45 replies to this topic

[gnowellsct]

Take a modern APO and mask it down to 40 mm, you are now in the aperture range of Galileo. But unlike Galileo you have a number of things that he would have killed for including a nice diagonal and fancy eyepieces.

And the truth is if you take something like the vixen sd-81s and mask it down to 40 mm it works very well as a telescope and you can see quite a bit of detail on Jupiter and Saturn.

But Galileo famously could not make out the rings of Saturn.

Once someone suggested in these fora to see what Galileo saw you should take a piece of Saran wrap and stretch it across the front aperture. I think it might be true! Has anyone tried it?

I am also aware that some of our resident optical history experts have contended that Galileo's telescopes were diffraction limited. Given Galileo's inability to make out Saturn's rings I wonder if that's really plausible. I suppose it would be plausible if the maximum achievable power of the telescope was low.

But I'm wondering more about the Saran wrap simulation. I guess one could even try that during the day.

Greg N

Edited by gnowellsct, 21 May 2022 - 12:30 PM.

[vtornado]

I was thinking about trying saran wrap, but for a completely different reason.

Trying to observe through an open window and keep heat from billowing out.

There still could be a boundary layer at the film so it may not work for that reason.

 

Astro film seems not to be degrading, and it is all wrinkly ???

 

Could the opticians of the day make give a lens a nice figure?

Edited by vtornado, 21 May 2022 - 12:42 PM.

[petert913]

I believe Galileo's first telescope was 40mm, but he had to mask it down to 25mm due to poor lens design.

[jmillsbss]

We do better than Galileo to observe through our 8x50 straight through finders.

 

I think the biggest difference would be light pollution.  I've observed, with direct vision, a number of galaxies through my 4" doublet from dark sky sites.   I struggle to see more than a gray smudge with averted vision in a 10" scope from my Bortle 5 home skies looking at those very same targets.

 

So take the 8x50, stop it down if you want, rub your finger on the side of your nose and attach saran wrap to the objective using nose grease as glue and get to the darkest skies and maybe, maybe, now you're getting close to Galileo's setup!

 

We complain over field curvature and coma and chromatic aberration and flare, yet we have access to some of the most advanced optics ever dreamed of, even in the early 20th century.

 

I'm intrigued looking at telescope histories.  Some of the crazy designs like the "tubeless" string telescopes and the enormous, even compared to today's massive dobs, polished silver mirrors and speculum metals of 36" and 48".

 

The Leviathan of Parsonstown and the aerial telescopes like those built by C. Huygeens just blow my mind and keep my occupied when the clouds are relentless.

[gnowellsct]

Posted here by accident: please move to refractors

[oldtimer]

I have made my best attempt at making a scope similar to Galileo's design. After much searching, I found a 40mm 1150mm plano-convex lens.. I stopped i down to 30mkkm making it a F38. Finding a suitable concave lens proved impossible so I settled for a Huygens. It does produce about a 1 degree FOV which is larger than Galileo' s scope. I have taken this to public events on a very tall tripod so that straight through viewing can be done. I explain to the public that although the size of the scope is a good model the image it produces is probably better than what Galileo was seeing.(better optics).

 

Gary (oldtimer)

[JamesMStephens]

The February 2009 Sky & Telescope has a box by Damiano Fedeli describing a replica of one of Galileo's telescopes based upon examination of one of Galileo's surviving instruments.  The primary is a plano convex singlet 37-mm diamater with 980-mm focal length, and the eyepice is a 22-mm diamete biconcave lens with a (negative) 47.5-mm focal length.  The objective would have been stopped down to a mere 15-mm, for an f/65 instrument.

 

I've read references that pointed out how contemorary lenses (made for eyeglasses) had a small central portion that was usable, requiring stopping down to very small apertures.  I'd thought references would be easy to find, as well as information on the replica telescope, but no luck.  The S&T box shows a poorly resolved image of Saturn.  It's elongated, but doesn't show rings.  The Dawes limit for a 15-mm aperture is 7.7 arcseconds.  Could a diffraction limited 15-mm instrument could resolve the rings?  I think this would be just outside its limits.  

