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1 What is Spectroscopy?
It
is the science of recording and analysing electromagnetic radiation
(eg light) in terms of its wavelength (colour).
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2
Why is Spectroscopy so important in Astronomy?
Astronomy
is rather unusual among the sciences in that it is almost entirely
observational rather than experimental. With the rare exception
of a few meteorites and samples of moon rock, just about everything
we know about the universe and the objects in it comes from
analysing the light (and similar electromagnetic radiation such
as radio and X rays) coming from it. Spectroscopy is a key tool
in this process, revealing the physical and chemical processes
which drive the formation, structure and evolution of the components
of our universe.
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3
What is the STAR ANALYSER?
The
STAR ANALYSER is a high efficiency blazed transmission diffraction
grating designed to make recording spectra easy using a telescope
and webcam, video or CCD astro-imager. Like any standard 1 1/4
inch filter, the device screws into the nosepiece of the imager
and allows the recording of the spectrum of any object that
appears star-like in the field of view.
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How does it work?
The
STAR ANALYSER diffraction grating intercepts the light from
the telescope and deflects (disperses) it into a line on the
camera detector according to the colour or wavelength. Longer
(redder) wavelength light is diffracted more than shorter (bluer)
wavelengths.
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What is special about the STAR ANALYSER?
The
STAR ANALYSER has a number of design features which make it
particularly effective and easy to use with popular low cost
webcams, video and astro-imagers.
It
contains a replica diffraction grating made from a high quality
high efficiency blazed master. This means that a much higher
proportion of the light is directed into the spectrum compared
with lower cost unblazed or holographic film gratings for example,
allowing far fainter objects to be recorded.
It
is physically identical to a standard 1 1/4 inch filter. This
means it can be screwed onto a standard camera eyepiece nosepiece
or included in a filter wheel.
The
dispersion has been matched to the small CCD chip sizes found
in these cameras. This means the complete spectrum and the zero
order (straight through) image of the object can be imaged at
the same time, simplifying object identification and spectrum
calibration.
The
delicate grating surface is protected by an antireflective coated
glass cover. This means that it can be cleaned if necessary.
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6
What kind of objects can I record?
The
spectrum of any type of object can be recorded provided it is
bright enough and appears approximately point like in the field
of view. This includes stars of course and planets (provided
a relatively short focal length telescope or camera lens is
used). Compact planetary nebulae also make interesting targets.
The STAR ANALYSER, mounted in front of a wide angle camera lens,
can also record extended objects such as bright comets. If you
are lucky you might even catch a meteor spectrum using this
technique!
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What can the spectrum tell me about an object?
The
broad shape of the spectrum gives information about temperature.
(For example the spectrum of cool Betelgeuse is quite different
to that of hot Sirius) Narrow absorption (dark) or emission
(bright) lines tell us about the chemical elements that are
present and how they are behaving. (For example the spectrum
of Betelgeuse reveals the telltale signature of molecules in
its atmosphere, while stars like Vega show absorption lines
due to hydrogen atoms.) Shifts from the expected wavelength
of lines can give information about the way the object or different
parts of it are moving. The blue shift of hot gas rushing towards
us after a supernova explosion and the redshift of a distant
quasar due to the expansion of the universe are examples of
the kind of processes which it is possible to record using the
STAR ANALYSER
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8 What type of camera can I use?
(See
also "How faint an object can I record")
Because
the spectrum image is so compact, (all the available spectrum
information can contained within a length of just 500 pixels)
just about any electronic camera (without a lens) which can
be fitted to a telescope in place of the eyepiece can be
used. For brighter targets webcams, solar system imagers
such as the Meade LPI or Celestron Neximage etc and video
cameras are ideal. The recent crop of economical long exposure
cameras such as the Meade DSI and those made by ATIK and
SAC Imaging will greatly increase the range of objects which
can be recorded, as will more conventional CCD astro cameras.
Megapixel digital SLR cameras can also be used and the large sensor
size means that the spectra of many stars in the field
can be imaged simultaneously . For precise scientific
work a monochrome camera is easier to calibrate for intensity
as there is no need to correct for the three colour filter
responses. Colour cameras however produce beautiful spectra,
displaying the actual colours of the spectral lines. Note however
that the infra red end of the spectrum will not be recorded
if the infra red blocking filter present in most colour cameras
is left in place.
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9
What type of telescope can I use?
(See
also "How do I focus the spectrum")
Just
about any telescope or even a camera lens can be used provided
the object is bright enough and is reasonably stellar in
appearance. The simple arrangement of placing the grating
in the converging beam of the telescope produces some aberrations.
