Colour processing advice

[Matt S]

Dear experts!

Grateful if you could provide a few tips regarding colour processing in StarTools v1.8.

I’ve attached a recent image of NGC2403. It was taken with my wedge-mounted CPC925, at F6.3, using an ASI294MC Pro (OSC). The image comprises ~2:30hrs integration (2min subs) and is calibrated using darks, lights and biases and stacked in DSS. RGB channels were aligned in DSS, and the ‘second option’ was selected when loading in StarTools.

As you can see, I’m struggling with the red colour. This target has plenty of Ha regions, which I can only seem to process as a flashy neon-esque pink: very Buck Rogers, but not what I’m really after!

I’m under Bortle 4(ish) skies, although I have local interference by neighbours’ lights etc. I’m away from home and unable to upload the full data to my Dropbox account. I’ve tried sampling the galaxy and, separately, sampling the stars to set ‘white’ correctly.

Given the above, can you offer any tips?

Thanks for reading and clear skies.
Matt

[BobGaul]

What I do is make a star mask (only the stars are selected), open the color module and click "sample". This tells star tools that the stars are supposed to be white (on average). The back to the mask and select invert (all but the stars selected). Now back to the color module where the color balance set for the stars is now applied to the rest of the image.

[hixx]

Hi Matt,
Your whole image seems to have a purplish tint, even the stars and the background
Before You reach color module, please make sure to neutralize the background with WIPE. Do NOT perform background neutralization or RGB channel calibration in the stacker (DSS)! Also, what might turn weak stars into purple is strong Denoising

Furthermore, H-II regions will show the purple color, as they are purple indeed! Only H-a is red, but throw in H-b (486nm =blue-green), H-y (436nm= blue) and you'll get this pink/purple blend depending on how strong individual lines are...

Clear Skies,
Jochen

[Matt S]

Gentlemen
Thanks for the advice. I had another crack at processing NGC2403 last night: the result is attached. It looks more natural at least. I used the larger stars to ‘sample’ the white balance etc.
I’d still like to bring out more of the red star forming regions in the spiral arms. Can you offer any other tips? Would an L-Pro/broadband filter help, noting I’m under Bortle 4/5 skies. Or is it simply that 2.5hrs of integration time is just not enough..?
Thanks for reading this.
Clear skies
Matt S

[Mike in Rancho]

How are you getting your initial color balance settings? Though I will say the less neon version is an improvement, I think.

Bortle 4 is pretty dark to me, but 2.5 hours can still be pretty short for a galaxy, and f/6.3 isn't crazy fast like a RASA or anything. More integration will probably allow better Wipe and AutoDev, and maybe not so many other techniques being implemented to enhance the target and perhaps overcook it.

What star forming regions are you looking for that are red? I thought most were kind of blueish. But if there are Ha regions here, you can capture those if you have the right filter, and it would be right down the alley for NB Accent.

[Matt S]

Hi Mike
Thanks for the post.
In terms of colour balancing etc, I’m following the instructions for the proper use of DSS, using the ‘second option’ when first loading the stacked image into ST, and then following the star sampling method.
Wrt the ‘red colour’ I’m actually referring to the HII regions in the spiral arms. These are normally rendered as being bright red in other images, however this may be a touch of ‘collective artistic licence’.
I was tempted to use an Optolong L-Pro filter, thinking that might help, but given I’m under Bortle 4/5 skies, my impression is that this would be a waste of time.
Clear skies
M

[Mike in Rancho]

Hi Mike
Thanks for the post.
In terms of colour balancing etc, I’m following the instructions for the proper use of DSS, using the ‘second option’ when first loading the stacked image into ST, and then following the star sampling method.
Wrt the ‘red colour’ I’m actually referring to the HII regions in the spiral arms. These are normally rendered as being bright red in other images, however this may be a touch of ‘collective artistic licence’.
I was tempted to use an Optolong L-Pro filter, thinking that might help, but given I’m under Bortle 4/5 skies, my impression is that this would be a waste of time.
Clear skies
M

I'm not sure I've seen too many bright red, at least as far as the little Ha globules in galaxy images. They often seem to be pink or purple. Though some galaxies might have some really cool red structures - M106 perhaps.

But yeah what you say you are after sounds perfect for NB Accent, if you can capture some separate Ha or duoband data. And one of the options within is between a more red result or a more "Balmer purple" result for the Ha areas you want to poke through into the galaxy.