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

The main characteristics of the detector system and CCDs.

Field of View	3 square degree (2.2 degree field of view)
Pixel scale	0.27 arcsec/pixel (15 microns)
Detector	62 2048x4096 science CCDs, 520 megapixels total
Read-out noise	7e-
Read-out time	30-40 sec
Dark-current	~<25 e-/pixel/hr (spec)
Dynamical range	16 bit
CCD Gaps	3.0 mm (201 pixels) along long edge (e.g., between S4 and N4); 2.3 mm (153 pixels) along short edge (e.g., between N4 and N5)
Cosmetics	Good to excellent.  On average, each CCD has 0.05% bad pixels and the worst CCD has 0.39% bad pixels.
Filters	6 filters now available (ugrizY; see Sec X.X)
Saturation	linear to better than 1% to 130,000 to 240,000 e-, depending on the CCD [1]
Gain	5 e-/ADU
Geometrical distortions	: very small maybe <0.08%
Raw data format	Fits (with extensions),1 GB/file, ~600 Mbyte/file compressed
Telescope aperture	4m
Telescope focus	Prime
Instrument F ratio	f/2.7

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.

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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

Filter	Central wavelength (nm)	FWHM (nm)	Avg absolute transmission
u	:350	:75	>:85%
g	475	150	>85%
r	635	150	>85%
i	775	150	>85%
z	925	150	>85%
y	:1000	:110	>85%

Transmission curves: ASCII Tables [3]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.

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

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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

Filter	Offset (microns)
u	1.5
g	0
r	0
i	-44.5
z	-141
y	-161

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):

Filter	Limiting mag	Saturation Limit (in 2 sec)
u
g
r
i
z
Y

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.
 

2. The DECam Hardware

2.1 The CCDs

The DECam has 62 2048x4096 pixel CCD detectors for science images and 12 2048x2048 pixel CCD detectors for guiding, alignment and focus.  The CCD detectors are 250 µm thick, which is about 10 times thicker than conventional CCDs.  This thickness greatly improves their quantum efficiency in the near infrared (800-1100 nm).

As seen below, the number of bad pixels varies from CCD to CCD, with the worst CCD having ~0.4% bad pixels.  The average percentage of bad pixels per CCD is ~0.05% and the median is 0.02%.

Figure 2.1.1  Left: The DES 2k x 4k CCD module.  Right: A histogram of the percentage of bad pixels in the 62 2kx4k DECam CCDs (Flaugher et al. 2012, SPIE).

Figure 2.1.2 shows the layout of the CCDs on the focal plane as well as a typical flat field image.
 

Figure 2.1.2.  Right: The DECam focal plane populated with 72 CCDs.  Left: A flat field image from the DECam.

The schematic layout and dimensions are detailed in Figure 1.2.1 which also gives the dimensions of the gaps between the CCDs.

2.2 The Cooling System

The DECam CCD array is cooled autonomously and can operate for extended time periods (months) without human intervention.  This is achieved by using a liquid nitrogen (LN2) closed loop, two phase circulation system.  The LN2 process tank contains approximately 200 liters of LN2 and this LN2 is circulated to the imager vessel heat exchanger and back.  As the LN2 cools the imager, some evaporates in the process as it absorbs the camera's heat.  Therefore, inside the imager vessel, the LN2 is separated into liquid and gaseous phases.  The liquid phase continues to circulate through the cooling system whereas the gaseous phase is condensed using two cryocoolers, and returned to its cold liquid state.

The cool down time is 4 hours before the CCDs can be read out, and another few hours before the temperature is stable within the requirements of -100°C +/- 0.25°C.

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Figure 2.2.1.  The main components of the DECam vessel, showing (left) the vessel, the readout crates, the vacuum interface boards (which transfer the CCDs’s signals from inside the imager to the readout crates), vacuum pumps, and liquid nitrogen ports.  Also shown (right) is the internal heat exchanger cooling ring used to cool the CCD array.  The heat exchanger is not normally visible due to the protective vessel casing.

The temperatures of the CCS are displayed in the GUI (fill in here).....  The CCDs should be between -95°C and -105°C.

More information on the DECam cooling system can be found in SPIE 2012 Commissioning the LN2 system at CTIO [7], SPIE 2010 Cooling the Dark Energy Camera CCD Array [8] and SPIE 2008 The Dark Energy Survey CCD Imager Design [9].