The advantage of the low dispersion used in the STAR ANALYSER
is that these are kept to a minimum. There are some trade offs
with focal length. Less aberration is introduced by using long
focal lengths but the resulting larger size of the stellar
image will tend to limit the resolution. In practise, the STAR
ANALYSER performs well with the typical focal lengths found
in amateur telescopes, though in poor seeing conditions or
at focal lengths over 2m, a focal reducer will generally improve
the spectrum sharpness by reducing the size of the star
image.
Some
additional care is needed when focusing using an achromatic refractor
as with these instruments not all colours (particularly Violet
and Infra red) focus at the same point. Particularly with a short
focal length achromat, you may find that not all the spectrum
can be brought to focus at the same time and you may need to
concentrate on one segment at a time. (The spectrum will appear "fish tailed" at
the violet and or red ends) Reflectors, including Cassegrain
designs do not show this effect. Apochromats are affected less
and, if corrected for imaging including the Infra-Red region,
may show no chromatic aberration. Indeed the STAR ANALYSER is
an interesting way of checking the chromatic correction of your
telescope!
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10
How faint an object can I record?
Like
normal astronomical imaging, it depends on the size of telescope,
sensitivity of the camera you are using and your sky conditions,
but because the light from the object is spread out across the
camera detector, objects need to be perhaps 5-6 magnitudes brighter
than for a normal image. (This is one of the reasons professional
telescopes tend to be so huge!)
In
practise, the spectrum of the brighter planets and hundreds
of stars down to mag +4 can be recorded using a modest 8 inch
(200mm) scope and a sensitive webcam or planetary imager such
as the Meade LPI or Celestron Neximage. Given good sky conditions,
and using the same aperture telescope with a sensitive monochrome
CCD imager such as the ATIK 2HS, SAC8, DSI Pro, Starlight Express
MX5, SC3 modified webcam etc, recording the spectra of objects
down to mag +13 is possible, allowing the measurement of quasar
redshifts and the classification of bright supernovae to be
performed! Integrating Video camera such as the Mintron 12EV,
Stellacam etc and long exposure colour imagers such as the Meade
DSI, SAC7 ATIK 1/2C, modified colour web cams etc will reach
intermediate magnitudes, to record bright comets or the fascinating
supernova candidate Wolf Rayet stars for example.
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What wavelength range can I record?
The
range depends on the response of the camera sensor and whether
an infra red blocking filter is fitted. An unfiltered CCD sensor
will typically record from less than 400nm in the violet to
beyond 800nm in the infra red (Even colour sensors with any
infra red filter removed will record into the near infra red,
as the colour filters built into the chip are transparent to
infra red)
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What is the resolution of spectra produced by the STAR ANALYSER?
At
first sight one might think that by using a grating with more
closely spaced lines or mounting it further away, one could
increase the resolution as much as one liked. In practise though,
there are several other factors which limit the resolution for
this type of configuration. The most significant ones are.
The
size of the star image, which depends on the stability of the
atmosphere and increases with the focal length of the telescope.
Distortion
of the image due to chromatic coma, which is independent of
the grating or mounting distance but is worse for lower focal
ratio telescopes.
Field
curvature which changes the focus along the spectrum and is
worse for higher dispersion gratings mounted close to the camera
sensor.
The
size of the camera pixels.
Because
of these limitations, the resolution of this type of spectrograph
is restricted to typically 1/50 to 1/100 of the wavelength (eg
5-10nm at 500nm), independent of the diffraction grating design.
It does mean however that by choosing the grating design and
mounting distance with care, the whole spectrum and the undeflected
star image can be fitted on the chip without losing any of the
available resolution, which makes the spectrograph more sensitive
and easier to use.
(Spectrographs
used by professional astronomers achieve greater resolution
by adding a slit and collimating optics, but these are much
more complex and cost many tens of times more)
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13
How far from the camera should the STAR ANALYSER be mounted?
(See
also "What are the optional spacers
for and will I need any?"
The
further away from the camera sensor the STAR ANALYSER is placed,
the more spread out the spectrum becomes. If you mount it too
close, you will lose resolution. Too far away and you will not
be able to fit the whole spectrum in the camera frame. In practise
the minimum recommended distance in mm is 4x the pixel size
in microns (eg for a ToUcam webcam 4x5.6=22.4mm) and the maximum
while still fitting the spectrum (diagonally across the chip)
up to 700nm is 12x the sensor diagonal size in mm ie
55mm
for 1/4 size sensors like the ToUcam, Neximage, ATK1 etc
75mm
for 1/3 size sensors like the ATK2, LPI, DSI, SAC8 etc
100mm
for 1/2 size sensors like the Minton 12V1 EX etc
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What are the optional spacers for and will I need any?
