Re: Mark IV mount

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Tue, 19 May 1998 15:35:28 -0500, Tom Droege <droege@fnal.gov> wrote:
*>Chris and all,
*>
*>I am building a complete system.  Much like buying an LX-200 from the
*>store.  This is pretty much like I did with the Mark III.  For the most
*>part I sent everything needed except a computer, a pump, and a garbage
*>can.  This helped get almost everyone on line promptly.   

Building a good mount is no simple task. I'm actually encouraged that
Tom has redesigned it a few times. It shows he's working hard to make it
good.

*>I agree.  I have to build a focus scheme.  This involves allignment to 
*>a milliradian or better.  Once I work out how to allign the camera and the
*>telescope, aligning two telescopes is duck soup.

Let's see, that's 57/1000 or .06 degrees, about 3 minutes of arc. That
should be doable, even with gears. HOw do you plan to drive the declination
axis, with any corrections for various errors?

*>>It seems that having a mount that can only hold two scopes means you
*>>would never want a "triple".  If the mount could hold four scopes
*>>a triple would become a reasonable option again.
*>
*>Yep, just the R does not seem to add much.

Balancing three vs. balancing four: kinda depends on the design and
its inherent symmetries.

*>Possibly I am building an English mount.  I mostly try to work from first
*>principles and design something that fits all the needs.  As Ross says,
*>the devil is in the details.  We are only a week or two away from a 
*>drawing, then you can see what I have in mind.

Tom, do you have access to some old amateur books called "Amateur Telescope
Making"? From a series in Scientific American from about the '30's and '40's?
It's been reprinted recently, but the old books are usually available as a
set of three for about $50 to $100.  They talk about all KINDS of mounts
in the series. Some old university libraries have them around. I have a
set, could get another. Or you can root through Sky and Telescope on some
rainy day for odd designs - generally from the 1970's backward. Just
looking at the pictures, in my experience, is helpful. There is
a recent book, "Unusual Telescopes", I believe: a small volume on
various amateur and professional designs. Again, thought provoking.

Yoke mounts are not so bad: you lose part of the northern sky, but you
get two points of stability. With tandem telescopes you can even get back
some of the northern sky, and eliminate some counterbalances. A clever
arrangement of three telescopes could "straddle" the center axis, costing
you only some of the northern and far southern skies.

If you like a lot of "iron", you can use sheer mass to make it stable. But
I can't imagine you shipping "iron" around. So your use of
aluminum extrusions and sheet makes sense. Especially as you say you
are using the mount itself as an enclosure and cable run.

There are a number of "stock" mounts for sale, but they are to my
knowledge all GEM's or maybe forks. Are you really set up with a small
machine shop to crank these suckers out?
 
*>I plan to design a "house" as part of the whole design.  I will build one
*>and mount it on my roof deck.  I will provide plans to anyone who wants to
*>build a similar design.  It will probably be built from 1/4" plywood, with
*>a 1/2 or 3/4" base.  But the house is optional.  There are a bunch of 
*>stepper motor and limit switch channels available for those that want to
*>do their own thing.  This will be part of getting people involved.  I will
*>ship the camera when I see a picture of it's new house.  ;^)
*>
*>Tom Droege

Amateurs love to see other people's mounts and their construction. I'm
sure your mount would win a prize at Stellafane, if only for creativity.
Sometimes I wish I had a mechanical engineer's degree instead of an
electrical degree: I only know enough about mechanics to get scared off!
Post your notes when you can, Tom, and don't let us bug you too much
if you get odd comments about it.

Herb Johnson

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