 

Jim

[John Gauvreau]

Obtaining good quality glass for his scopes was a great challenge for Galileo.  If we are talking about the 20 power scope with the red and brown leather and gold embossing, then the lens was a piece of glass about 37mm in diameter but was stopped down to about 17mm to reduce aberrations. 
 

When I made my replica one of the biggest challenges was getting glass bad enough to mimic the view.  In the end I used plastic lenses and although the optical arrangement is not the same as the original scope, the apparent field of view (very small!), the magnification and the optical quality are all close to the original, so people looking through my replica get a good idea of what Galileo was seeing.

 

Edit - Jim was posting the correct sizes at the same time I was writing.  Thanks Jim!

Edited by John Gauvreau, 21 May 2022 - 02:08 PM.

[Taosmath]

In the Intro Astronomy Class I teach, for one of the activities I have my students make 40mm refractor using an Achromat from Surplus shed, some plumbing parts and an 10mm SVBony EP.

 

While the optics are considerably better than Galileo used in Sidereus Nuncius  (e.g. Achromat, much wider FOV, larger Exit pupil) its light gathering is similar and so is the magnification (about 15x, though Galileo later used greater magnifications)

 

 

 

You can see the Galilean moons and you can see somethings strange is happening with Saturn (If you 'cheat' and use a good 4mm EP, you can see the rings), plus you can observe the moon and the phases of Venus, so I assign students projects to observe those and reflect (Ha!) on what Galileo achieved with his scope in 1610.

 

The larger FOV is particularly useful, since it's already a challenge for the students to find Jupiter in the scope, so a 4 degree FOV really helps - I doubt I'd get many students see anything at F30+ (The white rectangles of plastic on the top of the tube are a sighting system)

 

After that I have them look for some easy DSO's, M31, M42, M45 etc. etc.

 

It's a worthwhile activity for my students.  It combines some rudimentary understanding of how a refractor works, finding some basic celestial targets, a bit of digging into history of science and how one man with primitive equipment managed to upset the authorities at the time, plus at the end they have a small refractor they can take home & play around with, before we get out the 'Proper ' telescopes.

[TOMDEY]

Cool, Greg, et. al !

The February 2009 Sky & Telescope has a box by Damiano Fedeli describing a replica of one of Galileo's telescopes based upon examination of one of Galileo's surviving instruments.  The primary is a plano convex singlet 37-mm diamater with 980-mm focal length, and the eyepice is a 22-mm diamete biconcave lens with a (negative) 47.5-mm focal length.  The objective would have been stopped down to a mere 15-mm, for an f/65 instrument.

 

I've read references that pointed out how contemorary lenses (made for eyeglasses) had a small central portion that was usable, requiring stopping down to very small apertures.  I'd thought references would be easy to find, as well as information on the replica telescope, but no luck.  The S&T box shows a poorly resolved image of Saturn.  It's elongated, but doesn't show rings.  The Dawes limit for a 15-mm aperture is 7.7 arcseconds.  Could a diffraction limited 15-mm instrument could resolve the rings?  I think this would be just outside its limits.  

 

Jim

Thanx for the tech specs, Jim

 

Well... I just dug up these from my cornucopic Pandora's Box >>>

 

Now for the most difficult part... a tube made of "strips of wood and red leather". Without that specific build, any results I report will be entirely meaningless.    Tom

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[vtornado]

I have made my best attempt at making a scope similar to Galileo's design. After much searching, I found a 40mm 1150mm plano-convex lens.. I stopped i down to 30mkkm making it a F38. Finding a suitable concave lens proved impossible so I settled for a Huygens. It does produce about a 1 degree FOV which is larger than Galileo' s scope. I have taken this to public events on a very tall tripod so that straight through viewing can be done. I explain to the public that although the size of the scope is a good model the image it produces is probably better than what Galileo was seeing.(better optics).

 

Gary (oldtimer)

 I am thinking about making one for out reach.  I can't find a roughly 40mm x 1000mm lens.  Where did  you get yours?  A few botique optics houses have them for $$$,

probably because they are 1/4 wave blah blah blah

 

if I find an air spaced 60mm achro doublet of 1000mm focal length,  Can I use one of the elements?   I could stop it down. to 40, although that lens would be MgFl coated.

Edited by vtornado, 21 May 2022 - 03:10 PM.

