Columbia University Astronomy and Astrophysics: MDM 8K CCD

MDM Observatory's 8K CCD array project based at Columbia Astrophysics Laboratory: COMMANDS FOR TAKING EXPOSURES

Arlin Crotts e-mail: (arlinastro.columbia.edu)
Telephone: (212) 854-7899

BASIC COMMANDS FOR PRODUCING IMAGES:

There are nine DetCom commands which must be properly set up before taking new exposures, and which will be remembered by succeeding exposures (which might require only a "go" command. These are:

etime <sec>
Set exposure time, in seconds; <sec> can be a floating point number, but currently must be at least 3.

etype <c>
Choose image type o)bject, b)ias, d)ark, f)lat, c)omp or x for focus.

filename <fileroot>[????][$]
provides a image root name <fileroot>. If one wants the filename to increment in a number of N digits, one should follow the fileroot with N question marks ("?"). If one wants the filename to indicate what kind of exposures was taken, add a dollar sign ("$"), which will be replaced by the file type "o", "b", "x", "c" or "f" depending on the type specified in "etype". There are no spaces in the expression <fileroot>[????][$]. If one simply wants to take a temporary exposure, forego the question marks and dollar sign e.g. filename test, and the filename counter will not be incremented. (The file will be overwritten if one continues issuing the same command - although the most recent version of the file with the duplicate name would be rewritten to test.LAST in this case.)

fileno <n>
sets the value of the file number counter to "n", for use by "filename" if a counter was assigned using "?" characters. The file counter is reset whenever a new filename command is issued, so must be set manually using fileno to the desired value.

header <c> <val>
Assign image FITS header entry `<c>' ("comment", "observer", or "object") to `<val>'. Note that Telescope Control System information is automatically written to the header if the 8K/TCS link has been established. To do this, type:

tcs on
This will establish communications between the 8K and the Telescope Control System and allow the telescope position, etc. to be entered into the 8K image FITS headers. You will need to do this for every DetCom session before starting exposures. The command "tcs off" will suspend communications.

Set the mosaic readout format using "raster":

raster <xc> <yc> <xs> <ys> [<xb> <yb>]
- or -
raster CENTER <xs> <ys> [<xb> <yb>]
- or -
raster FULL [ BIN2 | BIN4 | BIN8 ]

Where <xc> <yc> is the center of a region of size <xs> by <ys> pixels. The current detector has a maximum image area of 2048 by 4096 pixels, so the center must be in the range [1:2048,1:4096], and the size must remain within this as well. The 4 parameters can be replaced by `FULL' to choose the full detector. Or, the first 2 parameters can be replaced by `CENTER', followed by <xs> <ys>.

Optionally two more arguments giving the x and y binning factors <xb> <yb> can be specified, or the single word `BIN2' `BIN4' or `BIN8' can be used for equal binning in x and y. (Actually the word `BIN' can be omitted if a single number follows `FULL'.)

Regardless of the syntax you use to enter the raster, a string will be generated with the equivalent raster command, using the `CENTER' `FULL' `BIN2' keywords if possible, and will appear on the status bar. This indicates the raster that will be used with the next `go' command. Changing raster/binning after a readout does nothing to the current image in memory, even if you do it before saving the exposure to disk.

When all of the above are completed, one can take a produce an image with

go <n>

where <n> is the number of images desired in a sequence (one image by default). An exception is in the case of a focus frame ("etype x "), for which one issues a series of "go" commands followed by a "readout":

go
go
go
.
.
.
go
readout

where one changes the telescope focus and pointing between exposures. The series of "go" commands produces a multiple exposure for the stars in the field which can be used to find the best focus in the sequence. One can use this technique if one intentionally wants to take multiple exposures, as some observers do for variable sources or standard stars.

If you need to abort an exposure, press "control-C" while the exposure is integrating. DO NOT PRESS CONTROL-C DURING CCD FLUSH OR READOUT, else you will quite probably need to restart DetCom or worse. After hitting control-C and you will be asked if you really want to abort the exposure. If you say "no" the exposure will simply resume. (This is the way to pause an exposure, due to clouds, etc.) If you say "yes" you have two choices:
1) you can type "readout" to complete the exposure with the current exposure time, which is probably less than what you requested with "etime" - you will be informed how long the actual time will be before you make this choice; or
2) you can simply start a new exposure, destroying any information from the current exposure. You may want to "flush" the CCD, depending on how much signal was collected.

