Well Ian my secondary needs recoating so we decided to make another secondary with a slightly better figure than the last and yeh,we like playing around with class.
I cut a few secondary mirrors in one go. Here is a pic of the trepanning process
Wow! cutting your own secondary. That not bad. While I am happy to have a go at building a scope using commercial optics, I don"t think I would have go at a primary let alone cutting and grinding a secondary.
I presume you achieve a flat surface by gringing and polishing between 3 surfaces??
I presume you achieve a flat surface by gringing and polishing between 3 surfaces??
How flat do you think you have got it.
Cheers,
I might be getting of the topic here but anyway cutting 6 to 8 secondary in one go allows me to pick the one with the closest flat, we test this with an optical flat and then proceed with the figuring there is no grinding unless the class has scratches on it.
I am not sure at what wave length it is at the moment but I am not in a hurry for it.
Well after 12 months of planning and construction my string scope Dob is finally in a useable form. Still a couple of jobs to do:
1. Padded canvas cary bag.
2. Some form of lid, I am still thinking about this.
3. New shroud and dew shield once I have used the temporary ones for a little while.
My observing buddy (17 yo son) and I had both scopes out for a decent observing session last night. Seeing was quite good although transparency was down a little on the previous night.
I am quite pleased with the stringscope.
The good news:
Jupiter @ 280X quite spectacular showing cloud swirls in the large equatorial bands and quite a number of additional bands during moments of steady seeing. Split Antares at 100X, very clear at 200X. The homunculus in Eta Carina showed signs of structure at 100X and up.
I was able to do some star testing. There is no sign of turned edge.
The not so good news:
While star testing I noticed what appears to be a minor zone within the inner third of the mirror radius. I guess this is probably OK as the images look OK to me and the intra and extra focus patterns look fairly closely matched.
There is some noticeable glare around bright stars which is less noticabe in the other scope with the same EP. I suspect this might be the degraded mirror coating. I need to look at this further.
Attached photo's show the completed scope side on plus equipment drawer and electrical panel.
I have got to build another one now. Mmmmm ....... I think I might be hooked!! As might be expected my observing buddy is of the view the next one should be 16" with a full set on Naglers. I am much more inclined to rebuild our existing solid tube dob to gain additional experience before attempting a larger one, 12.5" perhaps. As for the full set of Naglers, he has my full support to go and get a job for that one!! Of course, I will happily use them once he has bought them.
Balance with heaviest EP, shroud and 400mm dew shield has worked out just about right. This was after I moved the after I moved the altitude bearings 5 mm about a week ago. Holds any position down to horizontal, ever so slightly front heavy if anything. If I want to fit a 30 or 50mm finderscope or indulge in a 24 Panoptic, I will have to use a spring "virtual" counterweight.
Ian,
Great Scope!!
I've not seen this " String Scope" design before. It could be the perfect solution for my 10" that stalled a couple of years ago. I notice you haven't fitted a backboard on the upper assembly. Has this been a problem ?
I am not sure what you mean by a "backboard". The cloth shroud and foam dew shield prevent stray light from entering the focuser. If I operate without the shroud and/or dew shield then I would have to fit a 200mm diameter baffle on the UTA opposite the focuser. I have found the black rip stop nylon shroud wooks well except when a car drives close by and a little glare is then noticeable.
The "truss" is made of "Plus 450" archery bowstring triangles, held under 10kg tension by 16mm aluminium tubes in spring loaded sockets.
