The fourth instrument cooldown was started October 29 and the first all-up cryogenic tests commenced on October 30 and continued to mid-November. Using the spectrograph focus stage all the spectrograph modes were focussed and excellent quality argon lamp spectra acquired in all four testable spectrograph modes (0.8-2.5 micron cross-dispersed mode, low-resolution prism mode, short wavelength single-order long-slit mode, long wavelength single-order long-slit mode). The guider array does not have a focus mechanism so the offset between the warm and cold focus is calculated and adjusted before cooling. A very good cold focus was measured. All the cold mechanisms were extensively exercised.
During the cold test the effects of flexure were measured by observing spectra of lamp lines in the spectrograph and the position of the re-imaged slit in the slit-viewer. The specification is for spectral movements of less than one pixel and re-imaged slit movements of less than one pixel on tilting from zenith to 60 degrees over. Flexure testing of the cold cryostat on the tilt-table revealed movements of up to 10 pixels in the spectra and up to 4 pixels in the re-imaged slit. Similar movements were measured once the instrument was warm. These movements were largely removed by stiffening the springs in the grating turret and slit wheel mechanisms, and by balancing the rotation of the grating turret and adding two supporting rollers (for a total of three) to the grating turret. Other modifications included re-assembling the focus drive with a new ball drive and delrin gear to reduce vibration and moving the spectrograph focus 2mm closer to the spectrograph lens to place the optimum focus in the centre of the focus range. By mid-December spectral movements had been reduced to 2 pixels in the east (telescope equivalent), 0.5 pixels in the west, 0.5 pixels in the north and 0.75 pixels in the south, tilting to 60 degrees from vertical. Movements of the re-imaged slit were all less than 0.5 pixels.
In December, installation of the calibration box on top of the vacuum jacket completed fabrication and assembly of the cryostat. Argon lamp lines from the calibration box were used in the flexure tests. The only fabrication remaining is for modifications to the IRTF's multiple instrument mount and counter-weight system so that SpeX can be mounted to the telescope.
Also in December, we took delivery of Aladdin 2 array SCA47672. We have concluded that this is the best of the 1024x1024 PAIDAI arrays and it will be used in the spectrograph. The Aladdin 2 512x512 array SCA46509 was chosen for use in the guider/slit-viewer and the other three 512x512 arrays were returned to the PAIDAI project. (For technical details of both arrays see previous quarterly reports.) At the close of the quarter SCA47672 was being tested in the lab test dewar at 30K.
|Argon lamp spectra taken with the 1024x1024 mux in the short wavelength cross-dispersed mode. Spectral lines from 0.8-1.0um are detected in the lower two quadrants of the mux. Longer wavelength lines fall in the upper part of the mux but are not detected since the mux cuts-off at about 1.0um.|
|Flexure testing of the cryostat on the tilt-table. The cryostat is shown tilted 60 degrees from vertical towards the equivalent of west if mounted on the telescope.|
|Another view shows the calibration box on the top plate of the cryostat.|
|A close-up view of the calibration box. A sub-plate containing lamps, integrating sphere, fold and illumination optics is at the left, the pick-off mirror and cryostat entrance window are at the top, and the CTI 1050 closed-cycle cooler and liquid nitrogen fill-port are at the right.|