The guide camera on the WFCam instrument has two purposes. First and foremost is to act as a guide camera for the Wide Field Camera. In the guiding mode, a Guide On command is issued from the TOI to the TCC. The guide camera then takes an image and looks for stars. Assuming that stars are found, it then attempts to fit a Gaussian to each star, compute their centroids and report this information in order of brightness to the TCC. The TCC then selects a guide star and instructs the guide camera to take a continuous series of images. For each image the location of the centroid of the selected star is calculated and reported to the TCC. The TCC compares the centroid location of the current image to the original image and then computes the offset and applies a correction to the telescope servo's. The process continues until the GUIDE OFF command is issued. Images are stored in he Guider Images directory on the Guide Camera PC. This directory is cross mounted onto jupiter where the images can be viewed using either the TOI guide image viewer or IRAF and XIMTOOL.
In addition to guiding, the WFCam Guide Camera is used to generate pointing models for the telescope. This is done using a script on the Alpha computer. 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.
Currently, cross mounting the guide images on jupiter requires running the script "mgcam" This script will ask for a the root password. If you need the root password, please see the observatory manager. This is obviously a security problem and we hope to eliminate the need for the root password soon.
Once you have run the mgcam script, you can find the images in /mnt/gcam/.
To Be Written
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.
As of this data (June 27, 2001) the model created using the WFCam Guide Camera cannot be used for the HRCam. There are some differences between the pointing between the two sides which make the models slightly different. We hope find a way to adjust the model so that it can be used on both sides but for now, we need to use separate models.
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.
To collect pointing error data you need to log into the Alpha computer and issue a command at the command prompt. But before you do this, you need to get the telescope up and running using the TOI. I recommend that you start the TOI, switch to the WFCam, slew to any object in the sky and start the instrument rotator. Take an initial image to make sure that you are close to the object you selected. Once you have done this preparatory work you are ready to start data collection. To do this, follow the following instructions. You can also take a look at the TCC documentation, the link for which is shown below but it is slightly out of date.
telnet charon" and log in as TCC. If you need the password, see the observatory manager.
set def tgrid". This directory contains the default grid files used for data collection. The names are somewhat descriptive of the form of the grid but you should take a look at them using the "type <filename>" command. Look at the top of the file and it will tell you how many points are in the catalog.
set def tpterr).
dopterr". You will have to answer a number of questions, most of which are obvious but make sure that you tell dopterr to pause 10 seconds before the next exposure, otherwise you will get streaked stars. Write down the name of the pointing error file which will be given to you. You will need this later to compute the pointing model.
getdat instead of indat and then follow the procedure.compute the pointing m