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  1. #1
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    Default Adding a reticle to a 2" moonfilter



    So I had an idea today that it would be cool to add a reticle to a 2" moon filter we have at the observatory so we could accurately point out small features on the moon, or identify certain craters for people.

    i.e. We would just center the cross hairs on a certain crater or feature and could tell people "In the center, we have blah blah blah".

    So, any ideas how to do this? I'm sure its stupidly simple, but I don't wana mess up the filter learning how to do it without consulting some people first.


  2. #2
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    Adding a reticle to a filter probably isn't going to work, because filters aren't placed at the focal plane of the eyepiece. (If they were, you would see every speck of dust on the filter.) Because you want the wires to be in focus at the same time as the image from the objective, they have to be located at the focal plane.

    You would have to add the reticle to an eyepiece. If you have an old cheap eyepiece that you don't mind tinkering with, you could try it. You would start by finding out where its focal plane is. If it's in the empty part of the barrel, you're in luck. You can add fine wires in the barrel at exactly the focal plane.

    On the other hand, some eyepieces contain an integral barlow lens. In that case, the focal plane is inside the eyepiece, between the lenses, and you'd have to open it up to add the wires.

    This is not a project to try with your prized Panoptic or Nagler!

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  3. #3
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    Default

    EDIT: Keith answered while I was typing...
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  4. #4
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    Default

    Quote Originally Posted by KeithBC View Post
    Adding a reticle to a filter probably isn't going to work, because filters aren't placed at the focal plane of the eyepiece. (If they were, you would see every speck of dust on the filter.) Because you want the wires to be in focus at the same time as the image from the objective, they have to be located at the focal plane.

    You would have to add the reticle to an eyepiece. If you have an old cheap eyepiece that you don't mind tinkering with, you could try it. You would start by finding out where its focal plane is. If it's in the empty part of the barrel, you're in luck. You can add fine wires in the barrel at exactly the focal plane.

    On the other hand, some eyepieces contain an integral barlow lens. In that case, the focal plane is inside the eyepiece, between the lenses, and you'd have to open it up to add the wires.

    This is not a project to try with your prized Panoptic or Nagler!
    Thanks Keith.

    Would wires in the image plane of the mirror work? (i.e. the object plane of the eyepiece).

    I can't mess with our eyepieces, as some are very expensive and we only have a few in the 2" size for the main scope, but i could make some kind of insert for the focuser...

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    Default Adding a reticle.

    I am not sufficiently knowledgeable about optics to tell you where a reticle should go.

    I did, however, successfully substitute a reticle with super fine, etched graduations for the course and thick cross hairs that came with my finder scope. Before the modification, the cross hairs were so thick they actually blocked objects!

    Surplus Shed has dozens of reticles to choose from and the prices are often as little as $2.00.

    One nice feature of using etched reticles is that the graduations light up with side lighting.

    Clear Skies

    Art


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    Default

    Quote Originally Posted by Jovian View Post
    Thanks Keith.

    Would wires in the image plane of the mirror work? (i.e. the object plane of the eyepiece).

    I can't mess with our eyepieces, as some are very expensive and we only have a few in the 2" size for the main scope, but i could make some kind of insert for the focuser...
    When the eyepiece is focused, the image plane of the mirror and the focal plane of the eyepiece are identical. That's what being in focus means. Since that location is inside the barrel of the eyepiece, either ahead of the first lens or between the lenses, it would be pretty difficult to attach the wires to the OTA.

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  8. #7
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    Default

    OK, I think there's mix-up in terminology (on my part). I was thinking of the focal plane as something else.

    Well in any event, I can probably measure where the focal plane is with a simple laser pointer (assuming it is real and not virtual) and thin piece of paper for a screen.

    Next time I'm up there I'll give it a shot, and if it looks like I've got some room to work with I'll post back here for more ideas.

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    Default

    To find the focal plane of the eyepiece, take a strip of paper and fold over the end to from an "L". Now insert the paper into the eyepiece and find the position of were the end of the paper is in sharp focus when you looking into the eyepiece. That is the position you need to place the recticle so when the image is in focus the recticle will also be in focus.
    Kellner eyepieces are very good for using with recticles since their aperture stop is located near the end of the eyepiece barrel and this were the cross hairs or reticle is placed.

    All the Best,
    - Dave

 

 

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