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International Occultation Timing Association * European Section * |
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International Occultation Timing Association * European Section * |
The IOTA Occultation Camera (IOC)
Preliminary Operation Manual (01/2002)
ATTENTION!
The following is essential for a proper installation of the camera! Please do it carefully, read the instructions! The camera will not work properly if it is not set with great care!!!!!
Setup the delay times for the ADC (T command)
Depending on the speed of the connected computer system different delays have to be set. After typing the command T the AD-Delay value can be edited. It defines the time interval up to the AD conversion is finished (about 10 µsec). If the value is too small, the conversion is not ready at the time of interrogation by the running program. Therefore meaningless values are read from the ADC. If the value is set too high, time is wasted and the speed of readout operation decreases. To give an example, for a Pentium with 133 MHz the camera needs a delay value of larger than 90, for a 80386 with 40 MHz a value of larger than 22 may be appropriate. This delay has to be determined for each computer system independently by the user. The second value which can be set by the t command is a delay for the sample and hold circuit (SH-Delay) inside the ADC. Because the amplifier on the control PCB of the camera has a certain rise time, the output of the amplifier may not be interrogated too soon after the CCD chip has shifted a new pixel value to its output register. Furthermore, in order to remove certain noise structures -on the image originating from the power supply of the computer system, the SH-Delay value can be varied according to the users system. If the value is too large (more than 50 perhaps), it slows down the operation of he camera system. If the values are changed during operation, they are not stored at the end of program execution (eXit command). However they can be stored in the CCDR.INI file. The setting of the t values during program execution can be prevented, if the last number in the appropriate line of the CCDR.INI file is set to 0 instead of 1.
Setting up the camera for everyday's work:
The power supply: The power supply can be used for operation from 90 to 264 V AC with 50 or 60 Hz and from 7 to 15 V DC . Therefore it is highly versatile to function from nearly all power sources you can think of. It needs about 30 to 50 Watts, about 3 Ampere from a car battery at 12 to 14 V.
Two different operation modes can be used:
The AC main power is used:
Before you connect the power supply with the mains, be sure that the voltage at your home plug is from 90 to 260 V alternating current at 50 to 60 Hz. Connect all cables of the camera system. Connect the big black cable for the power supply with the connector on the back side of the power supply with your plug. Switch the two little switches (the red one and the metallic one) to their lower position. Switch on the main power switch on the backside. It is black and has a 1 and 0 written on it. The internal fan starts its operation and the green lamp on the front side should be on. The thermoelectric cooling of the camera is now operative. If the red switch is in the upper position, the thermoelectric cooling is off as well as all other voltages for the controller box and the CCD. It has the same effect as switching the main power switch off, only the fan is running and the green lamp is on.
A DC power source from 7 to 15 Volts is used:
The power supply can be connected with a DC source using the little round plug on the backside. The red and black cable is ready to be used with a car cigarette lighter plug. Red is positive and black is negative. It is a 2.5mm connector with the plus sign connected to the inner round pin. Be sure to look for the right polarity! The power supply is protected with a diode in the DC power line, but anyway! Before connecting the DC source, the red and the metallic switches should be both in lower position. After connection of the DC power source, the red little switch has to be set in the upper position for running the camera from a DC power source. At this time the thermoelectric cooling of the CCD is running. A 4 amp fuse (European format) is used for protection. The green lamp is always off during DC power operation. In order to start normal operation of the system, the metallic little switch on the backside of the power supply can now be set to its upper position . The red light will go on and the camera is fully powered. The switch disconnects all voltages necessary for operation of the controller box and the CCD head. Only the thermoelectric cooler is running with the metallic switch in lower position. Before using the camera it should have been on for about 10 to 15 minutes to guarantee photometric operation
ATTENTION!
During DC operation the cooler mounted on top of the power supply can get up to about 70 de centigrade during normal operation! Do not prevent heat exchange by putting anything on top of it! There are no serviceable parts inside the power supply, so there is no need to open it. Opening cancels your warranty of the whole camera system! If you would like modifications of the system, please ask the developer group of the camera.
No other cables besides the one delivered with the system can be used. Especially the 5 pin DIN cable can not be exchanged for a shorter one. Serious damage may result of the CCD chip and the TEC element
If the delay values of the camera program have already been set for your computer system, you can now continue as follows: The controller box has two switches, only one (the one close to the center) is connected. The other one is for further modules already built in. The connected one controls the amplification of the amplifier. In the lower position (at the 0 mark) the camera has a sensitivity of around 11 electrons per one digit on the pixel intensity, at the upper position the sensitivity is around 32 electrons per digit. The TC245 CCD delivers about 4 µV per electron. By the amplifier it is converted to 224 µV or to 77 µV per electron. The ADC has a conversion of about 2.44 mV per digit. This gives a overall conversion rate of 11 electrons per digit or of 32 electrons per digit. These values are only approximate ones, they have to be measured for each CCD chip separately. To know these values is very important for photometric work, therefore the user should determine these values from time to time. For most work, the switch should be in the lower position at 11 electrons per digit, however with bright light sources the upper position should be used (32 electrons per digit). In the lower position, the chip can not be overexposed before the pixel are turning red on the computer screen. If the lower sensitivity is used, the maximum charge storage per pixel can be exceeded, so the chip can be overexposed without the pixels are turning red. This could seriously disturb your photometric work. Blurring may occur as well. So carefully check your light values. In the two first displaying screens (the one where information is displayed at the right side of the image area), the values for MEAN, SIGMA and REF are displayed as they are coming from the ADC, whereas in the third mode (one information line below the display) the values are multiplied by 10 in order to give the approximate numbers of electrons collected. The actual number of incoming photons per pixel has to be considerably larger, because the quantum efficiency of the chip can vary depending on the wavelength of the incoming light from 0% to about 65%.
