PDO – Lightcurve Program Details
The lightcurve program at the Palmer Divide Observatory has evolved considerably since
it first started in 1999. The number of telescopes that have gone in and out the doors is
a big staggering upon reflection. I think I could have another two or three large scopes
hay two wires extend out
from the house to the 0.5m (20" building). A KVM switch in the 14" building
allows the two computers to share keyboard, monitor, and mouse. Despite temperatures below
0, the computers seem to operate well. The computers are in enclosed cabinets with a small
light bulb to generate heat. This is both for temperature control and to keep the larger
insects and rodents out.
Exposures
Exposures throughout the program have been, on average, 90-120s, depending on the
brightness of the target asteroid. As fainter targets came into the program, the exposures
have been increased up to 240s (4 minutes). All exposures are unguided. Generally, the
0.35m scopes can reach good SNR values (> 50, 0.02m) down to 15.5 with 180s. The 0.5m
has worked down to 18th magnitude but is usually limited to 16.0-16.5 at 180-240s for >
50 SNR. A master dark is automatically subtracted from each light frame after the exposure
is taken. Flat fields are applied using the batch imaging process in MPO Canopus prior to measuring the images.
Software
Telescope and camera control is done with MPO
Connections, a custom program with simple scripting written at the Palmer Divide
Observatory. It is capable of sending a telescope to several targets, maintaining focus,
changing filters, and all the other requirements of a research level astrometry or
photometry program.
Image processing and measurements are done with MPO
Canopus. This is another commercially available program written at PDO that was the
first to include Alan Harris' industry standard Fourier period analysis algorithm in a
program for general use by amateurs.
General Program Description
Asteroids are chosen by first determining which targets within range of the equipment
are near opposition and favorably placed for lengthy overnight runs. Special attention has
been given to the Hungaria group since 2004. This group consists of high albedo inner main
belt members, thus allowing the statistical sample of asteroids to include smaller members
than might be otherwise possible. As an aside, initial observations and analysis at the
PDO has lead to the discover of six binary asteroids since October 2004, five of them are
Hungarias.
Once the list of potential targets is made, it is usually reduced by filtering out
those asteroids with already well-known lightcurves. Sometimes, however, even those with
known periods are observed, either as a check of the original results if they are somewhat
dubious or to assist with shape modeling project. The filtering is done by referring to
the list of lightcurve
parameters maintainseries of V images is taken
of the target field as well. Using a method outlined by Richard Binzel, the clear
observations can be converted to reasonably good V magnitudes (0.01-0.03m accuracy with
good technique). Once the data are on the standard system, the initial nightly zero points
can be set more accurately and the data from multiple sessions successfully merged.
Once the script is started, I periodically monitor its progress, making sure that focus
is holding - sometimes it changes rapidly and needs adjusting before the auto-focus
command is reached in the script (usually once an hour). Besides that, I can and often do
go on to other things such as reading and watching TV (mostly "Law and Order"
reruns and football/hockey).
Data Reduction
Data reduction is done with MPO Canopus.
For each image, the following information is stored in a database:
UT Date/Time of mid-exposure
Air Mass
Instrumental magnitude of the target and comparison stars
Catalog derived magnitudes of the target and comparison stars (optional)
SNR of the target and comparison stars (to allow computing the estimated error per
observation)
Average magnitude of the comparison stars
Comparison - Asteroid magnitude
Differential photometry techniques are applied to the data reduction. Several
comparisons are used (two minimum, and up to five) to provide additional stability to the
average value of the comparisons and to assure that there will be at least one comparison,
preferably two, that is not variable.
In addition, the distance of the asteroid from earth and its predicted magnitude are
kept as part of a larger record associated with all the data for a given night's run.
These are used to determine the corrections required for phase angle differences and
light-time corrections. The mean value of all the averages for the comparisons is also
stored. This can be adjusted per session so that all data is eventually referenced to a
common, but arbitrary, zero-point, i.e., the comparison value used for all data points,
even over several nights is the same.
Period determination is accomplished using a routine based on the FORTRAN program FALC
developed by Harris et al. This performs a Fourier Analysis on the data, allowing
different parameters such as number of harmonics, period, size of period steps, etc. to be
held constant while others are varied. This routine is also included in the Canopus software. Finally, a plot of the raw data
or phased (all data merged into a single cycle from 0 to 100% of the derived period) is
generated. This plot can be saved as a Windows BMP for reproduction and manipulation at a
later time.
If you would like more information about the details of the asteroid lightcurve
program, equipment, or software at the Palmer Divide Observatory, please drop me a note. |
