Previous: Setting the image format
Up: Setting the image format
Next: Setting up the IPCS
Previous Page: Setting the image format
Next Page: Setting up the IPCS

Setting up CCDs

The format of the CCD images obtained by the DAS is highly flexible. You can choose the size of the basic CCD frame, and the number of planes per image cube in three-dimensional work, and then reduce the volume of data by windowing and binning.

The default image size --- full-frame read-out --- is normally set to be the full area of the CCD chip plus some extra pixels --- overscan --- which indicate the bias level. However, the default size can be set to be a sub-set of the CCD chip or to be very much larger than the size of the chip. You must set this default size when you first use a detector channel. The system keeps records of the normal frame-size for each CCD and will suggest this value when prompting you for the frame size.

The ING CCDs can perform binned read-out on either axis or on both at once. You invoke binning by specifying the number of pixels binned together on each axis. The binning factors may be different on each axis but each factor must be an integer from one to ten inclusive and must be compatible with any windows you have set up (see below). Binning reduces the read-out time by the product of the binning factors, but if you bin in you lose the benefit of the faster read-out speeds: the CCD drops back to STANDARD speed. To turn off binning, set the binning factors back to (1,1).

You can also reduce the size of your images, and the read-out times, by setting windows for the read-out: only the pixels within the windows pass through the CCD electronics into the DMS. Up to four windows may be set at one time, but windows may not overlap and no window may extend beyond the basic CCD frame. If two or more windows are active, the read-out time is reduced but the images on disk are full-frame with blank pixels between the windows.

Windows and binning may be combined in the same read-out, but the parameters must be made compatible. The size and position of the windows is specified in terms of un-binned CCD pixels and the size of each window must be a multiple of the binning factor in each axis.

You can choose the speed at which the CCD reads out to suit your observing programme. Faster read-outs have higher noise and you should consult your support astronomer for the most recent figures. The allowed read-out speeds are (in increasing order of speed) SLOW, STANDARD, QUICK and TURBO. A fifth speed, NONASTRO, is only useful for engineering work.

Each time you log in for observing, you must use one of the format-setting commands before trying exposures. If you have recently observed on the channel in question, the DAS will have saved the last format you used and you can recover this with the reformat. If there is no suitable stored format, you will have to set one up with the setup_ccd command. It usually necessary to issue setup_ccd or reformat whenever the CCD controller or the DMS has been reset in hardware.

The usual image size for the various CCD chips are stored in a database and will be retrieved by the DAS and presented as defaults when prompting for the image dimensions. Usually, it is best to accept these defaults. If you choose to override the defaults, this fact should be noted in the log.

To bin up CCD data, use commands such as

ICL>  bin red 2 2 
   ICL>  bin aux 2 1
You can set initial binning factors with setup_ccd.

To set a window on a CCD, use the window command. For example, when using Isis with an -pixel CCD and the spectrum running in the direction you might use

ICL> window red 1 1280 200 0 350
to capture a 200-pixel strip about the spectrum that runs the full length of the chip. The parameters are, in order, channel name, window number, window -size, window -size, window -offset and window -offset. The offsets are the number of pixels by which the window is displaced from the origin of the CCD; i.e. a window with offset (0,0) includes part of the first row and column of the CCD. The window number can be from 1 to 4 and you can reuse a window number - this eliminates the previous window of that number from the system. Setting a window to a size of zero destroys it.

You can temporarily disable windowing, return to reading out the whole CCD, and then re-enable the windows:

ICL> disable_win red
   ICL> run red 10
   ICL> enable_win red
Note that enabling and disabling the windows does not change the binning.

To change speed, you may either use the command rspeed):

ICL> rspeed red  standard
   ICL> rspeed blue quick
or you can use the name of the speed as the command followed by the name of the detector channel:
ICL> standard red  
   ICL> quick    blue
Valid speeds are slow, standard, quick, turbo and nonastro in increasing order of speed. At the time of writing, the speeds slow, turbo and nonastro are under test and are not recommended for observing.

The normal exposure cycle for CCDs includes a preflash, but the available CCD chips do not need a preflash, so the preflash length is usually set to zero. If you do need to preflash, the length of the flash is set with a command of the form

ICL> preflash aux 0.05
where the second parameter is the length of the flash in seconds. This command does not illuminate the CCD but all subsequent exposures will include the preflash.

Previous: Setting the image format
Up: Setting the image format
Next: Setting up the IPCS
Previous Page: Setting the image format
Next Page: Setting up the IPCS

Mon Oct 3 21:59:29 WET 1994