Hardware
The image acquisition program transfers the data from the camera to the computer and vice versa by the parallel interface of a standard printer port. In order to let the camera operate with even relatively old computer systems, the hardware has been designed to connect to the oldest concept of a parallel interface. Therefore 9 lines are used for output and only 4 lines can be used for input to the computer. The parallel port must have the address 0378H in the computer, otherwise please contact your local IOC dealer. The 0378H address is the most common used address of a parallel port in the PC, if the PC has only a single printer port, it is usually assigned to that address.
In case, the camera does not work at all with your computer, please check first the used printer port by attaching a standard printer to it, if it prints properly. If the printer doesn't work, the problem is with your computer, not with camera! Check your setup menu in the PC (very often accessed by pressing the DEL key during booting), if the parallel port is setup in the right way. You must look sometimes into the manual of your motherboard in the PC, but very often the menus in the SETUP-programs are self-explaining. The interrupt line of the printer is not used for communication from and to the camera.
If the camera has been connected correctly to the computer, at startup of the IOC program the camera should be running and producing images on the monitor. The very important reference value (REF) has to be set between 0 and a few hundreds. It should never have negative values or values being very large. It has to be kept in mind, that the total range of values the camera can deliver is 12 bit (=4096). The data values available for the real image is the difference 4096 - REF. Therefore it is important not to adjust the REF too high, because this reduces the dynamic range of the image acquisition.
After an image has been taken, it had to be corrected by a dark frame and a flat field. These operations for single images stored in FITS can be done by any standard image analysis program such as MIDAS or IRAF. The dark frame image and the flat field image have to be taken according to the following description:
a) Dark frame
A dark frame corrects for the number of electrons accumulating during the exposure time of the image, which are caused by a temperature dependent dark current. It has to be kept in mind, that the dark current varies from pixel to pixel on the array. Therefore the recorcded number of photoelectrons measured for each pixel has to be corrected. For this correction an image has to be taken, which has the same exposure time and temperature as the image itself. The dark frame has then be subracted from the original image.
b) Flat Field
The light sensitivity of each pixel in the array is slightly different from each other. Therefore the sensitivity has to be normalized. Even more, vignetting problems or the optical system itself may add another light variation over the array. In order to dcetermine these losses, a perfectly equal illuminated area has to be imaged. This causes in many cases severe problems, the astromnomical literature is full of examples how to generate perfect flat fields. In any case, the recorded flat field has to be corrected with a dark frame with the same exposure time as the flat field. The exposure time of the flat field (and the dark frame of the flat field) has not to be the same as the original image.
The following operation has to be performed with an image analysus system:
CI = (OI - DF(OI))/(FF-DF(FF))*MEAN(FF-DF(FF))
CI: Final corrected image to be used in further calculations
OI: Original Image
DF(OI): Dark frame of original image
DF(FF): Dark frame of flat field
FF: Flat field
MEAN: Mean value of all pixels in an image
The final image, corrected for dark frame and flat field effects is the only product, which can be used for photometric calculations! All other images can only be used for 'having a nice day (or night)', but not for any kind of serious astronomical work.
Recorded movie sequences have to be corrected accordingly, special software to do this is available from your local IOC dealer (as you already suspect!).
This is already confirmed by the selection of large great overviews, as we all know.
Commands:
A: Adjustment of the size of the read-out area on the chip.
First the x-value xa, second the y-value ya is set.
P: Position of the left upper corner of the read-out area on the chip.
First the x-value xp, second the y-value yp is set.
The setting of the area size has an influence on the read-out time of the chip. The larger the area, the longer the read-out time.
The corners of the read-out area are depending on the values in the A and P command as follows:
upper left : xp yp
upper right: xp+xa yp
lower left : xp yp+ya
lower right: xp+xa yp+ya
As default values xp=1, yp=1 and xa=100 and xa=100 are set.
E: Setting of exposure time (ET) in seconds. The real exposure time the program uses is always equal or lower as typed. It is the time necessary to find the last full time unit based on 1/18.2 seconds. That is the time clock interval of the internal computer clock for IBM compatible computers. If the selected ET is longer than the image interval time (IIT) the exposure ends with the end of the IIT. In that case the time is marked in red on the screen.
