To double check the final dimensions of the Folded Stevick-Paul optical tube assembly I laid out three 4'x8' sheets of plywood on the ground and drew out the optical path at full scale. The white rectangles in the photos are full scale drawings of the mirror cells that will be bolted onto the tube assembly. Doing this allowed me to double check all the dimensions and angles that I would need for cutting up the 1" box-section steel for the OTA and fine tune the whole support structure.
The photo, below, shows the light path of the convirging cone of light to its focus point, and how it passes close to the tertiary 10" flat folding mirror (in fact, through it's cell!) This mirror cell will have to be built in such a way so as not to impede the light path. Cooling fans on the tertiary mirror will heve to be ducted away from the light path to avoid turbulence.
It is interesting to note that in faster (f/12 and faster) versions of the Folded Stevick-Paul design the light cone actually grazes the edge of the tertiary mirror, which needs to be very carefully bevelled to minimise the obstruction. This is another reason I decided to design my FSP at f/15. I had hoped to make an f/20 system, but that would have meant building an even bigger observatory! Also, I can *just* test a 14" f/15 mirror inside my house. The mirror must be tested at its radius of curvature (twice its focal length) which is 10.5m!!!
2 comments:
Phil,
This looks like an amazing project - When is the expected completion? That dome really looks the business, I don't think I would have the patience see it through. Can't wait to see what you plan for the slit, how you're going to rotate the dome etc. Best of luck - I will follow this with interest.
Hi John,
Yeah, it's a bit of a monster! I had originally thought I'd have the whole project finished inside a year... two years ago! Life has conspired against me! However I am making a bit of a concerted effort at the moment to get the job finished this year...ish.
Realistically, I probably won't get a suitable window of weather to erect the floor, walls and dome over the winter (way too wet and windy!) but I am getting all the pieces in place for rapid assembly in the spring.
I am currently working on the steel framework for the two optical tube assemblies and will build the large fork mount after that. Once those jobs are done, I can calculate the off-set for the concrete pier and cast that into the ground. After that, it's basic carpentry.
My plan for the dome shutters is a bit special (an perhaps wishful thinking!) They will not sit proud of the dome as on most other domed observatories, but will be flush with the skin of the rest of the dome so that they do not break the nice symmetry of the structure... which means that I will have to come up with a means of elevating them out from their recessed 'home' position before they can be slid open. I have a couple of ideas for doing this, but it may involve an 'iterative' trial and error approach!
Phil.
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