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CRPIX values in double windowed frames

What to do if you have wrong CRPIX values in your header

The CRPIX header keyword is, together with CRTYPE and CRVAL, the set of three keywords which define the coordinate system associated to that image. Briefly speaking, while CRTYPE defines the kind of coordinates to be used (like pixel, RA, Dec..), CRPIXn sets which physical pixel will be considered as the reference pixel and CRVALn stores the value of the logical reference. More details on this topic may be found here.

The current CCD control software of SuSI2 limits to a maximum of two the number of frames which can be correctly handled each time a MEF file is created.

In particular this implies that, taking an image with SUSI_img_obs_DoubleW template and setting at least one of the two windows to span both chips, all the image extensions beyond the second one will have incorrect CRPIXn (n=1, 2 for x and y axis respectively) header keywords.

The scientific reliability of data is not compromised. However, all the commands dealing with the positioning of the subframe with respect to the whole SuSI2 field may not properly work for the extra images. Furthermore, it should also be noted that it is usually possible to align the FoV such that each of the two windows will fall on a single CCD.

Below you can find the description on how the CRPIXn are set and how to change their values from within ESO-Midas and Iraf. The case of binning 1 will only be considered. For the other binning values, simply divide the final CRPIX by the value of the binning and round the result to the nearest greater integer.

For SuSI2 raw frames, the reference pixel which will be considered the first (1, 1) logical pixel (i.e. CRVALn = 1) is located at physical pixel (12, 12).

Thus, in general, for a bin1 windowed frame, with window starting at physical pixel (x_win, y_win) on CCD#45, CRPIXn should be:

CRPIX1 = 12 - x_win + 1
CRPIX2 = 12 - y_win + 1

If, instead, the lower edge of the window falls on CCD#46:

CRPIX1 = -2132 - x_win + 1
CRPIX2 = 12 - y_win + 1

In case the window covers both CCDs, it will be splitted into two sub-windows, one for each CCD, and the CRPIXn values will obey to the corresponding rules presented above, where x_win will be for CCD#45 the actual lower edge of the window, while for CCD#46, x_win == 1, i.e. CRPIX1 == -2132. The picture below shows a possible case, where the red rectangle represents the window spanning both CCD chips.

Under Iraf, CRPIXn values can be modified issuing the command hedit <image> CRPIX1 <new_CRPIX1_value> , where <image> is the name of the image whose header should be modified, and <new_CRPIX1_value> is the new value for the CRPIX header keyword. Type help hedit at the Iraf prompt for full documentation on the task.

Also under Midas the procedure is quite straightforward; at the prompt just type: WRITE/DESCR <frame> CRPIX1 <data>, where <frame> is the name of the frame and <data> is the new value for CRPIX1.
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