Re: [APML] Shooting the Sun

["Alson Wong" <alsonwong@earthlink.net>]

----- Original Message -----
From: "Frank Schwartz" <fschwartz@rcn.com>
To: <astro-photo@seds.org>
Sent: Sunday, September 02, 2001 8:02 PM
Subject: [APML] Shooting the Sun


> I used to have an 8" LX-50 with a full aperture solar skreen, so I knew
how
> to handle that, but I have recently purchased a 10" LX-200 (used), and it
> came with a Kendrick Kwik Focus.  Also, there was an 80mm Badaar solar
> filter insert for the Kwik Focus.  Now, since I'm not working with full
> aperture, do I do anything different when calculating for exposure?  For
> instance, technically, it's only an 80mm scope, but it's still a 2500mm
> focal length.  Is there anything I should be taking into consideration
here?
> Using an exposure calculator I have, I came up with between 1/500 and
1/1000
> for the shot.

Since you are shooting at the same focal length, but with a much smaller
aperture, your f-ratio is much greater.  2500mm aperture with an 80mm focal
length is an f-ratio of f/31, so your exposures will be about 10 times as
long compared to f/10.

> I have read (on Kendrick's site) that this is supposedly better than full
> aperture because you don't have the central obstruction.  Is there any
truth
> to this?  I would have thought you could get better resolution with the
full
> 10"...

Full aperture does provide better resolution.  Smaller aperture is less
affected by seeing, however.  I use one of the 82mm Kwik Focus filters in my
C-9.25, with pretty good results.  The sunspot and Mercury transit images on
my Web page were taken with it at an f-ratio of f/78 with eyepiece
projection.
--
Alson Wong
Riverside Astronomical Society
     http://www.rivastro.org/
Visit my Web page at:
     http://home.earthlink.net/~alsonwong/index.htm


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