First published in the Bulletin of the Astronomical Society of South Australia,
August 2014. Expanded version
(www.assa.org.au)
by Martin Lewicki
Telescope users are usually well aware
of the benefits of advanced optics available in their instruments
today. Compared to the primitive optics that astronomers contended
with in the early days of telescopic astronomy, today's high
precision optics yield sharp, high contrast views that we now take
for granted.
Prior to the invention of achromatic lens in
1733 early astronomers had to contend with views plagued by chromatic
aberration, often referred to as “false colour”. Chromatic
aberration and to a lesser extent spherical aberration are natural
image defects of simple lenses. They produce views of celestial
objects surrounded with unwanted colour fringing due to the inability of a simple lens to bring all colours of an image to the same focus. This results in lower image contrast and washed out colours. Yet these telescopes opened up a new universe
that included the revolutionary discoveries of Galileo with his small
15mm (1/2-inch) aperture lens. Later larger versions up to 200mm
(8-inch) objective lenses were built by Huygens, Cassini and Hevelius. These telescope
makers realised that extremely long focal ratios were required to
diminish the chromatic aberration suffered by these simple objective
lenses. They discovered that if aperture was doubled then focal length had to be squared, (or four times longer) to keep the chromatic aberration in check! As a consequence of desiring larger apertures their telescopes correspondingly grew to unwieldy lengths,
up to 46 meters such as Hevelius' “150-foot telescope”!
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| Hevelius' 150-foot (46m) 8-inch (200mm) Telescope 1673 |
In order to get the feel of what it may
have been like to use a non-achromatic telescope of the day I decided
to build a small long focus refractor with only simple lenses like
what would have been available in the early 17th century.
I felt a 30mm (1.2-inch) objective diameter of 1000mm (“three-foot”)
focal length would be a good start. Thus my telescope while smaller
than that used by serious observers of the time might have been about
the size owned by a well to do dilettante with astronomical
interests.
I purchased a surplus 30mm plano-convex
lens over the internet for a few dollars. After a trip to Spotlight
for some spent cardboard material tubes of the right diameter I made
a cardboard cell for the objective lens, fitted it to one end of a
cut-to-length “three-foot” tube and inserted a smaller diameter
draw tube for the eyepieces in the other end. The flat side of the
plano-convex objective faces inward. The plano-convex lens is preferred
as it has less spherical aberration than the more common double
convex lenses. Indeed, two of Galileo's surviving telescopes have
plano-convex and plano-concave for the objective and eyepiece
respectively, though this may have been to simplify
fabrication by fashioning one instead of two curves on each glass.
The eyepieces I use are old Tasco of
the Huygens design in keeping with the type used in the 17th
century invented by Christiaan Huygens himself. The Huygens eyepiece
comprises of two short focus plano-convex lenses appropriately spaced
with flat sides facing the eye to give a wider field of view than a
single lens. My eyepieces are focal lengths of 38mm, 23mm and 12.5mm
giving 26x, 43x and 80x. Finally a light baffle was placed in the
tube to reduce internal light scatter. In all, the optics of my three-foot
telescope use simple lenses; no achromatics, no coated lenses, in
keeping with the 17th century theme.
It was time to try my scope out on
objects in the celestial firmament. First targets were stars. I was
surprised to find they could be focused sharply enough so that the
airy disk (sometimes known as the diffraction disk) was apparent.
Savvy telescope users know this is a sign of good optical
performance. Point sources like stars will focus at best to a small
disk thanks to the wave-like properties of light. Only a slight amount of false colour was
evident surrounding the stars. Brighter stars and higher
magnifications show more false colour but the airy disk was still
clearly evident.
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| Snapshot of moon at 26x with camera at eyepiece. Some colour fringing is evident around the lunar limb and craters. |
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Next was the Moon. Again, surprisingly
good with only slight false colour fringing around the lunar limb and
shadowed craters. I felt the view at 26x was quite acceptable with
sufficient contrast revealing a wealth of detail in craters and
mountains. Saturn was next. At 26x its ball and ring were easily
evident and at 43x the ring was distinctly separated from the ball.
But one could understand how Galileo was perplexed by the
'handles' of Saturn. If I had not known that they were rings my
first impression would have been rather like that of Galileo. Some
false colour fringing was seen as a blue and red halo around the edges of
the planet and its ring which increased at higher magnifications. Later in the year Jupiter was visible.
The four Galilean moons at 26x were crisp points and the disk of
Jupiter itself just barely hinted at the equatorial belts. Like
Saturn, Jupiter showed a certain amount of false colour around its bright
disk.
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| Snapshot of Venus 26x |
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Venus also graced the evening sky. I
was able to trace its phases in my telescope over the closing months
of 2013 toward its inferior conjunction. The planet's brilliance
however exacerbated the false colour so that it was surround by
glorious halos of red, yellow, blue and purple. It was with Venus
that it was plainly evident the telescope was not corrected for
chromatic aberration! Like many observers I found Venus easier to
appreciate in a daytime sky where contrast is lower. Mars however at
it's current apparent diameter of 14” (late 2013) shows little more than a
orange disk at all magnifications.
However my biggest surprise was how
well this simple-lens telescope performs on “close” binary stars.
Stars are limited to those with a separation of 3.9 arcseconds - the
so-called Dawes limit for a 30mm aperture. My first target was Alpha
Centauri. The two components are now 4.5 arcseconds apart (2013). My telescope
revealed their duplicity. At 43x they appeared as two airy discs of
unequal size almost touching surrounded by several airy rings. The
Alpha Crucis pair was even more intriguing. At 43x and 80x the two
components at 4 arcseconds apart sat close together with a thin line
separating their two airy disks.
