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Is there a reasonable way to reverse a Telrad?

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#1 derangedhermit

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Posted 23 November 2012 - 06:19 PM

I'd like to mount it backward and be looking from the battery end.

I guess I'd need to cut the angled plastic holding the glass off, turn it around, and re-attach it, right?

Or would it be easier to convert it to vertical, cutting a hole in the bottom, cutting off the top part, replacing the mirror with the glass, and adjusting the glass directly?

#2 Mirzam

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Posted 23 November 2012 - 06:33 PM

I don't suppose you would be willing to tell us why you want to do this?

JimC

#3 kfrederick

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Posted 23 November 2012 - 06:35 PM

On mine my eye is at the battery end. I think

#4 derangedhermit

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Posted 23 November 2012 - 06:48 PM

On mine my eye is at the battery end. I think

I think it will work better for you if you turn it around :grin:

#5 derangedhermit

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Posted 23 November 2012 - 06:49 PM

I don't suppose you would be willing to tell us why you want to do this?

JimC

Will you share your ideas if I do?

Lee

#6 Mirzam

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Posted 23 November 2012 - 07:41 PM

Maybe. I might have to destroy a Telrad to figure out what would or would not work. I think I have a spare.

JimC

#7 mconnelley

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Posted 24 November 2012 - 03:56 AM

Hello:

Converting a Telrad to use vertically may involve a few challenges since you do need that lens to collimate the beam coming from the reticle inside. The easiest way I can see to accomplish what you want is to remove the glass, then cut off its supports an glue them back on backwards.

Cheers
Mike

#8 derangedhermit

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Posted 24 November 2012 - 07:52 AM

Maybe. I might have to destroy a Telrad to figure out what would or would not work. I think I have a spare.

JimC

Please don't do that on my account.

#9 derangedhermit

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Posted 24 November 2012 - 08:02 AM

Converting a Telrad to use vertically may involve a few challenges since you do need that lens to collimate the beam coming from the reticle inside.

Cheers
Mike

Ah yes, I didn't have a Telrad in hand and had forgotten about the lens.

#10 StarStuff1

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Posted 24 November 2012 - 02:11 PM

Just buy a Rigel QuickFinder. :foreheadslap:

#11 derangedhermit

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Posted 25 November 2012 - 08:18 AM

Just buy a Rigel QuickFinder. :foreheadslap:

I've owned three, and I think that's my limit. I prefer the Telrad's view.

#12 Mirzam

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Posted 25 November 2012 - 09:22 AM

I also prefer the Telrad view to the Rigel but have used several Rigels in order to save weight on the UTA. My favorite of all is the Televue Starbeam, which seems to have no parallax shift at all.

To make the change that you were thinking about on a Telrad actually looks like it would be very easy. Remove the standard angled glass pane (it slides out of the holder). Then get a slightly smaller width pane that will fit in between the existing plastic uprights on either side of the projection lens. Try to get a good friction fit between the uprights. Play around with the angle of the new pane until you are happy and then silicon it into place.

Let us know how it works out.

JimC

#13 greenglass

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Posted 25 November 2012 - 03:13 PM

Darlet

#14 derangedhermit

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Posted 23 December 2012 - 02:01 PM

My favorite of all is the Televue Starbeam, which seems to have no parallax shift at all.


What determines the amount of shift in these devices?

To make the change that you were thinking about on a Telrad actually looks like it would be very easy. Remove the standard angled glass pane (it slides out of the holder). Then get a slightly smaller width pane that will fit in between the existing plastic uprights on either side of the projection lens. Try to get a good friction fit between the uprights. Play around with the angle of the new pane until you are happy and then silicon it into place.

Let us know how it works out.

JimC

The slot for the glass is at a 45 degree angle. I intend to cut the sides supporting the glass at 45 degrees, making the glass support sides into a right angle isosceles triangle. Then I'll use silicone to attach the existing glass onto the new trimmed side. If I don't like it, I can always cut the silicone and put the glass back into the slot. BTW, my glass pane is (was) help in by some kind of weak adhesive from the "factory".

