Preparing a CCD frame

A variety of operations need to be performed on a CCD frame before the reduction process described in Figure can commence. These include reorienting the image, subtraction of the CCD bias, removal of cosmic rays, and division by a flat field frame to remove pixel to pixel gain variations.

The FIGARO conventional orientation requires that orders lie horizontal with wavelength increasing to the right, and with bluer orders at the bottom. AAO CCD frames must be rotated and flipped:

    $ ROTATE image=object output=obj_rot
    $ IREVY  image=obj_rot output=obj_final

The bias level can be obtained from the overscan region of the CCD frame using ISTAT, and removed using ICSUB. If time--variations of the bias occurred during readout, the overscan rows can be EXTRACT-ed, SFIT with a suitable polynomial, normalized with ICDIV, and ISXSUB-tracted.

If the CCD has a small non-linearity when the intensity level is near saturation (65535 ADU), after removing the bias the non-linearity can be removed using CCDLIN. This requires knowledge of the non-linearity parameter , which is known for AAO CCDs but is negligible for most spectroscopic data.

Cosmic rays can be eliminated using CLEAN, BCLEAN, or BCFCLEAN algorithms. The cosmic ray may be a small fraction of the signal (but not of the noise) and difficult to see. It is worth temporarily removing the inter-order gaps and the intensity variations along the slit before cleaning the image. A simple procedure for doing this is as follows:

• IXSMOOTH with a broad Gaussian (about 30--60 pixels) to obtain a reference frame in which the intensity variations in the cross-dispersion direction are retained and those along the dispersion direction are smoothed.
• IDIV-ide the original frame by the smoothed frame to give a flattened image in which the cosmic rays are obvious. Note that each cosmic ray has dark wings attached to it. This is an artifact of the smoothing operation.
• CLEAN to remove the cosmic rays.
• IMULT-tiply the cleaned image by the smoothed reference image to recover the original intensity variations minus the cosmic rays. This process will also remove the dark residual wings on either side of the cosmic ray strikes.

   $ IXSMOOTH IMAGE=object SIGMA=100 WIDTH=50 OUTPUT=smooth
   $ IDIV     IMAGE=object IMAGE1=smooth      OUTPUT=temp
   $ CLEAN    IMAGE=temp                      OUTPUT=temp
   $ IMULT    IMAGE=temp   IMAGE1=smooth      OUTPUT=object

If several similar images are available, say from a series of flat field or stellar exposures, then one of the images can be used instead of the smoothed image as a reference. Cosmic rays on the denominator image will appear as dark spots on the image to be cleaned but can be ignored in the cleaning process.

Flat field frames are obtained by taking repeated exposures of a continuum source and then adding them. The effects of non-uniform illumination of the detector are removed by dividing the frame by a smoothed reference frame. Overscan columns can cause errors in the smoothed flat field near the edge of the frame. To overcome this problem it is necessary to first subset the image to remove the bias columns and then superset the frame later to restore the proper dimensions. The following sequence illustrates the principal. In this sequence it is assumed that the individual flat field frames have been reoriented, cleaned and added into the final frame named `flat'. In this example, the original frame has dimensions of x= 584, y=386.

   $ ISUBSET image=flat ystart=1 yend=386 xstart=1 xend=576 output=flat_sub
   $ IXSMOOTH image=flat_sub  sigma=100  width=50  output=smooth
   $ IDIV  image=flat_sub  image1=smooth  output=flat_sub
   $ ISUPER image=flat_sub xsize=584 ysize=386 xpixel=1 ypixel=1 output=flat