(meteorobs) Perseids

[Ed Majden]

On 10-Aug-07, at 10:19 AM, stange34 at sbcglobal.net wrote:

> Hello Ed.
>
> I have been to your website many times. Very interesting.
>
> Have you lately developed an inexpensive way to get into meteor 
> spectrographing? Or is it still a costly operation with diffraction 
> gratings or prisms etc.
>
> I would someday soon.... like to try it with a spare low light(0.1Lux) 
> cameras being triggered by aerial light movement from HandyAvi.
>
> YCSentinel
>
> YC Sentinel:
	This is a difficult question to answer.  Precision replica 
transmission gratings can be expensive for the average amateur.  A 
50X50 mm grating costs around $700 to $800 US$ last time I checked.  
Large format cameras such as 4X5 or larger are preferred as they will 
provide good dispersion with an appropriate lens.  Such cameras are 
only capable of recording spectra from -3.0 magnitude meteors and 
brighter so one can use a lot of film before your lucky enough to 
capture a spectrum.  This can get expensive.  Another problem is 
getting the spectrum measured.  A typical Perseid or Geminid spectrum 
probably won't provide much new information as there are many spectra 
secured from these showers.  A professionals time is limited so they 
are mostly interested in measuring very good high dispersion spectra.  
There are fewer spectra from minor showers so they can be of interest 
but capturing one is not easy.  A spectrum of a possible meteorite 
dropping fireball would be of great interest.  Less expensive small 
replica gratings are available from Optometrics LLC, the largest being 
50X50mm square.  They cost around $165 US$.  I am testing one of these 
and they look promising but so far I have obtained only one faint 
spectrum.  I hope to get a better one if it ever clears up around here.
	Fainter meteor spectra, (to around + 3.0 magnitude) can be obtained 
using a 2nd or 3rd generation image intensifier and recording the video 
with a camcorder to a VCR.  New intensifiers can get costly but 
sometimes surplus items show up on eBay.  Problem is getting one 
exported out of the U,S.A. if you don't happen to live there.  In some 
cases you need an export permit.  Other countries manufacture 
intensifiers, Russia, China, etc but I have not tested any of these.  I 
understand that Jiri Borovicka's group uses Russian intensifiers for 
some of their work.
	I'm in the process of trying to get a spectrum with a Canon D20 DSLR.  
This will eliminate the high cost of using film.  The main problem here 
is the small size of the ccd chip and length of exposure before noise 
becomes a factor using an un-cooled ccd device.  Such ccds also have 
poor sensitivity at the blue end of the spectrum and you have to remove 
the IR blocking filter to get into the near IR region.  Because of the 
small size of the ccd dispersion is a problem.  I discussed this with 
Dr. Jiri Borovicka and he suggested using high dispersion even if you 
only capture part of the spectrum.  Time will tell if this is worth 
doing.
	Some professionals have experimented with cooled ccd cameras like the 
SBig ST-8 cameras and have met with some success.  Longer exposures are 
possible with a cooled ccd but this size of the chip still imposes 
limitations on spectral dispersion.  This again can get expensive.
	I hope this answers some of your questions.

Ed Majden

http://members.shaw.ca/epmajden/index.htm