CFHT Information Bulletin, number 38, First Semester 1998

The creation of well registered, precision apertures in a 2-dimensional field mask has been done for several years at CFHT using a YAG laser coupled with computer controlled step motor stages¹. The apertures must be correlated to the objects of interest as seen in a direct image on the detector. This requires mapping the detector coordinates to the mask so that only the light from astronomical objects of interest will be collected. In order to minimize the time between taking the direct image and using the mask in the multi-object spectrograph (MOS), an efficient process and reliable equipment must be used. This has been the driving force behind several significant improvements. The latest hardware upgrade is described here.

Maintaining the mask material at the cutting laser nominal focus is of critical importance when making a mask that contains a sequence of several dozen apertures. In the current configuration of MOS the mask field is 69 mm by 77 mm. The foil is constrained by a clamping method around the circumference so that the foil is locked into the holder guaranteeing registration during the cutting process. This method of constraint has the advantage that foils can be easily and quickly inserted into the holder without over-constraint. The disadvantage of this method is that coil memory of the foil and gravitational sag due to an unsupported structure are not taken out. Consequently, as the foil is moved under the cutting laser, the operator is required to monitor and adjust the relative distance of the cutting laser focal lens assembly, if necessary, using a manual linear stage to which the laser focal lens is attached. To do this in real time while the foil is moving and the laser is cutting requires some practice and skill that a visiting astronomer may not have time to acquire.

To overcome the difficulties in manually focusing the cutting laser, a Selcom SLS 5010/100- YA laser displacement measurement system and a Newport DC-servo stage have been purchased and integrated into the existing focus stage assembly. Figure 36 shows the assembled system removed from the YAG laser. The laser displacement system (LDS) operates at 780 nm with a 100 mm stand- off and 10 mm measurement range. Laser light is generated and sent out one aperture of the LDS. If stand-off conditions are met the laser light will reflect off the target and enter another aperture of the LDS and be received by an internal Position Sensitive Detector (PSD). The PSD then relays information to the signal processor which, with the host processor, generates analog and digital outputs. The loop is closed when the analog signal is read by the controller and an error signal is generated from a standard P.I.D. algorithm. The resulting correction voltage is applied to the DC- servo stage which then moves both the attached LDS and the laser focus lens to bring the cutting laser back into a nominal focus. Figure 37 shows the attached DC-servo stage without the LDS. While this LDS to servo interaction is occurring the Galil controller is running a background task monitoring the LDS output to ensure that it is in a valid region of measurement. To protect the servo stage from runaway and the mask from poor quality cuts, the background task looks for several conditions: a valid stand-off signal from the LDS, the servo stage physical limits, and X-Y stage values that are appropriate for either a MOS or OSIS mask. If any of these conditions are not met, the controller suspends movement of the DC-servo stage until LDS, servo stage and X-Y stage conditions are restored to the valid region. For example, the homing sequence will always take the foil out of range producing a LDS stand-off invalid signal. Attempts to cut an aperture slightly out of the mask field of view might also create an invalid signal and suspend the servo loop.

Due to the optical nature of our black anodized aluminum the longer wavelength LDS coupled with its automatic surface reflectivity compensator make it ideal for our particular application. At wavelengths below 700 nm the aluminum foil is about 6% reflective to reduce the amount of scattered light within the spectrographs. However, at 780 nm reflectivity increases to 40% for the matte finish side of the aluminum and 50% for the shiny side. This is a decided advantage especially when working off the normal axis as is required by our mechanical setup. The 16 kHz sampling frequency and 2.5 micron resolution make the LDS far superior to manual focusing capabilities. Upon verification of the manual focus stage value, once the system is set up the operator is able to load a mask, start the cutting sequence from the computer and then walk away. Monitoring of the system can be done either from within the LAMA room or from the control room where two monitors are dedicated to showing both the magnified cutting spot and a general fixed view of the electronics rack. Progress, completion of the mask, or problems can easily be watched from these control room monitors eliminating the need for constant attention by an operator. Once this system is proven to be reliable, fine tuning of the cutting parameters will be done to produce high quality cuts in the least amount of time. This is our goal for the immediate future of the Nd:YAG mask cutting laser at CFHT.

The author would like to thank Jeff Ward for the excellent design and integration of the electronics. The effort towards making this project happen is greatly appreciated.

1. Bernard Di Biagio, Etienne Le Coarer and Gerard Lemaitre, "2D Mask generation by YAG laser for multi-object spectroscopy at CFHT : LAMA", SPIE Vol. 1235 Instrumentation in Astronomy VII (1990), pp. 422-427.