Technical aspects of the new AAO/UKST H interference filter

Basic features of the UKST H interference filter

Basic design: A 3-cavity design of ion-assisted deposition (IAD) of refractory oxide material on both sides of a Schott RG610 R-band filter.

Filter size: mm. A single element `monolithic' H interference filter.

Clear Aperture: Approximately 305 mm diameter.
Coatings were deposited within a circular aperture to mm of the edges of the square blank.

Measured filter thickness: 5.53 mm.
The filter was easily accommodated inside an existing UKST plateholder with only minor modification. The filter is actually 1 mm closer to the focal surface than existing filters, maximising image quality.

Central Wavelength: Å
The central wavelength variations incorporate the effects of the f/2.48 beam.

FWHM of filter bandpass: achieved, i.e. a 1% filter.

Blocking: 0.01% of peak out-of-band transmission over the range 2000 Å- 6990 Å.

Transmitted wavefront: better than /4 achieved.
Confirmed by interferogram (see fig.2). Peak-to-valley range of only 0.193 waves with an rms of 0.040 waves. Results integrated from 2732 points measured over the clear aperture using a He-Ne laser at 6330 with aperture size of 51 mm.

Peak Transmission: across the clear aperture with 5% max variations (see fig.3).

Temperature effects: Central wavelength temperature shift: 0.035Å/C.
An extreme operating range of -5C to 30C corresponds to a blueward shift of 1.23 Å.

Coating refractive index: 1.973.

Surface quality: S/D (scratch/dig)= 80/50. A measure of the number of minor surface imperfections per unit area.

Humidity tolerance: The filter meets or exceeds the US military standard for protection against humidity.

Polarisation effects for off-axis rays: At large incident angles the filter peak will broaden and split into two peaks whose polarisations are 90 degrees apart. Plots provided by Barr Associates reveal that with 10.64 degrees angle-of-incidence in collimated light the s- and p-planes of polarization have bandpass shifts of only 1.8 Å with respect to each other.

Physical filter deformation: The filter is quite heavy such that some physical deformation of the filter under gravity might be expected when in the telescope. Any sag is in the same sense as the existing curvature of the focal surface. Barr Associates have not provided any details but indicate that any minor sagging present will not create any problems. Certainly the image quality across the entire filter surface is excellent.

Coating toughness: The filter surfaces can be cleaned with a lint free soft cloth soaked in methanol, acetone or ethanol. The coatings also meet or exceed military standards for moderate abrasion resistance.

  
Figure 2: Interferogram of the full uncoated RG610 filter substrate taken in transmitted light as provided by Barr Associates. The interference lines are quite parallel with little distortion. The interferogram confirms the transmitted wavefront is better than 1/4 wave per 25 mm.

  
Figure 3: Transmission map of the coated H filter as provide by CSIRO National Measurement Laboratory as part of the independent filter acceptance tests, re-produced with their permission. Note the very high and uniform levels of filter transmission () over much of the circular aperture.

Environmental issues

The H focal surface filter mounted in a dedicated plateholder only becomes uncovered inside the protected telescope tube of the UKST and is thus less prone to external environmental effects such as high humidity since dry nitrogen is purged over both emulsion and filter at the focal surface. Off the telescope the filter is stored in a dry inert atmosphere which affords further protection ensuring filter longevity. Such a filter can be interchanged with any other during the night so no observational flexibility is lost.

Operation over bright of moon

Because of the narrow bandwidth of the filter, deep exposures can be taken in brighter sky conditions. Initial tests indicate that the H survey could be undertaken during currently unscheduled bright of moon nights when the UKST is otherwise idle. This offers the potential for making more effective use of the telescope.

© Copyright Astronomical Society of Australia 1997