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Hi,I would like to display an image with z1 and z2 of my choice and overplot wfpc2 apertures and a compass on it. If I open the image from within IRAF, I can choose
z1 and z2 but cannot specify the fk5 values of the apertures.If I load the image in stand alone ds9, I can overplot the apertures. But, how do I now specify z1 and z2? What does the task user within scale do? is ther any way
I can specify z1 and z2?
thanks
Aparna

The "Scale Parameters" option at the bottom of the DS9 'Scale' menu should let you set the z1/z2 values manually, I don't believe there are any XPA access points or command-line options that would let you do this automatically. The menu on that popup panel will also let you control some parameters used in the DS9 zscale algorithm which is the old version. IRAF currently uses (by default) 1000 points evenly grid over the image to compute the z1/z2, the old version (i.e. DS9) uses 600 points on 120 lines and so the sampling and resultant values will be slightly different.How are you overlaying the WFPC aperture? Tasks like WCSCTRAN and SKYCTRAN can help you convert between image and sky coordinates. The COMPASS task in STSDAS.GRAPHICS.SDISPLAY can overlay a compass for you.-Mike