A focal length of 122 feet!

[AMcD]

I attended an event at the Royal Society tonight for work.  During the reception prior to the event I spotted this.  It is an objective lens by Huygens, presented to the Society in 1691.  The label says the lens results in a focal length of 122 feet.  Too big for my observatory.

 

[ONIKKINEN]

I wonder if this was an "aerial refractor" type lens with the lens sitting on top of a pole or a tree and the eyepiece handheld taught behind it with some rope attached to the top. Seems awful long for something to be built to a tube with any rigidity.

I swear im not making that up, that actually did happen.

[allworlds]

I'm not sure if Huygens's 120 footer was an aerial telescope or not. He also made what he called "tubeless" telescopes - what we would now call a truss-tube, of sorts.

There is a drawing of Hevelius's 150 footer, https://commons.wikimedia.org/wiki/File:Houghton_Typ_620.73.451_-_Johannes_Hevelius,_Machinae_coelestis,_1673.jpg

That's what you had to do to get the chromatic aberration under control before achromats were invented. Or you could build a reflector - with mirrors that were about 66% reflective at best and needed the tarnish polishing off every few months and after you do that a few times you've wrecked the figure.

[Steve Ward]

A simple Google search soon takes you where you need to be ... https://www.google.com/search?q=huygens's+120+foot+telescope&sxsrf=ALiCzsbRE5P87ByvRQDpOK4a7RWQJ9M0zw%3A1665720541409&ei=3eBIY9bOGOHE8gLF3JvYDw&oq=Huygens's+120+foot+telescp&gs_lcp=Cgdnd3Mtd2l6EAEYAjIHCCEQoAEQCjIHCCEQoAEQCjIECCEQFToKCAAQRxDWBBCwAzoHCCMQsAIQJ0oECE0YAUoECEEYAEoECEYYAFDDCVialAFgmagBaANwAXgAgAF0iAHVCpIBBDEyLjOYAQCgAQHIAQjAAQE&sclient=gws-wiz

 

Edited October 14, 2022 by Steve Ward

[ONIKKINEN]

7 hours ago, Steve Ward said:

A simple Google search soon takes you where you need to be ... https://www.google.com/search?q=huygens's+120+foot+telescope&sxsrf=ALiCzsbRE5P87ByvRQDpOK4a7RWQJ9M0zw%3A1665720541409&ei=3eBIY9bOGOHE8gLF3JvYDw&oq=Huygens's+120+foot+telescp&gs_lcp=Cgdnd3Mtd2l6EAEYAjIHCCEQoAEQCjIHCCEQoAEQCjIECCEQFToKCAAQRxDWBBCwAzoHCCMQsAIQJ0oECE0YAUoECEEYAEoECEYYAFDDCVialAFgmagBaANwAXgAgAF0iAHVCpIBBDEyLjOYAQCgAQHIAQjAAQE&sclient=gws-wiz

 

Didn't really take me anywhere special, nor did a browse through the google suggested articles explain much.

This was quoted in two sites of the results:

Quote

In his telescope, there was no tube and only the air frame. Huygens named this type of telescope ‘tubeless’. However, his giant 120 feet long telescope was in fact difficult to use.

So was it in fact an aerial telescope or not? "tubeless" sounds like it kind of has to be.

The picture results all seem to point to the monstrosity of Hevelius' 150 foot refractor and no mention of a 120 footer.

[cloudsweeper]

150 foot focal length.  Wow!  But from basic physics, isn't that just a sheet of plane glass?  😉

Doug.

[allworlds]

Well now you've made me want to figure that out.

For a singlet lens the radii of curvature are quite easily related to the focal length. Guessing a refractive index of 1.5 and assuming it's a symmetric lens, the radii of curvature end up equal to the focal length actually.

https://physics.stackexchange.com/questions/168749/radius-of-curvature-and-focal-length

Using the equation for a circle y^2 = r^2 - x^2

1/3 foot away from the centreline of the 122 foot focal length the lens edge is 0.00046 feet lower than the centre. That's about 5.5 mils.

I wonder how Huygens measured it? Or maybe he didn't exactly - maybe he just ground lenses and saw what focal length he got?

[Ags]

He'll need a focal reducer for imaging.

[tomato]

122 feet, maybe that’s why it is still in it’s presentation box.

[Mr Spock]

How things have changed. I wonder what they would have made of a small apo?

[Ags]

I wouldn't mind sending my grab and go setup to Galileo, assuming DHL deliver to the Renaissance. 😀

I wonder what the shipping costs would be?

All I ask in return is that he sends me a few small, entirely inconsequential sketches of what he sees...

Edited October 17, 2022 by Ags

[Mircea]

ONIKKINEN

Yes, you are right, in all articles and books I read, and where it is mentioned this instrument, it is quoted as an ''aerial telescope''.

This monster lens was actually tested and used but I guess not often.

If my memory serve well, this aerial telescope was tested by Robert Hooke.

And it was for sure used to gage by comparison the quality of the six inch Newtonian presented in 1721 to the Royal Astronomical Society  by Capitain John Hadley.

The image was deemed to be brighter in the aerial refractor but the Newtonian won hands down due to the five feet focal length.

 

There was needed a painful training and technique to use such an instrument, detailed by Huygens when he described the telescope with which he discovered the rings of Saturn.

It seem there was the habit to hand held the eyepiece like a loupe but I believe that happened only at what we call today a ''star party'' and only for very bright objects like the Moon.

If of Keplerian type, one was getting a magnification of 122x with an eyepiece of one feet focal length. We should keep in mind, with the Keplerian eyepiece, the observer should hold his eye at a distance equal to the focal length of it minus one third of an inch. If you keep the eye closer, you will get  a strong aberration of ''kidney bean''.

So I set, at most , to two feet the focal length of those hand-held Keplerian eyepieces. At 60x, I guess, something could still be seen on the Moon.

Star parties happened also in the past, like we see in these woodcut of 1690 by Jan Goerre  of Amsterdam.

 

Mircea

 

 

[maw lod qan]

On 17/10/2022 at 14:24, Ags said:

I wouldn't mind sending my grab and go setup to Galileo, assuming DHL deliver to the Renaissance. 😀

I wonder what the shipping costs would be?

All I ask in return is that he sends me a few small, entirely inconsequential sketches of what he sees...

You forgot to request, signed.

[Mircea]

That's good idea, you would arrange yourself pretty well with a couple of Galilei's autographs ! 😍

Just be careful don't let him know, it was a tricky person. 

Mircea