NEW! Project WWW-site for the Dark Energy Survey
In November 2003 CTIO issued an Announcement of Opportunity to develop a major new instrument for the Blanco 4-m telescope. Letters of intent were due by March 15 2004. We received a single letter of intent from the Dark Energy Survey (DES) Consortium (Fermilab, U. Illinois, U. Chicago, LBNL, CTIO/NOAO) to build a very large mosaic CCD camera for the Blanco prime focus. The Dark Energy Camera would bridge the gap between the Blanco telescope and the LSST in time (2008-2012); in A-Omega (telescope collecting area x area of sky), by this criterion being nearly a factor of 10 more efficient than the Blanco + Mosaic II, but a factor 10 less efficient than the LSST, and in science; the new capability fits neatly along the track ESSENCE, CFHT legacy -> Blanco Dark Energy Survey -> LSST surveys -> SNAP/DESTINY surveys. The DES would occupy 30% of the Blanco time over 5 years, producing a deeper set of data than the SDSS, over a similar area. A Powerpoint version of the Science Project can be downloaded.
The DES team submitted their proposal by the due date of July 15 2004, it describes an instrument that although optimized for the Dark Energy Survey will also be useful for a wide variety of science projects by the general community. The proposal was reviewed by an external expert panel (Blanco Instrumentation Review Panel, BIRP), which made a recommendation that the instrument be built. Presentations were also made to the NOAO User's Committee. more detail. The consortium is presently simultaneously in a design & Development phase, searching for an additional partner to round out the resources required, and iterating on a Memorandum of Understanding with NOAO. At CTIO we are reviewing possible improvements desirable for the Blanco telescope in order that it optimally accommodates the DEC (and NEWFIRM before it).
Along with other dark energy initiatives, the project will be presented to the Dark Energy Taskforce in mid-2005.
We are actively
soliciting views and suggestions
from our community.
The actual specifications of the DEC will evolve somewhat over the next few months; some possible trades are highlighted in the questions below. Notable in the tabled comparison are the large gain in sky coverage, the very high red sensitivity of the CCDs, and the short readout time compared to the Mosaic II instrument. Note that the very large size of the filters, and the restricted prime focus cage environment, places restrictions on the number of filters than can be loaded at any one time.
PARAMETER DEC Mosaic II unit
Optics FOV 2.1 0.9 degrees diameter
Corrector passband 320-1100 340-1000 nm
Number of pixels 503 66 million
Image scale 0.27 0.27 arcsec/pixel
Area of sky in 1 exposure 2.8 0.35 square degrees
CCD type LBNL SITe
QE 300 nm 5 8 %
QE 400 nm 60 68 %
QE 500 nm 75 80 %
QE 600 nm 80 86 %
QE 700 nm 88 80 %
QE 800 nm 90 70 %
QE 900 nm 92 40 %
QE 1000 nm 60 11 %
QE 1050 nm 10 1 %
Number of loaded filters 4 14
Read time 20 150 seconds
1. The near term evolution of Blanco instrumentation, and that offered on other facilities available to the US community is described HERE. During 2005 we will only offer the wide-field instruments Mosaic II, ISPI and Hydra on the Blanco. If we assume that we start sharing NEWFIRM with the Mayall from 2007 onwards, then the Blanco instrument complement in 2009 could be either (i) DEC and NEWFIRM, or (ii) DEC, ISPI and Hydra. Comments?
2. The present design has no Atmospheric Dispersion Corrector. ADCs add complication, and generally cut out the ultraviolet, the very wavelength range that they would otherwise correct. An observing strategy would be to plan one's program to do the bluer passbands at lower airmass than the redder passbands. Note that it is planned to guide through the science filters, with guide CCDs in the dewar. Comments?
3. The filters are enormous and expensive ($100K each). Only four (nominally SDSS g r i z) can be loaded at a time, and installing other filters will be a daytime job. However changing from one installed filter to another will likely be rather quick. Comments?
4. What other filters are essential? Would you like to be able to use smaller filters, sacrificing some of the field?
5. The present corrector design is all fused silica and thus has excellent UV transmission. However our final design might use some optical element that restricts the UV, if we find there are cost and/or performance advantages in doing so. Is it important to have good response below 320 nm? 340 nm? 360 nm? 380 nm?
6. Would you observe with this camera for your science? Would you like to observe yourself, or would service observing be acceptable?
7. The camera will have pipelined data reduction into the NOAO archive. Do you think you would use data from the camera in your science, after the standard propriety time of 18 months? The DES frames would be available after 12 months, do you think you would use this data set?
8. The DES will take 30% of the time over 5 years, imaging 5000 square degrees mostly centered on the SGP, and with a repeated survey of 40 square degrees. Since much of integration time will be in the z-band, the survey can use bright as well as dark time. The DES will thus take a large part of the available time over the months August-January. Note that part-nights scheduled for other projects would need to use the same griz filters as the DES. Comments?
9. Is there some other aspect of the design you would like to comment on?
10. What other questions should we be asking?
Please respond to Alistair Walker