I finally completed the rebuild of my 16" F4.5 scope. The original structure was intended as a temporary solution to enable star testing of a second hand Meade DS mirror.
Fairly traditional truss tube dob design with a few differences:
1. Large alt bearings 2 X mirror diameter (800mm). Assists with balance and alt/az control force harmonisation. These are becoming increasingly common in European ATM scopes, not so much in the US.
2. Triangular mirror cell with the entire cell tilting for collimation.
3. Whiffletree mirror edge supports.
4. Slotted tongue and bolt truss attach points top and bottom.
5. Integral filter slide
6. Integral dew heater wiring. No cables frompowered ground board to powered secondary cage and integral secondary mirror hearer wiring.
7. All plywood end grain hidden with Blackwood edging.
8. Secondary cage is all glued plywood - no Kydex
9. Nested design where the secondary cage resides inside the mirror box for transport and storage
Only had the scope out for one observing session at Wiruna last Fri/Sat. Worked well in very heavy dew for a 3 hour gap between cloud and fog!
Last edited by stringscope; 18-08-2010 at 10:47 AM.
Reason: spelling
Many thanks for your comments Guys. Lots of fun building it, took a bit longer than I had hoped - usually does.
I am not happy with the appearance of the altitude trunions and I am planning to build new ones with 18mm ply inside and 18mm Blackwood facing on the outside including the spokes. I have a "spare" billet of Blackwood here but I am still working out how best to do this.
Finder is fabricated from a SurplusShed 80mm F5 objective lens, plumbing fittings, 2" drawtube from a GSO newtonian focuser, UO AMICI prism and an old Celestron 32mm Plossl fitted with crosshairs. The Mounting brackets are WO 100mm guidescope brackets. 12.5X80 with a 3.5 degree FoV. FoV is limited due to vignetting by the UO AMICI prism 20mm aperture limit. To achieve a larger RACI FoV with this objective requires a WO 90 degree AMICI " prism that weighs nearly 1/2kg.
Hello Mark, yes, I should have added: 0nce I was able to star test the mirror at the SPSP (original scope was finished @10PM the night before leaving for Wiruna), it clearly required refiguring. Mark had a look at the mirror during the SPSP and took it away with him for some expert TLC followed by recoating with Issac. The secondary is an excellent unit from Protostar. The result has been very pleasing optically and as the mirror is only 38mm thick, cools fairly quickly.
I now have a structure that goes someway to matching the optics.
A photo of the original scope and some additional detail of the whiffletree edge supports with the front baffle removed. The closeup image includes a dummy mirror used during construction.
The 4 front mirror retaining clips are 3mm ply disks that rotate to allow mirror removal. The reason for large ply clips is in the event of a big bounce during transport, the large clips spread the load and the ply has some flex, hopefully preventing any front surface chipping.
Hi Ian, good to see you got the mirror tested and in a scope. Looking forward to your test once it is refigured. I always thought it needed something, but didn't know what.
Hi Lester, Mark refigured the mirror a couple of years ago and I have been using the refigured optics in the original structure since then. Image is now effectively seeing limited. Once the mirror T is close to ambient T, star test is excellent and shows excellent definition, contrast etc. The mirror cell in its original configutation had a sling that I has never very happy with and occassionally generated astigmatism. To date I am very happy with the reworked mirror cell with the whiffletree edge supports.
Hi Andrew, Eventually I relented and fitted wheelbarrow handles. At the end of the day with a 10kg mirror, and without going to composites or welded aluminium structure, the minimum mirror box weight I could come up with was about 20kg. Given the dimensions of a mirror box that force one to lift/carry this weight about 300mm away from ones body, I could see this eventually generating back trouble. I now have a ramp for the van and wheel the scope in and out of the van. -Luxury - The 1.8 metre handles are a bit of a pain to store at home though.
The dew heater electronics are mounted in the filter slide box above the filters. I was hoping this arrangement would obviate the need for a filter slide heater. At Wiruna in heavy dew followed by fog, I noted the filter slide box exterior remained nearly dew free for the entire night and there was no tendency for the filters to dew up. My only concern with this arrangement is the potential for out gassing from the warm electrical components leaving a film on the filters.
