What is OPTIC anyway? --------------------- The Orthogonal Parallel Transfer Imaging Camera (OPTIC) consists of two Lincoln Lab CCID28 2K x 4K CCDs mounted side by side in a dewar. The gap between them is 1mm, approximately 72 15-um pixels. 2048 2048 <===============> <===============> --- +---------------+ +---------------+ | | CCD1 lower | | CCD3 lower | 516 (sic) | |---------------| |---------------| | | | | | 2052 | | | | | | CCD1 upper | | CCD3 upper | 1536 | | | | | | | | | | --- |---------------| |---------------| | | | | | | | | | | | | CCD0 upper | | CCD2 upper | 1536 2052 | | | | | | | | | | |---------------| |---------------| | | CCD0 lower | | CCD2 lower | 516 --- +---------------+ +---------------+ <===============> <===============> These CCDs have a number of interesting properties, chief among which are that they are capable of "orthogonal transfer", i.e. the charge can be noiselessly clocked horizontally as well as vertically, which permits tracking of image motion and fast readout of guide stars. Another feature is that they are each split into four regions which can be clocked independently, making the 4K x 4K focal plane divide into the eight regions illustrated above. Each of the "CCDs" above reads out through its own amplifier, acting like an independent CCD, even though the metrology of the pixels over each CCID28 is seamless. The pixel size is 15 um which translates to about 0.14 arcsec at the UH2.2-m or WIYN telescopes. Readout time is about 25 seconds unbinned, or 8 seconds binned 2x2. The gain is about 1.4 e/ADU and the read noise is 3.5-4.0 e-. The readout from each quadrant of OPTIC is rotated and flipped in the resulting FITS file to make a consistent picture. The physical gap between the chips is *not* part of the FITS file, and the bias regions for all four quadrants are found over on the right hand side of the FITS image. Although the four regions in each chip are fundamentally the same, the "lower" regions enjoy the ability to reach the amplifiers without disturbing the upper regions. Therefore the "lower" regions are potential locations for guide stars, and the control software will permit you to specify zero or one stars in each lower region to be used as guide stars. When exposing, the software quickly clocks a small patch which you specify around each selected guide star first horizontally and then vertically over to amplifier. This does not disturb the image in the upper regions or any unselected lower regions. The software then does a readout of the small patch, analyzes the guide stars, possibly performs shifts on the non-guide regions, and then pauses for another guide star image to collect. There is a utility called "mkfinder" (described in detail in Manual_utilities) which can be used to make finding charts which have an OPTIC footprint overlaid. It is strongly recommended that you make a finding chart for each field you observe. How is OPTIC started? --------------------- Once OPTIC has been mounted on the telescope (see Manual_startup), the user normally logs into the host computer "mkbill" as "obs" (usual password for obs accounts at the UH2.2-m), and uses "startx" to bring up X Window. Choose a window to run the OPTIC communications program and type "otcom". Assuming this has no errors, it sets up communications to the DSP through the device driver and interface and is ready to accept commands which you type at it. Otcom runs as a server as well, so that you can alternatively control otcom from a GUI called "otgui". This GUI serves a dual role: it displays the current status of the camera and it accepts user input, formats it into otcom commands, and sends it to otcom. Commands sent by otgui are echoed back in the log window, so you can learn otcom's commands by seeing what otgui is sending. Upon startup you need to download code to the DSP. This is done by "df" or by using the "Init" menu entry in the GUI. If this is successful you need to turn on the "high voltages", using the command "pon" or the "Init" menu entry. A fresh download requires you to reset the regulation temperature of the CCD using the "temp" command; "temp -105" is a good choice. You are now ready to observe with OPTIC. Summary: (log in as "obs") % startx - typed at the shell % otcom - typed in a window designated for otcom's use % otgui & - typed in a different window 001> df - typed at otcom (or executed via the GUI) 001> pon - typed at otcom (or executed via the GUI) 001> temp -105 - typed at otcom (or executed via the GUI) If there are no errors, the GUI should display only soft pastel colors. Bright red indicates that something is wrong and you should read Manual_gui to understand what is not right. Simple point and shoot observations ----------------------------------- Although OPTIC has many unique capabilities, it works perfectly well as a pair of conventional CCDs. There are four GUI blocks you will want to set. The first block defines the file prefix and file number. The default file prefix is "/tmp/ccd", but the place you want to save data is "/b0/optic", since /b0 has approximately 65 Gbyte available for observations. A possible choice might be to create a directory for each night, e.g. "030120", and a short file prefix, e.g. "f". The file number will automatically increment with each observation, but you might want to set it to something else. The second block shows the current filter and you can click on one of the buttons to change filter. The third block allows you to set the exposure time and the readout format. Two buttons labelled "1x1" and "2x2" will give you the entire