Preparing Data Frames

Data frames to be run with INVENTORY commands should be clean. It is desirable that they do not contain any dark spots situated close to objects. This could result in an unpredictable outcome! Bright spots are less dangerous. They are partly detected by the SEARCH/INV command, but the CLASSIFY/INV command usually classifies them as defects, unless they look like galaxies. Unwanted frame defects can be removed using the MODIFY/PIXEL command. Frames that have not been properly cleaned or even frames which have not been cleaned at all can also be used but the user should in this case check visually which objects may be affected by defects.

Irregular or sloping background is not a problem. One has only to know if background variations are related to changes in sensitivity or are caused by some additive factor. Depending on what is the case, the keyword FIELDVAR should be set to 0 (sensitivity changes) or to 1 (additive factor - default).

Unused parts of CCD--frames should be removed with the help of the EXTRACT/CCD command. There is a problem when valid data is on a circular image. In this case it is convenient to set the unused part of a frame to the value close to the average background value or to the value of low cut. This can be done with the help of a properly shaped mask. The same holds true for other cases of non--rectangular images.

When more than one frame of the same field is analysed, it is convenient to align all frames to common physical coordinates. This can be done by using commands such as CENTER, ALIGN, or REBIN.

When a user plans to manually select the standard stars for point spread function determination, he should identify these stars by means of MIDAS command GET/CURSOR STARS/DES before using the ANALYSE/INV command.