The AAO Wide Field Imager & 
Prime Focus Upgrade Page


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


Whats's New?

PFU and WFI Commissioning - December 2000

The commissioning run for in December was quite successful. Updates on the WFI/PFU commissioning status can be found
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.

Finally we will  be carried out service observing. The approved programmes are here.

AAT Wide Field Imager and Prime Focus Unit Cookbook & Notes

A very drafty set of notes which will be updated as  knowledge of WFI & PFU progresses.

WFI SDSS Filters

The WFI SDSS filters have arrived. These g,r,i,z filters are 162 x 162mm in size, and 5.0mm thick (actually the r is 4.7mm thick). They have been manuafcatured by Barr Associates to the SDSS recipes. A u filter was not acquired, as the difference between our CCD's q.e. and that of the reference system's would be sufficiently different that no matter what we do, there would be significant colour terms. The WFI Schott glass U filter will be used instead.
 

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 following images of the WFI focal plane have been provided by GL scientific. Yes it does exist! The mosaic is the flattest Gerry Luppino has ever constructed - look at the reflected light patterns accross the chip boundaries.
 

WFI on Bench WFI Focal Plane
WFI Dewar and Electronics viewed from rear (click on image for larger view) WFI Focal plane (click on image for larger view)

Further images are ozcamexternal.jpg and ozcamstraps2.jpg (ozcam is Gerry's quaint name for WFI).

Overview

The Wide Field Imager (WFI) is a joint project being carried out by the AAO,  RSAA(otherwise known as Mt Stromlo) and the University of Melbourne to provide and 8Kx8K CCD imaging mosaic for use at the 3.9m AAT and the SSO 1m.

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


  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.

*
Top and side-on views of the WFI dewar showing the eight CCDs in place
At the centre of the system is a 123 x 123 mm focal plane mosaic of eight three side buttable 2048 x 4096 15 micron pixel CCDs which AAO and MSSSO are procuring as a result of a being partners in a consortium of astronomical institutions funding development of such devices at Lincoln Labs in the USA.

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.

Partners

The following organisations are involved in the WFI project:

Contractors

The following are the major contractors that will help in the WFI project:

Performance

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 .

The WFI Focal Plane

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.

WFI CCD QE Curves
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"
G
B
5.81
2.13
0.75
Assumed U-band sensitivity for detectors the same as 
measured MITLL2 - colour terms will differ though.
B 24 300s 1.5" D
G
B
11.78
6.80
2.20
Asumed mean B QE for WFI is 55% compared with 35% for MITLL2
V 24 300s 1.5" D
G
B
7.17
5.25
1.86
Assumed mean WFI QE similar to MITLL2
R 24 300s 1.5" D
G
B
5.61
4.76
2.39
Assumed mean WFI QE similar to MITLL2
I 23 300s 1.5" D
G
B
4.74
4.53
2.9
Assumed mean WFI QE similar to MITLL2

 

Links


Prime Focus Upgrade (PFU)

The Prime Focus Upgrade will provide (for the first time) an automated facility for the use of CCDs at the AAT's f/3.3 prime focus. A specification was developed for this project in 1997, in which a filter wheel, travelling blade shutter and auto-guider were required. In 1999, this specification was revised so that PFU will now provide filter wheel and shutter facilities alone, and WFI's on-board guide CCDs will be used for guiding when WFI is in use. Guiding will not be possible with other imaging cameras, unless they provide their own guide facility.

In January 2000, the first commissioning run for PFU took place. A detailed report on the results from this run, as well as a picture gallery of PFU images is available
 

PFU Specification

The current specification can be obtained as a  Postscript file