First proper galaxy effort

[MattJenko]

This was a 35 x 30 second run against M31 last night, quite early, so a degree of LP, but before the moon rose.

Pentax K-500 DSLR using Evostar 80ED + flattener.

20 darks, 15 bias and 20 flats using a white t-shirt + laptop screen.

This has only been stacked in DSS and converted to greyscale in PS with a tiny stretch only. Everything I touched made this worse, so requesting some advice on how to proceed please.

Things I have already noticed:

1. Not in perfect focus
2. Not got distance between flattener + sensor correct (elongation)
3. I have overexposed the galaxy cores.

This is my first plea for imaging help, so let me know if I have missed any pertinent info out, and thanks for looking!

Matt

[IanL]

1. I'd be surprised if you have really overexposed the core with your set-up and 30s exposures.  With a similar setup (80ED, FR, Canon 500D) and 30 minute exposures I have a much tighter core. It might be that your stretch is a bit too much to start with.  Try a less strong initial stretch so that you get a tighter core and then use curves to try to tweak out the galaxy arms without further stretching the core.  Failing that what ISO setting did you use? It is tempting to use a really high ISO but this can lead to loss of data on bright objects.

2. Not sure your flats are really working, I can see a dark circle (dust shadow) at the edge of the galaxy arms between 9 and 10 o'clock, and another one slightly further out just before three o'clock. Check the flats by stretching one of them to see if you can identify the same shadows in the corresponding positions.  If they are there then you may need to do something in DSS to make them more effective, but it is possible that the dust has moved between lights and flats being taken (if your camera has sensor auto-cleaning like the Canons do, you need to disable it before taking the lights until after the flats so the dust stays in one spot throughout.

3. Not much you can do about focus this time round - just something to work on for next time.  There are lots of tips for improving focus so ask if you need them.  Regarding the spacing, it is pretty critical with the 0.85x FR which I assume you are using.  You can have a go at cropping the image to remove the worst elongation in the corners but ultimately you need to ensure you have the right adaptors, extension tubes and spacers to get it spot on.

4. Any reason why you have converted to monochrome?  You should have some decent colour in there which will add depth if you can bring it out.

[MattJenko]

Thanks for the reply Ian, much to contemplate and try out tonight.

I'll have another crack at the stretch. I was using ISO 800, so given that you have been doing significantly longer exposures than me, I agree I am very unlikely to have blown out my galaxy core.

This was my first effort at flats, and I think you are also right about the auto-cleanser, as I had to replace the batteries during my session (another lesson!)  and it does indeed give a good solid shudder on startup!

I usually use a Bahtinov mask, but I had already got this in view and nothing was bright enough to use it properly, so I eyeballed it, which I do regret. I'll ensure a nearby bright star is used prior to final framing next time.

I converted to monochrome simply because the original image was quite impressively reddened, which I think flummoxed me, as everything I tried to do to remove it resulted in a quite dreadful mess. I will have another go with the benefit of some sleep and time to think a bit less wine influenced.

Thanks again for taking the time to respond, much appreciated. I'll buy you a pint sometime, as I am just outside Colchester too.

[MattJenko]

Took your advice. The 'overexposure' was a result of me carelessly dropping from 32bit tif to 16 bit tif. My flats actually do have those dust bunnies in although maybe slightly in a different position. I will experiment with DSS to see. For now, I have used PhotoShop Fill to cover them up. Not very well, but enough to significantly improve the situation.

[IanL]

OK looks like you are getting somewhere now!

You should be able to fix the red colour cast easily enough in PS.  (I'm not a PS user myself, I do all my processing in PixInsight but the procedure should be valid).  You need to take the linear image (i.e. before you do the initial stretch) and look at the histograms for the red, green and blue channels separately.  You should see high peaks somewhere to the left of each of them - these are the background sky (see my post here if you are unfamiliar with histograms http://www.blackwaterskies.co.uk/2013/12/how-to-interpret-image-histogram.html).

What you want to do is apply a small histogram stretch to two of the channels to line up the peaks with the third one - typically you just need to move the ends of the histogram transformation lines slightly.  Once the peaks line up you should now have a (very dark) grey background rather than a background with a (very dark) colour cast.  N.B. I am sure there are functions in Photoshop for equalising/balancing the colour channels which can achieve the same result in a different way, but the benefit of trying to equalize the background peaks is that you have a (fairly) objective measure of the correct colour balance, whereas using tools adjusting the foreground colours is harder to get right by eye.

Once you have the colours balanced, then you would apply the big histogram stretch to all three channels as you have done above and you should find that the colour balance of the image is just about right.  Of course you can use more histogram adjustments, curves or colour balancing tools to tweak it if needed.

Next up you might try masking the background and then increasing the saturation of the galaxy and stars to make the colour more visible.

[MattJenko]

It would be simply rude of me not to try out what you suggest.

I stretched the Red to match the Green/Blue and then did a full stretch of them all. I realised that my 32->16bit conversion was not done well and this time I flattened the layers first and then used the 'Exposure and Gamma' method for doing the HDR Toning rather than the default 'Local Adaption', which kept my hard work at 32-bit pretty much intact.

I haven't tried masking the galaxy yet, its on the list to practice. Once again - many thanks, I have learnt more doing this than I have in weeks of fiddling aimlessly about with adjustments.

[IanL]

Glad to offer a bit of inexpert advice!  Photoshop is not my thing (haven't used it since the '90s ) so apologies for being a bit vague about the details!   I think you could go a lot further in toning down the red in the image but it it definitely getting there now.  To be honest, learning how to process the images is the key to this game.

One site that might be worth a look for PS tutorials is this:  http://www.astropix.com/