Cross-hairs on a plossl?

[acey]

I want to put cross-hairs on a Plossl eyepiece. I have a surplus 10mm Sirius eyepiece and I've removed the silver barrel - it looks like the only other thing I can remove now is a blackened inner retaining barrel with notched end. From diagrams I've seen, it looks like this barrel should go straight onto the lens elements. So once I remove it, will the lens elements be free to drop out (or become misaligned), or will I find some kind of retaining ring beneath the inner barrel? Should the cross-hairs be applied to the top of the blackened barrel?

[The Sailor]

Hi Acey, you can purchase an eyepiece with crosshairs but from what I see of the one I have you could DIY. Unscrew the silver barrel, get a washer that will fit where the barrel sat, the centre hole as large as possible and as thin as possible. Solder two pieces of copper wire to form a cross, set into the space and replace the barrel. If anyone can come up with a better plan?

Jim

[Doc]

I have a plastic crosshair thingy that screws into the end of a 1.25" eyepiece it's made by TAL.

Perfect for what you need, i've seen them on E-bay sometimes for £15.

[peeceeTal2M]

SCS Astro used to sell a cross hair that screwed into bottom of eyepiece like a filter, think it was around £10 -£12. Might be worth giving them a ring .

[acey]

Main thing is that I want the cross-hairs to be in focus (hence at the focal plane) and thick enough to be visible without illumination: I already have a Meade reticle eyepiece but the thin lines need LED illumination to be visible. And I don't want to spend any more money if possible!

Mick - do the cross hairs on your Tal thingy come to focus? Does it need to be screwed right up inside the barrel?

[barkis]

I once used a piece of hard clear plastic from an old CD case.

I scribed a pair of parallel lines at right angles to each other, as close as a millimetre apart. I used a new Stanley blade, and made one pass for each line along a steel rule. It is easy to go off line, so care has to be the watchword.

The lines cross in the middle, and form a small box, which is where the guide star sits, if that is the purpose of the excercise, as it was for me.

It is fairly easy to turn the piece into a disc to sit inside the field stop of an eyepiece. I illuminated the cross, by an attenuated Red LED shining across the lens of the refractor I was using as a guidescope.

[NGC 1502]

The cross hairs need to go at the focal plane, which is where the field

stop is, that defines the edge of the field of view. If they are anywhere

else they will be blurred or invisible.

The field stop on Plossls is visible when looking up the chrome barrel.

The field stop on many more complex eyepiece designs lies between

the lens elements and is not accessable without dismantling, not recommended.

Regards, Ed.

[spaceboy]

I have a plastic crosshair thingy that screws into the end of a 1.25" eyepiece it's made by TAL.

Dito

Found it comes in useful for setting up polar alignment and finders.

PS Yes they do come into focus...Just

[ollypenrice]

I have a home made finder EP like this and having the soldered wires close to the bottom element seems fine. However, heat can make it go crinkly!! Still OK for a finder though.

Olly

[acey]

Question is - how do I reach the focal plane? The pic shows the 10mm plossl with the chrome barrel removed and the flat plane of the field stop exposed. At the centre of it is a raised, threaded barrel. If I put a wire across this raised barrel, it is out of focus when seen through the eyepiece. In order to have the wire in focus it would, I presume, need to be flush with the field stop. This would necessitate removing the threaded barrel. But is there anything else holding the lens elements in place? My fear is that if I loosen this barrel then the elements will become misaligned. And even if I could safely remove it, I have no means of cutting it down to make it flush with the field stop: I would need to find an alternative way of holding the elements in place.

[Capricorn]

No reason optically why the focal plane and the field stop are coincident. You may already have removed sufficent to be past the focal plane.

The tube may well be acting like a baffle so will not be at the focal plane. Also the field of view will be so small for the actual lens in the eyepiece that the tube cannot be a field stop.

[acey]

By cautiously moving a wire inside the retaining barrel I've established that the focal plane is coincident with the flat plane we're calling the field stop (at least to within a very small distance).

