Jump to content


Making finder crosshairs - advice please?

Recommended Posts

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 :)

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

  • 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!

Link to comment
Share on other sites


This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.