Goodricke-Pigott Observatory

Ex stellarum lumine ac peritia, scientia”

5500 West Nebraska Street
Tucson, Arizona 85757
(520)578-8564

gpobs@mindspring.com

Roy A. Tucker, Observer

Operations supported in part by a 2002 Eugene Shoemaker Grant from The Planetary Society

Last updated 21 January 2009

 

Observatory History
Observatory Instrumentation
MOTESS
Observational Programs
Observational Techniques
The Observer

Observatory History

Goodricke-Pigott Observatory, a privately owned facility, was formally dedicated on October 26th, 1996 and observations began that evening with imaging of Comet Hale-Bopp. Although construction had begun in 1990 and observations had been made previously with various instrumentation, it was at this time that the full system, consisting of a Celestron C14 telescope and Southwest Cryostatics Model PV camera, was fully functional and ready for observations.

The observatory was named for two amateur astronomers who lived in York, England in the late eighteenth century, John Goodricke and Edward Pigott. These observers discovered some of the first known variable stars such as Delta Cephei, Beta Lyrae, Beta Persei, and Delta Scuti. Remarkably, John Goodricke correctly explained the variability of Beta Persei, also known as Algol, as an orbiting dark body periodically passing in front of the star and blocking a portion of its light. Such an object is now known as an eclipsing binary star. Two years later, in 1784, John Goodricke was awarded the Royal Society's Godfrey Copley medal in recognition of his research. In 1786, he was elected a Fellow of the Royal Society only two weeks before his death at the age of twenty-one. His accomplishments are all the more remarkable when you realize that he was totally deaf from a very early age. And this at a time when deafness was equated with mental incapacity. John Goodricke overcame great obstacles to learn things that had never been known before. Both of these gentlemen have been a great inspiration to me. This observatory is dedicated to the memory of two early amateur astronomers, their friendship, and their love of the sky. For further information, I refer you to the article on page 400 of the November, 1978 issue of Sky & Telescope magazine.

Observatory Instrumentation

For observations of random areas of the sky, a Celestron C14, 0.35 meter aperture, f/11 Schmidt-Cassegrain is available. The optics have been upgraded by optician Robert Goff of AXE (Astronomically Xenogenic Optics, Tucson, AZ). The clock drive has been upgraded by the Edward R. Byers Company. The residual two-minute, ten arc second periodic error is corrected by a Santa Barbara Instrument Group ST-4 star tracker attached to a Celestron C90 telescope mounted to the side of the main telescope.

The Southwest Cryostatics CCD cameras mounted on the observatory telescopes are currently equipped with Scientific Imaging Technologies (SITe, Beaverton, OR) TK1024 thinned, back-illuminated CCD imaging devices. This device is a 1024 square array of 24 micron pixels, 24.576 millimeters on a side. Peak quantum efficiency is about 90% at 640 nanometers and is above 80% from 400 nanometers to 800 nanometers. The QE is still above 10% at 300 nanometers and 1100 nanometers. The field of view at the C14 f/11 focal plane is 21.6 minutes of arc. An unfiltered four minute integration with the C14 will typically show stars as faint as magnitude 20.5.

A new instrument began operation in April of 2001. Dubbed 'MOTESS' for Moving Object and Transient Event Search System, it consists of three identical 0.35 meter telescope/CCD camera combinations.

Stellar occultations and similar rapidly occurring events may be recorded using a monochrome CCD video camera and S-VHS video recorder which can also provide time and date stamping.

A UBV photoelectric photometer with pulse counting electronics may be used for precision photometry. Currently, the photometer uses an unrefrigerated 1P21 phototube. A Hamamatsu R1414 phototube has been obtained and will be installed along with thermoelectric cooling devices in the future.

MOTESS

The past several years have seen remarkable advancement in the participation of amateurs in the field of minor planet research, primarily due to the easy availability of good CCD cameras and ever more powerful computer equipment. As the search programs extend to ever fainter objects, both amateurs and professionals must refine their methods to improve both magnitude penetration and areal coverage. A prototype instrument consisting of three identical telescope/camera combinations has been constructed and will help explore how this improvement in capability may be economically accomplished.


