Wednesday, September 11, 2013

A Diagonal For The Galileoscope

The Galileoscope has become one of my favorite experiment telescopes. I should explain what I mean by "experiment". I use this telescope for things like video imaging, solar work, testing eyepieces, etc. But it is more than just a test telescope. It can be used for some observing work. 
The chief complaint about the telescope, though, is the lack of a star diagonal. If the telescope is raised higher than 35° from horizontal, it begins to become difficult to use, especially is you use a shorter tripod. Unless the average Galileoscope user has access to a taller tripod, the only real objects are those that are closer to the horizon.
Adding a star diagonal to the Galileoscope is tricky. The draw tube focus is very limited. Adding a diagonal is not practical without modifications. There are a few eyepieces that can be used, but very few. 
Some users have modified the telescope to take a focuser. To me, that just doesn't seem very practical. In order to add a focuser, the tube has to be cut, and the last thing I want to do is weaken the tube's structural integrity. This is also something that the average Galileoscope user might not want to do, and almost everything I do I want others to be able to. 
The simplest solution is installing a transit lens. Some users have used Barlow lenses, but they add length and therefore more magnification. The trick here is to increase the focal length just enough to make using a star diagonal practical. 
The lens I used was a 29mm plano concave (PCV) with a -53mm focal length. This is actually leftover from my Galilean project. I placed this ahead of the diagonal and used the Galileoscope's 20mm Plossl. This makes it close enough to not increase magnification too much, the result being roughly 1.5x to 2x. The target for the first full test was a waxing crescent Moon, and the results were very good. The amount of focus play is sufficient to allow the use of additional eyepieces. 
The diagonal I used was a heavy 1 1/4" older Meade, which was tricky. At higher angles, the weight of the diagonal kept pulling the draw tube focuser out. A couple of times, the star diagonal nearly fell out. The solution would be to use a plastic diagonal. Surplus Shed and other outlets sell inexpensive units that should be much lighter. 

There is still some tweaking that needs to be done, however this simple modification greatly improves the usability of this telescope. When the tweaks are made, they will be posted here.