Help understand over and under exposure on histogram

[turfpit]

bluesilver

At the start of an imaging session, I use FITS Liberator (v3) with a test frame to:

• Ensure I have a correctly exposed histogram (Min > 0, Max < 65535) with help from the clipping boxes on the right hand side of the window.

• Check the Image headers to ensure I have correctly set up all the FITS entries for camera/optics etc.

• Apply the ArcSinh(ArcSinh(x)) stretch to check the image.

The histogram shown is FITS Liberator v3. There is also a FITS Liberator v4 available now.

• v3, 5Mb download, Windows

• v4, 75Mb download, Windows, Mac, Linux (.deb & .rpm)

For processing though, I use Pixinsight.

You might be interested in viewtopic.php?p=29445, Eta Carinae and the Keyhole Nebula, Ha/OIII stacks processed with PixInsight.

Dave

[bluesilver]

Tried 120 second exposures last night, got a better looking change in the histogram, I did flatten out a little towards the left, but still had a lot of saw tooth edges to it, yet to process the 120 second image.
Will have to re search through SharpCap to see how to load all images in and let it run through them so it can stack them up to see if I can get a better looking histogram, loading a single 120 second image in, the histogram just doesn't look right.
Still reading up about the Fits Liberator.
This is the final but very quick processed image of the 180 second exposures that were way over exposed and scaled now to post on here, processed in Pixinsight

Keyhole Nebula2.jpg (745 KiB) Viewed 1258 times

[turfpit]

Peter

That looks a decent image. Stars not bloated, star colours evident, plenty of faint detail. Now you will be able to relate the SharpCap histogram to the likely outcome.

Dave

[oopfan]

Peter,

This looks fine. Do you consider this over-exposed? How so?

In astrophotography, the objective is to capture the nebula, from the faintest wisps to the brightest bits. The stars are a side-effect. Very few cameras have the dynamic range to capture it all without saturating stars. Fortunately, most post-processing software packages have a Star Size Reduction tool made for this specific purpose.

So depending on your goal:
1. If astrophotography, then expose for the nebula and to heck with the stars.
2. If photometry, then expose for the stars to prevent saturation and to heck with the nebula.

If I were you, I'd be very happy with this result.

By the way, I'm not exactly sure what I'm looking at. Is this a single 120s frame or a single 180s frame or a stack of 120s frames or a stack of 180s frames. How large is the stack?

We use notation that states it succinctly:
1x120s = a single 120s frame.
1x180s = a single 180s frame.
6x120s = a stack of six 120s frames.
4x180s = a stack of four 180s frames.

Often we incorporate the notation in the file name. For example "Keyhole 6x120s".

Brian

[turfpit]

Peter

A small adjustment of contrast/saturation (with PixInsight). Of course at this stage in processing it becomes 'adjustments subject to taste'. Bortle3 and a modern low noise camera make a good combination it seems.

I would be interested to see the screenshots of FITS Liberator for a single 120s and 180s frame (there is much to be learned from the Min/Max values).

Keyhole Nebula4.jpg (909.94 KiB) Viewed 1237 times

Dave

[bluesilver]

Appreciate the replies and advice, I will get to FITS Liberator and learn how to use it, just been a tad busy with work and taking advantage of the few clear nights.
The image was a stack of 55 x180 seconds, calibrated with flats, darks, bias.
I have just done 36 x 300 seconds with a Optolong L-Ultimate filter
I plan to extract the Ha data from the filter stack and then add it to the red channel of the 180 second stack, then combine it all back together.
Still reading up on how to do this, but have the basics.

[oopfan]

Peter,

A stack of 55x180s. I was thinking you were able to do that in a single 180s frame! Great work. Excellent guiding.

Brian