Re: Mark IV mount

[Tom Droege <droege@wwa.com>]

There are several ways to approach a design. One is to do it right.  The
problem is that to "know" you are doing it right takes a full career for
each part.  For example, when I was designing the fork mount I was worried
about the load on the ball bearing mount.  It is the rotational component
that gets you.  You can reduce the bearing load by making the mount larger.
 So I went off to find out what a reasonable load on the bearings would be.
 A trip to the internet produced a huge array of results.  There are as
many bearings out there as there are resistors.  I know about resistors,
but not about bearings.  So I have to be conservative, or enter a carreer
as a bearing designer.  This is only one of the things that caused me to
give up on the fork mount.  


At 07:12 PM 5/19/98 -0400, you wrote:
>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.
>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.

It is not getting an idea about a mount that is a problem.  It is
implementing a particular mount and working out the details.  Trying to
make it as small as possible so that it will fit on my deck (and someone
else's deck).  Trying to build it with not so many parts.  OK, I have delt
with this machine shop long enough to know that the average part costs me
$20.  So the fewer the parts the fewer the $20s.  Of course a complicated
part, like the camera head back, might cost $200 in quantity 10.  But that
is sort of the range of things.  As Ross says, the devil is in the details.  
>
>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

This is out.  Arne is a Polaris fan, and wants to be able to look at it.  
The design in TN-39 got around this, but had a real problem with balance.
I did not like how the RA drive came out. 

>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.

Here is why I use aluminum.  The shop would rather cut steel.  But I can
work aluminum in my little shop, so if something comes back not quite like
I had expected, I fix it.  After a while, the shop gets to know this, and
it figures into the price.  so we both end up with a good deal.  Also
anodiced aluminum has a nice appearance, and is nice to work with.   Also
there is no rust, so I don't have to worry about finish and painting. 

>
>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 just send the work out.  The trick is to produce good drawings along the
way so that the next batch is right.  I think we are on out third ream of
drawing paper and the second set of ink cartriges for the plotter.  I also
try to make as many parts alike as possible.  So if I have to parts the
same except one has a few extra holes to mount a motor, both parts get the
holes.  It is cheaper to make two of a more complicated part than two
different parts where one has an extra hole.   I could try to explain why,
but it would not make sense to anyone.  

Tom Droege