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Subsections

The GenI CCD Camera (Imaging)


THE GENI CCD CAMERA SYSTEM (IMAGING)

(Rev. August 17, 2011)

System Description

The GenI CCD system uses the Generation I controller developed by San Diego State University (Leach group) and now managed under the company name of Astronomical Research Cameras, Inc. (ARC). The detector is an SI424A scientific grade CCD imager manufactured by Scientific Imaging Technologies, Inc. (SITe).

The SDSU controller is mounted directly on a CCD liquid nitrogen dewar with a serial fiber optic communications link leading from the controller housing to a PCI interface card in a Sun Ultra5 computer. For operation with the GenI controller, the toggle switch on the back panel of the computer should be set to ``GEN I''.

The power supply for the controller is a box mounted on the east side of the 40-in telescope tailpiece, and the box has a switch which must be turned on for the controller to operate. This power supply is plugged into an AC outlet on the tailpiece which receives power only when the CCD switch on the bottom rack panel in the control room is also turned on. The controller operates an electromechanical shutter mounted inside the tailpiece, either under software control or by way of a toggle switch on a black box near the controller power supply.

The Sun Ultra5 control computer is named crux and uses the Sun Solaris2.8 operating system with the CDE window system. The user interface to the CCD controller is the a program called Voodoo developed by SDSU, executed with the command juju, which runs a version of the program which has been modified locally for use with the GenI controller and CCD on the 40-in telescope at Fan Mountain. Images are stored on disk in FITS (*.fits) format and can be transferred from disk to magnetic tape (4mm DAT DDS4). A typical full frame requires 8.6MB of storage. Images can be displayed and analyzed using IRAF tasks. This manual does not explain the details of using IRAF, but IRAF has an extensive built-in help facility, and full documentation is available on the web at the IRAF Project Home Page.

CCD Camera Specifications

Table 1 summarizes the current configuration of the CCD camera, chip, and dewar


Table 1: Current configuration of the CCD chip and camera.
Spec Value
Dewar IR Labs
  liquid nitrogen cooled, 1 ${\ell}$ capacity
  heating resistor
  hold time $\sim36$ hours
Chip SITe 2048 $\times$ 2048 CCD Imager
  back-illuminated, thinned to enhance blue response
Operating Temp. unstable above $-100^\circ$ C
  optimal operating temperature $\sim -110^\circ$ C
  lowest achievable temperature $\sim -134^\circ$ C
  temperature readout may be erratic
Format 2048 (cols) $\times$ 2049 (rows)
  24 $\mu$m square pixels
Field of View $12.5^\prime\times 12.5^\prime$
 on FMO 1-m 1 pixel = $0.365^{\prime\prime}$
CTE 0.99998-0.99999
Dark Current negligible at $-110^\circ$ C
Full well $>150,000$ electrons/pixel
Readout 85 s for full frame (2049 rows $\times$ 2088 columns)
  maximum ADU = 65535
  single amplifier readout mode
  no on-chip binning
  no subarray capability
Low Gain Gain: 3.84 $e^-$/ADU, Readout noise: 8.9 $e^-$
High Gain Gain: 2.06 $e^-$/ADU, Readout noise: 16.9 $e^-$


Fig. 1 is a plot of quantum efficiency vs. wavelength for the CCD in the GENI camera.

Figure 1: Plot of CCD chip quantum efficiency vs. wavelength. The relevant curve for our chip is the top solid curve.
\begin{figure}\begin{center}
\epsfig{file=geni/QE.ps,height=3.0in} \end{center}\end{figure}

The Dewar

Description

The CCD is mounted on a cold finger in an evacuated chamber behind a fused silica window in a liquid nitrogen dewar and is cooled by direct contact of the cold finger with liquid nitrogen. The dewar, manufactured for ARC by Infrared Laboratories, Inc., has a capacity of 1 ${\ell}$ and a hold time of about 36 hours. The dewar is capable of cooling the CCD to a temperature as low as about $-134^{\circ}$C, but normally a heating resistor in the dewar is used to regulate the CCD temperature to some optimal working value between $-110^{\circ}$ and $-100^{\circ}$ C.

A small electric fan with a rotary on/off switch is mounted on the tailpiece of the 40-in telescope to blow warm air through a tube to a connector near the top of the CCD dewar to produce a continuous flow across the dewar window to prevent fogging. It should normally be left running constantly while the camera is mounted on the telescope.

Normal Operation

  1. At least 24 hours before your scheduled observing night, send email to Nick Nichols (nichols@astsun.astro.virginia.edu) and ask him to make sure the CCD dewar is filled and the defogging fan is running. Top off the dewar at the beginning of the night, and again at the end of the night as a courtesy to the next observer if observations are scheduled for the following night.

