Restore star colour in PixInsight

[Albastars]

I'm a newbie at this and, as recommended in another thread, I tried a star cluster just after full moon last week.

I imaged Caroline's Rose - NGC 7789 last week with the moon at 62%.

• Canon 800D - modded but not full spectrum
• WO Z73ZS (430mm)
• 4hrs integration (120 x 120secs) plus 30 bias, 30 darks and 30 flats
• ISO 200

I usually expose for nebulae at ISO 400 and 3 min subs but went with the above to prevent blowing out the stars.  That didn't work too well it seems.

I've tried Photometric Colour Calibration and the Repaired HSV Separation script that I have seen recommended but I cannot get colour back in the stars.  After sorting the background and gradient plus a bit of deconvolution I'm happy enough with the image - it just lacks colour!  A stacked and cropped (no other processing) xisf file is attached if anyone has the time/inclination to look at it. Can this image be rescued?

 Re the exposure:  I found this lovely pic on Astrobin - https://www.astrobin.com/ey3xlq/ - 30sec subs at ISO 800.  Are 120secs too long no matter what the ISO?

2BREF_crop.xisf

[wimvb]

Have you tried arcsinh stretch as the first stretch? This process is best, imo, at keeping star colour.

[City9Town0]

I'm with Wimvb... and a pixelmath blend of Histogram stretch, arcsinh, and masked stretch may help.

[wimvb]

1 hour ago, Albastars said:

Are 120secs too long no matter what the ISO?

All else being equal, 120 s @ iso 200 should be equivalent to 30 s @ iso 800, and a good as choice as any for a first try. The best exposure time depends on many factors, including optics, tracking accuracy and sky conditions. As long as stars sren’t overexposed in your subs, you should be able to get colour in your final image.

Edited August 30, 2021 by wimvb

[wimvb]

Arcsinh stretch followed by curves transformation shows plenty of colour.

The reason why the star colour in the cluster is less obvious in your images is most likely due to the difference in telescope. Your telescope has a focal length of 430 mm and is a refractor with minimal diffraction. The reference image was taken with a 650 mm reflecting telescope. Reflectors create star spikes and diffraction due to the central obstruction. Both will bring colour into the image.

Atmospheric differences and tracking differences, degree of (de-)focus and collimation play a role as well, because they affect how stars appear in an image.

[Albastars]

Thanks Wim and City9Town0.  I tried arcsinh initially but it didn't work for me - I was a bit too aggressive I think.  On seeing Wim's processing (thanks for taking the time) I gave it another go and got a better result.

Great info Wim about the differences between scopes.  It would seem some of my problems are due to my expectations!

For my own curiosity I'll try again but with a shorter exposure time.

Thanks again.