Wow, thanks for all the positive comments!
Seems like most people on here image either at >= 500 mm focal length, or <= 24 mm... hopefully this can be my little niche
Simon - err yep, I did indeed sell the Astrotrac
, but then I bought it back in exchange for something else... long story! I then sold my EQ6 but now that I'm hitting my head against the AT's limitations I'm regretting that too
Mike - thanks! The Coalsack + Dark Doodad is my favourite too... I'm a huge fan of dark nebulae.
I think you guys give me too much credit for the processing (it's rather rudimentary at the moment), but here's what I did (from memory):
Step 1. Lightroom RAW conversion:
* turn off all automatic processing (brightness/contrast/black point/etc)
* set manual white balance
* enable lens profile correction and adjust settings
* reset contrast curve to linear
* export as 16-bit TIFFs for import into Deep Sky Stacker
* repeat for darks/flats/bias/etc frames.
Step 2. Deep Sky Stacker stacking:
* load light/flats/dark flats/darks/bias frames
* tweak the star detection threshold to aim for about 1000 detected stars (too few seems to result in incorrect alignment, same with too many - or it crashes DSS)
* use the middle light frame as reference
* stack 100% of the light frames and see what the result looks like
* if the stars aren't pinpoint across the field (i.e. alignment errors), then figure out what went wrong and/or which light frames to omit
* for the Rho Ophiuchi shots, I had to drop 4 frames for it to stack perfectly
Step 3. Deep Sky Stacker stretching and curves:
* click the checkbox to move RGB sliders simultaneously
* increase the black point until any clipping starts to occur (i.e. you see a pixel in any channel appear at 0/black on the histogram)
* unclick the above checkbox, and each the black point for the other channels so that the RGB peaks all line up
* click apply to see what the data looks like (you may have to adjust the midpoint slider of the curve so that it's not too bright/dark) - make sure the dark nebulosity isn't clipped
* leave the white point alone as the bright stars are already overexposed
* set saturation to 20% (there will be a huge colour cast over the image) and then carefully tweak the black points of each RGB channel to cancel out the colour cast
* fiddle around with the curves sliders to get a nice balance and contrast between background, dark nebulosity, bright nebulosity, and stars
* tweak saturation slider
* export as 32-bit TIFF
Step 4. Photoshop tweaking:
* mask any edges or corners that didn't get flat-corrected properly
* clone out the dust spot in the corner that didn't get flat corrected
* perform a fine-grained Levels or Curves (as 16 bit image) tweak
* vibrance/saturation tweak to boost colour of nebulosity
* boost red+magenta channel saturation to compensate for reduced H-alpha (e.g. somewhere between +15 to +30)
* convert to 16-bit using exposure 0 gamma 1
* resize image, e.g. 1200x800 for web
* apply local contrast enhancement of dark dust lanes and nebulosity (e.g. unsharp mask with settings 12%, 12 px, 0)
* export as JPG
That's it! It's all really quite straightforward, perhaps except for the stretching and curves step in DSS. It's a very, very finicky step where even just having a slider a couple of pixels in the wrong position messes up the whole image.
When the curves sliders are set right, you can adjust the dark dust, nebulosity, star intensity, and star field background individually on separate sliders.
If you'd like to give it a go yourself on my data, you're very welcome:
*
RAW file from a single sub taken with my SLR (28 MB)
*
converted TIFF file from Lightroom - one sub (result of Step 1) (126 MB)
*
stack of 31 subs from DSS - no curves/stretching (result of Step 2) (202 MB) - download and load this file into DSS if you want to play with stretching/curves
*
output TIFF from DSS after stretching and curves (result of Step 3) (125 MB)
Enjoy!
