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Homebuilt 17th century Looooong Refractor

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#1 raym.

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Posted 03 September 2008 - 11:39 AM

Using a 40 mm X 4000 mm fl positive meniscus lens from Surplusshed, a 1 1/4 X 5 inch achromat for an eyepiece, and THREE six foot long McMaster Carr 4 inch diameter shipping tubes...Ive built a pretty decent working 17th century Refractor like Hevelius would have used in 1647. Ive always wanted to do this.....and finally, well, that long focus lens from surplus shed, seeing that on their catalog....set the wheels in motion. I sectioned up one of the shipping tubes, and sliced an inch ont of it on the table saw. That coiled tighter, and allowed it to slip halfway into one, and halfway into the other tube, giving great stregnth. The thing balances on a dime and dosent flex. Its just under fifteen feet long....
So...Howz it on stars? And, HOW DO YOU USE IT??

Well, Im on the third floor, with a good high window in a big room. It has a good south east view. So, things like scorpio, and Jupiter, are in view now. I can balance the scope with about 7 feet of it poking out the window, and balance it so it rests near the eyepiece on a convinent piece of cat furniture that is nearby. Once you get the view, its really really very good! I was totally surprised how hair trigger the focus actually is. Using the 5 inch achromat for an eyepiece, it gives 33X. The brightness and image are exactly what you would expect from a 1.6 inch objective telescope, bright, sharp and detailed. The field of view is very very narrow. It was only about the width of juiters moon system. There was no visible chromatic abberation, so at F 100 I guess that is no longer a factor!

Speaking of Jupiter...Very good to have that in view with this scope, it is a good object to look at. All the moons were visible, last night two were very close together, and they were two pinpoint dots. The oval shape of the planet was apparant and I do think I seen two bands. The planet was quite bright. Moving to Antares, the red star shown with pinpoint power, less flashing than usual. It was a very nice view. very crisp and bright.

Finding 'your eye' can be tough, I pushed the eyepiece lens up further into the tube so pretty much your eye rests on the tube end about five inches up from the eyepiece.

It was quick and cheap to build, and neat to see that this ancient telescope design still actually does work. Thats a photo of Jupiter taken by holding up the camera to the eyepiece area.

Posted ImagePosted Image

#2 BluewaterObserva

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Posted 03 September 2008 - 02:44 PM

To cool....

Would you think about another, maybe a little larger aperture?

I have toyed with this idea every time I see an 80mm F/30 or slower objective.

#3 Michael Miles

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Posted 03 September 2008 - 02:54 PM

I agree - way cool. Have you checked the tube droop with a laser? I know, not very 16th century...

Michael

#4 raym.

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Posted 08 September 2008 - 07:48 AM

I had my first opportunity to take a look at the Moon with this scope last evening. VERY impressive! There was no trace of residual color, and crater detail was amazing. I ramped the power up to 60X, and still had nice detail and a fairly bright image. Using a 1 inch fl. hastings triplet for an eyepiece, this gave 160X and it was amazing that though dim, crater detail was astonishing. So, thats 100X per inch, using a simple positive meniscus lens.

This has taught me that the astronomers of the late 17th century were able to use this type telescope to great advantage. Chromatic abberation had been taken care of, and the use of very high magnification was very possible.

Bracing the scope only at the balance point and the eyepiece end made it very steady, and it was easy to follow the motions by just moving the eyepiece end a tiny bit.

Very much fun, I encourage anyone who has an interest in these very early telescopes to get one of these lenses and give it a whirl.

I think that I would be able to add at least 6 more feet to the scope and still have it stay rigid. Now....only glitch is finding a quality 2 inch X 200 inch fl. positive meniscus lens!!

#5 spaceghost

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Posted 08 September 2008 - 10:54 PM

raym, how easy is it to find Jupiter in your scope? Can you sight down the tube easy enough?

