Re: Big Dobson
From: Shneor Sherman (shneor_at_my-deja.com)
Date: 11/18/04
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Date: 18 Nov 2004 07:53:51 -0800
MitchAlsup@aol.com (Mitch Alsup) wrote in message news:<e90782f7.0411161518.13886bcc@posting.google.com>...
> "eric" <eric.archambeault@videotron.ca> wrote in message news:<bwHld.4355$SC.8773@wagner.videotron.net>...
> > Hi,
> >
> > I would like to know if someone know a telescope maker for 30" or bigger
> > Dobson (with tracking, navigation system and/or computer control and/or CCD
> > tracking system). Obsession dont make the 30" anymore, and Starmaster limit
> > is 28"... cost must be under 35000$US
>
> After a quick survey on the web. If you want a 30"+ scope, you are
> probably
> going to have to grind and polish it yourself. The only people making
> 30"+
> optics are selling the primary for more than you budget for the whole
> scope.
>
> John Hudec tells a story about the 36" optics he made for Obsession.
> He tells
> about how these objectives just about killed him (in a literal sense),
> they
> are big, unwieldly, extremely heavy, and easy to damage. In addition,
> where
> are you going to send it when it needs a new coating?
>
> >
> > Thanx for info!
> > Eric
>
> You should also be aware, that for the most part, a 30"+ scope is for
> those
> objects you can't reach in a 16-20" scope. You just can't get a short
> enough
> focal length to get many of the larger sky objects in the lowest power
> EP,
> nor does the atmosphere put up with the lowest power a 30 throws very
> often.
> With a 30" F/5 you have a 150" focal length causing a 31mm Nagler to
> have
> 123X minimum power, thereby constraining you observations from 123X to
> 600X.
>
> You should visit a star party where a 30"+ scope resides (or New
> Mexico Skies
> and rent the 30"er) to see just how big these suckers are. Hint:
> although
> the scope can fit in an Excursion sized SUV, the poles will not, thus
> man-
> dating a trailer of some 16 feet. Nor will the ladder fit in any
> reasonable
> sized vehicle (to reach the eyepiece/focuser.)
>
> If you haven't do so already, I suggest you start with "The Dobsonian
> Telescope" by Berry and Kriege. Reassess you desires and cost
> structure,
> and then seriously think about building it yourself.
>
> I happen to have a 20"er that I built myself, I wanted to build a
> 25"er.
> I am glad (now 5 years later) that the 20" optic fell into my hands
> before
> I could find a 25" optic.
>
> Mitch
I have had the good fortune to be able to occasionally view through a
30" Starmaster owned by a consortium here in Northern California. It's
an f/4.5, With a 31mm Nagler, the true field of view is 42 arc-minutes
- not a really wide field, but wide enough for all but the largest
telescope objects. It's true that you need near-perfect conditions to
get the maximum out of this telescope, but even average conditions
provide spectacular views, expecially when it comes to galaxy
clusters, for example.
While it is quite difficult, I have seen one person put it together
and take it down. But it normally requires two people to do part of
these tasks. This particular telescope rides in a small trailer; the
ladder can ride on the roof of the vehicle that pulls the trailer
(depending on who has it at any time).
As for me, I have a 22" that I am happy with and which fits in my
Mazda 323 hatchback, with all the accoutrements.
Clear skies,
Shneor Sherman
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