Originally posted by XLXW Very impressive photograph for a 200 mm lens of M51. I don't understand by what you mean by this language: "20 x 118 seconds, ... 3 darks, 10 flats" in your description.
Translation
- series of 20 pictures with exposure of 118 seconds each (called "lights").
"darks and flats" are correction frames that help to opitimise the results of the stacking process.
- 3 "darks" = 3 pictures with same parameters as the normal exposures (iso, aperture, exposure time etc.) at same temperature but with lens-cap on lens. Darks contain the sensor noise. They are later used to subtract noise from the "lights" when the pictures are stacked. If you use the built-in noise reduction for long exposure you do not need darks but each picture needs double time then. Better use darks that are conveniently shot at the end of the session while cleaning up the location.
- 10 "flats" = 10 pictures with same camera/lens combination that contain vignetting, stains, dust etc. of the optical system. I make my flats in cloudy daylight with white t-shirt over lens against the grey sky, focus set to infinity. Like the darks the flats are used during the stacking process. Aim is to neutralise vignetting and other imperfections that come from the lens and sensor.
A 4th type of correction frames, the bias frames records the noise that the sensor produces while reading outvthe data. I do not use bias frames bedaus in my opinion the bias must also be a part ofvthe noise in the darks.
By stacking lights, darks and flats the erratic noise ist reduced while the static signal (e.g. stars) is opitmised.
Try it, its quite easy, software is for free (e.g. DeepSkySacker) and results are dramatically improved, also in wide field astrophotography. There are same good tutorials on youtube.
---------- Post added 04-25-15 at 11:10 ----------
Originally posted by mikeSF Phantastic!!!