Advanced TTF Data Analysis

Here we briefly describe the reduction of TTF data within the framework of iraf versions 2.10/2.11. The TFred package written by D.H. Jones (Mt. Stromlo/ESO VLT) contains a set of tasks especially for this purpose. You can download individual TFred routines from here. The routines are particularly relevant to multi-object surveys. A similar set of routines is under development for the analysis of diffuse emission line sources. Frankly, the TTF provides narrowband images such that the user should be able to undertake the analysis alone. But, in the presence of a strong night sky line, the slowly variable background induced by the phase effect may require a specific routine, or discussions with JBH.

TFred is a package of iraf tasks specifically designed to aid in the analysis of the stack of narrowband frames produced by a tunable filter. While most of the tasks are self-contained, a few call external fortran code and uses  (by E. Bertin) for object detection and photometry. Many of the tasks may have a broader use in the analysis of astronomical data from Fabry-Perot interferometers.

The analysis of multi-object tunable filter data has three stages: (1) preparation of images, (2) object detection and selection of emission-line candidates, and (3) flux calibration. The procedures of each stage are summarised in what follows. Note that all tasks within the TFred package begin with a `t'; those without are standard iraf software. A full list of TFred tasks is given at the end.

(1) Image Preparation

Initially the raw scan frames must be prepared for analysis. This includes the removal of the bias level and pixel-to-pixel variations, and the fitting and subtraction of night-sky rings. Images are then aligned to a common reference frame and trimmed. A duplicate set of frames is created and degraded to a common worst seeing. Then frames are co-added into combined scans, of which there are different versions arising from the use of median filtering or straight summation. Versions are also generated from the smoothed (seeing-degraded) and unsmoothed frames. Each of the final combined scans have noise-edge masks computed and applied to the image corners. This last step is necessary to stop the object-detection software from setting a detection threshold that is biased by the statistics of pixels outside the image area. In summary:

(2) Detection and Selection of Candidates

By this stage there are three scans: (1) a summed (uncleaned) scan with the original variable seeing, (2) a summed (uncleaned) scan with frames smoothed to a common worst seeing, and (3) a cleaned scan with frames smoothed to a common worst seeing. From here, the steps of object detection, photometry and selection are straight-forward and efficient. Most of the subsequent analysis works with object catalogues rather than images, so processing time is fast. In summary:

(3) Flux Calibration

With selection of emission-line candidates finalised, all that remains is for the measured fluxes to be calibrated in terms of physical units. Initially, standard star scans (taken on the same night as the science frames) must be reduced in order to obtain flux calibration constants for that night. These calibrations can then be applied to the object catalogues in a single step. In summary:

Some final catalogue processing may be necessary to sort objects by field and to apply final selection cuts.

Summary of Tasks in TFred

The following is a guide to the tasks available in TFred. In addition to the tasks used in general reduction, there are others for more specific applications.

 tapcheck  Display a scan mosaic and draw object and sky
  apertures on each frame for inspection.

 tapdraw  Draw the different apertures used for
  object photometry onto a mosaic of object scans.

 tbias  Bias-subtract all the images in a scan.

 tcalibrate  Calibrate the fluxes from a set of narrowband frames
  using the previous photometry of standard stars.

 tchoose  Select entries in the head catalogue based on some
  specified criterion, and then select the matching
  entries from the secondary catalogues.

 tcircle  Superimpose any circle (and its centre) onto an
  existing image.

 tcombine  Combine multiple scans into a single deep one.
  Option of creating cleaned or straight-summed output
  frames in either fits or iraf format. Further option
  of applying zero or noise corner masks to images.

 tcray  Produce an ASCII table of all pixel locations
  in a scan affected by cosmic-rays or ghosts.

 tcull  Create, display and label strip mosaics for candidate
  inspection and editing. Automatically modifies object

 tdeep  Combine all of the narrowband images in a scan
  to produce a single, deep, narrowband frame.

 tdisplay  Display a sequence of narrowband frames, one by one,
  with the option of using imexam on each.

 tdouble  Check for double-counting of objects by .

 tedge  Automatically measure and add the correct amount of
  noise to the edges of frames. Doing so removes
  the shadow of the circular field stop.

 tesone  Select only single-band detections from a raw
  object catalogue.

 tespect  Extract object fluxes from raw detection catalogues
  and sort into narrowband spectra.

 texpand  Reverse the work of tshrink. Automatically
  seek all images in a directory and uncompress them,
  with the option of converting to either fits or iraf

 tff  Take a list of raw flatfield images and bias-subtract
  and normalise them, ready for use.

 tflip  Flip one or more images arbitrarily.

 tflat  Form flatfields from a whitelight cube.

 tflux  Measure standard star fluxes from a stack of
  narrowband frames.

 tfwhm  Measure the FWHM of one of more objects through
  a series of narrowband frames.

 tgalactic  Compute the galactic extinction correction at a
  specified wavelength, given the visual extinction A(V).

 tgauss  Degrade a stack of narrowband images to the same
  seeing, after computing the convolution kernel required
  in each case.

 thist  Draw a histogram of sky values for a region selected
  through the Ximtool display. Option of working on
  a stack of images in sequence.

 tmakefile  Create a file list of image names.

 tmask  Create images of an artificial circular aperture mask
  and/or a noise-corner mask.

 tmcomb  Apply zero and/or noise-corner masks to a stack of
  existing images (combined: cleaned and/or summed).

 tmodap  Replace the object fluxes previously measured with
  small apertures with larger ones.

 tmosaic  Automatically create a strip-mosaic directly from an
  object catalogue. Also has a chart-making capability.

 tmulti  Automated application of noise-corner masks to image

 tnewmake  Create file lists with the option of file skipping,
  (otherwise same as tmakefile)

 tnoise  Get coordinates of a few specified sky regions and
  measure the RMS noise within those regions for every
  image in a narrowband scan.

 toffsets  Measure mean pixel offsets of a set of images from a
  reference image.

 tonecal  Calibrate single-slice candidates to physical flux
  units and correct for the aperture used.

 tpath  Edit the image pathname in the headers of a series
  of images

 tpay  Like tcray, except with the additional option of
  small sub-image regions to expedite testing.

 tpipe  Reduce images from a set of scans in a fully automated
  fashion, from raw telescope output to flipped, bias-
  subtracted and background-flattened images.

 tpull  Pull out the cosmic-ray and ghost-affected pixels in
  a stack of images and write these locations to file.

 tratio  Take the ratio of two images through the fluxes of
  a set of stars, common to both.

 tregions  Display an image in Ximtool and read-off image
  regions selected by the cursor. Then write these to file.

 tringSub  Same as tring but performs subtraction of
  background instead of division.

 tsample  Do image statistics on a stack of images.

 tscale  Normalise object fluxes to a common level and select
  emission-line candidates from the catalogues.

 tsex  Run  on all images in a narrowband scan.

 tsingle  Automatically combine up to four sets of narrowband
  scans into individual, deep images.

 tstar  Pipeline reduce standard star scan (bias and flatfield).

 tstrip  Create a strip-mosaic of object images through a scan.

 ts2  Like tstrip except the scans are put into two columns.

 ts3  Like tstrip except the scans are in three columns.

 tshrink  Search a set of directories for all images, convert
  them to fits (if not that already), and compress them.
  Opposite of texpand.

 twavelengths  Generate a table of wavelengths from a specified
 sequence of TTF plate settings (Zs-values), and the wavelength
calibration information. Also corrects off-axis calibrations for phase

 tzzz  Like tmosaic, except that data is extracted from
  full reduced catalogues in final form.

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