  2. Filling the dewar takes about 15 minutes if it is still cold from the previous filling, but up to 40 minutes if starting from room temperature in summer. It takes about 4 hours for the dewar to cool from room temperature to the optimal operating temperature of $-110^\circ$ C, so it is important to allow sufficient time for the cooldown before observing. You should therefore have the dome and catwalk doors open (in order for the air inside the dome to reach temperature equilibrium with the outside air, which is necessary for good seeing) and the dewar filled before sunset.

  3. To fill the dewar, attach the connector at the end of the hose from a 25$\ell$ LN2 tank to the CCD dewar and fill the dewar with liquid nitrogen until you see liquid spilling out the side vent of the connector. It takes some time for the hose to cool sufficiently to allow nitrogen to pass without evaporating, and the connectors will become coated with frost. When done filling, unscrew, cover, and stow the fill tube and nitrogen tank. Then attach the spill-tube to the CCD dewar. The spill-tube will prevent nitrogen from spilling from the dewar at moderate angles; however, when moving the telescope to large zenith distances such as when the lens cover is removed or replaced or during dome flats, nitrogen will still spill out. This is normal, but try to avoid it when the dewar is full. You can avoid some spilling by removing the telescope cover before filling the dewar, if conditions permit.

  4. To avoid condensation or frost on the dewar window, be sure the defogging fan is running.

Potential Problems and the Dewar Vacuum

As of January 2007 the GenI and GenII CCD dewars have new vacuum valves which share a single new gauge which can be connected to either dewar. The vacuum is good for several days without pumping as long as the dewar is not allowed to warm up. The LN2 hold time is about 24 hours.

As a routine, keep filling the dewar every 24 hours or so as long as the camera is in use on the telescope. Leave the camera control software up and running on crux to check the temperature, with temperature regulation set for $-110^{\circ}$C. For the GenI dewar be sure the defogging fan on the telescope tailpiece is running to keep frost from forming on the dewar window. This requires the CCD switch on the rack panel in the control room to be ON, to supply power to the camera controller and the defogging fan.

The equipment for reading the vacuum gauge can usually be found in the storeroom on the dome floor level, or in the spectrograph room. It consists of a power supply transformer wired up to a 9-pin D connector and a digital multimeter. The D connector should be plugged into the connector on the vacuum gauge (before plugging in the power supply). When the meter is switched on to the 2VDC scale the voltage should ideally read 1.000V, which translates to roughly 0.01$\mu$ (0.01mTorr). Every increase of 1V is a factor of 10 in pressure, so 2V would be $\sim0.1\mu$, 3V is $\sim1\mu$, and 4V is $\sim10\mu$. According to the dewar manual, problems (such as outgassing and difficuly holding LN2) set in when the pressure reaches 5$\mu$, so for our purposes the vacuum is lost if the reading is over 4V. In practice, the dewar will probably not hold LN2 unless the reading is 2.5V or less.

To pump the dewar, connect the stainless steel hose from the vacuum pump to the dewar flange with a Quick Flange (QF) connector, but leave the dewar valve closed. The seal is made by compression of an O-ring between mating flanges by finger closure of a wingnut on a metal clamp, and the connectors on the dewar and the pump hose should be kept sealed with cover flanges when they are not connected to each other.

Plug in the vacuum pump to a 220 VAC outlet, using the extension cord if necessary. Press the PUMPING button to turn it on. The vacuum pump is a two-stage pump system which includes a controller. The roughing pump operates by itself first. The turbopump should spin up automatically when the roughing pump has lowered the pressure far enough for the turbopump to safely operate. Allow the turbopump to evacuate the hose for at least 30 minutes. After that time, if the turbopump is spinning (check the speed indicator if you can't hear it), the pressure should be low enough to safely open the vacuum valve on the dewar. (If the turbopump is not spinning, do not open the dewar vacuum valve! If you cannot find a leak in the hose or fittings that you can repair, the pump may need maintenance.) If the dewar pressure reading is not less than 3V ($\sim1\mu$) after 3 hours, there is probably a leak of some kind which must be fixed before the camera can be used.

When the pressure reading has dropped to 2.5V, close the dewar valve, turn off the vacuum pump, and fill the dewar. Ideally, the dewar will fill completely and the pressure reading will drop to 1.0V ($\sim0.01\mu$). If the dewar does not fill completely in less than 20 minutes, let it cool down for an hour or more, check to see that the pressure is still low and pump again if necessary, then try filling it again. As long as the dewar is kept filled and the pressure reading remains less than 2.5V the camera should work properly. When turning off the vacuum pump, wait until all rotor motion has stopped completely before unplugging the cord from the power outlet.

