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Re: (IAAC) FWD: What should I buy? (rich field scopes)



At 09:46 PM 6/16/98 -0400, Sue French wrote:
>From: Jeff Medkeff <medkeff@c2i2.com>
>OTOH, contrast-reducing effects such as scatter (caused by air-glass
>surfaces and increased by the use of coatings) and internal reflections
>(which, though decreased because of coatings, are still quite obvious with
>the Naglers), will be far more deleterious - and obviously so to the
>observer.
>    It's not obvious to me.  In fact, I find the veiws in Naglers generally
>sharper, brighter, and clearer on both planetary and deep sky targets.
If internal reflections in the Naglers are not obvious to you, you aren't
paying attention. Point your Nagler-equipped scope at any 1st magnitude or
brighter star and check them out. They arrange themselves along a line
through the center of the field in a straight line, radiating from the
position of the star; with a Nagler 20mm there are four quite obvious
reflections and several secondary ones that I did not count.
The scatter numbers for the Naglers are well known. With eight air-to-glass
surfaces, coated with the same coatings in use on a UO ortho (four air to
glass surfaces), the scatter is roughly half again as much. Scatter
severely reduces an observers sensitivity to small color contrasts; this is
not likely to be noticeable during scotopic observing exercises, but is
quite apparent to critical observers of the planets. Al Nagler is one of
these; he had much to say about this at Riverside, at the eyepiece panel
that we were both a part of.
Since the light transmission of Naglers is a few percent less than the
Orthos, and the design is not as well corrected as orthos across the field,
it may seem that the Nagler is "sharper, brighter, and clearer" than a good
ortho - especially at low power on a fast scope, where other aberrations
will overwhelm the correction of the ortho - but in reality the perception
is nothing but fantasy.
--
Jeff Medkeff          | Acting Assistant Coordinator
Rockland Observatory  | Association of Lunar and Planetary
Sierra Vista, Arizona | Observers, Solar Section
On the web at http://shutter.vet.ohio-state.edu/

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