AstroNotes 2017 March Vol: 56 issue 03

Member News . A Simple Objective Prism Spectrograph . AstroNotes Bookshelf . Book Review . Next Meeting . Centre Information

Pages: 

6

Download PDF version: 

AstroNotes

The Newsletter of the Ottawa Centre, RASC

Volume 56 - No 3 - March 2017

Member News

Congratulations! The National Awards committee has recommended three Ottawa Centre
members for the National Service Award in 2017: Chuck O’Dale, Mike Moghadam and Simon
Hanmer. Read more about the Service award and their individual citations here. These awards
will be presented at the GA banquet this summer.

Date change: The Ottawa Field Naturalists will present Howard Simkover speaking on Meteors
and Meteorites at their monthly meeting on March 28 at 7:30 pm. Location: Salon B, K.W.
Neatby Building, Central Experimental Farm, 960 Carling Avenue.

A hearty welcome to last month’s new members: William Harrison, Sheng-Wei Lin, Patrick
Sabourin, Martin Viala, and M. M. Vankalapati. We look forward to seeing you at our meetings
and answering any questions you might have.

It’s not too late to sign up for Janet Tulloch and Gordon Webster’s Art and Astronomy weekend
workshop at the Ottawa School of Art’s Orleans campus March 3 – 5. The workshop includes a
field trip to Petrie Island, weather permitting.

Fibre art by Janet Tulloch, recently displayed at the Mississippi Valley Textile MuseumAstroNotes


A Simple Objective Prism Spectrograph

By John Thompson

What is described here is a simple prism spectrograph that attaches to the front of a camera lens, in
my case a Canon Xt DSLR, Baader modified for extended red sensitivity. It was made from plans from
an astronomy magazine, probably Sky and Telescope, but possibly Modern Astronomy, one which I
subscribed to in the 1970's.

It is a relatively simple, cheap project which can be made by anyone with modest crafting skills. It
consists of two 45-45-90 degree prisms, a pair of which is found in each side of a prismatic binocular to
turn the image upright without reverting it. They are placed with long sides facing each other at a 60
degree angle, one corner of each touching at the top (see diagram). I held the assembly together with
corrugated cardboard and epoxy glue, and glued a metal bracket to the bottom to support it, bent to
place the final device's optical axis in line with the optical axis of the camera lens. I also glued a 1”
piece of tubing to the last surface, facing the camera lens, to keep out stray light so only light from the
prism assembly will reach the camera lens. I use a standard 50mm lens, but someone more
adventurous could try this with a longer focal length lens, which would in principle give more detail in
the resulting spectra.

Camera
Starlight
Binocular Prisms

Spectrographs come in many types – prism or grating (or a combination called a “grism”), high or low
resolution, direct-vision or deviation. For prisms, they may be single or multi-prism, with different
kinds of prisms, and for gratings, they may be transmission or reflection and possibly “blazed”, which
increases their efficiency. They also vary by where the device is placed – in front of the telescope or
camera objective, or near its focus.

This device is a low resolution objective prism spectrograph. It has the disadvantage of producing non-
linear spectra, as do all prisms, making it difficult to calibrate if one wanted to measure wavelengths of
features, but has the advantage of being highly efficient with its use of light compared to gratings,
which produce multiple spectra with much of the light not being diffracted at all (the so-called zero-
order spectrum). Assuming a 4% loss at each glass surface due to reflections, 84% of the incoming light
goes into each stellar spectrum. Coated prisms would improve the efficiency to more that 90%. This
device bends the light by about 45 degrees, making it tricky to aim, so I drew marks on the housing as
an aid. First magnitude stars can be seen through the camera's viewfinder, even after being dispersed
into a spectrum.

Sourcing the prisms

The prisms must be from a porro prism prismatic binocular. You can identify these by noting the lens
barrels are not in line with the eyepieces. If they are, it is either a Galilean binocular, like an opera
glass, or it uses a different kind of prism system not suitable for this project. If you can't find cheap or
inoperable binoculars to disassemble, Surplus Shed in Pennsylvania is a good source – their prisms cost
as little as $2-3 each. They also supply 60-60-60 equilateral prisms, one of which would probably do as
well as two binocular prisms. Those made of flint or dense flint glass will give the best dispersion.

