SharpCap Forums

I first attempted it 3 years ago:
viewtopic.php?p=4487#p4487

That was one of my first LRGB efforts, but as you can see my refractor is horrible at focusing blue. Since then I developed a technique for capturing color with only red and green filters, and a Wratten #12 for luminance. Now, having eliminated blue wavelengths, stars are better behaved:
Depending on which monitor I view it in, I think it is a little bit too "cold". I'll play with that and post a new image when available.

William Optics 71mm f/5.9
Atik 314E CCD
Optolong RG filters
GSO Wratten #12 filter
Unitron Model 142 GEM
DIY Active Guiding with PHD2 and Raspberry Pi
3D-printed Newtonian Mask for diffraction spikes

Gain: 0.21 e-/ADU
Full Well Depth: 13.4ke-
Read Noise: 5.3e-

W12: 15x120s
R: 16x240s
G: 10x240s

Total integration time: 2.23 hours

78% Moon, 110 degrees away.
Altitude +30 degrees at start, +2 degrees at end.

Bortle 5 normally, but probably Bortle 6 with the Moon tonight.

APP 1.8.2
Affinity Photo 1.9.2.1035

Brian

Looks great
Here on my monitor indeed the cluster looks indeed a bit less red/warm when compared to other NGC 6791 captures I did see. In your capture it leans a bit towards purple?

Menno

Hi Menno,

That is the one thing that I dislike about color astrophotography: color balance. This time I relied 100% on Star Color Calibration, but I understand that it can be fooled. An image can skew blue if there are more red stars than blue. Also, everyone's monitor is different.

Sara Wager says that she literally takes days to tweak color balance. The trick is to make an adjustment, and then walk away from it. When she returns, she bases the next adjustment on her first impression. She says you can't give your brain the time to convince itself that the image is something that it is not. I too find this to be true. The longer I look at my image, the better it looks!

Brian

Hi Brian

So true all that.
More then once I thought I had it right after hours of work. Then looking at it the next day and deciding to start over
Also, (ambient) lighting in the room. For the first, rough processing steps it doesn't matter here. But the finer processing I really do when it's dark(er) outside. This is also where I calibrated my monitor for. Will therefor also look at this again tonight
And with capturing also why the advise for white balance with R50/B50 is so important I think. And then with processing not altering the color balance (except in some cases the background), only enhancing it when necessary.

And yes, I use/try the Photometric Color Calibration of SiriL and with M31 that gave very weird results

Menno

Hey Menno,

I reprocessed the image using a small shift from blue to red. I double checked star colors against a photometric database.

What do you think?

Brian

Hey Brian

This looks better IMO. Stars still have a hint of blueish and the cluster looks better now.
This is also a kind of gut feeling btw. This version "feels" better

Menno

Menno,

Are you talking about red stars having a blue fringe? That is due to using the same exposure for both the red and green filter. My sensor is much more sensitive to green wavelengths than red, therefore stars are slightly larger with the green filter. APP has an option to correct for chromatic aberration. I'll have to check that box next go-round. Another solution is to use different exposures for red and green. I've done that in the past. It has worked well, but it also means more dark frames. If I had a ZWO ASI1600MM, then I would purchase their matching LRGB filter set. That then enables a person to use the same exposure for RGB. Mine are Optolong where the transmittance is equal in each band. Another complication is that my pixels are a little large for my focal length, therefore faint stars are box-like. All of these things add up to make a less-than-optimal image.

Brian

Brian

Ah no, I was comparing it with the first image. The stars there were very blue. The 2nd one has way better star colors: white with a hint of blue which is good I think

Menno

Menno,

The cluster is full of red stars. That I know. It's not surprising that Star Color Calibration skewed the image towards blue after seeing all of those red stars! The cluster really pops now.

Brian

Sometimes I really do need to describe things better :p
I did mean the stars surrounding the cluster. Those were blue and now white and thus indeed the cluster itself now indeed red