Almost right! Ninety minutes of exposure time, not 1 single exposure. For the signal to noise ratio, one 5 minute exposure is equivalent to stacking five 1 minute exposures so you can stack as much time as you need to get the details you want.
What you're saying is 100% correct that over time you lose accuracy. Focus isn't as much of a problem once you have it in the region but there's a whole mess of things that will ruin your accuracy. The mount itself tracks at the rate the earth turns but there's periodic error in the gears on the mount, there's the inherent error in the accuracy of the mechanisms themselves, error in your polar alignment which is the basis for the coordinate system your mount uses (you kinda aim for the north star but in reality Polaris actually does little donuts around it) and then there's the accuracy of your calibration on where your mount thinks it is in the world and where it should be looking. Probably a few more I haven't learned about yet too. At shorter focal lengths, these errors aren't very pronounced, but at longer focal lengths the same error becomes more pronounced because it's over a greater distance. You can take shorter exposures to compensate which is what I'm doing now at 45 seconds a pop or your can get better gear.
There are autoguiders that are a secondary camera with a wider field of view than your primary (sometimes you have a secondary scope for them) that latch onto a guide star of your choosing and try to keep it centered in its sights. They tell your mount if it's moving too fast or too slow to keep the guide star centered and compensate for a good chunk of error and I think the software runs on your laptop. Usually imagers will see a substantial jump in image quality especially at higher focal lengths once they start doing it. Additionally there are higher precision research grade mounts that have better manufacturing, instrumentation, software and alignment processes that actually map the sky which will help eliminate drift so you can shoot up to 15 minutes for one exposure. This bad boy here is owned by someone in our society for the low, low price of $9.000
https://www.optcorp.com/software-bisque ... gQodXgkOzQ. And that's not even counting the rest of his gear. His tripod for example that this mount goes on is another 2k I believe
overall he probably has 20-25k worth of equipment including his 5k monochrome CCD camera, the filter wheel, his laptop, his telescope, his guide camera, his guide scope, any software he has to pay for and any cables / eyepieces / field flatteners / etc that he uses.
But once you know what you're doing and have the right equipment, you can get some baller images like this one that probably requires around 10 grand worth of equipment looking at the list of the guy that posted it. I'd love to get to this point some day.
I occasionally read "if you were looking for a cheap hobby, this isn't it", but honestly, unless you're gonna take up by the numbers needlepoint I feel like most things we're interested in require investments in time and money to master. Hopefully it will be well spent!