" filter <n> [<filtername>] "
Move filter wheel to filter position <n>, where <n> is an integer from 0 to 5. The filter number <n> will be written into the FITS header.

To move the filter wheel, simply use the filter command. DetCom should not start an exposure until the filter wheel has stopped moving.
(For observers who want to be sure of their filter selection, the best way to assure that the filter has arrived in position when you want to begin your exposure is to enter the identical command, filter <n> [<filtername>], AGAIN e.g., using the up-arrow; if the filter wheel is still moving the second command will tell you so in a popup box. If an observer is nervous about whether the wheel arrived at appropriate filter, one can go out to the dome, flip on the front panel switch. BE SURE THE "GO" BUTTON IS OFF OR THE WHEEL WILL MOVE TO THE POSITION SET BY THE ROTARY SWITCH. Look at the position indicator lights. These lights decode the position correctly and can be trusted. If no light is illuminated, the wheel is off position - which is unlikely. If one needs to be absolutely sure, one can open the approximately 20cm square filter access panel on the underside of the filter wheel turret by turning both captive screws and reading the filter number of the filter slot that should be centered on the panel hole. The slot number should be 3 + the current filter number, modulo 6 e.g., 2 if the current filter is 5. Do not touch any filters and be sure to replace the filter access panel immediately. ALL OF THE ABOVE PRECAUTIONS ARE VERY UNLIKELY TO BE NECESSARY.)

The filter wheel is run by the GLACE box mounted on the side of the MASTER controller. If the box is ON, the filter wheel can be controlled by the GLACE front panel. If the box is off, the filter wheel is run by computer within DetCom. We have now added a sense line to tell DetCom that the front panel is on and that the computer is locked out. This way people won't have problems with the shutter not opening or the filter wheel not turning from computer control if they forget to turn the front panel off before returning to the control room.

The filter only turns in ONE direction. DO NOT FORCE THE WHEEL TO TURN WHEN INSTALLING FILTERS! The sense of the rotation is in increasing filter number (0 to 1 to 2 ... to 5 and back to 0, etc.). It takes approx 70 secs to reach the filter farthest from the position where you are located (e.g. going from filter 0 to filter 5).

Front Panel Control of filter wheel:

- Be sure the GO button is NOT in the GO position.
- Turn the GLACE box front panel ON
- Select the filter you want to go to using the rotary switch.
- Put the GO button in the GO position - the filter will turn to that position and stop.
- The appropriate red filter LED will light and stay on.

NOTE: You can leave the GO button in the GO position, but if you do so, the filter wheel will attempt to move to the filter selected by the rotary switch as soon as the front panel is turned on. This is not a problem - you just want to know what is going on. We prefer to leave the GO button off so as to avoid when turning on the front panel to see what the indicator lights read.

DISPLAYING 8K IMAGES

In addition to a merged, co-added square FITS image showing the entire array area (which is a FITS file with "-B8" appended to the image root name e.g., <fileroot>[????][$]-B8.fits) and generated by default each time the image is read out, one can generate one's own merged images using mdm8k . (Note: the mdm8k comand is not currently installed.)

To convert an image into a form that is easy to display as a single file, use the command mdm8k at a Unix prompt:

mdm8k <input> <m> <n> <output>

where <input> is a wildcard expression describing the 8 files produced by a single exposure, <m> has the value 1 (or 0) depending whether you want an approximate bias level to be subtracted in the displayed image (or not), <n> has the value 1, 2, 4 or 8 describing the displayed images binning (in addition to binning set by "raster"), and <output> is the resulting output image.

These merged images (default or mdm8k) will display the individual 2Kx4K CCD images in the following configuration:

7 | 6 | 5 | 4
0 | 1 | 2 | 3

where CCDs 4 through 7 are displayed "upside-down". In the usual 8K mounting arrangement on the telescope will put North towards the right (towards CCDs 3 and 4) and East towards the bottom (towards CCDs 0 through 3).

