometries ideally suited for use with cameras: satellite located to the right or left of a pair of stars, or any configuration of a satellite and three stars.
The reduction algorithms of both geometries require the physical length on the image between one pair of stars, and between each star and the satellite. The program accepts lengths measured using a ruler or calipers, or the pixel coordinates of the satellite and stars, from which it will compute the required lengths.
D.9.2.1 Measured Length
To work with lengths, select option "Length" in the Geometry frame. The Position frame will display several input controls, depending on the number of stars selected in the FOV.
In the case of a satellite located to the right or left of two reference stars, the Position frame displays input boxes for the measured length on the image between stars A and B, star A and the satellite, and star B and the satellite. Use the option buttons that appear at the bottom of the frame to specify whether the satellite was "Right of AB", or "Left of AB".
In the case of a satellite and three reference stars, the Position frame displays input boxes for the measured length on the image between stars A and B, star A and the satellite, star B and the satellite, and star C and the satellite.
D.9.2.2 Pixel Coordinates
To work with pixels, select option "XY" in the Geometry frame. The Position frame will display several input controls, depending on the number of stars selected in the FOV.
In the case of a satellite located to the right or left of two reference stars, the Position frame displays pairs of input boxes for the x and y pixel coordinates on the image of the satellite and stars A and B.
In the case of a satellite and three reference stars, the Position frame displays pairs of input boxes for the x and y pixel coordinates on the image of the satellite and stars A, B and C.
D.9.2.3 Accuracy Tips
In principle, using three stars should be more accurate than using two stars, but both can yield accuracy significantly greater than 0.01 deg cross-track, when used with suitable reference stars.
Measured lengths are scaled according to the distance between stars A and B. For this reason, it is recommended to choose star A as the one closest to the satellite and star B as the furthest from the satellite.
For two reference stars, A and B, if the satellite is between them, and is close to star A then an exact in-line or right angle configuration is best. Exact in-line can be forced by setting the measured length BS to the smallest non-zero value, i.e. 0.0001, but this should only be done when the satellite is between A and B. If the satellite is outside of AB, setting BS to zero will cause unreliable results.
If the satellite is between A and B and not in-line, then an equilateral geometry would minimize error. Close to in-line compounds the measurement uncertainty.
Using three reference stars, it would be optimum for minimizing measurement errors to get the satellite within the triangle formed by A, B and C, and all the lengths, AS, BS, and CS close to the same size.
If the satellite is near star A and the other reference stars are positioned to help reduce measurement error, a three star position will be superior to a two star. For instance, this will work well:
S
A
B C
WARNING! Avoid using three reference stars that are exactly in line. This will probably never occur in actual practice, but using it in a reduction would yield unreliable results.
Pixel coordinates may yield greater accuracy than measured lengths, but both will yield excellent results, when used with care.
In tests, repeated measurements of an on-screen length using calipers accurate to 0.001 inches, varied by 0.002 or 0.003 inches, indicative of the actual accuracy. This corresponds to a linear angular error of 5 seconds of a degree. Depending upon the geometry chosen to calculate a position from these measurements, error is estimated as anywhere from 60 seconds of a degree, worst case for an unfavorable geometry, to 20 seconds worst case for a more favorable geometry. Most situations will be more accurate than this, on the order of 10 seconds of a degree.
Due to the expected consistently high accuracy of camera observations, there is no provision to enter estimates of position uncertainty on a per observation basis. Instead, the program uses the same fixed value as for coincident appulses, i.e. zero miss-distance. The default uncertainty is fixed at 0.005 deg, which may be changed by editing line 12 of ObsReduce.ini.
D.9.3 Important Notes
IMPORTANT: ObsReduce interprets traditional geometry selections and position entries in accordance with the orientation of the FOV, as discussed in Section D.16. FOV Settings Frame.
IMPORTANT: Moving the satellite may result in automatic clearing of positional entries, and other data, as discussed in D.15.1 Automatic Observation Clearing.
If you seldom report optical data, you may instruct ObsReduce to skip past all of the optical fields during data entry, by entering n at line 24 of ObsReduce.ini.
Magnitude is entered via two text boxes, the meaning and labeling of which is determined by the reporting format.
In IOD format, the first box is labelled "Magnitude", and is used to enter the satellite's magnitude at the time of the observation; the second box is labelled "Unc +/-", and is used to enter the magnitude uncertainty. The default uncertainty is 1, which can be changed by editing line 23 of ObsReduce.ini.
In UK and RDE format, the boxes are labelled Brightest and Faintest, and are used to enter the satellite's magnitude extremes during the minute centred on the time of the observation. In the UK format, if both magnitudes were the same, only the brightest is reported, and the field for faintest is left blank. ObsReduce respects those rules.
Use this box to enter the satellite's period of brightness variation, in seconds.
The Optical Code specifies the object's optical behaviour. The choices depend upon the reporting format. You may set the default value at line 25 of ObsReduce.ini. If you usually do not report this or any other optical data, you may prefer to set the default to a blank space.
Pressing the Reduce button results in the following actions:
- reduces the positional observation based upon the coordinates of the selected reference stars and specified geometry and position
- formats the reduced observation in the chosen reporting format, and displays the result
- if the object's elements have been loaded, computes the cross-track and time difference between prediction and observation. See below.
Obs - Pred consists of the position cross-track and time difference between observation and prediction, with the track compensated for Earth's rotation in the interim. The cross-track difference is displayed in the same units as the position uncertainty, as specified in the ObsReduce.ini file at line 6.
These values are computed whenever the Reduce button is pressed and orbital elements have been loaded. Their sole purpose is to aid in finding and confirming the reference stars used in an observation. If you make several observations of the same object on a pass, it reasonable to expect them to have roughly similar cross-track and time differences, If not, then it is worth reviewing the input data to ensure that no data-entry or other processing error has been made.
One pitfall to be avoided is attempting to "improve" observations on the basis of the predicted track or the cross-track and time differences. Observations are made at the eyepiece, and cannot be improved afterward, by ObsReduce or any other means.
The only meaningful measurement of observation error is through the orbit determination process, in which a number of observations made over a period of time are analysed to produce a new set of orbital elements that best fit the observations.
The difference between predictions made with the new orbit and the observations, called residuals, are the best indication of the absolute and relative accuracy of the observations.
Differences between prediction and observation are to be welcomed - indeed they are the reason that we observe - to provide the data by which ever-changing orbital elements may be updated. Attempting to "correct out" the differences during the reduction process obliterates the very data that we seek to obtain.
D.12.1 Use As Early/Late Button
If the difference between the observed and predicted time is significant and you have additional observations to reduce, pressing this button will copy the time difference into the Satellite Early / Late text box (located to the left of the Plot Satellite button, so that all subsequent satellite track plots are compensated for Earth's rotation corresponding to early/late interval. In the vast majority of cases, this will place the reference stars near the centre of the FOV (field of view), thus speeding the task of identifying the correct stars.
The elset frame displays the age of the orbital elements used to compute the satellite's track, and an estimate of the uncertainty in the predicted time of the pass. This information is intended as a rough guide to the reliability of the prediction; it is probably most useful to observers who have a practical understanding of orbital elements, and how their accuracy degrades over time.
The elset age is the number of days from the epoch of the orbital elements to the time of the observation.
The prediction time uncertainty, shown in the box labelled Tunc, is the number of seconds that the object would be early or late, assuming that its rate of orbital decay is uncertain by 10 percent.
The prediction time uncertainty provides a reality-check of the time difference between observation and prediction, displayed in the box labelled Time, found in Obs - Pred frame.
For example, if the prediction time uncertainty is 1.5 s, then I should not be surprised to find that the time difference between my observation and the prediction is of the same order of magnitude.
