Finding focus

18 March 2019

You might have noticed that telescopes, particularly small refractors, have multiple uses and configurations. 

Mostly, of course, they're used as standalone telescopes. Normally they are used with a diagonals, but sometimes you don't want a diagonal in there. For example, you're trying to look at something near the horizon, or actually on ground. Or you're photographing through the refractor, including using it as a guide scope for autoguiding.

This is where the challenge begins. Sometimes, if you take the diagonal off a telescope, and put the eyepiece directly in its place, or if you swap the eyepiece for a camera, you won't be able to get it to focus. In these cases, you'll need a spacer, which is a ring that simply adds distance between the focuser and the eyepiece.

These two photos show instances when I've needed spacers. The first is for my main refractor, which wasn't shipped with a diagonal at all. In order to figure out how much space I needed, I had to set up an eyepiece on a separate small tripod and actually measure the gap between the eyepiece and the end of the focuser.

The second photo is a guide scope that I decided to re-purpose as a finderscope. The guide scope was built for a camera, so the sensor had to be at the point of focus. An eyepiece has to stand a bit back from the focus, and I had to add this space. You can see the spacer - it's that wide black collar that's holding the eyepiece.

Another very common problem, particularly for owners of Newtonian reflectors is that they can't get a DSLR close enough to get focus, even though an eyepiece does work. This is a more knotty problem, as you need to remove space rather than add it. In this case, a low-power Barlow lens might be necessary, or some other solution.

Talk to your friendly telescope shop owner. There might be a bit of head-scratching, but we should be able to get you focused again!