First published in the Bulletin of the Astronomical Society of South Australia,
August 2014. Expanded version
by Martin Lewicki
August 2014. Expanded version
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 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. 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”!
|Hevelius' 150-foot (46m) 8-inch (200mm) Telescope 1673|
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 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. 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.
Next was the Moon. Again, 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
'appendages' of Saturn. If I had not known that they were rings my
first impression would have been rather like that of Galileo. Some
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 colour around its bright
|Snapshot of moon at 26x with camera at eyepiece. Some colour fringing is evident around the lunar limb and craters.|
|Snapshot of Venus 26x|
|Sketch Alpha Centauri and Crucis at 80x|
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 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” 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.
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.
|Comet C/2014 Q2 Lovejoy with field stars to magnitude 8|
|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 enable17th 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.
|Snapshot of Sun through eyepiece.|
- Objective lens obtained from Surplusshed http://www.surplusshed.com/
To see this simple lens act as a 1000mm telephoto lens see:
What's Next?A test rig with 2-inch (50mm) planoconvex lens objective x focal length 2500mm
working at f50
(update 9 February 2016)
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.
Materials from hardware shop and piece of timber from garage. Total cost is about AU$20 for tube assembly. The 50x2500mm planoconvex objective from OptoSigma US$49.50. They have 50mm up to 5m focal length!
Sky and Telescope
Astronomy’s Neglected Child- The Long refractor – R. Berry, Feb 1976
A Telescope of the 17th Century - A Binder, Apr 1992
Stargazer - Fred Watson
Seeing and Believing – Richard Panek
All About telescopes – Sam Brown 1975 (Edmund)
Could Jean-Dominique Cassini see the famousdivision in Saturn’s rings? arXiv:1309.1711v1
including various sources of miscellaneous lens