SCT & unfiltered solar-projection

Re. the venus transit.
If we want to use our 5" and 8" SCT for projecting a unfiltered
image onto a white screen, for how long might that be done,
without building up heat and damages to the SCT ?
30 secs. ? several minutes ?
Pete.

"Bino Bing" wrote in message
. ..
Re. the venus transit.
If we want to use our 5" and 8" SCT for projecting a unfiltered
image onto a white screen, for how long might that be done,
without building up heat and damages to the SCT ?
30 secs. ? several minutes ?
Pete.
Consider reducing the light entering the scope, with a simple aperture mask.
The problem with an SCT, is that the whole tube is enclosed, and the light
is getting more concentrated all the time. If you think of an 8" SCT, by the
time the light is refected back to the secondary, this (less the losses in
the reflections/transmissions), receives all the light from about 45 square
inches at the front of the scope, concentrated allready into an area of
about 4.5 square inches. The mirror assembly is also receiving direct
sunlight on it's 'back' as well. This can lead to the secondary adhesive
melting, or if the scope is aimed a little 'off' so part of the light hits
the secondary casing, this can get very hot, and crack the corrector. If you
reduce the effective aperture to only about 2.5", you can still make
excellent projections (remember to cover your finder scope), and the amount
of heat energy is not so bad...
A small refractor is simpler and safer.

Best Wishes

On 2004-06-05, Bino Bing wrote:

If we want to use our 5" and 8" SCT for projecting a unfiltered
image onto a white screen, for how long might that be done,
without building up heat and damages to the SCT ?
30 secs. ? several minutes ?

Very close to zero. I wouldn't do it at all.

Very close to zero. I wouldn't do it at all.

Hi:

I agree. Don't do it. If you don't have a Solar filter, find someone who does
locally and arrange to hang out with them. Using an SCT for projection is risky
at best--there's just too much chance of causing internal damage.


Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

Baader Planetarium AstroSafety Solar Film is inexpensive. You can make
an offset filter cell to place over your corrector plate. The cost is
alnost next to nothing. Why risk damaging an expensive OTA?

Matthew Ota

Bino Bing wrote:

Re. the venus transit.
If we want to use our 5" and 8" SCT for projecting a unfiltered
image onto a white screen, for how long might that be done,
without building up heat and damages to the SCT ?
30 secs. ? several minutes ?
Pete.

Bino Bing wrote:

Re. the venus transit.
If we want to use our 5" and 8" SCT for projecting a unfiltered
image onto a white screen, for how long might that be done,
without building up heat and damages to the SCT ?
30 secs. ? several minutes ?
Pete.
No NO NO! Just say No. Don't do it, don't even consider it.

David Moody