One of my first astrophotographs
I was chatting to someone a few days ago, and the subject of old photos came up. I have very few: this one is from the late 1970s.
At school, we had a 4" Unitron refractor on an equatorial mount. We could take photos of the Moon, but deep sky objects were beyond us.
Apart from the cost, taking single, long exposures was supremely difficult. The longer the exposure, of course, the better the photo, but how do you keep the telescope on the target?
To guide the telescope, a second scope was mounted on the main scope. This one had more magnification, and an eyepiece with a crosshair. The operator would peer through the eyepiece for the entire time, staring unblinking at a star. The operator would compensate for drift with the mount's slow-motion controls, but these were the only movements they could risk.
If the operator lost the guide star, the photo would be ruined. Because of the long exposures, a wasted photo would cost of a lot of time.
It was a terrible job. By the end of the night, the operator would be exhausted, stiff as a board and with sore eyes, having been hunched in odd positions, unmoving, for hours at a time.
The development of digital sensors and processing allows us to simultaneously get accurate guiding and super-long exposures.
Autoguiding is exactly the same process as before, but the computer does it. Cameras are cheap and sensitive and the computer is more attentive.
To get the equivalent of an eight-hour exposure, software integrates the series of subexposures by "stacking" them.
So if your guiding goes wonky for a few seconds because of wind, cable snag, someone walking past or some sort of mechanical inaccuracy like a bit of grit in the gears, we can throw out that individual subexposure. Losing five minutes is not normally a great disaster.
Incidentally, you can take a Moon photo as good as this one these days with a mobile phone and something like a saxon 709AZ3. Using your own mobile phone, you'd have change from $400.