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| Two 3d printed grating holders with iron dust doped epoxy resin. (The Seestar S30 uses magnetism to affix external filters). - Credit: Jolene McSquint-Fleming. |
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| The SolidCad generated grating holder model for 3d printing. - Credit: Jolene McSquint-Fleming |
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| The 50 lines/mm transmission grating (sourced 'off the shelf' from China via the Internet for less than £10) sitting on a laptop next to a 3d printed grating. - Credit: Jolene McSquint-Fleming |
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| Ken Harrison's Excel Spreadsheet (SimSpec V4_4a) - Credit K.M. Harrison |
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| The completed Seestar S30 compatible objective lens mounted 50 lines/mm. transmission grating glued to the 3d printed iron dust doped holder awaiting attachment (upper face down) and magnetically to the Seestar smart telescope. Credit: Jolene McSquint-Fleming (distantly related by marriage to Bob Fleming). |
"Our ever resourceful engineer at the JPO, Jolene Mc Squint-Fleming, has been beavering away in the JPO's 'Clean-room' to make a Mark 4 prototype objective lens mounted slitless low resolution transmission grating spectrometer for the Observatory's Seestar S30 smart telescope. Previous prototypes have been partially successful in that they delivered stellar spectra using an 80 lines/mm grating. Unfortunately the star (zero order) and its first order spectrum could not be contained within the longest side of the Seestar's deepsky imaging sensor. This is not essential but it does aid spectrum calibration by having a zero point together with the first order spectrum, blue through red, in one field of vision. The JPO team recommend, to anyone wishing to develop an interest in stellar spectroscopy, Ken Harrison's excellent book 'Astronomical Spectroscopy for Amateurs' (The Patrick Moore Practical Astronomy Series) Paperback – 9 Feb. 2011
by Ken M. Harrison (Author).
The next to last of the above images shows Mr Harrison's Excel spreadsheet which given the application inputs of a telescope, camera and diffraction grating calculates a number of useful spectral related features and confirms whether or not the zero to red first order spectrum will fit on the camera sensor.
The night sky may be appreciated in many ways. For some, just looking up at the beauty of the stars and planets is enough but for others finding out how the Universe works is a strong driving force.
In the distant past, astronomers and scientists believed that all humanity could ever know about stars were their position in the sky, their colour and their apparent and relative brightness. It is no exaggeration to say that the science and practice of spectroscopy changed this severely limited expectation and has provided not only comprehensive information about galaxies, stars, and other cosmic matter but also about the space between them. It might be said that 'Spectroscopy' provided both the foundation and prime tool for the development of the science of 'Astrophysics'.
The early days of 'Spectroscopy'
The “fathers” of astronomical spectroscopy are generally considered to be:
Joseph von Fraunhofer (1787–1826)
He built some of the best early spectroscopes and discovered the dark absorption lines in the solar spectrum (now called Fraunhofer lines) around 1814.
His work laid the foundation for using spectra to study the Sun and stars.
Gustav Kirchhoff (1824–1887) & Robert Bunsen (1811–1899)
In the 1850s–1860s, they showed that each chemical element produces a unique spectral “fingerprint” of emission and absorption lines.
They were the first to identify chemical elements in the Sun and stars using spectroscopy, proving that the stars are made of the same elements found on Earth.
Together, Fraunhofer, Kirchhoff, and Bunsen are usually credited as the founders of astronomical spectroscopy.
So what are the Jodrell Plank Observatory goals for the use of spectroscopy and what tools can we use to achieve them?
Kurt and his team have already designed and made a 500 lines/mm spectrometer and have captured and processed stellar spectra for a number of stars using the JPO 127mm Meade refractor. The results have been quite encouraging but capturing and processing data using this telescope-spectrometer is very resource and time hungry. As a consequence the number of stars for which spectra have been obtained from the JPO is quite small.
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| Comparative Spectral Profiles for the Stars Altair, Alshain and Tarazed captured with the 500 lines/mm spectrometer attached to the JPO's 127mm Meade refractor. (three we did earlier) |
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| 500 lines/mm. Spectrometer utilising the QHY5-11 mono camera. Credit: J.McS-F and Kurt Thrust. |
The team would like to do more spectroscopy combined with standard imaging using kit that can be quickly and easily deployed. If the Seestar S30 can be converted for spectroscopy by the addition of an external objective mounted transmission grating, quite wide star-fields might be first imaged and then have selected star spectra captured. Hopefully, if we can get this bit of kit to work we will be able to work faster and smarter.
A Seestar objective lens spectrometer using a 50 lines/mm set up will provide lower resolution spectra than the 500 lines/mm spectrometer attached to the eyepiece end of 127mm refractor but working smarter will result in the spectral investigation of more stars!
