WIRO Cassegrain Guiding and Acquisition Module (GAM)
GAM electronics here
The WIRO Cassegrain GAM is used to acquire an astronomical target and position it onto the fiber-optic integral-field unit (see figure below). A motorized slide within the GAM enables the user to position one of three pick-off mirrors into the beam. These include: 1) a solid mirror for target acquisition, 2) a pellicle (92%/8%) beam splitter, and 3) a perforated mirror for offset guiding while the light of the target goes to the fiber-optic array. A small SBIG-237 CCD camera is mounted on an x-y stage on the side of the GAM and can be used to either acquire the target object using the solid mirror or for guiding with either the pellicle or the perforated mirror. A calibration assembly is mounted opposite the guide camera such that light from the calibration lamps is imaged onto the fiber array via a small mirror cemented onto the back side of the larger solid mirror. The fiber optic array is located beneath the 3-mirror stage and can be rotated to any desired position angle. Both the fiber optic array and the guide camera are located behind a 0.33X focal reducer such that the 100 micron fibers of the array correspond to 1.0 arcsec on the sky. The right-hand figure shows the fiber-optic array viewed through the focal reducer. The resulting scale is 9.96 "/mm corresponding to 0.07 arcsec/pixel and a field of view of 46 X 34 arcsec with the ST-237 CCD. However, the GAM x-y stage (see figure) has a range of +- 50 mm such that the searchable field with the GAM is a little over 17 arcmin on a side.
The motorized, 3-mirror slide of the GAM is designed around 2 precision, linear slides manufactured by Velmex Inc. The x-y stage and the GAM superstructure were designed by Mike Pierce and Phillip Haynes of the UW Physics and Astronomy Department with help from Steve Hodder of the A&S machine shop. Steve was also the chief machinist for the project. The electrical cabling was done by James Weger of the Physics and Astronomy Dept. The figure below at left shows the mechanical layout of the GAM. The figure below at right shows a close-up of the 3-mirror slide.
Once the telescope is positioned to the coordinates of a target the final positioning is done using the SBIG-237 camera. This is typically done using the #1 (solid) mirror using a series of exposures to determine the position of the target and then determining the offset necessary to place the target onto the center of the fiber optic array. Once the telescope is positioned the GAM slide is positioned to bring the #3 mirror (with the central hole) into position such that the light from the target reaches the fiber optic array. In this configuration the light from the surrounding region is diverted to the ST-237. The x-y stage can then be moved in a "cruise mode" (see below) until a suitable guide star can be found and guiding begun. Alternatively, the GAM stage can be positioned to the #2 position (the 92%/8% pellicle) and the guiding can be accomplished using the object itself provided it is sufficiently bright. The integral-field, fiber optic array is mounted on a rotational stage located below the 3-mirror slide and behind a 0.33X focal reducer. The position-angle of the array can be chosen if desired and positioned accordingly.
The graphical user interface (GUI) was designed and programed under Visual Basic 6 by M. Pierce and P. Haynes. The interface communicates over ethernet to a Compumotor 6K8 stepper motor controller. The figure below shows the GAM GUI. The left-hand portion of the GUI is associated with the GAM and the right-hand portion of the GUI is associated with WIRO-spec, the optical VPH spectrograph. The top-most text box shows the IP address of the 6K8 controller. The next set of buttons are used to connect/disconnect to/from the CK8. The right (GAM) side has buttons for "jogging" the X-Y stage and a text box is included for setting the default step size. At present 10,000 steps corresponds to roughly 10 arcsec. To the right of text box is a button for bringing the X-Y stage back to 0,0. This is essential for acquiring the next target since otherwise the ST-237 will not be aligned with the fiber optic array. The next set of three buttons allow the GAM mirror stage to positioned to one of the three positions. Note that the current position is highlighted in green. Below that are two buttons that allow the fiber optic array position angle to be set to any desired position.
The three-position stage is positioned using the buttons on the GAM interface. Note that the light from an object only reaches the fiber-optic array using the "full" or "pellicle" positions and that calibration exposures can only be taken with the mirror stage in the "full" position. The latter position allows the light from the calibration lamps to be reflected off of a small mirror mounted on the back of the full mirror and then onto the fiber optic array. If the target is sufficiently bright, guiding can be accomplished using the light from the object itself via the "pellicle" position. However, it is expected that in most cases the object will be sufficiently faint that this will not be possible and a suitable nearby guide star will be needed. In this case, the object exposure will be acquired with the "thru" mirror and the x-y stage positioned such that a suitable guide star for the ST-237 can be found. Note, before moving to the next program object be sure to move back to the "full" mirror or it may be very difficult to locate the object since the light will now be going to the optical fiber.
The guide camera consists of an SBIG ST-237 CCD camera. The camera is located behind a second 0.33x focal reducer which produces a scale of 9.96 arcsec/mm. The 7 micron pixels of the ST-237 are thus 0.07 arcsec and the field of view is approximately 46 X 34 arcsec. However, the range of the x-y stage is ±50 mm giving a searchable field of nearly 19 arc minutes. The step size of the x-y stage motions can be adjusted by the user but the default of 10000 steps corresponds to about 10 arcsec. Note that a +x motion moves the stage East (the star West) while a +y motion moves the stage North (the star South). The ST-237 is operated in a "guide mode" using a commercial software package (Maxim DL) which provides a guide signal based upon centroiding of the stellar image. The guide signal consists of four simple TTL signals representing ±x-y offsets which are then fed to the WIRO control system via the hand-panel input. Currently there is no need to use the X-Y stage as it tends to be problematic. The pellicle is used to take spectra and the the guider uses the object itself to guide on.
Using the GAM
The calibration lamps are contained within a light-tight housing mounted on the "back-side" of the GAM (opposite of the ST-237 CCD). The currently available lamps include PHOTRON CuAr ThAr and Neon These are projected onto the fiber-optic array using a set of lenses and a small mirror mounted on the rear of the solid mirror. As a result, the solid mirror position must be selected from the GAM interface in order to use the calibration lamps. The desired lamp can be selected using the buttons on the GAM interface. The figure below shows the open calibration housing.
1) When ready to take a comparison
exposure begin by selecting the GAM "full mirror"
2) Select the desired calibration lamp using the appropriate buttons on the GAM interface.
3) Take an exposure with WIRO-Spec and adjust the exposure time as necessary.
4) When finished, be sure to turn off the calibration lamp and move to “pellicle” before taking any data!
Guiding with the
Once the object is positioned onto the optical fiber and the mirror stage is positioned in the "pellicle" position the user is guide on the object. The procedure is as follows:
1) In MaximDL Select the "guide" mode with an exposure of at least 10 sec, with Autodark selected and binned 3x3. Be sure to enter the declination of the object.
4) Once a full frame is taken, position the centroiding box on the selected star and hit "start" to begin guiding.
5) Verify that the guiding is satisfactory and then begin the object exposure with WIRO-Spec.
For more details see Guiding with the ST-237.
Last Modified 3/2/2006
Michael Pierce (email@example.com)