(meteorobs) Perseids and forest fire.

[Ed Majden]

on 8/11/04 11:01, Wayne Watson at sierra_mtnview at earthlink.net wrote:

> Hi, Ed. What sort of spectographs do you have?
> 
> We have some similar problems with forest fires here in N. Calif. Yesterday
> there 
> were 34 fires in the entire state, and one 1000 acre fire about 20 miles from
> here. 
> It seems to be out now.
> 
> Ed Majden wrote:
> 
>> I plan on setting up a couple of spectrographs for tonight's Perseid
>> max.  Not only do we have to please the weather gods but we have to contend
>> with a forest fire not all that far from here.  No property is at risk but
>> the fire has generated a lot of haze and smoke.  A giant Martin Mars water
>> bomber is on the job so I expect they will have it out by tonight. At least
>> I hope they will.
>> 
>> Ed Majden
>> Courtenay, B.C.
>> Canada
>> 
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Hi Wayne:
    I have a number of different cameras that have been converted to
spectrographs by adding a dispersive element, prism or transmission grating,
in front of the lens.  These are slitless spectrographs, the narrow trail of
the meteor acts as a slit and provides the spectrum.  One is a converted
F-24 surplus Aero camera converted for 4X5 film.  It has an 8 inch focal
length Pentac F-2.9 lens and is fitted with a ~30 degree dense flint
objective prism.  Modern spectrographs use transmission diffraction gratings
as dispersion is nearly linear, where with a prism dispersion is good at the
blue end but crowded at the red end.  I have a selection of gratings of
varying efficiency that are used on various cameras, mostly 2-1/4" square
format.  The best grating is a B&L 52X52mm 600 g/mm precision blazed replica
transmission grating. On an 80mm fl lens this provides a dispersion of
around 207 a/mm.  I also have an inexpensive thin film type Learning
Technologies holographic grating with 750 lines/mm.  I would estimate that
it is about 1 magnitude less efficient than a precision blazed replica glass
grating.  The AMS also funded an Optometric thin glass grating with 300 g/mm
that I am testing for meteor work.  I believe the largest size available is
50X50 mm in size so this limits the size of the lens you can use.  I will
use this on a 2nd generation image intensifier video spectrograph to record
faint meteor spectra.  At least that is the plan.  I bought two large
reflection gratings on eBay, one is 4 inches square and the other is
somewhat larger.  One has 600 g/mm and the other is 1200 g/mm.  A fellow
from the BAA demonstrated that you can use a large reflection grating for
meteor spectroscopy by having a camera look down at it, reflecting  the sky
into the lens.  I have not experimented with this as yet but hope to use
them to obtain high dispersion  spectra with long focal length surplus aero
lenses.  Note: I mention, surplus and used.  I operate mostly without
funding so this is how I keep costs affordable.  Used stuff seems to work as
well as new if your selective in what you buy. It is hard to find good used
gratings however and the thin film holographic grating is a compromise.

Ed