With the installation of an IR array provided by CTIO, both the optical and IR channels of the Ohio State University-supplied camera ANDICAM on YALO are now functioning. This enables simultaneous optical and infrared imaging, including dithering in the IR channel while the optical channel integrates. However, ANDICAM still has only half of its CCD working due to amplifier problems. As a result of providing the array, a nominal engineering-grade device obtained as part of the OSIRIS upgrade, the CTIO observing time share with YALO + ANDICAM has increased to 27.5% and some access to the Galactic Bulge is now available. See the accompanying article for details and a near-term observing opportunity.
In July, the Ohio State team installed the IR array, a 1024 × 1024 HgCdTe HAWAII array from Rockwell. This permitted a checkout of the IR channel end-to-end. The internal mechanisms (dither/focus mirror and filter wheel) have been thoroughly exercised and appear quite reliable. The dither/focus motion in particular functions very well; the motions are accurate and repeatable, which should allow for straightforward assembly of dithered images. The IR channel is sufficiently parfocal with the optical filters that no internal refocus amongst the possible filter combinations is required (that is, the IR and CCD channels are "locked" together in focus, so only the telescope focus should ever be adjusted).
The array has very low read noise (~ 11 e-) and is flat to ~ 20% (most of the variation is a gradient towards the right-hand-side or north edge of the array). There is a group of about 300 dead pixels and ~ 50 scattered dead/hot pixels over the array. The system throughput is about as expected, suggesting the array has reasonable quantum efficiency for devices of this type. Measurements of a grid of standards spanning all four detector quadrants and ~ 15% of the area of the array show very good reproducibility (< 1% rms in the photometry). These tests were limited by weather, and a larger grid covering more of the detector is planned.
Sensitivity of the infrared channel is as expected. A web page to assist observation planning is under construction. A rough guide is that H = 13 is easy (high S/N in a few minutes), H = 15 is possible (high S/N in 30-40 minutes or so), H = 17 is tough (poor S/N in an hour); anything fainter should be done elsewhere. Numbers at J and K are similar (a bit higher at J, a bit lower at K).
Preliminary use of the instrument over the first few nights suggests that only occasional sky measurements are required, which should improve observing efficiency. Optimal observing strategies (exposure times, co-adds, dither spacing, etc.) will be defined during July and August. All observing is done in queue-scheduled mode by a telescope operator/observer.
On the optical side, we had hoped to reactivate the "dead" half of the CCD.
Unfortunately, we were unable to configure the device for science-quality 2048 × 2048 operation. The full device is now reading out, but the left-hand-side (south) does not contain science-quality data. Stars can be seen on that half of the CCD, but there are several problems that prevent the data from being useful. Principally, the effective full well of that side of the detector is only about 1000 DN (roughly 15 times smaller than the "good" side); there may be other systematic problems with the data that we don't fully appreciate. Users should note that, because of the reconfigurations of the readout, the half of the detector that is "good" is now different from before (the "good" half is now the right-hand-side or north edge). Users will need to modify their data reduction routines to change the "good" part of the CCD. The good part is obvious on inspection.
There is some further good news. The new control system is working very well. Our observers have assimilated the changes and seem comfortable with taking CCD, IR, and DUAL mode images. We have a good set of web tools to help the operators with observation planning and will soon integrate the nightly operations with user supplied parameter files for the observations.
The dual-channel imaging capability is nearly magical to watch. Seeing the CCD integrating while the IR channel reads out and dithers around the sky is fascinating -- especially when the images arrive on the data reduction computer and you can look at optical and IR images of the same field taken at the same time.
Darren Depoy (Ohio State),
Bruce Atwood (Ohio State),
Jerry Mason (OSU), Rick Pogge (OSU),
Ken Sills, (OSU), René Méndez (CTIO)