Imaging camera parameters and description

Introduction

A detailed description of the imager can be found in Gunn et al. (1998) and in the project book. This page summarizes the most important information needed to understand the data it produces:

imaging camera parameters
filter curves
arrangement of CCDs on the imaging camera
CCD nonlinearity

Imaging camera parameters


Photometric CCDs	30 2048 × 2048 SITe/Tektronix 49.2 mm square CCDs, arranged in 6 columns parallel to the scan direction and 5 rows perpendicular to the scan direction
CCD read noise	< 5e^- pixel^-1 (overall system is sky limited)
Image frame size	2048 × 1361 pixels (13.51 × 8.98 arcminutes)
Image column separation	25.17 arcminutes
Detector separation along column	17.98 arcminutes
Focal-plane image scale	3.616 mm arcmin^-1
Detector image scale	3.636 mm arcmin^-1
Pixel size and scale	24 μm; 0.396 arcseconds pixel^-1
Filters	r i u z g scanned in that order, 71.7 seconds apart
Integration time	54 s
Operating mode	drift scan
Field distortion	<0.1 arcseconds over the entire field
Field size	2^°.5
Flux calibration	Standard-star fields at 15^° intervals along scans, tied to BD + 17^° 4708, atmospheric extinction determined by the PT
Astrometric CCDs	22, 0.25 × 2 inches, above and below CCD columns; r filter plus 3 mag neutral density filter, 10.5 second integration time

Transmission curves

The photometric flux calibration web page is essential reading for those wishing to understand the SDSS photometric system. There is a long page describing transformations between SDSS (ugriz) and Kron-Cousins (UBVR_cI_c) photometry.

Average wavelengths of SDSS filters
`u`	`g`	`r`	`i`	`z`
3551Å	4686Å	6165Å	7481Å	8931Å

SDSS system response plot

The solid response curves show the througput defining the survey's photometric system, which includes extinction through an airmass of 1.3 at Apache Point Observatory. For reference, the dashed curves do not include any atmospheric extinction. These are sometimes loosely referred to as "filter curves" although they do include the full system response from atmosphere to detector.

filter curves for ugriz filter set

Tables

Tables of camera sensitivity through each filter are available as html tables and as ASCII tables: u.dat g.dat r.dat i.dat z.dat

The columns of the table represent:

wavelength
on the sky sensitivity looking through 1.3 airmasses at APO for a point source. These define the survey's photometric system.
sensitivity under these conditions for very large sources (size greater than about 80 pixels) for which the infrared scattering is negligible (the infrared scattering only affects the thinned detectors used for ugri and among these, it is negligible for ug; hence this column is different from column 1 only for ri)
the response of the third column with no atmosphere
assumed atmospheric transparency at one airmass at Apache Point Observatory

Arrangement of CCDs on the camera

Arrangement of CCDs on the imager photo of the imager CCD arrangement

Last modified: Mon Apr 10 21:31:08 BST 2006