"Alan W. Craft" wrote in message
...

There's one other thing about the 5" SCT that you should
know. Notice that in the advertisements the telescope's corrector
plate is rarely, if ever, visible. It's always pointing away, and with
very
good reason, as the secondary obstruction is, comparatively...HUGE!


If you have obstruction-phobia, this will alarm you. It's about 1.8 inches
(vs. 5 inches aperture) -- that is, about 36% of the diameter, or 13% of the
area.

But why should this be objectionable?

Light loss? Well, the C5 still has the same effective aperture as an
unobstructed 4.6-inch. The 13% light loss costs you 0.15 of a magnitude in
light grasp.

Diffraction? Well, that works both ways. It slightly reduces contrast on
certain types of planetary detail (so that the C5 is comparable to perhaps a
3.5-inch apo refractor), but it improves the view of double stars. My C5 is
very good with double stars.

"Light scatter" or "Loss of contrast"? None, apart from what I just
mentioned. Those are widespread misconceptions about the effect of central
obstructions.

In particular, people sometimes say, "A 36% (diameter) obstruction costs you
36% of the contrast." It does nothing of the sort. The obstruction
*removes* some of the light; it doesn't scatter it randomly over the image!

See my computerized telescope book for more about this, including computer
simulations.


P.S. By "C5" I mean Celestron 5, the same optics as the NexStar 5i.

--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope