Newsgroups: sci.astro

> in hpscit:sci.astro / dave@arcturus.as.arizona.edu (David A. Harvey) 
  writes:
 My article deleted...

>A 70mm refractor "blowing a Questar out of the water"???!!!!?   


	Looks like I found a hot button here.

	Yep, it happens, sorry to disturb your comfort about Questar being
"The Best" optics but reality sometimes intrudes no matter what advertizing
or folklore leads one to believe. Gad it is hard to have ones preconceived
notions challenged isn't it? I've been an active observer for some 32 years
and have become much less impressed but what should be true as opposed to
what I have observed.  

	A Fluorite Apochromat is a VERY different critter than a Fraunhoffer 
doublet. They are also different from the non-ED Christen triplets.


>I don't think so Jack.  Unless you got a really bad Questar - the Questar's
>has an aperture advantage of 27% over your 70mm job (not counting the 
>central obstruction) and just and that alone says it will have better 
>resolution than the 70mm.

	Can you say unobstructed?
	    Central obstructions are not a desirable optical attribute,
	    reduced contrast results.

	Can you say scattered light from mirror coatings?
	    Evaporated metel mirror coatings look like sandpaper at high 
	    magnification, they scatter light throught the field further 
	    reducing contrast.

	Ever hear of chromatic aberation? 
	    Do you think a thick singlet meniscus corrector adds color?
	    You bet it does.

	The Gregory-Maksutov is a compromise design at best.
	    The Questar secondary is an aluminized spot on the spherical 
	    miniscus corrector. Good for production, not opticaly optimum.
		Ask your "Master Opticians", f/15 helps here though.

	Think of a Questar as a folded 3.5" f/15 refractor with one thick non-
	color corrected element, two light scattering mirrors, and a spherical
 	magnifying secondary. Add another mirror or two more air-glass inter-
	faces for a star diagonal or prism. You get both chromatic and reduced
	contrast problems in one package.

	Aperture is not the whole story.

	Some of us like a wider field, f/15 does not do it. The little Apo'
	will do both low and high magnification nicely. 

	How long are the exposure times for astrophotography at f/15? The 
	Fluorite Apo has a field flattener-focal reducer that takes it to f/5.


>Have you ever looked through a Questar?  As an owner of a 3.5" Questar and a 
>4" Astrophysics refractor - both of which have been tested by Master Opticians -
>and both test out better than a 8th wave at the wavefront - the images are
>comparable (resolution wise).  The 4" has brighter images and less light 
>being pumped into the secondary difraction ring but they are nearly the
>same resolution wise.

	Many times, I have used several Questars, both 3.5" and 7". I've also 
used a few OTI Quantums, two 4" and one  6" model, and other Maksutov systems
up to 11 inches aperture. I own a 5" Maksutov-Cassegrain.

The Questar owner didn't like it either that night.  We could see a darker ring 
around the polar cap and the fact that the cap was split in several places 
was not seen in the Questar.

>Your 70mm can not be "better" than a good Questar.  I'll put my 3.5" Questar 
>up against your 70mm Vixen any day - bets are my Questar will "blow your
>refractor out of the water."

	I would not recommemd you offering to trade "pinks" on such a 
comparison with an apochromatic refractor, you might not like the results. 
You may want to do some research into the optical qualities of a Gregory-
Maksutov (Questar) and some of the Fluorite Apochromatic refractive systems. 

	Use whatever telescope makes you smile, but don't claim something
can't be true just because you may not have seen it or someone told you it
could not be. 

	Oh yeah, you may want to make the effort to use a small Fluorite
Apo sometime, you may well be impressed.



	Jack Zeiders