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Re: [APML] Tele Vue & Field Flatteners for Refractors?
Hi Jim,
As Chris mentioned, the "Ranger" 1.1x version induces chromatic aberration. The Pronto/TV85 .8x flattener is APO and so produces sharper stars, but does have somewhat more light drop-off towards the corners since it's reducing the light cone. But both are restricted to a clear aperture of only about 37mm due to the I.D. restriction of their male T-thread end, which is the main source of the vignetting in each.
Much of the "blue halo" part of the chromatic aberration from the Ranger unit can be greatly reduced by use of a UV-cutoff filter like the 2" versions available from companies like Baader, Astronomik, Schuler, and others. If you use an IDAS LP filter, it also cuts off in the blue end at about 410nm, so it acts as an effective UV-cut also.
Additionally, if you have RegiStar, you can realign the individual RGB channels to further reduce the lateral component of the chromatic aberration of the Ranger flattener:
http://home.comcast.net/~jeboud/xchrom.htm
Unfortunatly, RegiStar can have a problem with some starfields, and *at times* may not realign the channels as well as it usually does. But it's always certainly worth the effort--- I even use it to realign the RGB shift due to atmospheric dispersion in images of somewhat low-altitude subjects taken with the APO Pronto/TV85 flattener.
This image was taken with a TV85 using the Ranger flattener, _no_ filter, just RGB aligned with RegiStax:
http://home.comcast.net/~jeboud/m101wdfld.htm
A real pain about the Ranger flattener is that it's female end _isn't_ T-thread, so you'll have to also buy the 2" adapter from TeleVue that mates up to it. I also found that adapter to have the smallest O.D. of any 2" accessory I ever owned, and so it was a sloppy fit.
In the end, I found myself shooting only with the Pronto/TV85 .8x flattener. It gives better images and I can live with the vignetting.
John Boudreau
http://home.comcast.net/~jeboud/astro.htm
----- Original Message -----
From: "Jim Vineyard" <mojjv@hotmail.com>
To: <astro-photo@seds.org>
Sent: Monday, February 02, 2004 7:31 PM
Subject: [APML] Tele Vue & Field Flatteners for Refractors?
>
> Hello All,
>
> I'm trying to figure out the *real* difference between the "Ranger" and
> "Pronto & TV-85" field flatteners. Both are described on Tele Vue's page as
> follows:
>
> http://www.televue.com/engine/page.asp?ID=169
>
> I'd much prefer to use the "Ranger" version, both for price ($84) and
> because it does not reduce the focal length but actually increases it
> slightly by 1.1x. The thing is, it is listed for use on quote: "the Ranger,
> Pronto and TV-85."
>
> The "Pronto and TV-85" version costs $265 and reduces the f.l. by 0.8x
> *BUT* is listed for use on ANY brand refractor from 400mm to 600mm.
>
> I want to use a flattener on a non-Tele Vue brand scope of simular
> aperture, 600mm f. l. with a 2" focuser integrated T-thread adapter. I
> don't want vignetting, or I'd at least like to reduce it to a minimum on
> 35mm film. This'll be my first refractor since my first-ever scope some 12
> of them ago! It's a semi-apo.
>
> So can I get away with using the "Ranger" field flattener?
>
>
> Mucho - Thanks In Advance (M-TIA) for any guiding wisdom, or even an
> alternative brand which might work!
>
>
> Jim Vineyard
> www.deepskyphotos.com
>
> (I'm once again behind some 2 to 3 hundred APML postings and need to catch
> up! Plus there's that CCD list. I don't know how you guys do it, fast
> computers and speed reading?)
>
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