UVEX Spectroscope

The UVEX spectroscope is a new instrument enabling amateur astronomers to study stars in near ultraviolet (for hot active stars) and infrared (for cooler stars and comets).

Designed by Christian Buil and developed by a team of French amateur astronomers, the spectroscope uses mirrors to avoid chromatic aberration which causes colour fringing in simple lenses. The optical design is based on the Czerny-Turner spectroscope shown below. A cylindrical lens is used to reduce the astigmatism inherent in the design.

The spectral range is 3200 to 8600 Angstroms or more, depending on the camera and telescope. Ozone in the Earth's upper atmosphere absorbs light strongly below 3200 Angstroms. The optimal resolution is R=750 (using a 300 lines/mm grating) but R=3600 can be achieved using a 1200 lines/mm grating.

Construction

The construction cost is around £300 for the optical parts (plus entrance slit) with most of the structural parts made by 3D printing. The optical parts are listed below. Plans, downloadable parts and tutorials are available on the project website (http://uvex-spectro.tech).

Part	Description
Grating	ThorLabs GR25-305 (300 lines/mm)
Mirror M1 (fl 150mm)	ThorLabs CM254-074-G01 ?
Mirror M2 (fl 200mm)	ThorLabs CM254-100-G01 ?
Cylindrical lens	ThorLabs LJ1934L1-A

Astronomical targets

The ultraviolet (UV) spectrum is important in the study of Be stars, which are hot stars (temperatures around 12,000 to 30,000 Kelvin) with prominent lines such as hydrogen in emission. The Hydrogen alpha line shows double-peaked emission at high resolution (R~10000) indicating a rotating disk of material around the star. According to a recent paper (Cochetti et al 2020), a significant number of Be stars also show a second Balmer discontinuity which can be investigated using the UVEX spectroscope.

Links

• UVEX project website (French/English)
• Christian Buil's design (French)

A foray into Spectroscopy

In January I acquired a `Star Analyser 100 spectroscope', a diffraction grating which screws into a 1.25-inch eyepiece just like a colour filter. Well, here are my first results captured with an Imaging Source DFK21 camera and processed with Valerie Desnoux's Visual Spec software which is excellent (and free!).



I used Sirius (spectral type A0V) to calibrate the image which has a resolution of 10.5 Angstroms per pixel. The final spectrum (green) is shown between 4000 and 7000 Angstroms (note my response curve has sadly missed the Halpha line at 6563 Angstroms) based on 17 seconds of raw video. The black curve shows a reference red supergiant spectrum of class M2.

Also shown for interest are the Balmer series of H lines (red) and Ti lines (blue).

Links

• Valerie Desnoux's Visual Spec page.
  
• Robin Leadbetter'sspectroscopy links.
  
• More spectroscopy links
  

GLAST telescope in orbit

NASA's latest space-based observatory, the Gamma-Ray Large Area Telescope (GLAST), is now successfully in orbit around Earth at 565 kilometres altitude. Launched on Thursday 11th June 2008 by a Delta II rocket at 17:05pm BST (12:05 EST), it is currently undergoing tests to ensure all the instruments are working correctly.

It carries two main instruments: the Large Area Telescope (LAT) and the Burst Monitor (GBM). The first instrument will be used to reconstruct images at very high energies (photons with up to 300 GeV which are radiated by the most energetic events in the Universe). The second instrument is used to detect short-lived sources such as the Gamma-Ray Bursts of which around 200 will be discovered annully (around the same rate as Swift, which is dedicated to discovering GRBs).

Links

GLAST
Swift

Mega stellar mergers

I recently happened upon a report discussing an unusual binary star system in the Magellanic Clouds, which are prominent in the southern sky. In fact, the Large Magellanic Cloud is brighter than the Orion nebula despite being 100 times more distant, and is a tremendous star-forming region containing many massive stars, novae, and a famous supernova known as SN1987A. The binary, with the unexciting name of LH54-425, is made up two young, hot blue stars with 62 and 37 times the mass of our Sun in a close orbit. In fact, the stars revolve once around each other every 2.25 days and are almost in contact.

Due to the disparity in their masses, co-discoverer George Sonneborn of NASA's Goddard Spaceflight Center believes that in the not-too-distant future (that is, within a few million years!) the two stars may merge together, creating an even more massive star. The result may appear similar to the hyper-luminous eta Carinae (about which more in a later post) which is probably the most luminous star in our own Milky Way galaxy. Astronomers do not know exactly when or how such stars merge, but it probably occurs roughly once per century in the Milky Way for massive stars, and much more frequently in stars of lower mass.

Hot on the heels of this news from the LMC (Large Magellanic Cloud), astronomers at the telescope have been able to weigh a star about 7000 light-years away in the star cluster NGC3603 (see picture). The result: a truly enormous 114 times the mass of the Sun! This star is also a member of a binary system; its partner is a comparative lightweight at 84 solar masses! Whether these stars will eventually merge is anyone's guess; it is quite possible that the giant star will lose so much gas it will never grow large enough to absorb its companion.

Links

Star duo on New Scientist Space
Super stars create new Sun