Re: ATM 10" Flex Mirror

["Jim Miller" <jim@jtmiller.com>]

major improvement in truss blocks can be had by using moonlite connectors at
www.focuser.com.  no fancy angle drilling, just bolt and go!

jtm (just a customer!)

----- Original Message -----
From: "Jeff Morgan" <jeffmorgan@mindspring.com>
To: <atm@shore.net>
Sent: Friday, October 19, 2001 9:42 AM
Subject: Re: ATM 10" Flex Mirror



Dennis & Doug:


I'm in the finishing OTA finishing stages on my flex project, and thought I
would offer these comments.

The decision on sonotube or truss is really just an issue of being able to
store and transport at tube that is going to be in excess of five feet long.
My project is a 10" f/5, but my car is a Honda Accord. So, I went truss.

As far as mirror thickness goes, it just means that your center of gravity
would be a bit higher (less ballast low), requiring a taller mounting. If
you can get your hands on some sonotube and weigh it, should be a simple
matter to calculate a weight and balance. My guess is the truss arrangement
will allow you to keep things lower to the ground, and more stable.

On this minus side, I found that making the lower clamp blocks was the most
tedius part of the project. You'll need a forstner bit, a good drill press,
a spindle sander, and lots of patience. Even then, the blocks won't all be
the same. Some of the holes won't be exactly vertical. These small errors
multiplied over the length of the truss pole makes for some frustration.

No experience yet on the cooling issue. If anything, I think the more open
mirror box of a truss scope would have fewer challenges. Dirt on mirrors is
inevitable. But let's keep things in perspective - with good cleaning
technique dirt is only an occasional inconvenience. I have provided for two
3" fans mounted on one side of the mirror box with a louvered vent on the
opposite side, still untested of course.

Good Luck!

> Date: Thu, 18 Oct 2001 16:35:57 -0400
> From: Dennis Woos <wooscons@sover.net>
> Subject: ATM 10" Flex Mirror
>
> As our 6" f/8 flex mirror dob performs well, my son Doug and I are going
> to try our hand at flexing a 10" f/6 plate glass mirror, 7/8" thick.
> While we are grinding the mirror, we have started to think about a
> couple of issues.  1)  Should we build with sonotube, or move to a new
> (for us) truss-tube design?  How well would a truss scope work with a
> relatively light mirror?  Is it worth the extra work?  2)  If we build a
> truss-tube scope, then how does the problem of bringing/keeping the
> mirror at ambient change?  For our 6", we followed Adler's example and
> have mounted a fan blowing across the mirror.  This works well.  If we
> go with a truss design, then do we still need a fan blowing across the
> mirror?  If we do, then it does not seem possible to filter the air, and
> this doesn't sound good.
>
> Any and all advice will be appreciated!  BTW, here is the low-down on
> the 6" f/8.  We bought the spherical mirror, mirror cell, spider, and
> 1.5" secondary from Telescope Warehouse for $100.  Of course, we didn't
> use the mirror cell, and the secondary is a bit too large but we used it
> anyway.  The diffraction rings look to us to be identical both sides of
> focus. We can see Trapezium E pretty easily, and we have seen F.  Four
> moons of Saturn are pretty easy, and it seems that with averted vision
> other moons are just visible, but this may be our imagination.  The
> Cassini division is very black and clearly seen all the way around, and
> there is other stuff going on in the A ring.  There is a lot of detail
> visible on Saturn itself.  We need to observe/learn more of Saturn to
> say more.  Shadow transits on Jupiter are easy, and the GRS is visible
> but washed out.  In fact, all color on Jupiter is kind of washed out. at
> least compared to what we expected.  However, we recently looked at a
> CCD image taken through a Zeiss 6" f/15, and it was very similar to what
> we see.  Anyway, we are happy with the scope, and are looking forward to
> pulling the 10"!
>
> Dennis and Doug