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Subject: [M]: RE: Variable Tele-Extender
From: Richmann, Michael
Reply To: mapug@shore.net
Date: Tue Dec 09 08:39:42 1997

Take a look at Michael Covington's "Astrophotography for the Amateur".
All of the requisite formulae are there and should answer your
questions.
As for "true magnification", convention is that the length of the
diagonal
of the film negative constitutes the focal length of the so-called
"normal"
focal length lens for a given format. In the case of the 35 mm
negative,
this is about 43 mm. By that convention, a 23,400 mm lens would be
about 540 power but you're sacrificing light gathering ability and image
quality to get that magnification from that particular eyepiece. The
numbers
you quoted by themselves seem reasonable. I've used a 26 mm eyepiece
to achieve an 8000 mm focal length at f/40 while trying to shoot Saturn.
Didn't turn out too badly except that the B&W Kodak T-Max emulsion
was quite grainy even when developed at the nominal ISO 1600...

Mike

> -----Original Message-----
> From: robert rossland [SMTP:robert.rossland@eu.e-universe.com]
> Sent: Monday, December 08, 1997 7:54 PM
> To: mapug@shore.net
> Subject: [M]: Variable Tele-Extender
>
> I have a question for the group. I just received my Meade Variable
> Tele-Extender and I was looking for information on what the
> magnification would be with a specific eyepiece and a 35mm camera.
> I found the following information at the Astronomics web site.
> http://www.astronomics.com/Content/accessories/photo/tele-x.htm
>
> "For example, if you use a 12mm eyepiece on a 2000mm focal length f/10
> Meade 2080 or Celestron C8,
> your magnification factor will be 11.7 (152mm projection distance,
> minus
> 12mm eyepiece focal length,
> then divided by 12mm). Your projection focal length will be 23,400mm
> (2000mm multiplied by 11.7); your
> photographic magnification will be 570x (23,400 divided by 50); and
> your
> projection focal ratio would be
> f/10 multiplied by 11.7 - or f/117!"
>
> My question is: is this photographic magnification of 570x true
> magnification, or is it only apparent magnification on the 35mm
> negative? Since a 12mm eyepiece with a 2000mm focal length will only
> give you 167x for visual observations.
> I will be using a 7" mak (2670mm F.L.) with a 26mm eyepiece. The
> variable tele-extender has a range of 115mm-152mm. That would give me
> 183x-259x, can this be correct !!!
>
> Also i'd like to hear from anyone with experience photographing the
> moon
> and planets with a 7" mak.I'll be using fuji 800 super G plus, so any
> help with exposures times would be very helpful.
>
> Thanks,
> Robert