2.3 The Readout Controllers

The DECam CCDs are read out using the Monsoon [10] system, an open-source image acquisition system designed by the National Optical Astronomy Observatory (NOAO). The electronics consists mainly of three types of modules, the Master Control Board, the Acquistion Board and the Clock Board.  The Clock Board generates the clock rails for the readout of 9 CCDs (there are 13 clocks per CCD and these clocks move charge either vertically or horizontally).  The Acquisition Board provides bias voltage to the CCD and digitalizes CCD output signals. The Master Control Board is designed to control the clock and video boards and communicates the Monsoon crate with higher level communication modules.  The DECam Monsoon system provides a total of 132 video channels, 396 bias levels and about 1000 clock channels in order to readout the full mosaic at 250 kpixel/s speed with 10 e- noise performance.

More information on the DECam Monsoon controller can be found in Castilla et al. 2010 [11].

2.4 The CCD Shutter

The shutter for the DECam consists of stepper motors that drive the blades via belt drives.  This synchronized motion of the shutter blades is advantageous for fast exposures.  The absolute timing of an exposure is measured to a precision of 10 millisecond.  The shortest exposure time is limited to ~5 milliseconds and the exposure duration accuracy is ~1 millisecond.  The exposure start precision is <50 microseconds. (doc-db-750 and 2071-v6).  The DECam shutter is about 2 m long, and 0.76 m wide.

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Figure 2.4.1.  The DECam shutter, pictured with its Bonn University (Germany) manufacturers.

2.5 The Filter Changer

The filter changer provides positions for eight DECam filters.  These filters are housed in four stacked cassette mechanism sub-assemblies, and each cassette houses two filters (see Figure 2.5.1).  Compressed gas moves the filters into place and back out from the aperture. The air cylinders producing the force of the compressed gas have integral air cushions at the end of travel to absorb energy of motion and integral needle valves for safety and speed control.

It takes less than 10 seconds to exchange a filter for another and the DECam filters can be changed while readout is happening.  The filters are always within 0.125 mm of the optimal position.

Figure 2.5.1 Left: The filter changer mechanism Right: An individual filter and filter cell

The current position (as of Jan 23, 2013) of the filters is as follows:

even side                       odd side position

c1f2: g-band in cell 7    c1f1: i-band in cell 3
c2f2: r-band in cell 6    c2f1: z-band in cell 4
c3f2: Y-band in cell 8   c3f1: u-band in cell 5
c4f2: pinhole                c4f1: block

c1f1 is slot 1
c1f2 is slot 2
c2f1 is slot 3
c2f2 is slot 4
c3f1 is slot 5
c3f2 is slot 6
c4f1 is slot 7
c4f2 is slot 8
 

More information on the DECam filter changer can be found in SPIE 2010 Large Format Filter Changer Mechanism [13].

2.6 The Guider CCDs

Guiding with the DECam is accomplished using four 2kx2k CCDs on the north and south sides of the science field.  The field of view of each CCD is about 9.2 arcmin on a side, and at a given location, suitable guide stars are almost always available without moving the telescope from the desired position. 

The acquisition of a guide star is based on stars that (1) have few saturated pixels (2) are far way from the border of the image (3) are without neighbors and (4) are point sources.  SEXtractor is used to facilitate this process.  In each of the four guide CCDs the best reference guide star is chosen and an tracking correction is calculated.  An example plot showing the output offset (error signal to the telescope control system) is shown in Figure 2.6.1.  Therefore it is straightforward to visually see how well the guide CCDs are adjusting the telescope tracking and also to discard or adjust any problematic guide CCD.

Figure 2.6.1. The output offset calculated from each guide star in each guide CCD.  This offset is used to correct the telescope control system and ensure precision telescope tracking. 

The first time slewing to a new field, the process time to find a guide star is about 1.3 seconds.  After this, a region of interest much smaller than the full CCD (the default is 50 x 50 pixels), is saved as a region of interest fits file, and the guider can analyze images in 0.06 seconds.  The precision of the tracking depends on the quality of the guide stars, as well as the number of guide CCDs used.  Table 2.7.1 shows the measurement of the tracking precision using the DECam Guider Software.

Star Magnitude	1 Guide CCD	4 Guide CCDs
15	0.044 pixels	0.038 pixels
16	0.074 pixels	0.049 pixels
17	0.103 pixels	0.056 pixels
18	0.143 pixels	0.082 pixels
19	0.220 pixels	0.095 pixels

Table 2.7.1 Precision of the tracking error as a function of the brightness of the guide stars and the number of guide CCDs used.

There are three configurations that can be used for guiding:

1.  Auto -- The guider continuously reads new images, tracks guide stars and sends correction signals until asked to stop guiding.
2.  Self -- The guider analyzes the next (first) image and finds the best guide star for each of the four guide CCDs. 
3.  User -- The guider presents the image to the observer, who then selects the guide stars.

The Auto guider is the fastest and easiest, and this configuration is recommended.

The DECam guider manual can be found here (there is a draft of this manual, I will add the link when a more robust version is ready). 

2.7 The Focus and Alignment CCDs

The focus and alignment system of DECam is automatic--the camera actively controls the focus and alignment so the user can sit back and enjoy in focus images.  The adjustments made to ensure precision focus are hexapod tilt (2 axes), hexapod translation (2 axes), hexapod piston (focus) (1 axis) and primary mirror support system astigmatism (2 axes).  These adjustments are made using eight 2K by 2K CCDs, which are defocused by either 1.5 mm extra- or 1.5 mm intra-focally.  As exposures are taken, the donuts (out of focus star images) on each 2k x 2k CCD are analyzed for coma, astigmatism and focus (see Figure 2.7.1).  A Zernike analysis of the wavefront error at the focal plane measures the Zernike parameters, which are then turned into hexapod and primary mirror corrections.  “Time is precious,” Physicist Aaron Roodman of SLAC National Accelerator Laboratory says. “The nice thing about this system is that it will focus automatically every minute."

Figure 2.7.1  Simulated out-of-focus star images (donut) images, resulting from imposing aberrations on the primary mirror. Left donut shows the comatic aberration resulting from the optical axis being not perfectly aligned.  Right donut shows the astigmation resulting by moving the primary mirror closer to the focal plane by 300 microns.  The middle donut shows a simulated DECam donut, smeared to simulate 0.75 arc-second seeing and pixelated to simulate the CCD pixel scale of 0.27"/pixel.

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Figure 2.7.2  The corner of a section of a DECam image, showing the 2k x 2k focus and alignment CCD with out of focus stars.  The out of focus stars are used to ensure that science frame CCDs has stars which are perfectly in focus.

More information on the focus and alignment CCDs and Zernike analysis can be found in Roodman (2012) [15].  Note that the highest consumption per capita of donuts is in Canada [16].
 

2.8 The Hexapod

The DECam hexapod consists of six jacks to finely tune the x,y,z (lateral, longitudinal and vertical) linear movements as well as the three rotations pitch, roll and yaw.  Because of temperature variations and telescope movement, the DECam longitudinal position, transverse positions and tip-tilt angles change slightly relative to the primary mirror between exposures.  The DECam hexapod is driven by the wavefront sensor CCDs and from laser alignment sensors that provide active control of focus and alignment.  The position of the camera barrel and focal plane is adjusted, positioning the DECam in an accurate and stable optical alignment and ensuring very precise and repeatable focus, tip-tilt, and transverse movements.  Generally the hexapod makes very small (~50 microns and 5 arcseconds) adjustments in transverse motion, in focus, and in tip-tilt, after every exposure to keep the camera aligned with the primary mirror.  (doc-db-1213-v7)

Figure 2.8.1  The DECam Hexapod

2.9 Correctors

There is no Atmospheric Dispersion Corrector (ADC) available with the DECam.  The Earth's atmosphere disperses the light from stars significantly when observing away from zenith. The effect is greatest and similar at U and B where the stellar image is stretched, for example, ~0.5" at a zenith distance of 45° (1.4 airmasses), and 0.9" at 60° (2 airmasses). We should test the blurring for long exposures at high zenith distances during commissioning.

3. The DECam Software

3.1 SISPI (Survey Image System Process Integration)

SISPI is DECam's read-out and control system.  Figure 3.1.1 shows a schematic overview of SISPI.

Figure 3.1.1 The SISPI Components shown in a block diagram. (DES-doc-1965-v8)

An exposure sequence starts when the observer sends a request to the Observation Control System or OCS (center of block diagram in Fig. 3.1.1).  The OCS first queries the state of the instrument and sends commands to the telescope control system to slew the telescope to the given location, to adjust the hexapod controller, and to load the requested filter.  Next the OCS preps the front end electronics to receive a new image. The OCS opens the shutter for the length of the exposure, and upon exposure completion, assigns an image builder process to assemble all pixel streams into the full image. The electronics are triggered to readout the CCDs. Image data flows from the DECam CCDs (Focal Plane) and the Monsoon front end electronics to the Image Acquisition and Image Builder systems before it is recorded on a storage device and handed over to the NOAO data transfer system (Data Management).  At a rate of 250 kpix/s it takes about 17 seconds to transfer the data from the focal plane to the computers of the Image Acquisition system. During this time the telescope can slew to its next position. 

Further details on SISPI can be found here [17].

