wpo - review of the Rainbow Optics grating + grating efficiency
My
review of the Rainbow Optics grating is in the June'99 issue of Astronomy
Now v13;#6; p17/18 and is summarized below. Many of my spectra
on this homepage are via the Rainbow grating. Newcomers to stellar spectroscopy
will find the Philip's Colour Star Atlas E2000 by John Cox &
Richard Monkhouse; Geo Philip Ltd - 1991; ISBN 0540 012521; invaluable
- all stars to <mag 6 are coloured according to their spectral class
with much useful data in the text.
Below shows the Barlow-like tube [terrestrial erecting devise] supplied with 70mm Bresser Skylux refr converted to a simple spectroscope by adding a slit and grating.
The images below record dispersion of a red monochromatic laser pointer through four different transmission gratings - the top two disperse most light efficiently into a single bright 1st order spectrum [1] of modest dispersion with sample spectra of M57 - note faintness of zero order and brightness of 2nd order OIII-2 and Ha-2. In the inefficient bottom pair the zero order [Z] is the brightest with little photon energy forming the spectrum which is also overly dispersed and faint - thus exposures will be exceptional long or confined to the few brightest stars with amateur telescopes.
The Fingerprint of Stars......a review of the Rainbow Optics grating by Maurice Gavin
To casual viewing stars look much alike. Apart from differing brightness and a hint of colour in the brighter examples, starlight hides its secrets well. Normally the professional astronomer uses a complex instrument called a spectroscope to extract this information but the Rainbow grating, in its remarkable simplicity of application, opens that door to the amateur as well. The Rainbow grating is a transmission type of negligible thickness and can form a spectrum without special optics. There is a caveat - it only works with points-of-light. In the absence of a slit to isolate a sample of light, extended objects like the planets or the Moon will be hopelessly blurred. The colours will still be there but the fine telltale absorption lines crossing the spectrum will be lost.
At the telescope....
The Rainbow grating simply screws into a standard eyepiece filter thread
for visual use at the telescope’s focal plane. The star’s spectrum will
be a thin line of colour and a small cylindrical lens (part of the kit)
clips over the outside of the eyepiece to convert the spectrum into a rectangular
band. This makes it easier to spot any lines crossing the spectrum.
With patience the spectra of a few dozen stars should be resolved.
For those without a telescope the Rainbow grating can be attached immediately before a camera lens. A fixed tripod and exposures of about 60 seconds will suffice to record the brighter stars and planets. The Rainbow grating really comes into its own when attached to the nosepiece of a film or CCD camera at the telescope’s focal plane. Much fainter stars can be targeted and numerous dark absorption lines, even bright emission lines in suitable candidates, are readily recorded. A drive is not essential - the image may be trailed across the camera focal plane via a static telescope. Driving the telescope to track the stars will permit longer exposures to record spectra of much fainter stars. For CCD users there is the additional advantage that results are seen on the computer screen immediately the exposure is completed. The length of the spectrum depends on its separation from the detector surface which is adjustable via suitable extension tubes.
Technical spec....
By comparing the length of the spectrum (against a grating of known
specification) it is estimated the Rainbow grating has 200 lines per mm
or a total of about 5200 lines across its 26mm effective diameter.
The estimated 75% overall efficiency of light into the main spectrum (to
quote the manual) seems reasonable. [Christian Buil estimates ~66%
into 1st order]. This is a high order of efficiency for a transmission
(non-reflecting) grating. The Rainbow grating would appear
to be a sample from a large holographic grating of excellent quality.
Unlike reflective gratings which are extremely delicate and easily damage
the Rainbow grating, protected between two glass covers, is a robust and
practical devise.
Conclusions....
A 12-page A5 booklet by Jim Badura - Rainbow designer - provides useful
hints and tips on its use. In concentrating almost exclusively on its visual
use it largely underestimates the remarkable power of this devise which
is even capable of a little backyard science for those so inclined.
The Rainbow Optics grating is available from True
Technology Ltd.....
Text and image below posted on CloudyNights Forum [Variable stars - 2005 Nov 23 msg #699925] following a member's request for further info.
"Thanks for the interest;-). I've attached a sampler showing steps to create a star's spectrum profile. Alternatives include AstroArt and Vspec but I mostly use the SX camera controlling software that includes an option to profile intensity in a single row of pixel across the CCD. This is done on-the-fly as the image is downloaded - can't get a quicker spectral fix than that! A few minutes are spent to ensure the spectra are truly horizontal before a session begins - this unnecessary with either AstroArt or Vspec.
The session always begins with an A-type star like Altair that has hydrogen lines of known wavelength - see http://www.astroman.fsnet.co.uk/fraunhofer.gif .
No IR block filters are used - the beauty of CCDs is their natural IR sensitivity [SX CCD's span 3500A in near UV - 8500A in near IR] and record red stars for example that 'disappear' visually but remain bright in IR as the spectrum shift redwards to right. My favourite spectral stars are the exotic Wolf-Rayet with their emission line spectrum. Starters for explanation of spectra via http://www.astroman.fsnet.co.uk/begin.htm" Maurice Gavin [aka Nytecam]
[c]Maurice Gavin 1998-2005