Author Topic: 'Newt:real-world laser raytrace ?'  (Read 151 times)

Joseph Garrison

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'Newt:real-world laser raytrace ?'
« on: December 24, 2017, 06:42:38 PM »
Intrigued by the success of my concrete, visible-light testing of eyepieces such as AFoV, EP, & ER using a visible light & smoke setup, I have been wondering the way to do something like find some visual affirmation of the way secondary sizing & position could be optimized in my existing job, the rebuild of my 17.5" truss Dob.

I have played a bit with a 'big brother variation' of my EP test rig, but the distances between the mirror elements makes actually seeing what is happening between pri-sec-focuser hard using this strategy.
So I thought I'd discuss my idea for a 'real-world laser raytrace' setup with you optics wizards here and see what you think about it.
Let me state that I am NO optics magician, I need to see and touch for it to soak in,
Second, I know & have utilized Newt-for-Web & comparable newt layout applications.
I have read it about, including Mike Lockwood's excellent dissertation on the topic of secondary sizing.
They allhelp but the 'show-me experimenter' in me needs some sort of hands-on to get a real-world grasp of exactly what it means to my specific system.

What I want to test for is:
1-secondary big enough to capture the entire primary light cone, and at what distance ?
2-where do I need to place both secondary and Focuser for both secondary dimensions ?
3-how will this all play out in an LR newt at radically different mounting angles ofsecondary and focuser ?

Of course I want the smallest secondary I will get away with, but not at the cost of losing Primary light cone, ie.  I don't wish to have a 17.5" primary acting like a 16".
Hopefully this illustration is self explanatory.  The notion is to utilize lasers positioned to strike the farthest useable edges of the primary, just within the bevel, coming in perpendicular to the primary.  Their mirrored beams should specify the outer edges of the primarys light cone, which in turn should allow me to determine which secondary works best for my setup, and at what distances (d1 & d2)
I love your thoughts on this.  If you think I'm all wet please inform me.  I understand this may appear over-the-edge to you men who will point to a mathematical solution to this, but I would like to "see" it.  I hope this may help others using the same questions better picture these basic design questions.  I have a buddy (Jeff) who is pondering these same questions with me right now, I'm sure there are other people.
In case it ends up that this little setup works it will be a massive help for me as I delve into my own eventual plan to make my Dob a low-rider, where secondary dimensions, angles, and distances go totally nuts.
. . .Newt for internet doesn't cover that !

Thanks in advance
CS
Bob



napephona

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Re: 'Newt:real-world laser raytrace ?'
« Reply #1 on: December 27, 2017, 02:16:13 PM »
What is wrong with mathematically setting up the equations and solving them? Agreed that it isn't sexy but it will give you all the degrees of freedom for a general case and thus every thinkable way.

halubicom

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Re: 'Newt:real-world laser raytrace ?'
« Reply #2 on: December 30, 2017, 05:37:35 PM »
Quote
What is wrong with mathematically setting up the equations and solving them? Agreed that it isn't sexy but it will give you all the degrees of freedom for a general case and thus every thinkable way.
Tos etup and align the lasers correctly to trace out the edges of the field will require several alignment aids and accompanying calculations such that the effort and or costs involved greatly exceed the cost of a few sheets of paper that would be required for a graphical solution accompanied by a few calculations.

tenpaseper

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Re: 'Newt:real-world laser raytrace ?'
« Reply #3 on: December 30, 2017, 08:24:12 PM »
Bruce and I are on the same side so it seems.  Honestly: of you can set up and solve a first degree equation you are already there for a good guesstimation for a Newtonian.

telschronexic

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Re: 'Newt:real-world laser raytrace ?'
« Reply #4 on: January 02, 2018, 12:17:01 AM »
And don't forget that you want to do more than have full illumination for the on-axis object.

Saul Zhang

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Re: 'Newt:real-world laser raytrace ?'
« Reply #5 on: January 09, 2018, 11:56:44 PM »
Quote
...What i want to test for is:
1-secondary big enough to capture the entire primary light cone, and at what distance ?
2-where do i need to position both secondary and Focuser for the two secondary sizes ?
3-how will this all play out in a LR newt at radically different mounting angles ofsecondary and focuser ?