The
STAR ANALYSER has been designed to work with the smallest commonly
used sensor chips and the range of camera nosepiece designs
commonly encountered. Some cameras have larger chips and/ or
shorter nosepieces and so would benefit from an increase in
distance of the device from the sensor. The minimum recommended
distance (in mm from the sensor to the tip of the nosepiece
where the STAR ANALYSER screws in) is 4x the camera pixel size
in microns. For example, for the ToUcam webcam, the distance
would be 5.6x4=22.4mm. If you find that the distance for your
setup is less than the minimum value, we recommend purchasing
sufficient spacers to bring the distance above the minimum.
Each spacer adds an adjustable 7-10mm. To date, the following
cameras are known to benefit from the use of spacers.
Meade
DSI 1 spacer (Note the DSI pro does not require a spacer)
Mead
LPI 2 spacers
Celestron
Neximage 1 spacer
If
you are not sure if your particular setup requires spacers,
please e-mail us with details of your camera and the sensor
to nosepiece tip distance and we can advise you.
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15
How do I focus the spectrum image?
Just
as in conventional astronomical imaging, focussing can be tricky.
Focussing the zero order image of the star will get you somewhere
near, but you might find you need to wind the focus in a touch
more to achieve the best focus of the spectrum. If any narrow
absorption or emission lines can be seen, these can be used
to sharpen the focus. If there are no obvious features then
focussing to narrow the width of the line can get you nearer.
Once you have good focus, it can be useful to note for future
reference how much the focuser had to be moved compared with
the zero order star image focus point. (Note that it may not
be possible to obtain good focus at all wavelengths simultaneously
when using an achromatic refractor due to the telescope design)
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16 How can I turn my narrow spectrum image into a wide strip?
Many
image processing programs have all you need.
First
rotate your spectrum image so it is horizontal and crop the
image so it just shows the strip with the spectrum (it will
typically only be a few pixels high)
Resize
the strip to the same width but only one pixel high, then resize
again to the same width but 30 pixels high and voila! A professional
looking strip spectrum which will make the spectral lines much
easier to see.
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17
How can I calibrate my spectrum?
The
STAR ANALYSER has been designed to make calibration as easy
as possible. With most configurations you will be able to capture
the image of the star as well as the spectrum in the same frame.
The star image is your zero point. Because the dispersion of
the STAR ANALYSER is low, the wavelength is essentially proportional
to the distance along the spectrum. (Note that this is not true
for higher dispersion designs used in the same configuration
and is a particular problem for prism based spectrometers) All
you have to do is measure the number of pixels from the centre
of the star image to a feature of known wavelength in the image
(eg a Hydrogen Balmer line in a star such as Vega or a Telluric
line due to the earth's atmosphere, which can be seen in many
spectra). If you divide the wavelength by the number of pixels,
you have a calibration constant (in angstroms per pixel or nm
per pixel) which can be used for all your measurements provided
you do not change your setup.
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How can I convert my spectrum into a graph?
There
are commercial and freeware programs which can take the image
of a spectrum, measure the pixel values along the line and convert
them into a graph or a data file for use in a spreadsheet. For
example the freeware program Visual Spec http://astrosurf.com/vdesnoux/
is particularly powerful with several advanced functions to
aid calibration and analysis of your spectrum. The popular commercial
astronomical image processing program AIP4WIN http://www.willbell.com/AIP/Index.htm
also has features for spectroscopy
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Can I use the STAR ANALYSER for visual observations?
Yes,
though the length of the spectrum will be rather short unless
you can mount the STAR ANALYSER some distance before the eyepiece.
This can be achieved for example by screwing it into the ingoing
side of a diagonal threaded for filters. (Note that achieving
sufficient distance may be a problem with some telescopes, particularly
Newtonians where inward focus travel tends to be limited) A
cylindrical lens fixed over the eyepiece can be useful to spread
the width of the spectrum and make the lines more visible. Tapping
the eyepiece can produce a similar effect.
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Where can I share my results with other users or get advice?
You
can get support by
e-mailing us or alternatively, why not join the Yahoo group staranalyser
where you can get support, meet other users and share and discuss
your results?
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