Re: Photographing geosynch satellites
Wed, 24 Jul 1996 11:32:10 -0400

On 23 Jul 1996 Ron Lee provided some helpful tips on photographing
geosynchronous satellites.  For those interested in narrowing down the
various combinations and permutations involved in this (film speed, exposure
speed, f/ratio, satellite magnitude, etc.), I might suggest a book on
astrophotography by Barry Gordon called, appropriately enough,
"Astrophotography."  I believe it is still available through most popular
astronomy magazines.

While the book itself (at least not the edition I have) does not discuss
satellite photography (though aspects its sections on meteor trails is
applicable), it does discuss the various issues involved in photographing
point sources, meaning stars, (as opposed to extended objects such as
planetary disks, the moon, galaxies - even the sky itself).  Photographically
speaking, a geosynchronous satellite is no different than a faint star,
except that is remains fixed locationally with respect to the Earth.  

Recording point source images on film, according to the book, is function of
absolute lens aperture, not focal length.  (The reason being the image of a
point source cannot be magnified; thus increased focal length has no baring
on the size of the image recorded for a point source.  For larger objects,
however, such as the shuttle and Mir, obviously there is a point when focal
length will show these as extended objects, and no longer as point sources,
in which case different rules apply.)  

As an example of how Gordon's "fx" system works, lets assume the
geosynchronous satellite in this case is of mag. 10.  The book assigns mag.
10 stars an fx value of "22."  The idea would then be to find the exposure
time for this object, using the book's "fx" table, based on ones chosen lens
and film speed.  This is done by adding up their assigned "fx" values,
subtracting the total from 22, and using the remainder to find the exposure
time a cross tabulated on the book's "fx" chart.  

As an example, Ron mentioned he used his C-8 as a tripod.  Instead for this
example, I'll use the C-8 as the lens as well.  Since the geosynchronous
satellite is a virtual point source, we're interested in the C-8's aperture
(8 in. or 20 cm)  not it's focal length.  The book's chart assigns an "fx"
value of "-1" for a 20 cm lens aperture.  Using 200 ASA/ISO film, which the
book gives an "fx" value of "4," we add those two (4 + (-1)) and subtract
that from 22 to get a total of 19.  We would then look at the chart's
exposure column to see which exposure provides and "fx" value of 19 -- which
turns out to be about 2 seconds.  (The book actually provides a range of
values between roughly 1.5 and 2.5 seconds.)  Thus, to photograph this mag.
10 geosynchronous satellite using a C-8 at prime focus and 200 speed film
would take an exposure of about 2 seconds (within a margin of error).  

I took Barry's class on astrophotography at the Hayden Planetarium in NY a
number of years ago (he has now retired out to New Mexico), and found the
"fx" system he creates for the basis of determining exposure times very
helpful.  In hindsight, I should have been using it all along in even
photographing simple wide-field satellite trails, since his "fx" system even
works for determining sky fog limits, which have cost me more that a few
photos.  If I'd bothered to sit down and figure out, using the "fx" value for
an urban sky, how long I could keep my shutter open before the light polluted
sky washed out a given satellite's trail, I'd have saved a lot of film.

 - Jim Cook