Afocal system

[Steve King]

I am a new user to the free OSLO program.

I am setting up a Galilean telescope focussed at a finite distance (11
feet). I want the system to have an angular magnification of 3x but
can't figure out how to do it. I'm tracing rays "backwards" by going
through a 3mm radius aperture (simulating the eye iris) then going
through the negative lens then through the positive objective. I have
tried used the GENII Ray Aberrations (from Optimize menu). This does
optimize, but, the magnification does not stay at ~3x. I have tried
both AFOCAL and FOCAL modes without success.

Any suggestions?

Steve

[Dr. Eck]

Gallilean telescopes aren't used much any more because the exit pupil
position is virtual, so you end up with a very limited field of view,
but I'll assume that you have a good reason to use one. To make a
Gallilean telescope, you need a plano-convex (PCX) and a plano-concave
(PCV) lens. For a 3X magnification, the PCX lens needs to have a
focal length 3X that of the PCV. Let's start with a focal length of
75 mm for the PCX and -25 for the PCV.

The diameter of the PCV lens needs to be big enough for the 6 mm
diameter of the eye's iris plus some edge space for mounting, so let's
make it 10 mm diameter. The PCX lens needs to be at least 3X 6 mm
diameter, so we'll pick 25 mm, which is readily available. A very
common optical glass is BK7, which has a refractive index of 1.517, so
we'll use that.

To calculate the radius of curvature for the two curved surfaces, just
multiply the focal length by the refractive index, so we get 38.775
for the PCX and 12.925 for the PCV. Thickness is not very critical,
so we can make it 4 mm for the PCX and 2 mm for the PCV.

The only thing left is the distance between the two lenses, which will
change depending on the distance from the object to the telescope. We
can start with the sum of the focal lengths (75mm + -25mm = 50mm) and
let optimization get it right.

Here's the lens file, assuming an infinite object distance:

// OSLO 6.6 48454 6878 5836
LEN NEW "No name" 0 5
EBR 9.0
ANG 0.0000572957795
DES "OSLO"
UNI 1.0
SNO6 "geniierf_lt 0.206997208521 25.0 1.00 -0.90 0.90 0.80 -0.80 0.80
0.70 1.0 1.0"
// SRF 0
AIR
TH 1.0e+20
AP 9.9999999977e+13
NXT // SRF 1
GLA BK7
RD 38.775
TH 4.0
AP 12.0
NXT // SRF 2
AIR
TH 46.0636433075063
AP 12.0
NXT // SRF 3
GLA BK7
TH 2.0
AP 5.0
NXT // SRF 4
AIR
RD 12.925
TH 10.0
AP 5.0
NXT // SRF 5
AIR
AFO 1
AMO ANG
WV 0.58756 0.48613 0.65627
WW 1.0 1.0 1.0
END 5
DLRS 3
OPDF 1.0e-12
OPOC "opcb_abs"
VAR NEW
V 1 2 0 TH 0.1 10000.0 1.0 0.0009
END
OPE NEW
O 2 "OCM2" 1.0 "PU"
END

Save the above text as Gallilean.len and open it in OSLO. In the lens
spreadsheet, you'll see that thickness 2 is variable. And if you open
the Optimize>Operands dialog, you see that the only aberration with a
non-zero weight is PU, the marginal ray angle. We want this to be
zero for collimated exit light, so the target is just OCM2.

To optimize for your finite object distance, change the OBJ thickness
to the appropriate number and click on Ite at the top of the text
window.

HTH

Steve Eckhardt

On Apr 25, 9:22 pm, Steve King <steveandkathyk...@sbcglobal.net>
wrote:

[Stephen King]

Steve, Thank you for taking the time to provide this detailed and complete answer to my question. I loaded your lens file and ran it in OSLO.  If i only allowed the space between the lenses to be variable the optimization worked fine and the magnification remained at 3x.  However, when I added all four lens surfaces as variables the optimization no longer held the magnification fixed at 3x. I found the OPIC for OSLO - EDU optimization file and saved it to the Private\ccl directory.  I then used OPIC to optimize.   I setup the system "backwards" with the eyepiece first and the objective second.  I placed a 6mm diameter aperture (to simulate the eye iris) 6mm before the eyepiece.  I used the Focal mode with the object at infinity and the image at 3350mm (my desired imaging distance).  A max field angle of 12 degrees (eyepiece side) was assumed.  The final magnification I settled on was 2.4x, so, I set the OPIC operand #3 [Chief Ray y-height (PYC)] target as      OCM3 - 297 where                     [tan(12)/2.4] x 3350 = 297. A weight of 10 was sufficient to maintain the magnification at 2.4.  I also put a weight on the overall lens length to prevent the system from getting too long. With this approach I was able to maintain the magnification while optimizing all four lens surfaces and the lens spacing.  Ultimately I ended up going to an achromatic objective which reduced the aberrations by ~5. Using OPIC to optimize provided the needed paraxial targets.  I'm not very familiar with programming in OSLO so this was the easiest solution for me. Hope this makes some sense. Thanks again for taking the time to help.  I don't know if you ever did any teaching, but, your clear style would make you a very good teacher. Regards, Stephen King Retired Physicist Raytek Corp. --- On Mon, 5/7/12, Dr. Eck <dr.e...@gmail.com> wrote:

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