Considerations for OB Preparation

Correct Filter List in LBC OB-generating GUI

The latest release of the OB GUI (21 November 2007, available at and click "Binaries" which will start a download of the zip file: has the correct filter names.

Set OBSERVER to lbtsdt (must be lower case), PROPID to the PI name

Because of the way the archive access is structured, all of the OBSERVER flags in the OBs need to be set to lbtsdt. Also, to aid in sorting through the archive we recommend that the PROPID be set to something unique for your program (many are using PI_Name).

NOTE for Extended Source Targets

The normal procedure we have been using for relatively empty fields is to use the science OB to send the preset and then abort the remainder of the OB. When on target, we have been using a generic OB for obtaining the image necessary for active optics analysis. For science targets of potentially significant extent (M81, NGC objects, nearby galaxy cluster cores, nebulae) we recommend that you choose a nearby field with stars of sufficient brightness (~10-13 mag) for the image analysis and generate an OB specific for your project to take the image needed for active optics correction.

The code is pretty flexible on the star magnitudes and will automatically select everything unsaturated, with sufficient SNR and not confused with adjacent pupil images. The majority of random 'blank' high-latitude fields typically have one or more usable stars. Avoid placing any bright extended sources on the upper half of chip 2, near the center of the mosaic. The active optics field needs to be close to the science field (only a small delta-elevation between the two fields) to remain well-corrected on the science field.

We can continue to run with the current procedures for the January SDT run, but this would be a useful modification for future SDT runs. This OB should be have the following parameters set (at the bottom of this page a template Vfastextra.ob is available to download; you need edit only the lines below TEC_RA and TEC_DEC to contain the RA and DEC, both in decimal degrees, of the desired pointing).

Parameter Value
Class Type FOCUS
RA & Dec coords for your chosen star field.
Y region 3201 - 4608
Filter V-Bessel
Focus start pos -0.80 mm
N exposures 1
PROPID as appropriate for your project (see above)

Run active-optics (AO) correction procedure every half-hour.

The collimation and focus of the primary mirror and LBC are most strongly correlated to changes in elevation, but also include a temperature term. In rapidly varying conditions you may want to consider running the AO recollimation and focus procedure more frequently than 30 minutes. Conversely, in stable conditions or meridian crossings where the elevation is changing only slowly, it can be run less frequently. LBTO currently recommends that the AO recollimation be run every half-hour as well as any large slew to a new target field. The easiest way to maintain this pace of recollimation and refocus is to design your OBs to execute in about 30 minutes.

Keep individual exposure times to ~5 minutes.

This is also tied closely to changes in elevation. Collimation and focus are corrected by a look-up table in the PSF sub-system. However, these corrections are only applied at the beginning of an exposure. Generally, a change in elevation of only ~1.5 degrees is sufficient to notice aberrations in the image due to mis-collimation and defocus. Thus, LBTO currently recommends that exposure times be kept to a maximum of 6 minutes in any filter. Tracking fields straight east or west, where the elevation is changing most rapidly, is where you need to be the most careful. Observations during meridian crossings, or of fields at high declination can relax this exposure time limit.

Note that the 20-minute U-band exposure was taken at a meridian crossing where the elevation was only changing slowly. But it would be less likely any queue-scheduled SDT observation could be so closely timed.

Avoid observing above an elevation of 80 degrees

There are currently some unknown rotator tracking problems that have been seen at high elevation. They are being worked on, but for at least the January 2007 SDT run it would be best to restrict all observations to elevations below 80 degrees.

-- DavidThompson - 03 Jan 2007, -- JohnHill - 09 Jan 2007
Topic revision: r8 - 07 Jan 2008, OlgaKuhn
This site is powered by FoswikiCopyright © by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding Foswiki? Send feedback