Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mso.anu.edu.au/~bessell/Filters/Filters.ps Äàòà èçìåíåíèÿ: Thu Jan 22 02:20:30 2004 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 02:54:24 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: arp 220

Filters
Filters are used in astronomy for two main pur­
poses, to either lower the overall intensity of
the light or to restrict the wavelength range of
the light that is being measured. When the
eye was the only astronomical detector, filters
were scarcely used, but astronomy has always
adopted the new technologies of the day, be
it photography, photomultiplier tubes, image
tubes, CCDs or IR arrays. As the new detectors
invariably have a wider wavelength response
and/or improved sensitivity than previous de­
tectors, filters became very important in order
to match previously used bandpasses and to
sample the flux of astronomical sources at dif­
ferent wavelengths thus providing some astro­
physical information on the source's tempera­
ture and other properties. Some filters work by
selective absorption and transmittance: these
are the colored glass filters. Others work by se­
lective reflection and transmittance: these are
usually referred to as interference filters.
An interference filter comprises a multi­
layer of thin films that selectively transmits or
reflects light of particular wavelengths. They
fall into two broad types, bandpass filters
(usually narrow bands) and edge filters, that
transmit above or below a certain wavelength;
dichroic beamsplitters are edge filters. The
Fabry­Perot tunable filter is a unique interfer­
ence filter that can be used to spectacular ef­
fect on emission­line objects. Finally, there are
filters used for measuring polarized light and
studying astronomical magnetic phenomena.
Colored glass filters
The first filters used in astronomy were colored
glass (Schott or Corning) or colored gelatin
(Kodak). These were used with photographic
emulsions to define B, V, R, and I bands. Col­
ored glasses had been made for thousand of
years for expensive glass vessels and ornaments.
Nowadays, colored glasses are produced in one
of two ways, either by ionic coloration or by
absorption and scattering from a suspension of
colloidal particles that are produced in the glass
and controlled in size by heat treatment after
an essentially colorless glass is made. In Fig. 1
are shown some sample transmittance curves of
these glass filters.
The ionic glasses are made by dissolving
particular salts (such as cobalt or nickel oxide)
in glass. The ionic coloration of the UG (vio­
let) and BG (blue) series of glasses produces a
spectral transmittance curve resembling a bell­
curve of half­width between 100 and 200 nm
but most of these glasses also transmit red light
beyond 700 nm as well, thus the violet glasses
appear purple to the eye.
The second type of colored filter glass are a
series of sharp edge (or short wave cut­off) fil­
ters, the WG, GG (sulphur and cadmium sul­
fide), OG (cadmium selenide) and RG (gold)
series which absorb light blueward of a quite
sharply defined wavelength. The filters are
made with their short wave cutoffs ranging from
400 to 800 nm in steps of 20 nm. To the eye,
the WG glasses are essentially colorless, the GG
series go from colorless through light yellow to
dark yellow, the OG are orange­red and the RG
series are rose to ruby in color.
Schott also make two special (nearly) color­
less glasses (BG39/40 and the KG series) that
can be used as long wave cutoff filters; however,
these glasses have long wave cutoffs that fall
more slowly (over about 250 nm) than the short
wave cutoffs rise (over about 50 nm) thus pro­
ducing an asymmetrical spectral transmittance
curve when used together to define a band­
pass. Where a steeper cutoff is desirable, it
is necessary to use an edge interference coating
(see below) in combination with a Schott short
wave cutoff filter. An important use for the
BG39/40 and S8612 glasses is also as a ''red­
leak'' blocker, absorbing light beyond 700 nm
where most of the Schott blue and violet glasses
transmit. Polished copper sulfate crystals or a
copper sulfate solution have also been used in
astronomy as a UV transmitting blue filter to
block red leaks, but the improved UV trans­
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mission of S8612 glass, although not as high as
that of copper sulfate, has enabled it to replace
copper sulfate for most imaging applications.
