Beginning of the Night Lucifer Start-Up Procedure

Currently, the telescope pointing drifts by tens of arcseconds from day to day. Since the acquisition camera in the AGw has only a 25 arcsec field of view, these offsets can move the guide star completely off the CCD. This guide will explain what the OSA needs to do to correct for these errors and get Lucifer observers up and running at the beginning of the night. It requires the OSA to work in cooperation with the Lucifer observers in order to complete the following steps.

See the special instructions in BeginningOfTheNightLUCIFER for using two stars to correct the Azimuth Zeropoint (IA). These special instructions superced the general details described below.

Step by Step Cookbook

  • After the telescope has been readied normally, the enclosure is open, and any preliminary data (such as flat fields) has been taken, you will need to ask the Lucifer observer to send a preset to a pointing star in parallactic angle mode. It is important that the preset be sent in parallactic angle mode so that the coordinate system seen on the Lucifer camera lines up with that of the telescope (AZ-EL). This makes our correction of errors in the pointing model much easier. Preferably they should choose a star that is near their first science field (in order to save time slewing later), and neither too low or too high in elevation. A star between 40 and 80 degrees elevation would be ideal.

  • Once the telescope has arrived at the wavefront sensor star and the mirror has collimated, ask the Lucifer observer to take an image of the star.

  • The star will appear on the Lucifer read-out and it will have some position in the image. Ideally we want this star to be nearly perfectly in the center of the image, and this is what we will be trying to achieve. Ask the Lucifer observer to determine the x-y offset of the star from the center of the image (in arcseconds). See figure 1 below for an example of what it will look like.

+insert figure 1 here

  • Once the observer has told you how many arcseconds the star would need to be moved in x and y to the center of the image you will be able to determine how to modify the pointing model to center the star.

  • Open a command prompt on the TO Station, and type ptlist. This will display the current pointing model corrections that are being applied to the telescope pointing. It will look something like this: (disregard the numbers they will likely be different in your case)

* Pointing Model Terms *

IA -286.93580

IE -46.00000

NPAE 17.99860

CA -16.50000

AW 5.10060

AN 24.39890

TF 21.06980

  • The only two values that we care about in this case are CA and IE. CA is the pointing model correction (in arcseconds) applied as an azimuth offset of the optics relative to the EL axis, and IE is the correction being applied to the elevation axis. Now we need to determine by how much we need to change CA and IE in order to center the star in the image.

  • In the following figure we'll overlay the picture above with representations of the CA axis and the IE axis. As you can see CA is alligned along the image x axis, and IE along the y. However, CA becomes more positive in the negative x direction, and IE becomes more positive in the negative y direction. Though that may seem complicated, its actually quite easy.

+insert picture 2 here

  • In figure 2, you can see that the star is below and to the right of the center of the image. In order to center the star, we want to move the star up and to the left. To do this the camera needs to shift down and to the right. Imagine you are looking through your own camera at home, taking a picture of an object. If you want the object to move to the left in your viewfinder, you have to move the body of the camera to the right. This works on the same principle.

  • Now we'll add the values that the Lucifer observer gave to you (see figure 3 below). We'll say that the star is 16 arcseconds to the right of the center, and that it is 25 arcseconds below the center. Now since we want the star to move to the left and up, we want the camera to move to the right and down. Moving the camera to the right (positive x direction), corresponds to the negative CA direction. In essence, we need to make the CA more negative in order to move the camera to the right. By the same token, moving the camera down (negative y direction), corresponds to the positive IE direction. This means we need to make IE more positive in order to move the camera down.

+insert picture 3 here

  • From our above ptlist values IE = -46.0 and CA = -16.5. We've decided that we need to make CA more negative, so we will take the x offset value that our observer calculated for us (16 arcseconds) and subtract it from our current CA value. The equation looks like this: The CA we want = -16.5-16 = -32.5. By the same calculation our IE needs to be more positive so we add the y offset value from the observer to the current IE value. Like this: The IE we want = -46 +25 = -21.

  • Now that we know the values that we want IE and CA to be, we simply need to modify the pointing model to our new values. To do this, in a command prompt on the TO Station we will use the command ptmodify. So to change IE we would enter the following: ptmodify IE -21 where -21 is the value that we calculated. To change CA we would enter: ptmodify CA -32.5.

  • Now ask the Lucifer observer to take another image of the star. The star should now be centered in the image. Now that it is centered, the guider should be able to find it and guide on the star.

  • Ask the observer to send a preset for a wavefront sensor star in active mode. An active mode preset will attempt to guide on the star, and it will begin to take wavefront sensor images in order to correct the focus of the image.
Topic revision: r5 - 16 Feb 2010, JohnHill
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