Observers Manual for Megacam on the MMT

Megacam Observers Manual

Original: December 16, 2004

Revised: January 4, 2005

B. McLeod, N. Caldwell, G. Williams

The latest version of this document may be found on the web at:

Table of Contents

Megacam Observers Manual

Table of Contents

I. Introduction

II. Megacam Description

III. Observing Instructions

IIIa. Quick Start Up Procedure

IIIb. Detailed Start Up Procedure

IIIc. How to Take Images: mice

IIId. Typical Observing

IIIe. Focusing, Collimating & Guiding.

IIIf. Displaying an image.

IV. ds9 Information

V. Automatic Data Logging

VI. Data Storage

VII. Filters

VIII. Exposure Time Calculator

IX. General Comments and Information

X. Problems

XI. Data Reduction for Megacam

I. Introduction

Megacamis a large mosaic CCD camera with a 24'x24' field-of-view, made up of 36 CCDs with 2048x4608 pixels. Its 0.08" pixels offer good sampling under the best seeing conditions at the MMT. Megacam has 8 available filter slots and in-dewar guidingand focusing.

Figure 1 The Megacam focal plane array; 36 CCDs with 2048x4608 pixels.

Figure 2 Cut-away engineering drawing of the Megacam instrument.

II. Megacam Description

Megacamis a CCD imager currently installed at the f/5 focus of the 6.5m MMT. It was designed and built by a team at the CfA. It employs 36 “science” CCDs and two “guide” CCDs. All of these are thinned,backside-illuminated E2V (ex-Marconi, ex-EEV) CCDs, with a 2048x4608 pixel format. The Megacam plate scale is 0.169 mm/arcsec so each 13.5 micron pixel is 0.08 arcsec on the sky. The field-of-view of each CCD is 160”x 360”, for an effective science field of 24’x 24’. The science CCDs are mounted adjacent to one another, with a 1mm gap (6 arcsec on the sky) between them in one dimension. The centers of the guide CCDs are mounted about 20 arcmin off axis.

Each science chip is read out with 2 amplifiers;the unbinned readout time is approximately 34 seconds and the binned 2x2 readout time is approximately 10 sec. Currently there is also an overhead of 25 seconds for writing to disk. Digitization is done with a 16 bit ADC, so data values up to 65,536 are recorded. Dark counts are normally insignificant, but are high after the controller has been powered up, and also after the chips have been exposed to very bright light (such as room light). QE curves for the chips are shown in Figure 3 below (also available in pdf format).

Figure 3 QE Curves for the Megacam CCDs.

The guide CCDs are read out with a separate set of electronics and different computer. The guide CCD can see light when the shutter is open OR when the shutter is closed through special openings in the shutter. There is no pickoff mirror, and no TV guider or acquisition for Megacam. For guiding, before the shutter to the science CCDs is opened, a single 0.5 second full acquisition exposure is taken, on which a star is identified in each of the two images. Subsequent 0.5 second exposures are taken automatically for guiding during the science exposure.

The CCDs are in a LN2 dewar, which has a hold time of about 30 hours. This dewar is mounted to the Megacamtopbox.The topbox, shown in Figure 4, houses two filter wheels and the shutter. Each filter wheel can accommodate 5 filters. The available filters include the Sloan u', g', r', i', and z' filters, and a set of guider blocking and polarizing filters(see Section VII. Filters) for the transmission curves and instructions for using other filters.

Figure 4 Megacam topbox.

The topbox shutter is a dual blade design, similar to the type used in SLR cameras. This allows accurate exposures over the large focal plane for times as short as 0.100 seconds. Short exposures are accomplished by using a narrow opening between the plates, and scanning this slit across the CCDs.

Because the MMT is an Alt-Az telescope, the focal plane rotates, and therefore a derotator takes that rotation out for imaging purposes. The typical procedure for Megacam is to use a position angle of 0˚ when observing objects south of δ=31˚41’19.6” (the latitude of the site) and a position angle of -180˚ for objects north of that declination. This aligns one side of the CCD array with North. For a position angle of 0˚ North is to the right and East is up. When using these position angles an object can be tracked from horizon to horizon without hitting a rotator limit (±175˚).

The Sun station called Packratis used to acquire the science images., while Cfaguider acquires the guider images. Note that different keyboards are required for the different computers. Images are taken by using a graphical user interface called mice. These images are stored on a 1 Tb disk on Packrat, in multi-extension FITS format. The directories are called /data/ccd/MEGACAM/yyyy.mmdd (e.g., /data/ccd/MEGACAM/2005.1014). The files themselves will have names like a0001.fits, where "a" is a prefix that the observer can select.

III. Observing Instructions

IIIa. Quick Start Up Procedure

1.)Log on to packrat as user "megacam". Ask a staff member for the location of the password.

THERE ARE TWO OPTIONS FOR OPENING THE MEGACAM GUIS

______

OPTION a - Recommended - Open the GUIs on fiber.
______
2a.) Log on to fiber as user "john" (megacam passwd).

Start the window manager.

startx

Open a terminal window by clicking on the shell icon on the toolbar at the bottom.