The actual instrument telescopes are conventional Newtonians with 35-centimeter aperture, f/5 primaries of low-expansion Astrositall material and 8 centimeter minor axis secondaries of fused quartz. A temperature-compensating optical support structure using the differential expansion of steel and aluminum rods eliminates the need for focus changes during the course of an evening's observations. Since the telescopes are intended to be directed towards a fixed azimuth and elevation for months at a time, there is no problem with structural flexure due to changing orientation and a relatively lightweight structure has been implemented.

To facilitate directing the instrument at the desired point in the sky, each telescope is supported by a short yoke English mounting fabricated from commonly available structural steel and plumbing fittings. There is no clock drive mechanism or precision slow motions other than some threaded steel rods and nuts to rigidly hold the instrument in the declination and hour angle positions. At those rare times when the instrument pointing is changed, those steel nuts may be "tweaked" with a wrench to effect a certain amount of coarse adjustment. One full rotation of the declination adjustment nut effects a motion of approximately 5.85 arcminutes. Fine positioning is accomplished using dial gauges to measure motions of the telescope primary mirror cell relative to the observatory floor. 0.001" corresponds to six seconds of arc.

Another unusual aspect of the telescope design is that focussing is accomplished by moving the primary mirror. This is a somewhat tedious process but fortunately, due to the temperature compensation mechanism, must be done only at long intervals. First, the primary mirror is adjusted so as to be perpendicular to the optical axis. Three dial gauges indicate the positions of the three adjustment screws on the primary mirror. During the focussing process, the differences in the readings are maintained so as not to change the collimation while identical increments of displacement are applied to the three screws. Approximately an hour is required to fully accomplish the iterative focussing process.

Three home-brew cameras based upon an existing design (Tucker, R. A. 1995, "A Public Domain CCD Camera Design", Bull. American Astron. Soc., 185, #63.04) have been constructed using thinned, back-illuminated 1024x1024 CCD imagers from Scientific Imaging Technologies (Beaverton, Oregon). These TK1024 devices have 24 micron pixels and, in combination with the telescope system, produce an image scale of 2.83 arcseconds per pixel. The resulting field of view is 2898 arcseconds or 48.3 arcminutes or 0.805 degrees.

These cameras are capable of being operated in continuous scan mode and the equivalent integration time at the celestial equator is approximately 193 seconds. The combination of integration time and aperture should reveal stars as faint as 20.5 magnitude. Scanning at the celestial equator permits examination of just over 12 square degrees per hour. In normal operation, the three telescopes are aimed at the same declination but spread in Right Ascension at intervals of 15 to 60 minutes to produce a data stream of image triplets separated in time that reveal moving and time-varying objects. At this time, the instruments are centered on +05 degrees, 0 minutes declination. The separation between the three instruments is about 20 minutes of Right Ascension.


The presence of unattenuated bright moonlight greatly reduces the dynamic range of the images, perhaps even producing saturation of the CCD. Before each camera is a filter slide mechanism that permits insertion of a clear filter of anti-reflection-coated BK7 glass or a color filter of photometric “R”, "V" or "I". Consideration was initially given to the use of neutral density filters to attenuate the light levels during times of bright moon but it was quickly realized that color filters produce the desired attenuation and also provide useful color photometry. The filters are usually arranged so that image triplets ordered in time in "R", "V", and "I" are obtained.

The instrument shelter is a conventional roll-off roof design, eight by twelve feet in size, optimized for the enclosure of three instruments that are pointed in the general direction of the celestial equator. The view of the sky is restricted by eight-foot high walls that provide protection from artificial light sources in the vicinity of the facility. An electrically-powered garage door opener is used to automate the motion of the roof. Although this mechanism extends across the center of the opened aperture, the telescopes can easily be positioned to avoid any obscuration of the optics. Fundamental considerations in the design of the shelter were low cost and unattended operation.