#6 DAVIDG

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Posted 09 September 2008 - 09:10 AM

There is a nice article by Richard Berry in Sky and Telescope in Feb 1976 "Astronomy Neglected Child, The Long Refractor" about how he also made one. He ground the lens himself from a piece of plate glass. It also discusses how he mounted the instrument.

- Dave

#7 refractory

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Posted 09 September 2008 - 10:47 PM

One might get around a number of issues by getting a small very high quality mirrored optical flat and mounting it ahead of the objective, allowing you to keep the OTA at one orientation in terms of altitude- perhaps some sort of control so you can reorient the mirror from the eyepiece end in one or two dimensions. An old carousel of some sort for azimuth and you could stay conveniently seated.

Jess Tauber

#8 raym.

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Posted 10 September 2008 - 08:05 AM

It is very very easy to find visible stars and planets with the scope. Sighting along the tube, just like a rifle. The field of view is about the diameter of the moon at 33X. I had thought about using the mirror arrangement, but felt that would entail much more work, especially when trying to follow the motions. My shop is 70 feet long, that is, if I could find a 70 foot fl. lens, I may try the mirror arrangement. I think F 100 is the optimal situation for a simple lens. Later in the season, when Uranus comes into my narrow part of the SW sky, I'm going to make a try for it.

If anyone out there would be willing to grind me a decent 20 to 24 foot two inch lens, Id love to get one! Back in my teen years, I tried to make a lens from a flashlight cover glass, but the optical performance was horrible. That was a 21 footer, and I mounted it using a gathered together setup of oak runners my Dad had been using for boat building. It was braced on the edge of the Garage. Not a real success by any means, this new version is FAR superior.

#9 Tavi F.

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Posted 03 June 2011 - 05:10 AM

Yes, these long refractors are intresting and gives good images (specially in visual mode). I have made one such telescope for myself, in the latest version, using a 2.4m (~8ft) focal length (F) biconvex lens at 40mm opening (D)... so, obtaining a "focal ratio" of 60.
I make the comparation, in terms of chromatic aberration, between objective lenses of different diameters by defining "the chromatic aberration coefficient: x".
x=N/D,
where: N=F/D... focal ratio.
This is a simple equalization, presuming that similar glass was used for each lens.
Of course, the manufacturing precision and the profile are also important. The meniscus profile is not too good at reducing the spherical aberration, but in cases of very high focal ratio (like Surplusshed's meniscus 40mmx4000fl) this aspect is negligible.
The 17th century opticians/astronomers adopted a x=~1.6...1.65.
In my opinion, a singlet objective with x=1.4 to 1.7 gives good images. If x>=2.2, then the views through the refractor are very good indeed!
For x=1.5: a 40mm aperture lens should have N=x*D=60, but a 70mm diameter objective needs a N=105 (or F=7350mm)!

I took some photos in afocal mode, with a Canon A530 fixed behind the eyepiece (Plossl f=25mm).
First, the optical tube in provisional form:
Posted Image

1. A terestrial mosaic (levels adjustment and resized), target at 58m (~63yds):
http://img5.imagesha...ceabiserici.jpg
2. A crop100% from another frame, unprocessed... an artificial star:
http://img844.images...ala58mcr100.jpg

3. A street light at 53m (~58yds), mosaic, unprocessed:
http://img812.images...annelneprel.jpg
note: USlim means the resolving power for 40mm entrance pupil.

And finaly, some astro pictures through a slightly shorter focus lens (F=2030mm; D=38mm), all are processed.
4. Moon, mosaics of singles frames:
http://www.flickr.co...in/photostream/
http://www.flickr.co...in/photostream/
http://www.flickr.co...in/photostream/

5. Jupiter, a stack of 98 frames:
http://www.flickr.co...in/photostream/

#10 highfnum

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Posted 03 June 2011 - 05:54 AM

i did one 5 inch 40 feet
here is shot of moon thru it


http://www.cloudynig...php?photo=19933

#11 Ed Holland

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Posted 03 June 2011 - 03:16 PM

What a great thread!

