[RCC] M81 & M82

Reg Pratt:
Bring the background levels up a bit so that they aren't clipping. I'm sure you did that to help clean the background up but see if you can reduce the clipping a little bit. You could try making a red mask to bring the HA in M81 out a little better. Try stretching your stars with Arcsinh stretch to help with color. This is a solid first light!

Thanks Reg, 

thanks I will try those tips, I did arcsinh stretch the stars a bit but probably could go harder on that. Color masks are probably the thing that would help th emost on the HA in M81. 

Cheers,
Jeremy

Reg Pratt:
Did you stretch the stars with the main image? My usual flow is to remove the stars while the image is linear, stretch them with HST a bit, then finish the stretch with Arcsinh. Stretch in small iterations. You'll know if you've gone too far because the stars will start looking cooked.

I did some simple histogram and curve stretches on the main image. Then did arcsinh on the star image following StarXterminator. If I go too far it looks a little too cooked for my taste, but I think I can dial it up a bit still.

Reg Pratt:
Doing 180s subs  in sky that bright is certainly working against you. You could always go back and do a few minutes of very short subs to ensure they aren't saturated and use those instead. You'll have much better star color then.

Good idea, I've never done separate star lights, but here it would make a lot of sense. Too bad the forecast is cloudy for the next week

Reg Pratt:
Did you stretch the stars with the main image? My usual flow is to remove the stars while the image is linear, stretch them with HST a bit, then finish the stretch with Arcsinh. Stretch in small iterations. You'll know if you've gone too far because the stars will start looking cooked.

I did some simple histogram and curve stretches on the main image. Then did arcsinh on the star image following StarXterminator. If I go too far it looks a little too cooked for my taste, but I think I can dial it up a bit still.

Like Reg, I extract stars while linear and stretch the stars and nebulosity separately. For what it's worth, for stars, I:

1. Extract luminance from the star image
2. Stretch luminance using HistogramTransform until you get the star density/size you want
3. Stretch color star image using Arcsinh stretch
4. Look at the cores of the larger stars in the Arcsinh stretched image, if they are blown out, repair using the Repair HSV Separation script
5. Combine stretched luminance and repaired Arcsinh stretched image using LRGBCombination
6. If needed, adjust color saturation of the final combined star image using CurvesTransform

I see a lot of good advice here but I don’t see much about what immediately jumps out at me:  The colors aren’t right!  You aren’t alone in this regard.  I see gobs of images of these two objects all the time with the same problem.  PCC is pretty much an obsolete tool.  The intent was good but the implementation just wasn’t right.  SPCC is a much better tool but it’s not perfect either and the results that you get can depend on how you apply it to the data.  I’ve found that it generally gives more accurate results if applied to high quality RGB data.  Applying it to LRGB data can be problematic.

THE number one rule for getting the colors close to right in galaxy imaging is to do a sanity check against what we know about how stellar populations are distributed within spiral galaxies.  Young, hot, blue super giants commonly dominate the colors in the spiral arms and older, cooler, yellow/orange/red stars dominate in the core.  M81 is a classic standard spiral galaxy and if your processing shows it as a mostly yellow object, something is wrong.  IMO, you don’t even need SPCC to get a result that is close to correct if you simply use the ColorCalibration tool with M81 as the “White Reference” and a patch of black sky as the “Black Reference”.  Obviously if SPCC is working correctly, it should give a very similar result—and if it isn’t, you’ve got to figure out why.

Finally, I completely agree that getting the background set properly is super important.  If you use PS, adjust the background to a level of about 15 and you’ll be in the right place.

John