Homebuilt 17th century Looooong Refractor
Posted 03 September 2008 - 11:39 AM
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 03 September 2008 - 02:44 PM
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.
Posted 03 September 2008 - 02:54 PM
Posted 08 September 2008 - 07:48 AM
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!!
Posted 08 September 2008 - 10:54 PM
Posted 09 September 2008 - 09:10 AM
Posted 09 September 2008 - 10:47 PM
Posted 10 September 2008 - 08:05 AM
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.
Posted 03 June 2011 - 05:10 AM
I make the comparation, in terms of chromatic aberration, between objective lenses of different diameters by defining "the chromatic aberration coefficient: x".
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:
1. A terestrial mosaic (levels adjustment and resized), target at 58m (~63yds):
2. A crop100% from another frame, unprocessed... an artificial star:
3. A street light at 53m (~58yds), mosaic, unprocessed:
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:
5. Jupiter, a stack of 98 frames:
Posted 03 June 2011 - 03:16 PM
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
Posted 04 June 2011 - 03:20 AM
I designed a 60mm achromat that uses B270 crown, and SF1 flint. It has a focus of 94.43", and radii as follows:
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!)
Posted 04 June 2011 - 04:08 AM
Posted 04 June 2011 - 05:34 AM
most used for solar observing
Posted 04 June 2011 - 05:45 AM
Ed Jones makes a 4 inch 80 footer
most used for solar observing
Posted 04 June 2011 - 03:54 PM
then look at distance to lens
that a 9 inch prime focus size - no eyepiece
Posted 04 June 2011 - 04:53 PM
Posted 04 June 2011 - 05:37 PM
the longest i ever heard of was 600 feet FL
somewhere in France
Posted 04 June 2011 - 08:24 PM
12-60 footers can be mounted with tube and tacle mount
Posted 05 June 2011 - 02:46 AM
Posted 05 June 2011 - 04:38 AM
i had one on gallery
took it off
ill have to look
Posted 05 June 2011 - 04:40 AM
but mirrors were awful in 1600's
this -> folding refractor