To: "Jeremiah J. Burton" ,

From: Gary Seronik 
Reply-To: Gary Seronik 


Hello Jeremiah:

At 08:46 PM 5/20/2003 -0600, Jeremiah J. Burton wrote:
>
>I have an old Meade 8" dob with a closed & heavy mirror cell.  It will
probably be a while before I can replace the existing cell.

I would advise you to make that a priority -- those scopes are thermal
nightmares.  Not only is the cell design a disadvantage, but they have also
stuck several pounds of iron behind the mirror as counterweight.  Like the
mirror, this too will radiate heat all night long.  It should be moved to
the outside of the tube.

>
>I saw an article in the September 2000 sky & telescope, that talked about
thermal problems.  It suggested putting a vent above the mirror to help
with cooling.  This seems pretty easy.  Would it be worthwhile to put a fan
in the vent?

That's what the article says to do and it is absolutely necessary -- the
vent by itself will not be enough to allow the mirror to reach ambient.


>If so, should it blow out or in?  How big should the vent & fan be, & how
should they be possitioned?

The article you cite (which is in the January 2002 issue, by the way)
describes all this in detail, give it another look. If you don't have that
issue, try your local library or download it from SkyandTelescope.com.

>
>when I do get around to installing a new- cell, what is a good one to get?
 I've heard that the University Optics cell needs some modification.  what
kind of modification?  Should I use silicon to attache the mirror?

You can do that and avoid using the clips, but modification to the cell
isn't really necessary, though it can be made easier to adjust.

Better yet -- I'd suggest making your own mirror cell.  It's not that
difficult and you will learn something.  The double-plate design I
described in the May 2002 issue of Sky & Telescope should work well for
this telescope.

Good luck!

Regards,
Gary Seronik

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