by Tom Glenn » Thu Jul 21, 2022 6:32 am
Thanks for all of the comments! Below are a few answers that may be of interest.
De58te wrote: ↑Sat Jul 09, 2022 3:46 pm
Just out of curiosity, is Tom Glenn any relation to astronaut John Glenn?
No relation, just coincidental names!
VictorBorun wrote: ↑Sat Jul 09, 2022 11:46 pm
However in the video frame posted Saturn's pixels are 60/255 gray and the ISS's pixels are 255/255 white.
So the ISS is just 4 times brighter.
And solar panels are 60/277, no brighter than Saturn.
The tonal values you quote above are correct, with the solar panels and Saturn both at about 60/255, and the brightest parts of the ISS saturated at 255/255. However, the image is
not presented as linear (most images are not) and so the relationship between 60 and 255 is not 4x. Instead, a nonlinear curve (gamma of approximately 2.2) was applied globally to the image. This means that the difference between 60 and 255 is actually about 23x. Interestingly, the difference in magnitudes between the two objects would imply an approximate 28x difference in brightness, but that assumes point sources, which these are not. In fact, the ISS is very tricky to expose because it has inconsistent specular highlights that vary considerably from pass to pass. I estimate it was overexposed by about 1 stop (2x) here, so the difference in brightness between Saturn and the ISS as observed in the image is probably about 46x difference. The image had to be exposed for the ISS, which leaves Saturn almost
invisible in the raw linear file, but it was revealed upon the gamma transformation, as shown in the comparison set of images below.
Linear data:
Gamma transformed data
VictorBorun wrote: ↑Sat Jul 09, 2022 11:46 pm
could the ISS be partialy shadowed?
The ISS was completely out of Earth's shadow. You may be thinking of images in which you can see the ISS exhibiting shadows that are cast by the
spacecraft itself, which are quite common, and very interesting to observe.
alter-ego wrote: ↑Sun Jul 10, 2022 11:48 pm
I
SS is not in the shadow.
Per
Heavens Above and the "challenging image" link, it exited the shadow at 4:25:06am PDT (1m 17s before capture). I used the site coordinates that Tom listed at the link.
You are correct, but out of interest for readers I would point out that when retrospectively analyzing orbits using Heavens Above, the results become less accurate as time passes because the program uses the current orbital elements (at the time of your query) to produce the charts, and those are only accurate near the epoch. So even in your star chart, which was produced well after the event, we can see some slight discrepancies with respect to both position and time for the ISS if we compare to the chart I produced at the time of the event.
Thanks for all of the comments! Below are a few answers that may be of interest.
[quote=De58te post_id=324042 time=1657381597 user_id=141631]
Just out of curiosity, is Tom Glenn any relation to astronaut John Glenn?
[/quote]
No relation, just coincidental names!
[quote=VictorBorun post_id=324051 time=1657410380 user_id=145500]
However in the video frame posted Saturn's pixels are 60/255 gray and the ISS's pixels are 255/255 white.
So the ISS is just 4 times brighter.
And solar panels are 60/277, no brighter than Saturn.
[/quote]
The tonal values you quote above are correct, with the solar panels and Saturn both at about 60/255, and the brightest parts of the ISS saturated at 255/255. However, the image is [i]not[/i] presented as linear (most images are not) and so the relationship between 60 and 255 is not 4x. Instead, a nonlinear curve (gamma of approximately 2.2) was applied globally to the image. This means that the difference between 60 and 255 is actually about 23x. Interestingly, the difference in magnitudes between the two objects would imply an approximate 28x difference in brightness, but that assumes point sources, which these are not. In fact, the ISS is very tricky to expose because it has inconsistent specular highlights that vary considerably from pass to pass. I estimate it was overexposed by about 1 stop (2x) here, so the difference in brightness between Saturn and the ISS as observed in the image is probably about 46x difference. The image had to be exposed for the ISS, which leaves Saturn almost [i]invisible[/i] in the raw linear file, but it was revealed upon the gamma transformation, as shown in the comparison set of images below.
[b]Linear data: [/b]
[attachment=2]ISS-Saturn-TGlenn-linear.jpg[/attachment]
[b]Gamma transformed data
[/b][attachment=1]ISS-Saturn-TGlenn-gamma.jpg[/attachment]
[quote=VictorBorun post_id=324051 time=1657410380 user_id=145500]
could the ISS be partialy shadowed?[/quote]
The ISS was completely out of Earth's shadow. You may be thinking of images in which you can see the ISS exhibiting shadows that are cast by the [i]spacecraft itself[/i], which are quite common, and very interesting to observe.
[quote=alter-ego post_id=324082 time=1657496913 user_id=125299]
I[b]SS is not in the shadow. [/b]
Per [url=https://heavens-above.com/]Heavens Above[/url] and the "challenging image" link, it exited the shadow at 4:25:06am PDT (1m 17s before capture). I used the site coordinates that Tom listed at the link.
[/quote]
You are correct, but out of interest for readers I would point out that when retrospectively analyzing orbits using Heavens Above, the results become less accurate as time passes because the program uses the current orbital elements (at the time of your query) to produce the charts, and those are only accurate near the epoch. So even in your star chart, which was produced well after the event, we can see some slight discrepancies with respect to both position and time for the ISS if we compare to the chart I produced at the time of the event.
[attachment=0]Heavens-above.jpg[/attachment]