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:

• Wavelength calibrate Z-step versus wavelength. Ideally, this step has already been done at the telescope, before the scan was taken (tsample).
• Create bias and flat-field images (imsum, tff).
• Bias-subtract, flat-field, flip and subtract sky-rings from images (tpipe).
• Align images and trim (toffsets, imalign).
• Find centre and radius of best circle to match circular field aperture stop, if present (tcircle).
• Create zero corner mask using the dimensions found by tcircle (tmask).
• Measure the changes in seeing/psf through the stack (tfwhm).
• Convolve the full stack to the worst seeing (tgauss).
• Combine images to produce two scans: one cleaned and the other straight summed (tsingle, tcombine).
• Create a deep image from all individual narrowband frames (tdeep).
• Obtain locations of many sky regions, free of objects (tregions).
• Create a zero edge mask (tmask).
• Measure the sky noise, create noise-edge masks and apply these (with the zero edge mask) to each respective stack (tmulti). Ideally, the aperture edge should be rendered undetectable.

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:

• Check the change in seeing through the convolved stack - should vary at the level of no more than ~ 1% (tgauss).
• Detect objects and photometry in each of four pre-selected apertures (tsex). Objects are detected using the uncleaned, unsmoothed images. Photometry however, is done on the matching uncleaned but smoothed frames.
• Create a file of central wavelengths corresponding to each image (twavelengths).
• Sort individual object photometry into TTF spectra (tespect).
• Compile a cosmic-ray/ghost pixel location file for all images in the stack (tpull).
• Photometrically register frames and extract double-detection emission-line candidates (with and without continuum). Write these to separate candidate catalogues (tscale).
• Check raw candidate catalogues for cases of double-counting by the object-detection software. More often than not, double-counting is not found, and so the task is run more as a check (tdouble).
• Inspect double-detection candidates to ensure no cosmic-rays or ghosts have evaded previous detection. Remove them from the candidate catalogues if found (tcull).
• Extract single-detection candidates from catalogues (tesone).
• Check candidate aperture corrections and replace object fluxes with those from the next largest aperture in cases where selected apertures are too small (tmodap).

 

 
 
 
 
 
 
 
 
 
 
 

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:

• Bias-subtract and flatfield standard star frames, using appropriately trimmed versions of the bias and flatfield frames obtained earlier (tstar).
• Photometer standard stars and correct for airmass and effective passband width. Also correct wavelengths for phase effects associated with the position of the standard star in the beam (tflux).
• Flux calibrate emission-line candidates to physical flux units (using standard star calibration constants). Correct for airmass and Galactic extinction (tcalibrate).
• Calibrate the fluxes of single-detection candidates (tonecal).
• Create strip-mosaic and chart images of finalised candidates (tmosaic).

 

 
 
 
 
 
 
 
 
 
 
 

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
  catalogues.

 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
  format.

 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
  stacks.

 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 (Z_s-values), and the wavelength
calibration information. Also corrects off-axis calibrations for phase
effect.

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