projection TV lenses

The other day at a garage sale a projection TV was for sale. Looking at the
projector lenses I wondered if they could be used to make a refractor. The
TV was faulty and
probably can't be fixed for a resonable price.

They appeared to be about 90 or 100mm diameter. Considering that the
projection reflects off a mirror and onto the screen, the FL would have to
be
maybe 400mm or so. Any ideas on this?

On Sun, 16 May 2004 22:17:07 GMT, "Thomas" wrote:

The other day at a garage sale a projection TV was for sale. Looking at the
projector lenses I wondered if they could be used to make a refractor. The
TV was faulty and
probably can't be fixed for a resonable price.

They appeared to be about 90 or 100mm diameter. Considering that the
projection reflects off a mirror and onto the screen, the FL would have to
be
maybe 400mm or so. Any ideas on this?

My guess would be that their correction for chromatic aberration is somewhere
between poor and non-existent. Since in the TV each is only focusing a
relatively narrow part of the spectrum, such correction is not necessary. They
will also be designed to produce a large, flat field (in projection) which will
probably translate to some unusual distortion when used in telescopes.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

" My guess would be that their correction for chromatic aberration is
somewhere
between poor and non-existent. Since in the TV each is only focusing a
relatively narrow part of the spectrum, such correction is not necessary.
They
will also be designed to produce a large, flat field (in projection) which
will
probably translate to some unusual distortion when used in telescopes.

_________________________________________________


Ok. Sounds good

"Thomas" wrote in message news:GwTpc.7566$9P6.98@clgrps12...

" My guess would be that their correction for chromatic aberration is
somewhere
between poor and non-existent. Since in the TV each is only focusing a
relatively narrow part of the spectrum, such correction is not
necessary.
They
will also be designed to produce a large, flat field (in projection)
which
will
probably translate to some unusual distortion when used in telescopes.

_________________________________________________


Ok. Sounds good

I emailed with someone who tried it, and Chris hit it on the head --- a
waste of time making it into a scope.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************

"Thomas" wrote in message news:DLRpc.16863$0e6.2779@clgrps13...
The other day at a garage sale a projection TV was for sale. Looking at the
projector lenses I wondered if they could be used to make a refractor. The
TV was faulty and
probably can't be fixed for a resonable price.

They appeared to be about 90 or 100mm diameter. Considering that the
projection reflects off a mirror and onto the screen, the FL would have to
be
maybe 400mm or so. Any ideas on this?

Projection TV lenses tend to have a relative aperture of about f/1,
and have many waves of residual aberration. Some of the newer ones
for HDTV are quite good, and some are now fully color corrected, but
may not be available in garage sales for some time!

Brian
www.caldwellphotographic.com

You've got your choice of three, the red, green or blue.