DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Tim,
I would double check the focal length on the Moon or the Sun. Even though the hill was a couple of miles away it wasn't at infinity and that will make a difference in the focal length at infinity. I'm a little surprised you came up with 86" vs something closer to, or actually greater then 90" since the lens is suppose to by f/15. Lens maker are usually really good at hitting radii or the performance of the lens will greatly surface. Usually the focal length is off by no more then 1% or less. Also since the hill wasn't at infinity, the focal length you measured is longer then what it will be when the lens is focused on an object that is really at infinity. Are you sure you had crown element facing skywards ?
I calculated that if the lens had 90" Fl at infinity and the hill was really 1.5 miles away, the focal plane should be at 90.9". ( Correction, it should be 90.085", Thanks Mike)
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
Edited by DAVIDG (07/02/09 10:31 PM)
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1571
Loc: Fort Worth TX
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Actually the focal plane shift for a 90" EFL paraxial lens imaging at 1.5 miles is 0.085" (image distance is 90.085"). Think you left out a zero 
Mike
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Quote:
Actually the focal plane shift for a 90" EFL paraxial lens imaging at 1.5 miles is 0.085" (image distance is 90.085"). Think you left out a zero
Mike
Thanks Mike, sure did drop a zero.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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hehe
Okay, well I only got so far as to cut out my baffle/bulkheads, I haven't cut the walnut plywood yet.
It's marvelous out tonight, so I will set up again after dark.
Hey, downtown LA has all kinds of lights and is about 7 miles in a straight line from my hilltop. How bout I try that?
I like it because I can set up horizontally.
-Tim
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Oh, by the way...
My 6" f/10 has a 54" tube. The cell is mounted recessed in the sky end, and I have a University Optics 2" focuser on the other end. That thing is almost identical to a Meade 680, so it's about 4" high by itself, and with another 3.5" added (needed) for a 2" diagonal, I'm right up around 61.5" from the center of the lens.
I'll still give it a check tonight, though.
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Okay, sighting on downtown lights, I get 89 inches from the back of the cell to the base of the RKE 28mm the stop where it goes in the focuser).
THe cell is 2.5" wide, so if the objective is considered in the middle of that, then I get 91.25" focal length, or f/15.2. If f is measured from the rear of the achromat, then the FL is 89", or f/14.8.
Safe to assume f/15 and build accordingly?
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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I would say it's a f/15 and build to that focal length. By the way you can put those lights from LA to good use. You can get a good feel for the spherical correction of the lens by looking at a Mercury street-light as far away as possible. Use a green eyepiece filter and ronchi grating with at least 100 lpi. The mercury line at 546nm will be isolated by the green filter and that is were your lens should be corrected it show no spherical abberation so with three ronchi lines showing they should be nice and straight.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8305
Loc: Bremerton Washington
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Quote:
I would say it's a f/15 and build to that focal length. By the way you can put those lights from LA to good use. You can get a good feel for the spherical correction of the lens by looking at a Mercury street-light as far away as possible. Use a green eyepiece filter and ronchi grating with at least 100 lpi. The mercury line at 546nm with be isolated by the green filter and that is were your lens should be corrected it show no spherical abberation so with three ronchi lines showing they should be nice and straight.
- Dave
Now that's a slick trick Dave.
Too bad I don't have any street lights close by... 
No...that's a good thing. 
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Well, I'm definitely making this up as I go along!
I was going to use this "workbench" to lay up the tube slats, but it's so hot out there that the 1/4" plywood will likely warp out of shape if I do...
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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So, I just used it to cut the plywood to the length of the tube, plus a quarter inch or so for trimming.
It's Walnut plywood, but it's got mdf as a core, which I didn't notice when I brought it home. This means I need to be pretty careful working with it, lest I go through the thin veneer while working on it.
I had bought a new carbide blade for my table saw, and ripped the slats on it, with a 20 degree bevel (9 sided tube).
Yesterday, I made these "bulkheads/baffles" out of some 3/4" chinese plywood scraps I had laying around. I decided to use these instead of some 1/4" plywood, so I'd have some strength for things like rings and/or tube weights and guide scopes, and I'm spacing them every 12" just so I'll remember where they are when the tube is together!
But I quickly remembered the good old days when I made my 8" Springfield tube this way... The imprecision of making the bulkheads means I'm going to wind up a little big as I wrap the slats around them, or worse, a little small. So, I plan to tape up 5 of the 9 sides first, and see if the bulkheads will fit without either spreading the sides apart or leaving a gap on the inside between the bulkheads and slats.
If they do (either thing), I'll cut them into "gussets" so I get a good fit at all the corners.
I numbered every facet of the bulkheads and each side so I know where everything goes. Now, I'm going to go out there and glue up the first 5 sides.
Here's a pic of the baffles and the pattern I made them from, on my neatly-kept saw table!
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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plyscope
sage
Reged: 11/23/06
Posts: 326
Loc: Perth, West Australia
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Looking good Tim.
A trick I used was to make a pattern from thin sheet metal and then clamp all the baffles and pattern together and sand the sides of the baffles down to the metal edge of the pattern. The baffles were close enough in size then that the tube sides were all equal.
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Andy
6" f15 refractor
90mm f16.7 refractor
Edited by plyscope (07/03/09 05:33 PM)
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Andy:
That's a good suggestion. I may try that as opposed to cutting my baffles up. On the other hand, if I cut them into thirds, maybe they'll all happily fit without any additional sanding.
Here's the first 5 slats taped up:
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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And here's where I dry-labbed them together with the first baffle. Already, there's error before I get to the last slat on the left, and the next one on the right is going to have issues as well...
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Hm....
I sanded these to the tracing of the pattern on a 12" disk sander. I wonder if I were to just take all the baffles and sand the line off, if that would be enough for it all to fit?
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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I think I'll go and tape up the remaining 4 sides so I can lay them on top of the other 5 with the baffle in place. then I'll try sanding that baffle to where it fits, and when it does, go for it with the rest of them!
-T
im.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Last dry lab clampup before I pour glue over everything!
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Glue in the grooves!
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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First "half" glued up.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Now, I'm pretty sure I don't have any flat black paint that I can brush on. Only spray cans.
May have to hop in the Model A and head off to the hardware store for some.
I do have some satin house paint, but it's latex. if I thought the rustoleum would stick to it, I'd brush that on, then spray down the tube once it's all together to knock down the "highlights". But I'm pretty sure it won't stick.
-tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1421
Loc: Highland Park, CA
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Hah! Found some. A quart of flat rustoleum.
So, here I am painting the inside of the first half.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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