CCDPACK XREDUCE bulletin article

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A new window on CCD data reduction

Users of the new version of CCDPACK (V2.0) might be forgiven for suffering a little culture shock, for this release not only introduces a new graphical user interface (GUI) but also breaks the mould of running a series of programs to reduce your CCD data.

Rather than presenting you with a button to "flat-field" your CCD frames, for example, the new interface concentrates on organising and categorising your data. Once this is done, the necessary processing steps are deduced and executed automatically. The approach bears more resemblance to the software you might run on your PC than to traditional astronomical data reduction software.

The decision to do things this way was determined in large part by the response to the last Starlink Software Survey, which showed a need for both software and documentation to be more approachable, tutorial and intuitive. In the new CCDPACK, software and documentation have therefore been closely integrated -- into a form of "interactive cookbook" that guides you through each decision, presenting and describing the options available at each stage.

Using this, you should be able to achieve high-quality results quickly and easily, even if you initially know little about CCD systems. Along the way, you will be introduced to each of the important factors that influence CCD data reduction, so that you quickly learn how to make the crucial decisions that affect your data. Even for more experienced users, the new interface presents a convenient and time-saving alternative to running the underlying CCDPACK programs directly.

These programs (makecal, calcor, makeflat, etc.) remain at the core of the new system but have been augmented by a new schedule command that decides how to reduce your data from the information you supply (using your preferences if several approaches are possible). Reductions can even be stopped mid-way and continued later from where you left off. It is schedule that lies beneath the GUI and allows it to talk about instruments, filters and data frames, rather than the low-level details of how to process the data. This scheme differs from the equivalent ccdproc procedure in IRAF/CCDRED in that it completes the reduction in one pass and of course you don't have to know much about it anyway; if you use the GUI.

Infra-red data reducers shouldn't feel left out as the GUI also has features for handling infra-red array data. These include the option to remove a zero bias contribution (a necessary fudge) and to re-use images as flatfields. CCDPACK also has facilities for registering, aligning and normalizing frames so that large mosaic images can be produced. It is fully described in SUN/139 which can be viewed on-line from the xreduce GUI or by using the ccdwww command.

Figure 1: The main window of the CCDPACK GUI (xreduce).

Probably the first thing you will want to do here is pull down the Help menu and select "On Window". This starts a World Wide Web browser that lets you read about the window and CCD data reduction in general. Selecting `On Context' tells you how to get help about a particular part of the window -- just place the pointer over the bit you're interested in and press the F1, F2 or Help key.

Figure 2: CCDPACK already knows about the characteristics of many CCD cameras and this window allows you to select one of these standard configurations (or you can define your own).

This also allows xreduce to understand your data better. For example, if you have WHT data from around 1994-ish onwards, then its header information contains enough detail to allow it to be reduced automatically. All you need to do is identify your data files (this is done in a separate window) and tell xreduce to reduce them.

Figure 3: If your CCD camera isn't already known to CCDPACK, then you will need to supply some information about it. The most important parameters are the extents of the light-sensitive area and the bias strips. If you don't happen to know these, then help is at hand -- in this window you can pan and zoom the image and select the regions you want by dragging the mouse.

Figure 4: Unless your frames already contain this information, you may have to identify which ones are "targets" (i.e. astronomical objects), "flats", "bias" frames, etc. This is done in this window, where you can also categorise frames by filter colour. When you've finished, this information is recorded inside the frames themselves -- so you won't need to do it again.

Figure 5: The last window in xreduce where you choose final options before a reduction is scheduled and executed.

Note the save disk space option which deletes frames after processing. Selecting "lots" will reduce your disk space requirement to just a few frames more than the raw data, although your original data will be lost (you should keep a tape copy). Selecting "some" saves less space but retains the original data.


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