Re: [ATM] New 16" ultralight telescope

["Alan Scott" <alanne1956@hotmail.com>]

I want to mention that one of the design goals for DS-4 was light weight.  
There is about a 5 to 1 ratio between weight in the upper "cage" and weight 
in the mirror box.  In other words, for every 1 pound you add to the upper 
cage, you either must add 5 pounds to the mirror box, OR increase the size 
of the bearings.  And 17" diameter sheets of glass are probably not that 
light.

Second goal was to have a good deep space scope.  The wires are virtually 0% 
of the area of the upper cage, thus don't decrease the light at all.  I need 
to do more observing before I can comment on the spikes, but I believe that 
they are pretty insignificant.    Wire spiders are also easy to remove, wrap 
with a shirt, pack in a suit case, and ship by plane.  The secondary goes in 
the eyepiece case, and the secondary "ring" would screw down onto the top of 
the mirror box - or go in the bottom of that suit case.


alan


>From: Mike Lockwood <melockwo@uiuc.edu>
>To: atm@atmlist.net
>Subject: Re: [ATM] New 16" ultralight telescope
>Date: Fri, 21 Apr 2006 12:22:34 -0500
>
>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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