Hi Steve, are you using a UV/IR filter? The 183 needs one.
1. Exposure length depends a lot on the target - something bright like Orion will only want short exposures - I do between 10 & 90 secs, blending to bring the core out. But with a dimmer target 5 min or longer is quite doable.
Now, the other settings.
Gain: Tricky question, maybe someone with 183 can answer better. But unlike newer cameras, the 183 doesn't have a HCG mode where a specific setting causes a large drop in read noise & a return on things like DR & full well. For you, using 0 gain won't be much different to using 120 gain except in brightness. You will get a little more noise at 0 gain, but by far MOST noise is from light pollution!
Temp: I use 0° for a couple of reasons - (a) Diminishing returns as you push the temp lower, working the cooling harder for not much advantage. (b) It is much easier to reach 0° throughout the year, allowing consistent usage. This is important to me, especially in summer where night temps can be over 30°. (c) Cooling doesn't need to work so hard, especially over long sessions. (d) Thermal/read/sensor noise is really nothing compared to the noise caused by light pollution. Dr Robin Glover from SharpCap goes through this in detail in a lecture that I can link if you like.
2.
Bias: I don't use bias with ZWO cameras & even they say DON'T use bias frames.
Quote:
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Unlike CCD cameras, to ensure you properly remove glows, you should only subtract just the master dark. Do not use a master bias.
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Flats: These are really individual things, depending on the method you use to obtain them. I use a white cloth + tablet or tracing pad. With APT, it has a tool for calculating the flats, but you can use the histogram to get decent flats - you want an unstreched flat to peak between about 1/3 to 1/2 from the left. You may also want to do dark flats, which are exactly the same settings as flats but with the lens covered.
Darks: Good thing with cooled cameras you can do these at any time. You just need to make sure all the camera settings are the same. I take my camera off, cap it & place it inside a box & let it do it thing.
3.
Vignetting: This is strange. With the sensor size of the 183 you shouldn't be getting vignetting even if using 1.25" connections. All I can suggest is going through the setup & ensuring everything is connected properly & there is nothing that could be blocking the light path. Weird.
4. You're post processing will change very little really. As John says, your 183 & DSLR are really both OSC cameras.
Hope this helps, cheers,
Mark