These pages are to inform AAO users of the planned functionality of the
AAO's Prime Focus Upgrade (PFU) and the AAO/MSSSO/U.Melb Wide Field Imager (WFI).
|Useful Links and other WFI Resources
PFU and WFI Commissioning - December 2000
at the AAO WFI/PFU Commissioning Plan and Status page. New dark current measures, gains, read noises, SCTEs and
sensitivity estimates are now available. Also the Direct Imaging calculator now includes measured parameters for WFI and the triplet corrector.
WFI SDSS Filters
Each filter has a 152mm x 152mm clear aperture. They will be mounted
in their own filter holders, and will be available for use on the 40" so
long as they are never removed from their holders. These filters have been
assigned numbers #90-#93 in the AAT
Filter Catalogue. Throughput profiles can be obtained there.
WFI Focal Plane is assembled.
The Prime Focus Upgrade is an AAO project to provide upgraded facilities at the f/3.3 AAT prime focus for the mounting of CCD detectors. In particular, it is required on order to mount the WFI and the AAT, and to provide filter wheel and shutter facilities.
The Wide Field Imager will provide a focal plane CCD detector system with a size of 123 by 123 mm, 8192 by 8192 pixels, made up of a 4 x 2 array of 2048 x 4096 pixel thinned, back-illuminated CCDs, to be used on both the Anglo Australian and ANU 40inch Telescopes. The output of the system is pixel data in the form of multi-extension FITS files written on an archiving medium which is part of the system, and/or transported to a device of the user's choice over a Local Area Network, or fed into an automated archiving and reduction pipeline where available.
The focal plane will be housed in an evacuated vessel (the "dewar") and will be cooled to about 170 K by the use of liquid nitrogen refrigeration.
The dewar fits onto exposure controllers located on the AAT Prime Focus (ie the AAT Prime Focus Upgrade) and the 40" Cassegrain Focus, each comprising a filter wheel with at least four positions, shutter and their control electronics frames.
Autoguiding will be accomplished by the use of one of eight small guide CCDs located on the focal plane next to the scientific detectors. (At earlier times it had been planned to provide a seperate guider head in the AAT's Prime Focus Upgrade. This has had to be removed from the plan, so the WFI guide CCDs will be used on the AAT, as well as on the 40").
Two San Diego State University second generation CCD controllers will be mounted on the dewar to read out the CCDs and transfer the data over fibre optic links to the data acquisition computer system.
The control and data acquisition computer system will consist of two Sun workstations with enough capacity to efficiently handle the data stream from the CCD controller for display, storage and archiving, plus data reduction. The use of two computers will allow the time-critical control of CCD readout to be separated from the user interface and data reduction computer. It will also allow user interface and data reduction computers to be set up permanently at both telescopes, optimally configured for each organisation's data pipelining procedures. The control and data handling software will be the MSSSO CICADA package, suitably adapted to deal with reading out CCD mosaics through the SDSU controller.
The AAO Direct Imaging calculator can be used to do signal-to-noise calculations forthe WFI at the AAT Prime Focus with the triplet corrector. The sensitivity guestimates below should not be used any more. For other updated performance details see AAO WFI/PFU Commissioning Plan and Status .
Note that the SCTE numbers quoted below are only useful in a relative sense. They were not determined at the WFI operating temperature. See the AAO WFI/PFU Commissioning Plan and Status page, for images showing the SCTE performance based on dark images and cosmic rays.
The following figure compares the QE Curves measured for 4 of the these devices with the MITLL2 and MITLL3 in use at the AAO. w19c1, MITLL3 and MITLL2 (before coating) all share a common red-optimised AR-coating, producing poor blue QE. The QE in the blue for the Phase 2 devices is better, though not as good as that seen in (for example) EEV devices. On the other hand, these devices generally have excellent red QE, good fringing performance (twice as good or better than the AAO's TEK1K) and the best CCD amplifiers ever constructed.
Using these curves, we guessed that that U-band performance of the Phase 2+ devcies in WFI should be as good as the MITLL2 (though obviously with a different colour term behaviour), and overall B-band QE should be 50-100% higher. Based on that we would predict the following sensitivities.
AAT WFI Sensitivities Guesses
DO NOT USE THESE
Use the AAO Direct Imaging calculator
Passband Mag Time Seeing D/G/B S/N Comments U 23 300s 1.5" D
Assumed U-band sensitivity for detectors the same as
measured MITLL2 - colour terms will differ though.
B 24 300s 1.5" D
Asumed mean B QE for WFI is 55% compared with 35% for MITLL2 V 24 300s 1.5" D
Assumed mean WFI QE similar to MITLL2 R 24 300s 1.5" D
Assumed mean WFI QE similar to MITLL2 I 23 300s 1.5" D
Assumed mean WFI QE similar to MITLL2
In January 2000, the first commissioning run for PFU took place. A detailed
on the results from this run, as well as a picture
gallery of PFU images is available