Last Updated June 17, 2002
I just started work on my next challenge. This will be a 25" f5 Folded Newtonian portable scope incorporating an ultra thin mirror. I've wanted to make a big dob for quite a while but I hated the idea of climbing up a tall ladder. As well, the cost of buying a scope such as this was out of the question so that only left one option... Design and make a scope to my preferences. So here are my design goals.
Design Goals:
Large aperture with a useful magnification range of 80-350 power.
Eyepiece height low enough that I would not need a ladder when viewing the zenith.
Portable in a mini-van without consuming all of the space. This means a max packing height less than 44".
Fun and challenging project which will keep me busy and out of trouble for the next 1.5-2 years.
Total project cost not to exceed $1000 US. Otherwise it will violate goal #4.
In order to achieve goal #2 I need to fold the light path back onto itself by the use of an 8" flat. Effectively my eyepiece should be at eye level. I found the following wonderful program to perform the calculations http://www.oldham-optical.co.uk/telescope calculation 95.zip necessary to position the flat and diagonal to ensure that the diagonal does not further obstruct the light by remaining in the shadow of the flat. The total light loss due to the 8" flat is only about 12 percent.
Before starting this project I used a wonderful program called Plop written by Professor David Lewis of the University of Toronto. Plop is an automated design optimization program for mirror cells. The program can be downloaded from here. Plop tells me that my 25" by 1" thick mirror supported using an optimized 27 point mirror cell will have a Visible RMS Error of 2.21e-6. This value gives me enough margin to proceed with the project. As well I've found a few other people on the net who have accomplished similar feats with thin mirrors and, who are willing to guide me in this endeavor. One individual made a very nice 20" mirror out of 7/8" glass!
Thumbnail view of the conceptual design. When set up the eyepiece will be at 68" height. When packed up the overall height will be 43". Not shown are the truss tubes. I also plan to have wheels mounted to the bottom of the scope for easier transport. The total weight of the scope will be approx. 108 lbs (33 lbs Primary, 35 lbs mirror cell, 16 lbs baltic birch, 8 lbs truss tubes and 16 lbs for the secondary cage).
Project Cost Breakdown:
