DECam Overview

1. 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 characteristics 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

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 separated 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.  The detector position numbers (e.g., "S4" or "N4" near the center of the field) are useful for identifying specific CCDs to look at in the multi-extension images, like "display DECam_00153116.fits[N4]"). 

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 preserved 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.

Approximate characteristics of 620 mm diameter DECam Filters

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.

Filter Focus Offsets relative to the r filter

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 across the entire field of view.  Distortions are below 1% and the photometric accuracy and homogeneity across 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.
 

1.5 Safety Precautions

The DECam science CCDs are sensitive to over-illumination, and failures of DECam CCDs may occur if the CCDs detect too much light.  Although there are focal plane protection diodes installed that will shutdown the power if too much light is detected, these are a last-resort safety net which we do not want to employ.

1.  Twilight sky flats (evening or dawn) are forbidden.

2.  On-sky observing will start no earlier than 30 minutes after sunset and will finish no later than 30 minutes before sunrise.    The telescope operator will advise the astronomer of these times.

3.  Before the dome is opened, at any day time between the above limits, the person opening the dome will check on the DECam Guis whether VSUB is OFF and the blank filter is in the beam.  This includes opening the dome to allow the telescope dome to ventilate prior to observing, and when extra light is needed in the dome for daytime work.

4.  Before turning on the lights in the dome, check that no calibrations are underway and that the shutter is closed and VSUB is OFF.

5. Please use the "Calibrations warning signs" on the console and in the elevator.    Any calibrations running in the morning after observing must finish before 08:00 a.m. unless  arrangements have been made with the TelOps Manager the previous day.