Author Topic: cross hair eyepiece?  (Read 1114 times)


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cross hair eyepiece?
« on: July 29, 2017, 01:22:57 PM »
I am in the middle of making a finder scope, for my 127mm APO.

Using a 200mm camera lens with a tube extender that carries a cheap surplus 25mm plossl eyepiece.
I now need to add in a pair of cross hairs. I'll be using some 0.1mm diam copper wire. I don't have any thinner wire.

I have a really cheap cross hair finder that came with the old 6" Newtonian and it can't focus on the cross hairs at the same distance as focusing on the stars. The cross hairs seem to be in the wrong location. The cross hairs are almost invisible when the stars are in focus.

I have tried to measure the distance from a focused object (the stands of paper fibre in a white page) to the face of the screw in connector on the 25mm plossl. I am around 8.6mm to 9mm from the paper to get good focus. (laid the eyepiece on a 9mm drill and on a 11/32" drill)

Is 8.8mm the distance I should set the new cross hairs from the eyepiece so that I can focus on the stars and be able to see the cross hairs?

Since the holes I will have to drill through the aluminium holder will be much bigger than the wire. I can adjust the hair location slightly If I don't get it quite right at first attempt.

Many thoughts welcome.
« Last Edit: July 29, 2017, 01:30:37 PM by watson »
Sky-Watcher eq6 pro
Explore Scientific 127mm f7.5 Apo
Orion UK europa 250mm f4.8 Newtonian
Canon 450D, not modified (yet)
2" 40mm & 28mm and 1.25" 16mm Explore EPs
Altair 2" diagonal and 2" Barlow


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Re: cross hair eyepiece?
« Reply #1 on: July 30, 2017, 07:36:13 PM »
Hmmm well fair play to you for trying to make you own, I'm not suprised these
are tricky to make and I must confess I don't have any advice to offer.
Tal 200K