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Crossway

cross hairs in scope

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I am using a telescope home made from  redundant binos....a 50mm object lens and a 20mm erecting eyepiece. from a telescope.  Works very well terrestrially which is what it was intended for  but I would like to use it also as finder scope on my reflector,  I don't think cross hairs are essential but I would like to experiment down this line  anyway.... problem is I don't seem to be able to locate the exact place to introduce them.  I would have thought that the natural focal point of the eyepiece would be the spot but clearly that is not it.

Has anyone any advice on this one 

Pete

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The cross hairs are usually placed at the field stop of the eyepiece. If you can fit cross hairs to a tube that can slide inside the eyepiece barell then you can slide it to the best focus.   ?

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Most EPs in binoculars have the field stop at the end of the barrel, so you can attach it there. I have in the past glued to actual hairs across the barrel and made a useful finder for my scopes. More recently I got a neat crosshair etched into glass from the "Surplus Shed" for next to nothing: https://www.surplusshed.com/category/Reticles

 

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Thanks  for replies and pleased to have confirmation that this procedure is perfectly doable.  I haven't heard the expression  field stop before but obviously this sounds as if it would be located at the extremity of the focussing eyepiece. Have I got this right?

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Thanks too.

I was pondering on knocking the glass out of an old filter and putting some thin wire crosshairs across that; then screwing the whole lot into the end of a 25mm plossl.

I’ll give it a try and post if it works.

Paul

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Looking forward, Paul73, to see  how you get on with this.  A good idea to use the filter holder but just wondered if the "hair" is a bit on the thick side.  Perhaps my  plan to use genuine hair won't work either but nothing ventured,,,,,,,,,,,,,,, ! 

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One precision finder eyepiece! 

I’ve been dragged out to a Burns night dinner, so can’t test until later:

3001AA8A-CAA4-467D-8F65-26C5C3747203.thumb.jpeg.230921d26eb95096a441499a6ccb8375.jpegEF738FEF-0822-4080-8C13-34B1538A2403.thumb.jpeg.63385ecc424ebb24d9d1a529f9a481a9.jpeg

 

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Needless to say. It didn’t work! 

At least I now know the location of the Field Stop. ?. In the generic “Super Plossl” (it isn’t), the field stop is a couple of mm before you hit the first lense.

I made a washer out of milk bottle plastic  with three little holes for the two wires  (leaving plenty of slack). The whole thing was clamped in place with a retaining ring from a, now dead, filter. I then pushed the wires to about the right spot, tied the single wire to the “V” and did fine adjustments with a pair of tweezers until sharp and roughly central.

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Works a treat!

Paul

7C988151-DBE6-4ACB-8515-BE10A9374E42.jpeg

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I've made 100's using human hair and have the bald patch to prove it.    ?

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Years ago, I made cross using BosTik adhesive. It's quite 'stringy' and you can stretch it across an aperture to make quite nice cross hairs.

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I've made cross-hairs using real hair and hot-melt glue.

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Some good ideas  folks have.   And I now know what/ where  the field stop is !!   I am persevering with human hair  but had a trial run using fibre optic filament from an old  Christmas tree illumination.  Worked OK but  the filament is much "fatter" than you would think  when magnified in the FS  zone.  Also, I think that  the cross hair image would be more functional if slightly out of focus  and therefore placed outside the FS point.  More experimenting.

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Babies hair is very thin. When my son was little I did cut a few hairs and used it in one my eyepieces, added a LED and it works fine.
After that I did the same as Bruce and again added a LED. Shining the red light into the perimeter of that sheet of acrylic That 'crosshair' was nicely visible. Had to add a potentiometer because of : to bright.
Although transparent acrylic is optically pure I never used it in my main telescope.

In the early days professional astronomers kept spiders at home to use the spiders' silk strands as crosshair. VERY strong..!

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E E Barnard used hair from a donkey called Pinto for his finder scope on Mt Wilson in 1905/6. From here he took his famous images of the dark nebula of the Milky Way.

My daughter has a long haired Guinea Pig called Ted. He might be willing to donate a couple of strands...

Paul

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