FAQ

Mosaic II FAQ
Frequently Asked Questions

I identified an object exposure as a domeflat. How do I correct this after the image is written to disk?

You need to do two things: (a) change the name of the file, and (b) change the information in the header describing the "image type".

• change the name with something like
• rename dflat2015.fits obj2015.fits
• change the image type in the header with the msccmd command, which is needed to change the information in ALL extensions of the MEF file. Use something like
• msccmd "hedit $input OBSTYPE object" obj2015.fits

What is the linearity of the Mosaic CCDs like?

The linearity should be good to <1% up to 20,000 and 40,000 adu, depending on which CCD. The point at which each amplifier saturates is posted on the main CTIO Mosaic web page. The amplifiers probably begin to go non-linear within 10% of these values. Note that there are SIGNIFICANT variations, so one must be careful when specifying the saturation value.

How do I fix bad headers? (when TCS & WCS info isn't present)

Before fixing any headers, save a copy of the original image.

• To add parameters RA, Dec, Filter,epar fixhdr1

• To add any other parameters, epar ccdhedit

How do I take apart MEF files (extract individual CCD frames)? How do I make an MEF file from individual CCD frames?

The tasks to use are mscsplit and mscjoin.

How do I interpret the WCS in the Mosaic image headers?

Lindsey Davis described some of the details of the WCS calculations in this email about Mosaic WCS computations.

How do I correct for the difference in pixel area for photometric measurements?

Frank Valdes described this correction in an email about using the task 'pixarea' in the mscred package.

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Last updated: April 2010 by AMK