DISPLAYING IMAGES USING Saoimage

To convert an image into a form that is easy to display as a single file, use the command mdm8k at a Unix prompt:

mdm8k <input> <m> <n> <output>

where <input> is a wildcard expression describing the 8 files produced by a single exposure, <m> has the value 1 (or 0) depending whether you want an approximate bias level to be subtracted in the displayed image (or not), <n> has the value 1, 2, 4 or 8 describing the displayed images binning (in addition to binning set by "raster"), and <output> is the resulting output image.

To display the output from mdm8k, type at a Unix prompt: saoimage <output>

The image will be loaded into the SAOimage buffer and displayed on the screen. Note the top row of buttons in the SAOimage toolbar. The button selections in the bottom row of the toolbar, as well as the functions of the mouse buttons, will change depending on which of the five buttons in the top row of the SAOimage toolbar you have currently selected. Here is a brief description from the help page for saoimage:

In "Scale" mode, the mouse buttons control blinking of displays (saved by clicking with the same mouse button in the blink submenu button).
In "Color" mode, mouse dragging stretches and shifts the color map as per the contrast/bias, threshold/saturation, and gamma submenu selections.
In the color graph window, color table vertexes may be added or moved by clicking or dragging with the LEFT (red), MIDDLE (green), or RIGHT (blue) mouse buttons. The graph will update continuously only if tracking is turned on.
In "Cursor" mode, the mouse controls the size and locations of cursors. The rule is: LEFT button for position, MIDDLE button for size, and RIGHT button for angle or deletion (depending on the cursor).
In "Pan" mode, and in the pan window (regardless of mode), the LEFT button selects the center of the image, while the MIDDLE button selects the edge of the display, zoomed from the given center. When either function is dragged, the rectangle in the pan window shows the area that would appear in the display when the button is released.

For more detailed instructions, type help saoimage at the UNIX prompt.

DISPLAYING IMAGES USING IRAF
Create a window by typing

xgterm &

in one of the terminal windows, and move to the xgterm window. Also, create a display window:

ximtool &

In the xgterm window, change directories to the location where images are being stored:

cd /local/data/images

That directory should contain a file called "login.cl". If not, create such a file by running mkiraf. Next, start IRAF by typing:

cl

You probably want to increase the size of the display window image buffer to a size larger than the default by typing:

set stdimage = imt4096

To convert an image into a form that is easy to display as a single file, use the command mdm8k at the IRAF prompt:

!mdm8k <input> <m> <n> <output>

where <input> is a wildcard expression describing the 8 files produced by a single exposure, <m> has the value 1 (or 0) depending whether you want an approximate bias level to be subtracted in the displayed image (or not), <n> has the value 1, 2, 4 or 8 describing the displayed images binning (in addition to binning set by "raster"), and <output> is the resulting output image. (This discussion assumes you have used mef off to produce non-multiextension FITS images. If not, you need to use the IRAF msc package.)

To display the output from mdm8k, type at the IRAF prompt:

display <output> 1

(If you don't like the greyscale range chosen by default, you can choose your own white and black pixel values by using:

display <output> 1 zs- zr- z1=<blackvalue> z2=<whitevalue>.)

One can pan around the image using the middle mouse button in the panner inset in the upper right of the ximtool window, and one can zoom using the middle mouse button in the main ximtool window. One can change the contrast windowing using the right mouse button. By choosing the "Control Panel" in the "Options" menu, one can manipulate the image in many other ways.
Much more information about ximtool can be found here. (PostScript, 118kB)

Using the command

imexam

one can interrogate the image e.g. find the image width by pressing the "a" key when the circular cursor is positioned on a star. The output produced includes three different methods of measuring the full-width-half-maximum of the image in the last three columns of the text outout in the IRAF window. There are many other useful keystrokes, which are explained for you if you press the "?" key within imexam when the cursor is in the ximtool window.

DISPLAYING IMAGES USING Vista (NOT CURRENTLY INSTALLED!)

To display the output from mdm8k (or the default merged file), type at a Unix prompt: Vista

then within Vista, read the image with: rd 1 <output>

and display the image with: tv 1

You can change the greyscale windowing by adding the black and white pixel value after "tv 1", and you can adjust the image contrast, zoom and centering using the mouse.