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| Sketch Alpha Centauri and Crucis at 80x |
The pretty binary Alberio did not
disappoint. The two components showed their delightful orange and
blue-white colours due to their spectral types K3 and B8. At 26x
false colour hardly interfered with their appearance. A selection of
other binaries such as the orange dwarfs 61Cygni, the wonderful Theta
Eridani pair (8.3”), Beta Tucanae (30”) and the Beta Monocerotis
(7.2”) gave pleasing views.
Next were deep sky objects. Under light
polluted urban skies they were pale versions of themselves due to the
small aperture. But away from light polluted skies at 26x the Orion
Nebula showed a diffuse mist with its characteristic structure but
with only three of the four trapezium stars apparent. Clusters such as
such as M7 and M41 look like a fine sprinkling of stars with no hint
of false colour. Eta Carina nebula clearly revealed its “L-shaped” misty form
and the Jewelbox its “A” shaped asterism. In December 2014 Comet Lovejoy (C/2014 Q2) graced the evening sky. From my light polluted suburban location I was pleased to pick the comet at 26x shining at magnitude 7.5 drifting through the constellation Puppis. The sketch reveals stars down to magnitude 8 (labelled without the decimal point). An advantage of long focal ratio telescopes is that the background sky darkens more than a star making fainter stars more visible than otherwise.
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| Comet C/2014 Q2 Lovejoy with field stars to magnitude 8 |
Finally,
in order to get the best sharpness out of my 17th
century telescope I attached a modern photography-grade deep orange
filter to the eyepiece. These filters were not available to 17th century
observers. But the filter made an immediate difference. The light through the filter is now nearly monochromatic and
can be sharply focused. The Moon looks perfectly sharp with virtually
no trace of false colour! The outlines of Jupiter and Saturn snapped
into sharpness along with brilliant Venus showing clean views of its
phases with little false colour. However for such a small aperture
the orange filter blocks more than half of the light so it is next to
useless for faint stars and deep sky objects. Had 17th
century astronomers been able to make precision optical glass flats I
imagine they would have benefited with “stained glass” versions as
filters to sharpen the views through their much larger light-capturing apertures.
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| An orange filter dramatically improves sharpness. | |
Clearly
these 17th century telescopes were capable of revealing
detail in planets and stars that paved the way for new discoveries.
Essentially, a simple lens telescope will perform to the same
resolution and almost same contrast as an achromatic lens of the same
aperture - if the focal ratio is long enough. This enabled
17th century astronomers to make significant discoveries
with their simple glasses at the end of their long tubes. Discoveries of Saturn's moons Titan, Tethys, Dione and Rhea, the ice caps of
Mars and Cassini's division in Saturn's rings were made with these
pre achromatic telescopes. These telescopes were used by the likes of
Edmund Halley, and Robert Hooke who discovered Jupiter's Great Red
Spot and William Gascoigne who invented the
filar micrometer in 1633 and used it to prove the Moon's orbit is
elliptical. Even when the colour-corrected achromatic lens finally became widely available in the 1750's the long telescope persisted for some years because
flint glass, one of the components of the achromatic lens, was
difficult to manufacture with consistent quality to make large
aperture objectives.
My long skinny telescope
attracts attention when I bring it to star parties. Many viewers
comment on the clarity of the moon, planets and stars seen through
the three-footer and were surprised to learn learn that it cost about
$25 to make mostly from recycled and second-hand materials. For
convenience especially for star parties I added a 45 degree mirror
diagonal for comfortable viewing. I also added a small finder
to help with aiming. To complete my range of eyepieces I also made a
second draw tube fitted with a single plano-concave
lens of minus 70mm
focal length. When inserted in the eyepiece end it becomes a Galilean
Telescope producing a 14x upright image that is
optically similar to Galileo’s early telescopes. It allows viewers
for example to appreciate how Galileo saw the Moon and Jupiter with
its satellites in its very narrow field of view. The telescope thus proved to perform
rather better than I and most people would have expected. Visitors and
in particular children are often intrigued with the views possible
through this simple-lens home-made astronomical telescope. ***
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| Snapshot of Sun through eyepiece. |
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| Pencil sketch of Moon at 26x with orange filter |
Notes:
Objective
lens obtained from Surplusshed http://www.surplusshed.com/
Hevelius
telescope image - Wikipedia
To see this simple lens act as a 1000mm telephoto lens see:
https://sites.google.com/site/simplelenstelephoto/
What's Next?
A test rig with 2-inch (50mm) planoconvex lens objective x focal length 2500mm
working at f50
(update 9 February 2016)
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| Two 1000x50mm PVC tubes joined by a 50mm collar making a 2m tube. Cardboard focus tube slides in at right to make a 2.5m telescope. The join is braced by a u-bolt tightened by wing nuts to keep tube (near) perfectly straight. | | |
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| Materials from hardware shop and piece of timber from garage. Total cost is about AU$20 for tube assembly. | | |
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| The 50x2500mm planoconvex objective from OptoSigma US$49.50. They have 50mm up to 5m focal length! | |
Hand-held views of the moon and some bright stars look promising. Now to make a mount!
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References
Sky and Telescope
Astronomy’s Neglected Child- The Long refractor – R. Berry, Feb 1976
A Telescope of the 17th Century - A Binder, Apr 1992
Books
Stargazer - Fred Watson
Seeing and Believing – Richard Panek
All About telescopes – Sam Brown 1975 (Edmund)
Telescope/Optics links
http://galileotelescope.org/Galileo_telescope/ (for Huygens eyepieces. Search eBay too)
