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:
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:
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:
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.
Display a scan mosaic and draw object and sky
apertures on each frame for inspection.
Draw the different apertures used for
object photometry onto a mosaic of object scans.
tbias Bias-subtract all the images in a scan.
Calibrate the fluxes from a set of narrowband frames
using the previous photometry of standard stars.
Select entries in the head catalogue based on some
specified criterion, and then select the matching
entries from the secondary catalogues.
Superimpose any circle (and its centre) onto an
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.
Produce an ASCII table of all pixel locations
in a scan affected by cosmic-rays or ghosts.
Create, display and label strip mosaics for candidate
inspection and editing. Automatically modifies object
Combine all of the narrowband images in a scan
to produce a single, deep, narrowband frame.
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 .
Automatically measure and add the correct amount of
noise to the edges of frames. Doing so removes
the shadow of the circular field stop.
Select only single-band detections from a raw
Extract object fluxes from raw detection catalogues
and sort into narrowband spectra.
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
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.
Measure standard star fluxes from a stack of
Measure the FWHM of one of more objects through
a series of narrowband frames.
Compute the galactic extinction correction at a
specified wavelength, given the visual extinction A(V).
Degrade a stack of narrowband images to the same
seeing, after computing the convolution kernel required
in each case.
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.
Create images of an artificial circular aperture mask
and/or a noise-corner mask.
Apply zero and/or noise-corner masks to a stack of
existing images (combined: cleaned and/or summed).
Replace the object fluxes previously measured with
small apertures with larger ones.
Automatically create a strip-mosaic directly from an
object catalogue. Also has a chart-making capability.
Automated application of noise-corner masks to image
Create file lists with the option of file skipping,
(otherwise same as tmakefile)
Get coordinates of a few specified sky regions and
measure the RMS noise within those regions for every
image in a narrowband scan.
Measure mean pixel offsets of a set of images from a
Calibrate single-slice candidates to physical flux
units and correct for the aperture used.
Edit the image pathname in the headers of a series
except with the additional option of
small sub-image regions to expedite testing.
Reduce images from a set of scans in a fully automated
fashion, from raw telescope output to flipped, bias-
subtracted and background-flattened images.
Pull out the cosmic-ray and ghost-affected pixels in
a stack of images and write these locations to file.
Take the ratio of two images through the fluxes of
a set of stars, common to both.
Display an image in Ximtool and read-off image
regions selected by the cursor. Then write these to file.
Same as tring but performs
background instead of division.
tsample Do image statistics on a stack of images.
Normalise object fluxes to a common level and select
emission-line candidates from the catalogues.
tsex Run on all images in a narrowband scan.
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.
Search a set of directories for all images, convert
them to fits (if not that already), and compress them.
Opposite of texpand.
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
except that data is extracted from
full reduced catalogues in final form.