Using the spectrograph

It is first necessary to get a good focus on the stars without the spectrograph in place. A manual lens
should have an infinity mark – the focus should be at or very close to this. Automatic lenses, which are
more common, will not have this mark, but can be focused in manual mode on a bright star using the
viewfinder or the LCD screen. When the correct focus has been found, a piece of tape can be placed
on the lens barrel to maintain the focus. The spectrograph can then be attached to the camera. Mine
is constructed so the dispersion is vertical, so the device must be aimed so that the sky motion moves
the stars in a horizontal direction to widen the spectrum from a line. This is the motion of stars on or
near the meridian facing south. Stars in the east and west move at an angle to the horizon, so if the
camera is not tilted accordingly, the spectra will have diagonal features. Getting good spectra in the
north circumpolar region is tricky but can be done.

You will have to experiment to get the best exposures for different brightnesses of stars, which
depends on the ISO rating, sky darkness, lens aperture and length of exposure. For my sky (4 th
magnitude or better), I typically use ISO 100 or 200, f/3.5 and 15 seconds to get spectra of 1 st to 3 rd
magnitude stars.

Results

Here are two spectra obtained of winter sky objects. More spectra and a discussion of their meaning
and interpretation will follow in next month's AstroNotes.

M42, showing Hα, Hβ and O-III emission. Note the difference between a diffuse object’s spectrum vs. a
stellar, or point source

Sirius, showing Hα, Hβ and Hγ Balmer absorption lines and possibly other faint lines in the red.


AstroNotes Bookshelf

Edited by Janet Tulloch

Each month our librarian, Estelle Rother, chooses one book from our library of about 800 books to feature. The library is located in a cabinet behind the Aviation Museum theatre and is open immediately after meetings. You can also consult the Centre’s website for most of our titles.

Estelle’s Pick for February
Visual Astronomy of the Deep Sky
By Roger N. Clark (Cambridge University Press, 1991)


Book Review

By Pat Brewer

Black Hole Blues and other songs from outer space
by Janna Levin (Alfred A. Knopf, 2016)

his book tells the story of the development of the Laser Interferometer Gravitational-Wave Observatory, or LIGO.
LIGO in its newly advanced form has been in the astronomy news recently with the detection by its two massive observatories of gravity waves resulting from two black hole collision events. The story of the journey from the early experiments used to develop the technology, through to the building of the two 4 km-long “L”-shaped observatories, covers many personalities and many years of development.

At times through the book it seems that original  architects, Rai Weiss, Kip Thorne, and Ron Drever might never succeed in their project due to their own conflicts and the challenges of getting such a large and expensive government-sponsored project off the ground. Author Levin spends a great deal of time describing their personalities and conflicts, as well as those of other key people working on the project. In some ways the book is a psychological
examination of their attempts to work together, or at least towards the one goal. Her research is based on extensive interviews both by her and from the Cal Tech Archives.

As we now know, the project was completed, and the observatory has already proven successful. The epilogue to the book includes early details of the first discovery in the late summer of 2015. The
astronomy press has suggested that the success of the observatory could lead to a Nobel Prize for the three founders. This book moves along well under author Janna Levin and makes an interesting story
out of the development of the two massive observatories. The only shortcoming in the book is the lack of any photographs or diagrams, both of which would have enhanced the read. The book is currently
available in book stores and online. A paperback edition will be released in April 2017.


Next Meeting

7:30 PM Friday March
3, 2017 at the Canada Aviation and Space Museum (directions). Note there is a $3 parking fee for museum parking. The meeting runs until 9:30 pm.

Part I: Hugo Lama will be giving a talk on ancient star charts.
Part II: TBA
PLUS: all our regular meeting features: Ottawa Skies, 10-minute Astronomy, Observer Reports, and of course, the beloved Door Prize!

All RASC monthly meetings are free and open to members and non-members alike. Refreshments will be available and this will be a wonderful opportunity to meet new friends who share a common
interest and chat in a relaxed, stimulating and fun environment. Please join us!
If you cannot attend in person, follow the proceedings over our live stream here.


Centre Information

General enquiries: contact@ottawa-rasc.ca

The Ottawa Centre 2017 Council

President: Tim Cole (president@ottawa-rasc.ca)
Vice President: Mike Moghadam
Secretary: Chris Teron (secretary@ottawa-rasc.ca)
Treasurer: Oscar Echeverri
Centre Meeting Chair: Kelly Jordan (meetingchair@ottawa-rasc.ca)
Councilors: Carmen Rush, Gerry Shewan, Jim Sofia
National Council Representatives: Brian McCullough, Robert Dick
Past President: Gordon Webster

2017 Appointed Positions

Membership: Art Fraser
Fred Lossing Observatory Director: Open
Smart Scope Director: Open
Hospitality: Art & Anne Fraser
Stan Mott Library: Estelle Rother
Ted Bean Telescope Library: Al Scott
Webmaster: Open
AstroNotes Editors: Karen Finstad & Janet Tulloch (astronotes@ottawa-rasc.ca)