Background sky: effect on histogram

Discussions of using SharpCap for Planetary Imaging
Post Reply
windytops
Posts: 3
Joined: Fri Aug 14, 2020 9:49 am

Background sky: effect on histogram

Post by windytops »

[1] Is there a way to remove or largely remove the background contribution to a planetary histogram? My histograms usually show a peak at both black and white ends. I assume that the background sky helps form the left side peak and the planet the right hand one. At present I clip off the histogram to the left of the black peak. I do this because the histogram of the sky alone often mirrors this end of the curve closely. But since I am interested in faint grey marks in the atmosphere of Venus I am worried that this process could be invalid- I might be losing some information. Therefore I do not extend the clipping zone any higher. Is there a way to subtract the all the sky, in a similar way say to dark frames?
[2] I try to reduce any invalidity by making the ROI box very similar to the planet size. This reduces the sky contribution, but there is still some in the corners of course. And most conditions do not allow a really tight fit to the planet. Would it be a possible, as a long term development, to have a circular ROI option and hence reduce the captured background even further?
User avatar
admin
Site Admin
Posts: 6630
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Background sky: effect on histogram

Post by admin »

Hi,

I'm afraid I'm struggling to understand why it would make any difference.

If you are using the histogram as a tool to adjust exposure then either the brightness of pixels in the planetary disk is well separated from the sky background brightness or it isn't (two separate peaks or two overlapping peaks). Tinkering with the selection area will just show different views on the same data, but won't change anything fundamental.

If the histogram shape is actually the primary result of your measurements then maybe that makes more sense, although even then I'm not sure.

Perhaps a couple of screenshots showing what you mean will help make it clear?

cheers,

Robin
windytops
Posts: 3
Joined: Fri Aug 14, 2020 9:49 am

Re: Background sky: effect on histogram

Post by windytops »

Venus is not conveniently positioned at the moment, and the weather has not been kind to me either. So I cannot promise a histogram example yet.
But Venus detail is often best seen at a time when the background sky level is not negligible [except in Ultra Violet] So the histogram appears to take the form of two overlapping peaks. In Post-processing, [Registax] I do the clipping and find that the background darkens and the planet limb sharpens. But the phase also changes because clipping reduces the terminator. My worry was that in clipping off the light grey of the sky, I might be also clipping off some of the the light grey of the surface markings. But you seem to be reassuring me that that will not be the case. Thanks.
User avatar
admin
Site Admin
Posts: 6630
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Background sky: effect on histogram

Post by admin »

Hi,

I think the clipping will really only affect the terminator/edges - the brightness of any interior part of the planet will be well above the clipping level - although there are brightness variations they are small compared to the difference between planet and background.

Hope you get some better weather soon!

Robin
Post Reply