Darks and bias frames can be taken anytime and aren't subject to same focus, in fact the camera doesn't even need to be mounted on the telescope. As for flats, the focus does need to be reasonably close but not critical so noting the focus position at the end of any session or simply not moving the focuser should get you close enough. Then you simply need to find a means of evenly illuminating the CCD chip like by using a light box, tee shirt flats or any number of other ways. In fact I'm taking flats now in the observatory using a piece of display graphics backlit film over the end of the telescope with a white cloth illuminated by means of two lamps about 5' back with one on each side at maybe a 45 degree angle. The only reason being that I now have some high clouds and that doesn't do well for sky flats. There are plenty of ways to do these calibration frames but it will take some time and not necessarily quality imaging nights. Darks on cloudy nights indoors and same with bias frames. Build a library of re-usable frames. I usually image with 15, 20 and 30 minute exposures and can easily do -35 degrees during winter months but usually do -35, -30, -25, -20 and so on depending on the season. My darks and bias frames are usually done about every 5-6 months depending on temperature range of the current season. I only take bias frames yearly. Flats as needed. I use a rotator so sometimes I take at different rotated angles if the other flats aren't working satisfactorily. Depends how much of a purist you are. But you need calibration frames for decent images or in my opinion you're wasting your time.
