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Making finder crosshairs - advice please?


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Dear wise ones,

I have recently acquired a nice TS-Optics 60/234 RACI finder. This is a pleasure to use but has one big disadvantage -- it has no crosshairs so centring an object precisely is impossible.

I'd like to remedy this but I don't know where in the optical train I would put something. Not next to my eye obviously, and I presume not at the end of the dew shield. Maybe at the far end of the eyepiece, like a filter?

Can you advise where and what I might use?

thank you in advance :)

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Using two thin copper wires might give the effect you're after ? Something like this:

wrapped lense.jpg

Simple enough to do, it won't damage your finder and you can remove it whenever you want.

Edited by Akiainavas
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I believe the finder in question has interchangeable eyepieces in which case the question is indeed about how to add crosshairs to an eyepiece, and as has been said, they need to be at the focal plane of the eyepiece, so that they will be seen in focus. I'd just make a couple of points:

On a  traditional eyepiece such as a plossl the focal plane is easily accessible by unscrewing the silver barrel: cross hairs are applied to the field stop, as has been said. A caveat is the state of your own eyes: if, like me, you're a spectacle wearer then something at the focal plane may not look in focus unless you're wearing your spectacles. So if, like me, you observe without spectacles, you need to find exactly where crosshairs should be positioned to be in focus for your eyes. I'm short-sighted, and by looking through the eyepiece held up to daylight, while placing a needle point at the field stop, I established that crosshairs would need to be slightly below (away from the eye), a distance I could achieve with a metal washer (shim). So my procedure was to apply crosshairs to the washer, then glue that to the field stop. I used fine fishing line.

In complex wide-angle eyepieces there is typically an integral negative lens (Barlow) at the field end, and the focal plane is not accessible unless you completely dismantle the lens (not advised). So for a crosshair eyepiece you want to use a plossl or similar, preferably a cheap spare one in case of mishap.

If you observe at a truly dark site, crosshairs can be potentially difficult to see against the dark sky, so should not be too thin. At a typical light polluted site they can be as thin as you like, since they'll be seen against a bright sky.

If you don't want to make your own crosshair eyepiece they are available commercially.

Edited by acey
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Thanks very much guys. Knew you'd come up with the goods :) Much obliged.

I do indeed use Plossls for the finder so I will give it a go. Now where can I get some hair...

ouch

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  • 3 weeks later...

Update -- hair worked fine. Believe it or not my (head) hair was even a bit thicker than would be ideal!

What didn't occur to me was that the cross would need to be precisely centred, because my finder focuses by turning the eyepiece and there's no lock on it. A bit of sellotape on the focuser was enough to sort this out.

I am now finding stuff twice as fast and very happy. Thanks!

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