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Light Cone & Object Image ??

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#1 kram2105

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Posted 17 November 2012 - 04:52 PM

I built a folded 6 f12 refractor. I started with a tapered paper representation of the light cone i wanted to fold. 6 inches wide at the objective, 72 inches long, comming to a point at the focal plane. at 36 inches there is a 3 inch dia. mirror on a 10 degree angle bouncing the light back toward the objective. it was later that i learned about object image at the focal plane and i began to wonder if the 3 inch mirror should have been bigger. if a fellow atm'r could offer some guidance i would be much obliged.
my questions are, how much of full feild am i getting
( 100% 80% etc ) and also, does the black retaining ring around the mirror act as a baffle? Thanks for your time Mark F. Bristol, Pa

#2 Sean Cunneen

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Posted 17 November 2012 - 06:06 PM

Have you built it yet? Is your tube baffled as well? You won't be missing much, maybe in the 10% range if you keep the design as-is and everything else is okeedokee. Cutting off the outside of the image won't affect things too much and in some cases, it can help. The edge of an objective is often where the errors are, so having a smaller secondary can act like a mask and keep some of those errors at bay. Also, planetary targets, bright nebula, etc won't be affected by a smaller illuminated field.
What you'll want to check is vignetting with any baffles or your focuser tube. That's where the monsters hide...

#3 GlennLeDrew

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Posted 17 November 2012 - 06:27 PM

At the half focal point (midway between objective and focus), your 3" mirror just accommodates the on-axis light cone. Therefore the circle of full illumination is if course a point. But due to the considerable separation of the obstructing edge of the mirror, the shadowing is quite diffuse and so the illumination fall-off is very gradual. Without drawing it up, my intuition says you'll have something near 70% illumination at the edge of a 2" circle. Even a 3" image circle would have not much less than 50% illumination at its edge.

#4 kram2105

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Posted 20 November 2012 - 10:34 PM

The scope is finished with no baffles. I used a tube with black flocking inside so i don't see the need for baffles.
thanks for your input, i also have a Jegars 6" an f10. I'll use it to compare brightness and see what i get. TY Mark

#5 kram2105

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Posted 20 November 2012 - 10:41 PM

You mention "draw it up", exactly what i need to learn to do. I have a book "all about Telescopes" but the part about ray tracing was a bit complicated hense my simple approach. in any case the mirror can be replaced with a larger size.
TY Mark

#6 GlennLeDrew

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Posted 21 November 2012 - 11:28 AM

Mark,
The simplest ray trace has just one light cone converging to a point at the field center. But one's field of view is not just a point, but rather is a circle defined by the eyepiece's field stop, which for 1.25" eyepieces can be as large as 27mm, and for 2" eyepieces 46mm. Draw at the focus a line which represents the largest field stop you will use. Now draw a new light cone converging at each end of this line. You now see the full envelope of light cones which firm image points across the full field.

Where your 3" mirror was placed at the halfway point, you'll note that it was just barely large enough accommodate the on-axis light cone only. Edge-of-field light cones have small portions missing the mirror's edge.

Hopefully this ultra-brief lesson will get you going with your ray tracing adventures. Sam Brown's books are quite possibly the best of their kind, with clear illustrations galore, for those of us who are more visually oriented.

#7 nytecam

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Posted 22 November 2012 - 03:36 AM

I started with a tapered paper representation of the light cone i wanted to fold. 6 inches wide at the objective, 72 inches long, comming to a point at the focal plane.

The 'cone' shouldn't come to a point but at the focal plane = width of the intended EP fov - then the folded points on the cone = min size 2nd/3rd plane mirrors' major axes :p

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#8 AV in CMH

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Posted 23 November 2012 - 10:39 PM

Hello,

Interesting project.

You might consider cutting a paper pattern of the light cone, with a .5 to .75" dia at the far end. With a paper cone you can experiment with different folding patterns.

One pattern I have seen used is attached. I like this type of folding as it brings exit pupil up by 90* without using a star diag.

Good luck, have fun,

Anthony

PS drawing is not to scale.

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#9 bremms

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Posted 23 November 2012 - 11:52 PM

In one word, yes it is too small. One nice feature of a refractor is a wide unvingetted field. You don't want to cut off the light cone, you are wasting aperture. Mirrors tend to be worse at the edges. Best thing to do is, 4 inch mirror and put in a couple of baffles. That way you have an unobstructed light path over the whole field. And, you won't be using the edges of the mirror near COF.

#10 dan_h

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Posted 24 November 2012 - 02:00 PM

In one word, yes it is too small. One nice feature of a refractor is a wide unvingetted field. You don't want to cut off the light cone, you are wasting aperture. Mirrors tend to be worse at the edges. Best thing to do is, 4 inch mirror and put in a couple of baffles. That way you have an unobstructed light path over the whole field. And, you won't be using the edges of the mirror near COF.


That would be a good solution for the reasons stated. Another option is to move the 3" mirror further back from the objective but if the tube is already constructed that isn't possible.

dan






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