Is it better to under-expose or over-expose?

[Stellaris]

Hi, I'm fairly new to astrophotography.
I'm using a SkyWatcher 150/750P, with a SW NEQ3-2 Pro Goto, and a Fujifilm x-t3.
I'm wonderwing whether its best for me to overexpose or underexpose?

[vlaiv]

There is no such thing as underexpose in astrophotography.

Overexposure is not a good thing because you loose information, but, again, overexposure is not the same thing as in daytime astrophotography and there is a good chance that some parts of image will be overexposed even in what we would normally consider proper exposure.

With astrophotography we have very powerful tool at our disposal - which we don't usually use in daytime photography (in general it is best to "unlearn" all you know about photography when doing astrophotography as most if not all concepts from daytime photography are inapplicable and often wrong in astrophotography). This tool is called stacking.

With daytime photography we take only a single exposure and as such - we might be concerned with under / over exposure.

With stacking and astrophotography - we simply don't think in terms of under / over exposure.

We control single exposure length in such way as to overcome read noise of our camera with some other noise source (often sky noise but can be thermal noise in DSLR / non cooled astro camera). Once you swamp the read noise - result after stacking won't be different no matter how long individual exposures are as long as total cumulative exposure is the same.

We deal with over exposure by simply using shorter exposure and then using only parts of shorter exposure in places where we actually have pixel saturation. This is mostly cores of very bright stars, but can sometimes happen on cores of very bright DSOs - M31 Andromeda galaxy  and M42 Orion Nebula being two prime candidates for this type of "core burnout".