12 replies to this topic

[Ovidiu Catalin]

Hello fellow stargazers, in this topic the main subject we will debate is about old 17th century refractors and modern replicas of 17th century telescopes single lens refractor also observations through this instruments single lens refractos where used at the beginning of the 17th century until the middle of the 18th century.

 

With very long focal lenght's these fancy scopes where at that time very long do to the extremely long focal lenght design to minimize the chromatic aberations.

 

I have many single lens telescopes made, over the past 10 years, i built a great variety of single lens refractors with focal lenght's up to 4m, tubeless and closed telescopes.

Now i have 3 scopes with great quality a 35/2000mm plano convex refractor named Hevelian telescope, a 26/1980mm meniscus refractor these two are tubeless scopes used as a aerial telescope, a very good quality 27/1250mm closed tube single lens refractor biconvex simetric objective made by Tavi F and 2 aerial very long objective 3500mm and 4008mm mm positive meniscus lens used without eyepieces for solar projection and prime focus observations without eyepieces.

 

The 26/1980mm long refractor with a very good quality meniscus lens from Surplush. 

Edited by Ovidiu Catalin, 08 May 2020 - 12:12 PM.

[Ovidiu Catalin]

Testing the great 26/1980mm Hevelian 2 refractor truss tubeless aerial telescope design with a long board 1800mm used as support for my objective lens cells, the reflexions are invizivile at bright objects but not good for low magnitude objects in the night sky it works on stars up to 4 magnitude at this setup, the objective lens full diameter 40mm and is used at 26mm diameter aperture.

 

 

Edited by Ovidiu Catalin, 08 May 2020 - 12:31 PM.

[Jon Isaacs]

Ovidiu:

 

Very interesting work you've done.  It reminds me of some of the projects in Sam Brown's All About Telescopes.  I like projects like this.  

 

If I am understanding this correctly, the telescope in the photo has a 26mm aperture and a 1980mm focal length, I calculate a focal ratio of f/76. 

 

I calculate a depth of focus at f/76 of 13mm, 1/2 inch.  That might require a two speed focuser but one quite different from the one's we are used to, this one would need an overdrive rather than a microfocuser.  Your simple lash up should work just fine.

 

I am curious what Venus looks like in your scope.  Using equation used to evaluate achromats, focal ratio/aperture (inches), this scope would be about 76.  Do you see chromatic aberration?

 

Jon

[Ovidiu Catalin]

Thanks, the meniscus objective is a single lens the image remains focused even when you move the eyepiece 1cm in the focuser, i don't need a micro focuser for this scope, chromatic aberations are very little only at the edge of the moon and on Venus egde but spherical aberations at a magnification of 77x and up to 100x is small very unique for a meniscus lens some lenses that i used with this shape the image es where very week and at hig magnification of 80x to 100x the spherical aberations where very strong.

Here is Sirius at 100x with the 26mm refractor.

The phone amplfy some cromatic aberations that where not there only at the egde of Venus do to the lens of the camera. 

 

 

This image of Sirius was taken with Huawei p9litte afocal 20mm Kellner eyepice with some camera zoom 5x.

 

 

Venus with the 26mm refractor at 116x, 17mm plossl eyepiece, afocal, camera Huawei p9litte, post processing. 

Edited by Ovidiu Catalin, 09 May 2020 - 01:50 AM.

[Ed D]

Great project!  Thank you for sharing it with us.

 

Ed D

[Ovidiu Catalin]

The greatest project was made in 2014, then i purchased two singlet objective lenses from Tavi F a 28/1250mm and 40/2400mm the 28mm was a success i have made a very nice vintage refractor with a copper tube mounted on a eq1 mount the refractor is stil used today, its a marvelous instrument the first of is kind in Romania in that time no one has been able to made a permanent construction of a singlet refractor using a different lens shape rather than a eyeglass lens (meniscus).

 

 

 

1 https://1.bp.blogspo.../s1600/DSCN4461.

 

2 https://1.bp.blogspo...600/IMG_2020051

 

3 https://1.bp.blogspo...s1600/IMG_20200

 

4 https://1.bp.blogspo...s1600/IMG_20200

 

5 https://1.bp.blogspo...1521+-+Copy.jpg

 

6 https://1.bp.blogspo...5252_227+-+Copy.

