Calibrations:

AFTERNOON+TWILIGHT: The following table summarizes the recommended calibration exposures for the grisms+gratings supported within the Low-Redux pipeline.

Binning: For the blue-side observations, we present values assuming binning 2x2 (spatialxspectral). We recommend this level of binning when using the 1200 grism to reduce the readnoise by a factor of sqrt(4)=2. For a 1" slit, binning by 2 in the spectral dimension implies a sample of 4 pixels. The spatial plate-scale on the blue-side is 0.135" per native pixel. Therefore, 2x binning will lead to Nyquist sampled seeing only for FWHM ~ 0.6".

Flat fielding: Our approach for flat fielding these spectroscopic observations is to divide up the flat fielding into a pixel-flat which represents the intrinsic pixel-to-pixel response variations of the CCD (i.e. the response to a smooth source), and an illumination flat, which represents the larger scale illumination variations due to non-uniformities in the width of the slit, vignetting, etc. This separation is clearly not perfect, but our experience is that the illumination flats significantly improve sky subtraction.

The pixel-flat for the blue-side is constructed in a novel manner for the high resolution grisms. One observes the twilight sky with the grism in place but without a slitmask (i.e. "direct"). This smears the solar spectral features sufficiently that the flat can be properly normalized before applying a pixel-to-pixel correction. We use the twilight sky because the internal and dome lamps both emit too little UV light. Although one might record a few thousand counts with these lamps, we have confirmed that this is dominated by scattered red light (e.g. one observes fringing!).

For the red side, dome flats are better than internal flats for correcting the pixel-to-pixel variations because they reduce the level of scattered light. However, one may desire to take internal flats on the sky during the night (or twilight) to remove fringing. The same dome flats can be used for the illumination correction although observations at twilight may be preferable.

• Organize the data
  1. Create a 'Raw' directory -- mkdir Raw
  2. Place all of the Raw frames in the Raw directory
  3. gzip the files (optional)
• Create the plan file
  1. Launch IDL from above the Raw directory
  2. Run long_plan :: This code parses the headers of each raw file and creates an ASCII file which summarizes the data. It attempts to guess the 'type' of each frame (e.g. arc, domeflat, science).
         Example: IDL> long_plan, '*.fits*', 'Raw/'
• Edit the plan file (default: plan.par)
  1. Consider copying plan.par to a new file
  2. For longslit observations, slitthresh is automatically set in the code to be 0.10. For multislit observations you may specify your own value of slitthresh by including this parameter in the plan file. The default for multislit observations is currently 0.02. For short slits, lower slitthresh as necessary.
  3. Remove unwanted images from the .par file.
  4. Inspect and edit the image types for the various exposures. (Listed in column 3 in the plan file) For the LRIS spectrometer there are 5 types of interest:
     
     

     Image Types

     Type	Description
     arc	Exposure of the arc lamps taken with the instrument configured for spectroscopy (slit + dispersing elements).
     domeflat	Flats used to calibrate the pixel-to-pixel variations in the CCD. Blue side: At high dispersion (600 or 1200 grism), one should observe the twilight sky without a slit. This is the only means of getting enough blue counts in a spectrum that does not have sharp features. At lower dispersion (300 grism), one can acquire a dome flat. Red Side: One obtains 'standard' dome flats. Note: The code currently attempts to distinguish between domeflats and twilight flats by considering the time the exposures were taken and the position of the dome slit relative to the pointing. There is no header keyword that specifies the dome lamps being on or off. Therefore, pay extra attention to these files to make sure they are labelled correctly.
     twiflat	Flats used to calibrate the larger scale variations (i.e. greater than 5 pixels) in the illumination pattern of the CCD. Spectrum of the twilight sky through the longslit or slitmask.
     iflat	Flats used to calibrate the pixel-to-pixel variations in the CCD. Domeflats are always preferred to minimize scattered light, but internal flats can be used if necessary.
     bothflat	If the user specifies this designation, the flat image will used for both pixel and illumionation corrections (i.e. it will be treated as a domeflat and a twiflat simultaneously). This is useful in cases where twiflats were not taken and the user wishes to make an illumination correction.
     science	Spectrum of an astronomical object. This includes 'standard stars'.
     std	If the user specifies this designation, this frame will be used as a crutch for tracing other objects. It will be reduced as if it were a science frame.

     
     
     
  5. Insure that the necessary calibration files exist. Files of 'arc' type are always needed. In addition, it is ideal to have both domeflat (or iflat) and twiflat files for each grating/grism/slit combination; this gives the best calibration of pixel-to-pixel variations in the CCD and illumination function. However, if only domeflats are available, these may be used for both the pixel-to-pixel calibs and the illumination calibs by changing the 'domeflat' type to 'bothflat' in the plan file.

All of the reduction steps are run by the code long_reduce
    Example: IDL> long_reduce, 'plan.par'

This routine:
1. Stacks and process the flats
2. Identifies the edges of the longslit
3. Calibrates the various arc images
4. Processes the science frame (flattens)
5. Finds/traces objcets within the slit
6. Extracts using a non-parameteric optimal extraction algorithm