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Re: (meteorobs) Moon nights

Cathy Hall wrote:

> I'm also interested in filters.  I was fascinated by the article on filters for
> observing comets - in the book 'Observing and Photographing the Solar System' by
> Dobbins, Parker, and Capen.  They talk about, among other things, use of filters
> to reduce the effect of scattered light.

I'll start my answers a little early.  I have little experience in
photographing meteors, but I intend to try it from downtown Toronto when
I move.  That ought to be a hoot!  But I have worked on
scientific/military instruments where enhancing signal to noise was the
purpose of life.

...use of filters to reduce the effect of scattered light ...

A common misconception.  I will sound dogmatic, but trust me, the
principles are very simple.

For scattered light originating infront of the filter, the best thing is
to turn out the lights.  (I am NOT advocating the use of rifles to kill
cobra lights!)  I mean lights like your back door light, flashlights,
etc.  Most people already know this.

The next measure is use a very well designed and implemented baffle to
protect the filter from stray light.  Typically a well protected lens
will show vignetting, but that can be removed by an operation called
flat fielding during processing.

Finally, you then need to measure your sky background, which will likely
vary with moon phase, angular separation from the moon, dusk, dawn,
pointing direction, and lastly spectrally.

Only the spectral variations can be handled with a filter.  Given the
meteor spectra, brightness, and location all of which vary and usually
unpredictably, your measured knowledge of the sky background (where else
did you think you were going to get it :-)), and your film/lens
performance, just maybe you can select a filter that rejects spectral
areas were the effective sky response and the effective meteor emmisions
are in an unfavourable ratio.

With any luck, you'll have enough signal left to photograph a meteor!

So when you try to answer your other questions, you must view them in
the context of your sky background.

For example, I may find my self in Toronto where I can only look NW, and
the light pollution is so bad that who cares where the moon is or what
the phase is!  Conversely, in southern Sask, the skies are so incredibly
dark you can shoot with practically any setup and get better results
than anywhere in southern Ontario.


Scattered light behind the filter is the number one reason for the good
lens and internal baffling found in quality cameras.  Even inexpensive
35mm SLR's are pretty good though.

> My questions are:
> 
> 1.  Do other groups out there use special films and filter combinations for
> meteors to combat moonlight?  If so, what are they?

Shoot 90 degrees of azimuth away from the moon where the sky is
darkest.  As mentioned, you have to know your sky.  (ie. is moonlight
significant?)

 
> 2.  With regard to the comments on high-velocity meteors, do they all have
> strong spectral emissions in the near-infrared?  In other words, what showers
> might benefit from near-infrared film, and what showers from more normal
> response film?  And how about low-velocity meteors - what showers would seem to
> benefit from what types of film... ?


Simplistically, you can estimate the relative emissions from a meteor as
a black body heated to some temperature.  But when you start taking
about emmision lines, that goes out the window.  You want to get an
answer from some one knowledgeable on meteor spectra, so you might try
looking for books and titles by Peter Millman.  Peter has pasted on, but
he was always very interested and active in meteor spectral work as a
member of NRC.  There may also be IMO handbooks on this.  There is one
on photography, well worth the money!

I also believe that one of us from Vancouver Island does meteor spectra,
maybe he can help us.

For what it is worth, you can buy filters which will isolate single
spectral lines to give you the ultimate in signal to noise, but the
signal may be too weak to record.  Besides even the elcheapo filters of
this kind start about $5,000 US.

 
> 3.  Has anyone ever looked at the possible use of filters for visual work, say,
> for moonlit nights?  Some sports utilize tinted glasses, depending on sky
> conditions.  For example, trap and skeet shooters, viewing fast-moving painted
> clay disks, both in daylight and at night under lights, wear different tinted
> glasses to best increase the contrast of these fast-moving objects against the
> sky.  Has anyone ever tried, say, lightly tinted yellow glasses to counteract a
> bright moonlit sky for visual meteor observing?

As a former target shooter, I can assure you that the selection of
yellow filters is to correct for inherent deficiencies in the eye. 
Target shooters like high resolution.  That accuity is found in the
yellow to yellow-green spectrum.  Aditionally, the eye is poorly
compensated for focus in the blue and violet spectrum.  
Wearing yellow filters ensures your eyes can focus tightly with out the
blurr caused by the violet and blue spectrum and with some red rejection
as well.  

For driving, for safety reasons, the red should be left in, thus
"blublockers" are preferrable here.  As for different tints, well the
customer is always right.  If they work, they work.  Target shooters are
usually very picky about extracting any percieved or real advantage they
can find.

Now camera lens do have similar deficiencies, and the moment you
consider broadband to include infrared, most lens are going to defocus a
some points in the spectrum unless it is specially designed for the
job.  Note that minus violet filters are widely used in all areas of
photography to the point where they are mounted on the camera full time
by some.

Camera film is inherently highly sensitive to blue and violet.  You have
to get inventive to entend its response to cover the visible spectrum. 
As I mentioned in an earlier post, the films reciprocity characteristics
are a dominant factor, potentially far more than spectral sensitivity.

We have few films to choose from for high quality work because what we
want as a reciprocity characteristic is opposed to the desires of 99 and
44/100% of the photographic users.  If I wasn't clear, please just ask. 
If you are worried about suppressing sky background in meteor
photography, the correct film reciprocity characteristic is far, far
more important than filters.

Hope this provides some insight into a potential can of worms.  :-)

Regards,

--
John Ohrt,  Regina, SK, Canada
johrt@ibmdot net
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