Exposure question (RGB imaging)

[billyharris72]

Hi all:

This is going to sound like a silly question (as I think I know the answer) but I thought I'd sample the collective wisdom on here before I risk another night's frustration (too few clear nights for experimentation without at least checking I'm on the right track)!

I've recently purchased the Astronomik LRGB filter (31mm) set as an alternative to the ZWO filters. I'[m using these on an ASI1600. These ZWO filters had been giving me problems - lots of scatter on the blue, and big halos with the Ha filter. I've used the new Ha filter, and despite being 12nm I have to say I much prefer it to the ZWO option - stars are nice and tight with no hint of the ugly halos that the ZWOs were giving me (though there is a tad less contrast).

In operation, I've found the Astronomik filters to be fine - no major noticeable issues in the field. Using the focus assist on both Sharpcap and AstroArt I was getting comparable star FWHM with all filters, and things seemed to be behaving well.

I finally got round to processing some data (IC405) at the weekend, only to find absolutely horrible chromatic issues, with the stars having very clearly defined, dense, turqoise halos. I also noticed that AstroArt, when set to automatically estimate colour balance, was combining RGB in 3:1:1 ratios. In the individual RGB stacks, both green and blue had markedly larger FWHM values than the red, but also, the background level for red was only about a third of that for the green and blue.

My working hypothesis is that this has to do with the response curve of the ASI1600 sensor, and that the larger FWHM may be due to relative over exposure causing the stars to bloat (does that happen like that)?

With this in mind, does it make sense to reduce exposure times for blue and green, while increasing for red? Would that affect FWHM, and is getting these (and presumably the background levels) to match liekly to help with the colour fringing issues?

Thoughts and advice appreciated.

Thanks,

Billy.

 

 

 

 

 

[ollypenrice]

I don't think you're alone in struggling with star halos and colour balance with a CMOS camera. One of our robotic users is finding similar things. The ideal solution would be to find someone gettng good results on your kind of target with the same camera and to ask them how they are doing it. Maybe Rodd on here would be able to help.

I don't like the Trichromy combining feature in AstroArt 6 and have reverted to V5 where, with my setups, I can just combine at parity and then run a DBE for excellent results.

Best of luck with it.

Olly

[eshy76]

Hi Billy - I use different LRGB filters to yours, but I also have the ASI1600.

For me in Surrey (Bortle 7-8), I use exposure times of 30 seconds for G and B and 60 seconds for R. That is at Unity gain (139). These get me to about 1500-1700 ADU (on a 16-bit basis) per sub per filter - I have to expose twice as long for the R filter to get the same ADU per sub. My approach is dithered shorter subs (needed due to light pollution) and lots of them for stacking.

Or for your problem - I expose for half the time with G and B as the subs saturate faster with those filters.

To rule out over saturation, you certainly try lower exposure times for G and B - what kind of ADU are you getting per sub and what is your gain? Assuming you are using the default bias offset of 50, then the maths suggest you should be able to drown out the read noise at unity gain with subs with background as low as 1400 ADU - you can judge your exposure time based off that as a lower limit and your own level of light pollution.

I hope this helps!