How far to process an image (M13)

Hi Guys,

I was very grateful for the balanced and honest feedback you gave me in my previous post (DSS vs Siril), so I thought I pursue that line.

After stacking with Siril, applying the wonderful photometric color calibration, asinh stretch (1000) and the default deconvolulation, I imported to GIMP in tif format:

I then only applied 2 commands, I raised the black level to well below the histogram (no clipping, I hope):

And then I applied an s-curve to brighten the brightest parts, while leaving the background untouched:

This was the result:

While I obviously need to fix my collimation, I look forward to your candid comments and maybe any suggestions on what more I could do to improve the image.


DSLR: Nikon D5000,
Frames: 60 x 180 secs @ ISO 1600 & temp 284-5 K
Darks: 40 x 180 secs @ ISO 1600 & 288 K
Biases: 100 @ ISO 1600
Flats: 100 @ ISO 100 (Minus flat-biases also @ ISO 100)
Celestron C8-N with goto mount & FL 1000 mm
Guided with QHY5 and guidescope FL 327 mm



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There is a histogram tool in siril that can do what you do in Gimp, maybe you can use one tool less in your processing pipeline. The asinh transformation is a good start. Without nebulae it’s harder to see if information from the background is lost, but at the end of stacking, the background should not be completely black (0 value), otherwise it may mean that your preprocessing is wrong, which often happens because of the bias being removed twice.

Hi Vincent,

So here is is with the Siril histogram. The image looks great, but there is a lot of background noise, mostly fixed pattern. I find I have more control in GIMP:


If I could control it more precisely in Siril, that would be great.



This kind of pattern appears frequently with CMOS sensors. To make it disappear, moving the telescope throughout the acquisition of the target is recommended, that would force different photo-sites of the camera to be used for the same image pixels, resulting in blending and smoothing out of the pattern. This is called dithering in astronomy, the fact of moving a few arc seconds every few images to have a more random registration than with very good guiding.

The other thing is that here the background is very bright so making it a little darker would make the noise appear less. Unfortunately I think that removing the noise and chrominance noise from the background is mostly a question of increasing the total exposure, but you already have a lot for this object here. Make sure you use the dark optimization since your darks and frames are not taken at the same temperature.

Hi @vinvin ,

Thank you very much for your detailed points.

Dithering. I did think I had dithering active here, however, on examination of the PHD log, I discovered that I hadn’t worked. I need to check. I’ve never really got it to work satisfactorily.

I think I did use dark optimisation, but I’ll double check. I normally allow the temperature to vary by up to 5 K or so. More than that, I retake the darks. I understand that noise doubles for every 12 K. If I had a cooled camera, obviously it would be very different. :frowning:

Longer. My light frames are 180 s x 60, so 3 hours. Do you think if it were,say, 12 hours, I would get more signal and less noise?



Dark optimization is made for cameras without stabilised temperature. When it varies by a few degrees between takes, it compensates the levels of the dark and generally works very well. Even for more than 5 K.
I think 3 hours should be more than enough for M13… I assume something else is wrong, maybe it depends mostly on the camera, I don’t know how this one behaves. Logically with more exposure there shall be less noise, but I’d try to perfect the current image and see if it’s too noisy or not at a normal sky level before thinking about adding hours. Good luck :slight_smile:

Hi @vinvin

I really like the way Siril stretches the histogram, apart from the red fixed-pattern noise. I can’t think of any way of removing it, except maybe re-running the data capture with dithering actually working. So next time I’m really going to push the dithering aspect and see if I can get it properly.

I’m sure there are more sophisticated wayts of editing with GIMP, for instance using masks and layers, I’m just not familiar enough with it do it properly.