
WHAT'S YOUR ANGLE? PHOTOGRAPHING CONSTELLATIONS (*)

Let's photograph some constellations tonight! I've got my 35mm camera, film, and
tripod ready. Hmmm, what lens(es) should I bring?

First of all, let's answer these questions:

 1) At this time of year, what constellations are in the night sky?
 2) What time do I plan to shoot?
 3) Do I have my star chart or atlas ready?
 4) Do I want each constellation to fill the frame or do I want to use one lens
only? This would show the relative size of each constellation, small or big.

Well, questions number 1 and 2 are answered by number 3, and question number 4
is answered by you. [For argument sake this example will not be motor driven.]

As you may all ready know Cygnus is larger than Lyra. What lens will cover the
full extent of Cygnus? This brief Basic program should help in your planning:

 100 REM  ANGLE.BAS
 110 REM  WHAT'S YOUR ANGLE?
 120 REM  For finding IMAGE SIZE and FORMAT COVERAGE of Telephoto Lenses
 130 INPUT "Focal Length of Lens (MM)";F
 140 INPUT "Angular Size of Constellation (DEGREES)";T
 150 S=F*T/57.29578
 160 FS=2*ATN(11.45/F)*57.29578
 170 FL=2*ATN(17.1/F)*57.29578
 180 PRINT "LENS F.L.:  ";F;"MM"
 190 PRINT "IMAGE SIZE: ";S;"MM"
 200 PRINT "FORMAT COVERAGE (35mm film): ";FS;"by";FL;"DEGREES"
 210 END

Example: CYGNUS (**)

 Focal Length of Lens (MM)  50
 Angular Size of Constellation (DEGREES)  22

 LENS F.L.:  50 MM
 IMAGE SIZE: 19.1986 MM
 FORMAT COVERAGE (35mm FILM): 25.7967 by 37.7614 DEGREES

So, this tells us that using a 50mm lens with a 35mm camera, the long dimension
of Cygnus would be about 19mm on the film which covers an area of about 25.8 by
37.8 degrees. That's just about right! Of course you can run the program again
to find the image size of the "width" of Cygnus, and try different lenses to see
how big Cygnus would be. Why not try a 28mm or 1000mm? What happens?

Armed with a good star chart and the above program, you can plan ahead and know
exactly what you're going to get without any second guessing!

Here are some common lenses and the area of the sky each covers:

 LENS (mm)    35mm FRAME (deg.)
 ---------    -----------------
   28           44.5 by 62.9
   50           25.8 by 37.8
  135            9.7 by 14.4
  180            7.3 by 10.9
  200            6.6 by  9.8
  450            2.9 by  4.4
 1000            1.3 by  2.0
 2000            0.7 by  1.0

 =======================
 NOTES:

  * Adapted from ASTRONORTH Vol. 3 No. 9, Sudbury Astronomy Club
 ** From SKY ATLAS 2,000.0, Sky Publishing Corp.                                                                                   