Appendix 6 - Filters.

1. Filters Available
2. Filter Holders and Wheels
3. Filter Bandpasses                  

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The AAO's imaging filter `inventory' is currently somwhat confused. It is hoped this page will eventually list all the AAO filters available for imaging, their usual locations, how they were made, and what their throughput is (both in table and plot form).

Until that happens, however, if this page does not contain the information you are looking for, you should contact AAO staff for help. Steve Lee (sl@aaocbn.aao.gov.au) will almost always be able to answer your question - but requests should be directed through Chris Tinney (cgt@aaoepp.aao.gov.au), who is the AAO astronomer responsible for imaging.

Filters Available

The filters all come in one of two types - they are either interference filters, or broadband filters made with Schott glasses. For broadband applications, the interference filters generally have better throughput and a `squarer' profile than the Schott glass filters. Of course, the standard photometric passbands are generally not very square, making the interference filters a poorer match to them. In general, however, 4m telescope imaging projects are more interested in going deep, than high precision photometry, in which case the intereference filters are obviously to be preferred. In practise they are used almost exclusively.

A range of `narrow-band' interference filters are available. Some of these (in particular the Taurus and Taurus Tuneable Filter, or TTF) are very good, having peak transmissions of over 90%. Some are quite poor, with peak tranmissions of only 50%, so be careful which filter you choose.

The tables below list the available sets of filters, and includes the `recipes' for the filters (where available) and the appropriate focus offsets for use with the DOUBLET CORRECTOR.  Section 3.1.6 discusses the difference between triplet and doublet focus.  The Filter Bandpasses section contains hyperlinks to tables of bandpass information, and GIF files containing graphs pf the band passes.

Of all the filters listed below, only the KPNO set and the f/1 set see much action.

• KPNO interference BVRI set + CuSO4- this is the default broadband set which will be installed for f/3.3 and f/8 imaging unless you specifically request otherwise.

• 50mm square Schott glass filters, anti-reflection coated.

• Halpha Filter Set - the details here are drawn from the 1989 CCD User's Guide.

• LDSS 100mm diameter BRI Schott glass Set (usually installed in LDSS, but may be removed for other imaging applications if required).

• 50mm square Gunn-Thaun g (PASP, 88, 543)
   

• 50mm diameter B,V,R,I,V+R Schott Glass f/1 Set

• 50mm square `Steve Lee' Schott Glass Set

• ANU 75mm diameter BVRI Schott Glass Set (now permanently on loan to AAT).

• ANU 160mm diameter UBVRI Set (only available if requested from ANU and not required by ANU Observers on 40" or 2.3m telescopes). All are 5mm thick

• TAURUS and TAURUS Tuneable Filter (TTF) filters. The TAURUS filters are 3.0 inches in diameter. The TTF filters are 4.85 inches in diameter. You can obtain more information about the TTF filters from the TTF Home Page, maintained by Heath Jones (MSSSO) and Joss Hawthorn (AAO). In particular, have a look at the Filters & Masks page.

• Photographic Narrow-band filters. There is a collection of large, narrow-band filters originally designed for use with photographic emulsions. Currently there is little experience with the use of these filters for CCD imaging, however, there is no reason to expect they cannot be used. The bandpasses and central wavelengths are from the manufacturer's data, as seen in an f/3.3 converging beam. The filters (except for Halpha which is 10" x 10") are all 5" x 5" square, and mount in special single slides. The `Focus Offset' values should (according to the Photography with the AAT manual) be appropriate for use alone with a CCD detector. Values in parentheses should be checked in good seeing.

Filter Holders and Wheels

f/3.3

All filters mount in a slide which is inserted into the camera head a few centimetres under the CCD Adaptor Plate. Filters (eg. the photographic interference filters, the 160mm ANU filters, or the 100mm LDSS filters) can be inserted into these slides singly. Alternatively, there are three wheels which can be installed in a modified filter slide and inserted into the Camera Head.

