"LIVE FROM THE STRATOSPHERE" P R O J E CT U P D A T E #17 PART 1: Preparing for a flight and how infrared objects are located _______________________________________________________________________ PREPARING FOR A FLIGHT By Ben Burress, Tracker Operator There can be a great deal of preflight preparation work for a Tracker Operator, or there can be very little, depending on the nature of the objects the astronomers want to observe. The easiest research flights are those where the objects that the astronomers are interested in are bright, starlike objects (stars, planets, bright asteroids and the like). These objects are easy to find using an acquisition chart, and once spotted the only thing to do is to instruct the Video Star Tracker to grab it, move it to the astronomers' "boresight" (the spot in the camera field where their instrument is looking), and tell them to start taking data. Most often, however, it's not quite that simple, as with the flight tonight. Tonight (September 27, 1995) we will be flying what I judge to be a "typical" KAO flight plan. On board is astronomer and instrument owner Dr. Al Harper of Yerkes Observatory, with guest investigators Drs. Jackie Davidson and Harold Butner. Their list of objects include IRC +10420, L 1455, Saturn, L 1031B, IRAS 23568+6706, TMC 1A, GG Tau, and M 42. IRC +10420 and Saturn are objects of known infrared brightness, and will be used by the investigators to calibrate their detection system, so that they will know how much signal from their system corresponds to how much brightness from the rest of the objects on the flight. Preparation for most of the objects on tonight's flight involves several activities on my part, the first of which was to obtain a list of the objects and their precise celestial coordinates from the astronomers. I have several optical and infrared catalogs in my office which I can use to find coordinates and possible past measurements of infrared brightness. However, quite often there are a number of points of interest to an infrared astronomer within a single object, and, not being able to read their minds, I must make sure that I prepare my charts based on their own list of pointing positions. Once I have the coordinates, I consult our files of finding charts and materials which have been developed over the years to see if charts for the objects exist, and if so, I pull them out. There are two basic charts I bring along for every object: an "acquisition" chart and a "tracker" chart. The acquisition chart covers a region of the sky about 8 by 12 degrees and contains stars of 9th magnitude and brighter. This chart I use for initially finding (acquiring) the star field of the object. The tracker chart's dimensions are about 25 by 18 arcminutes (the full moon is about 30 arcminutes in diameter as seen from the Earth). This chart contains stars as faint as 14th magnitude, which is about the faintest magnitude that is visible through the tracker telescope/camera. The tracker chart also contains plotted points for all infrared positions that the investigators will observe. If a particular object on the flight's list has never been observed on the KAO, then I must generate a new acquistion and tracker chart set. Prior to 1990, these charts were made by hand, plotting on graph paper positions of stars and objects. It was time consuming work, often taking as much as an hour to create one set. Today, however, we use a computer program called "Pickles", which reads the Hubble Space Telescope Guide Star Catalog from a CD ROM and plots the stars it reads. The HST GSC is a catalog of precision star coordinates created for use with the Hubble Space Telescope, and contains over 20 million stars. This catalog is well suited for use by Tracker Operators on the KAO. The star coordinates are very accurate, and the catalog contains stars as faint as 16th magnitude, much fainter than our cameras can see. Once I have a complete set of charts, I must go about the most time consuming part of preparation: generating a list of offset guide star coordinates for the Yerkes D.O.G. The D.O.G. is a Digital Offset Guider; a motorized X-Y stage that moves the main telescope's focal plane camera around within the focal plane (actually there is a fiberoptic bundle connecting the D.O.G. and the focal plane camera, but the effect of moving the bundle is the same as if the camera itself were being moved around). The purpose of the D.O.G. is analogous to steering a boat to an unseen destination by using other visible landmarks. Say you know what direction you want to sail your boat, even though the island you are bound for is beneath the horizon. You might use another island which is in view and calculate how far off your bow (or stern) you need to keep that island in order to maintain your desired course. With the D.O.G., the known coordinates (in Right Ascension and Declination) of the invisible infrared source and of a nearby visible star are fed into a computer, which calculates exactly how far the D.O.G.'s motors must be driven, both horizontally and vertically, to place that star precisely on the guiding mark in the focal plane picture (in other words, once the D.O.G. is offset to the calculated position, by placing the Guide Star on the focal plane camera's guiding mark you are actually placing the infrared source position on the investigator's boresight (sensitive spot). Thus for each object which must be "offset tracked" I must find one or more suitable stars (bright enough to see in the focal plane camera image, and close enough to the infrared source position so that the D.O.G. can reach it). To meet the requirements of a "suitable star", the star must be at least 12th magnitude in brightness and reside no more than 10 arcminutes from the infrared source. In most cases, finding a guide star is not difficult; in most areas of the sky there are enough stars of suitable brightness and close enough together to be "dogable". Once a complete list of object and guide star coordinates is gathered, the list is typed into the computer for reading by the D.O.G. program. The last thing that I do in preparation for a flight is to generate tracker overlays. Though the D.O.G. can offset to stars in the focal plane and allow fine guiding, the focal plane field of view is small (only a few arcminutes), and recognizing a star field in that field of view is difficult without the use of the tracker telescope. The tracker telescope/camera system is a small cassegrain reflector with a sensitive video camera at the focus. This scope is mounted on the frame of the main telescope, and is coaxial with the main mirror's optical axis. As I said, the field of view of the tracker telescope is a little smaller than the area of the full moon. On the video monitor at my station on the plane, the tracker telescope image has a scale of one arcminute per centimeter, which is exactly the scale at which we create our tracker charts. I create a tracker overlay by placing a transparent sheet of plastic over my tracker chart and tracing the positions of stars and infrared source positions with a grease pencil. In flight I will use this overlay by holding it up to the video monitor and lining up all of my grease pencil marked stars with the real stars on the screen. Although I can't see the optically invisible infrared objects on the video monitor (the tracker camera being sensitive to optical light only), by overlaying my grease-penciled map on the real stars, I can see where on the screen the infrared objects reside. Some day, perhaps on SOFIA, the old plastic tracker overlays will be replaced by computer generated star positions. So, with this arsenol of charts and coordinates, I walk onto the plane and hope very hard that I haven't forgotten anything....