Which subs make a better image?

[PhotoGav]

I call upon your experience, please...

I am working on an image of M31 and have a fair bit of data (decent, though not necessarily from perfect skies, I've rejected quite a large number of hazy subs already) collected so far. My question is: which subs would it be best to capture more of to make an improvement to the final image? So far I have (approx):

L 9 x 1200s

L 13 x 120s

Ha 6 x 1200s

R 12 x 600s

G 12 x 600s

B 12 x 600s

The forecast is for a decent clear sky tonight, so should I add perhaps another half an hour to each of R, G, B & L 120s and then give the rest of the night to L 1200s or should I just do a whole whole night of L 1200s thereby adding a few more hours to it?

Basically, will just more Luminanace data give a better end result or would more colour data as well be of more help to the final image?

I'm very new to LRGB imaging, so hope that you can help me learn!

Thank you.

[riklaunim]

It's hard to tell what's the best exposure time and frames count. If a camera has very low read noise and is cooled so that dark current isn't a noticeable factor (like Atik 314L+ or other similar Sony ICX camera) then even relatively short frames will catch "all" the signal from the target. More noisy camera would need longer exposures to bring signal above noise level (if the noise isn't rising as fast as the signal).

With my Atik314 imaging I recently started shooting many 3 or sometimes 5 min frames, and even at 3 min I can get some "empty space" on the left side of the histogram. Multiple stacked frames nicely cancel out the noise and in general it works. If the luminance is good, color may be less good, or shot binned to make it even quicker.

[PhotoGav]

Thanks for your reply @riklaunim. I've decided to try and get a bit more of everything, though things seem to be conspiring against me... What started out as a crystal clear evening has turned cloudy. Hopefully it will clear up in a short while and I can start gathering some subs.

[ollypenrice]

If you know the tricks you can get away with far more lum than colour. In Ps stretch your RGB and process it for saturation and low noise, not detail. Then prepare your L with a hard strech and good contrasts and sharpness. A very deep L layer will give very thin colour if just dropped onto the RGB in one go so try a partial application. While the RGB is underneath with the L on top at, say, 20% opacity, up the RGB saturation, put a very slight blur into the L (say 0.5), flatten and consider this your new RGB. Re-apply the L at a higher opacity and repeat. And repeat the whole thing again. This way you tease up the colour intensity to fill the deep L layer.

Olly

[PhotoGav]

Thank you Olly, I will try that approach.

I started trying to process the RGB yesterday evening and am struggling to pull the colour out evenly. In your opinion, which is the best approach to processing RGB, to merge the three unprocessed stacks into one RGB file and then stretch and tweak that or to process each individual colour channel stack first and then merge the three and carry on tweaking?

As for the blues that I keep on seeing in the outer rings of M31 pictures, where do they come from? I don't appear to have those colours in there! Are they just made up?!

I am ever closer to booking in to Les Granges... How many days would you need to turn me into a master mono CCD data processer??!!?

[ollypenrice]

Thank you Olly, I will try that approach.

I started trying to process the RGB yesterday evening and am struggling to pull the colour out evenly. In your opinion, which is the best approach to processing RGB, to merge the three unprocessed stacks into one RGB file and then stretch and tweak that or to process each individual colour channel stack first and then merge the three and carry on tweaking?

As for the blues that I keep on seeing in the outer rings of M31 pictures, where do they come from? I don't appear to have those colours in there! Are they just made up?!

I am ever closer to booking in to Les Granges... How many days would you need to turn me into a master mono CCD data processer??!!?

Without any doubt at all you should combine the RGB before stretching. 

Indeed before stretching my combined RGB I run it through DBE and SCNR green in PI and only then do I stretch it. I tend to stretch the RGB using a pure log stretch (moving the mid point to the left) though I use custom shaped curves and often several of them in layers to stretch the luminance.

Processing is something that is best learned/taught one to one. That's how I learned (from guests in the early days) and you can make remarkably fast progress. I'd like to sign up for some one-to-one from a few people, that's for sure.

The blues in the outer part of M31 are just there in all the data I've used. (Some we shot here, some came from Les Tilley in the USA and some from Tom while he was in Spain. It isn't made up. On the other hand I find mine too cyan and insufficiently magenta. Try as I might I can't lose this effect. I'd ike a bit of the Gendler-magenta effect on this target.

 lly

[PhotoGav]

Brilliant Olly, thank you so much for your kind advice. I feel better prepared to try again now! Yes, the Gendler image is pretty darn good! Watch this space for my effort soon!

As for master classes in the art of processing - definitely on my Astro To Do List now!