I: Setting of image interval time (IIT) in seconds. The real IIT the program uses is always equal or lower as typed. It is the time necessary to find the last full time unit based on 1/18.2 seconds. That is the time clock interval of the internal computer clock for IBM compatible computers. If the selected IIT is not long enough to do all computer operations (storing the image, displaying, doing houskeeping etc.) it automatically is set to longer times. In that case the time used is marked in red on the screen.
Display function have no influence on the stored image values, they are only used for display.
O: Offset selection. The selected value (negative values are possible!) is subtracted from the real image data before displaying. Therefore to compensate for a bright background a positive value may be typed in. If the background is too dark to see the background noise as it is may be necessary for checking the camera functions, a negative value can be used.
+: The image values for display after subtraction of the offset value is devided by
-: 1, 2, 4, 8 and so on depending on the displayed number (0,1,2,...). This number can be incremented or decremented by using the + or - keys.
Different option can be set by the Q command as follows.
Q:
First option is binning mode:
1: Each pixel is readout without binning Pixel size is 8.5 times 19.75µm.
2: Two adjacent pixels in x-direction are binned externally. Equivalent pixel size is 17 times 19.75 µm.
3: Three pixels are binned internally. This is the fastest mode, but with lowest spatial resolution in x-direction. Equivalent pixel size is 25.5 times 19.75 µm.
After entering the number, the next option is
Window measuring functions:
(0): The mean value of all pixels inside the ROI is calculated and displayed. The standard deviation sigma is calculated as well.
(1): The sum of all pixel values in the ROI window is calculated and displayed. The standard deviation sigma is calculated and displayed as well.
(2): The maximum of the pixel values in the ROI window is calculated and displayed. The standard deviation sigma is calculated and displayed as well.
(3): The total
intensity above background in the ROI window is calculated as
follows:
A 1 pixel limb is defined outside the ROI. The mean of
all pixels values in this limb area is calculated. This is assumed to
be the background for an object inside the ROI. From each pixel value
inside the ROI the assumed background mean is subtracted and the
remaining values are added. This result is displayed. The function
can be used in order to determine intensities of objects in a very
fast way, if the background is smooth on a scale of the size larger
than the ROI. Particularly in astronomy it can be used for a rough
determination of a stellar intensity. If it is used on a noise
continuous background without any special bright object, the
displayed values may float around zero.
Next option is background subtraction, default m ode is 0.
Window background subtraction:
(0) No background subtraction is performed.
(1) The mean value of the pixels inside the ROI is calculated and subtracted from all pixels in the image before displaying it. Therefore a background subtraction is performed automatically, if the ROI is in the right position for the determination of the background. If it is not used properly, it can be the cause for a totally dark (=blue) image on the screen. However it has to be mentioned, that it only influences the display of the image, the original data for storing are not effected at all.
R: Repeat mode switches from yes to no and vice versa. If mode = yes, the camera records one image after another automatically, if mode = no, the camera is in single frame mode, each image is recorded by pressing the space key. The R switch works during acquisition and during replay as well.
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M: Start movie modes:
A: Acquisition mode:
Type file name: The filename for recording has to be typed in. It can be in the current directory (the home directory of the camera) or everywhere else, then the filename has to include the full path.
# of images: The total number of images in a file is selected, the file works as a ring buffer. After the last image is recorded, it starts at the beginning of the file. THAT MEANS THAT THE OLD DATA IN THE FILE ARE OVERWRITTEN !!!!!
Stop of data acquisition is done by pressing the ESC key.
R: Replay mode
Type file name: The file name of the recorded image file has to be typed. It can be in the current directory (the home directory of the camera) or everywhere else, then the filename has to include the full path.
Stop of data replay is done by pressing the M key and afterwards the R (replay) key.
S: Single FITS image recording mode
The image displayed is saved as a FITS file. Part of the FITS header is given as in the CCDR.INI file. OBJECT and COMMENT keywords can be filled out in the following two lines. The filename is chosen automatically. It consists of the up to four letters set by the FILE command overwriting the default values set by the CCDR.INI file. A four digit image number is added automatically after the letters as well as the .FIT extension. After storing the 4 digit number is incremented automatically.
In the MODE line the status is displayed as follows:
NORMAL: Running with display but without acquiring
ACQUIR: Aquiring images automatically
REPLAY: Replaying images from a previous recorded file.
Information output of the Program:
Using the . key the display of information can be changed from the full menu over a reduced menu to a status line at the bottom of the display.
A very basic intensity measurement window has been implemented. It can measure the mean intensity and standard deviation of all pixels, the sum of all pixel values as well as the maximum intensity in the window. The mode can be set by the window statistics option of the Q command. The different modes are set as follows:
mode = 0 mean value and standard deviation
mode = 1 sum of all pixels (integral)
mode = 2 maximum intensity value in the window
The window can be moved around in the image by using the cursor keys and it can be enlarged by using the curser keys in combination with the SHIFT key.
The measurement results are updated automatically and displayed in the MEAN and SIGMA line (measurement value and sigma where applicable).
X: Exits the program with a question: do you really want to exit...
Answer Y to exit or any other key for continuing operation of
the program.