3.2 Logging In and Starting Up

Before you can begin to take data you must log in.  When you click for the first time on any DECam GUI, you will be required to login. The user name is DECamObserver and the password is the proposal id of the observations taking place.  This password is valid only during the days of the run. It expires automatically afterwards.  By default, you will be logged in with authentication level user. At this level, you can watch the system, but you cannot control it. To control it, you will have to change your authentication level to observer.  This is done in the Observer Console, the first of the GUIs discussed below.

3.3 GUIs

The Graphical User Interfaces (GUIs) can be accessed from any browser once on the CTIO network at: http://system1.ctio.noao.edu:7001/apps [18], but full support for all the apps is currently provided only for specific browser versions.  The GUIs of most interest to observers are highlighted below, and more details on all GUIs can be found here [19].

OBSERVER CONSOLE

The observer console (Fig. 3.3.1) is the main GUI to operate the system.  The Configure button (top right) is needed when you first start the system. The status display in the upper left corner informs you about the system status and if SISPI is ready to take an exposure. The element next to the status display is used to for exposure and setup control. Currently it consists of three tabs labeled System Control, Exposure Control and Runtime Control. The middle part of the page is reserved for information about the exposure queue on the left and current and past exposures on the right. The GUI elements on the bottom of the page can be collapsed to have more space for the exposure information. When visible the section in the bottom left corner show an animated view of the image data flow through SISPI. Messages from the OCS are displayed in the text field in the bottom right corner.

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Figure 3.1.1.  Observer Console GUI

To take an exposure with DECam you have to submit an exposure request to the SISPI exposure queue, by selecting the Exposure Control view from the tab selector.  The top rows consist of parameters for your exposures.  Fill these out and add these to the observing queue by clicking the Add button.  Breaks can be added by pressing the Break button.  You may also upload a script containing a list of exposures and breaks. The button Enable Auto enables the auto pilot to set up the queue for you. To start running the queue, hit Go. To do a single exposure bypassing the queue, hit Step 1.  When your observations are in progress, the queue on the left shows the next exposures, the panel at the bottom shows an schematic view of the system as the current exposure goes on, the table on the right/center of the window shows the list of exposures already done or in progress. It includes information about the location of each image on disk.

A complete description of the Observer GUI can be found here [21].

COMFORT DISPLAY

A ds9 window on the comfort display monitor shows the last image taken, updating automatically as new images are completed. It shows all science CCDs and focus CCDs. A lower resolution png file (scaled down images of 1 graphic per crate) that can be easily displayed remotely is also produced and you can see it in the Comfort Display web GUI (Figure 3.3.2).  All of the images taken will be automatically transferred to the observer2 work station and can be more closely examined (i.e., with IRAF) from there.

Figure 3.3.2 Comfort Display GUI.

 

IMAGE HEALTH

This GUI displays detailed statistics on each CCD for each image.  Depending on which of the three image health views are selected in the bottom left corner, a color code indicates each CCDs (1) noise level, (2) sky level and (3) seeing.  The mean, variance per amps for the overscan and data regions are also displayed.  Clicking on a CCD will pop up the image of that particular CCD and the values for that ccd will display on the right panel.

Figure 3.3.3 The Image Health GUI.

ELECTRONIC LOGBOOK

DECam has an electronic logbook, in which exposures automatically are recorded.  Observers are encouraged to make comments.  Certain alarms will be automatically added to the logbook.  The logbook can be accessed either through SISPI, or independently, by pointing your browser to the url: http://system1.ctio.noao.edu:8080/ECL/decam [22] .  A login is required to post (but not to view) logbook entries.

Briefly, additional GUIs include:

GUIDER, which shows the region of interest for the guider CCDs,

ARCHITECT CONSOLE, which shows all the SISPI nodes and the status of their components,

VARIABLE VIEWER, which allows the user to monitor shared variables,

EXPOSURE TABLE, which shows the list of exposures taken recently,

EXPOSURE BROWSER, in which you can query the exposure table,

ALARM HISTORY, which lists the alarms generated by the system,

INTERLOCK VIEWER, which shows the status of the various SISPI interlocks,

ICS, which shows the status of various instrument control system components (i.e., shutter, filter changer, hexapod, etc.)

TELEMETRY VIEWER, which displays time series of variable relevant to the operating status of the system (i.e., the LN2 tank level and pressure, the CCDs temperatures and voltages, etc.)

SCRIPTS EDITOR, which allows the creation of an observing queue, including multiple exposures and dithering

and further details can be found here [19].

Evaluating, Recording and Reducing DECam Images

To be filled in later with help from the DECal team.