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solaryellow
member
Reged: 09/28/09
Posts: 51
Loc: New Milford CT
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I have a couple copier lens telescopes I made for fun, basically junk, but for the kids to knock it around, it is fine.
I have a 108mm Plano Convex lens that came out of a high end camera.
had a 89mm plano convex about 1" apart from it, as well as some wierd FLAT lens that was about an 1" thick sandwiched with that.
I scrapped the flat lens, and plan on using the 108mm lens by itself.
I did notice that if I hold the 89mm lens about 15 inches behind the 108mm, everything is focused nicely.
I know if I use both lenses, the focal point will be about 25 percent shorter than if I just use the single lens.
What is the best way to determine the FL of the large lens.
And if I plan on using both Plano/Convex lens, is there a method for spacing on those as well?
I am thinking I might have to get fancy and build a PVC rig with several slots that I can just rest the lenses in various spacings, and test with an EP, but perhaps there is an easier way.
I figured using both lenses may reduce Chromatic abberations as well.
Any suggestions?
Thanks..
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16" Meade Starfinder,GEM AC version. home
16" Meade SCT McCarthy Observatory (JJMO)
Tak FSQ106 JJMO
5" Meade refractor/ Coronado filters.
60mm Bushnell refractor, my first since early 80's
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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How about just focusing an image of the moon or Jupiter on a piece of paper and having a friend measure off the distance? Or do you need something more precise?
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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solaryellow
member
Reged: 09/28/09
Posts: 51
Loc: New Milford CT
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I did that using the light in my house.
held lens and focused spot on white paper.
Was 12" for 89mm and 12.5" for 108mm
So 317mm for the 108mm is ok, but I want to incorporate the 89mm in the there as well.
2 plano convex lenses, curved side pointing towards stars on both.
I figure the 2 together will work better.
But trying to get the right distance of the 2 together is where I am stuck.
The 2 could be spaced from as close to 6" to maybe 24" apart in the optical train, and still focus an image depending on how far my eye is from the 2.
Should I just take the 108mm, and using the 317mm FL, and draw the cone it would make, and measure to where the cone walls are 89mm and use that for the placement of the second lens?
In theory, I saw this layout on someone elses site and it shows the Focal point now at a point in between the second smaller lens, and the original 317mm focal length.
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16" Meade Starfinder,GEM AC version. home
16" Meade SCT McCarthy Observatory (JJMO)
Tak FSQ106 JJMO
5" Meade refractor/ Coronado filters.
60mm Bushnell refractor, my first since early 80's
Edited by solaryellow (10/18/09 01:04 AM)
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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 640
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To get the effective focal length of a combination: focus an object subtending a known angle and measure the image: then divide the width of the image by the angle to get the focal length.
Clear? - well, I thought so... A constructed example: suppose the moon is an even 0.5 deg wide as seen from your place on Earth (a good planetary program will give this angle to an unreasonable accuracy). The focused image turns out to be 3.14 mm. 0.5 deg is 0.5/(180/pi) radians : 3.14/(0.5/57.30)= 360 mm.
This method does not assume anything about placement or thickness of the lenses of a combination, but you should be able to measure the image accurately.
One way is to capture it on an imaging chip with known pixel size (like a web cam minus its lens): suppose the pixel size is 5.6 micron and you count 561 pixels: the width is 561*0.0056=3.14 mm.
Did you want it to be easy too? 
Nils Olof
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solaryellow
member
Reged: 09/28/09
Posts: 51
Loc: New Milford CT
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Quote:
To get the effective focal length of a combination: focus an object subtending a known angle and measure the image: then divide the width of the image by the angle to get the focal length.
Clear? - well, I thought so... A constructed example: suppose the moon is an even 0.5 deg wide as seen from your place on Earth (a good planetary program will give this angle to an unreasonable accuracy). The focused image turns out to be 3.14 mm. 0.5 deg is 0.5/(180/pi) radians : 3.14/(0.5/57.30)= 360 mm.
This method does not assume anything about placement or thickness of the lenses of a combination, but you should be able to measure the image accurately.
One way is to capture it on an imaging chip with known pixel size (like a web cam minus its lens): suppose the pixel size is 5.6 micron and you count 561 pixels: the width is 561*0.0056=3.14 mm.
Did you want it to be easy too?
Nils Olof
Sorry Nils, without images I am lost 
Well, I understand what your saying somewhat.
What I did was slap the 108mm in a temp cell, a 4" pvc coupler, which allows just enough room for some nylon bolts to hold it nicely.
The 89mm just happens to fit perfectly inside a 3" pvc cooupler.
With just the 108mm lens, there is a wild sort of abberations, chromatic and other nonsense, but when I put the 89mm 3" behing the 108mm, it looks pretty sharp, even to the edges.
So laying it on a table and holding an EP up to the focal point of the smaller objective, which was about 12" (305mm)
I now placed the larger 108mm (317mmFL) lens in front and made some back and forth adjustments, and realized, that the distance of the EP from the 108mm lens was still at 12" no matter where I placed the 2 lens spacing, it all still ended up with a EP about 12" back to get focus.
The only difference was, the closer the lenses were together, the further back the EP had to be.
I found that a 3" spacing of the 2 lenses allowed me to at least get the EP focuse point far enough back to get some plumbing in there to make it work.
The lens was a Melles Griot. looked like something nice to tinker with.
We will see.
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16" Meade Starfinder,GEM AC version. home
16" Meade SCT McCarthy Observatory (JJMO)
Tak FSQ106 JJMO
5" Meade refractor/ Coronado filters.
60mm Bushnell refractor, my first since early 80's
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2270
Loc: Kent, UK
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I work on efl = product/sum
so efl = (focal length A x focal length B) / (focal length A + focal length B minus the separation between them).
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
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solaryellow
member
Reged: 09/28/09
Posts: 51
Loc: New Milford CT
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I just used that formula.
108mm lens is 330mm, I remeasured using ceiling light with etch surface, so I could focus on paper more clearly.
89mm is 280
280 X 330 = 92,400
280 + 330 - 76.2mm spacing = 533.8
92,400 divided by 533.8 is about 173mm FL with that setup.
This seems to be correct, based on telescope mechanics website saying, that 2 plano convex lenses used together will reduce FL to half the distance of the single lens.
173mm is close enough to half of 330mm, so I think I have it right....
thanks for the formulas.
Dennis.
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16" Meade Starfinder,GEM AC version. home
16" Meade SCT McCarthy Observatory (JJMO)
Tak FSQ106 JJMO
5" Meade refractor/ Coronado filters.
60mm Bushnell refractor, my first since early 80's
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hwhall
member
Reged: 09/22/08
Posts: 188
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> I did that using the light in my house.
> held lens and focused spot on white paper.
To be more precise, you'd like to use a light source far enough away that the light rays can be considered parallel. A light source inside the house probably appears as slightly diverging rays. A measured FL that way ought to be a bit longer than for parallel rays.
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solaryellow
member
Reged: 09/28/09
Posts: 51
Loc: New Milford CT
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Thanks, I will try that when the moon or sun is out.
It has been cloudy.
I understand what you mean about inside light, but I was able to see the etchwork design come to focus nicely from the light fixture itself, and not so much the light.
It kind of had me thinking of some other ways to test, like a decal on a window and letting the light shine in, and focusing the image on the wall opposite the window.
But your right, something further away will be more accurate.
--------------------
16" Meade Starfinder,GEM AC version. home
16" Meade SCT McCarthy Observatory (JJMO)
Tak FSQ106 JJMO
5" Meade refractor/ Coronado filters.
60mm Bushnell refractor, my first since early 80's
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