Re: Estimating Magnitude

Leigh Palmer (
Sat, 30 Aug 1997 10:44:40 -0700

>I'm curious as to how magnitudes of satellites are judged.  Is this
>just knowing magnitudes of stars and then comparing?  I would
>like some advice from those who think they estimate magnitudes
>fairly well.

I judge the magnitudes of satellites by comparing them to known stars.
There is a problem, however. The true photometric measurement of stars
is relatively easy because they are very slowly moving and relatively
slowly changing. Satellites vary and move rapidly across the field of
view. There may be psychophysical reasons for estimates made of the
magnitudes of moving objects being systematically brighter than those
of relatively stationary objects, but a search of the literature made
at my request by a colleague here in psychology turned up no good work
on this question. Of course photometry can be done (and probably is
done) using a telescope programmed to follow a satellite. I have done
conventional stellar photometry using a 72-inch telescope in Cloudcroft,
NM, owned by the Air Force which would have done the job nicely, and I
think probably did. It is an az-alt-az-mounted behemoth which could be
oriented with one azimuth setting chosen to minimize the variation
required in the altitude during a programmed slewing of the 'scope in
the second azimuth.

Whenever I make a magnitude estimate I use stars in the same
field of view and try to find a comparable star. Liesurely comparisons
with several stars in the field, the standard method for stars, won't
work for satellites - they just move too fast. I have a computer
application that permits me to simulate the satellite pass and
identify stars with which the satellite has close encounters
(appulses). The application also has an extensive database of stellar
magnitudes, so comparisons are relatively easy to make. I usually
simulate a pass I want to observe beforehand. When I get serious
(which is rarely for satellite observations - they are just fun for
me) I take out a cassette recorder and a radio for recording time and
observing data. If I want to I can recreate the satellite pass later
and compare it to the simulation.

Another approachable method would be the direct video recording of a
satellite pass using an RFT (richest field telescope) and hand guiding
it. Simulation and rehearsal would be required for the success of this
method. I don't know if anyone has tried it, but it would certainly
work on the brightest satellites. Comparisons could then be made at
more liesure, and light curves could even be constructed.

The programmable telescope is well beyond the means of even the most
well-financed amateur, but computer simulations are relatively cheap.
I get as much fun out of predicting passes or reconstructing them on
my computer as I get observing them. I realize that I have gone far
from the answer to the original question, but I think I'll leave this
all in the discussion for comment by others.