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RE: [OM] Taking pictures of Mars

Subject: RE: [OM] Taking pictures of Mars
From: "Ralf Loi" <ralf_loi@xxxxxxxxxx>
Date: Tue, 26 Jun 2001 18:04:52 +0200

Olaf,
here are quick answers to your questions.

>How did you manage to avoid the star trails then for such long exposures?
...
>> Since I have a telescope but not the motor, I take photos moving the
telescope manually,
>> so every minute gained is good.

>Does that work well? This doesn't sound straightforward to me.
Nonetheless,
>I guess it takes a really long exposure to capture stars on the frame that
>are not very bright.

My equipment for astro photos is a telescope on an equatorial mount and the
camera with lens mounted in parallel.
If placed right, i.e. with good polar alignment, the equatorial mount allow
to follow the "motion" of the sky simply rotating one gear only: this is
what the motor do (if not placed right... this is another problem!). So I'm
a motor, in this case. But I have to know how fast rotate the gear: this I
know because I watch continuosly a star with a particular eyepiece that has
a cross in it, so I can easily mantain the star (guide star) in the middle
of the field. 10/15 minutes is the maximum amount of time that I can afford
for both arm/eye fatigue. I found that with a focal of 135mm this scenario
works good, I also took some photo with the 300mm, but in this case it is
very difficult.
An equipment not too expensive is a "blind" motor with camera attachment
that you can put on a tripod. This combo does not require user intervention
and if right placed (again...) it allows you to use a 100/200 mm tele and
use up to 15/20 minutes exposure. Only to mention one, Vixen make one.
The great advantage of using a long exposure is that you can collect more
photons, thus seeing on the film also very faint stars: with the 135/2.8 I
was able to record stars that are only a few brighter than ones that I can
see with a telescope with an aperture of 100 mm (the 135/2.8 has an
aperture of less than 50 mm).

>Excellent, that's good news! Perhaps a stupid question, but does pushing
the
>film two stops affect the reciprocity failure characteristics of the film?

As far I know, pushing the film does not affect the reciprocity defect.
There are methods used for minimizing the defect but there are not easy
(very low temperature, or "washing" the film before exposure in a nitrogen
bath...).

Ralf Loi



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