Re: Starlink brightness observations of Russell Eberst posted by Ted Molczan

From: Anthony Mallama via Seesat-l <seesat-l_at_satobs.org>
Date: Sat, 29 Feb 2020 01:07:19 -0500
Thierry,

Thank you for replying to my posting about Starlink magnitudes. Your
accurate video data is just what will be needed in order to precisely model
the brightness of these satellites.

"This is consistent with the video I have taken on Feb 22nd of several
units (launched Jan 7th) where I identified two populations, mag 2.5 for
the brightest (including Darksat) and 4 to 4.5 for the dimmest:"

Since this video was taken only six weeks after the launch, perhaps the
satellites were not yet in their final orientation. That could lead to the
two populations of magnitudes. On January 16 I saw 59 of the satellites
launched on January 7, and I noticed a large brightness distribution.

"So far, I got much brighter magnitudes than the ones measured by Russel,
but I don't understand why."

One possible explanation is that you are measuring brightness for
satellites at higher elevations above the horizon. As I mentioned in my
original posting, the shape and orientation of the satellites causes them
to be seen face-on when overhead but edge-on when near the horizon.

When I correct my observed magnitudes to a distance of 1,000 km those
recorded at elevations > 40 degrees average 5.8 magnitude, while for 30 <
elevation <= 40 they average 6.1, and for elevation <= 30 they average 6.7.
If you adjust my magnitude 5.8 to an altitude of 550 km the result is 4.5
which agrees rather closely with the high elevation findings you reported.

So, one interesting experiment for your video equipment would be to record
some low elevation passes. If my hypothesis about the Starlink orientation
is correct, then the magnitudes should be fainter even after they are
adjusted to a standard distance of 550 or 1,000 km.

Best regards,
Tony Mallama


On Friday, February 28, 2020, Thierry Legault <legault_at_club-internet.fr>
wrote:

> Hello
>
> I have taken new videos Wednesday evening from Versailles, France, between
> 19:00 and 19:45 UTC.
> The satellites are all from Nov 11th 2019 launch: #1056, #1054, #1062,
> #1053, #1017, #1067, #1058, #1943.
> Altitude range at culmination is 630 km to 555 km (altitude above horizon
> 62° to 88°).
>
> Careful examination of videos and comparison with many stars in the field
> show very consistent magnitude for all units: 4.2 plus or minus 0.2 at
> culmination. Brightness is fairly stable during a passage, without any
> flare.
>
> This is consistent with the video I have taken on Feb 22nd of several
> units (launched Jan 7th) where I identified two populations, mag 2.5 for
> the brightest (including Darksat) and 4 to 4.5 for the dimmest:
> http://www.astrophoto.fr/starlink_20200222_magnitude_fb.jpg
>
> So far, I got much brighter magnitudes than the ones measured by Russel,
> but I don't understand why.
>
> Note: the videos are taken with Sony A7S with Atomos Shogun in 4K low
> compression ProResHQ 800mbit/s.
>
> Regards
>
> At 15:33 26/02/2020, Anthony Mallama via Seesat-l wrote:
>
>> Ted,
>>
>> Thanks for sharing Russell Eberst's magnitude data. I found that the
>> average 1,000 km magnitude for the satellites at the final 550 km altitude
>> is 6.8. When that is adjusted to 550 km it becomes magnitude 5.6.
>>
>> Your plot shows a moderately weak correlation between brightness and phase
>> angle (again for the satellites at the final 550 km altitude). So, these
>> data do not indicate a very pronounced backward or forward scattering of
>> sunlight - more like a diffuse scattering. Additional data at high and low
>> phase angles would provide a more definitive result though.
>>
>> Since Starlink satellites themselves are shaped like a table top, I would
>> expect to see a fairly strong dependency of distance-corrected brightness
>> on elevation above the horizon. The satellites are seen face-on
>> (presumably
>> brighter) when overhead but edge-on (fainter) when near the horizon, if I
>> understand their orientation correctly.
>>
>> There could also be a relationship between brightness and the azimuth
>> difference between satellite and sun - due to scattering. (Similar to
>> phase
>> angle though not quite the same.) Altogether, the brightness model might
>> be
>> fairly complex.
>>
>> Question: If the satellite elevation and azimuth turn out to be important,
>> can those quantities be determined for past observations? I ask because
>> positions from TLEs become inaccurate pretty quickly. So, I'm wondering
>> whether az and el should be recorded at the time of observation.
>>
>> In my opinion, the brightness of Starlink satellites is an important issue
>> (ask any astronomer) and a major way in which we observers can contribute
>> to our field of study. Anyone who wants to make visual brightness
>> estimates
>> can find instructions for stars at aavso.org. The same method can be
>> applied to satellites.
>>
>> Lastly, I can send you a spreadsheet with my first 20 magnitude
>> observations if you want to include them in your analysis. They're not in
>> the format you're using though as they include az and el.
>>
>> Best regards,
>> Tony Mallama
>> _______________________________________________
>> Seesat-l mailing list
>> http://mailman.satobs.org/mailman/listinfo/seesat-l
>>
>
> Thierry Legault
> www.astrophoto.fr
>
>
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Received on Sat Feb 29 2020 - 00:07:55 UTC

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