Of course, the decay-rate uncertainty itself is uncertain, so it may differ greatly from the 10 percent assumed by ObsReduce, which in turn affects the time uncertainty estimate.
Fortunately, the prediction time uncertainty varies in direct proportion to the decay uncertainty, so if there is reason to believe that the decay uncertainty differs from 10 percent, it is easy to mentally adjust the time uncertainty displayed by ObsReduce.
For example, if the displayed time uncertainty is 0.5 s, but you suspect that the decay uncertainty is 50 percent, not the 10 percent assumed by ObsReduce, then mentally multiply 0.5 s by 50/10, which yields a time uncertainty of 2.5 s.
If the elements are fairly recent, and known to be reliable, then ObsReduce's assumption of a 10 percent decay-rate uncertainty probably is reasonable. Otherwise, the uncertainty is likely to be much higher, but that is a matter of judgemental.
The FOV Centre frame contains the controls to centre the field of view for a given date and time, anywhere in the sky, in coordinates of R.A. and Dec or AZimuth and ELevation.
This capability is required when reducing observations of objects for which orbital elements do not exist, or are very inaccurate. As long as the coordinates in the immediate vicinity of the reference stars are known, they can be displayed in the FOV, and the observation reduced.
It is important to note that the FOV centre date/time is distinct from the observation date/time, even though they usually have the same value. Keeping them distinct enables the object to be moved back and forth along its predicted track, which necessarily changes the prediction time, while preserving the observation date/time.
By design, whenever the observation date/time is entered or edited, the FOV Centre date/time is automatically synchronized to match, on the assumption that the predicted track will be sufficiently accurate to place the reference stars near the centre of the FOV at the time they were observed.
R.A. and Dec or AZimuth and ELevation entries take effect when the mouse cursor leaves the text box, such as when the Enter key is pressed, or the mouse is physically moved out of the box.
The controls in the Move Satellite frame enable an object to be moved backward or forward along its predicted track.
There are two sets of increment/decrement buttons: one that moves the satellite in increments that are some percentage of the diameter of the FOV; the other in increments of time in seconds. The size of either increment may be changed at run-time.
Both methods ultimately involve changing the time. When moving in increments of FOV diameter, the time increment is computed internally by dividing the angular increment by the object's angular velocity.
I believe that most users will prefer to move objects in increments of the FOV diameter, because angular velocity is automatically taken into account, ensuring that the object moves in constant, predictable steps, everywhere along its track.
A powerful feature is the ability to specify the element of time to be incremented, selected using the "by changing" option buttons: FOV Time or Early/Late.
If the time difference between the observation and prediction is small, or if it is large, but its size known and entered in the Satellite Early / Late text box, then incrementing the FOV Time (i.e. the date/time in the FOV Centre frame) will produce accurate plots.
If the satellite is significantly early or late by an unknown interval, and the location of the reference stars is unknown, but you have a good sketch of them and a few surrounding stars in the field of view, then follow the steps in Section C.1.3 to use Move Satellite to help bring them into the FOV.
D.15.1 Automatic Observation Clearing
Whenever you enter an object's geometrical and positional data, ObsReduce remembers the coordinates of the centre of the field of view. If a subsequent Move Satellite command causes those coordinates to move out of the field of view, then ObsReduce clears the positional entries and resets the time and position accuracy to their default values.
Similarly, causing stars that have been selected to exit the field of view, causes them to be automatically de-selected, clears the results of any reduction, clears the positional entries, and resets the time and position accuracy to their default values.
The controls in the FOV Settings frame enable the user to change the diameter, limiting magnitude and orientation of the field of view.
The FOV diameter may be varied between 0.1 deg and 20 deg. The default value may be set at line 26 of the ObsReduce.ini file. The limiting magnitude may be set no brighter than 5. The default value may be set at line 28 of the ObsReduce.ini file.
