Spectrographic time budget

The spectrographic observation cycle (nominally 1 hour) is divided among 6 major components. About 10% of the time is allocated to moving the telescope to the zenith and exchanging the spectrographic cartridges. Another 8% is used to move to the field and to calibrate the spectrograph, acquire the guide stars and begin guiding. About 7% is used to take 4 highly binned spectra in a 2 x 2 square centered on the targets. These data are used to improve the spectrophotometric calibration of the spectra. The balance of the cycle, 75% of the time, is used to take three long integrations at full resolution. Percentages do not add to 100% because of rounding.

Summary of spectrographic cycle	Operation (sec)	Percent
Move to zenith and exchange cartridges	365	10
Move to field, calibrate and acquire field	296	8
Observe 2 x 2 raster (spectrophotometry)	255	7
Redshift exposure 1	960	25
Redshift exposure 2	960	25
Redshift exposure 3	960	25
Total time for cycle	3796	100

The budget below gives the breakdown of the table above. Basic operations are those that are most likely to be necessary. Contingent operations are those that are not anticipated but may be necessary depending on the performance of the telescope and instrument. This table was revised after an internal review held August 4, 1997 in Seattle.

Operation	Basic operations (sec)	Contingent operations (sec)	Notes
Move telescope to instrument exchange position	60		1
Deploy flatfield screen*			2
Engage axis brakes, disable servos, turn on exchange light	5		3
Push telescope stop button, turn on low-level lights	5		4
Move spectrographic cart to receive used cartridge	10		5
Raise lift, unlatch used cartridge, lower lift	120		6
Shift cart to place new cartridge over the lift	10		7
Raise lift, latch new cartridge, lower lift	120		8
Move spectrographic cart to the holding location	20		9
Turn off low-level lights, release telescope stop button	10		10
Turn off exchange light, disengage axis brakes, enable servos	5		11
Move telescope axes to estimated observation halfway point	60		12
Focus telescope (open-loop)*			13
Focus spectrograph (open-loop)*			14
Focus spectrograph (closed loop)		240	15
Take and read out spectral calibration data (arc exposure)	60	60	16
Take and read out flatfield exposure	60		17
Acquire and analyze guider calibration frame(s)*			18
Retract flatfield screen*			19
Open loop point to spectrographic field	20		20
Acquire and analyze guide star images 1	20		21
Correct telescope offset	2		22
Focus telescope (closed-loop)		60	23
Acquire and analyze guide star images 2	20		24
Offset slightly to measure guide bundle orientations	2		25
Acquire and analyze guide star images 3	20		26
Correct offset, scale and rotator angle	10		27
Acquire and analyze guide star images 4	20		28
Confirm proper position; commence guiding	2		29
Observe 2 x 2 raster (spectrophotometry)	255		30
Redshift exposure 1	900		31
Read out exposure 1	60		32
Adjust focus, etc., as required*			33
Redshift exposure 2	900		34
Read out exposure 2	60		35
Adjust focus, etc., as required*			36
Redshift exposure 3	900		37
Read out exposure 3	60		38
Adjust focus, etc., as required*			39
TOTAL TIME (seconds)	3796	360	40
TOTAL TIME (minutes)	63.3	6	41

* This operation occurs in parallel with the task above.

Notes:

1. TCC.
2. The flatfield screen deploy time is 10 seconds.
3. Control passes to the MCP and the cool observer.
4. Cool observer.
5. Cool observer.
6. MCP.
7. Cool observer.
8. MCP.
9. Cool observer.
10. Cool observer.
11. MCP. Control of the telescope is passed to the warm observer and the TCC. The SOP script for the next observation is started.
12. The telescope is moved to the estimated mean altitude, azimuth and rotator angle of the observation for wavelength and flatfield calibrations. This minimizes the effect of instrument flexure on the calibration.
13.  
14.  
15. This assumes two 60 second arc exposures and two 55 second readouts. This should not be necessary.
16. If the calibration is stable, this item can be deleted. Otherwise, the arc exposure will likely consist of a 30 second integration followed by a quick 28 second readout, half the normal time. The contingency is in case it is necessary to expose 60 seconds to get adequate signal to noise and to use a normal 55 second readout because the quick readout does not provide a good calibration.
17. The flat-field exposure can be read out with a quick 28 second readout, half the normal time.
18. Exposures with flat field illumination of guide fiber bundles and the sky fiber bundle are used to determine where the images of the ends of the fiber bundles fall on the guider CCD. The TCC attempts to center each guide star in its guide fiber bundle. It monitors the sky brightness in the sky fiber bundle.
19. The flatfield screen retract time is 10 seconds, i.e., shorter than the readout time.
20. A typical observation is approximately 15 degrees long on the sky. Generally, this will be less than 30 degrees in azimuth and rotator and much less in altitude. Consequently, the move from the halfway point to the start will be about 15 degrees and often somewhat less.
21. First, the offsets of the telescope altitude and azimuth axis are corrected. This should put all the guide stars on the guide fibers.
22.  
23. Focus should not be necessary unless this is the first cartridge of the night or a long delay has occurred after the previous cartridge.
24. Second, a pair of images are taken with an offset of about 2 arc seconds between images. This allows the rotation angle of the guide fibers to be measured. If the rotation angle does not change from cartridge to cartridge, this step can be deleted.
25.  
26.  
27. Third, with the rotation angles known, altitude and azimuth offset, image scale, and rotator angle can be solved for and corrected.
28.  
29.  
30. Four exposures are obtained in a 2 x 2 raster centered on the targets. The offset is 1.5 arc seconds in each direction in altitude and azimuth. Each exposure consists of a 60 second integration followed by a 3 second heavily binned readout in parallel with a 2 second pointing offset.
31. The lengths of the three redshift exposures is adjusted for galactic extinction.
32.  
33. The telescope is focussed in parallel with the readout of the CCDs.
34.  
35.  
36. The telescope is focussed in parallel with the readout of the CCDs.
37. The length of this exposure is adjusted based on guide camera measurements of sky brightness and atmospheric extinction. The aim is to produce a uniform limit to the survey outside the galaxy.
38.  
39. The telescope is focussed in parallel with the readout of the CCDs.

Date created: 07/30/97
Last modified: 08/22/97