Re: ATM 22" Mirror

["John Sherman" <shermj@netzero.net>]

>I've found it takes a lot of "horsepower" to cool my 15"
>full thickness mirror.  I use two 26 cfm fans blowing on the face of the
>mirror, and one 85 cfm blowing on the back, and the early indications are
>it's the best modification I've ever made to my scope. It took nearly 140
>cfm to get my 15" to cool down in a
>reasonable period of time and perform up
>to it's potential.  Before I took these steps I was PLAGUED by thermal
>problems and substandard imaging.


Alan,

Those thermals are, indeed, a big problem. I have a fan blowing on the back to
cool the mirror. Also, I have a small fan which takes air out of the top of the
mirror box through a hole. I can adjust the speed of either fan. At 170x or so,
I like to rack a star out of focus till the donut of light is seen. Then, I can
easily study the thermals inside my scope (and above my scope). The best I've
seen was with neither fan on, and there was only a very thin wisp of heat rising
up from the center of the mirror. The seeing was good that night, also, but my
image did not improve. Of course, I had to use the fans to get the mirror cooled
down to that level. Certainly, even more fans would do the job quicker.
Some people say you only have to see one super high resolution view through a
telescope, and then you realise the shortcomings of all the others. These
quality scopes are not abundant. Fortunately, I own one. So now I can easily
tell what my other scopes are supposed to show me.  Good thing, bad thing...
Also, recent reliable testing convinces me that my 22" mirror has to be
refigured, that it is my biggest problem. I just need to figure out who is going
to do it, and how soon will it be done??
Thanks for your help,

John










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