 

7 https://1.bp.blogspo..._235615_059.jpg

 

8 https://1.bp.blogspo..._235000_895.jpg

 

9 https://1.bp.blogspo..._234359_417.jpg

 

10 https://1.bp.blogspo...102129284817982

Jupiter with the 27mm refractor afocal, 10mm eyepiece with nikon coolpix L310 camera, post processing

 

12 https://1.bp.blogspo...er_28mm_180.jpg

Jupiter with the 27mm refractor afocal, 10mm eyepiece with nikon coolpix L310 camera, post processing

 

Here are some images taken by Tavi's F with the same objective lens 28mm aperture, made by him from that kit, pictures belong to him.

 

https://live.staticf...f84abf0be_o.jpg

https://live.staticf...042bacdb5_o.jpg

https://live.staticf...7482fd279_o.jpg

https://live.staticf...3962ed9b8_b.jpg

https://live.staticf...6f008f01a_b.jpg

Edited by Ovidiu Catalin, 11 May 2020 - 06:01 AM.

[musicengin]

Beautiful scopes, and beautiful images!  I will enjoy following your projects.

[Ovidiu Catalin]

What was optics like in the 17th century? In fact, there are various reasons for the poor quality of the optics of the seventeenth century.
First of all the glass was not good, because the material was not homogeneous (the index is not constant in the glass), full of bubbles and striae: the only way to get rid of this was at that time for opticians to select a part of the glass and reduce the thickness of the lenses. Second, the quality of grinding and polishing in the seventeenth century was mediocre, only a few opticians could make a good grind without scratches and digging. Poor polishing has been the source of diffusion defects and reduced image contrast. Third: the lenses were not coated with antireflex and some reflections affected the image. Regarding the surface figure (None of the surfaces were tested at that time, it was a kind of "blind polishing): hand-polished lenses had defects such as non-revolution (equivalent to astigmatism) or defects at the edge, being too low - spherical aberration mainly for lenses polished with paper or leather) by reducing the diameter with a diaphragm lens (diameter <0.7 x lens diameter), those defects were canceled. The fifth theory of Descartes' optics said that the shape of a lens must be a kind of ovoid surface impossible to polish and test at that time, and the closest shape was the spherical surface that can be obtained naturally. If the sphere is very close to the ovoids (better than lambda / 4) no aberration remains. This is obtained for the number F around 8 or 10. So why did astronomers want to make long focal lengths with long tubes? The reason is mainly the chromatism! And not too many people give this explanation. In fact, if you increase the F ratio to 20 or 100 you will see that the chromatism becomes smaller than the Airy disk and the chromatic aberration is somewhat reduced. The problem at the time was to polish long and very long focal length lenses. Hartsoeker was working on this subject in France at the time. It was difficult because the F / 100 lens means that the depression is very small (around 10 microns for a 50 mm lens) and the lens is almost flat. Making molds in the seventeenth century with such precision was almost impossible.

Here are some tests on a objective lens made by Rudolph, J.G in the 17th century, aperture 25mm, 824mm FL with focogram and ronchi optical tests.

http://www.dioptrice...=Rudolph, J.G.

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[Ovidiu Catalin]