Combination filters and bandpasses
The ionic colored glasses define broad bell­
shaped bandpasses in the violet, blue or green
regions; however, the choice is restricted. Bet­
ter definition has traditionally been obtained by
combining one of those filters with one of the
long pass glass filters, or by using a long pass
glass filter and the red sensitivity cutoff of the
detector, for instance the particular sensitiza­
tion of a photographic emulsion (O, G, D, E, J,
F, N) or the fall­off in the sensitivity of a pho­
tocathode (S5, S11, S20, GaAs). But with the
use of wide wavelength response CCD detectors
and the restriction of photographic emulsions
to the broad band Tek Pan film, it is now nec­
essary to have filter defined bandpasses. This
is also true in the IR and the UV where in
the past, the atmospheric windows were often
partly used to define the edges of the bandpass
resulting in the bandpass differing between sea­
sons and with observing altitude. The standard
U, B, V & R bandpasses can be defined using
only glass filters but better R and I bandpasses
can be made using interference edge filters for
the red cutoffs. The bandpass filters for the
Sloan Digital Survey have been defined in this
fashion, the short wave cutoff coming from the
Schott color filter and the long wave cutoff from
an interference edge filter. It is necessary to use
BG39 with other BG glasses and S8612 with
UG glasses to eliminate the red leak of those
glasses.
Combination glass filters have been used
successfully with CCD systems to match the
standard UBVRI system (see Magnitude Scales
and Standard Systems). The thicknesses of the
glasses can be adjusted to tune the passband
for better results with detectors having differ­
ent wavelength responses.
Practical consideration
From a practical viewpoint, many of the BG
and all the UG glasses are difficult to use. They
are hard to polish, tarnish in air easily and
the high concentration of ions in the available
glasses often means that they are brittle and the
required blue or violet color is produced with
only a thin piece (1mm or less) of the glass.
This is unfortunate, as it means that it is diffi­
cult to produce good optical quality glass sand­
wiches in a large enough size to cover the focal
plane of the large Schmidt telescopes where it
is desirable to have such a blue and ultravio­
let filter for use with the new Tek Pan film.
However, up to 25 cm square filters suitable for
CCD arrays can readily be made. Large glass
pieces are usually unavailable from stock and if
ordered have to await a new melt. Some glasses,
eg. UG2 and BG37 are no longer made.
Neutral density filters
Although astronomical objects are generally
very faint, sometimes it is necessary to atten­
uate light. The NG series of Schott filters
provides a large range of attenuation and are
especially useful when it is better to absorb
the light rather than reflect it away. However,
the attenuation is not especially constant with
wavelength and the glasses absorb shortward
of 400nm. For astronomical objects therefore
it is usually preferable to use filters made from
fused silica that have been coated with a metal­
lic alloy. This provides excellent attenuation,
that although not identical for all wavelengths,
is usually close enough to be easily calibrated.
These coated fused silica filters can be used
from the UV to 2 microns. For IR wavelengths
germanium substrates are used. These filters
are also made with a wedge or radial gradient
of attenuation that is useful in non­quantitative
situations.
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Interference Filters
Thin film coating technology has matured
and excellent interference filters are available
from many manufacturers using computer de­
signed coatings and computer controlled coat­
ing plants.
Bandpass filters
Bandpass interference filters are made by coat­
ing a glass or fused silica substrate with alter­
nating layers of –=4 reflective and –=2 trans­
mitting dielectric materials to make a single or
multiple Fabry Perot interferometer. The spac­
ings between the layers is designed to enhance
the transmittance at the wavelength of inter­
est. By varying the reflectivities, the number
of layers and the number of Fabry­Perot cavi­
ties the shape and width of the bandpass can
be altered.
The more cavities, the squarer the bandpass
is. Square bandpasses are desirable for emission
line photometry so that the response is uniform
across the field (changing angle) and for a range
of radial velocities. But for some astronomi­
cal purposes, such as broad band continuum
photometry, square bandpasses are not desir­
able as they produce too great a response dif­
ference as a particular spectral feature moves
in and out of the band due to radial velocity
differences or chemical and physical differences
between objects. Similarly, square bandpasses
can also result in big differences when a filter
is replaced by another whose bandpass is not
precisely the same. For that reason, combina­
tion glass filters, or glass and interference edge
filters have been preferred for continuum pho­
tometry. However, rapid advances in coating
technology, including rugate filter technology,
now enable arbitrary shaped bandpasses with
high throughput to be made. It is likely that
interference filters in the future will increasingly
replace most colored glass filters.
Interference filters are produced as a glass
sandwich, usually between two colored block­
ing glasses. Because of the hygroscopic nature
of the coatings the edges are sealed with epoxy
and encased in an aluminum cylinder. The fil­
ters are normally between 5 and 10mm thick
depending on the diameter and the manufac­
turer.
