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tommyhawk13
professor emeritus
Reged: 09/28/07
Posts: 693
Loc: Jacksonville, Fl
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I've acquired a beast of a lens that was advertised as 5" diameter and 18" focal length from a Buhl Mark IV opaque projector. If the advertised numbers are correct, that's a fast f/3.5 with plenty of light gathering capability.
I have no idea how bad the coma will be, but the color seemed good to my eyes when looking at the moon.
It came with a 1/8" aluminum plate that I clamped to a big tripod, and aimed it at the moon. I had to hold my head back about 32" to reach focus.
I plan on mounting the lens on the end of an 8" sonotube, and putting a home made 4x5 back on the other end to make a rotating astrograph. Does this seem like a worthwhile project, or am I dreaming?
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Meade Starfinder 8,Meade SN-8 OTA, Orion Atlas, and a handfull of film cameras
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Mike Lang
member
Reged: 07/25/09
Posts: 15
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Tom:
I believe I have the same (or similar) lens - 5" 18 inch focal length (F3.6) - made by Baush & Lomb. I acquired it about 30 years ago at a trash dump - it was in perfect condition. It was in a cardboard box that I kicked as I walked by, the box hardly budged, so I picked it up, openned it up, got a big ole' grin, and I'm sure my eyes sparkled like I had just come across the Hope Diamond. The lens is coated a nice blue color. The black anodized lens cell is mounted in a short length of PVC pipe. I made a Plossl eyepiece from a pair of binocular objectives, a diagonal from a mil-spec front surface mirror (in an aluminum box, duct-taped together), and a plumbing-parts helical focuser. The thing looks like an absolute nightmare and weighs 15 pounds, but the views through it are incredible. Only problem is it suffers from kidney bean - if your eye isn't perfectly centered over the eyepiece. But I "fixed" that problem by placing a pipe cap over the eyepiece (about a half inch above the lens) with a quarter inch hole in the middle of the cap. You can press your eye right up to the cap without touching the glass, and get a really wide field low power view. I've used it for looking at comets and other wide field objects (Andromeda is really nice in it from a dark sky). The Moon will blind you, and, there's no CA - it's really unbelievable ! (well, it is really low power) I think it's only about 9X, but when it's collimated and focused, it's surprising how nice the views are. (of course, when it's off, it's just silly bad!) I tried every kind of eyepiece I could get my hands on over the years, Televues, Meade SWA's, etc... (didn't try an Augen - just didn't seem right to put something that expensive on this thing). The only thing that works is my cobbled up Plossl (the lenses are edge-blacked, and masking taped together with a Gnats hair between them). I would suggest checking a nearby plumbing supply or a plumber who works big industrial jobs and see if they have a scrap pile. That's where I got my tubing - from a job site that had a bunch of cutoff pieces. Then I got a cap at a plumbing supply, and the other parts were Home Depot. I've got it on a big ugly green military surplus fluid head plus tripod that looks like it carried a camera to take pictures in WW2, but it holds this beefy thing quite well. I call it my "Sewer Scope" because the "OTA" still has the manufacturers original words "6 Inch Sewer" painted on it. Front lens cap is from a big coffee pot, and the cap on the eyepiece is bright orange, from a can of shaving cream. My wife even likes it in the living room because it's so "Road Warrior". Star parties are always fun because folks look at it and laugh, then look through it and get all teary-eyed (especially when I tell them it all cost about 20 bucks) I think you're gonna like what you've got, because you have the right kind of attitude - calling it a "beast of a lens".
I'll post a picture of my conflaburation in the next few days, for motivation, or fear.
Mike
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tommyhawk13
professor emeritus
Reged: 09/28/07
Posts: 693
Loc: Jacksonville, Fl
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Thanks, Mike. No fear here. My "camerera" will look like the unwanted child of a newtonian reflector and a 5" refractor. I found a concrete form tube at Home Depot that fits perfectly in Orion's 8" rings. Apparently their 8" forms are shipped in various dimensions so they can slip one inside the other.
For now, I am going to keep an eye out for a 6" sewer pipe just to work out the true focal length, and I'll eventually work up a focuser for it.
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Meade Starfinder 8,Meade SN-8 OTA, Orion Atlas, and a handfull of film cameras
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Mike,
Little wonder the views are so nice. By using a pair of bino objectives, the focal length of the eyepiece you made must be no longer than about 60mm. Assuming this to be the case, the magnification would be 7.6X. If your eyes have pupils that open up to a full 7mm, the working aperture of the 'scope is at most 53mm. That's a far cry from the real aperture of 127mm, and represents a huge amount of completely unused glass and a lot of dead weight. You might as well have used a small 'scope's 50mm objective with a focal length of 450-500mm (F/9-10); it would deliver essentially identical views in a *vastly* lighter package.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Mike Lang
member
Reged: 07/25/09
Posts: 15
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Glenn:
Fascinating ! (however, what a bummer) So any ideas how to remedy this ? I have tried (a long time ago) different eyepieces that were supposed to work down to around F4 - but the images were lousy as I recall (coma, CA, couldn't focus). But thinking back on that experience, probably 20 years ago, I didn't appreciate how critical collimation is for a "telescope" this fast. So I'm wondering if I were to go back and redesign this thing so I can adjust collimation, if those eyepieces would actually provide a usable image, and without throwing away all the photons.
