What is an "astrograph" ?
Tony
other telescopes or just a fast visual tube ? Hope this isn't too stupid a
question, but I'm seeing the term used a lot and not sure why...
Thanks,
Tony
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Michael A. Covington
> other telescopes or just a fast visual tube ? Hope this isn't too stupid
a
> question, but I'm seeing the term used a lot and not sure why...
It's a telescope specially designed for astrophotography -- not all that
common. The main desiderata would be: ability to focus with a camera body
attached in place of the eyepiece (some Newtonians won't); uniform coverage
of a 35mm film frame, or in many cases something larger; and color
correction suitable for photography (which nowadays mainly means you can't
get away with as much error in the deep violet as you can in a visual
instrument).
--
Clear skies,
Michael A. Covington / AI Center / The University of Georgia
Author, ASTROPHOTOGRAPHY FOR THE AMATEUR
http://www.CovingtonInnovations.com/astro <><
Phillip
Tony <NOS...@crosswinds.net> wrote in message
news:8l0bkv$ffo$1...@nntp9.atl.mindspring.net...
> Technically, just what is an "astrograph" scope? How is it different from
> other telescopes or just a fast visual tube ? Hope this isn't too stupid
a
> question, but I'm seeing the term used a lot and not sure why...
>
Hi Tony.
An astrograph is a 'scope designed specifically for astrophotography.
Depending on the design, some astrographs are not very suitable for visual
observing. For instance, in order to make a newtonian reflector better for
photography, you generally have to use a larger than normal secondary mirror
which increases the central obstruction. It affects contrast for visual
use, but will have a larger fully illuminated field to fill the frame on
film. There is usually some sort of trade-off involved.
Phillip
Breen
"Phillip" <jph...@charter.net> wrote in message
news:sn7fi2s...@corp.supernews.com...
fros...@my-deja.com
especially and, usually, expensively designed for taking astro-pictures.
The idea is to provide even illumination across a film-frame-size flat
field at fast f/ratio, without color shift and other optical problems.
This is difficult and expensive to do. None of these things is as
important for visual observing as for photo use. Film is very
unforgiving. The term "astro-graph" implies that the instrument is
designed for photographic purpose as much or more than for visual use.
In article <8l0bkv$ffo$1...@nntp9.atl.mindspring.net>,
"Tony" <NOS...@crosswinds.net> wrote:
> Technically, just what is an "astrograph" scope? How is it different
from
> other telescopes or just a fast visual tube ? Hope this isn't too
stupid a
> question, but I'm seeing the term used a lot and not sure why...
>
> Thanks,
>
> Tony
>
> --
>
> When replying, please replace NOSPAM with RANGER in E-Mail address.
>
>
Sent via Deja.com http://www.deja.com/
Before you buy.
Tony
Tony
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<fros...@my-deja.com> wrote in message news:8l221t$vcm$1...@nnrp1.deja.com...
Paul Schlyter
Michael A. Covington <See http://www.CovingtonInnovations.com for address> wrote:
>> Technically, just what is an "astrograph" scope? How is it different from
>> other telescopes or just a fast visual tube ? Hope this isn't too stupid
>> a question, but I'm seeing the term used a lot and not sure why...
>
> common. The main desiderata would be: ability to focus with a camera body
> attached in place of the eyepiece (some Newtonians won't); uniform coverage
> of a 35mm film frame, or in many cases something larger; and color
> correction suitable for photography (which nowadays mainly means you can't
> get away with as much error in the deep violet as you can in a visual
> instrument).
.... or at least could, when astrographs first were constructed, and
when photographic plates were sensitive to only blue light.
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It is an oversimplification to state that an astrograph is a
"telescope specially designed for astrophotography". A Schmidt
telescope, for instance, is a telescope specially designed for
astrophotography -- but a Schmidt telescope is not an astrograph.
An astrograph is a REFRACTOR with a short focal length and a wide
field of view, intended for photgraphic use. Often the objective of
an astrograph has more than two lenses. A Schmidt telescope or a
Newtonian aren't astrographs, even if they are equipped for
photography, because they aren't refractors. And the Newtonian
doesn't even have a wide useful field of view.
The Observatory of Stockholm has an old 40 cm astrograph with a
4-lens objective and a usable field of view of several degrees. You
should avoid using red-sensitive plates on that instrument. In a
building nearby there is a double refractor, with a 50 cm visual
refractor and a 60 cm photographic refractor (both lenses are
acromats), both having a focal length of 8 meters. If you observe
visually with the photographic refractor, you'll see a very obvious
red "aura" around any bright object. The 60 cm photographic
refractor isn't an astrograph though, because its field of view is
too narrow.
--
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Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se or paul.schlyter at ausys dot se
WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch