The DECam: Overview

1. The Dirt on the DECam: Overview

The DECam is a high-performance, optical, wide-field CCD imager for the prime focus of the Blanco 4-m telescope
at CTIO.  This DECam user manual provides information on the charachteristics of the imager, as well as the hardware and software components.  We welcome any comments and suggestions on the manual; please address these to our user support group at decam-helpctio [dot] noao [dot] edu.

1.1 Main Characteristics

Table 1.1.1 The main charachteristics of the detector system and CCDs.

1.2 The Focal Plane Array and DECam CCDs

The DECam field of view is one of the largest in optical astronomy.  The field of view is defined by 62 CCDs of 2048x4096 pixels each (1 pixel corresponds to 0.27 arcsec on the sky).  There are also 12 2k x 2k CCDs, 8 used for monitoring focus and alignment control and 4 used for guiding the telescope.  The detectors are seprated by gaps of 201 pixels in rows, and 153 pixels in columns.  The field of view is shown in Figure 1.2.1.

Fig. 1.2.1  Physical layout of the DECam.  The 2k x 2k CCDs labeled as "F" will be used for focus and alignment control and those labeled as "G" will be used for guiding.  Between the CCDs are gaps of 201 pixels in rows, and 153 pixels in columns.

BEWARE that the gaps between the CCDs are real, so be sure not to loose your object in a gap.

1.3 The Filters

Physical Dimensions: The DECam filters are 620mm (~24.5 in) in diameter with a thickness of ~13mm (0.51 in).  This size was chosen to ensure that the image quality is perserved over the full 2.2 deg wide field of view.  The filter coatings are highly uniform and designed to minimize in-focus ghost images from internal reflections between coating layers.

Transmission curves: ASCII Tables that numerically describe the transmissions are available on the DECam Web Pages. Note the transmission of the DES g and DES r filters at 557.7 nm are less than 1% to avoid contamination from a bright night-sky emission line.

Figure 1.3.1.  The Asahi measured filter transmission curves (doc-db-5847)

Figure 1.3.2.  The DES g filter is handled in the CTIO clean room.  It is composed of fused silica and weighs about 22 lbs.

1.4 Limiting Magnitudes and Performance

The exposure time calculator (ETC) is reasonably well calibrated, and its use is encouraged for exposure times.  The limiting magnitudes for a point source under average conditions (1 arcsec seeing, 1.2 airmass):

The image quality of the DECam CCDs is excellent accross the entire field of view.  Distortions are below 1% and the photometric accuracy and homogeneity accross each CCD is < 5%.  The number of bad pixels varies from CCD to CCD, the average is ~0.049% bad pixels and the worst CCD has 0.39% bad pixels.