Operating the CCD Camera

Login

  1. Log onto crux as user genicam with password juju&u$r.

  2. Before proceeding, insert a blank tape into the DAT drive and enter the command mt -f /dev/rmt/0n status in any terminal window to verify that the tape drive is working. (Check the label on the tape drive for the device name currently in use.) You should get a message resembling:

    crux% mt -f /dev/rmt/0n status
    Sony 4mm DAT tape drive:
       sense key(0x6)= Unit Attention   residual= 0   retries= 0
       file no= 0   block no= 0
    

    If you don't get the above message try power cycling the tape drive. Saving your data to tape is one of the last and most important things you'll do at the end of the night, so it's best to make sure this will go smoothly at the outset.

  3. The home directory in which you will be working on crux is /crux/genicam. In this directory is the file login.cl, a startup file used by the IRAF program, and the subdirectory juju, which is used to store setup files for the camera controller.

    In addition to the Sun internal 19GB disk, there is also an external 34GB disk attached to crux which appears as a directory called /data. All raw image files should be stored in the subdirectory /data/genicam. In this directory (i.e. after entering cd /data/genicam), create a unique subdirectory for your images.

Starting IRAF

First start the DS9 image display program by entering ds9 & at the prompt in a terminal window. Then open an xgterm terminal by entering xgterm &. In the xgterm window, from directory /crux/genicam, enter cl to start IRAF. To see a help page for any IRAF task, enter help task at the cl> prompt. One way to run any IRAF task is to enter epar task, edit any parameters that you want to set or change, then type :go and hit RETURN. Tasks can also be run directly from the IRAF command line.

Starting the Voodoo Program

  1. Start the modified version of the Voodoo camera control program by entering the command juju in a terminal window. The Voodoo Main window should appear on the screen (Fig. 2).

    Figure 2: The Voodoo Main window.
    \begin{figure}\begin{center}
\epsfig{file=geni/jpics/JMain.ps,width=4.0in} \end{center}\end{figure}

  2. Some configuration parameters for Voodoo may be set using the popup windows available from the menu bar of the Main window. First select Setup from the menu bar to bring up the Setup window (Fig. 3). Load the file /crux/genicam/juju/juju.setup and click Apply to initialize the camera controller, then close the Setup window.

    Figure 3: The Voodoo Setup window.
    \begin{figure}\begin{center}
\epsfig{file=geni/jpics/Setup.ps,width=4.0in} \end{center}\end{figure}

  3. The Subarray popup window will not do anything until the necessary readout instructions have been added to the code that is downloaded to the processors in the GenI controller. The Voodoo Focus Sequence will not work either, since it depends on the same subarray readout instructions.

  4. Select Parameters from the menu bar to bring up the Controller Parameters window. Select the Temperature tab and set the array temperature control to -110 C (Fig. 4). Click Apply Above and close the Controller Parameters window.

    Figure 4: The Voodoo Controller Parameters window, Temperature tab.
    \begin{figure}\begin{center}
\epsfig{file=geni/jpics/Temp.ps,width=4.0in} \end{center}\end{figure}

  5. Select Debug from the menu bar, then open the Developer Parameters window by selecting Development. Select the Gain tab, set the Low video gain button, then click Apply Above (Fig. 5). Close the Developer Parameters window.

    Figure 5: The Voodoo Developer Parameters window, Gain tab.
    \begin{figure}\begin{center}
\epsfig{file=geni/jpics/Gain.ps,width=4.0in} \end{center}\end{figure}

  6. If you would like your image headers to contain FITS keywords other than those required for basic formatting, you can use the FITS window (Fig. 6). If TCSLink is checked, the FITS header parameters labeled Universal Time, Local Sidereal Time, Equinox, Airmass, Hour Angle, Right Ascension, and Declination are updated automatically over a serial link to the telescope control PC at the beginning of each exposure or whenever you click Update. The filter parameters labeled Filter 1, Filter 2, Filpos 1, and Filpos 2 will be updated also, regardless of the setting of the TCSLink checkbox. The FITS header parameters with white backgrounds may be edited manually.

    Figure 6: The Voodoo FITS window.
    \begin{figure}\begin{center}
\epsfig{file=geni/jpics/Fits.ps,width=6.0in} \end{center}\end{figure}

Filter Control System

The filter wheels inside the telescope tailpiece above the CCD camera shutter can be operated from the filter control panel on the south face of the tailpiece when the switches are set to LOCAL or from the filter PC computer in the control room when the switches are set to REMOTE. Filter wheel A (the lower one, also known as filter wheel 1) has 4 openings spaced at 90$^{\circ}$ intervals and holds 6-in square filters. Filter wheel B (the upper one, also known as filter wheel 2) has 6 openings spaced at 60$^{\circ}$ intervals and holds 4-in square filters.

