Re: ATM low cost secondarys

["Richard Schwartz" <richas@idt.net>]

how big does it need to be?   the distance from the focal plane to the
center of the diagonal is about 8".   the mirror is f/5, and the diameter of
the focal plane is about 2" if you have a big focuser.    so the mirror
minor width needs to be about 2" + 8" / 5 = 3.6".   the length is 3.6" x
sqrt(2) = 5.1".   there is a more accurate formula, but 3.6" x 5.1" is close
enough.  you might reduce the diagonal a bit if you have an xtra low profile
focuser.  this wood work by reducing the 8" distance from the center of the
diagonal to the focal plane.

you can go to a dime store such as woolworths, and just get a simple flat
mirror that women use for shaving when their mustache is too heavy.  this
kind of mirror is silvered on the back.  you can cut it with a glass cutter
to a 3.6" x 5.1" rectangle.  this will work, but the closer you get to an
ellipse, the better.  so break the corners off of that wrecked angle to make
an octagon.   use a carbide whetstone to bevel the edges so you don't get
cut, but take care not to knock the silvering off of the back.   that is the
lowest cost secondary you can get.   but it is not the best.  most telescope
builders spend a lot of money for a thick mirror that is aluminized on the
front surface and is flat to 1/17.43829 light waves.

you have to take care to mount the secondary so that it is not bent, so that
it does not get astigmatism.  some experts in the atm list will tell you to
glue the diagonal in with silicone.   i don't know what is best any more
because everybuddy says something different.



----- Original Message -----
From: "natsp" <natsp@alltel.net>
To: <atm@shore.net>
Sent: Monday, August 21, 2000 11:20 AM
Subject: ATM low cost secondarys


>
> looking for a secondary for the 14.5-f5  focal legnth 72.650  16" tube..
> i was told i needed 2.8" secondary.
> why so big?
> should it be elipticle?
> can i get an inexpencive one?
> thanks
> stan
>
>
>