### 5 How to reduce your data now

If you’re now wondering how you should go about reducing your data and don’t want to read this very long document, then this is the place to start. Later sections are rather more technical or describe data registration and related topics and should really only be consulted when the need arises.

#### 5.1 Using the CCDPACK data reduction GUI

If you are sitting at an X display then type the commands:

%  ccdpack
%  xreduce &

These commands will start the CCDPACK GUI. The GUI will lead you through the stages necessary for organising your data so that it can be scheduled for processing and will start a background job to do the reduction. The GUI has a context sensitive help system, so pull down the Help menu and choose On Window to get information about your immediate concerns (the help will actually appear in a hypertext browser – netscape by default). Hopefully this should be enough to get you started but read the next few paragraphs if you have time.

The underlying purpose of XREDUCE is to get sufficient information gathered about your data so that it can plan a reduction schedule. It can do this in one of two ways. Either you can select from a list of known telescope/detector combinations, or enter all the information required yourself (with lots of help of course). XREDUCE just really needs to know what package options to use, where a few geometric features of the CCD are (if needed) and the types of the various input frames (it needs to know which frames are bias, flatfields, darks and the real astronomical ones, called “target frames”).

If you’re fortunate then your data will contain a description of itself in a form that XREDUCE can understand. In this case all you need to do is identify the detector and then go on to list all the frames you want to process. If your data is from a detector with limited information available (such as the geometries of the various parts of the CCD) then you can select this and all you need to do is then inform XREDUCE which of the input frames are which frame type. Check the known detectors under the Options menu in the main window.

If neither of these options is available to you then all isn’t lost as configuring XREDUCE is easy. All you need to do is set some general options (press button General Options), define the CCD characteristics (there’s graphical help available if you need to define any of the CCD geometries, check under Options in the CCD Characteristics window) and then go on to organize your frames into their types (button Manual Organization). To set the reduction running look under Setup and Run.

No matter how you inform XREDUCE about your data you still have to follow pretty similar routes:

(1)
set up the package (probably using both the General Options and CCD Characteristics windows)
(2)
“import” your data frames (probably using one of the Manual Organization or Using FITS Headers windows)
(3)
say how you want to debias your data (this is done in the Setup and Run window, where your options will be restricted to those possible).

Debiassing is done by two basic methods, either by subtracting bias frames or using the bias strips. If you have bias frames then use them, if you don’t look for dark strips down the sides of your data; these are the bias strips (have a look at Figure §1). If you have bias frames and bias strips then use both (this is the zeroed master, offsetting to bias strips option).

The reduction is run independently of the GUI and its output is kept in a log file xreduce.log, so exit XREDUCE when you are finished (this is after you have accepted the options under Setup and Run) and the reduction will continue.

One final piece of information concerns the nature of the “data frames” as they have been called so far. These may be of any kind (as described in section §6) providing you have initialized the CONVERT package before starting up XREDUCE. If you have not initialized CONVERT then you will only be able to use data frames that are stored in the Starlink NDF format and their names will consequently have a file type of ‘.sdf’. When using these names in the GUI do include the ‘.sdf’ extension, this is contrary to how you would deal with these names in “normal” NDF processing programs where you would not include the file type.

##### 5.1.1 GUI problems

If you have a ungraceful exit from XREDUCE you may have some CCDPACK and KAPPA processes left running. You will now have to kill these by hand. How you do this is system dependent. On BSD type systems (like Digital UNIX and Linux) try:

% ps ux

and on system V machines (Solaris) try:

% ps -ef | grep \$USER

and look for the processes named ccdpack_res and kapview_mon (and perhaps others, such as ndfpack_mon). Get rid of these using the kill command followed by the process identifiers (usually the number in the second column).

#### 5.2 Command-line reduction

There is a command-line interface with similar abilities to XREDUCE if you do not have access to an X display (or prefer a command-line anyway) it is started using the command:

% reduce

REDUCE it isn’t quite as capable as XREDUCE so you’ll have to work a bit harder. Consult the description in the appendix for a little more about this routine, but it’s probably worth trying it out and failing first.