Creating Sky Images

 

Example 1: Create a 512 512 pixels image (see note 1):

Midas 001> BIN/IMAGE #512, #512 input=events output=ima1

    Example 2: Create an image with pixel size of 30 sky pixels (see note 2):

Midas 001> BIN/IMAGE 30 in=events out=ima2

    Example 3: Create an image with pixel size of 10 arcseconds (see note 3):

Midas 001> BIN/IMAGE 10 arcsec in=events out=ima2

    Example 4: Create an image with pixel size 10 from a selected box in the sky (see note 4):

Midas 001> SELECT/BOX (-100,-200) to (100,300) BIN/IMA 10 *events *smalima1

        Example 5: Create an image with pixel size 5 from a box selected via the cursor on a reference image (see note 5):

Midas 001> LOAD/IMAGE ima2 cuts=0,5
Midas 002> SELECT/BOX cursor BIN/IMA 5 in=events out=smalima2

              Example 6: Create an image from specified amplitude channels (PSPC data), time intervals and a circular portion of the sky selected via the cursor (see note 6):

Midas 001> SELECT AMPL >11 SELECT TIME 10000,20000-
           SELECT/RING cursor BIN/IMAGE 30 *events *image

Example 6a: Eliminate the first 5 ``raw'' amplitude channels from an HRI observation (containing UV contamination) and bin the result into an image:

Midas 001> SELECT RAW >5 BIN/IMAGE 16 *events *image

                        Example 7: Create from the total field three images at three different spatial resolutions (see note 7):

Midas 013> BIN/IMAGE [20][30][60] in=events out=big,right,small

    Example 8: Create three images in three different energy bands (PSPC data only) with pixel size of 30 sky pixels (see note 8a). Create from them a true color image and load it to the image display (see note 8b):

Midas 001> SEL AMPL [10,50][51,100][>100] BIN/IMA 30 *events *soft,medium,hard
Midas 002> LOAD/RGB soft medium hard rgb 0,4 0,5 0,4

              Example 9: Create an energy image with the same parameters as in example 2 (PSPC data only, see note 9):

Midas 001> SET/PROJECTION ENERGY
Midas 002> BIN/IMAGE 30 input=events output=energy_image
Midas 003> SET/PROJECTION INTENSITY

      Example 10: Create an image with the same parameters as in example 2, corrected for vignetting/effective area and dead time (PSPC data only, see note 10)

Midas 001> SET/PROJECTION CORRECT
Midas 002> BIN/IMAGE 30 input=events output=corrected_image
Midas 003> SET/PROJECTION NOCORRECT

       

Note 1

This command creates a fixed size sky image of all the photons listed in the input Photon Event Table events.tbl: only the image dimensions are specified. In this example the program selects a binsize so that all the photons fit into the 512 512 ima1.bdf file. The limit on your image size depends on the available memory and disk space, but remember that source detection commands (see Spatial Analysis chapter) still requires in input images of exactly 512 512 pixels.

Note 2

In this case the constraint is set to the image resolution: only the pixel size is specified. Here the output image ima2.bdf is made of pixels which are 30x30 times the original sky pixels (this would turn out to be a 512 512 image in the case of a ROSAT-PSPC pointed observation).

Note 3

As in example 2, only the binsize is expressed in arcsecond. Available units are degree, arcminute, and arcsecond.

Note 4

This pipeline of two Projection commands creates an image from a specific portion of the field of view. A spatial SELECT command precedes the BIN one. The numbers in brackets indicate the sky pixel coordinates of two opposite corners of the box. The * symbol is a short form for indicating the input and the output filenames.

Note 5

In this sequence of commands the sky portion to be selected is defined manually via the graphic cursor. A previously created sky image ima2.bdf is loaded into the image-display for reference.    

Note 6

In creating a sky image the photons might be generally filtered according to any arrival times, energies, and positions on the detector face or on the sky. In this example amplitude channels greater than 11 (not included) are not selected.

Note 7

Here 3 images with different spatial resolution are created. To remove the squared brackets here would generate an unfriendly error message (it cannot be created a single image having three different bin sizes).

Note 8a

Up to 15 images can be created at the same time. In this example it is shown how to create 3 images simultaneously at 3 different energy bands. The squared brackets are required in order to process the three selections in amplitude independently. To remove them would turn out in a single selection of all photons having amplitudes greater than 9! (the two commands SEL AMPL 10,50 51,100 >100 and SEL AMPL >9 are equivalent).

Note 8b

True color images can be produced from three images in different energy bands. Here the images soft.bdf, medium.bdf and hard.bdf, created by the previous command, are used. A special matrix of pointers, rgb.bdf, is produced and displayed through a special Look Up Table (contained in the system table rgb2.lut) generating a real color image on the image display. The cuts values, specified in the example for the three components, must be considered indicative. In order to get a good true color image, some tuning of the cuts is often required. Keep in mind that it is a mistake to treat true color images as data images (for instance smoothing them, or running source detection tasks): these are coded images, and the pixel values are just ``pointers to color''.

Note 9

Here is shown how to create the so-called energy images (i.e. their pixel values are the average energy channel of the photons contributing to each pixel). The ENERGY projection mode must be set before the creation of the image. Don't forget to get back to INTENSITY mode afterwards. Look Up Table energy.lut is especially suitable for displaying this kind of images. Keep in mind that it is a mistake to run source detection tasks on energy images -- they are not intensity images!

Note 10

Warning: the Projection correction mode is not applicable for WFC and HRI data. Only PSPC data can be corrected in this way. For HRI and WFC data alternative procedures are offered (see commands CORRECT/..., section 4.3.4, page ). As soon as Projection is set in correction mode, all the output binned datasets are corrected for vignetting/effective area and dead time effects. Data correction is a delicate matter, so read carefully section 4.3.4! The default correction is not always the best you can choose (it could be not appropriate for your data). Keep in mind that it is wrong to run source detection tasks on corrected images!

More details about image creation can be found in section 4.2.1 on page . In order to get familiar with the projection command syntax it is strongly suggested to read thoroughly section 4.3 also.