Re: star tracking was Re: coma was Re: [OM] 35-80/2.8

Subject: Re: star tracking was Re: coma was Re: [OM] 35-80/2.8
From: "John Petrush" <petrush@xxxxxxxxxxxxx>
Date: Tue, 10 Aug 1999 21:32:30 -0400
Acer Victoria <siddim01@xxxxxxxxxxxxxxx> wondered:
> Come again? Since the 35 is wider, why doesn't it trail earlier? I mean,
> with a wider angle of view, you have more stars farther from the Pole
> star, and since every other one goes around Polaris, the farther they are,
> the faster their linear velocity (to the eye on Earth) to keep a constant
> angular veloc. I'm prolly completely and utterly confusing matters<shrug>

When John P wrote:
> >>>With a fixed 50mm lens, at mid-latitudes, stars will begin
> to trail in about 20 seconds, with a 35mm lens, trailing begins in about
> 30 seconds.<<<

With a wider angle lens, the field of view is greater, hence more trailing
is needed to cover the same distance on the film itself.  Also, as one moves
from the equator to the pole, it takes longer to trail since the star
scribes an ever smaller circle nearer the pole.  If a star were exactly at
the pole you could expose all night since it would never change relative

A fair "rule of thumb" approximation for the maximum duration of an exposure
*at the equator* is 700/f, where f is the focal length in mm.  The threshold
increases as one gets closer to the poles, or more precicely, the
declination you are pointing at.  The table below gives some decent starting
values to experiment with:

focal length            FoV (deg)          Maximum time (sec)
     (mm)                                    at declination
                                         0       50     75
   28                   44 x 63          25      38     95
   35                   36 x 52          20      31     76
   50                   26 x 38          14      21     53
   85                   15 x 23          8.1     13     31
  135                  9.7 x 14          5.1     7.9    20
  200                  6.6 x 9.8         3.4     5.3    13
  300                  4.4 x 6.5         2.3     3.6    8.9

Source:  Astrophotography 2nd Ed.  by Barry Gordon pp 34

As you can see from the table, as focal length increases maximim time before
trailing decreases.  Also, as declination increases so does maximum time
before trailing.

John P
there is no "never" - just long periods of "not yet".
there is no "always" - just long periods of "so far".

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