Depending upon the season, this instrument produces 1 to 1.5 gigabytes of image data per clear night (150 to 220 image triplets, each covering 0.64 square degrees). It is not possible to examine all of these images intensively by eye in less than ten to twelve hours and so it is necessary to rely upon computer image processing to search for interesting objects. One computer, a Pentium-class PC-compatible, is dedicated to acquiring images from the three cameras. Although operated in continuous scan mode, the image stream is sliced up into 1024 squared FITS images for convenience. The images are then passed over an ethernet connection to a second computer for analysis and archiving on portable hard disk drives.

Currently, moving object recognition is provided by a software package called PinPoint by Robert Denny of Mesa, Arizona. On the basis of user-adjustable parameters, this software can automatically find moving objects and generate astrometry reports for the Minor Planet Center.

The same region of sky is searched night after clear night, a strip of sky 48 arcminutes wide at a particular declination, currently +05.00 degrees, limited at the west and east ends by evening and morning twilight. Because of their relatively slow motion along a primarily east-west line, Main Belt asteroids are generally observable on multiple nights. Co-adding the three images from one night and comparing the result with the co-added images from other nights permits the recognition of slowly-moving outer-solar-system objects. This strip of sky will be a window of observability through which smaller bodies of the solar system will drift over time and be noted. I call this the "Fly-paper" or "Field of Dreams" strategy.

Although initially intended as an asteroid and comet search instrument, the archived data have been processed to find variable stars. The first two-year survery produced over 26,000 variable star candidates. This MG-1 catalog of variable stars may be found at the GNAT (Global Network of Astronomical Telescopes) website.


This instrument has been built by a single person working in his spare time over the course of two years for an investment of about $12,000. It should be possible for other amateurs and modest professional institutions to duplicate this system if they are strongly motivated. I consider this system to be essentially an engineering prototype. Later versions of this design will incorporate changes based upon lessons learned from it.

You can see that this type of instrument has no expensive precision mounting. The telescopes are fixed in position and scan the same strip of sky each night. The cost of the instrument is primarily in the optics and the cameras, those components that are required for the detection process. A precisely pointing and tracking mount, while very expensive, does not detect a single photon.

Interestingly, based upon discussions with friends at SpaceWatch and Catalina Sky Survey, it appears that future versions may utilize five telescopes instead of three. The threshold of detection of a moving object using three images is about three sigma above the surrounding sky. For four images, it is about 1.6 sigma. For five images, it can be a remarkable 0.6 sigma! For a modest 67% increase in cost, it is possible to emulate the performance of a telescope with twice the aperture but without the complications of a larger telescope (sacrifice in field-of-view, more complex camera, etc.)

At the end of July 2002, The Planetary Society provided a Eugene Shoemaker Grant to help support the continuing MOTESS observational program. This grant was very helpful in purchasing the CD-R discs used as an archival medium and permitted me to distribute the data to volunteer participants for examination and measurement. Funding was also included for periodic maintenance of the instrument to assure that it is operating at peak performance. I wish to express my sincere thanks to the Society for their grant and their expression of confidence in my efforts.

Observational Programs

Currently, the primary observational program is the search for Near-Earth Objects. Begun in May of 1997, the program has so far scored six successes: 1997 MW1, an Aten-class asteroid, was discovered on 29 June 1997; 1998 FG2, an Apollo asteroid, was discovered on 21 March 1998; 1998 HE3, another Aten, was found on 21 April 1998; Aten asteroid 2003 UY12 was discovered on 17 October 2003; Apollo asteroid 2004 MP7 was found on 26 June 2004; and Amor asteroid 2008 SP7 was found 23 September 2008.

The first comet discovery occurred on 13 September 1998. The very first scan on the first night after the end of the summer monsoon season revealed P/1998 QP54 (LONEOS-Tucker). Initially spotted in late August by the LONEOS search program of Lowell Observatory, it was mistaken at that time for a common Main Belt asteroid. The second comet discovery occurred during a four-day break in our summer monsoon weather on the 23rd of August 2004. C/2004 Q1 (Tucker) was of magnitude 15 and moving almost due north in Cetus. Examination of the previous night's images provided a second night of astrometry almost immediately.

