ALP-5 Calypso WFCam Pointing Model Generation

Introduction

In addition to guiding, the WFCam Guide Camera is used to generate pointing models for the telescope. Pointing model generation uses a command file script and an add-on to the TCC software to automatically collect position data on a large (or small) number of stars. This script uses the TCC to instruct the telescope to slew to a the star nearest each point on a user defined grid. Once the slew is complete, the guide camera takes an image, locates the star in the field and computes the centroid location, which is transmitted to the TCC. The TCC uses the computed centroid position to compute the pointing error and write the data to a file which can then be opened with TPOINT and used to create a pointing model. The process takes approximately 1 minute per star so data for a large, dense pointing model can be created in a relatively short time. The process is very simple and reliable assuming that you have a reasonably good model to begin with so that the stars fall somewhere on the guide camera.

To use DOPTERR there must be an existing model which is good enough that a majority of the stars fall on the guide camera. For periodically refining existing models, this should not be a problem. If a major change has been made to the telescope (such as removing and replacing the primary or secondary mirror) then it will be necessary to take a manual set of pointing model data using the TOI. A total of 12-15 stars around the sky should be adequate for the preliminary model.

The DOPTERR proceedure also requires that the guide camera location and scale factor be accurately defined in the instrument block. The procedure for creating the instrument model is described in the TCC manuals. It also means that a camera without a guide camera (i.e. the HRCam) unfortunately cannot use this automatic data collection routine.

Instructions

Make sure that you insert the ND filter on the guide camera

The pointing model creation process uses FK5 Stars in the 5-7 magnitude range. These stars are much too bright to be used straight into the guide camera which was optimized to guide on stars 11th magnitude and dimmer. The result is that all stars in the pointing model catalog saturate the camera and are rejected by the guide camera software. In order to perform the automated pointing model creation it is necessary to insert an ND 2.0 filter in front of the guide camera. This filter reduces the intensity by the equivalent of 5 magnitudes so that very few of the FK5 stars saturate the camera.

1. Locate the neutral density filter. It is located in the base of the tool chest in a small black box.
   
   
   
   
2. With the rotator in the park position, loosen the two screws which clamp the guide camera tube into the WFCam body (Figure 1).
   
   
   
3. Carefully slide the guide camera upwards until the tube slides out of the mount.
4. Slide the neutral density filter into the Guide Camera Tube with the three clamp screws away from the camera so that they can be accessed for tightening. The Neutral density filter should be inserted until it is just flush with the end of the tube.
   
   
   
   
5. Tighten the three set screws to secure the filter in position.
   
   
   
   
6. Insert the guide camera back into the WFCam body until the bottom of the Focus Guide label is flush with the mount. The guide camera should stop at this position allowing the two set screws to be tightened.

To remove the filter, reverse the above procedure.

Start the telescope

The next step is to start the telescope. Go through the normal axis initialization procedure and open the mirror cover. Make sure the the WFCam is the selected instrument and test the guide camera to make sure that it is functioning properly. Slew to a convenient object and make sure that it is in the field and that the rotator is tracking. Once you have verified that the telescope is operating properly, halt the axes.

Run DOPTERR

The next step is to run DOPTERR. During this procedure the telescope will be controlled completely by the TCC. The TOI can be left running and it will display the status of the telescope, but no interaction with the TOI is necessary. If for some reason it becomes necessary to halt DOPTERR, press < ctrl >c in the DOPTERR window to kill the process.

Now sit in front of the Sun workstation and type the following

• Open an xterm window.
• $ telnet Charon and log in as TCC.
• $ set def tgrid
• Type DIR and select a GRID file. The last number in the file name indicates how many points are in the grid. If you TYPE /PAGE the file, the top of the file has a description of the grid points.
• $ dopterr tpterr:pmod_ddmmmyy.dat
• DoPtErr_DoPause [false] false
• DoPtErr_DoRetry [true] false
• DoPtErr_MaxAltForCorr [80] 80
• DoPtErr_NExtraCorr [0] 1
• DoPtErr_SlewPause [2] 10
• DoPtErr_OffPause [2] 10
• Az/Alt grid file?TGRID:< gridfile name>

Thats all there is, now go have a cup of coffee and it will be done in a couple of hours. You can watch the telnet window and it will display the progress.

Creating the Model

Once the data collection is complete, it is time to create the model. These instructions will show you how to create a quick model. If you want a more accurate model, then you will have to read the instructions in TPOINT and try different coefficients.

• $ set def TDAT
• $ TPOINT
• * GETDAT tpterr:pmod_ddmmmyy.dat
• * call calypso
• * plton
• * Enter Graphics Terminal Name? TEK_4010
• At this point open a TEK window and switch to TEK mode
• * call A9 {This will plot out the stars and their errors after the fit of the model
• Print the model
• Switch back to VT mode
• * outmod TDAT:WFTELMOD.DAT
• * quit

That is it, the new WFCam model has been created.

The above procedure uses a very basic pointing model and it is good if you have a small (less then 50 stars) data set. If you have a large data set then it is recommended that you replace the "call calypso" command with "call calypsowfcam" command.

Creating an HRCam model

It is possible to create a useable (but not optimal) HRCam model with a simple modification to the WFCam model. To create an HRCam model from the WFCam model, copy the WFTELMOD.DAT file to HRTELMOD.DAT.

$ copy WFTELMOD.DAT HRTELMOD.DAT

Next you must edit the HRTELMOD.DAT file by adding 47 arc seconds to the IE coefficient and 25 arc seconds to the IA term (these are the zero offset terms for the elevation and azimuth encoders). If you are familiar with the VMS editor you can edit the file in your xterm window using;

$ edit HRTELMOD.DAT

If you are uncomfortable with the vms editor, ftp the file to the SUN and edit it and then ftp it back to the Alpha.

Installing the model.

There are two ways to install the new model. You can park the telescope, exit the TOI and then restart the TOI. This will cause the TCC to re-read the pinting model files. If you do not want to quit the TOI you can install the model immediately. First, make sure the telescope is not tracking or slewing by pressing the STOP botton. All axes should report "Halted" in their status windows. Next, open a TCC command window and enter the following command;

SET BLOCK TELMOD/IN=TDAT:WFTELMOD.DAT

or, if you are running the HRCam

SET BLOCK TELMOD/IN=TDAT:HRTELMOD.DAT

That is all there is to do. The new model is has been installed.

NOTE: The SET BLOCK TELMOD command is run automatically each time you switch instruments so you only need to enter this command for the instrument currently in use.