Allow packrat to display on fiber.

> xhost +packrat

On a window onpackrat start the GUIS

> startmegacamguis
______

OPTION b - Open the GUIs on packrat (only if fiber is not working)
______
2b.) Start the Megacam Status GUI.

> megacam &

Start the SHUTTER/FILTER MONITOR GUI.

> topper &

Start the MMTO Status and Exposure Status display GUIs.

> mountdisplay &

> expdisplay &

Start the paddle GUI.

> paddle &
______
3.) Home the filter wheel and shutter.
- in the Megacam Status gui push "Home"
4.) Scan the filter wheel.
- in the Megacam Status gui push "Scan"
5.) Start the observing gui.
domice
6.) If there has been a power cycle of the CCD controller, reset the
detector server.
- in the mice gui push "Detector Up/Down" to the right of
"camera servers" at the top of the gui. (You may also need to push the
"Hk Up/Down" button - to be tested)
You must take a "throw-away" image because the first image after a
a detector reset will be bad.

______
7.) Log in to alewife as "mmtobs".
8.) Log in to cracker from an alewife terminal.
> ssh megacam@cracker
9.) Start a vncserver on cracker.
> vncstart
10.) Exit from cracker.

> exit

11.) Start a cracker vncviewer from alewife.
> vncviewer cracker:1
12.) Start the postproc server from a terminal in the cracker vnc session.
> postproc
13.) Start a ds9 window from a terminal in the cracker vnc session.
> ds9
14.) Start xgterm, and start IRAF in the xgterm.
> xgterm -sb
> cl
______
15.) Log into cfaguider as "guider" (megacam passwd).
16.) Open a terminal window (Right click --> Programs --> Terminal).
17.) Restart guider software.
> restart_guider
This can also be used to kill and restart the guider if it is already running.

18.) If the guider GUI doesn’t appear open another terminal and start it manually.

> guidegui

IIIb. Detailed Start Up Procedure

Megacam observers will typically want five windows displayed:

1.) megacam - displays the status of the megacam servers.

2.) topper - displays the status of the shutter and filter wheels.

3.) expdisplay - displays the exposure status.

4.) mountdisplay - displays the telescope status.

5.) paddle - allows the observer to offset the telescope.

These windows can be displayed on either packrat or fiber. Displayingthem on fiber conserves desktop space on packrat.To start them all on packrat run the individual programs in the followingorder:

> megacam &

The ideal configuration for the megacam window is for all the boxes to be colored green.

Several of the boxes are red upon startup. The directions which follow indicate how to start the servers which make all the boxes green. Ifduring observing something appears to "hang" check the Megacam Statuswindow for red or yellow boxes. If any of the lights are red check thefollowing:

1. Detector: Click Reset

2. Guidecam: See below

3. Telescope: Telescope operator needs to start telserver

4. Topserv or Scanserv: Click Reset; If no action check power to topper

VME crate in rack

5. Homed: Click home; if you get an error, click Clear and try again

6. Scanned: Click scan after doing home

7. ESTOP: Is emergency stop pushed?

8. Filt load: Is either filter load button on topbox set to Load?

9. Temp fault or amp fault or Curr overload: Is topbox power on?

To start the detector server do one of two things:

On packrat type the following:

> ccdmegacam restart

OR

In the Megacam Status GUI push the "Reset" button to the right of

of "CCD:".

Start the topper, mountdisplay, and expdisplays from a packrat terminal.

> topper &

> mountdisplay &

> expdisplay &

In the Megacam Status GUI push "Home" which initializes the shutter and filter wheel. You will see the shutter and filter wheel move in theSHUTTER/FILTER MONITOR window.

In the Megacam Status GUI push "Scan" which rotates both filter wheels around to scan the barcodes. You will see the filter wheels rotate in theSHUTTER/FILTER MONITOR window. When this is complete the filter names willappear on the GUI.

Occasionally, one of the status displays will stop updating. In this case close all the GUIs and do startmegacamguis again.

IIIc. How to Take Images: mice

Data taking is done through the observing GUI/catalog manager calledmice, a graphical interface to the CCD controller and telescope.The mice GUI, shown in Figure 5, allows the observer to observe in a traditional manner (one image at a time) or to perform a complex observing sequence using a “Dither Tool”. Rather than entering separate, time-consuming commands to do things such as move the telescope, acquire a guide star, change the filter, enter an exposure time, and type in an object name, the Dither Tool refers to an established catalog to perform all those functions, and more. Hours of observations can be planned in advance and executed without astronomer intervention, in principle.

To start the mice interface type:

> domice

The mice interface is split into a top, middle, and bottom section. The top and bottom sections have a gray background and the middle section has a light blue background. The top is used to control the instrument power and servers and to home the top box components. The status of each subsystem is indicated with color using the color convention of all the MMT f/5 instruments; green means OK or RUNNING, yellow means HUNG (perhaps only temporarily), and red means NOT RUNNING. The observer generally will not change anything in the top section of the mice GUI. However, if any of the servers go down during observing they can be restarted by pressing the appropriate server button.

