by Arthur Babcock
Editor's note: Readers of the MIRA Newsletter know that MIRA's main research instrument, the 36" telescope on Chews Ridge has undergone an extensive refurbishment and upgrading in the last two years. While extending and improving the telescope's performance is a never-ending process, the main job is now complete, and the Newsletter's focus in the next few issues will be the research uses to which this fine instrument is being put. We begin this series with a user's eye view of the upgraded telescope.
From the operational point of view, the most significant aspect of the MIRA professional telescope's new capabilities is that the astronomer stays in the warm control room, on the north side of the Oliver Observing Station. Not only are the telescope's main functions such as positioning, tracking, and focusing controlled remotely, but so are its many instruments and ancillary equipment.
A research session begins in evening twilight as the astronomers scurry around the observatory preparing the building and the telescope for use. We must check the pressure in the dry nitrogen system that prevents dew from forming on the telescope optics. The first-floor air compressor is turned on and the astronomer checks the level of power in the storage batteries. Depending on the night's research program, we may need to replace one of our specialized electronic charge-coupled device (CCD) cameras with another. After a last check to ensure that the relative humidity and wind strength are low enough for safe observing, the roof is rolled back and various pieces of equipment on and around the telescope are plugged in and switched on.
In the control room, the three computers that control the telescope are switched on and the instrument comes to life. As the control system is initialized, the telescope begins to track to compensate for the Earth's rotation and the focus is set to the exact point where it was at the last use. Then, as the twilight deepens, we command the telescope to slew to a bright star. This is the one point at which the astronomer needs to return to the observing deck; because of a few uncertainties such as the time in the control computer's clock, the control system usually needs some human intervention to find the first star of the night. One of the astronomers returns to the telescope, sights the setup star through the 4-inch finder, and uses the control paddle to center the star in the crosshairs; after that, the telescope can be positioned from the control room for the rest of the night.
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The data-collecting end of the 36" telescope. Visible in this view are the many cables added to the telescope during the upgrade. The black box near the center (arrow) houses the guiding CCD. |
By this time, one or two computers controlling the various CCD cameras will have been switched on. They cool the camera sensors to -20ºC or colder in order to minimize electronic noise in the camera. The next step is to move to a medium-bright (10th or 11th magnitude) star to focus the telescope. Even though the focus is where we left it the previous night, temperature differences require focusing each night. This operation is accomplished with whichever main imaging camera is being used; the camera software is set to take a continuous series of short (3 second) exposures of a single star. Using the control paddle located in the control room, we change the focus slightly (there is a digital readout of the focuser position) while the camera software shows us a continuously updated graphical representation of the star image's shape. When the star image is as small and as bright as it is going to get, we make a note of the focus setting and the quality of the night's seeing in the logbook, and proceed with the observing program.
The next step is to slew the telescope to the first target. This can be done in a number of ways. First, the astronomer can enter the target's coordinates in the control program and command the telescope to move to that position; this is the most practical method when the target is a field centered on a particular coordinate rather than a specific object. When, on the other hand, we are interested in a particular object, such as the planetary nebula M57, the most convenient way to get there is to use the computer running The Sky software [see Donna Dulo's article in the Fall 2001 Newsletter _Ed.]. This program displays a sky chart and a set of crosshairs depicting the telescope's current position. To move to a new object, one has only to point at the target, click the mouse button a couple of times, and the telescope slews to that point on the sky. During the first few months of the system's operation, it was very hard not to run to the telescope deck to see if the telescope was actually going where we wanted and not crashing into the furniture, but after a few hundred successful operations of this kind, we have begun to trust the equipment.
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Arthur Babcock makes a note during an observing session with the 36" telescope. |
If we need to take a long exposure of the target, the telescope must be guided to correct tiny errors in its motion as it tracks the object across the sky. This has always been a necessity with telescopes. At MIRA, where the star images are especially small and precise, these corrections can be quite delicate. In the past, the astronomer performed this function by watching a guide star in an eyepiece and keeping it centered on the crosshairs by pushing buttons as the star drifted this way and that. A moment's inattention could ruin hours of work. At best, this was a boring task, and at worst, it produced real physical discomfort. Astronomical lore is full of tales of astronomers' efforts to control unruly telescopes. It is said that Milton Humason, Edwin Hubble's assistant at Mount Wilson, would actually climb aboard the 100" telescope at times to use his body weight to correct especially large errors in the telescope's motion. Moreover, the exercise necessarily took place in an unheated observatory, and one can sympathize with the unfortunate astronomers who guided telescopes for hours at a time in sub-zero weather, their tears freezing to their cheeks. Indeed, a MIRA astronomer once suffered the misfortune of his eye sticking to the eyepiece!
These heroic days are gone forever, thankfully; nowadays, we use a separate CCD camera to perform the guiding automatically. The MIRA offset guider positions a tiny mirror to direct the light from a single star into an auxiliary CCD camera, which takes an exposure every few seconds and measures the exact position of the star in its field. If the star should drift a bit from one exposure to the next, the guiding CCD issues a command to the telescope to move in the opposite direction, and the star returns to where it should be.
In practice, of course, there are complications and difficulties not apparent from this somewhat schematic account of the autoguiding procedure. Many adjustments may be made to the guiding program, and we are still in the process of discovering the settings that give the best guiding.
In the morning (or earlier if the session was curtailed by moonrise or bad weather), we close everything up and drive back down the hill, often with between 100 to 200 megabytes of data and thoughts of a warm bed.
In the next issue, Dr. Whitney Shane will report on MIRA's research project on gravitational lenses.
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Last updated 2/17/02 DMC