Observing Operations | Reviews | Survey Management

Sloan Digital Sky Survey Review of Observing Systems and Survey Operations

Photometric Telescope Observers Program Overview
Eric Neilson April 10, 2000

Introduction

The Monitor Telescope Observers Program, or MOP, provides the interface for the observers on the Photometric Telescope (or PT), and implements automated observation of standard stars and secondary patches (transfer fields). All functions are available through a command line interface, and most are available through a GUI (graphical user interface) as well. Procedures common to routine observation are mostly automated.

Functionality

Basic telescope, filter wheel box, and camera control. Using MOP, the observer can send basic commands to the telescope, filter wheel box, and camera. These include (but are not limited to) slewing the telescope, setting the filter wheel, opening and closing the shutter, and reading out the CCD.
Target field management. MOP allows the user to load lists of targets into memory, create observing plans using these targets, and then carry them out. MOP loads the locations and contents of primary standard, pointing, and secondary fields from the mtstds product, and allows the user to load other targets, called "manual targets," from a file owned by the observer. These manual targets can be added interactively or by directly editing the manual target file. MOP can assist the observer in choosing targets by keeping track airmass at extrapolated time of observation, keeping track of which fields have previously been observed at what airmasses, and planning pairs of observations at different airmasses for use in extinction measurements.
Manual and semi-automated observing. An observer may observe by selecting a desired field (or list of fields), and MOP will observe them in sequence. The resulting images can be examined interactively. The observer may also make observations to measure the focus, correct the pointing, and take grid tests. On a more basic level still, the observer may observe by directing the pointing, selecting the filter, opening the shutter for the desired time, and reading out the CCD.
Automated observing. MOP can add targets to the planned target list either at the request of the user, or automatically as it completes observations. MOP attempts to choose targets such that a suitable number of primary standards, in a range of colors and at a range in airmasses, are taken within a sequence of observations, and that any observations not necessary for primary standards are of needed secondary standards. Because the planned target list remains editable by the observer, the automatic selections may be modified by the observer as desired.
Calibration. MOP will take automated sequences of dome flats and biases, and assist the observer in taking twilight flats by measuring the count levels and suggesting exposure times based on a modeled twilight.
Log creation. In addition to the report file required by the pipeline, MOP assists in the creation of a manual log by providing an editor window with appropriate fields for the observer to fill out. Many operations in MOP, particularly the taking of observations, generate log lines which are automatically appended to the manual log. At the end of the night, MOP can send the log to the appropriate mailing list.

Status

MOP is currently usable, and most of its desired functionality is implemented and working. In its current state, in the course of a typical night, an observer will

uncover the telescope, use MOP to point the telescope to the dome screen, turn on the lamp, and begin the taking of flat field exposures. MOP does not need attention from the observer while the dome flats are being taken.
instruct MOP to take the desired number of bias frames, and wait for the biases to complete
open the dome (MOP is not involved here)
point the telescope to an appropriate point in the sky for twilight flats, and use MOP to take twilights. MOP requires user confirmation of the exposure time for each flat as it is taken, but assists the observer by returning the count level of each flat as it is taken, and providing suggested twilight exposure times based on a model
load a list of fields MOP should not observe. This list includes previously completed secondary patches, and may need to be updated by hand
enter into MOP the stripes and coordinates along these stripes for which secondary patches are to be observed. These may be changed at any time during the night
select an FK5 star, observe it, and correct the pointing based on where it falls in the image.
using MOP's tools for selecting targets, create a list of 7 or 8 target fields to begin the nights observing
take a focus frame of the first target in the list, and adjust the focus accordingly
turn on the "auto-picker" in MOP, and begin observing; as each field in the initial list is observed, and appropriate field will be chosen by MOP and appended to the list
wait for the sun to come up. In this time, the observer should periodically
1. check the focus, and adjust it if necessary
2. note weather and observing conditions in the log as they change
3. check images and log to catch ensure everything is working properly
as the sky becomes bright enough, take twilight flats as described above
cover the telescope, and close the dome
email the log to the appropriate mailing list
write data to tape

This procedure usually functions without problems. There is a bug which occasionally (once every several nights of observing) causes GUI to partially freeze and stops the automatic observation. Operation can be restored using one command at the command line. The problem may already be fixed, but its intermittent nature means that a number of additional nights of observations with the fix are necessary to establish this.

Several currently incomplete enhancements are desired for improved performance and observer convenience:

fully automated twilight flats. Improved modeling of the twilight to facilitate this is being worked on, but may prove impossible due to night to night variability in the color of the twilight sky. It is likely that this feature would not generally be useful, as dome flats in all filters except u are likely to be suitable. Automated twilight flats in the u filter only would be much simpler.
improved automation in secondary field selection Currently, the observer must keep track of which secondary fields need to be observed, and which are completed. A database is being developed which will keep track of which secondary fields have been successfully taken, which have been attempted but have not yet been determined to be of suitable quality, and which of the unobserved fields have the highest priority. When this product is complete, MOP will examine it do determine its choice of secondary patches.
integration of partial array reads with automatic target selection. Although MOP may perform partial array reads at the request of the observer, the readout size is not automatically adjusted for each target field. If in the future MOP will automatically read out only the central region of the CCD when primary fields containing only one primary are observed, time will be saved on primary fields.
automatic focus. Focusing currently requires observer attention. Options for automatic focusing, such as mapping of focus as a function of temperature, are being explored. Note that the focus is sufficiently stable that, on nights with good seeing, near critically sampled images can be maintained throughout the night with only one focus adjustment after the initial focusing for the night.

Performance

Except for sunrise and sunset, when the observer must open and close the dome, and initiate the taking of bias frames and flat fields, MOP mostly succeeds in automating a nights observation. The observer must still "keep watch" throughout the night to keep the telescope in focus, monitor weather conditions, and ensure that MOP continues to run. The errors which cause MOP to stop automated observations are being tracked down and solved. Possible strategies for automated focusing are being explored.

The PT does not currently observe secondary patches at the rate necessary for the survey. Refinement of the exposure times (which are currently known to be much longer than necessary) and increased use of partial array readouts will significantly speed up observation.

Conclusion

MOP's basic functionality is complete: an observer may take a nights data conveniently and efficiently, needing only periodically to check up on the observations. However, several features important for the convenience of the observers are not yet finished. Completion and integration of the secondary patch database will become particularly important as the survey progresses, as it will become very difficult for the observers to keep track of which secondary patches need observation. Work on these features , and the general stability of the program, is progressing steadily.