Re: [APML] Coma corrector needed ?

["John C. Mirtle" <jmirtle@shaw.ca>]

Regardless, a flattener is required on anything faster than f6. Even f6 can
benefit, but the coma free field is very generous  without one. The model
made by Baader (marketed by Celestron) was great at f6, see Scott Justice's
web page for details. The Lumicon model is simple to use with a Lumicon
off-axis guider, just screw it onto the bottom. There is little change in
focal position, and almost no change in the focal length of your system. The
Paracorr multiplies you focal length by 1.15X, this probably helps reduce
apparent vignetting of the smaller lens. It also attaches to the camera's
T-ring, and sticks out like a sore baseball bat. It cannot be used with
conventional off-axis guiders, but perhaps with a guiding head. It also
alters the focal position of the camera - it seems to be geared to achieving
focus with a camera on a visual Newtonian. Guide scopes can be successfully
be used with Newtonians, but not many of them. Lumicon or Celestron get my
vote.

my 2 cents (CDN)

John Mirtle
Calgary, Ab. Canada

----- Original Message -----
From: "Tom & Lou Krajci" <krajcit@3lefties.com>
To: <astro-photo@seds.org>
Sent: Friday, February 15, 2002 5:52 PM
Subject: Re: [APML] Coma corrector needed ?


> From: "Mike & Dianna" <galaxy@ncweb.com>
>
> > I have a home built 8" F3.5 newt for AP only . When I researched coma
> > correctors, Lumicons and the TeleVue parracor, I found that the Lumicon
> has
> > a larger area of glass than the Televue....therefore less vingetting.
>
> I disagree.  The spacing for the Lumicon corrector is farther from the
film
> plane than for the Televue corrector.  For an f/4 type of light
cone...they
> both should give roughly the same 100% illuminated field...about 1 inch in
> diameter.  Because the Lumicon corrector is a bit farther (3 and a little
> inches versus 2 and a little inches for TeleVue)...the rate of
illumination
> fall off for the Lumicon may be a bit less severe outside the one inch
> diameter circle...but it's probably not a big difference.
>
> Also, the TeleVue will give tighter star images if you plan to do visual
> work with your scope at higher powers.  The Lumicon is corrected for photo
> work only, and will show mushier images for visual work.
>
> Tom Krajci
>
>
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