Illuminated reticule eyepiece

During the latest lockdown in Melbourne, I was at home, but had taken with me a small Newtonian telescope and a guide camera. I was wondering what sort of photo of Jupiter I could get with simple and inexpensive equipment.

I put the scope up onto my NEQ6 mount, but it didn't have a finderscope. Complicating matters, the guide camera was set into the focuser of the telescope and it was nicely focused. I didn't want to move it. How was I going to see where the scope was pointed? How was I going to align the scope to the sky?

Reusing old equipment

I have an old guide scope at home - an Orion. It's not as good as the saxon one, but it does the job. I've also got an ancient illuminated reticule eyepiece which I rescued out a junk bin at an astro meet once. I planned to use this in the guide scope.

The eyepiece was filthy. When I looked through it, all I could see was nicely-focused grit and dust across the whole field. I had to pull the eyepiece apart to clean it. But pulling it apart gave me a good idea as to how it worked.

How an illuminated reticule works

Have a look at my scribbled diagram. The main part of the eyepiece is a Plössl group (or it might be a Kellner, I didn't pull that apart). This is at an adjustable distance from a piece of flat glass which is etched with a double-crosshair. You can see this when it's illuminated from the side by a red LED. That's the silver part that hangs off the side of the eyepiece.

Once in the scope, you adjust the eyepiece so you can see the reticule in focus, then focus the telescope. This puts the focal plane of the telescope right on the etched glass, meaning you see both the image and the reticule in focus.

Putting it together

Once I had the guide scope, with its illuminated reticule, collimated to the main scope, finding planets was a snap. The NEQ6 was able to slew near to the planet, and from there I used the guide scope to put the target into the reticule. Once there, the planet was guaranteed to be in the camera's field.

Incidentally, here's one of the photos. Not bad for beginner equipment!