Science Operations

Tech Chip Guiding and Focus Adjustments

Unlike the Gregorian instruments, the LBCs do not have off-axis guiding and wavefront sensing (AGw) units. Instead, the LBCs are focused and collimated through the analysis of pupil images obtained with the science detector after each slew and approximately every 30 minutes (see LBC Active Optics), and guiding is done using one of the off-axis technical chips (tech chip 1). The other technical chip (tech chip 2) is used to make focus adjustments between each exposure. The 256 x 2048 pixel technical chips each cover a ~1 x 7.8 arcmin field and their position in the focal plane is described here.

Since the technical chips are behind the same shutter and filter as the science chips, (1) guiding and image analysis can only be started after the science exposure has begun, and (2) brighter stars will be needed for observations through lower throughput filters: the U filters (U-Bessel, SDT_Uspec), Y-FAN and the intermediate band TiO, CN and F972N20 filters.

Guiding with the LBC

Guiding is done only for exposures longer 40 seconds. Upon starting the science exposure, and after a few second delay, an 8 second exposure is taken with this chip, and it is automatically searched for stars, using the SourceExtractor software with rather stringent selection criteria (detection and analysis thresholds of 10 sigma). If no stars matching these criteria are found, the exposure time is doubled; this continues until at least one star is detected or the maximum guiding exposure time (64 seconds) is reached. When a number of sources are detected, additional criteria on their shape (SExtractor flag < 10), flux (between 5 and 60,000 counts) and magnitude are imposed, and they are ranked according to signal-to-noise ratio, SNR. Only stars with SNR >= 28 are used.

The guide star thumbnail image: The LBC User Interface displays a thumbnail image of the brightest guide star and reports the FWHM at the bottom of the display. Note that before a guide star is selected, the thumbnail which is displayed is the latest image in the buffer; this could be from the end of the previous night. Once a new set of guide stars is found, this image will be replaced by the current star image. When no guide star is found, a question mark replaces the guide star image. For the Red camera, the guide chip is slightly out of the focal plane and the observer should expect the reported FWHM to be larger than for the science images, by about 0.2″.

Binocular Guiding

During binocular observations the guide corrections are automatically optimized to offload as much as possible to each primary mirror before correcting the mount. Given that the technical chips are behind the shutters of each channel, and that both channels may not be exposing in sync, it is probable that at times only one side will be guiding. This has not produced any issues with image quality.

Tech chip focus corrections

Technical chip 2, which is used for image analysis, has been positioned 0.8 mm below the focal plane to produce extra-focal pupil images which are automatically stacked and analyzed to yield focus corrections which are sent to the primary mirror once the exposure is over and the shutter has been closed. This correction is relative to first stacked exposure, so if you only take one science exposure no correction is made. Both technical chips must use the same integration time, and the integration time on tech chip 2 is determined by that required by the guide star selection process. The image analysis works automatically and currently does not produce obvious feedback to the observer. To verify that it is running and sending corrections to the primary, the observer can check the value of Z04 in the LBC logfile, and the telescope OSA can monitor the value of Z4 in the PSF GUI.

The routine which analyzes the pupil images on tech chip #2 selects candidate pupils following the criterion that these have several contiguous pixels with values with greater than Flim=50 counts above the background level. The exposure time used, texp, is typically 4 seconds (now it is 8 sec). The pupils are annuli with dimensions of approximately 50 pixels for the outer diameter and 7 pixels for the inner diameter. With these assumptions, we estimate that the limiting magnitude for stars whose pupil images will be analyzed are:m(i) = -2.5 log10(Flim*Area/texp) + ZP(i) = -11 + ZP(i)

This gives the values listed in the table below.

Limiting Magnitudes for Tech Chip Guiding and Focus

Filter Guiding Focus1
mlim in 4 sec msat in 4 sec mlim in 4 sec
SDT_Uspec 18.5 11.3 16.3
U-Bessel 18.2 11.0 15.2
B-Bessel 20.5 13.4 16.9
V-Bessel 20.8 13.8 17.1
g-SLOAN 21.2 14.3 17.3
r-SLOAN 20.5 13.5 16.7
Filter Guiding Focus1
mlim in 4 sec msat in 4 sec mlim in 4 sec
V-Bessel 20.8 13.8 16.9
R-Bessel 20.6 13.8 16.9
I-Bessel 19.9 13.1 16.6
r-SLOAN 20.7 13.8 17.0
i-SLOAN 20.1 13.3 16.6
z-SLOAN 19.0 12.6 16.2
Y-FAN 17.8 13.2 13.6
F972N20 17.0 9.6

1. These estimates were determined according to the equation given in the text which precedes the table. These were checked against technical chip data on which at least one pupil was found, and are consistent if not a few tenths of a magnitude conservative.