1998 YF27 is a Mars-crossing asteroid with an orbital inclination of 46.4 degrees. Of the thousands of known asteroids, only a small number have higher inclinations. Perhaps not as exciting as an Earth-crossing asteroid, it's still quite a rare object.

If you have some means of examining FITS formatted image files and blinking them to look for moving objects, here are the zipped discovery images of the Apollo asteroid 1998 FG2, Aten asteroids 1997 MW1, 1998 HE3, high-inclination Mars-crosser 1998 YF27, and comet P/1998 QP54. I suggest the use of Astrometrica and one should blink all three images (two for 1997 MW1) to distinguish asteroid images from random 'hot pixels'. Be aware that some of the zip files are over 900 kilobytes and will take a while to transfer.

As an example of the images that the MOTESS instrument produces, see if you can find Apollo asteroid 2004 MP7 in these discovery images. Bear in mind that these images are 0.8 degrees across and the file is almost 5 megabytes even in zipped form. Notice the very nearby earth satellite passing by in one of the frames and the star clouds and dark nebulae of the Milky Way in the vast distance beyond. At the time these images were acquired, 2004 MP7 was about 3.3 million miles away. At its closest approach to the sun, it's almost within the orbit of Venus. At its most distant point from the sun, it is almost as far as Jupiter.

Observational Techniques

When I began my NEO search program in May of 1997, my observing procedure consisted of making a series of stare mode images in a north-south line using the camera with a SITe TK512 CCD. Emphasis at that time was to go as faint as I could, about magnitude 20.5. Under moonless conditions I would hit the background sky limit after about five or six minutes. I'm a little ways out of Tucson but I still have some light pollution. Stare mode imaging can be tedious and time-consuming: move the telescope, adjust the guider, operate the camera, repeat. In order to improve efficiency and increase sky coverage, I implemented two modifications to the system in the early autumn of 1997: I built another camera with a thinned, back-illuminated SITe TK1024 CCD and enhanced the control software to permit drift-scan imaging. Although the TK1024 has four times the number of pixels, they are 24 microns in size instead of 27 microns resulting in an increase in sky coverage by a factor of 3.1. At the celestial equator drift scan imaging will result in an equivalent integration time of 86 seconds, almost exactly the pixel dwell time for an object moving at a rate of 0.7 degrees per day. In other words, integrating for a longer period of time is a waste since the light will move onto a neighboring pixel and no longer contribute to the signal of the original pixel. A motion of about 0.7 degrees per day or faster indicates a potential Near-Earth Object. In practice I make three scans across a selected sky region and then compare the images using Astrometrica or PinPoint to look for moving objects. By this means I can cover approximately 1.6 square degrees per hour with the Celestron 14 telescope.

Normal astrometric observations are obtained using stare mode imaging. It is necessary to be able to point precisely at a particular point in the sky but none of the telescopes have encoders. I am able to point with the necessary precision by using a simple offset technique. Celestial coordinates, precessed to the same epoch, are found for the object to be observed and for an easily found reference star a short distance to the west and as close to the same declination as possible. I typically use reference stars that are 6.5 magnitude or brighter. I have determined that one rotation of the C14 declination slow motion knob corresponds to 320 seconds of arc. The knob is turned an amount appropriate to the difference in declination between the reference star and the object to be found. The clock drive is then turned off for a time equal to the difference in Right Ascension between the two objects. The object will then be located in the field of view where the reference star was originally.

The search for Near-Earth Objects requires a large field of view and the ability to see very faint objects. The final system characteristics are a compromise between these two requirements and are strongly affected by the background sky brightness. The C14 is an f/11 optical system. If I were to use an image compression optic to achieve an effective f/ ratio of 5.5, I would have four times the field of view but also four times the background sky signal and two times the shot noise (square root of four) thereby reducing my ability to see faint stars by 0.75 magnitude. Instead of seeing to magnitude 20.5, I would be limited to 19.75. The optimal image scale should approximately match the average star image size to the size of the pixel. Due to the optics, atmospheric seeing, and tracking errors, my typical star images are about three seconds of arc. The image scale of the C14 optics with the TK1024 CCD is 1.265 seconds of arc per pixel. During scan mode imaging, the CCD is operated in 2x2 binning mode to produce 2.53 second of arc effective pixels. Tests have shown that the C14 system reaches approximately magnitude 19.7 in scan mode operation. The MOTESS instrument is able to achieve approximately magnitude 20.5 due to the longer equivalent integration time of over three minutes.

