5 replies to this topic

[Rac19]

Having given up on actually tracking the ISS, I am now looking at tracking a Moon transit or near transit.

 

There is a forecast Moon grazing event coming up which raises the question of how to set exposure for such an event. The ISS is very bright but I think that the Moon is much brighter still. Should I set exposure to underexpose the Moon as I wait for the ISS to sweep past it.

 

EDIT: Some predicted passes in the next 12 months, as per SkyTrack.

 

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Edited by Rac19, 02 November 2024 - 10:56 PM.

[Winteria]

A google search of "ISS lunar transit" shows that when illuminated by the sun the station (mainly the radiator panels) is brighter than the lunar surface, so yes, you should underexpose the moon. 

 

Some examples:

 

https://spacestation..._Pan_2048px.jpg

https://youtu.be/tMppAqQaFJ8?t=11

 

I would post my exposure/gain settings for my ISS data and my lunar data to compare, but sadly I didn't save any capture log files for the latter.

[Rac19]

Thanks, the ISS is clearly brighter than the Moon.

 

What file format was used SER, FITS, AVI, PNG...?

 

Also, is the time gap between images as natively occurs during image capture or was it explicitly set?
 

Thanks again for the info.

[Winteria]

Thanks, the ISS is clearly brighter than the Moon.

 

What file format was used SER, FITS, AVI, PNG...?

 

Also, is the time gap between images as natively occurs during image capture or was it explicitly set?
 

Thanks again for the info.

The links I posted are not my data, so I don't know. Sorry.

 

SER or AVI would be best, and I imagine the time gap depends on the max fps of the cameras used for those sequences.

Edited by Winteria, 04 November 2024 - 11:19 AM.

[gregghallinan]

Hi Richard,

 

The Moon and ISS are the same distance from the Sun, broadly speaking, so the irradiance level is about the same. The ratio of apparent brightness is therefore just the ratio of the reflectivity/albedo of both objects. The geometric albedo of the moon is ~0.12. Solar panels are usually designed to have low reflectivity, but even those typically have reflectivity higher than 0.12, although the ISS solar panels are probably better than most. The rest of the ISS will be significantly higher in reflectivity, particularly the radiators. This is consistent with the video shared above. So, in summary, the Moon will be about the same brightness as the ISS solar panels and less bright than the rest of the ISS, so as Winteria suggests, under-exposing the moon is correct.

 

However, this assumes that the ISS is illuminated by the Sun from your perspective! Most of the time it will not be illuminated and you should expose the lunar surface as normal, because all you will see is a silhouette. Here's an example of the latter (not by me!) - https://www.youtube....h?v=TPyH-hgvwds

[Rac19]

Thanks for both responses. I think that it has clarified in my mind the issues that I have been wrestling with. The Moon and ISS may be about the same distance from the Sun but the latter is obviously much closer to Earth. I think that for a full Moon, the ISS is going to be in Earth's, at least for most of the transit, so exposure should be for a silhouette image.

 

The examples in the OP are all grazing passes so there will be no silhouette. In the first example, quarter moon at dusk, I think that maybe if I expose for the Moon, the ISS could be bright or overexposed. The rest are fullish Moons so that the ISS could be shaded to various degrees by Earth. One thing that StarTrack shows is when ISS will be sunlit so that should be useful.