This makes me interested in continuing a silly project I played with a couple of years ago - to make some simple lenses from acrylic plastic (Perspex / lucite). I used very simple tools and grinding materials made from household items (carefully supported emery paper, metal polish etc on tools made from wall patch compound) and was easily able to make convex & concave lenses - completely random focal lengths, of course. One advantage was that the material worked rapidly.

This technique would play easily into the looooong refractor experiments, since the curves required are so gentle. I think it would be quite easy to make a 3" lens. The smaller version I made (perhaps 1.5 or 2", I forget) seemed to be quite good - much better than a magnifying glass, and yielded reasonable images with a home made concave EP in a Galilean setup - I guess f/15 or f/20.

I think the viewss are so good because the objective is significantly above the atmosphere, so avoids a lot of the effects of seeing. Also the long tube gets you much closer to the target of interest ;)

Cheers,

Ed

#12 Mark Harry

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Posted 04 June 2011 - 03:20 AM

I like this thread, too! pretty good pictures, for singlets!

I designed a 60mm achromat that uses B270 crown, and SF1 flint. It has a focus of 94.43", and radii as follows:

R1- 40"
R2,3- -76"
R4- Flat

When green is in focus as a tiny dot, red and blue are almost totally within the airy disc. If anyone is interested, I can post details of plots later. It's designed to be cemented/oiled, so there's only 2 exposed glass surfaces. Glass is easy to obtain, and it's essentially apo performance with 19th century tech.
(steampunk comes to mind!)
Mark

#13 eric_zeiner

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Posted 04 June 2011 - 04:08 AM

This is a very cool thread and I would love to bang something like this together.

#14 highfnum

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Posted 04 June 2011 - 05:34 AM

Ed Jones makes a 4 inch 80 footer
most used for solar observing

#15 eric_zeiner

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Posted 04 June 2011 - 05:45 AM

Ed Jones makes a 4 inch 80 footer
most used for solar observing


:bigshock:

#16 Pinbout

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Posted 04 June 2011 - 03:46 PM

I thought it was 9.5 projection that's as good as a 4" scope.

solar projection

go down the page a while. :grin:

#17 highfnum

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Posted 04 June 2011 - 03:54 PM

look at image of sun on garage door
then look at distance to lens
80 feeties
that a 9 inch prime focus size - no eyepiece

#18 Pinbout

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Posted 04 June 2011 - 04:53 PM

after the first time I saw it, I was like...man I have to build one. :grin:

#19 highfnum

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Posted 04 June 2011 - 05:37 PM

yup
the longest i ever heard of was 600 feet FL
somewhere in France

#20 474747

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Posted 04 June 2011 - 07:53 PM

How do you mount one of these?

#21 highfnum

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Posted 04 June 2011 - 08:24 PM

the real long ones are arial
-no tube

12-60 footers can be mounted with tube and tacle mount

#22 seryddwr

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Posted 04 June 2011 - 08:39 PM

Do you have a pic of your 40 footer?

#23 Crayfordjon

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Posted 05 June 2011 - 02:46 AM

You could fold your refractor "Z" fashion, this would reduce the tube length to about a third. The flats do not have to be ultra precision, they need only be cheap commercial grade, that is to within 6 fringes not one sixth fringe as some will probably tell you. I have had a lot of experience with this and found that flats placed inside the optical systen do not have to be ultra precision and made from fused quartz etc etc. With your very long focus OG you can probably use two pieces of ordinary plate glass aluminized without any work on them at all. It is great fun to return to the old scopes that the world's greatest astronomers used to make all those fundimental discoveries. :cool: :cool:

#24 highfnum

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Posted 05 June 2011 - 04:38 AM

its set up like ed jones rig horizontal
with mirror
i had one on gallery
took it off
ill have to look

#25 highfnum

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Posted 05 June 2011 - 04:40 AM

robert hook was first to try this

but mirrors were awful in 1600's

this -> folding refractor






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