#15 derangedhermit

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Posted 23 December 2012 - 02:17 PM

Converting a Telrad to use vertically may involve a few challenges since you do need that lens to collimate the beam coming from the reticle inside.

Cheers
Mike


I looked at this some more. The mirror is no longer needed, so it could be removed, and the collimation mechanism from the mirror then mounted on the side of the LED/reticle assembly, with the knobs sticking out the side, ala Rigel. The lens/glass pane relationship is kept, but the lens has to be mounted on top of the unit, instead of the side. Basically, rebuild it to be configured like a large Rigel - although the Rigel is more integrated in design. It might be easier to use a new enclosure than try to hack the existing Telrad box.

I like this approach. The only drawback for me is that you cannot use stock Telrad bases, and have to come up with something else. I might make one sometime later, but will first make the reversed version, which is much easier to make.

#16 Dan McConaughy

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Posted 23 December 2012 - 08:40 PM

They sell a device for the Telra that diverts the light 90 degrees for vertical viewing. I tried it but didn't like it very much.

#17 GlennLeDrew

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Posted 23 December 2012 - 10:43 PM

Hermit,
What controls parallax error, or shifting of the reticle pattern with respect to the sky as the eye moves about laterally, is the accuracy of the spacing between the reticle and collimator. The reticle must be located at the focus of the collimator so that it is projected to optical infinity.

There's another wrinkle, although in practice minor. That the collimator is always a single element, there can be sufficient distortion of scale off-axis to result in some parallax error when the reticle is seen through the edge of the collimator. From my observations, lens-type collimators tend to be more prone to this, especially if the lens has a large aperture ratio (in order to accommodate a larger range of lateral eye displacement.) And the form of the curves on both surfaces matters as well.

#18 Spectral Joe

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Posted 23 December 2012 - 11:02 PM

Another issue is that unless the collimating lens is designed to have a flat field the outer edges of the reticle will not be collimated, and parallax will be apparent at the edges of the field. The N-3C gunsight, once used as a reflex finder by some, had a reticle that was dished, with its concave side toward the lens, which was an achromat.

#19 derangedhermit

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Posted 26 December 2012 - 12:03 PM

Hermit,
What controls parallax error, or shifting of the reticle pattern with respect to the sky as the eye moves about laterally, is the accuracy of the spacing between the reticle and collimator. The reticle must be located at the focus of the collimator so that it is projected to optical infinity.


Hmmm. I had intended to keep the Telrad's spacing from reticle to lens, but this brings up a point. The mirror used to align the Telrad with the scope has about 1/4" forward/back movement, changing the optical path length. I guess it would be nice to know just where that mirror should be, if the amount of error is sensitive to path length changes of 1/8" or so.

The vertical configuration, like used in the Rigel, with adjustment from the side for alignment, avoids this issue almost entirely.

There's another wrinkle, although in practice minor. That the collimator is always a single element, there can be sufficient distortion of scale off-axis to result in some parallax error when the reticle is seen through the edge of the collimator. From my observations, lens-type collimators tend to be more prone to this, especially if the lens has a large aperture ratio (in order to accommodate a larger range of lateral eye displacement.) And the form of the curves on both surfaces matters as well.


The Telrad lens is convex on top, spherical I assume, and appears flat on the bottom to casual observation. I haven't measured its f/ratio.

#20 GlennLeDrew

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Posted 26 December 2012 - 04:59 PM

When building or modifying a collimated sight, best to incorporate some way to tweak reticle-to-collimator distance. While aimed at a distant target, move the eye laterally and adjust until the reticle pattern remains stationary with respect to the target. Parallax will thus have been elliminated.

#21 Novaguy

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Posted 27 December 2012 - 10:31 PM

I sometimes look through the mirror that comes with the Telrad Dewshield (scopestuff). There is a fair amount of light lost and field of view lost in doing this, but it does work, reflecting the reticle image at a 45 degree angle.






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