|Answers to the first call for bids||first call for bids||Sloan survey||Sloan filters||Sloan tech. info|
The Sloan filter set selected for MegaCam
The u'g'r'i'z' sloan set has been selected for MegaCam. An additional b* filter is still under study (see the mail archive for details).
u' will be defined when the AR coating of the MegaPrime optics (wide field corrector, ISU plate and cryostat window) will be done and measured (should all be done by the end of January 2001).
A call for bids to Barr Ass. and OCLI is sent in mid November in order
to get a first idea on cost and delivery time for the MegaPrime filters.
We are using for now the CFH12K filter characteristics. Here is the summary of these characteristics:
The filters will have the following common specifications:
Please quote on both
(a) 300 mm x 350 mm, +0.0/-0.5 mm
(b) 300 mm x 300 mm, +0.0/-0.5 mm filters.
This size should include all mechanical packaging.
Thickness: ó15 mm including all mechanical packaging.
Clear Aperture: For (a) 284 mm x 337 mm or larger, for (b) 284 mm x 284 mm or larger.
Transmitted Wave front Error: ó 2 per 25.4 mm peak-to-valley at =632.8 nm
Surface Quality: 60/40 Scratch/Dig
Blocking: 10-4 from 300 nm to 1100 nm outside of the bandpass
Temperature: Central Wavelength (CWL) and Bandwidth (BW) are specified at 0 deg. C.
Beam: CWL and BW are specified for an f/4.14 beam.
Anti-reflection Coatings: Hard AR coating on both sides.
The individual filters will have the following additional specifications:
|Name||CWL (nm)||CWL Uniformity (nm)||BW (nm)||BW Uniformity (nm)||Average Transmission in Band|
|Z'||High pass: Cut-on at 850±20||±10||85%|
The uniformity specifications above refer to the amount of allowable difference in CWL or BW at any point within the clear aperture of the filter relative to the CWL and BW at the center of the filter.
A certificate of compliance will be needed with each filter supplied. This certificate will provide data to show that the filter meets the transmitted wave front error specification over the clear aperture (an interferogram is preferred) and will also include 10 evenly spaced transmission curve measurements over a full diagonal of the filter to verify the CWL, BW and uniformity of the filter.
In your quotation, please comment on any of the above specifications
that you don't feel that you will be able to meet and give a quotation
based on what you expect to be able to meet. Also please comment
on whether you would be able to make narrower BWs for the Hà and
Hà-off filters. The tolerance and uniformity specifications
should decrease in proportion to the BW decrease (the tolerance should
be 10% of the BW and the uniformity should be 5% of the BW for both the
CWL and the BW).
SDSS survey characteristics
The SDSS is expected to image nearly 104 deg2 of the northern high-latitude sky. The SDSS does not employ the standard filter system, but it rather adopts a filter system optimized for the survey. The 5 broad-band filters cover the full wavelength range of an optical CCD with very little overlap (see Figure 1). For a signal-to-noise ratio of 5:1, the limiting magnitudes are expected to be between 22 and 23 magnitudes, varying for the different filters. These magnitudes are on the AB system, referred to a spectrum with constant f (not an A0V star), and represent the fluxes a flat spectrum would have near the effective wavelength of the filters. The color data are obtained nearly simultaneously with a maximum time lapse of 5 minutes. Table 1 shows the filter characteristics of the filters used. A transformation from the Johnson UBV and Cousins RI system to the SDSS filter system has been estimated by Krisciunas et. al., 1998, by doing photometry of primary and secondary standard candles using a 1-m reflector and a nearly identical filter system(see Table 2).
|Filters||effective wavelength||full width at half maximum||Wavelength range||limiting magnitude for S/N 5:1|
Table 1: filter characteristics of the SDSS.
Table 2: Transformation equation between the Johnson UBV and
Cousins RI system and the SDSS system. (Krisciunas et. al., 1998)
Figure 1: The SDSS system response curves. The responses are
shown without atmospheric extinction (upper curves) and as modified by
the extinction at 1.2 airmasses (lower curves). The curves represent expected
total quantum efficiencies of the camera plus telescope on the sky.
NAME CENTER(nm) FWHM(nm) TMAX(%) DIMENSIONS(mm) TRACE -------------------------------------------------------------- u' 350 600 0.778 76x76x3 N/A g' 480 140 0.919 76x76x3 N/A r' 625 140 0.976 76x76x3 N/A i' 770 150 0.987 76x76x3 N/A z' 910 120 0.980 76x76x3 N/A
Filter Set (information from Barr Ass.)