I must emphasize the importance of using thinned, backside-illuminated CCD imagers in the search for Near-Earth Objects. A frontside CCD will have a peak quantum efficiency of about 40% as compared to a backside device with about 90% QE. The backside-illuminated imager will also have a substantially better response in the blue and near UV. The net effect is that a backside imager will produce about three times the signal of a frontside device when making unfiltered images of an object with a solar-type spectrum.

The discovery of my first NEO, 1997 MW1, after acquiring only 83 pairs of images was the most remarkable stroke of beginner's luck. My field of view with the original TK512 camera was only 12.1 arcminutes. Each image was therefore 0.04 square degrees. I had only searched about 3.4 square degrees to find my first object.

1998 was the first full year of scan-mode imaging operations and I was able to acquire over 17,000 images during my search efforts, covering about 700 square degrees of sky down to about magnitude 19.7. The MOTESS instrument covers approximately 100 square degrees per night (depending upon time of year) to fainter than magnitude 20. I am now able to search in one week an area of sky that previously required a year with the original C14 system!

Early MOTESS operations were relatively unsophisticated with various hardware and software elements of the system in a very developmental state. A night's images would require ten to twelve hours to search for asteroids and measure them, with many missed detections. Performance reached a far more mature level in the fall of 2003 with the Beta version of PinPoint 4.0. Fully released in January of 2004, PinPoint permits a single person to examine a full night's images, something like 110 square degrees, in only about two hours. During the two-month period from September to November of 2003, the MOTESS instrumentation permitted the discovery of 120 minor planets. During the year of 2008, over 68,000 astrometric observations were submitted to the Minor Planet Center.

The archived images acquired with MOTESS have been processed by a data analysis pipeline developed by Adam Kraus while he was a student at the University of Kansas. Adam is now a graduate student at the California Institute of Technology and his work with the data continues. His efforts have produced photometric histories of over 1.5 million stars in the first MOTESS field of view, revealing tens of thousands of new variable star candidates. The MOTESS telescopes are moved to point to another declination every two years to repeat the process of discovery for another strip of sky.

The Observer

I was born in Jackson, Mississippi in December of 1951 but most of my early years were in Memphis, Tennessee. From 1966 until 1978, I was a member of the Memphis Astronomical Society. My Bachelor's degree in Physics was obtained from Memphis State University (now The University of Memphis) in 1978. My Master's degree in Scientific Instrumentation was granted by The University of California, Santa Barbara in 1981. I was for three years a graduate student in the Planetary Sciences at The University of Arizona (1984-1986).

My interest in astronomy began when I was in high school in the fall of 1966. I read in an astronomy book from the school library about something called the 'Zodiacal Light' which could be best seen after sunset in the spring and before sunrise in the autumn. I decided to arise at 4 am, before morning twilight, so that I could see this phenomenon. I didn't see the Zodiacal Light that morning but I was so impressed with the quiet beauty of the morning sky and twilight that it became my habit for the next year to awaken at that time and enjoy the morning. As a result, I was a witness to the 1966 Leonid meteor storm. I received my first telescope, a three-inch refractor, on Christmas of that year.

I became acquainted with electronics at an early age since both of my parents repaired radios and televisions. I make a living as an electronic instrumentation engineer and I prefer working with astronomical instrumentation. I am especially interested in the stimulating challenge of research conducted with small aperture telescopes. My consulting work has taken me to such distant places as New Zealand and the Russian Six-Meter Telescope. I am currently employed as an engineer at the University of Arizona's Imaging Technology Laboratory.