Re: [ATM] 32" Focal ratio

[polaraligned <polaraligned@optonline.net>]

Hi Michael,


>My friends who have SCTs are always
> complaining about poor baffling and low contrast. Right away that raises a
> red flag with me.

This could also be compounded by other things like large central obstruction 
common
to SCT's, scattered light from corrector, etc.  These scopes are 
manufactured to
make the highest revenue possible for the company.

>
> I would strongly advise you to tilt the secondary to the side rather than 
> have
> it aim straight back at the primary (doesn't have to be a 90 degree tilt 
> in
> the optical path, but at least get the focal plane outside of the path of
> incoming light). Aside from making it easier to baffle effectively, you 
> can
> eliminate having to worry about temperature differences from your camera
> distorting the view.

You might be right here.  A camera looking straight up at the secondary flat
seems pretty easy to baffle when I sketched it out in AutoCad.  It is fairly 
close
to the secondary and gets some good coverage from it.  But placing a 
diagonal
teritary below it seems impossible to baffle.  As for the heat issue, there 
seems
to be conflicting opinions on it.  I don't want to learn a lesson the hard 
way here.
I would rather play it safe if I can, and aiming the secondary out to the 
side
may be the safest bet.

The only disadvantages I see are:
- a minor balance issue.
-Camera has to be set up when going from visual use.

The advantages over going at right angles are:
-Do not have to put teritary in for visual use
-Easy to baffle
-Don't have 3rd mirror for visual use (only 2 mirrors)
-Eliminate any possible problems from camera being in optical path.

So if I go this route I will need to determine the shape of the flat and how
to get it made correctly.  Having it the proper shape as opposed to just
using a round, like others have done, will reduce the central obstruction.
I seem to be settling into f/5 as this is as fast as I am comfortable going.
I guess the next step is to determine the tilt of the mirror.
It comes all down to being a big trade off of central obstruction vs. 
eyepeice
height.  The more you tilt your "diagonal" down, the larger the central
obstruction but the lower the eyepeice.
I need to figure this all out.
Does anyone have any advice on doing a folded 32" this way?
I am afraid to ask this, as I felt a lot of people were down on me on my 
initial
post.  They don't realize that I have the tools and skills to build this, 
and
also I am no stranger to large scopes as I currently have a 25" f/5.


>
> Another thing is to think about what pixel size you will be using. Since 
> you
> are designing the scope around the camera you can optimize the focal 
> length
> for the pixel size and field of view you want.

A f/5 at 160" focal legnth seems to be a pretty good match for the 24 um 
chips,
maybe just a tad long.  It is the same focal legnth that the 20"  f/8 RC's 
have, and
have been sucessful with using 24 um chips.  I don't think mating a camera 
up to
 this focal legnth will be a problem.  Ron Wodaski has some fantastic 
software for
matching cameras and scopes.  You can see the field of view (on an object 
like M51),
that a given camera will capture for a specific focal legnth.  Don't forget, 
the chips
are just getting larger and the prices are coming down.  Think of what these 
SBIG
research cameras would have cost just 8 years ago.


Thanks,
Scott


> Clear skies.
> -- 
> Michael Lindner
> http://www.starastronomy.org *** http://home.att.net/~mikel
> http://www.atmsite.org *** http://www.atmlist.net 

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