To load a filter into a filter wheel, first open the filter wheel access door (the rectangular panel above the filter control panel held shut by clamps) so you can see the filter wheels inside the tailpiece. Switch the filter wheel to LOCAL and use the SLEW button on the filter control panel to rotate it. Open the lock at the edge of the filter opening you select, slide the filter into the slot, then close and gently screw down the lock with your fingers. The filter opening in the telescope light path is the one diametrically opposite the one at the access door.

A Filter Control window (Fig. 7) has been added to Voodoo to provide an interface to the filter control system using a serial link between crux and the filter PC.

Figure 7: The Voodoo Filter Control window.
\begin{figure}\begin{center}
\epsfig{file=geni/jpics/Filter.ps,width=4.0in} \end{center}\end{figure}

  1. To control the filter wheels remotely, begin by turning on the filter PC (white tower) underneath the table in the control room. The filter PC boots to a DOS prompt.

  2. Enter cd c700$\backslash$lotion at the DOS prompt to get to the correct directory. Then enter potion to start the version of the filter control program that operates with Voodoo.

  3. The filter PC first checks for initialization:

    Assuming filter wheel positions 1,1 to start.
    If not, set positions on LOCAL ("GO HOME" buttons).
    Enter "Y" when ready to start.
    

    If necessary, go upstairs to the dome and HOME both filter wheels under LOCAL control. This sets filter positions 1,1 (filter openings numbered ``1'' in the telescope light path). Then switch both wheels back to REMOTE and go back down to the control room.

  4. Now click Init in the Voodoo Filter Control window. This must be done to initialize the filter wheel positions to 1,1 in the Voodoo software.

  5. To move the filter wheels, select the desired Filter Wheel position numbers with the radio buttons, then click Move. Voodoo calculates the necessary moves, sends commands to the filter PC, and updates the Filter Control display. Although the updates appear immediately in Voodoo, the filter PC monitor displays the actual move command while it is being executed and gives a confirmation when it is done.

  6. The numerical filter positions and the corresponding descriptive labels in the text fields in the Filter Control window are always updated automatically in the Filter Control window and included as FITS header parameters FILPOS1, FILPOS2, FILTER1, and FILTER2. The descriptive labels may be edited to match the filters loaded in the wheels, and these configurations may be saved and loaded as filter setup files with extension *.flt using the Load and Save buttons.

  7. The Home button moves the filter wheels to the initialized position 1,1.

  8. The Exit button causes the filter PC program to quit.

Scope Control System

A Scope Control window (Fig. 8) has been added to Voodoo to allow the user to load an object list and command the telescope to slew to a selected list object using the serial link to the telescope control PC.

An object list must be a simple text file with extension *.lst, with one line per object. The format of each object line is arbitrary and may include any number of fields of any reasonable length, except that each line must include the RA and DEC separated by whitespace (spaces or tabs) only, each in sexagesimal format with no whitespace padding.

Figure 8: The Voodoo Scope Control window.
\begin{figure}\begin{center}
\epsfig{file=geni/jpics/JScope.ps,width=4.0in} \end{center}\end{figure}

  1. In the Voodoo Scope Control window, click Load to select and load an object list file.

  2. Enter the Equinox of the coordinate list in the Equinox text field. This must be done only once per loaded list and may be changed at any time.

  3. To slew the telescope to a list object, swipe the coordinate section of the object line with the mouse (RA and DEC fields together) so that it is highlighted, then click Slew. The slew commmand will be echoed in the main Voodoo Information Window and the TCS will immediately slew the telescope. As always, a slew can be aborted with the Stop slew command (8) from the TCS Movement menu.

  4. Object lists cannot be edited or saved from the Scope Control window.

Taking an Exposure

  1. Check Save to Disk in the Main window and enter the full pathname of your image directory and a beginning filename for your images. If Auto Incr is checked, the numeric part of the filename will be automatically incremented with each new exposure. Otherwise you must enter a new filename for each new image.

  2. For normal exposures, check Open Shutter, enter the desired exposure time, and click Expose in the Main window.

  3. Display and analyze the images with IRAF and DS9.

Ending a CCD Session

  1. To copy your image files to DAT tape, change to your image directory in any teminal window, then use the unix command
    tar cvf /dev/rmt/0n .

    to write all your image files to tape. (This takes about half an hour for 100 images.)

  2. After your images have been written to tape, rewind the tape and take the tape drive off line by entering mt -f /dev/rmt/0 rewoffl, then remove your tape from the drive.

  3. Exit Voodoo (remember to home the filter wheels and exit the filter control system from the Filter Control window first), log out of IRAF, quit DS9, exit xgterm, and log out of the CDE window system.

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