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Subluminal
journeyman
Reged: 12/16/08
Posts: 8
Loc: Scotland UK
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Quick question about where to measure the FL of a doublet from when designing the tube; is it from the outer or inner surfaces or the mid point. The doublet I have is 48" FL and relatively thick, being a periscope lens, so where I measure from could have quite a significant effect.
Any answers muchly appreciated 
Thanks
Pete
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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The back focal length can be directly measured, and is what you use to set your tube length for your focuser, and your internal baffles (see attached diagram). Aim the lens at a very distant streetlight, the moon, etc. and measure from the back of the lens to focus.
For conventional achromats, the effective focal length (EFL) is slightly longer than the BFL. If you had the optical design (which we don't) you could calculate the EFL.
However, for a lens of unknown prescription, it's easy to measure EFL very precisely. Digitally photograph a pair of stars of known angular separation, preferably spanning 1/3 to 1/2 of your camera FOV. You need to know the pixel sizes of your camera in microns, and have software that can directly give you the (row,column) at each pixel. You also need software that can give you the angular separation between any two stars as one of its tools, but all the ones I'm familiar with can do that.
The separation of the star images in pixels is
D = SQRT[ (Star2 X - Star1 X)^2 + (Star2 Y - Star1 Y)^2]
The EFL is then
EFL (mm) = 206.2648D / (star separation in arc seconds)
Take several shots of different star pairs (Orion's belt stars, Pleiades, whatever will fit in your FOV), then statistically average the answers and you'll likely know your EFL to three decimal places.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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Subluminal
journeyman
Reged: 12/16/08
Posts: 8
Loc: Scotland UK
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Hi Mike
Thanks for the response, I do actually have drawings for these lenses that give radii for each surface, thicknesses for each lens etc and the FL when combined with the other element of the doublet i.e. 48". Not sure if they give refractive index tho I'll have to check. 
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Woo hoo  Put 'em up heyah, lessee what ya got. First guess would be BK7/F2 or BK7/F4, but could also be BaK1/F2, BaK1/SF5, and so forth. I would guess if you have the radii, thicknesses and airspace you probably have the glass data as well. 
Mike
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Subluminal
journeyman
Reged: 12/16/08
Posts: 8
Loc: Scotland UK
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Hi again, afraid these drawings didn't scan well so just going type all the info in...
Flint:
Material = EDF
Nd = 1.65108
v = 33.56
Combined focal length (with crown) = 44.7686"
Surface Regularity = 2 Fringes or better
r1 (Concave face) = 14.560"
r2 (Convex face) = 40.550"
d = .800
diameter = 8.150"
both faces AR coated (5350A)
Crown:
Material = LBC
Nd = 1.54065
v = 59.54
Combined focal length (with flint) = 44.7686"
Surface Regularity = 2 Fringes or better
r1 (Convex face) = 38.650"
r2 (Convex face) = 14.560"
d = 1.500
diameter = 8.150"
both faces AR coated (5350A)
As far as airspace goes I dont have any of the shims but from what I remember they were about thumbnail thickness so 25thou'ish??
I'll post a pic of them tomorrow.
Edited by Subluminal (11/08/09 05:46 PM)
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 970
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If you have access to an optical bench equipped with a nodal slide, you can quickly find the node location by rotating the table until the image is stationary. ( Is that not how it is done? I read about it, but have not practiced it). I have several Gaertner 1.5 meter optical benches. They are just sitting.
You can read about this method in ATM III, in the optical bench section by Horace Selby (RIP), with whom I once or twice spoke, but never met.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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I keyed your refractor into ZEMAX, using the six-digit glass codes you provided, and optimized the airspace. It came out 0.059", not far from what you remembered. The EFL is 44.61" and the BFL is 43.72".
The color correction is probably not good enough for planetary or high-power lunar viewing, or any CCD work. But it should give very nice low power views. You could use an eyepiece with an EFL as high as 35mm with this f/5.6 lens, which would give about 32X viewing. It will probably give pleasing views up to 80X or so.
Mike
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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This is your on-axis longitudinal color. The horizontal axis is the optical axis, and the vertical axis is the distance up in your lens pupil from the optical axis.
Notice that the blue and red light intersect at about 2.8" up in the pupil, or at about the 70% zone. That's good, although when designing I prefer to pinch them more at the 80-85% zone or so to balance the residual color blur against overall pupil area.
Green light kind of hangs with the blue light near the axis then begins to focus closer toward the pupil edge. Red and green have about the same undercorrected spherical aberration shape, while blue light is overcorrected.
Again, this should be a fine objective for low to medium power viewing.
Mike
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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And of course how could I resist optimizing the design? Using the same six-digit glasses, and letting the internal radii be different, I get the attached optimized longitudinal performance, with zero coma over the field. Notice the green light has essentially zero spherical aberration, while the blue and red cross each other at the 80% zone, which pinches the foci together better high up in the pupil.
It's an improvement over the original design, but even this tweaked design would not be all that hot at higher powers, say above 150-200X, due to residual unavoidable secondary spectrum. To improve this design one would have to change glasses to fluorites and abnormal flints, which would drive up the objective cost tenfold or more.
Let us know your viewing experiences with your objective when you get it up and mounted!
Mike
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Solar B
scholastic sledgehammer
Reged: 02/28/07
Posts: 916
Loc: By Edinburgh , Scotland
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WOW  incredible work Mike....well done indeed
As i live near Pete i will have the pleasure
of seeing this beastie when operational,i think Petes got
quite a bit of work to do on it yet.
Still it should,nt be too long until we can say
up periscope 
Brian
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" Gentlemen only ever use Refractors "
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Subluminal
journeyman
Reged: 12/16/08
Posts: 8
Loc: Scotland UK
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Hi Mike, thanks for all that super info, I'll strive to make good use of it. I've attached the promised picture and I know the first thing your going to say is it's flat at the bottom, its a feature of some periscope objectives the rest of the space is used for a conduit to get services and cables up the periscope. My idea is to fit a mask in front of the objective that either rounds the two corners out or makes a best fit (the "diameter" at 90 deg to the chord is 7.365").
I did get a very quick test done on this objective some while ago, it also suggested it might be quite good at low power, but we noticed that the image of the point source had two tails, the most likely cause of which was the corners of the "D", hence the idea to blank these areas off. The baffles would have to be similarly off-centre, but I think I have a way of controlling their position.
On the plus side I've found the airspace shims they're still attached
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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OK - good on the shims - will be interested to see what their thickness measures to and if it's 0.059" as the optimization suggests.
The missing flat side makes very little difference in transmission and star imagery, as you can see from the attached simulation. I'd just use it as is. You might need to blacken the flat sides with a Sharpie if you see any scattering off the Moon.
Up periscope! 
Mike
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Subluminal
journeyman
Reged: 12/16/08
Posts: 8
Loc: Scotland UK
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Thanks again Mike for all the info, very much appreciated
I'll post progress as I get on with it.
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