Re: [ATM] New 16" ultralight telescope

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

> I liked that wire spider of Alan's. But if someone is concerned with 
> spider spikes, why bother with a spider at all? I made a 16" dob and 
> had a a piece of Starphire glass cut and used it as protective front 
> plate and secondary holder. If I recall the glass was rated at 92% 
> transperant in the visible range and had zero affect visually on deep 
> sky objects.

I searched for that type of glass and it showed up as a type that is 
claimed to be "clearer" than normal float glass.  I didn't see any 
specs for flatness.  I doubt this is suitable for a telescope window 
on anything except a small telescope.  If one could hand-select flat 
portions of the glass, it is possible it might work, but I doubt it is 
as homogenous as real optical glass.

Assuming the glass may have had no effect on deep sky objects, a 
spider really doesn't have an effect either (unless you are imaging) - 
you will not see the spikes on deep sky objects.  With a spider you 
lose only a fraction of a percent of the light coming from the sky 
(rather than 8%), and there are no optical effects other than the 
diffraction.

I think it is on planets where spider diffraction may have the most 
effect - thick spiders cause quite a bit of diffraction and will 
degrade contrast - thin ones have almost no effect.  On the other hand 
a poor window will destroy planetary images.

      Mike Lockwood

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