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The Minor Planet Observer
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Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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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.

 

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This page was last updated on 01/19/11 04:38 -0700.
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