RE: [APML] Getting Started

["Jon Kolb" <jkolb@datawest.net>]

Hi Parker,

... Snip ...

> Also, any suggestions on a med size (80-100mm) high quality 
> refractor that won't be worth more than my car? 

How much is your car worth?  You can probably find a good deal on a used
Tak FS102 or Sky90.  The sale prices on these when bought new have
undercut the used market quite a bit as I recall.  Both are versatile
for photography and visual use.  The TeleVue 102 and 85 scopes are other
possibilies.  Tak camera mounts are superior, though, since they don't
vignette nearly as much and they have a camera rotator for changing
composition without changing focus.

... would the false color 
> through an 80mm ST achromat be awful if I found a cheep one 
> to use as a guide scope and to get my feet wet with prime 
> focus? 

Yes, but if you're just guiding with it, it shouldn't matter much.

> When I do look for a guide scope, will the 400mm focal 
> length be too short? 

Depends on what you're guiding and the focal length of the guiding
eyepiece.  I don't have Covington's formula at hand, but his book or
Robert Reeves' should give you the right guidelines.  For guiding
standard camera lenses up to about 300-400mm, a 5mm guiding eyepiece
should work.  I used to guide a 530mm scope with a 500mm scope at 100x
with no problems (other than a sore neck).

> And finally, assuming a relatively dark 
> sky (mag 5.5 or so), what's the typical skyfog limit with a 
> 50mm lens shooting at f/2.4 on E200? I'm just curious about 
> that one. I have only had the opportunity to shoot under 
> fairly bright skies (3.5-4 if you really try hard).

At that speed, you can afford to bracket and find out for sure what
works.  I'd start at 10 minutes and go up in 5 minute increments to
maybe 30 minutes max, but I'll bet the 30 minute shot would be pretty
fogged.  Still, in under two hours you'd have the bracketing done and
could decide for yourself the best exposure time.  You may find that you
don't care for the shape of the stars in the corners of the frame
resulting from coma, so shoot a few at f/4 and f/5.6 while you're at it.
Under dark skies, I've had good luck with E200 for 30 minutes at f/4.
Of course, you can also cut the exposures down about half, and have the
lab push the film one stop during development.

Sincerely,
Jon Kolb
Adventures in Astrophotography
http://home.datawest.net/jkolb/
jkolb@datawest.net





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