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Re: (meteorobs) Normal lens
In a message dated 97-03-21 17:45:58 EST, you write:
<< You could use a 35mm Camera with a 50 mm fl lens for spectroscopy but I
would not recommend it. The normal meteor spectrograph covers a region from
350.0nm to 650.0 nm. This is 300.0 nm long or 3000A. With a 600g/mm
grating on a 50mm fl lens you will have a dispersion of around 323A/mm
approx. Lets say, 300A/mm. This will give you a spectrum about 10mm long.
The H & K lines of Ca11 will barely be resolved. They are at 3938A (H) and
3934A (K) lines, only 4A apart. Other compound lines will just be blends as
they will overlap each other. You will not get much info from such a
spectrum. Using a telephoto lens will increase your dispersion but your
field of view will be so small, that you would be very lucky to record a
spectrum. It's difficult enough as it is. The minimum format I would use is
2 1/4 square with a 75mm fl lens. Even that is not great. 4X5 format with
a 150mm fl lens would be much better. High resolution spectra are what is
needed. Ed
>>
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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
and the lens and shutter mechanism is affixed. I simply bought a flat piece
of aluminum and cut down to the appropriate size to fit in the same place as
the lens board was. I drilled and filed an appropriate hole to accomodate a
135mm f/2.8 lens. I bought a used 135mm lens that was designed for a 35mm
camera....the brand didn't matter because I took it apart so that all I had
was the basic lens. I glued that sucker in the hole. Since there is now no
shutter, the image is circular and a little larger than when used with a 35mm
camera that cuts the image to a rectangular shape. Focusing is still done
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 pull it out manually...when it's dark and when the exposure is
done...I simply push it back in. 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. This
is where the Aero camera lens is desired...hopefully reasonably priced that
will meet my wife's approval.
With my 35mm camera, I sometimes screw in my prism box onto my 200 mm lens.
It's a little frustrating though because of the seldom bright enough meteor
seen...yet alone one crossing the camera's field of view. it's also a hassle
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. 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. The idea was to squeeze the bulb as soon as I see a bright
meteor...anywhere. I find this less for the brain to sort out in such a short
time. I've only tried this once. I think it will work, but the knowledge of
covering such a small area of sky tends to damper the enthusiasm of trying
for meteor spectra. Another reason for the 20 foot air actuated cable release
was to lessen the amount of star spectra clutter that was appearing on the
negatives during a 20 minute continuous exposure. I was afraid that if I
caught a meteor near the 20 minutes of the exposure, it would be ruined. I
suspect that as soon as I find a larger lens that could be used onto the 4X5
format, I will give it another go.
George Zay