image, unbinned or binned by 2, or you can specify a custom format using the "Other" button. The fourth block shows the time remaining in the exposure and has a button labeled "GO" for carrying out an exposure and "Clear" for clearing the CCDs. Other than this you can use the normal Atlas telescope guiding and you can issue familiar ?cdcom telescope commands by typing at otcom or entering the commands into the "Command:" window of the GUI. Summary (typed at otcom, or executed via the GUI): 001> fp /b0/optic/030120/f - select file prefix 001> fn 5 - select file number 005> filt 0 - choose a filter 005> et 100 - set the exposure time 005> sf 1 1 0 0 2048 2052 32 - set the readout format 005> clear - clear the CCDs 005> go - start an exposure Saving data ----------- Naturally you can use ftp or sftp to copy your data to another machine which has a tape drive. Alternatively mkbill has a CDR drive which can be used with to burn CDs. There are many way to burn CDs; here is a simple one which is reasonably reliable, although it does not check that you might overfill the CDR (normal capacity 7-800 Mbyte): mkisofs -R -J MY_DIR | cdrecord -v fs=6m speed=8 dev=0,0 - Don't try do this if you are loading the CPU, especially taking data! This will write the entire directory "MY_DIR" onto the CDR. Note that these will make "MY_DIR" the root of the CDR, i.e. a file MY_DIR/myfile will appear as /mnt/cdrom/myfile when you have mounted the burned CD. A typical night's observing, once compressed, will fit on four CDRs, taking about 10 minutes apiece to burn at 8x. Data reduction -------------- As far as I am concerned the bias level is quite constant, so I usually use the median of the bias strip as a constant bias level. Likewise, I do not think that the dark current has significant enough structure as a function of position to affect my observations, so I ignore it. If these are critical to you, take calibration data. No CCD is perfectly linear, and these CCID28's are no exception. There is a tradeoff that was made between linearity and noise performance. These CCDs are good to about 1% linearity between 300 and 30000 ADU. Beyond 32k ADU the four amplifiers behave differently, some becoming quite non-linear near saturation around 50k ADU (>3% decrease in ADU relative to signal). Likewise, take calibration data if your application depends critically on linearity. The OPTIC shutter is not extremely fast (although it's reproduceable) and the center of the field of view gets approximately 0.1 sec more exposure than the nominal exposure time. For precision photometry, therefore, I normally construct an exposure map from the ratio of two flatfields of 0.5 and 10 seconds illuminated by a stabilized dome flat. (For the 0.5 sec exposures it's important to pause at least 1 sec between shutter activations so that the shutter actually opens and closes and does not heat up too much.) There is a file called "optic_shutter.fits.Z" in /usr/local/inst/Data which is a 4x compressed image of the extra time an image gets above the nominal exposure time. The units are seconds. For OPTIC data taken in stare mode as described above, the normal dome flatfields or twilight flats or superflats are fine. However, data taken in OT mode is a bit different, since shifting the charge around during an exposure means that the charge found on a given pixel at the moment of readout was integrated on many pixels during the exposure. Therefore superflats are immediately problematic since different observations have different shift patterns and are not immediately comparable. What I normally do, therefore, is to take (unbinned) dome flats, and for each observation convolve the dome flat with the same shift pattern as took place during the observation. A utility called "conflat" does this efficiently (see Manual_utilities). This can then be used to remove the pixel-to-pixel non-uniformities from all observations. If desired, it is then possible to make a superflat and either subtract it (for fringing) or divide it (for bad pupil illumination) from each image. The expectation is that the scale of non-uniformities which the superflat is correcting is much larger than the scale over which OT shifting took place (which depends on the quality of the telescope guiding) and certainly much larger than the pixel-to-pixel non-uniformities which have been removed with the dome flats. How to learn more ----------------- Manual_contents lists the various manual chapters and can help you find the section you seek. You can read the Manual_general_hardware, Manual_general_software, and Manual_general_ccd to get a more thorough overview of how OPTIC is put together. The chapter Manual_tutorial goes through a sample observing cycle. It will give you a sense of how to use OPTIC's more advanced capabilities of tracking guide stars and removing image motion. Manual_gui describes the GUI in full detail, but it really should be read along with Manual_otcom_cmd which lists all the otcom commands in full detail. (There is also a Manual_commands which has a one line description of otcom command for quick reference.) Manual_track_predictions is a fairly complicated exposition which goes into the details of how otcom fits guide stars and then makes temporal and spatial predictions to follow the guide stars and remove image motion within the science regions. Manual_utilities describes some of the important and helpful utilities which can iad you in making finding charts, automatic log sheets, visualization of guide star video images, and flattening data which has undergone OT tracking.