Friday, June 20, 2014

Backyard Astronomy with a 17th Century Telescope

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.  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
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. 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.


Snapshot of moon at 26x with camera at eyepiece. Some colour fringing is evident around the lunar limb and craters.






 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 '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 disk.

Snapshot of Venus 26x







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” - the so-called Dawes limit for a 30mm aperture. My first target was Alpha Centauri. The two components are now 4.5“ 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” apart sat close together with a thin line separating their two airy disks.

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 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. 

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.

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. ***



Snapshot of Sun through eyepiece.


Pencil sketch of Moon at 26x with orange filter





Notes:
  1. Objective lens obtained from Surplusshed http://www.surplusshed.com/
  2. 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) 
    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!
    Hand-held views of the moon and some bright stars look promising. Now to make a mount!

    _________________________________________________________________________


    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://www.telescope-optics.net
    Wikipedia sources
    Articles

    Could Jean-Dominique Cassini see the famous
    division in Saturn’s rings? arXiv:1309.1711v1
     

    Software
    http://www.atmos-software.it/Atmos.html
    http://www.lambdares.com/oslo
     

    Optical Suppliers
    http://www.surplusshed.com/
    including various sources of miscellaneous lens 
    http://galileotelescope.org/Galileo_telescope/ (for Huygens eyepieces. Search eBay too)
     

18 comments:

  1. Hello Martin

    Congratulations ,very good ,very interesting article.
    Looking forward to read about the bigger 2 inch singlet refractor project.

    Regards, Mircea

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  4. Amazing work !
    I have made a 28/1250 mm refractor the lens was made by a optician and its a biconvex lens with a very high quality.
    I was eable to split the double stars: Castor, Algieba, Almach, Mizar, Albiero and many more.
    The highest magnification used is 125x with a 10mm plossl.

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  5. Excellent! You like myself were surprised what such simple optics can show. So far my observations with my new 50x2500mm are "hand-held" with one end braced on the eave of the house. Views of the moon at 50x are much brighter with higher resolution which indicates the lens is good quality. For double star and planetary work however will require making a mount. Looks like some version of the 17th century mast design will be in order.

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  6. excellent, can i use with this singlent lens 30mm/1000mm FL any eyepieces (plossl 4 elements) or is work just with huygens eyepieces that are not availables today.

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  7. You can get inexpensive Huygens eyepieces on eBay. Eg
    http://www.ebay.com.au/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xhuygens+eyepiece.TRS0&_nkw=huygens+eyepiece&_sacat=0

    Since your telescope operates at very long focal ratio (~f33) these eyepieces will perform very well. I find the Huygens give sharp stars right to the edge of their 30-degree apparent field in my telescope.

    You can of course use modern eyepieces like Plossl that will give a wider field of view, better contrast and colour due to their modern glasses and anti-reflection coatings. But I find part of the instructive fun of it all to use the simpler classical Huygens to replicate the viewing experience of the 17th C.

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  9. Martin try to diaphragm the 30/1000mm objective at 25mm and see how the image looks.
    And the 50mm objective at 40mm diameter.
    Make some diaphragms with different diameters.
    The Moon at 26x has to mutch CO at that power so if you reduced the objective size the image will be mutch cler, the same with de 50mm objective.

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  10. Ovidiu, yes I have tried a range of diaphragms on the 30mm and found the best overall performance was still at the full 30mm. While a smaller aperture stop reduces chromatic aberration it increases the size of the airy disk thereby reducing resolution and dimming the image too.

    I have run the my lens specs through the optical analysis software OSLO for my scope as well as the lens of Galileo, Huygens and Cassini telescopes. I found as they did, the best compromise was a circle of confusion about three times the size of the airy disk (3:1) that gives gives optimal performance. This is the ratio found in my 30x1000mm lens. By this criterion my 50mm lens should really be 2700mm fl rather than 2500mm to achieve this performance. So you are correct that a slightly a smaller, in this case 47mm stop should match this ratio for my 2500mm FL scope. I will have to wait until I have the mount built put this to the test.

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  11. I am going to build a new refractor with a diameter of 40mm and 2400 focal length this one will be a aerial telescope like used by Huygens.

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  12. I put your lens specs in OSLO and it reports a spot ratio (circle of confusion to airy disk) of 2:1 which should give better relative chromatic suppression than my 30mm scope (3:1).

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  13. I ordered a biconvex simplet objective witha very high quality made by Tavi F. the Romanian optican
    Lens specifiications here:
    A. Optical glass BK7 lenses
    Type 1. Specifications:
    - focal length (nominal) in G-channel: F = 2400mm (+10/-5mm);
    - total diameter: Dt = ~49.5mm;
    - center thickness: tc = ~8mm;
    - high precision optical diameter: Duseful = 40mm;
    - relative opening ("focal ratio"): N = F/Du = 60;
    - the chromatic aberration coefficient: x = N/Du = 1.5
    I will make an aerial telescope unsing this objective, making a tube and a mount for it is to hard for my resources.
    Do you have a email adress or gmail ?

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  14. How is your 50/2500mm project ? That board which you mounted the lenes in time will bend, its very difficult with long boards becaue you need 2 mounting points one close to the objective and the last close to the eyepiece, you can try a Aerial telescope design at this length.
    For the 50mm you will need a solid cardboard tube mounted on a board and supported by two frames,the firt one close to the objective will be a high ladder with 7 steps and the last mount a tripod adjustable height and with left and right adjustments.
    Very challenging it will be, i prefer the Aerial telescope method for my objective at 2.4m length.

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  15. Amazing work with de 50mm, greate idea

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  16. in this lens 30/1000 is difficult to distinguish witch face of lens is plano and witch face is convex : with naked eye faces of lens are similar. there is any method to do that ??

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  17. Amazing work. I've been experimenting with these simple telescopes for some time. My current project telescope, a 50mm f/20, is based on the Telescope 1609 Galilean, though I've modified it to take 1.25" eyepieces. Back in 2008, I did make something of a Galilean facsimile using a 30mm f/35, but it rarely gets used. Tonight, I have stopped the 50mm down to 30mm for some additional experimentation.
    Thanks for sharing!

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  18. Satish Patil Jalgaon India.
    Thanks for reminding me the struggle of Galilio

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