In the image below we see Foucault and Ronchi tests of four objectives achieved by Campani. The first test on the upper left shows the objective glass belonging to the instrument at the Orangery Museum, part of the Hessisches Landesmuseum Museum in Kassel, Germany. It is a large Campani telescope, which Landgraf Karl bought in Rome in 1700. It is signed along the rim with a diamond: "Giuseppe Campani in Rome year 1700 zum Cassellischen Kunst Hausse." It has a diameter of 78 mm used at a 50mm aperture and a focal length of 4.92 m.
Its surfaces are almost perfectly spherical and show only the normal spherical aberration. The two lenses on the right show the tests on the two large objectives of the aerial telescope. They belong to the National Conservatory of Arts and Metiers in Paris, France. Here we recognize that the edge was planned during the polishing process to a high degree. The result is a striking spherical overcorrection. However, as we will see below, this over-correction is only minor in the quality of the objectives.
   In the image below the tests on the top right being the first, the objective lens with a diameter of 135 mm, used at the diameter of 80mm and the focal length is 18.5 m. It is signed along the rim with a diamond: Giuseppe Campani from Roma Palmi 87 ".
  Also in the bottom right side we have the objective of 158 mm, full diameter used at 90mm and the focal length is 25.6 m. It is signed along the rim with a diamond: "Giuseppe Campani in Roma Palmi 120". In order to diminish the chromatic aberration in an invisible quantity, the first lens needs a opening of the diaphragm with a diameter of 80 mm and a second a diameter of 90 mm. Inside this aperture, the image distortion of the spherical overcorrection is so easy that the Rayleigh state fourth part of a wavelength is fulfilled. Therefore, we come to the conclusion that these objectives of the aerial telescope, although far from perfect in the modern sense, still gave good images.
This is exactly the same reason I explained in my critical objections about the "Galilean telescopes" in Florence. Campani did not have any information on the state of his lenses in the region covered by the shutdown of the diaphragm, which he needed to eliminate the chromatic aberration. The aspherical aberrations of the lens surfaces are caused by an imperfect contact between the lens and the polishing surface, the fine paper glued to the sanding mold. If Campani observed with his star an imperfect image, then this would be an indication that the lens opening inside the diaphragm stop is not spherical. Therefore, he adjusted it and made a new polishing tool. He continued to repeat this process until the image of the stars became perfect.
Already during Campani's life and especially after his death (1715), some rumors were circulating about special secret methods or tools he used to achieve his goals. However, for me, the results of the objective tests indicate that his great ability and his tendency towards perfectionism are the ones that explain his great success.

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[musicengin]

I've been curious about this very topic for a while, thank you for illuminating it!

[Ovidiu Catalin]

 Last night, 19.05.2020, there was a confrontation between the two singlet telescopes, where we experienced in more detail the performances of the two telescopes on different celestial objects.
Venus is getting closer to us and this will mean his departure from the evening sky and his return to the morning sky, goodbye Venus was the first reply before the observations.
I started with the simple 27 / 1250mm symmetrical biconvex lens was a phenomenal experience at 50x, 25mm Kellner eyepiece Venus was very clear with sharp edges, at 127x 10mm eyepiece was very large and still clear at 73x 17mm eyepiece plossl the image is stil clear and very sharp the best magnification with the 27/1250mm refractor.
 I decided to take out the 26 / 1980mm Hevelian meniscus telescope to compare the two so I chose an almost identical power through the 26 / 1980mm 79x telescope with the 25mm eyepiece and with the 27 / 1250mm sinlget telescope i used a 73x 17mm eyepiece power, the results were very surprising, through the 26 / 1980mm telescope the chromatic aberration was smaller than through the 28mm telescope, Venus was bigger but just as clear, and through the 28mm singlet the image was sharper than that of 26 / 1980mm with more pronounced chromatism, here are the results below.
At double stars the 27 / 1250m telescope wins, i noticed the Castor double in Gemini separated with black space between 73x components with two very beautiful and well defined airy discs, and with 26 / 1980mm telescope the two 79x components were separated by a very thin black space, they were closer than through the 27mm, but being round with a nice airy disc. Singular stars have also been tested and look great with both instruments with a clear image of the airy disk and a perfectly round diffraction ring.

 

1 https://1.bp.blogspo...s+luneta+28mm%2

 

2 https://1.bp.blogspo...0518_213802.jpg

 

3 https://1.bp.blogspo...s+luneta+26mm.j

Venus with the 26/1980mm refractor, afocal 25mm Kellner eyepiece 79x, taken with Huawey p9litte.

 

4 https://1.bp.blogspo...neta+26mm+5.jpg

Venus with the 26/1980mm refractor, afocal 25mm Kellner eyepiece 79x, taken with Huawey p9litte 2

 

5 https://1.bp.blogspo...s+luneta+28mm%2

Venus with the 27/1250mm biconvex simetric objective refractor, afocal 17mm eyepiece, 73x.

[Ovidiu Catalin]

The 27/1250mm refractor its combation of 17th century and 19th century refractor with single lens objective and copper tube on a eq mount similar to 19th century refractors.

 

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Edited by Ovidiu Catalin, 20 May 2020 - 09:08 AM.

[Ovidiu Catalin]

Venus through the 28/1250 and 26/1980mm Hevelian singlet refractors, Huawei p9litte camera, afocal. 

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