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what is the focal length of a K&E 2022 alignment telescope?

rimcanyon

Diamond
Joined
Sep 28, 2002
Location
Salinas, CA USA
I looked on the instrument and in the manual, and cannot locate the specification for the focal length of a K&E 2022. I would like to be able to convert optical micrometer readings into angles, but to do that I need the instrument's focal length.
 
I do not have, nor have I seen reference to this value, But I just took a look at a cut away of the telescope construction, and scaling from the illustration, the distance between the objective and focusing lens centers is proportional to the barrel length as 85 is to 200.

Not much help, but something to begin from
 
Maybe this will help. I am NOT in any way associated with this company. For information only.

Paul

This 71-2022 Autocollimating Alignment Telescope is a K&E (Keuffel & Esser), but it is now made by Brunson. This one is fresh out of Calibration at the Brunson Cal Lab, and can be shipped to you immediately. Yes, this means this one can be overnighted to you by FEDEX! How fast depends on how much you are willing to pay for shipment. This is in a new Pelican case, and it meets all specifications for a 71-2022, which are: Magnification of 4X at zero focus to 46X at infinity Focusing range of zero to infinity Resolving power is 3.4 seconds of arc at infinity Field of View of 37 minutes at infinity, 42mm at zero focus Effective Aperture is 47mm Fully Coated Optics Cross-Pattern Reticle-Single lines Top and Right Fully-Erect Image in the eyepiece, which has diopter scale Optical Micrometers +/- .050", direct reading to .001" Telescope Barrel hardened & Stabilized Chromed Tool Steel Barrel Diameter of 2.2498" +0, -.0003"- A.I.A. Specs. Overall Length of 17 3/4" NSN 6650-00-021-9555

http://www.lightglassoptics.com/71-...by-KE-with-Calibration-Certificate_p_688.html
 
I do not have, nor have I seen reference to this value, But I just took a look at a cut away of the telescope construction, and scaling from the illustration, the distance between the objective and focusing lens centers is proportional to the barrel length as 85 is to 200.

Not much help, but something to begin from

Cal, I appreciate the hint, but I am not sure what to do with it. I assume there is some way to calculate the focal length based on the geometry, but I am not an optical engineer.

Paul, I have also seen the same kinds of information in the specs of the K&E manual for the 2022, but focal length is conspicuously absent. K&E used to sell an angle measuring attachment for the 2022, but that would not be necessary if one used the optical micrometer in conjunction with the focal length of the lens.

I can always try calling Brunson and see if they will tell me.
 
The barrel length on the RT&H scope in my inventory is 390mm
Scaling from the cut away, 200is to 85 as 390 is to 165.75. 165.75 being the distance from the center of the objective lens to the center of the focusing lense.

Are you not able to apply a gradient conversion to your set up due to unknown distances to the target? inches per inch and all that?

i.e. 10 min of arc is equal to .0029 inches per inch displacement. and so forth.
 
But I just took a look at a cut away of the telescope construction, and scaling from the illustration...
Sorry, but the physical dimensions of the lens are not related in any way to the location of the first and second principal planes. Those are the points that are used in the optical design.

Specifically, the intersection of the first plane with the optical axis defines the first principal point or "front node", designated H or H1, while the intersection of the second plane with the axis defines the second principal point or "rear node" (H' or H2).

By definition, all rays exiting the lens appear to emanate from the second principal point H2.

When focused on a subject at infinity, the distance from the rear node to the plane of focus is defined as the lens focal length.

The lens nodes are theoretical points, defined during lens design. They have no physical identity, i.e. they cannot be "seen". Consequently those positions can only be measured with optical instruments.

The location of the nodal points are not in any way constrained to be within the physical body of the lens. In particular, with long focal length lenses, the rear node may be way out in front of the lens.

- Leigh
 
The barrel length on the RT&H scope in my inventory is 390mm
Scaling from the cut away, 200is to 85 as 390 is to 165.75. 165.75 being the distance from the center of the objective lens to the center of the focusing lense.

Are you not able to apply a gradient conversion to your set up due to unknown distances to the target? inches per inch and all that?

i.e. 10 min of arc is equal to .0029 inches per inch displacement. and so forth.

Cal, my understanding of the problem may be wrong, but here it is:

An autocollimator focussed at infinity can measure angular displacements of the reflected image using the built-in optical micrometer. Those readings are independent of the distance from target to autocollimator (e.g. if I double the distance to the target, the angular displacement remains the same, and the micrometer reading is the same). However, the optical micrometer reading is in inches, not degrees. So to calculate the angular displacement, one needs to know the focal length of the instrument. I suppose that if I had an optical wedge and a projection collimator I could install the wedge on the front of the instrument and the micrometer reading could be used to calculate the focal length, but I do not have an optical wedge.
 
