Since the most recent Newsletter article on the CCD Mosaic Imager, we have had Mosaic testing runs at the 0.9-m in January, and the 4-m in November and January. The software has advanced, the system is more reliable, and we have conducted numerous tests.
During the January engineering run, we performed an
initial evaluation of the new corrector and its atmospheric dispersion
compensator (ADC). Although the seeing was variable during the run,
excellent subarcsecond images were obtained near the zenith with (or
without) the ADC in use. Images also were taken up to 70
from zenith in B.
At the larger zenith distances, the ADC improved the images as expected,
although the seeing at the time was about 1.3", so the improvement was
small. The software to operate the
ADC allows the observer to use it in a transparent tracking mode for long
exposures, or to set them to a null position where no correction is applied.
Caption: The left-hand image is a single 5 minute exposure of M33 taken at the 4-m. Images from the eight calibrated CCDs have been transformed into a single, geometrically corrected (for CCD alignment and optical distortions) image. The right-hand image is a combination of 5 shifted exposures which have been registered and median stacked. Trails from extremely bright stars remain but all other defects and gaps vanish. The two images are shown at the same size of 8K x 8K though the full combined image is 9K x 10K. Image reduction courtesy of Frank Valdes.
There remains one problem with the ADC that we expect to have largely eliminated before the 14-17 April engineering run. When using narrow-band interference filters, a faint image of the telescope pupil falls on the CCD with a diameter of about 10'. Depending on the bandpass and construction of the filter, this reflection typically manifests itself at a 3-5% level above the background. It arises from an internal reflection off the front surface of the rear element of the corrector despite the use of an extremely good anti-reflection (AR) coating. Our investigation suggests that similar four-element correctors currently in use should experience a similar effect; tests performed recently by Alistair Walker with the CTIO 4-m confirm this analysis.
For the purposes of the Mosaic, our approach to solving the internal reflection is to re-coat the critical surface with an AR coating specifically optimized for superb performance over the wavelength range where most interference filter work is done: from 5000 to 7000Å. To extend much beyond this range, interference filter users will have to rely on flat-fielding to minimize the effects of the reflection. During subsequent engineering runs, we will be exploring observing strategies to minimize the effect.
We currently have B, V, and R large-format filters for Mosaic. We are in the process of ordering several additional large-format filters. Depending on the pricing and vendor delivery schedules, we hope to obtain all or most of the following by June: Kron-Cousins I-band, Ha (80Å FWHM) and four redshifted filters at velocity intervals of 1800 km/s, [OIII] 5007 (50Å FWHM), and a continuum filter at 5300Å (250Å FWHM).
For each read of the Mosaic array, 128 Mbytes of data are generated. The readout time currently is 110 s (with one amplifier per CCD), but the time to handle the data and save it to disk requires an additional 4 minutes. Only then can one display the image and perform typical quick-look interactive analysis to assess the data quality. We consider this overhead of 6 minutes unacceptable.
The IRAF group has developed a strategy to improve the throughput of the Mosaic data pipeline significantly. A data messaging system, operating across computers, will take packets from the Arcon CCD controllers and ship them to our recently acquired Sun Ultra-Sparc 2. Clients on the Ultra will unscramble the packets from all the different CCDs/amplifiers, save the data to disk in an efficient new FITS format, and simultaneously display the data during readout to a new real-time image display. During the readout/display process, data can be accessed by all IRAF quick-look utilities (e.g., IMEXAMINE) to assess the data quality (e.g., focus, background level) even before the full image is finished reading out.
Preliminary tests of the messaging system, the fast ethernet link to the Ultra, and the new data format all appear promising. The new display system has not been developed yet, but should be available in the fall. In addition, the data pathway from raw images to astrometrically corrected and combined images has been verified. These tests utilize the new IRAF astrometry package, a new multi-amp version of CCDPROC, and the new multi-image FITS format that will be available under the next version of IRAF (V2.11). As an example, images of M33 were combined to produce the picture shown on the previous page.
We have additional 4-m Mosaic testing scheduled for February, April and July. In addition, three nights of shared-risk science have been scheduled at the 4-m in June/July and seven nights at the 0.9-m in June. We received eleven Mosaic expressions of interest for this time. Six of these programs had technical requirements consistent with current Mosaic performance; we have solicited and received detailed project descriptions from these proposers. We will finalize the disposition of these shared-risk blocks shortly.
The largest remaining task in the Mosaic project is the upgrade to science-grade CCDs from the current engineering-grade devices. As described in the December 1996 Newsletter, we have placed an order with SITe for thinned high-QE low-noise 2K × 4K CCDs. We do not know when we will receive eight suitable CCDs from SITe and have them optimized. Therefore, the operating assumption is that the current engineering-grade CCDs will remain the Mosaic detectors for the fall 1997 semester. However, we will do everything possible to expedite the CCD upgrade. For example, construction has begun on a copy of the Mosaic dewar. The science-grade CCDs will be installed in this new dewar in order to minimize the Mosaic downtime needed for the CCD upgrade. The dewar that is currently in use will become part of Mosaic Clone, which will eventually be deployed at CTIO.
We invite applications for shared-risk Mosaic time in the fall 1997 semester. Proposals should follow the normal application procedure. The proposals will be given a science grade by the TAC and also reviewed for technical compatibility with the current state of the Mosaic Imager by the Mosaic team. Proposers should list what filters they plan to use and the required photometric uncertainty at a given magnitude. As described above, there can be no assurance of the CCD upgrade occurring during the fall, but there is some possibility that it could occur late in the semester. Therefore, proposers should write their proposals for the engineering-grade CCDs; however, please feel free to let us know how the program would benefit if the science-grade CCDs become available. Programs with the greatest likelihood of being scheduled will have high science grades from the TAC, technical requirements that are consistent with the performance of the current CCD Mosaic Imager, and, to a lesser extent, observing goals that will aid in final Mosaic evaluation and commissioning.
Many users will need to choose between Mosaic and the standard CCD imagers at the 4-m and 0.9-m (T2KB and T2KA, respectively). In the September 1996 NOAO Newsletter, No. 47, page 27, we cautioned users on a number of deficiencies of the current Mosaic engineering-grade CCDs: they are unthinned and uncoated chips and therefore have poor sensitivity in the blue; the Mosaic CCDs are much poorer cosmetically than T2KB or T2KA; the effective readnoise can be as high as 30 e- over parts of the array. In addition, we only have a limited supply of Mosaic filters at present (see above). Finally, although the system is maturing, users should be aware that Mosaic is not yet as user friendly or reliable as PFCCD. Rough guidelines for using PFCCD versus Mosaic are as follows:
Feel free to consult with any of the undersigned if you are unsure of whether to apply for PFCCD or Mosaic for fall 1997.
For updates on the progress of the Mosaic project, check out the Mosaic web page at: http://www.noao.edu/kpno/mosaic/mosaic.html.
Taft Armandroff, George Jacoby, Todd Boroson (for the Mosaic Team)