MegaCam Surveys Working Group

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).

Characteristics chosen for the first call for bids

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:

Size:                         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
U' 350±6 ±3 64±6 ±3 70%
B 430±10 ±5 97±10 ±5 80%
V 545±9 ±4 88±9 ±4 85%
R 648.5±13 ±6 130±13 ±6 85%
I 830±19 ±9 193±19 ±9 85%
Z' High pass: Cut-on at 850±20 ±10 85%
H 657.5±1.0 ±0.5 10.0±1.0 ±0.5 85%
H-off 642.8±1.0 ±0.5 10.0±1.0 ±0.5 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
u' 3540 570 3250-3800 22.3
g' 4770 1370 4085-5455 23.3
r' 6230 1370 5550-6915 23.1
i' 7630 1530 6850-8400 22.5
z' 9130 950 8650-9600 20.8

Table 1: filter characteristics of the SDSS.

Transformation equation
r' = V - 0.42 (B-V) + 0.10
u'-g' = 1.33 (U-B) + 1.18
g'-r' = 0.96 (B-V) - 0.18
r'-i' = 0.99 (R-I) - 0.22
i'-z' = 0.67 (R-I) - 0.19

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.

SDSS u', g', r', i', z' filters

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
Note: SITe CCDs are not sensitive to the "Red leak".

u', g', r', i', z' combined trace

SDSS Filter Set (information from Barr Ass.)

SDSS Filter Set

Band CWL, Å FWHM, Å Construction
u' 3560 600 3 mm fused silica + coating
1 mm UG-11
1 mm S-8612 + coating
g' 4825 1380 3 mm GG-400 + coating
2 mm S-8612 + coating
r' 6261 1382 5 mm OG-550 + coating
I' 7672 1535 5 mm RG-695 + coating
z' 9097 1370 5 mm RG-830 + coating

  • Refractory oxide durable surface coating
  • Transmitted wavefront accuracy: 1/4 wave per inch at 633 nm
  • Parallelism: 30 arcseconds
  • S/D: 60/40
  • CWL tolerance: ±2%
  • FWHM tolerance: ±10%
  • Peak transmission: 70% for u'; 80% for g'; 85% for r', I' and z'
  • Out-of-band transmission: <0.01%, 300-1100 nm
  • Dimensions: up to 14 inch diameter or diagonal
  • Thickness: 5 mm
  • Operating temperature: 0°C 
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