Sloan Digital Sky Survey
Review of Observing Systems and Survey Operations
Photometric Telescope Overview
April 11, 2000
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.html
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.
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.
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
Review of Observing Systems
and Survey Operations
Apache Point Observatory
April 25-27, 2000