The orientation may be set to erect, inverted or mirror image, using the option buttons. It is important to note that the program interprets star selection, geometry and position entries consistent with the orientation. If the orientation is changed after a reduction has been made, the program automatically changes orientation and position entries to agree with the new orientation, such that the reduced observation does not change. The default orientation may be set at line 27 of the ObsReduce.ini file.
The IOD, UK, or RDE formatted result is displayed in the long text box near the bottom of the display.
Before saving your results, you should review your inputs to make certain they are correct. You may change any input and the result will be immediately reflected in the formatted observation at the bottom of the window.
For R.A. and Dec, ObsReduce enables you to change the number of significant digits after the decimal point from their default values, by using the increment/decrement buttons at the bottom of the window. UK format users can do likewise for the positional accuracy. To change your default values, edit lines 15 - 17 of ObsReduce.ini.
When you are satisfied with the accuracy and format of your results, you may save them by hitting the Save button.
As shipped, ObsReduce creates a directory named Observations as a subdirectory of the one in which it was installed. All four types of output file will be written to that sub-directory.
Results.obs is a text file log of all of your observations, in the chosen format.
For RDE (Russell Eberst) format, ObsReduce automatically writes the start-of-month header line, as well as the day of the month lines. It automatically keeps track of the need for such entries in a file called lastsessiondate.txt.
Details.obs is a text file log of the underlying details of your observations: coordinates and magnitude of reference stars, geometry, miss distance, as in this example:
2003 Jul 15 03:21:44.23 UTC 96072A 24680 (Base: 2003 Jul 14 23:36:23.42 UTC + 03:45:20.81 elapsed. Drift = 0.0 s/h) Star A: 14:30:30.5348 -04:03:54.715 (2000.0) Mag 7.41 Star B: 14:30:00.1397 -04:14:49.960 (2000.0) Mag 7.04 Object on line AB, 0.300 from A to B; Star Sep = 0.222 deg Object: 14:30:21.4050 -04:07:11.331 (2000.0) +/- 0.011 deg (0.050 X Star Sep) Obs - Pred: 0.036 deg X-track; 0.142 s early, relative 1.22 day old elset: 1 24680U 96072A 03194.91931763 .00014500 00000-0 15976-3 0 04 2 24680 97.8240 257.5084 0499000 278.3339 81.6660 14.82681039 09 24680 96 072A 2701 G 20030715032144230 17 25 1430357-040719 77 S
Session logs of the same information are stored in files named yyyymmdd.obs and yyyymmdd.det.
ObsReduce can only append to those files, so there is no danger that it will overwrite your data. Of course, it is prudent to make periodic backups.
You can change the location of the files, or cause some of them not to be produced, by editing lines 29 through 32 of ObsReduce.ini.
The star sep is the angular separation between the reference stars used in a reduction. It is displayed automatically each time a reduction is computed.
The standard positional accuracy is computed automatically each time a reduction is computed, as described in Section D.9.1.4.
D.21 Significant Decimals Buttons
The number of significant positions after the decimal of IOD and UK formatted results can be changed via the increment/decrement buttons located at the bottom of the window. Default values may be set by editing lines 15 and 16 of the ObsReduce.ini file. The program enforces the maximum and minimum number of significant positions as defined for each format.
UK format users may also vary the number of significant digits of the positional accuracy, using the increment/decrement button located at the bottom, right of the window. The default value may be set by editing line 17 of the ObsReduce.ini file.
The Clear Position button clears the entries made in the Position frame, the results of the reduction, all star selections, and resets the time and position accuracy to their default values. All else remains the same.
The same level of clearing occurs after a result has been saved.
D.23 Adding Stars to the Database
Although, ObsReduce's star database is derived from the Tycho 2 catalogue, which contains millions of stars, it is not complete, especially, but not exclusively, at the faintest magnitudes. As a result, you may occasionally find that a reference star you are certain exists, is not displayed by ObsReduce. Also, planets and asteroids appear star-like, and may occasionally be used as reference stars, but are not displayed by ObsReduce.