Already done by Jason K (reverse of your experiment):https://www.cloudyni...-3#entry4116531

Where to place the laser "pupil":http://www.vicmenard...rspectives.html (scroll down to: "Notes on matching a sight tube to your 'scope's focal length")

quiterhardpho

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Re: 'Newt:real-world laser raytrace ?'
« Reply #6 on: January 10, 2018, 11:06:05 AM »
Quote
Quote
What is wrong with mathematically setting up the equations and solving them? Agreed that it isn't sexy but it will give you all the degrees of freedom for a general case and thus every thinkable way.
Tos etup and align the lasers correctly to trace out the edges of the field will require several alignment aids and accompanying calculations such that the effort and or costs involved greatly exceed the cost of a few sheets of paper that would be required for a graphical solution accompanied by a few calculations.
Thanks for your comments. Certainly nothing is wrong with the mathematical solution approach. I have done that much using newt for web. What I am looking for is physical verification before I start cutting pieces for the new scope.Especially when I move on to the LR layout. So what I am hearing is that something like this would work if set up correctly ??

I am confident in my ability to set it up to the required degree of precision, I see the critical parts being only a perpendicular beam striking the edge , a properly offset 45* sec mirror, in line with the focuser. All of that is easily measureable, therefore physically verifiable..

As for 'sexy', I guess you could say that too. Pretty cool to actually SEE what the equations are predicting. For us experimenter type stuff like this is, well, pretty sexy ! A fun part of ATMg for me...Bob

coreanoguf

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Re: 'Newt:real-world laser raytrace ?'
« Reply #7 on: January 11, 2018, 01:00:03 PM »
Quote
Bruce and I are on the same side so it seems.  Honestly: of you can set up and solve a first degree equation you are already there for a good guesstimation for a Newtonian.

Certainly. I agree I can get a good guesstimation.
I have aguesstimation.
That is exactly what the Hubble builders settled for.
To this day I am still amazed that they did not perform a physical verification before the launch !

CS
Bob

Tommy Schmidt

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Re: 'Newt:real-world laser raytrace ?'
« Reply #8 on: January 11, 2018, 03:59:51 PM »
Quote
Quote

...What i want to test for is:
1-secondary big enough to capture the entire primary light cone, and at what distance ?
2-where do i need to position both secondary and Focuser for the two secondary sizes ?
3-how will this all play out in a LR newt at radically different mounting angles ofsecondary and focuser ?

Already done by Jason K (reverse of your experiment):https://www.cloudyni...-3#entry4116531

Where to place the laser "pupil":http://www.vicmenard...rspectives.html (scroll down to: "Notes on matching a sight tube to your 'scope's focal length")

Thanks much for that link Vic, exactly what I was looking for !
As usual, no surprise that there is 'nothing new under the sun'...
Looks like an exhaustive resource for me

CS
Bob

Steve Lewis

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Re: 'Newt:real-world laser raytrace ?'
« Reply #9 on: January 12, 2018, 06:19:01 PM »
Quote
...I am confident in my ability to set it up to the required degree of precision, I see the critical parts being only a perpendicular beam striking the edge , a properly offset 45* sec mirror, in line with the focuser. All of that is easily measureable, therefore physically verifiable...

In "reverse", the laser beam would be perpendicular to the edge only when the laser emitter/pupil is at the center of the focal plane and directed toward the edge of the primary mirror. But you're not interested in the focal point (axial) illumination--you're interested in the focal plane illumination. When the beam is emitted from above the focal plane (at the optimal pupil distance as referenced earlier in the link I provided) the outgoing beam from the primary mirror edge to the front aperture will not be perpendicular. This would be the scenario in your experiment where the laser beam is defining the edges of the various references (the unvignetted front aperture, the edge of the primary, the edge of the secondary, and the edge of the fully illuminated field diameter).

calbeyrefrows

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Re: 'Newt:real-world laser raytrace ?'
« Reply #10 on: January 12, 2018, 06:45:51 PM »
Quote
Quote
Quote
What is wrong with mathematically setting up the equations and solving them? Agreed that it isn't sexy but it will give you all the degrees of freedom for a general case and thus every thinkable way.
Tos etup and align the lasers correctly to trace out the edges of the field will require several alignment aids and accompanying calculations such that the effort and or costs involved greatly exceed the cost of a few sheets of paper that would be required for a graphical solution accompanied by a few calculations.
Thanks for your comments. Certainly nothing is wrong with the mathematical solution approach. I have done that much using newt for web. What I am looking for is physical verification before I start cutting pieces for the new scope.Especially when I move on to the LR layout. So what I am hearing is that something like this would work if set up correctly ??I am confident in my ability to set it up to the required degree of precision, I see the critical parts being only a perpendicular beam striking the edge , a properly offset 45* sec mirror, in line with the focuser. All of that is easily measureable, therefore physically verifiable..Bob
Actually the laser beams need to be parallel (for zero field angle) to the optical axis of the primary and not perpendicular to its surface. A pity because perpendicularity is assured by having the laser autoreflect onto itself.For a non zero field angle the laser beams need to be inclined to the optic axis. Achieving the required sub arc minute alignment isn't trivial.

