I have been
interested in Spectroscopy for a very long time, but never thought I would be
able to afford one - just look at the commercial models out there, not much
change out of $2000 for a low resolution model. Well I could always have
designed and built my own, but that is not yet in my skill set so when I
literally stumbled across this kit being offered by Ken Harrison, I thought
about it for about 2 seconds then put down my deposit. This kit is a sub
$900 US project (including postage) aimed at the intermediate level and offers
medium resolution spectra with the standard 600 l/mm grating. Ken has done
tests with gratings up to and including 1800 l/mm and was able to break R=10000
- now that's high resolution spectra (from an amateurs perspective)
Figure 1: Spectra L200 f/200 Littrow design Spectrograph at HHO
(You will notice the top of the Spectrograph is open. A small design issue
meant that this piece needed to be sent back for adjustment)
Targets
So, what was I planning to do with a spectrometer at HHO?
My first thoughts were for asteroid spectra and widespread determination of
asteroid Taxonomy but investigation showed that this is only possible if one has
a setup (even low resolution) that can see well into the IR. This is
beyond the capability of amateur CCD cameras although the L200 itself would be
up to the challenge. Next I thought I might be able to resolve binary
asteroids via radial velocity detection (just like exoplanets and binary stars)
but after doing the math I discovered that the wobble in a binary asteroid
system was typically less than 1m/s - an order of magnitude below what the
professionals can currently do in their exoplanet work. Once the L200 is
completed, calibrated and tested, I will choose it's purpose when I know what
the system is actually capable of.
There are a surprisingly large number of amateurs involved in
spectroscopy, but most are in Europe - specifically France and perhaps it is no
wonder that this is the source for information on the subject. Christian
Buil is the man and his web site holds a plethora of information (http://www.astrosurf.com/buil).
Something else to benefit the amateur is that the software needs to reduce and
calibrate spectra are free. Iris and VSpec (both available as links off
Christians web site)
First Light/Calibration
Here
is the L200 going through its 'first light'. The unit is sitting under
a fluro light bulb (one of those energy saver bulbs). It has a Meade
26mm EP 65mm from the opening (the calculated image focal point) and an
Olympus fe-340 digital camera sitting Afocally on the EP. The Olympus
is just a cheap pocket sized digital camera with 8Mpx CMOS chip. Its
longest integration time is 1/2 second so it's only useful for setup work.
The small black knob on the left side has a partner on the other side and
turns the shutter just above the slit. The shutter has a built in Neon
reference bulb for calibration. The big knob on the front controls the
slit width and the micrometer that controls the grating angle is located at
the bottom right, opposite the spectra exit point and EP.
The next step will be to connect an integrating CCD camera to the L200
and see what it can capture with a very narrow slit.
See details of the L200
Construction and Calibration here.
L200 fitted with an SBIG ST-8 CCD ready for action
