On this page I detail a way to make a simple "guidescope" for astrophotography
from a cheap Meade 8x50 finderscope. These finderscopes can be bought used
for around $40, and are easy to convert to a guidescope.
UPDATE: I finally had a chance to test the "finder guider" out. It worked as expected with one important note. For PHD to calibrate correctly I needed to calibrate on a star in the east, well away from the north celestial pole. I was trying to guide on a star that was quite near the north celestial pole, and even setting the PHD calibration step to the max (10,000) PHD still couldn't calibrate because the star didn't move enough. So I went to a star further in the east and away from the NCP and this time it calibrated in about 25 steps. So in order to use this guide scope and PHD, I'll first need to calibrate on a star in the east (or west depending on the object I want to image).
1. The eyepiece end of the finderscope unscrews from the finderscope tube.
2. A 2" to 1.25" slip in focuser adapter fits almost perfectly inside the finderscope tube. I had a William Optics 2" to 1.25" adapter laying around that worked perfectly.
3. In order to secure the 2" to 1.25" adapter to the finderscope tube, I drilled and tapped 3 holes in the finderscope tube spaced 120 degrees apart. I used a 8-32 drill and tap set, and secured the 2" to 1.25" adapter using three nylon screws. Both the tap set and the screws are readily available at any hardware store.
4. The one caveat for making a guidescope out of a finderscope in this way is that the guide camera needs to be one that slips into a 1.25" barrel. I use a Lodestar autoguider by Starlight Xpress. Other guide cameras that are not in a 1.25" barrel shape will not work because the focal plane is actually a little inside the back end of the finderscope and cannot get to focus. With a Lodestar or similar the camera can easily be slid in and out of the 1.25" adapter.
5. Focus is achieved by sliding the guide camera in or out of the 1.25" barrel. This is a screen grab of my neighbor's chimney that is about 200 yards away.