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Re: (meteorobs) Normal lens



You wrote:

>I don't think your definition of what constitutes a normal lens is the same
>as what it's normally referred to.  A normal lens for a 35mm camera is
>accepted at 50mm.  A 4"X5"'s normal lens is accepted at 6". And for a 2

Sorry, but you are miss reading the definitions of the different lens
catagories.

ie: Catagory "B" :  Normal lenses, with F-ratios of F-1 to F-3.5 and focal
lengths of 50mm, (the normal 35mm lens) to 200mm, (the common aero camera lens)
Both standard 35mm cameras and aero cameras fall into this catagory.

You wrote:

>You may be able to pick up Aero camera's or lenses, but it's not that easy.
>I've looked already in all of San Diego County...haven't found one yet . The

To get an aero lens, try placing a "Wanted to Buy" ad on the WWW on
ASTROMART or the ATM net.  I think the correct address for posting an ad to
Astromart is: astromart-errors@listsdot best.com.  That is how I got my F-2.5 -
12 inch focal length Kodak Aero Ektar for $125 US.  There is another outfit
called surplus shack but I don't know the address off hand.  You can place
an ad on the astro news groups also.  I'm sure you will come up with some
aero lenses that way.
 

You wrote:

>Out of curiosity, how long are your exposures when you are trying to make
>spectrographs?  Also, do you know of a source for large enough scientific
>grade prisms that have angles at about 30 degrees?  You've already indicated
 
I don't know where you can buy a standard 30 degree prism.  When I got mine
I bought a large 60 degree surplus prism by placing an ad in Sky &
Telescope.  Had it cut into two, 30 degree prisms and re polished and re
figured flat.  That cost me about $200 to have done. (1970).  I don't know
what it would cost today.  Gratings are a better choice, but new ones are
expensive.  I've been looking for used grating for quite some time, but so
far no luck.  The one I have is a Bausch & Lomb 600 groove per mm grating
with a rulled area of 52X52mm.  I got that back in the 1970's by placing an
ad in Sky & Tel. for $170 US.  A new one of that size today runs about $900
US.  If you find a source of inexpensive ones let me know!!!  You could try
cheap film replicas as an experiment but they won't be very efficient.  It's
har enough to get a spectrum as it is.  I've only secured a dozen since
1972.  The World List only included 318 spectra in 1958.  That is very much
higher now.

You wrote:  What are exposure times with spectrographs.

That all depends on how dark your observation site is.  I operate from my
backyard and can get away with 20 minute or so exposures before sky fog
becomes a problem.  At a good dark sky site you can go much longer.  If you
use an occultating shutter, ie. a rotating disk infront of the camera lens
that interups the light entering the camera, 1/2 on and 1/2 off you can
double your exposure times.  Getting a spectrum is much more difficult than
taking a direct picture of a meteor.  The meteor must be oriented parallel
to the thin edge of the prism or parallel to the grating grooves.  The
spectrum gets poorer as this angle changes.  If the meteor is at right angle
to the grooves of the grating or the thin edge of the prism you will not get
a spectrum as the light will be dispersed over itself.

Hope this helps you: Ed

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