[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: [ATM] Cassegrain vs. Gregorian (secondary testing) Best of BothWorlds

I am working on a 13.25" f/3.12 primary that will
become a 4-way telescope.  Here's the plan:

1. Prime-focus CCD.  Let's let my computer do the
looking so I don't have to put my eye to a cold
eyepiece barrel.

2. Visual-photo Newtonian.  I like galaxies and
faint-fuzzies.

3. Cassegrain.  I think the f.D will be about 18 here.
 I like the moon and sun, with the right filters. 
Great Orion also fits in here, as do several star
clusters.

4. Long-focus Gregorian.  I like planets and cheap,
longer-focus eyepieces.

The trick:  Put the Greg and Cass on the same focal
plane, parfocal to a millimeter or so, and make the
telescopes interchangeable at night with as little
screw-turning as possible.  Bonus points:  Put it on
an equatorial fork mount.

Sound like fun?  Stay tuned for pictures.  I just got
a cheap digital camera.

Kevin of Eastern Iowa
Seeker of the Darkness

ps.  Next project:  Large aplanat
--- "Richard F.L.R. Snashall" <rflrs@rcn.com> wrote:

> 
> 
> Bob May wrote:
> > The comment was made that the cass secondary with
> a e of -4+ would be easier
> > to build than a gregorian of -.5 would be and in
> this, I beg to differ.
> 
> This is not quite the comparison.  The Cassegrain
> with the same
> characteristics will have RoCs for the curves that
> are quite a
> bit longer than those of the Gregorian.  The actual
> amount of
> material removed will actually be less, even though
> the e value
> is larger (in magnitude).
> 
> > The methods that I know of for testing a cass
> secondary are the Hindle test
> > (large perforated sphere needed), through the back
> (need to grind and polish
> > the back to a near ROC of the front surface to
> minimize the errors from
> > refraction or run a raytrace to see what happens
> to the light as it goes
> > through the back surface and glass) or using a
> match plate (making a concave
> > surface to match the curve of the secondary).
> 
> And now the test I described.  The difference is
> that the lens
> setup must be zonally calibrated (against the
> perfect sphere)
> and that calibration must be subtracted from the
> final zonal
> differences.
> 
> > Bob May
> > http://nav.to/bobmay
> > bobmay@nethere.com
> > NEW! http://bobmay.astronomy.net
> 
> 
> -- 
> 
> Rick S.
> 
> http://users.rcn.com/rflrs
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
_______________________________
Do you Yahoo!?
Win 1 of 4,000 free domain names from Yahoo! Enter now.
http://promotions.yahoo.com/goldrush
_______________________________________________
ATM mailing list http://www.atmlist.net/