[ATM] spider rotation

[Arjan te Marvelde]

I think Jerry's right.
In my opinion the secondary mirror should be as close to the vanes as 
possible, to minimize the 'tuning fork' length.

To simplify construction you could shift the spider up and down the tube by 
moving the vane mounting along the tube wall. A few mm should suffice. 
Centering the spider in he tube isdone by adjusting vane lengths.

The spider itself then only needs tilt adjustment in two directions, 
sideways and up/down. When the point of rotation is close enough to the 
secondary mirror, the  centering will hardly change.

I came to this minimalist design, which works very well:
http://home.hetnet.nl/~artm/atm/rft/spider.html
http://home.hetnet.nl/~artm/atm/travelscope/images/result_sec.jpg

You can find other, similar designs on the web.

   Arjan

> About a week ago Arjan sent a link to such a design. Jerry must have 
> missed
> it.
>
> Arjan said...
> You mean this one: http://koti.phnet.fi/pulliy/second.html ?
>
> Notice the curves on the top of part #2 and the bottom of part #3. They 
> are
> centered at the center of the mirror's surface. Tilt and rotation is
> possible without changing the location of the center of the mirror 
> surface.
>
> More trouble than I would go through for something that I could do another
> way. I do not like the mirror being on the end of a tuning fork tine. May 
> be
> if the curve of part #2 was the top of the Mirror attach piece and
> adjustment along the main tube axis was by some other method than a
> relatively thin rod I could use this idea. I adjust rotation at the
> attachment to the spider, a tube almost as large as the diagonal minor 
> axis.
> Then tilt and depth in the main tube is done by spider leg attach points 
> to
> the main tube. Very solid that way. Adjustment lasts until the next time 
> the
> diagonal is removed from the tube for recoating.