T e c h n i c a l


The TTF is a Fabry-Perot interference filter, comprised of two closely spaced glass plates. The surfaces of these plates are polished to high optical precision. This precision is to ensure a high degree of flatness so that the surfaces can be kept perfectly parallel during operation of the instrument (as shown in the diagram below)



H. Jones

The development of the TTF has resulted from the recent accomplishment of several technological goals across different fields. These include:

    the ability of capacitance micrometry techniques to keep polished glass plates parallel to within 1/200 of a wavelength,

    the use of long-range piezo-electric transducers to alter the spacing between the plates over the unprecedented range of 8 microns,

    the application of reflective coatings through ionic bombardment.

All of these ensure that transmission profile and tuning accuracy of the filter can be precisely optimised for the intended operating wavelengths.


The figure below shows a cross-section through the Taurus-2 focal reducer that houses the TTF when it is in use. The entire assembly is attached to the telescope at cassegrain focus. The TTF sits in the beam at the point labelled `Fabry-Perot'. For a larger version of the image (where the print is legible) click here.


Courtesy of La Palma Observatory



TTF Tilts

The following screw tilts now work well at deflecting ghosts, (particularly now that the pupil wheel has largely been dismantled).

Measured from top (HIFI etalon; same for TTF):

    12.05 mm, 8.65 mm, 1.51 mm
This deflects to edge if 2" filters are in use. We have modified last value now by 1mm, (this deflects to edge if 3" filters are in use).



Pixel Scales

    f/8 . . . . . . . . . . . . . . . . . . . . . . 0.594 "/pix side
    f/15 . . . . . . . . . . . . . . . . . . . . . 0.315 "/pix side



Focal Lengths/Beam Sizes

    Anglo-Australian Telescope:

    Collimator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.9 mm
    Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127.8
    Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59.9


    William Herschel Telescope:

    Collimator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 660.3 mm
    Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .128.1
    Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60.6

The 1% extra beam size on the WHT corresponds to the "as made" f/10.9 telescope.
(Courtesy of Richard Bingham)


Technical Documentation

Technical information relating to the operation of TTF can be found in the following documents:


Maintained by Heath Jones