Ideas ? And by the way, the lens was made by "Projection Optics" - not B&L as I stated previously.
And here's a picture of the (apparently, aptly named) "Sewer Scope" ...
And, obtw, I've been testing an SV 80ED, and the low power image of the Moon is brighter in that scope than in the 127mm SS - duh ! (on me)
thanks !
Mike
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hwhall
member
Reged: 09/22/08
Posts: 188
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> the lens was made by "Projection Optics"
I have a Projection Optics lens that I would also like to do something with: 6 inch diameter, 30 inch focal length. "Transpaque" is also part of the label. The cell outer diameter is 6 3/8 inch & finding a nice fit tube has been a holdup. Most of the PVC sizes would need lots of shimming to get a fit.
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Mike Lang
member
Reged: 07/25/09
Posts: 15
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My PVC tube is 5.5 inches ID, and the lens is just a hair over 5 inches OD. So I cut rings from the PVC, split them so I could slide them into the OTA tube, and the lens assembly fits inside those rings (at each end of the lens assembly) perfectly. Then I have a set of two rings stacked on each other at the outside ends of the lens assembly, to keep it from sliding out of the OTA tube. But your lens is an odd diameter, so you might have to go with something like sonotube with plywood retainer rings, and wood shims to center the lens. Puzzling over Glenn's earlier post, I would suggest you pay more attention to collimation than I did, and make the focuser end of your scope be adjustable to be able to precisely center the optical axis of the focuser to the lens, and be able to "tip&tilt the focuser to get the optical axis through the focuser parallel and bore-sighted thru the optical axis of the lens assembly. At F5, you're that lens of yours should be "easier" to design a telescope arond than the F3.6 lenses Tom & I have.
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tommyhawk13
professor emeritus
Reged: 09/28/07
Posts: 693
Loc: Jacksonville, Fl
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I plan on using an oversized sonotube for the body, and 2 plywood discs with 5 3/8" centered holes to mount the lens.
The back will have a fixed plywood base with a very large opening, a second similar piece supported by 3 pieces of all-thread for collimation and precise focusing.
For the 4 x 5 back, I need to have a piece of etched or ground glass to focus at the point where the film will be.
My first thoughts for the actual back were to have the film sandwiched between a piece of clear glass, and a piece of 3/8" plywood laminated with black formica. I have to find a way to keep it light sealed until I mount it in the back of the camera. The normal 4 x 5 film holders have a peice of plastic that slides out when it is ready for exposure.
I'd just use thta, but it would have to have a vacuum pump to keep the film flat for long exposures, hence the glass sandwich.
Lots more to think about.
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Meade Starfinder 8,Meade SN-8 OTA, Orion Atlas, and a handfull of film cameras
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Mike Lang
member
Reged: 07/25/09
Posts: 15
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How about vacuum ? I came across vacuum pumps in the grocery store the other day. They are inteneded to be used for drawing the air out of sandwich bags - probably wouldn't pull much of a vacuum, but enough for your purposes if you had a film plate with lots of holes in it.
Search on JSP Astrocamera Vacuum Filmholder for clues.
But have you considered using a 35mm camera ? Something like an OM-1 with a Beattie Intenscreen would be a good start - if only to test your lens. Used OM-1's are going cheap these days.
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Andy Taylor
twistin' by the pool
Reged: 09/24/08
Posts: 437
Loc: t=0 UK
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Quote:
How about vacuum ? I came across vacuum pumps in the grocery store the other day. They are inteneded to be used for drawing the air out of sandwich bags - probably wouldn't pull much of a vacuum, but enough for your purposes if you had a film plate with lots of holes in it.
Search on JSP Astrocamera Vacuum Filmholder for clues.
But have you considered using a 35mm camera ? Something like an OM-1 with a Beattie Intenscreen would be a good start - if only to test your lens. Used OM-1's are going cheap these days.
I've heard of people using old refrigerator compressors as rough vacuum pumps...
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Equipment list of shame:
A strange heap of assorted junk that when thrown together and dragged out into the dark shows me the wonders of the universe...
And then dews up...
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tommyhawk13
professor emeritus
Reged: 09/28/07
Posts: 693
Loc: Jacksonville, Fl
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Quote:
But have you considered using a 35mm camera ? Something like an OM-1 with a Beattie Intenscreen would be a good start - if only to test your lens. Used OM-1's are going cheap these days.
I'm afraid that using 35mm would only tell me how the center of the lens performs.
I've made a lot of improvement using the Om-1 and a Yashica 35mm with prime lenses, and the SN-8, but I want to branch out to large format.
Using a vacuum pump like the ones suggeted, or an aquarium pump would allow me to use a real 4x5 back, and the ground glass would be there, I'm just trying to think outside the box, and cheaper. Maybe something will pop up on ebay.
Edited by tommyhawk13 (10/05/09 06:11 PM)
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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When doing some 4X5 experimenting in years past, I made a 'focusing screen' by taking an exposed and developed astro negative and putting it in a 4X5 holder. The holder had several holes drilled through it and positioned strategically across the field, which served as sighting holes for setting focus and determining if the rig was squared-on. Using a 7X loupe, the film grain worked reasonably well as a 'ground glass', and moreover was necessarily at the same focal position as an undeveloped sheet.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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