[John Gauvreau]

 I am thinking about making one for out reach.  I can't find a roughly 40mm x 1000mm lens.  Where did  you get yours?  A few botique optics houses have them for $$$,

probably because they are 1/4 wave blah blah blah

 

if I find an air spaced 60mm achro doublet of 1000mm focal length,  Can I use one of the elements?   I could stop it down. to 40, although that lens would be MgFl coated.

I couldn’t find such a small aperture with such a long focal length, so I used a shorter focal length lens and set it deep inside the tube, and then placed a clear window behind the mask at the objective end of the scope so that it looks like the lens is there.  The view through the scope still correctly mimics the magnification and apparent field of view.

Edited by John Gauvreau, 21 May 2022 - 03:17 PM.

[careysub]

 

I believe Galileo's first telescope was 40mm, but he had to mask it down to 25mm due to poor lens design.

The two extant scopes have a lens that is 51mm masked down to 26mm (FL 1330mm), and 37mm masked down to 16mm (FL 980mm). The eyepiece lens was 11mm FL -94mm and 16mm FL -47.5mm respectively.

 

It was not poor lens design, but the limited knowledge of the time about how to make good lenses, or even to fully understand what was possible. We have the advantage that the worst lens we have ever seen, or can obtain, is better than any lens that was made prior to 1650 (or probably even later). When you are accustomed to (in comparison) high quality lenses in cheap children's toys, it is difficult to appreciate that in early times it was not known with certainty that making a perfect lens was even possible.

 

https://bay-astronom...ELESCOPES_6.pdf

 

As Rolf Willich makes clear in his The Long Road To The Invention of the Telescope the breakthrough that created the telescope was the discovery that an aperture mask could produce a legible magnified image instead of one that was an amorphous blob.

 

If you have a long focus refractor of any kind a 16-26mm aperture mask could be used to an image similar to what Galileo saw (though it would be far sharper).

 

You can make a similar instrument for less than $5 of parts.

 

This 50x1000mm lens at SurplusShed is $1.50 and a close match to the larger scope objective:

https://www.surpluss...tem/L1918D.html

 

For the EP lens either of these 38mm lenses (-100mm at $1.5 0 and -50mm at $2.40):

https://www.surpluss...item/L1921.html

or

https://www.surpluss...tem/L1951D.html

could be used, adding a second aperture mask at the lens to reduce to the same aperture as Galileo.

 

Or if you want to splurge you could blow $4.50 on a lens that is a very close match to either:

https://www.surpluss...rt=&sortby= asc

 

You just need a cardboard tube - which would be historically accurate as that is the type of tube Galileo used.

Edited by careysub, 21 May 2022 - 03:58 PM.

[careysub]

 

 I am thinking about making one for out reach.  I can't find a roughly 40mm x 1000mm lens.  Where did  you get yours?  A few botique optics houses have them for $$$,

probably because they are 1/4 wave blah blah blah

 

if I find an air spaced 60mm achro doublet of 1000mm focal length,  Can I use one of the elements?   I could stop it down. to 40, although that lens would be MgFl coated.

Look at my post on above. You can order batches of 10 lenses from Surplus Shed  and get a 10-15% discount on each one. Also order a batch of 2" cardboard tubes (from elsewhere) and you can make kits costing less than $5 each.

 

To prepare the cardboard tubes for use I recommend getting a cheap flat black paint, diluting it so that it flows freely, then pour it through the tubes to blacken the insides. A ladle, a plastic container (to drain into) and place for them to dry and you can do a whole batch in 10 minutes.

 

For this you do not want achromat lenses. They weren't invented for another 100 years.

Edited by careysub, 22 May 2022 - 12:03 AM.

[CBM1970]

This is a very interesting endeavor.  One of the most impressive things about what Galileo DID see, in my opinion, is that he was not only dealing with very rudimentary equipment, but he also was looking at objects without much prior knowledge (for the most part) of what he would actually "see".

 

As has often been stated here on CN, observing the heavens with telescopes involves both the eye and the brain. As observers gain experience, they see more. I expect that part of this is due to time at the eyepiece, but part of it must also be due to expectations. I certainly see the GRS more easily when I know that I am observing at an hour when it is expected to be visible. I have similarly found many details in lunar, planetary, and dso observing by studying sketches and pictures and then looking for features that I saw in the sketches and pictures.

