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You start with the image to be calibrated in Buffer A (with the BLACK and WHITE levels adjusted to give a good view of the stars and the Cursor. With some input from you, JIMSAIP will construct a reference chart with star-like images and put it in Buffer B. You must then select a minimum of four matching pairs of stars between the two images and JIMSAIP will calculate the calibration factors for your image, including differences in scale, rotation between the images (try to keep the 'up direction' of your images within +/- 60 degrees from north), and any row or column shift. JIMSAIP will display the errors of a least squares fit among the reference stars (allowing you to select other stars if the errors are too great), the overall average error, the center of your image in RA and DEC, your pixel scale in arcseconds and your effective focal length measured in pixels (you multiply this number by the physical size of your camera's pixels to get the focal length in mm or whatever). Afterwards, the Examine Image display will show the cursor coordinates in RA and DEC and you can select any object or position in the field to measure.
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Upon selecting the Calibrate and Measure command, JIMSAIP will ask you for the center RA and DEC position of the reference field and for the width (in decimal degrees) of the reference field. You must have SOME idea of where your image is located and it must be within the bounds of the reference data file generated by either Guide 4.0 or the listing from STSCI. If you are measuring asteroids or comets, you probably made a finder chart to help locate the field in the first place and this finder chart can help with generating the reference field.
Often, you will want to make the reference chart a bit wider than what you know your image field to be to allow for little mis-alignments in centers. (After you have once calibrated an image, you will know your pixel size in arcseconds precisely and, hence, you can calculate the dimensions of your image field of view by multiplying by the number of pixels in each row and column.) JIMSAIP can handle the resulting difference in scale. If you make the reference field too wide, it may be difficult picking out star matches.
After you have selected the center RA and DEC and width of the reference field, JIMSAIP will construct a reference star chart and put it in Buffer B. It will use as the background level for the reference chart the BLACK level used for the CCD image in Buffer A, so you might want to adjust the BLACK and WHITE levels of your image in Buffer A before you start. You will then be put into the Examine Image mode with the CCD image in Buffer A displayed.
You will then be asked to supply a number of stars to use as references. You must input a minimum of four and may put in a maximum of 20. Because of problems with the GSC, this is not as straight forward as one might wish. See Precision Factors for more discussion on this point.
You next must select matching pairs of stars between the two images. You can 'flip' between the two images using the A and B keys to orient yourself, but you must always select the star in Buffer A before you select the matching star in Buffer B.
To select a star in either image, put the cursor box on or very near the star you want, and press the P ('Point' or 'Position') key. JIMSAIP will center the cursor according to the light distribution in the star image. As such, the results will depend in some degree on the box size, which you can adjust, and the noise of the background. Any time you use the P function you will have to first set the background threshold using the T ('Threshold') key. If your image has varying backgrounds from light pollution, nebulosity or whatever, you may want to reset the background threshold in a region nearby the star that more closely approximates the star's immediate surroundings. The reference field has a 'perfect' background and, unless you set a threshold in the CCD image field greater than the value of the reference star, you will always get perfect alignment in the reference field.
As the accuracy of your calibration depends on the selection process, you will want to exercise some care. Avoid close stars that you can't exclude by making the box smaller. Don't use too big a box as you will be more likely to include noisy background that could disrupt the centering process. With a little practice and care, you can achieve sub-arcsecond precision.
After you have selected your star following the procedure described above, press the N ('Next') key and JIMSAIP will switch you to the other image. If you are going from the CCD frame to the reference frame, select the reference star that matches the star you selected in the CCD frame. If you are going from the reference frame back to the CCD image, select a new star and repeat the process until you have selected the number of pairs you chose when you started. It is probably a good idea to select stars in a square or diamond pattern, but that is not essential.
When you use the N key to switch between images, JIMSAIP will leave an image of the cursor on the star so that you can see where you've been. If you don't like this, use the 'Esc' key to switch. The process will continue but without the 'mouse tracks.' You can always clear the 'mouse tracks' by pressing the R ('Rescale') key and accepting the default BLACK and WHITE levels. JIMSAIP will re-draw the image without the 'mouse tracks.'
If you get confused or do some stars out of sequence (or for any other reason), you can press the Q ('Quit') key and exit the whole process.
After you have completed the star pairs (in A-B sequence), JIMSAIP will calculate a least squares fit and will display the error of each star. Note any out of line figures because you may want to repeat the process avoiding the star that gave the worst result. Press any key to see the results of the calibration.
The average error in arcseconds is displayed along with values for the scale factor between the image and the reference, the rotation between the two fields, the row and column offset, the center of the field in RA and DEC, the pixel resolution in arcseconds, and the effective focal length in pixels.
Using the pixel resolution and the dimensions of your chip, you can easily calculate the width and height of your image field. With this, you can use the center coordinates and repeat the calibration, inputting the better values. If the rotation is small, the reference field will align very well with the CCD image. For best results, it is suggested that you repeat the calibration using the better input values.
Pressing any key will return you to the main menu. JIMSAIP will have saved the calibration factors in the image header, though, so that when you Examine the image, the RA and DEC information fields will be active and the position of the cursor will be displayed in RA and DEC. You can then move the cursor to any object in the image, use the P ('Point' or 'Position') key to center the cursor on the object and read out the object's RA and DEC. If you save the image (in FITS format), the calibration data will be saved along with it and the next time you load the saved image, the calibration will be in effect.
In the Examine mode, you can press the F10 key to record the measurements in an ASCII file (see Recording your data).