Both spectrometer set ups, however, work at 'low resolution'. Much more expensive equipment is required to undertake 'high resolution' spectral work which puts such activity outside JPO budgetary constraints.
What, in general terms, is the difference between the spectrum of a star and a spectral profile?
A stellar spectrum is the diffracted light from a star displaced and spread out by wavelength (from blue to red) in a number of bands (orders) which decrease in brightness as the orders increase (displacement from the star increase). The spectrum is the raw image captured by the camera and visible to the eye. You will notice that spectra are created on either side of a star by the grating. More expensive gratings direct more light to one side than the other. These gratings are referred to as being 'blazed'. It is usual to work with the right hand first order spectrum when creating the spectral profile.
The spectral profile is a graph of the intensity of the spectrum's light (photon counts by the camera sensor) (plotted on the Y axis) plotted against wavelength ( x axis). The profile is derived from the spectrum which requires careful calibration and processing. At the JPO we have traditionally used bright stars such as Vega in the constellation Libra, which have strong Hydrogen absorption lines, for calibration of target stars under investigation. For processing the spectral profile we use the excellent software BASS Project 1.9.9.
The useful information relating to a star is derived from the spectral profile. A profile may be best obtained from a black and white spectrum and for this reason our 500 lines/mm. spectrometer uses a mono QHY5-11 camera. The Seestar S30 by default captures colour datas.
What "low resolution" in stellar spectroscopy means
A low-resolution spectrum is one where fine details in the spectrum are blurred together. Instead of seeing very sharp, narrow absorption lines, you mostly see broad dips and bumps in the rainbow of starlight. So, you don’t get fine chemical fingerprints, but you do still capture the overall shape and large spectral features.
What we can learn from low resolution spectral profiles:
- Stellar temperature (the star’s “color” in detail)
The overall slope of the spectrum tells us how hot the star’s surface is.
Hot stars (like O and B types) peak in the blue/ultraviolet, while cool stars (K and M types) peak in the red/infrared.
Even at low resolution, the shape of the spectrum across visible light gives a good estimate of the star’s effective temperature.
- Spectral type and classification
Big molecular bands (e.g., TiO in cool M-type stars) or hydrogen Balmer lines (prominent in A-type stars) are broad enough to show up even in low resolution.
That lets astronomers assign a star to a spectral class (O, B, A, F, G, K, M).
- Surface gravity (giants vs dwarfs)
The strength and width of certain broad features are sensitive to whether a star has high gravity (main sequence dwarf) or low gravity (giant or supergiant).
For instance, pressure broadening is stronger in dwarfs, so hydrogen or calcium features look different.
- Metallicity in broad strokes
While you can’t measure detailed abundances, the overall strength of metal absorption blends can suggest whether a star is “metal-poor” (like in old halo stars) or “metal-rich” (like stars in the disk).
Extremely metal-poor stars stand out because their spectra look much smoother.
- Reddening and interstellar dust
Comparing the expected spectral shape (from the star’s type) to the observed shape tells how much starlight has been reddened by dust.
This helps estimate extinction and distance effects.
- Radial velocity (roughly, if the resolution is not too low)
At very low resolution, Doppler shifts are hard to measure precisely.
But with moderate low-res data (say R ~ 1000), you can still detect bulk shifts of broad features to see if a star is moving toward or away from us at tens of km/s accuracy.
- Special features or stellar activity
Strong emission lines (like H-alpha) show up even at low resolution.
This can reveal things like chromospheric activity, accretion (in young stars), or stellar winds.
What you cannot do with low resolution spectral profiles:
You can’t measure fine chemical abundances (like “this star has 0.2 times the Sun’s iron content”).
You can’t detect narrow line profiles that reveal rotation, turbulence, or precise radial velocities.
You can’t see isotopic ratios or subtle line blends.
Why low resolution spectroscopy is still powerful:
Even though it blurs the details, low-resolution spectroscopy is fast and efficient. Large surveys (like SDSS or LAMOST) have gathered millions of low-res spectra, which let us classify stars, map stellar populations across the Milky Way, and study galactic evolution.
So, in summary:
Low-resolution spectroscopy can reliably tell you a star’s temperature, general spectral type, luminosity class, approximate metallicity, effects of dust, and presence of strong emission features. It gives a broad but still very physical picture of the star, even if the fine chemical “fingerprints” are lost.
We are now already to test the prototype and await a clear night to undertake first light for this home-made bit of kit". - Karl Segin outreach coordinator at the Jodrell Plank Observatory.
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