Sorry, but the physical dimensions of the lens are not related in any way to the location of the first and second principal planes. Those are the points that are used in the optical design.

Specifically, the intersection of the first plane with the optical axis defines the first principal point or "front node", designated H or H1, while the intersection of the second plane with the axis defines the second principal point or "rear node" (H' or H2).

By definition, all rays exiting the lens appear to emanate from the second principal point H2.

When focused on a subject at infinity, the distance from the rear node to the plane of focus is defined as the lens focal length.

The lens nodes are theoretical points, defined during lens design. They have no physical identity, i.e. they cannot be "seen". Consequently those positions can only be measured with optical instruments.

The location of the nodal points are not in any way constrained to be within the physical body of the lens. In particular, with long focal length lenses, the rear node may be way out in front of the lens.

- Leigh

One could apply KISS and use a discription like this:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/foclen.html

Useful for a thin lens.
And as the information given is not for the instrument in question, Exact values are not relevant, only suggestive.


 
Cal, my understanding of the problem may be wrong, but here it is:

An autocollimator focussed at infinity can measure angular displacements of the reflected image using the built-in optical micrometer. Those readings are independent of the distance from target to autocollimator (e.g. if I double the distance to the target, the angular displacement remains the same, and the micrometer reading is the same). However, the optical micrometer reading is in inches, not degrees. So to calculate the angular displacement, one needs to know the focal length of the instrument. I suppose that if I had an optical wedge and a projection collimator I could install the wedge on the front of the instrument and the micrometer reading could be used to calculate the focal length, but I do not have an optical wedge.

There is an attachment lens that is fitted to the objective end of the barrel to allow direct angular readings.
Very convenient for direct evaluation of wedge in any "plano" optical element such as an interference filter.
Somewhat hard to find inexpensively. IIRC typical cost is north of $1200 when you can find one.

K&E part number 71 2302

When fitted, micrometers read directly to 1 second of arc over a 50 arc sec range.

I would like to know the optics specification of that accessory! ;-)
 
Useful for a thin lens.
Hi Cal,

The "thin lens" simplification is often used for preliminary design of a lens because it yields a simpler set of equations to solve. Thin lenses are theoretical abstracts that don't exist in the real world.

But a telescope is a lens system, not a single simple lens.
And the individual lens elements in the system are not "thin" lenses.

The simplifications must be discarded and real values used before an real lens can actually be fabricated.

- Leigh
 
Hi Cal,

The "thin lens" simplification is often used for preliminary design of a lens because it yields a simpler set of equations to solve. Thin lenses are theoretical abstracts that don't exist in the real world.

But a telescope is a lens system, not a single simple lens.
And the individual lens elements in the system are not "thin" lenses.

The simplifications must be discarded and real values used before an real lens can actually be fabricated.

- Leigh

You can't win, and there is a penalty for trying. Oh well!
And in this case, values discarded and replaced with absolutely NOTHING. So entertaining.
As mentioned, The example was made only for the sake of interest. No precision implied. But if the degree of precision required was sufficiently open, useful approximations could be made.
 
But if the degree of precision required was sufficiently open, useful approximations could be made.
Hi Cal,

I didn't understand the first part of your post.

The idea of the precision being "open" is the entire idea behind the thin lens equations. You can get the design in the ballpark with much less effort than doing all the accurate and time-consuming calculations.

Remember, lenses were being designed successfully many decades before modern computers were invented. Back in "the day", Nikon had rooms full of women doing identical calculations. They compared the results, and accepted them if they matched.

- Leigh
 
Hi Cal,

I didn't understand the first part of your post.

The idea of the precision being "open" is the entire idea behind the thin lens equations. You can get the design in the ballpark with much less effort than doing all the accurate and time-consuming calculations.

Remember, lenses were being designed successfully many decades before modern computers were invented. Back in "the day", Nikon had rooms full of women doing identical calculations. They compared the results, and accepted them if they matched.

- Leigh

Leigh, I did write that these numbers were scaled off a picture (in a book)! There are implications regarding precision when such methods are employed.
 
Leigh, I did write that these numbers were scaled off a picture (in a book)! There are implications regarding precision when such methods are employed.
Hi Cal,

Yes, I noted that.

My response was an attempt to explain why such measurements are irrelevant and invalid as regards the optical design of a lens, regardless of the accuracy with which they're made.

- Leigh
 








 
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