Re: [ATM] Hello list, some basic question for a new Dobsonian builder

["Bob May" <bobmay@nethere.com>]

Mike, what with our propensity to build rather than buy, I'd suggest that
you consider buying a kit for grinding your own primary mirror.  With a kit
at about $100, a Foucault tester for about $10 if you do some scrounging in
your woodpile and a coating at $40 or so, you will be able to do the focal
length you want without problems.  Grinding and polishing a mirror is more
boredom than technically difficult with the hardest part at the end when you
figure the mirror.  If you can find somebody in the astronomy club near you
that has done a mirror, he can usually be induced to show you how everything
goes.  This will mean that the telescope will take a lot longer to make tho
but you will be able to say that you did it all yourself.
I'll also note that you can do a sensibly perfect mirror the first time out
if you take care while the commercial mirrors are aften barely 1/4 wave on
the surface (what I call a 1/2 wave mirror) and sometimes not even close to
that.  And worse is that you don't have a choice of mirrors to find the best
one.
As to the EPs, I tend to differ from others as the availability of the 2X
barlows is high and this means that if you have a 25mm EP, you also have a
12.5mm EP with the barlowing of the 25mm EP.  Thus, sizes like a 25, 16 and
6mm would be a nice selection with the barlow, giving you sizes of 25, 16,
12.5, 8, 6, 3mm in your set.
The problem with EPs is that the distance from the mounting shoulder to the
focal plane of the EP is different between sizes for most makers of EPs.
This means that the focuser needs to move about to insure that the focal
plane of the EP matches to the focal plane (which doesn't move) of the
telescope.  Remember tht the whole system is an afocal system which means
that the light coming out of the EP is parallel light if the input light was
parallel light.  This parallel light path is what your eye needs to see the
image.
The original Dobson spider is indeed a kludge but you have to remember that
John Dobson intended his telescopes to be light buckets that show the more
expansive DSOs that are up there rather than doing higher power work like a
good job on planets.  As a result, the things like a full alignment really
aren't necessary to the degree that others prefer.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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