Shifts in bandpass
The bandpass of an interference filter shifts
blueward as the incident angle deviates from
the normal. This can be useful as it enables a
blueward tuning of the filter to be done. Typ­
ically a 5 degree tilt shifts the central wave­
length of a 656 nm filter by about 0.7 nm while
a 10 degree tilt shifts it by 2.7 nm. As most
imaging is done with a converging beam the
bandpass will broaden and shift blueward de­
pending on the f/ratio of the telescope and posi­
tion in the field. It is necessary to take this into
account when designing a narrow band imaging
filters.
Temperature will also shift the bandpass as
the distance between the layers changes. Typ­
ical optical interference filters at room temper­
atures show shifts of about 0.003% of the peak
wavelength per C. This is about 10 times less
than a typical glass edge filter, such as GG395
or OG590.
Edge filters
Unlike bandpass filters, edge filters do not con­
tain cavities. Generally they consist of quarter
wave and modified quarter wave layers that pro­
duce an 'edge' in wavelength space as the filter
changes from reflection to transmission. Fig. 2
shows examples of typical short pass and long
pass filters. Short pass edge filters are often
used with long pass glass filters to define as­
tronomical photometric bandpasses. Dichroic
mirrors are edge filters used at an angle of
45 degrees. In optical spectroscopy they are
normally used to reflect light blueward of some
limit into one spectrograph and transmit the
remainder into another. They are often also
used to reflect IR light to an IR instrument and
transmit the optical light to a guider.
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Fabry­Perot tunable filters
A FP interferometer has been introduced
at two observatories that has revolutionized
the way in which high efficiency interme­
diate to narrowband imaging can be car­
ried out. Advances in coating techniques,
precise spacing control of the interferome­
ter plates and an innovative charge shuffling
technique on the CCD detector have made
this possible. More details can be found at
(http://www.aao.gov.au/local/www/jbh/ttf/).
The filter can be tuned between 370 and 1000
nm and the resolution chosen between 100 and
1000. At low resolutions (below 300), normal
broad band BVRI filters can be used to block
sidebands. At high resolution (1000) a set of
intermediate interference filters are needed for
sideband suppression. Unprecedented observa­
tions have been made with this new instrument.
Rugate filters
A rugate filter is an interference coating in
which the refractive index of the coating varies
continuously through its thickness. This pro­
vides greatly enhanced flexibility in filter design
and extremely high levels of rejection. Multi­
ple passbands can be produced, for instance to
transmit two emission lines of interest, or to re­
ject certain wavelengths, such as the strongest
night sky emission lines or intense laser lines for
safety goggles.
Anti­reflection coatings
Typical glass surfaces reflect about 4% of the
normally incident visible and near IR light.
Silicon and germanium reflect over 60%. To
avoid light loss and problems associated with
reflected stray light from bright stars and the
sky, anti­reflection coatings are usually applied
to all optics including the filters. Multilayer
coatings can be designed to produce extremely
low reflectivities over a wide wavelength range
but for filters, a single layer coating designed
for the central wavelength of the filter is usu­
ally adequate. The most effective single layer
is a –/4 thickness of material whose refractive
index is the square root of that of the glass.
Bibliography
Information on glass filters can be obtained
from Schott Filter Glasses (Schott: Jena
Glaswerk Schott & Gen., Mainz); on opti­
cal interference filters from the Oriel Corpo­
ration (http://www.oriel.com), Melles Griot
(http://www.mellesgriot.com) and Barr Asso­
ciates (http://barrassociates.com) also for ru­
gate filters; for infrared filters (http://barr­
associates­uk.com).
Figure captions
Fig. 1. The transmittances of a sample of
colored glass filters. The thick curves are for
the ionic colored glasses UG1 (dots) and BG12.
The medium thick curves with symbols are
S8612 and KG3 used as red cutoff glasses. The
thin curves are three of the family of shortwave
cutoff glasses GG495, OG570 and RG9. msb­
filter1.eps
Fig 2. The transmittances of typical multi­
layer (interference) edge filters. The thin curve
is for short pass filter used for defining the
red edge of a passband or as a heat reflecting
dichroic filter. The thicker line is for a long pass
filter that could be used as a dichroic beamsplit­
ter in a spectrograph, reflecting blue light and
transmitting red light. msb­filter2.eps
MICHAEL BESSELL
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