I think a way to do it would be to cut a small plastic disc that I could thread down into the barrel to reach the focal plane. I would cut a circular hole in the centre of the disc and glue the cross hairs across it. Alternatively (so as not to impose any reduction of FOV) I could try and glue wires to the end of the retaining barrel, bending them inside such that they would traverse the focal plane. Either way sounds very fiddly, though.

[spaceboy]

I'm not sure with all the difficulties getting the cross hair into focus would the cross hairs not have to be accurately placed to be of any advantage anyway?. Like adjusting your optical finder, calibrating a laser collimator or adjusting the cross hairs of a rifle scope. Would you not have to have some way of adjusting the cross hairs into the center for accuracy???

[acey]

Accuracy isn't critical. Idea is not for centring/ polar alignment etc but to have an aid in judging approximate position angles of DSOs. I'm thinking of having a pair of wires along one axis as an aid to judging sizes. A step up would be a filar micrometer but that's a whole other ball game. The astrometric reticle eyepiece I already have needs illumination so isn't suitable: thick enough wires will, I hope, be visible without illumination.

Edit: The following thread at another forum has various ideas: in my case the best approach might be to cut notches into the protruding retaining barrel and have the wires lying across those. I particularly like the look of the "grid reticle" at the end of the thread, which is pretty much the sort of thing I'm after.

http://www.iceinspace.com.au/forum/showthread.php?t=12487

[acey]

This diagram from I. R. Poyser Telescopes - How to make a telescope shows the classic plossl design. If mine is made this way then the troublesome retaining barrel (a) is all that holds the lens elements in place.

[michael.h.f.wilkinson]

I found that glueing my hair across the field stop works well, much better than soldering copper filaments across. Hairs are thinner and can be repositioned while the glue is still tacky. Besides, no heat needs to be applied.

[Merlin66]

Is the field stop not mounted inside the chrome barrel?

I have a couple of old eyepieces which have the lenses built into a body section and the field stop ( an aperture ring) sits inside the chrome section...for a 10mm eyepiece the stop should be 10mm infront of the interion lens element.

A disk with notches, fine transformer wire (old transistor radios were good for that!) and superglue - job done!

[acey]

By comparing with a couple of other eyepieces I realised that the problem of the focal plane lying inside the field stop was due to my own eyesight. The solution was to remove the field stop and cut it to size.

I used an old 9mm Orion plossl that I dug out of my spares box. First I removed the chrome barrel and found the focal point by inserting a piece of bent wire into the field stop, carefully avoiding the field lens. This determined where I would need to cut the field stop, which I removed by simply unscrewing (no tools required). I was careful to leave the eyepiece lying horizontal, field lens upwards, since there was now nothing holding the lens elements in place. The field stop was easily cut with a hacksaw. My cut wasn't truly square but I found this didn't matter - thread placed upon the shortened field stop when reinserted was focussed across the field, so things weren't as critical as I feared.

For my reticle I used a steel washer whose aperture equalled the field stop diameter, and for the "wires" I used 0.1mm fluorocarbon fishing line (2lb breaking strength - the thinnest I could find in my tackle bag). On a piece of wood I fixed panel pins to hold the wires in the desired arrangement (a perpendicular grid having 2 wires along each axis). Having the pins well spaced made things easier and more accurate. I wound the fishing line around these pins into the desired configuration, fixed them taut, then slid the washer beneath a piece of paper at the centre where the wires met, removed the paper, and put dabs of glue around the edge of the washer to hold the wires in place. When all was well set I tested the view through the reassembled eyepiece and decided that two extra wires might be useful: these I added in the same way, then fixed the washer onto the the field stop with glue, screwed it back in, and the job was done.

All that remains is for me to see if the wires are visible without illumination against a dark sky! If so, then I shall be able to judge DSO angular sizes much more accurately. The grid I have made has various side lengths: I hope to measure these by timing the passage of stars of known declination across them. I would then be able to judge sizes as fractions of the determined grid spacings, or as multiples of the wire thicknesses themselves.