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(meteorobs) Re: predictions for Perseids 2001?

Mike,

Thanks for taking the time to thoroughly respond to my message. I guess
my being spoiled by transparent western skies is showing. I still don't
agree 100 percent that one should look straight up for any shower unless
the lower portion of the sky is grossly hazy or light polluted. But
that's OK as differing views offer the readers some insight into this
interesting aspect of observing. I readily concede that I am outgunned
by your mathematic prowess yet I am not totally unarmed as I have the
experience of 35 years of meteor observing behind me. 

I never meant for observers to look so low in the sky that the lower
portion of their field of view is wasted by viewing the ground.
Personally I want the bottom of my field of view to be just above the
horizon I am facing so I usually look at an elevation of 60-70 degrees
depending on the terrain on the horizon I am facing. To achieve the same
parameter a camera would probably be tilted 30-40 degrees high depending
on the lens.

I find that the atmospheric extinction at my observing sites is quite
low. Canopus rises only 5 degrees high here yet usually equals Rigel in
brightness. The stated extinction at 5 degrees elevation is 2-3
magnitudes (off the top of my head) yet the difference in magnitude
between these two stars is only 0.75 magnitudes. 

I guess the bottom line is that it all depends where you are located and
the current local atmospheric transparency as to how high one should
look and photograph in the sky. 

Clear and Transparent Skies!

Robert Lunsford



Mike Linnolt wrote:
> 
> On 6 Aug 2001, at 14:31, Robert Lunsford wrote:
> 
> > below your field of view. By aiming your camera low it is viewing a
> > thicker column of atmosphere which in theory should produce more
> > activity. The zenith is actually the worse area to view meteor activity
> > both visually and photographically.
> 
> When I read this my initial thought was it appears this way - the
> lower you look the more airmass you see therefore more meteors
> will be visible. However, on further analysis it appears not to be so
> in most cases. As you look lower, atmospheric attenuation and
> inverse-square law dimming of the meteors will reduce the
> numbers, (since on the average they are further away) but how
> much?
> 
> I did a quick back of envelope derivation and got the following
> results:
> 
> I assumed 3 factors governs the number of meteors seen as a
> function of elevation (or airmasses (X) where X=1/sin(elev)).
> 
> 1. the number potentially visible increases linearly with airmass (X)
> 2. atmospheric extinction reduces their average magnitude
> 0.5*(X+1)*A, where A is the average attenuation in magnitudes per
> airmass (typically 0.28 mag)
> 3. inverse square dimming in magnitudes is 5*log((X+1)/2)
> 
> Then, using the familiar "r" population index, we can write the
> meteor rate (n) as a function of airmass and magnitude dimming
> (M) will be n = Z * X * r^(-M) where Z is the rate at the zenith.
> 
> Combining (2) and (3) to get M= 0.5*(X+1)*A + 5*log((X+1)/2) we
> get n = Z * X * r^(-0.5*(X+1)*A - 5*log((X+1)/2 )
> 
> To find at what airmass (X) this rate is a maximum, we differentiate
> it with respect to X, set it to zero and solve for X.
> 
> This yields a quadratic equation in X:
> 
> (0.5*A*ln(r)) * X^2 + (ln(r)*5/ln(10) + 0.5*A*ln(r) - 1) * X - 1 = 0
> 
> Since X must be at least 1 airmass, this tells us that r < exp(2/(A
> + 5/ln(10))) for a solution to yield a maximum rate value at some
> elevation BELOW the zenith. So for a typical value of A=0.28 we
> see that r < 2.26.
> 
> So, in conclusion only the bright showers with population index <
> 2.26 would benefit from looking below the zenith to achieve a better
> rate. For a very bright shower like the Quadrantids with r=2.1, the
> maximum rate will occur by looking at airmass (X) = 1.19 or 57°
> elevation above the horizon. If you observe from a very transparent
> sky with A=0.15, for the same bright shower with r=2.1, the best
> occurs at X = 1.35 or 48° elevation.
> 
> While this is based on a simple model, it shows that only for the
> brightest showers could it benefit to center your view somewhat
> below the zenith, certainly not near the horizon. For average or
> fainter showers the best results will be observed at the zenith.
> 
> Sincerely,
> Mike Linnolt
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