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
had all that money gone into a well-planned facility. Aperture fever is a very infectious
and costly malady.
Below is my own brief history of time, the years since 1999, along with a general
discussion of the asteroid lightcurve program. If you'd like more details about CCD
photometry and lightcurve determination and analysis, then I recommend my book, "A
Practical Guide to Lightcurve Photometry and Analysis", available from
Springer.
Equipment
In 1999, lightcurve work started with a 0.25m LX-200 SCT using either an SBIG ST-7E
(enhanced blue chip) or ST-8 (standard chip). The camera was run at -10°, even though
lower temperatures could be reached during fall and winter, so as to keep consistency in
the images regardless of the season. In early 2000, a 0.5m f/8.1 Ritchey-Chretien was
installed in place of the 0.25m but in the same building. That was a very tight fit! In
fact, the scope could not be aimed below 40° to the east or west because it would hit the
walls of the observatory. Many telescopes and a couple of buildings built and torn down
later, the equipment situation has settled down to include three telescopes, the 0.5m R/C
and two 0.35m (14") LX-200GPS telescopes. The 0.5m is in a new building, meaning it
no longer hits the walls, while the two 0.35m scopes are in a single building in a line
going north-south. The 0.5m and one 0.35m scope use a Finger Lakes IMG-1001E run at 2x2
binning while the other 0.35m uses an SBIG ST-9E a 1x1. A focal reducer on that last scope
provides a reasonable FOV. All scopes have a pixel scale of about 2.5 arcseconds,
which is reasonable for the average seeing.
Each telescope/camera combination is run by its own PC located in the observatory
building. They are networked into the house via several routers and controlled from a
master computer inside via RAdmin, a remote control software. Only 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 maintained by Alan Harris and myself.
Once an asteroid is selected, a script is prepared that will automatically take images
of the asteroid all night, quitting when the asteroid reaches 30° in the west or twilight
begins. Additional items in the list periodically reset the telescope's position to keep
track with the asteroid, a sync to assure that the scope is aimed where it should be, and
an auto-focus to keep the images sharp throughout the run.
Usually, observations are done using a Clear filter. Calibration from night to night
then depends on the feature in MPO Canopus to
adjust the nightly zero points visually to get data from different nights to align. Should
the asteroid have a long period, this simple approach is replaced by getting calibration
images in the V filter of a nearby field with well-known magnitudes, e.g., from the LONEOS
catalog produced by Brian Skiff at Lowell Observatory. A short series 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. |