The middle section is mostly informational. The first line displays two CCD temperature measurements, a vacuum gauge “INIT” button, and an ESTOP indicator. The CCD temperatures will typically read approximately -120 C. During normal operation the ESTOP (emergency stop) indicator should read “on” indicating that none of the ESTOP boxes are depressed. The second line of the middle section displays exposure status information including image type, exposure time, and exposure status.

The third and fourth lines of the middle section display the status of the Dither Tool scheduler. The fifth line provides a means of clearing the Filter or Shutter in the case of a HUNG system.

Figure 5 The mice GUI; Startup tab selected.

Figure 6 The mice GUI; Standard Ops tab selected.

Figure 7 The mice GUI; Dither Tool tab selected.

The bottom half of the GUI contains three tabs. The “Startup” tab, shown inFigure 5, is used to start instrument servers and home the instrument components. Most observers will not need to use this tab because the Megacam subsystems are typically running.

The “Standard Ops” tab, shown in Figure 6, is the part of the GUI that is used for normal observations. The TELNAME, INSTRNAME, and DETNAME must be set correctly in order to save the proper telescope and camera information to the header. IMPORTANT: Make sure none of these tabs indicates “Test”. CONFIRM: The correct values should be “mmt_f5”, “megacam”, and “megasplit”. The second line displays information about the current observing program. These should be modified at the beginning of your run. The third line contains information about the current exposure. The “BINNING” and “GUIDING” entries must be entered by the observer. The drop down “GUIDING” box allows the user to choose one of three guiding modes; “current” to use the current autoguider setup, “manual” to use manual guiding, or “none” no use no guiding. The mice program uses the GUIDING information ONLY to notify the user if guide stars are lost. Therefore any form of guiding can be used at any time regardless of the GUIDING setting at the expense of a few pop-up windows.

The fourth line in the “Standard Ops” tab provides information about the state of a dithered observation.

The fifth and sixth lines in the “Standard Ops” tab are used to setup and perform manual observing. There are two boxes which allow the user to select the filters for both the upper and lower filter wheels. If you only want one filter make sure one of the boxes is empty. The upper filter wheel currently contains two polarization filters, a science blocking filter holder and a guider blocking filter holder. The lower filter wheel contains the Sloan u’, g’, r’, i’, and z’ filters. To block the guider detectors or the science detectors for calibration purposes choose gblk or sblk, respectively. The gblk filter should be used when obtaining twilight flats to minimize the scattered light when the sky is bright. The image type is set with the drop down menu at the left of the sixth row. The current options are given in Table 1.

Table 1 Megacam image types.

The first, last, and exptime allow the user to enter the first and last image in a dither sequence and the exposure time in manual mode. This row will vary depending on the image type selected. If a focus image is selected the default entries become first=1, last=7, and exptime=120, and two more entries, “start” and “step” are added to that row. The start and step entries are used to record the starting hexapod z-position and the step between each image.

If the image type is set to domeflat a button is added to the row to indicate whether the continuum lamps are on or off and to allow the user to turn them on. CONFIRM.

If the image type is set to bias no exposure time entry is available.

The “Title” button allows the user to enter the title for the current image. This will be written into the fits header with the keyword OBJECT. The mice GUI automatically queries the user for the title when the observer presses “Go”. The “Clear” button is used to clear the camera in the event of an error. If the “Go” button turns red because of an error during observing the user must press “Clear” to reset the system.

The bottom line of the “Standard Ops” GUI allows the user to PAUSE and exposure and then after pausing to ABORT, STOP, RESUME, or CHANGE the exposure time.

The last tab is the “Dither Tool” tab, Figure 7. This section of the GUI is used to set up an observing schedule. This schedule is very versatile as it allows you to offset the telescope, specify filters for each image, and specify exposure times for each image.

A typical dither which fills in the gaps between the CCDs is named line5.cat. This consists of a line of 5 images at a 15˚ angle and 50" offsets (confirm).

The configuration catalogs for mice are stored on packrat in:

/home/megacam/

To list the offsets for a catalog use the following command:

> column < line5.cat azoff eloff

To use the dither tool:

bring up the dithertool tab

a. Vanilla DITHER

keep the exptime and filter entries BLANK

b. More DITHER

enter 1 or more filter configurations - space separated

enter 1 or more exptime settings

c. Twilight Dither

I think an exptime of -1 will trigger mice to try an automatically compute the exptime

To take exposures:

a. Select the exposure type under the "object" button.

To AUTO DITHER

b. Select the dither file using the DITHER file select button

(or turn this off using the CANCEL option - field must be blank)

c. Select the starting/stopping row number from the dither table

d. If no exptime exists in the either file, enter it here

To MANUAL DITHER

b. Select the filter, azoff, eloff (or leave them blank and

they won't move)

c. One can optionally press the ConfigObs to apply these settings.

d. Set the exptime under the "object" (or other) exposure button

At the beginning of a sequence the software currently pops up a window saying "Error Can't Lock Guide Stars". Ignore this and choose"OK Start Exposure". When using mice try to choose guide stars which are close to the center of the guider chips so that they stay on the chip after a dither.