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Re: (meteorobs) Normal lens

>In a message dated 97-03-21 17:45:58 EST, you write:
>
>George Zay wrote:
>--------------
>This is what I've discovered. What I've done is this: I have an old press
>camera. The lens plate..dot called a "lens board" is removable. It's nothing but
>a flat piece of aluminum that's about 4" square.
 In the center a hole is cut

I have a 4X5 Crown Graphic with an F-2.5 - 178mm fl Kodak Aero Ektar mounted
on it's lens board as you describe.  It works, but there is a problem.  The
aero lens is heavy, and must be supported by a post or it would sag causing
out of focus images in part of the field.  It would be better to mount the
lens on a solid plywood box with a film holder at the other end.  I have
done this with an f-2.9 - 200mm fl lens and it works great.

>the same with the 4X5 camera. I'm without a shutter mechanism, but I get
>around this by using the sliding cover to the 120 roll film holder for a
>substitute.

I use a metal flap that swings over the lens as a shutter.  This will not
work if you are using a rotating or occultating shutter.  You could move
this flap to the back of the lens or place it just infront of the film.


 I pull it out manually...when it's dark and when the exposure is
>done...I simply push it back in.

This is a good way to do it too.

 In front of the lens, I have a rather large
>glass prism mounted in a small wooden box that screws into the front threads
>of the lens. I haven't used it much...yet...but would like to. I would like
>to find a slightly larger lens...preferably designed for a 4X5 camera.

An aero lens has a large diameter and requires a large prism.  If your prism
is  small you might consider a press camera lens rather than an aero lens.
An F-4.5 press lens of 135mm to 150mm focal length will work fine and these
might be easier to find in a used camera store.


 

>to advance the film every 20 minutes...especially when I'm already operating
>a guided camera and advancing the film every 30 minutes or less.

For meteor spectrosopy one does not generally guide the camera to follow the
stars.  The camera is mounted on a stationary mount and the stars are left
to trail.  There is less chance of screwing up a meteor spectrum because of
a guiding error.  Also, faint stars will produce faint spectra, thus less
clutter on your plates.


 You'd have
>to understand what is involved when I advance my camera's film. I once tried
>the scheme where I got a 20 foot long air actuated cable release with the
>squeeze bulb held in my hand. I hold it while meteor observing from the top
>of my observatory. 

You will have to be very quick to open a shutter in time.  Professional
systems have used photoelectric detectors to do this.  There is a problem
using your eyes to do this.  The brightness of a meteor can be very
deceptive.  I've seen meteors cross my cameras field that I was sure was
photographed, but nothing was recorded on the film.  Also, I've recorded
meteors that seemed too faint to be recorded.  The eye is most sensitive to
the green region of the spectrum and meteor radiation is generally stronger
in the blue region where the eye is not as sensitive.  This is especially
true of high velocity meteors such as the Perseids as the Ca+ lines are
strong in high velocity meteors.  Photoelectric detectors can be a problem
also.  Gale A. Harvey from NASA/LRC found that lightning on the horizon
would set off his camera shutters.

-----------------------------------------------------------------
Edward Majden                         epmajden@mars.ark.com
1491 Burgess Road                     Meteor Spectroscopy
Courtenay, B.C.                       CCD's - P.E.P.
CANADA  V9N-5R8                       Amateur Astronomy