OBSERVING MODES OF TTF

There are several technical problems driving the development of tunable filters for narrowband imaging over standard fixed interference filters. First, it is difficult and very expensive for manufacturers to produce high performance narrow passbands, particularly at resolving powers approaching 1000. This problem is largely circumvented by use of TTF in conjunction with a 5-cavity, intermediate bandpass blocking filter. Secondly, the TTF instrumental function has identical form at all wavelengths and all bandpasses. A comparison of two discrete bands at different wavelengths is moderated only by the blocking filter transmission at those wavelengths (which is normally flat in any case), and the ratio of the gap spacings. Thirdly, the same optical path is used at all frequencies. Furthermore, the ability to shuffle charge allows one to average out all temporal variations: atmospheric transparency, the contribution from atmospheric lines, seeing, detector and electronic instabilities.

We now describe some of the advantages to be had from the TTF shuffle capability in imaging emission-line sources.

Tuning to a Specific Wavelength and Bandpass

This allows us to obtain images of obscure spectral lines at arbitrary redshifts (see e.g. Jones & Bland-Hawthorn 1997a). We can also optimize the bandpass to accomodate the line dispersion and to suppress the sky background. The off-band frequency can be chosen to avoid night-sky lines and can be much wider so that only a fraction of the time is spent on the off-band image. Fig. 2(a) shows a charge-shuffled image of the planetary nebula NGC 2438 in [SII] lines at 6731 and 6717 Å. During this 12 min exposure, TTF was switched between the two frequencies 18 times while the charge was shuffled back and forth accordingly. In this way, temporal variations in atmospheric transmission are equally shared between each passband over the entire exposure time.

Shuffling Between On and Off-band Frequencies

With the new MIT-LL chips, we are able to image the full 10 arcmin field for two discrete frequencies. We can also choose a narrow bandpass for the on-band line and a much broader bandpass (factor of 4-5) for the off-band image so that we incur only a 20 % overhead for the off-band image. As with specific tuning (Section 3.1, above), multiple frequencies can be imaged in a single frame, the number of which is entirely arbitrary.

Time-Series Imaging

For time-varying sources, we can step the charge in one direction while only switching between a line and a reference frequency. For example, a compact variable source imaged through a narrow aperture in the focal plane forms a narrow image at the detector. We can switch between the line and a reference frequency many times forming a set of narrow interleaved images at the detector. The reference frequency is a measure of the atmospheric stability during the time series. Alternatively, if nearby reference stars are available for photometric calibration, then charge shuffling can be done at a single fixed frequency as demonstrated in Fig 2(b). For example, some x-ray binaries produce strong emission lines that vary on 0.1 Hz timescales. With a slit only 4 pixels wide, we can obtain 500 images in the emission line, interleaved with a further 500 images at a reference frequency. In this example, all of the CCD area is utilized because the charge is clocked in only one direction. The vertical shift takes about 50 s per row. Strictly speaking, this operation constitutes a `charge shift' rather than a charge shuffle. Once the chip is full, it takes 3 mins to read out the CCD.

If the read-out time of several minutes is critical to the measurement, an alternative method is time-series read out. For the example above, the four CCD rows comprising the slit are clocked downwards at the end of each exposure. However, the next exposure is delayed by the time it takes to read out the bottom four rows (200 ms). In practice, the slow shutter means that the time series mode is to be preferred for most applications over the `charge shift' mode.

Adaptive Frequency Switching with Charge Shuffling

When imaging spectral lines that fall between OH bandheads or on steeply rising continua, a powerful feature is the ability to shuffle between on and off bands but where the off band is alternated between two or more frequencies either side of the on-band frequency. As illustrated in Fig. 2, this can be used to average out either rapid variations in blocking filters or the underlying spectral continuum.

Shuffling Charge in One Direction with a Narrow Focal Plane Slit

This allows us to produce a long-slit spectrum of an extended source. While this is vastly less efficient than using a long-slit spectrograph, the capability is a fundamental component of establishing the parallelism of reflecting mirrors at few micron spacings (Jones & Bland-Hawthorn 1997b). We do this by taking long-slit TTF spectra of an arc-line, with a pupil-plane mask to isolate a particular section of the beam. The plates are adjusted until the line shows peak transmission at a common plate setting for all regions of the beam. It is then that the plates are parallel. Conventional methods are an order of magnitude slower by comparison.

© Copyright Astronomical Society of Australia 1997