Description

Description The Photometric Telescope calibrates transfer fields ("Secondary Patches") observed by the 2.5 m telescope and measures atmospheric extinction for 2.5 m calibrations.

The telescope is a commercial 20-inch Cassegrain reflector from DFM Engineering. After adding coma correctors to expand the field to one degree, it has the following characteristics: :

Primary aperture:   20 inches
System f-ratio: f/8.8
Field diameter 1 degree
Pixel size: 1.15 arc seconds
Pointing: 15 arc seconds rms
Uncorrected tracking: 0.5 arc sec drift in 5 minutes

More information is at http://www.jhu.edu/~sdss/PT/Optics/index.php

Instrumentation includes a six-position filter wheel, rotating sector shutter, flat field projector, Soric flat field screen, and a thinned AR-coated SITe 2048x2048 CCD similar to those in the SDSS camera and spectrograph detectors. The CCD is cooled with a mechanical refrigerator (an APD CryoTiger) and controlled by electronics similar to those on the SDSS imager and spectrographs. .

Commands to the CCD, telescope, and filter/shutter controller are sent from the unix-based observer's program ("MOP") to a terminal server in the dome. Each device is attached to a different port on the terminal server. CCD imaging data are returned via fiber optic directly to the custom data acquisition system. The observing procedure is ordinary all-sky broad band photometry. The standard star candidates (we have not yet declared a standard system) are mostly high quality non-variables from Landolt's UBVRI lists. System magnitudes will be tied to spectrophotometry of fundamental stars.

The system is functional.

The observer's software picks standard stars, moves the telescope, and gathers data. During the night, the observer selects the photometric transfer fields, monitors the sky, and checks for focus or other problems. At the beginning and end of the night the observer obtains calibration data (flats and biases) and opens or closes the dome.

Data are stored on a Unix (SGI Irix) computer until the end of the night when they're written to tape and shipped to Fermilab for processing. .

The system collects survey-quality data.  Known deficiencies against survey requirements are:

1. Observing efficiency is below the requisite one calibration set (four pointings) per hour primarily because too much time is needed for standard stars. The remedy will be to use sub-array readouts for the standards, reducing the readout time overhead. The observing system (MOP) has already been modified but the data reduction system (MTPIPE) needs to accept the new data format. This is a deeper problem than one might expect because the standard star identifications currently require position confirmation from many surrounding stars (a legacy from the old 24-inch telescope, which didn't point very well). We are developing single-star identification procedures based on the known pointing accuracy of the 20-inch.

2. Transfer fields are currently selected by the observer based on projected 2.5 m observing plans. Since routine observing does not allow full-time observers, we are implementing a system (database) that the observing software (MOP) will query to build a night's observing plan.

3. We are using a preliminary u'g'r'i'z' calibration based on USNO 40-inch observations and it will be a few months before we will have an "official" system declaration.

4. The staffing requirements are not known. We currently have a crew of non-APO observers (two JHU postdocs and a LANL scientist) dedicated to the telescope while it's in use. Within a year these observers will hand over their duties to SDSS observing staff. Although the 20-inch probably requires only 1/2-time attention while observing it's not obvious that frees up 1/2-time for other work if the other work cannot be interrupted so the staffing requirement is not known. This problem will be explored this summer and fall when existing APO staff will be trained to take photometry data while the 2.5 m is operating.

Long-term Robustness

We expect existing equipment to last the duration of the survey. It might be desirable to upgrade some items in the data acquisition system (especially as spare parts become scarce) but there is no fundamental reason to change.

Potential problems:

1) The dome temperature can be high in the summer. Controlling the heat will improve early evening seeing and reduce suspected problems with lubricants and sealants at high temperatures. Adding dome insulation and running the ventilation fan is probably a good idea.

2) Winter cold causes problems with the closed cycle refrigerator, namely it may refuse to restart after a power failure. We plan to install an uninterruptable power supply and also explore ways to warm the appropriate parts.

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Review of Observing Systems and Survey Operations
Apache Point Observatory
April 25-27, 2000