To mitigate such problems, ObsReduce supports the addition of stars to its database.
To add stars to the data base, create a text file named addstars.txt, and place it in the same folder (directory) that contains ObsReduce's star data files.
The data for each star added to the file must reside on one line, consisting of the following values:
1. Right ascension, 2000.0, radians
2. Proper motion of right ascension, mas/yr (i.e milli arc seconds per year)
3. Declination, 2000.0, radians
4. Proper motion of declination,(i.e milli arc seconds per year)
5. Visual magnitude
6. Comment or description (optional)
These values must appear in the above order, on a single line, separated by one or more spaces. If you do not know a star's proper motion, or the object is non-stellar, e.g. a planet or asteroid, enter proper motion as 0, for example:
0.6569371308 0 +1.55789494993 0 8.20 star
Since non-stellar objects change position rapidly, it is recommended to remove them upon completion of the reduction in which they were used. Alternatively, they may be prevented from being displayed by inserting a semi-colon at the start of the line, for example:
;3.3033984925 0 +0.06893177881 0 7.26 Pallas on 2005 Mar 09 at 10:51 UTC
Addstars.txt is read each time the simulated FOV is updated, so it is not necessary to restart ObsReduce for changes to the file to take effect.
Thank you David Brierley, Mike McCants, Ken Smith and Allen Thomson, for your encouragement, technical advice and alpha testing during the early development of ObsReduce.
Special thanks to Scott Campbell, who developed the algorithms used to reduce observations made with cameras, and performed the first beta tests of them.
Disclaimer: You install and use ObsReduce at your own risk. Ted Molczan accepts no responsibility for any adverse consequences arising from installing and using ObsReduce.
1. Download ObsReduce.zip, which contains all but ObsReduce's star files.
2. Download one of the following sets of star files:
Tycho85.zip (71,704 stars, 1.0 MB)
Tycho105.zip (540,312 stars, 6.6 MB)
Two additional Tycho star files exist; one contains stars to mag 12; the other contains all Tyhco stars. They are available only to positional observers who share their observations, and who have a demonstrated need. For further information, please contact webmaster@satobs.org.
For a given limiting-magnitude setting, regardless of the database size, there is no perceptible difference in the speed with which ObsReduce finds the stars to be displayed; therefore, the decision as to which set of star files to download should be made based on need, available disk space, and the speed and reliability of your Internet connection.
3. You may install ObsReduce in the same folder as a previous version, but before doing so, it is strongly recommended that you first make a copy of the old ObsReduce.ini file, then delete the old version of ObsReduce, using Windows' Add/Remove programs. ObsReduce.ini has changed from the beta version, but has many of the same entries, so you may wish to refer to it to help customize the new one. Eventually, I hope to eliminate ObsReduce.ini in favour of using Windows' Registry.
4. Copy ObsReduce.ZIP to a temporary directory, unzip the file, run Setup.exe, and respond to the prompts. You may receive an erroneous warning that you have insufficient disk space, which may be safely ignored. Some beta testers commented that Setup tended to issue overly conservative warnings, which they found best to ignore. You should respond according to your experience and confidence using Windows.
Unless you specify otherwise when prompted, ObsReduce will be installed at C:\Program Files\Obsreduce. You should make note of the installed directory, so that you may edit the initialization file located there, ObsReduce.ini.
5. To complete the installation, unzip the contents of the star files into the directory of your choice. As downloaded, ObsReduce expects to find the star files in the same directory in which it has been installed. If you choose to install them in a different directory, you must tell ObsReduce the path by editing line 34 of ObsReduce.ini.
6. ObsReduce comes with a sample orbital elements file, which may be outdated by the time you install it, so it is recommended to obtain a replacement containing the most recent available elements. Sources of orbital elements are listed here. To find your elements file, ObsReduce follows the path and name entered at line 33 of the ObsReduce.ini file.