LBC Scripting Basics

1) Introduction

2) Collimation and Co-pointing OBs

3) Guiding

4) Science OBs

5) Calibration OBs

 

Introduction

Observing with the one or both Large Binocular Cameras at the Large Binocular Telescope requires scripts that co-point and collimate the telescopes and then configure and execute a sequence of observations of the science target. All scripts or Observing Blocks (OBs) for LBC are in xml format and can be produced using the Observing Block Creation GUI provided by the LBC team.

A submission to the Partner Coordinator or the observatory should include:

  • A co-pointing OB (yourCopoint.ob)
  • A finding chart of the co-pointing target
  • Science OB/s (yourScience.ob)
  • A finding chart of your science field

Detailed instructions on producing valid copointing OBs and finding charts that can be used for both collimation and copointing are detailed below.

Science OBs should be 30-40 minutes in length, after which time the telescopes will need to be collimated again. For most programs science OBs will only need to specify Observing information, RA, Dec, PA, dither pattern, filter, exposure, and number of exposures. This page offers a step-by-step guide for how to use the OB GUI to specify this and other information.

Co-pointing OBs

Since LBC operates in a fully binocular sense it is critical to minimize the demands on the guide loop by making sure the mount correctly knows where it is pointing, ensuring the two sides are co-pointed, and collimating both mirrors. This can be achieved by allowing for a correction of the pointing and co-pointing, along with the collimation, near your science target field.

Select an easily identifiable (isolated, 10-12mag) source with proper motion corrected coordinates as the “co-point” target that you preset to in the co-point OB. This can be your science target itself if it is visible, or a star in the same field, or if your science target is significantly extended it could be a nearby field.

Prepare yourCoPoint.ob with the following parameters:

  • Choose a target that meets the following criteria:
    • Known, proper-motion corrected, coordinates
    • Unresolved point source (stars)
    • Field is uncrowded (not much contamination from overlapping pupils)
    • Field is free of significant background gradients (large galaxies, nebulosity)
    • Example of a bad field for DOFPIA would be a large galaxy like M51 or the heart of a Globular Cluster
  • Read full CHIP2 ccd, the others are not needed
  • This is NOT a focus OB!
  • 1.0 sec exposure
  • Same PA as your science target
  • Use r-SLOAN (Red) and V-BESSEL (Blue), same as dofpia

Make a finding chart (5’x5′ from DSS is fine) identifying the source

Co-Point OB

A suitable coordinate reference can be any easily identifiable, relatively isolated, source with accurate coordinates. Almost any star in the USNO-B1.0 catalog would do. In crowded fields it would be a good idea to provide a finding chart as well so that identification of the correct source is assured. Because the coordinate reference source is relatively bright, only a short exposure is needed for the pointing correction. This co-point OB should be set up with the parameters listed in the summary above.

Note that very crowded (i.e. Galactic plane) fields can be problematic for the collimation script as the out-of-focus pupils overlap. Select your fields carefully.

 

Lbcrangebal

Once collimated, re-send the yourfocus.ob and allow it to complete. An iraf script is available in the LBTtools/Observe package called “lbcrangebal” to do the optimal pointing and co-pointing corrections. This script takes as input the LBCB and LBCR filename time-stamps for the images taken in the co-point OB. It will read from the telescope control software two current pointing model parameters (IE and CA), and from the image headers the pixel coordinates of the mechanical rotator axis’ intersection with the telescope focal plane for both cameras. Tip-tilt requests to the two primary mirrors and an offset to the pointing model are calculated in a manner that maximizes the available range of the primary mirror while simultaneously correcting any common offsets from the pointing model with an offset to the mount pointing model.

5) You do not need a focus OB, only a co-pointing OB. It is a very simply OB to create, you only need a 1s exposure, the r (red) and V (blue) filters, chip 2, and a star with a well-known coordinate.

http://wiki.lbto.org/bin/view/PartnerObserving/LBCCopointAndCollimate

I’ve included an EXAMPLE of a copointing script (EXAMPLE_CPOB.ob) — you can simply change the coordinate field to the exact RA and DEC of a relatively BRIGHT source near your targets. You also should include a small finding chart for this object. Please note that the center of rotation/pointing center of LBC is in the center of the ARRAY, which is near the upper 2/3 of chip 2.

 

Collimation

Dofpia

Send the yourCoPoint.ob but interrupt it because you only need to send the preset to the TCS and do not need to take the exposures with LBC yet. Collimation needs to be done before the pointing correction as the collimation process for LBC is not done in a pointing-free manner in order to preserve range of motion of the primary mirrors. Use the IDL procedure dofpia to iterate on the collimation using extra-focal pupil images obtained by driving the primary mirrors -0.8mm down (away from LBC). Note that DOFPIA assumes that your collimation field contains only uncrowded stars and the background is reasonably uniform. Fields containing any significant nebulosity or a large galaxy are unsuitable. Any nearby (within a degree or so) field can be used that satisfies DOFPIA’s assumptions. Run dofpia with the /X2 switch to double the usual exposure time to 32 seconds (this will be made the default at some point). The script takes an exposures with both cameras and reads out only the top ~5 arcmin of chip 2, finds relatively isolated pupil images of sufficient signal-to-noise ratio and derives corrections to the wavefront. After waiting for the corrections to be applied, dofpia will check the corrections versus convergence criteria and iterate as necessary.

At the beginning of the night, and any other time you are out of thermal equilibrium dofpia will struggle to collimate the telescopes. Once things have settled down, dofpia will converge quickly at each new field, generally within two or three iterations.

 

Guiding

You do not need to select a guide star ahead of time, although it is often wise to visualize your field to be sure a guide star is on the technical chip which does the guiding. This is easily accomplished by running a few scripts to visualize your OBs. These scripts assume that you are running ds9 and perl.

First you can run the createregions1.pl script on your ob to produce region files for ds9 for all your offset positions:

./createregions1.pl nameofscript.ob

The region files will appear as: nameofscript_ob_0.reg

The if you run:

./ndds9.csh RA DEC

where coordinates are in standard sexagesimal format (i.e. 00 01 45 -15 20 45). DS9 should open with an image centered on the given coordinates and a catalog of stars labelled. You can overplot the LBC FOV using the files created by createregions1.pl by selecting in the ds9 menu: Regions –> “Load Regions”. A pop-up window will appear and you can then choose your .reg files.

 

Calibration OBs

 

A set of generic calibration OBs have been collected and are available in the tar file, Calib_OBs.tar, which you may download and edit as necessary. The top directory, Calib_OBs, contains the following subdirectories. README.txt files within each subdirectory describe the contents. These are linked below for convenience.

  • BIASDARK: OBs to take a series of biases and a series of darks. Note that these cannot be combined in one OB, because there is only one IRAF ‘imagetyp’ (OBJECT, FLAT, BIAS, DARK,…) allowed per OB. BIASDARK_readme

  • FOCUS: two OBs used to obtain extra-focal pupil images. FOCUS_readme

  • SKYFLAT: Observers should generate sky flat OBs using the mkskyflat script, available in this Mkskyflat1.tar file and available on most summit computers. For more information on how to use mkskyflat.pl and the approximate exposure time scaling between filters, see the mkskyflat_readme file.

  • STANDARDS: for standard stars. STANDARDS_readme

  • SUPERFOC. These ‘superfoc’ OBs step through focus and are usually not used during observing, unless the filter focus offset needs to be re-determined. SUPERFOC_readme

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