This is a very difficult question to answer for astro photos.   Firstly, let’s go back to basics. When taking a photo of a galaxy, nebula etc, you are trying to take a long exposure photo of a dim moving target.  This is a recipe for disaster.   Let’s highlight the pitfalls first…

Movement

If the scope get knocked whilst you are capturing an image, that image becomes bad, and shouldn’t be used in the final processing.   It will degrade the photo!   What this means is that the longer you keep the shutter open, the higher the risk of the photo being marked bad.   But with short exposure times, you may not capture all the detail.

Obstructions

The same goes for obstructions, the longer the exposure the more chance of a plane flying through the field of view, or worse… clouds.

Tracking errors

Again with long exposures, any tracking error will result in star trailing.  This is why Autoguiding is pretty much a given for deep sky astrophotos.

This is where two types of camera differ.  CCD cameras prefer long exposures, where as CMOS cameras prefer shorter exposures.    My camera, the ZWO 1600-MM-Pro, is a CMOS camera, so I can keep the exposure times short.    But what do I mean by short?

Well, it’s a question of relative settings.   It will be short compared to a comparible CCD image.

But there are some things that we need to know and understand to be able to set things up properly.

Firstly, there are the camera settings.    For my ZWO cameras, there are published documents, which list a value known as unity gain.

For the ZWO 1600-MM-Pro, this figure is 139

for my ZWO 290-MM this figure is 110

but what does this mean?    The scale on the graph read GAIN( e-/ADU)    HUH?    e- is the figure read from the sensors pixels.  ADU is known as Analogue Digital Unit.   So be setting the gain to 139, it makes the value of each pixel consistent with the value on the sensor. i.e. they are in unity.    Where does this number come from?   In the case of the ZWO camera’s it’s listed in the manual, and sometimes on the website selling the camera.

The simple way to think of it is that the gain is similar to the ISO of an SLR camera.  the higher the gain, or the higher the ISO of the film, the more effect the light has on the image.   In the case of a digital camera, that means that running a higher gain you’ll fill the pixel wells of the camera faster.

What’s a pixel well?   think of it that each pixel in the camera is a bucket.  Every time a photon of light hits the pixel, it registers a count on the camera’s pixel as the ADC (Analogue Digital Converter) is a 12 bit, that means that it’s possible for each pixel to have 4096 values.  A good exposure will mean that the dimmest object will have a value of 1, and the brightest will have a value of 4095.  This will ensure that the image doesn’t clip.

Turning the gain up on the camera will mean that these light buckets will up faster, turning it down will fill them slower.   slower is a longer exposure.

Lowering the gain will mean that more light is needed to fill up the pixel well.  Meaning a longer exposure is needed. The unity gain figure is supposed to be a sweet spot where the pixels will fill up fast, but without compromising the quality of the imge with noise.

This leaves the other value that you can set in the camera settings,  the Offset?   What exactly is this value?

Well, I didn’t know so I asked on my favourite astronomy forum stargazerslounge.com

Turns out that before I worry about setting the exposure, I should think about finding the correct value for the offset.   Basically, it’s the minimum value that creates and image without a pixel with a 0 value in, because if there’s a 0, we’ll never know if it’s a 0, or if the value is less than 0, but has been translated (clipped) to round it up to 0 as pixels cannot have a value lower than 0.

So, right now, I can’t tell you what the exposure length should be, there’s work to do to find out what the minium offset value should be for my camera.   Once that’s done, I’ll figure out how to find out what the “correct exposure” is for an image.  Then I’ll be able to work towards that.