Three such wheels are available:

1. A six position wheel - holds up to six filters,  either 50mm square or 2.25 inch diameter, and up to 11mm thick. This is the standard wheel used with the standard set of UBVRI broad-band filters.
2. A three position wheel - holds up to three filters, up to 75mm in diameter, and up to 11mm thick.
3. A three position wheel - holds up to three filters, up to 83mm in diameter, and up to 11mm thick.

f/1

The f/1 camera has its own motorised filter 'jukebox' into which 3 filters can be mounted before your run starts. It is a non-trivial excercise to change these during the night, so this should be avoided.

Filter Bandpasses

The following table contains hyperlinks to all the AAO filters for which band-pass information is currently available. The Files pointed to contain one line of header information, and the rest of the file is two ASCII numeric columns - the first is wavelength in nanometres, the second is throughput of the filter in % at that wavelength. The Figures pointed to are each about 7K in size. The Peak Throughputs are provided to give a rough comparison of the efficiency of different filters. The Approximate Bandwidths are FWHM - for asymmetric filters, like the glass R filters, it is only a crude estimator of the actual width, while for long pass filters like the glass I, it is meaningless.  To obtain the `actual' throughput you will get on the telescope for any of the broad filters, the QE curve of the detector must also be factored in.

	Set	Filter	Fig ?	File	Peak Throughput (%)	Approx. Bandwidth (nm)
	KPNO 50mm sq.	U(CuSO4)		aao_cus04.txt	79%	65
		B		kpno_b.txt	85%	115
		V		kpno_v.txt	77%	120
		R		kpno_r.txt	92%	135
		I		kpno_i.txt	85%	175
	50mm sq. glass	B		aao_glass_b.txt	56%	105
		V		aao_glass_v.txt	70%	90
		R		aao_glass_r.txt	73%	115
		I		aao_glass_i.txt	95%	-
		BJ		aao_glass_bj.txt	58%	105
		RF		aao_glass_rf.txt	38%	55
		IN		aao_glass_in.txt	72%	160
		B*		aao_glass_bs.txt	73%	190
		R*		aao_glass_rs.txt	75%	125
		RG695
		RG780
		RG830
		KG3
		O*		aao_glass_os.txt	85%	160
	LDSS 100mm Ø	B
		R
		I
	Gunn g 50mm	g
	f/1 Set 50mm Ø	B
		V
		R
		I
		V+R
	ANU 75mm Ø	B		anu_75_b.txt	59%	95nm
		V		anu_75_v.txt	78%	90nm
		R		anu_75_r.txt	73%	120nm
		I		anu_75_i.txt	88%	-
	ANU 160mm Ø	U
		B
		V
		R
		I
	H alpha Set	660nm
		669nm
		681nm
		690nm
		709nm
		712nm
	Photographic Interference Filters	Halpha 10x10
		[O II]
		H gamma
		He II
		He beta
		[O III]
		He I
		[O I]
		H alpha 5x5
		[S II]
		cont 365nm
		cont 460nm
		cont 540nm
		cont 643nm
	Taurus 3" Ø	469.5nm		taurus.469.2.dat	77%	2.6
		501.7nm		taurus.502.3.dat	95%	4.5
		587nm				3
		630nm				4
		658.3nm		taurus.658.4.dat	93%	4.5
		673.4nm		taurus.673.4.dat	97%	4.5
		872nm				1
	TTF 4.85" Ø Filters are summarised by this figure.
	TTF 4.85" Ø	I0 - 665nm				21
		I1 - 710nm		ttf.707.26.dat	87%	28.5
		I3 - 760nm		ttf.758.28.dat	89%	32.6
		I5 - 815nm		ttf.814.33.dat	76%	37.2
		I6 - 855				38
		I8 - 910nm		ttf.909.40.dat	89%	46.4

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This Page maintained by : Chris Tinney (cgt@aaoepp.aao.gov.au)
This Page last updated:  28 Feb 1996, by Chris Tinney