[APML] Re: Magnitude limit and exposure

["Renzo Del Rosso" <scrivimi@renzodelrosso.com>]

centaurus_a@virgilio.it Scrive: 

> Dear All, 
> 
> I'd like to try for the very first time to shoot prime focus.
Good luck and clear skies.
> Because I'm manual guiding I guess I'll try with a very fast film (any suggestion)

I tried for my first shoots with 800/1000 ISO films. The grain was visible 
but (I used a f 6,7 newton) the photos were reasonably valids.
> and a short exposure (not more than 20 minutes).
It's very hard for the first time guiding for more than 20 minutes. Then 
it's easy to reach 40 minutes or more. 

> Is there a way to calculate
> which magnitude I can reach depending on the scope f/ratio, film, reciprocity
> ratio and time? 
I think it's very hard to calculate the limit because it depends fron the 
trasparency and the darkness of the sky. 

> 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?)

With your scope I'ld try some globular cluster, at the beginning.
The F ratio is very close and you reach the sky fog limit in 20 minutes only 
in the backyard of the city.
In very dark skies (in Italy you can find them in very little sites), You 
must expose for 2 hours or more to reach the sky fog limit with a 200 ISO 
film. 

> My scope is a Vixen SP 102M (acromatic) f/10
I suggest you a focal reducer at F6,3. 

Clear skies
Renzo 

www.renzodelrosso.com 

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