| Long Exposure DSO Photo Techniques, Page 3 |
Post processing
After catching the photons, go with a good processing program, like Maxim DL to do your
preprocessing (flats, noise leveling, etc), aligning/stacking/combining of frames for each
filter type. Use Maxim DDP, or the FITS Liberator plug-in for Photoshop (CS or CS2) for
image stretches (log works well on star clusters, log-log works well on galaxies and some
globular clusters), and work with Photoshop to layer the LRGB frames and do final image
processing. Don't get carried away with the image stretch and make small adjustments with
curves and levels. Don't get carried away with any single adjustment, especially the
saturation control. If you want a truly cluttered image, like some of the Orion nebula images
you find on the web, just over stretch the image and over pump the saturation in
Photoshop, lots of dust and super saturated, cartoon like tones will emerge. These post
processing tools can yield anything from a nice natural looking image to a cluttered mess
of bright, gaudy tones, it's up to you to decide what satisfies you.
1) Don't over stretch your lum, especially if you are using DDP. The stars, which should
have a guassian intensity distribution will flat top, ring (the brighter stars when passed
through the DDP filter don't just flat top, the intensity rings), and bloat. The portions of the
scene that are dim will lack RGB support and just appear grey. Even the brighter portions
of the subject may appear washed out. The latter problem can be partially alleviated by
combining the RGB image with the lum to create a new lum (lum on bottom layer in PS, RGB
on top as a multiply layer, adjust density, typically 25-50%), then use it to form the LRGB. If
you have Ha, form a new lum as above, and even a new R as above, then combine the new
L and R with the BG to get a (LHa)(RHa)GB. If you also have OIII, make new G and B layers.
Adjust saturation and levels/curves on each layer combine. This will go a long way to
maintaining pleasantly high saturation subjects in your images.
2) Use an anti-pollution filter, like the IDAS, or others to get rid of sky gradients at the time
of capture. TAKE FLATS and biases to remove vigneting and dust doughnuts. You will still
need to remove minor gradients, but they will be much less noticeable. Gradient
Xterminator does a good job. While more complicated, the high order gradient removal
tools in PixInsight also work well. Sometime, even MaximDL's gradient removal tools work,
but typically not as well as the other two mentioned. If you spend some time on your
backgrounds, they will be nice and flat (uniform). The image will look like it was made at a
dark site, not your light polluted backyard.
MORE RAMBLINGS TO FOLLOW, like:
Buy a field flattener (FF) if your refractor makes weird shaped stars that seem to expand
from a common point in your photo. There are several manufacturers that make FFs for
specific scopes, others make FFs generally useful for a range of scopes. You may need to
buy a couple, or three, to find the one that allows your big frame camera to produce decent
looking stars out to the edge. If you don't want to spend the money on FFs, get a camera
with a smaller sensor, or plan on cropping away a part (maybe a lot) of that big photo.
If your reflector is doing nasty things to stars out at the edge, it's probably time to find a
coma corrector for it. I used the Televue Paracorr for a while, but didn't want to give up the
10-15% focal ratio (it effectively increases the focal length), so I switched to the Baader
MPCC. It does an adequate job (for smallish CCDs), without increasing the focal length.
There are other coma corrector (that are more expensive), that will allow for large
corrected fields, and even focal length reduction (look for a Wynn corrector/reducer, like
ASA's or Orion Optics).
Field curvature and coma, like poor guiding, are no excuse for weird looking stars, and the
resulting loss of resolution.
After catching the photons, go with a good processing program, like Maxim DL to do your
preprocessing (flats, noise leveling, etc), aligning/stacking/combining of frames for each
filter type. Use Maxim DDP, or the FITS Liberator plug-in for Photoshop (CS or CS2) for
image stretches (log works well on star clusters, log-log works well on galaxies and some
globular clusters), and work with Photoshop to layer the LRGB frames and do final image
processing. Don't get carried away with the image stretch and make small adjustments with
curves and levels. Don't get carried away with any single adjustment, especially the
saturation control. If you want a truly cluttered image, like some of the Orion nebula images
you find on the web, just over stretch the image and over pump the saturation in
Photoshop, lots of dust and super saturated, cartoon like tones will emerge. These post
processing tools can yield anything from a nice natural looking image to a cluttered mess
of bright, gaudy tones, it's up to you to decide what satisfies you.
1) Don't over stretch your lum, especially if you are using DDP. The stars, which should
have a guassian intensity distribution will flat top, ring (the brighter stars when passed
through the DDP filter don't just flat top, the intensity rings), and bloat. The portions of the
scene that are dim will lack RGB support and just appear grey. Even the brighter portions
of the subject may appear washed out. The latter problem can be partially alleviated by
combining the RGB image with the lum to create a new lum (lum on bottom layer in PS, RGB
on top as a multiply layer, adjust density, typically 25-50%), then use it to form the LRGB. If
you have Ha, form a new lum as above, and even a new R as above, then combine the new
L and R with the BG to get a (LHa)(RHa)GB. If you also have OIII, make new G and B layers.
Adjust saturation and levels/curves on each layer combine. This will go a long way to
maintaining pleasantly high saturation subjects in your images.
2) Use an anti-pollution filter, like the IDAS, or others to get rid of sky gradients at the time
of capture. TAKE FLATS and biases to remove vigneting and dust doughnuts. You will still
need to remove minor gradients, but they will be much less noticeable. Gradient
Xterminator does a good job. While more complicated, the high order gradient removal
tools in PixInsight also work well. Sometime, even MaximDL's gradient removal tools work,
but typically not as well as the other two mentioned. If you spend some time on your
backgrounds, they will be nice and flat (uniform). The image will look like it was made at a
dark site, not your light polluted backyard.
MORE RAMBLINGS TO FOLLOW, like:
Buy a field flattener (FF) if your refractor makes weird shaped stars that seem to expand
from a common point in your photo. There are several manufacturers that make FFs for
specific scopes, others make FFs generally useful for a range of scopes. You may need to
buy a couple, or three, to find the one that allows your big frame camera to produce decent
looking stars out to the edge. If you don't want to spend the money on FFs, get a camera
with a smaller sensor, or plan on cropping away a part (maybe a lot) of that big photo.
If your reflector is doing nasty things to stars out at the edge, it's probably time to find a
coma corrector for it. I used the Televue Paracorr for a while, but didn't want to give up the
10-15% focal ratio (it effectively increases the focal length), so I switched to the Baader
MPCC. It does an adequate job (for smallish CCDs), without increasing the focal length.
There are other coma corrector (that are more expensive), that will allow for large
corrected fields, and even focal length reduction (look for a Wynn corrector/reducer, like
ASA's or Orion Optics).
Field curvature and coma, like poor guiding, are no excuse for weird looking stars, and the
resulting loss of resolution.