The INT WFC Pointing Model

1. Single-star Calibration

This could be followed in case the observer has problems finding the target.

On the first night, the default pointing model should have been loaded in the initial TCS commands, and it is likely to be accurate. However, it is good to check it and make an additional correction with the following procedure.

First, check the lower shutter which should be in the low (default) position and point to a bright star (you can use one from the system catalogue.

SYS> agwin acq pos 1032 512 Sets readout to the centre of the autoguider chip.

SYS> autotv on Turns on continuous readout mode.

On the autoguider window mark a cross in its centre by selecting TOGGLE CENTRE CROSS from the SETUP menu.

USER> enter aperture 0 1065 -42 Changes the telescope pointing to the autoguider centre by redifining aperture 0.

USER> cal faint Sends the telescope to a suitable calibrate star.

When the HANDSET menu is enabled on the TCS window, then type <ctrl-z> on the TCS window to break out of it.

If the star appears right on the cross-hairs then the pointing is okay, and the rest of this section can be skipped. Otherwise, press the HANDSET button (INSERT key of the keyboard) to centre the star on the cross-hairs in the centre of the autoguider window. Use the arrow keys on the keyboard to move the telescope.

IMPORTANT: In case no star is visible in the autoguider window, check on the GUI if the camera shutter is open. Follow instruction about no stars seen in the autoguider window section of the troubleshooter.

Please note:

Given the orientation on sky of the autoguider field of view, arrows move the telescope in a way that can get the observer confused. So lower the moving increment rate on the TCS window to the lowest value and do a test first. Think of it as moving the centre cross to the star, and not the star to the centre cross.
Pressing an arrow key once moves the star by the increment size value shown on the TCS window, near the top. The increment can be changed by using the '<' and '>' keys.
After pressing a key, wait until the autoguider window has refreshed showing the new position.
In case the star is either too faint or saturated, then the exposure time can be changed by going to SELECT -> INTEGRATION TIME -> SELECT FROM MENU. If still saturating, considering using a narrowband filter, if there is any on the filter wheel.

Once the star is well centred, press HANDSET and type:

      USER> point calibrate           Writes the new pointing model to file.

      USER> cal anal zero             Loads the new pointing model

The rms of this solution is typically 0.01, because only one star is being used. Type 'Y ' to accept.

Prepare the telescope pointing back to the centre of chip 4:

       USER> enter aperture 0 -251 326

       SYS> autotv off                Turns off continuous readout mode.

IMPORTANT: If the pointing is still bad after the single star calibrate, then refer to bad pointing section in the troubleshooting guide. guide. First try zerosetting the telescope again, the default calibrate and then the 7 star calibrate if necessary.

2. Seven-star Calibration

For the ING SA supporting the first S/D night: the seven star calibration should be done always during the first WFC D-night. Please write down the resulting rms and send the value to the WFC instrument specialist.

For the observer: In case the single star calibration does not improve the pointing, then perform a manual 7 star calibration, here is the procedure:

 SYS> agwin acq pos 1032 512

 SYS> autotv on

 USER> enter aperture 0 1065 -42

(redefines aperture 0, i.e. the default pointing position, to the centre of the autoguider chip)

 USER> cal faint

(this runs the 7-star calibration script) The method to follow is exactly the same as that of a single star calibration (centering the star on the crosshairs, etc). However, instead of breaking out of the script using ctrl+z once the first star has been found, simply position it accordingly and press the HANDSET button to automatically slew the telescope to the next star. In case one star does not show up in the field (possible closer to the North pole where the pointing could be worse), then it could be ignored by typing "I". The script will then end after 7 stars have been centered. The resulted RMS should be smaller than 10 arcsec for a normal pointing model.

You will then be asked to accept the result. Follow with:

 USER> next

in order to re-centre the star using the updated pointing model.

Prepare the telescope pointing back to the centre of chip 4:

 USER> enter aperture 0 -251 326

 SYS> autotv off

This must be followed by the observed in case the one star calibration does not improve the pointing. Also, the seven star calibration must be followed by all ING students during the first S/D night of any new run. Please write down the RMS and report them to the instrument specialist.

The link perform a seven-star calibration include the procedure for calibrating the telescope pointingusng seven stars.

3. Full Pointing Test

We recommend to continue to run manualy the "Seven-star Calibration" test for at least 80 (ideally 100-120) stars.

This should be performed only by the ING staff or student support astronomers more rarely (about once every year) to re-measure the coefficients of the INT pointing model.

NOTE (Aug 2016): The next automate script does not work properly in case of big shifts in pointing (>10"), The script was built to observe a full grid of stars to make a pointing model is in the intics machine, in

SYS> pointing/autopnt_wfc

(click here for the ascii version)

It runs automatically and takes about 2 hours. When done, please inform the INT manager for the analysis of results.

The INT WFC Pointing Model

Contents

1. Single-star Calibration

2. Seven-star Calibration

3. Full Pointing Test