Re: [APML] Magnitude limit and exposure

["Stuart Heggie" <stuart.j.heggie@sympatico.ca>]

Nicola - I have the same scope! It is VERY slow for prime focus. You have shot the moon already I
trust? You will want a very bright object to shoot that with that scope.

I shoot 40 minutes at f/4 with 400 speed film with my newtonian and with my camera lenses. This
conversion to f/10 is not truly linear but for the exercise, if you shot 800 speed film at f/10 you
would need:

40min x 0.5 x (10/4)**2 = 125 minutes to hit the sky fog limit in a dark location. 20 minutes then
would get you about 1/6th the exposure you'd ideally want. This is going to be hard work for you I
fear.

Stuart
http://www3.sympatico.ca/stuart.j.heggie/Stuart.J.Heggie/
Flesherton, Ontario, Canada

----- Original Message -----
From: <centaurus_a@virgilio.it>
To: <astro-photo@seds.org>

Dear All,

I'd like to try for the very first time to shoot prime focus.
Because I'm manual guiding I guess I'll try with a very fast film (any suggestion)
and a short exposure (not more than 20 minutes). Is there a way to calculate
which magnitude I can reach depending on the scope f/ratio, film, reciprocity
ratio and time? I already know I won't reach the sky fog limit, but I'd
like to give a try to see if I can manage at least focusing and guiding
to obtain a reasonable result on some object (any suggestion on a bright
target?)
My scope is a Vixen SP 102M (acromatic) f/10

regards,
Nicola
http://astrofoto.laza.it

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