Re: [Fwd: ] ATMJ article on grinding a 6" by Russell and Germann

[spooner@page.az.net (Michael Spooner)]

Mel Bartels wrote:
> 
> Richard Baier wrote:
> 
> An article in issue #10 of the ATM Journal has caught my eye. Ouch! ;)
> 
> Dr. Mark D. Russell and Val German claim that that by using a precurved
> 6 inch precurved mirror blank, polishing pads and a "modified star test"
> the amateur can make a 5.75" mirror with apochromat like quality.
> 
> This is a very bold claim.  I remember eight or so years ago, at the Warren Rupp
> observatory in Mansfield, Ohio I viewed both Saturn and Mars through a
> 4 inch SP-C102F refractor. Saturn's rings were beautifully subdivided
> and Mars showed and impressive wealth of detail with a prominent polar cap.
> If the average person can make a mirror that will produce the similar views,
> the world is suddenly a much more interesting place!
> 
> Does anyone know what the authors mean by a "modified star test"?
> They did not elaborate...
> 
> They also make mention of displaying to the public at Riverside '95 and
> Astrofest '95 two eight inch reflectors to see if the general public
> could distinguish differing levels of optical quality.  Did anyone see
> these instruments and how did they perform?  What were their focal ratios?
> 
> On a different note, I have received a brochure from Obsession Telescopes.
> They claim that the Endurobrite coatings on their secondary mirrors,
> and I quote, "blows away ordinary enhanced coatings!".
> Is this claim legit or enhanced BS?
> 
> Thanks,
> Richard Baier

I took a look thru the two scopes that were set up at RTMC (tubes were 
tied together). The star test showed the differences in the mirrors 
readily. With the seeing at RTMC and possible tube currents, it would be 
difficult to tell if the scopes were the limiting factor. All I recall 
is neither scope had a steady airy disk (but I viewed fairly late 
with rougher seeing) and neither had star test extrafocal rings 
indicative of superb optics. I think their discription of the optical 
assemblies in the article is pretty good (though I might use average and 
above average). The seeing really needs to be good to see a difference 
in higher grade optics IMHO. If the mirrors were 8" then I believe the 
scopes were f/6 designs from memory of the lengths. An interesting test 
but I would have liked to see one more ultra high quality scope (I 
realize this was not needed for what they were exploring). 

I just set up a scope with an Endurobright secondary. It is instructive 
to hold it close to a flouresent light and view thru the back. I had the 
primary (11") coated with regular protected aluminum and I'm more 
interested in the potential difference between different coatings of 
supposedly similiar types. This primary definitely looks "dark" to me 
compared to my other mirrors for some reason. The Endurobright seems 
okay, but can't tell by my eye if it is much superior. The 11" is barely 
1/4 wave by the star test but Mars showed nice detail at 400x without 
too much glare. I'm hoping the wear and tear factor of the coating will 
be a good point. Also when it comes time to recoat flats in my folded 
refractor, I may go this route as the far blue/violet reflectance drops 
off rapidly and maybe this will help tune the color of the big lens <g>. 
With this scope the scatter may come into effect (if any). 

Mike Spooner