penliipamex

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Re: 'Newt:real-world laser raytrace ?'
« Reply #11 on: January 15, 2018, 05:13:18 PM »
Your ray trace is on the right path, now you just need to draw up all the other relevant light paths (ala):
Including the paracorr ! and the way it changes where the focal plane accepted by the EP lies.

bankrybettdog

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Re: 'Newt:real-world laser raytrace ?'
« Reply #12 on: January 18, 2018, 03:57:16 AM »
Honestly: even if you're interested in rigging something up, make a simple rig in which you can place the secondary and the primary and determine where you can see where the focal plane is (by focusing an object at infinity).
Look at the primary from the point where the primary fills the secondary completely, and by placing templates at the focal plane (cardboard pieces with circular holes) you'll be able to see the size of the fully illuminated field (it's the template through which you can still just see both the secondary and primary).

Using lasers isn't going to be very helpful, except perhaps in determining the axis to which the focal plane is square (but for this experiment extreme precision is not really needed).

brigtigeartgib

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Re: 'Newt:real-world laser raytrace ?'
« Reply #13 on: January 21, 2018, 12:04:01 PM »
Quote
Quote

<p class="citation">BGRE, on 19 Nov 2017 - 12:29 PM, said:<a href="https://www.cloudynights.com/index.php?app=forums&amp;module=forums&amp;section=findpost&amp;pid=8221924" rel="citation">[/url]<blockquote class="ipsBlockquote built" data-author="BGRE" data-cid="8221924" data-time="1511087397">

<p class="citation">Benach, on 19 Nov 2017 - 12:08 PM, said:<a href="https://www.cloudynights.com/index.php?app=forums&amp;module=forums&amp;section=findpost&amp;pid=8221915" rel="citation">[/url]<blockquote class="ipsBlockquote built" data-author="Benach" data-cid="8221915" data-time="1511086108">
What is wrong with mathematically setting up the equations and solving them? Agreed that it isn't sexy but it will give you all the degrees of freedom for a general case and thus every thinkable way.
Tos etup and align the lasers correctly to trace out the edges of the field will require several alignment aids and accompanying calculations such that the effort and or costs involved greatly exceed the cost of a few sheets of paper that would be required for a graphical solution accompanied by a few calculations.

Thanks for your comments. Certainly nothing is wrong with the mathematical solution approach. I have done that much using newt for web. What I am looking for is physical verification before I start cutting pieces for the new scope.Especially when I move on to the LR layout. So what I am hearing is that something like this would work if set up correctly ??

I am confident in my ability to set it up to the required degree of precision, I see the critical parts being only a perpendicular beam striking the edge , a properly offset 45* sec mirror, in line with the focuser. All of that is easily measureable, therefore physically verifiable..Bob

[/quote]

Actually the laser beams need to be parallel (for zero field angle) to the optical axis of the primary and not perpendicular to its surface. A pity because perpendicularity is assured by having the laser autoreflect onto itself.
For a non zero field angle the laser beams need to be inclined to the optic axis. Achieving the required sub arc minute alignment isn't trivial.[/quote]
I guess not very clearly but that is what I meant by perpendicular, (to the optical axis of the primary). I tried to show that in my sketch by the large square laid across the edges of the primary, defining an imaginary straight line across the concave face, to which the lasers would have to be perpendicular. No real square involved tho, that straight surface would be a measured height above the primarydiameter, from which the lasers would point downward. My articulation mechanism is two laser head positioners from old computer CD drives, one on either side over the primary, with their drive motors sync'ed together to provide matched radial positioning across the outer 3" of the primary.

esicnatka

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Re: 'Newt:real-world laser raytrace ?'
« Reply #14 on: January 22, 2018, 09:07:27 PM »
Quote
Honestly: even if you're interested in rigging something up, make a simple rig in which you can place the secondary and the primary and determine where you can see where the focal plane is (by focusing an object at infinity).
Look at the primary from the point where the primary fills the secondary completely, and by placing templates at the focal plane (cardboard pieces with circular holes) you'll be able to see the size of the fully illuminated field (it's the template through which you can still just see both the secondary and primary).

Using lasers isn't going to be very helpful, except perhaps in determining the axis to which the focal plane is square (but for this experiment extreme precision is not really needed).


Now that is an optical solution I had not even though of. Much simpler than the laser thing to say the least, it looks like it could answer all of my questions !
I will try this first
Bob