RE: [ATM] Rotating secondary

["Jerry" <wa4guu@verizon.net>]

--- artbianconi@blast.net wrote:
> 
> With a little effort, I might be able to make a
> device that allows me 
> to rotate the entire secondary and focuser 360
> degrees about the 
> sight axis of the OTA, without having to rotate the
> entire tube!

Art,


Doesn't sound do-able to me. At least not "reasonably do-able". There would
have to be some radial positions of the focuser that would be blocked by the
support from the lower telescope tube and the upper tube that supports the
spider. I am assuming that you do not intend to make the main tube large
enough for you to go inside it to observe. I can see how to go about making
the diagonal rotate on the spider. I just have a hard time figuring out how
to make the hole in the side of the tube rotate with it. I suppose you could
put the diagonal mirror on the sky side of the spider, or the spider could
be v shaped, apex toward the sky, to get the focus on the sky side of the
spider.  Or maybe the diagonal could be on the primary mirror side of the
spider at less than a 45 degree angle to direct the focus to the sky side of
the spider. Neither likely to be done in a way that could be considered an
"improvement". 

By what means would the focuser be linked to the diagonal? Would the focuser
also be supported by the shaft on which the diagonal mirror is attached? If
so I would wonder how much flexure there would be when I put that big 40 mm
wide-field eyepiece in the focuser. Also in addition to the diffraction from
the spider there would also be diffraction from the supports extending from
the shaft to the focuser. I think most ATMs would find that to be a
significant negative.  

These are probably a few reasons that those who want to be able to adjust
focuser position on demand while observing usually choose either to rotate
the entire optical tube assembly or to have an upper tube assembly with the
spider/diagonal/focuser that rotates as a unit on the front end of the tube.

I wouldn't use less than a 3 inch diagonal on a 15 inch f4.4 Newtonian
unless I intended to use it only at high power. If you got the focal plane
as close as possible to the tube axis and diagonal perfectly aligned on the
axis you would be doing something to get a 1 inch circle, slightly less than
1 degree, fully illuminated at the focal plane. The obscuration ratio would
still only be 20 percent with a 3 inch diagonal. If you want less than that
it would be a good idea to go longer than f 4.4.

Jerry





 






 




 

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