[ATM] New home built 12" ultralight dob

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

Oliver,
  Answers below:

>I'd like to know what you mean with a "torsional attachment". Does this 
>mean that you slightly twist 'n bend the trusses in order to fit them into 
>the hles you foresaw for them in the mirror box?
>
Torsional means ... that I don't know mechanical engineering worth anything. 
  No, they are not torsionally applied, there is no twisting at all that is 
necessary.  There IS a tensional component.  The trusses are dropped into 
the holes in the mirror box top, and come to rest 1 inch down, against a 
stop.  At this point, they are pointing straight up.  Then, pairs of trusses 
are pulled together at the top, creating a tension at the bottom of the 
trusses against the 1" holes in the top of the mirror box.  Turns out that 
it is strong enough that when asked about it, I hand people the telescope, 
set up, by one truss.  So, the whole 30 pound telescope is supported by that 
one truss tensioned into the wood of the mirror box top.  We all have to 
show off a bit!

Click on the right, upper picture of the main DS-3 web page for an example: 
http://www.xmission.com/~alanne/DS3Main.html.

>>http://www.xmission.com/~alanne/Astronomy.html
>
>Nice web site!
>
>I however have some questions on your mirror cell. My main concern with 
>your approach is that your coil springs apply pressure directly to the 
>wooden disks. This may severely deform your mirror's surface (no rotational 
>"tilt" freedom). I'd rather use a pair of "locking nuts" on your supporting 
>collimation bolts, with a washer between the locked nut pair and the 
>springs; this way the springs will apply much less deformation on the back 
>of your mirror. Of course, as you seem to have a rather thick mirror, this 
>deformation may get unnoticed.
>

I never thought of that.  Three answers.  First, with good skies, the major 
issues that I saw with the scope at 381 power was when de-focusing, and 
looking at the aire disk.  They were swimming slightly, after 9 hours in the 
open.  So, I conclude from real world tests that air currents on the surface 
of the mirror are my biggest problem with the mirror at this point.  Second, 
don't forget that double sided sticky tape has a spungy middle of a 
millimeter or so.  So, any irregularities in the wood are irrelevant.  In 
fact, I am not sure that I actually totally countersunk the screw heads.  It 
didn't matter.  Third, if I ever do use a thinner mirror, you have a good 
idea.  However, then the mrirror would flop around a bit as you go from 
compression to tension on the top screw of the mount, as you lean the 
telescope over.  As is, the scope holds perfect columnation between straight 
up and basically laying on the horizon.

By the way, isn't that a sweet mirror cell?  Credit where credit is due - 
the mirror cell and wire spider are from a local TAAS member, Jim Lawrence.

Alan Scott

p.s. - Sorry to start another thread - my e-mail program won't reply back 
correctly.  It is Hotmail.  Any ideas how to reply to a thread in Hotmail?

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