Andy Resnick said:
Flats... yeah, I've tried those. Unlike dark/bias/bad pixel frames, I spent at least a year (maybe even longer) trying to get flats to work with my telephoto lens and in the end just up. I'm not exactly sure of the underlying cause(s), but using flats always increased the amount of post-processing work I had to do:
1) Any dirt present on the flats and not in the lights results in not just incomplete correction of the lights, but the introduction of spurious bright spots.
2) The other image defect is hard to describe, but consisted of concentric rings of color, almost as though the colors of an archery target were superimposed over the image. I think it's related to the changing sky hue as it gets dark and also as the city light pollution commences.
Now I may try again with this lens, but not try too hard... :)
1) Yes, this is true. Any blemishes on the lens/optics must be consistent between lights and flats.
That's one reason why I take flats fairly frequently (about 1 set of flats for each deep sky target, and if I revisit a target after a significant amount of time has passed, that's a whole new set of flats). But yes, it adds another step in the post processing.
2) Hmm.
Not sure what caused that. But again, make sure that lens' aperture is identical between lights and flats. If the light source used for flats is too bright, and you try to compensate by stopping down the aperture, your flats are doomed. The aperture
must be identical between lights and flats.
Also, when in post processing, make sure the flat compensation is
multiplicative and not additive. For example, the master dark is subtracted from the lights (an inverse additive process). Don't do that with flats. The inverse of the calibrated flats (master flat) must be
multiplied, not subtracted.
And it can be tricky in theory to set up and configure correctly. For example, bias frames can be used to calibrate lights, flats and darks. But if
you've already [you haven't] calibrated your darks with bias frames, you want to be careful that you avoid calibrating your lights with bias frames too. Otherwise when calibrating your lights with the calibrated master dark frame you'd be double-doing (over correcting) your bias frame (i.e., read noise) calibration. Your software might automatically take this into consideration, but it has to be configured properly. (But it is okay to calibrate your flats with bias frames independently of all this.)
You mentioned you used APP (Astro Pixel Processor). I don't have any experience with that software. But I expect it can work with flats, as long as everything is set up and configured correctly.
Before I finish talking about flats, I can't imagine taking deep sky astrophotos without flats. Yes, it's an extra step or two, but one gets used to it. It's just a (relatively easy) part of my process now that I just expect and do without thinking much about it (because I'm so used to it).
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When looking for the dust mote culprits, you can probably rule out the front objective of the lens. Getting dust and muck on the front of the lens can reduce contrast and light gathering ability, but it probably won't have a significant effect otherwise. So the front of the lens probably isn't the place to look.
The back of the lens -- the side of the lens that points toward the sensor -- is a different story. That could be the place to look.
As a general rule, the bigger the dust mote looks on the exposure, the farther away it is from the sensor (for a given aperture setting). Dust on the sensor itself will just look just like a distinct piece of dust. Dust farther away will look bigger. Dimmer and blurrier, but bigger. (In my case, dust on my camera's sensor window [a few millimeters from the sensor] will appear a little bigger, rounder and blurrier than dust directly on the sensor. Dust on a filter on the filter wheel will appear even bigger than that, because it's farther away. If dust was on the objective itself, it would be infinitly big [or in practicle terms, vastly bigger than the entire sensor], thus it wouldn't look like a dust mote; it would just reduce contrast.)
Yeah, let's hope it's not inside the lens assembly.