 

Galileo's observations, for this reason, seem even more impressive to me.

[JamesMStephens]

I was hoping Tom (TOMDEY) would pick this up.  Yes, the eyeglass lenses are a great choice.  (And incidentally, Tom has constructed a great solar projector using them.) 

 

I've always wondered what Galileo used as a mount.  Probably the closest we can get is Scheiner

https://tinyurl.com/796fee2w

 

and this (which gives us insight into how far back the equatorial mount appears)

https://www.catholic...istoph-scheiner

 

Did Galileo consider mounting the telescope so secondary an issue that he never mentioned it?

 

Jim

[gnowellsct]

Did Galileo consider mounting the telescope so secondary an issue that he never mentioned it?

Jim

I don't know but I can tell you two things. The first is that the refractor was born used and made famous on some kind of alt az mount. And the second is that over in the refractor section there's a whole bunch of people who never got over it and have nothing but bad things to say about German equatorials. And they're going on about it right now.

I would also add that I started this thread in this section by accident and I'm waiting nervously for a moderator to tell me I'm a bad person. I'm not allowed to initiate in beginners.

[vtornado]

BTW I tried glad cling wrap on an 80mm f/11 refractor.

View was horrible.   Very blurry.  Astro-film it is not.

Window glass is a much better view.

 

I wonder if that is why in the instructions for AF it says not to stretch it tight.

It is optically flat, and stetching it would distort the thickness.

[vtornado]

I just purchased the lenses linked to by caresub.  It cost me $12.00 total.

 

50mm x 1000  It is a bi-convex.  Galileo's was a plano convex.

21 x -45 plano-concave. 

 

I can stop down the objective lens to 40mm.

 

As a proto type using some gift wrapping tubes

perhaps nested so I can adjust the focal length spyglass style.

For the final project perhaps use some PVC.

 

On ebay there is a pcx 25x1000mmm lens.  I have read where Galileo's objective

was stopped down. to 26.

 

If I read optical text book right plano convex should yield spherical abberation

if the incoming rays are parallel.  However how much SA is in a 50mm x 1000mm lens?

[gnowellsct]

BTW I tried glad cling wrap on an 80mm f/11 refractor.
View was horrible. Very blurry. Astro-film it is not.
Window glass is a much better view.

I wonder if that is why in the instructions for AF it says not to stretch it tight.
It is optically flat, and stetching it would distort the thickness.

But did you try it on the moon

[Ken Sturrock]

I'm waiting nervously for a moderator to tell me I'm a bad person.

Aw geeze. You gonna make me state the obvious?

Moving to Refractors....

[gnowellsct]

Well I'm glad to see that this thread got moved over to refractors. I initially posted it by mistake in beginners because I sometimes lose track of what form I'm in on my phone.

On the other hand I'm worried that we drag some beginners over here they are going to be sucked into the great vortex of the refractive lenses and will never reemerge.

Well that's okay there are worse Fates. I hope they left a pointer over in beginners I already got one inquiry as to where the thread had gone.

GN

[gnowellsct]

Was Galileo able to pick out the shadows of Jupiter's moons?

[gnowellsct]

Could the opticians of the day make give a lens a nice figure?

Well what you need to realize is that Galileo was in fact the optician of the day and must have ranked among the Masters to achieve what he did.

You are probably thinking well yeah but glasses were invented a couple centuries earlier. And there's the rub. To get the corrective effect that you need on the lens of your glasses you need a specific corrective curvature of maybe 5 mm. In other words when you're using your Renaissance glasses you're peeping through a small hole in a larger piece of glass which gives you the effect that you need.

The technical problem that had to be surmounted was how to create a larger curve that would yield a diffraction limited image. It had to be a larger diffraction limited curve in order to gather the light that would make the telescopes Superior to the unaided eye. By creating a lens of perhaps 17 mm Galileo was able to revolutionize not only optics but science and astronomy. Nonetheless he had a debt to the technique developed by the optician up in Holland whose name I'm forgetting. Someone here will know it. but Galileo just read the instructions and grounded lens and then started making his own modifications.

It's the original case of rtfm and then execute the project.

Greg N

[zsb04]

https://www.bhphotov..._Reflector.html

 

I think you can still find these around at places, would be interesting to use