Instruments & Operations



Preparing for Observations with LBC

This page contains information that is needed to prepare for observations with the LBCs.

Observing Block Files
Binocular vs. Single-Channel Observing
Considerations for Automatic Guiding and Focus Corrections
Elevation constraints

Observing Block Files

Observations with the LBC are carried out through scripts called Observing Blocks (OBs). OBs should be prepared before arrival at the telescope and submitted to the partner coordinator for review. The latest version of the software to prepare OBs was released on 6-April-2011 and is available from the LBC team home page.

A guide to using this software to create OBs is here

Back to Top

Binocular vs Single-Channel Observing

While observations are usually carried out with both LBCs simultaneously (binocular mode), one or the other can be used singly. This can be indicated in the OB, by editing the CCD parameters to turn OFF the detector on the channel that will not be used. However, in cases where single-sided observing may not have been anticipated and binocular OBs have already been created, the binocular OB may be used, but before running it, the observer should do several things:

Back to Top

Some considerations to allow and aid Automatic Guiding and Focus Corrections

There are two 256 pixel (512 but 256 rows are masked) x 2048 pixel technical chips on either side of the science array. Technical chip 1, next to science chip 1, is used for guiding and technical chip 2, next to science chip 3, is used for image analysis. Both technical chips are behind the same shutter and filter as the science chips. This means, first, that guiding and image analysis can only be started after the science exposure has begun, and, second, that brighter stars will be needed for observations through lower throughput filters: the U filters (U-Bessel, SDT_Uspec), Y-FAN and the narrow band filter F972N20.

This section starts with brief descriptions of how guiding and focus corrections are done with the technical chips in the LBCs and concludes with basic data needed to plan observations to enable guiding and active optics (focus) corrections.

Guiding with the LBC Guiding is done only for exposures longer 40 seconds. Upon starting the science exposure, and after a few second delay, a 4 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). If no stars matching these criteria are, 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, and the brightest is chosen to be the guide star. I adopt the criterion that SNR = 28 on a 4-sec exposure and use the exposure time calculator to predict, in each filter, the faintest stars which would be selected as guide stars. These limiting magnitudes will be deeper (by ~1.12 mag) for a 32-sec exposure.

The guide star thumbnail image: The LBC User Interface displays a thumbnail image of the 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 guide star 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".

Automatic Focus Correction The technical chip 2 which is used for image analysis has been positioned 0.8 mm below the focal plane to produce pupil images which are automatically analyzed to yield focus corrections which are sent to the primary mirror once the exposure is over and the shutter has been closed. 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 P03 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. 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 Image Analysis
filterguidingimage analysis1
mlim in 4 secmsat in 4 secmlim in 4 sec
filterguidingimage analysis1
mlim in 4 secmsat in 4 secmlim in 4 sec

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.

Adjusting dither positions:

The 256 pixel x 2048 pixel technical chips cover a rather small area of the sky (1 x 7 arcmin). Plan your dither sequence to place sufficiently bright stars on the guiding and active optics chips. The best plans will go awry if you do not correct the pointing before observing this field. It is highly recommended to correct the pointing offsets (CE and CA) and the co-pointing with the LBTtools.Observe task,lbcrangebal on a nearby pointing star before observing the field of interest.

The following tools are available to help the PI/observer select dither positions which will put bright stars on the technical chips:

Interactive Pointing function of the OB GUI: The OB preparation GUI has an Interactive Pointing tool which displays a DSS image and the outline of the LBC focal plane for each dither position. The active optics technical chip has a border region denoted by a lighter color blue or red (In the latest version of the GUI, GUIv2.0_20110406, this shaded region appears instead on the guide chip). No stars should be positioned in this region, since the ~ 45-pixel diameter pupil images will not be completely within the detector field of view.

An LBC template for ds9:

An LBC template for the four science and two technical chips is being developed for ds9. Not all regions of coordinates and position angles have been tested, but this template, LBC.template, is provided here to give information on the relative positions of the technical chips with respect to the science chips. This template was developed for the Blue Channel - for the Red Channel, the AO chip (tech chip #2) appears to be about 3" lower, with respect to the guide chip, than it is in the Blue Channel. Avoid selecting guide or active optics stars close to the edges of this template.

This template is for RA=0, DEC=0, and each line defines a ds9 box, with the columns: Xcenter (degrees), Ycenter (degrees), Xwidth (arcsec), Ywidth (arcsec), position angle. The first line corresponds to science chip #2, the second to science chip #1, the third to science chip #3 and the fourth to science chip #4. The size and position of technical chip #1 (guiding) is described on the fourth line and of technical chip #2 (active optics) on the fifth line. The technical chip centers are approximately 30" above the center of chip 2 and 13.89 arcminutes (833.5") to the right and left.

Binocular Observing: Important considerations when observing with both LBCs simultaneously:

Back to Top

Elevation constraints

Observations can be carried out at all elevations, however the effectiveness of the derotator very close to zenith, inaccuracies in the pointing and co-pointing models, and the mirror travel limits can compromise the image quality of observations made at extreme elevations.

At very high elevations, elevation > 85 degrees, residual rotational trailing has been seen in images. This may be partly due to pointing inaccuracies. Poor pointing translates to an inaccurate calculation of the rotator trajectory which may cause images trailed in rotation. Before observing a target near the zenith, correct pointing and co-pointing, by slewing to and observing a nearby pointing star and running lbcrangebal in the LBTtools.Observe package on the resulting images. More information is available in the LBC User Manual in the section on Beginning-of-Night Tasks