Bad Pixel Mask on the CTIO Blanco 4m

The Mosaic 2 CCDs have relatively few bad pixels, mostly around the edges and in CCD #3.
Bad pixels do not respond to light at all, or respond in a nonlinear way.
These can be removed from data using a bad pixel mask.
The current bad pixel mask in the mscred package is from 2001,
so we recommend using the updated bad pixel mask found here.







You can create your own bad pixel mask by taking appropriate flat field observations, or download the most recent mask image below.

Bad Pixel Mask Correction Files

Bad Pixel Mask for 8-channel mode, 16 June 2010

How To Use The Bad Pixel Mask

The bad pixel masks are not trimmed. Hence, your images must not be trimmed in order to use them. It is best to apply the bad pixel masks before making your master flats.

Here are the steps to use the bad pixel masks:
  • download the Bad Pixel Mask for 8-channel mode in the same directory as the directory you are reducing your images in.

  • Use these commands to change the header of your images, so that the image header keyword "BPM" will point to the new bad pixel masks:

    hedit *.fits[1] BPM BPM0610/bpm1_0610.pl veri- show+
    hedit *.fits[2] BPM BPM0610/bpm2_0610.pl veri- show+
    hedit *.fits[3] BPM BPM0610/bpm3_0610.pl veri- show+
    hedit *.fits[4] BPM BPM0610/bpm4_0610.pl veri- show+
    hedit *.fits[5] BPM BPM0610/bpm5_0610.pl veri- show+
    hedit *.fits[6] BPM BPM0610/bpm6_0610.pl veri- show+
    hedit *.fits[7] BPM BPM0610/bpm7_0610.pl veri- show+
    hedit *.fits[8] BPM BPM0610/bpm8_0610.pl veri- show+


  • run "ccdproc"
    here is an example:
    images  =        @V_flats.list  List of Mosaic CCD images to process
(output =                     ) List of output processed images
(bpmasks=                     ) List of output bad pixel masks
(ccdtype=                 flat) CCD image type to process
(noproc =                   no) List processing steps only?

(xtalkco=                  yes) Apply crosstalk correction?
(fixpix =                  yes) Apply bad pixel mask correction?
(oversca=                  yes) Apply overscan strip correction?
(trim   =                  yes) Trim the image?
(zerocor=                  yes) Apply zero level correction?
(darkcor=                   no) Apply dark count correction?
(flatcor=                   no) Apply flat field correction?
(sflatco=                   no) Apply sky flat field correction?
(split  =                   no) Use split images during processing?
(merge  =                   no) Merge amplifiers from same CCD?

(xtalkfi=      xtalk090401.txt) Crosstalk file
(fixfile=                  BPM) List of bad pixel masks
(saturat=                     ) Saturated pixel threshold
(sgrow  =                    1) Saturated pixel grow radius
(bleed  =               20000.) Bleed pixel threshold
(btrail =                   15) Bleed trail minimum
(bgrow  =                    1) Bleed pixel grow radius
(biassec=             !biassec) Overscan strip image section
(trimsec=             !trimsec) Trim data section
(zero   =            Zero.fits) List of zero level calibration images
(dark   =                     ) List of dark count calibration images
(flat   =                     ) List of flat field images
(sflat  =                     ) List of secondary flat field images
(minrepl=                   1.) Minimum flat field value

(interac=                   no) Fit overscan interactively?
(functio=             legendre) Fitting function
(order  =                    1) Number of polynomial terms or spline pieces
(sample =                    *) Sample points to fit
(naverag=                    1) Number of sample points to combine
(niterat=                    1) Number of rejection iterations
(low_rej=                   3.) Low sigma rejection factor
(high_re=                   3.) High sigma rejection factor
(grow   =                   0.) Rejection growing radius
(fd     =                     )
(fd2    =                     )
(mode   =                   ql)


  • It would be good to check the masks by displaying an image with the bad pixel mask overlaid:

    mscred> mscdisplay img.fits bpdisplay=overlay bpmask=BPM

Generate Your Own Bad Pixel Mask

  • Take Data: 7 Bias images, 7 Domeflats with short exposure times, 7 Domeflats with long exposure times.
    (The [OIII] filter works well to allow for a large dynamic range without using exposures that are too short. With the [OIII] filter you can take about 50s and 5s exposures.)

  • Zerocombine the Biases

  • Bias subtract the Domeflats

  • Flatcombine the domeflats with long exposure times

  • Flatcombine the domeflats with short exposure times

  • Divide the short exposure dome flat by the long exposure dome flat.
    (The iraf command to use is "mscarith")

  • Use "ccdmask" on each mosaic extension to create a pixel mask.

    mscred> nmisc
    nmisc> ccdmask DividedFlat.fits[1] bpm1.pl
    nmisc> ccdmask DividedFlat.fits[2] bpm2.pl
    nmisc> ccdmask DividedFlat.fits[3] bpm3.pl
    nmisc> ccdmask DividedFlat.fits[3] bpm4.pl
    nmisc> ccdmask DividedFlat.fits[5] bpm5.pl
    nmisc> ccdmask DividedFlat.fits[6] bpm6.pl
    nmisc> ccdmask DividedFlat.fits[7] bpm7.pl
    nmisc> ccdmask DividedFlat.fits[8] bpm8.pl

    The badpixel masks are bpm1.pl, bpm2.pl, etc.

  • Now you can display an image with the bad pixel mask overlayed:

    mscred> display DividedFlat.fits[1] 1 overlay=bpm1.pl

    or

    mscred> mscdisplay DividedFlat.fits bpdisplay=overlay bpmask=BPM

  • Visually inspect the image and find bad pixels. Add these bad pixels to the bad pixel mask:
    imreplace bpm1_0102.pl[431:431,2631:2631] 1. (to indicate a bad pixel) and imreplace bpm1_0102.pl[334:334,1:4096] 1. (to indicate a bad column)

Back To Mosaic II



Updated: May 2010 by Andrea Kunder

Staff Contacts:
Alistair Walker: awalker@noao.edu
Chris Smith: csmith@noao.edu
Knut Olsen: kolsen@noao.edu
Tim Abbott: tabbott@noao.edu