Screen an Image by a Mask

 

There exists a tool to create a mask for screening of an image. The   mask is a Midas image of the same size and with the same EXSAS descriptors as the image from which the mask was created by cursor, table or descriptor specification. It has the intensity 0 or 1 per pixel in case the pixel is screened or not. The mask has the proper form for Source Detection. The command syntax is

CREATE/MASK image mask [mode] [table] [disp]

 

• image is the name of the (displayed) image.
• mask is the name of the polygone mask to be created.
• mode is the input mode for the mask specification (CURSor, TABle or DEScriptor), the default is CURS. In case mode=DESC the image must be binned from a photon event table being selected by a SELECT/RING command. Then the mask boundary is determined by the outer radius of the ring.
• table is the name of the table with columns :X_SKY and :Y_SKY specifying the sky pixel coordinates of the polygone. The default is no table.
• disp is a display flag. If set to disp then the boundary of the mask will be displayed in the overlay (in green color), if set to nodisp then no boundary will be displayed. The default is disp in case of mode=CURSOR and nodisp else.

Examples:

CREATE/MASK image1 mask1
CREATE/MASK image1 mask2 TAB polygo
CREATE/MASK image  mask DESC ringevents

Presently there is one mask image in the calibration area containing the window support structure of the ROSAT PSPC detector, EXSAS_CAL:mask0thick.bdf. It contains the support struts with   a thickness of 2 arcmin. In order to proper position this mask image a command is available.

ADJUST/MASK image mask [rot_angle x_trans y_trans] [attitude] [load]

 

• image is the name of a reference image. It must be of size image pixels and contain the general EXSAS descriptors. Such an image is created with the command CREATE/SOURCE_DETECT_IMAGE.
• mask is the name of the mask to be rotated and displaced. It can be the mask containing the ROSAT PSPC window support structure, EXSAS_CAL:mask0thick.bdf.
• rot_angle is the rotation angle applied to mask. The default is to determine it from the attitude table attitude, if this parameter is specified, else rot_angle=0.
• x_offset is the translation in x-direction in image pixels applied to mask. The default is to determine it from the attitude table if specified, else x_offset=0.
• y_offset is the translation in y-direction in image pixels applied to mask. The default to determine it from the attitude table if specified, else y_offset=0.
• attitude is the name of the attitude table from which the mean rotation angle rot_angle and x_offset, y_offset are determined (the mean over the times interval specified by the TIM_SEL descriptor). The default is no attitude table.
• load is a load flag for the image multiplied with the rotated and displaced mask. If set to l[oad] this image will be loaded, if set to n[oload], it will not be loaded. The default is load.

An image mask.bdf will be created.

The mask created with ADJUST/MASK as described above does not take into account the spacecraft wobble. Therefore this method may not be very satisfactory if high precision is required. To get a PSPC mask which is corrected for the spacecraft wobble one has to use the command CREATE/WOBBLED_MASK.   This command uses the attitude information from the attitude table to first create an image showing the fractional illumination of the field (complete obscuration = 0, no obscuration = 1) by the PSPC window support structure. This image is then converted to a 0,1 mask by applying an (adjustable) threshold. The drawback of this command compared to ADJUST/MASK is that the excution time can be considerably longer. Syntax:

CREATE/WOBBLED_MASK [image] [mask] [threshold] [attitude] [keep]

• image is the name of a reference image. It must be of size image pixels and contain the general EXSAS descriptors. Additionally it has to be binned with the standard PSPC binsize of 30 sky pixels per image pixel. Defaulted to image1.bdf.
• mask is the name of the output mask. Defaulted to mask.bdf.
• threshold is the threshold to be applied for the conversion of the image showing the fractional illumination to the 0,1 output mask. Defaulted to 0.95 - this means that every pixel obscured more than 5% of the time will be set to 0.
• attitude is the name of the attitude table. The default is attitude.tbl.
• keep is a flag which determines whether the intermediate image showing the fractional illumination will be kept on disk (file name shadow.bdf). Defaulted to nokeep. Any input not beginning with the letter N or n will set this flag to keep.

Note:CREATE/WOBBLED_MASK uses internally the EXSAS command CREATE/EXPOSURE. Any screen output relating to instrument or exposure maps should be ignored.

Any active time selections of the reference image (descriptor TIM_SEL) are taken into account.

Results are slightly different for PSPC B and C. The detector ID is taken from descriptor DETECTOR_ID of the reference image.

In principle the intermediate image shadow.bdf can be used to estimate a correction for the source fluxes for sources partially obscured by the PSPC window support structure.   But keep in mind that this can be only a rough estimate since the point spread functions are extended, and the model for the rib structure used internally by CREATE/WOBBLED_MASK can only be an approximation (there is e.g. no energy dependence take into account) - the reality is more complicated than all models.