Wavelength calibration (for broadband filters)

The TTF produces something which approximates to a monochromatic field only at the lowest resolving powers. The BTTF and RTTF were both specified with this in mind. Conventional broadband filters allow the TTF to be used at any wavelength in the optical window. The filters currently available at the AAO are shown below:

The MITLL CCD response curves are only illustrative; the Tek response curves are as measured by J.R. Barton.

An important problem is how to calibrate such wide filters. There are few, if any, arcs which provide a few widely spaced emission lines that can be identified unambiguously. A swarm of emission lines would be ground up by order confusion.

Method 1.  At least for the BTTF blockers, the narrowband filters (shown in green below) provide a  useful calibration. For the RTTF, there is a narrowband filter at 872nm not shown here.  The edges of the intermediate blockers are also useful for calibrating the broadband blockers. The black lines indicate the transmission profiles of both TTFs.

Here, we illuminate the instrument with whitelight and use the narrow filters as broad emission lines. We can even use the filter width or filter tilt to determine the approximate wavelength dispersion. Now scan the filter profile in sausage cube mode. The spectral reduction steps are the same as those for the intermediate filters shown here.

This is what the HeII (468nm) filter looks like, scanned coarsely through two orders with the BTTF (in sausage mode). In etalon "z" units, the FSR is 243.

Method 2.  There is an alternative approach to calibrating the broad filters. Choose the intermediate filter closest to the spectral region of interest and wavelength calibrate this filter at low resolution.  To determine the required etalon "z" value, extrapolate the filter to the required wavelength, out of the intermediate band if need be.  This turns out to be the most accurate method of calibrating broadband filters.

Advanced tip: It's worth noting that one can establish the lambda-z  relation for low resolution work using a single, shuffled arc image, rather than having to wait for sausage cubes to complete. A vertical cut through the centre of the image shows the narrow band filter profiles. The subtlety is only to determine in which row the shuffle image starts and finishes. Once you have this, simply offset the start and end "z" values so that the y-z  relation looks right. If you know the centre and bandpass of your narrow filter, you have the wavelength offset and dispersion in z. This is precisely how I calibrated the KPNO V filter for Hewett & Warren's QSO lens experiment in August 1998. When the 580/18nm filter finally arrived, the calibration was found to be correct to within 5%, which did not matter for the broad TTF bands used.