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40mm Crosshair eyepiece

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Is there such a thing as a 40mm crosshair or reticule eyepiece? All such eyepieces I have seen have much smaller focal lengths, I guess because they are mainly used for AP guiding.

The reason I ask is that I am using a second hand 60mm Vixen Guidescope as a finderscope. Combined with a 90º erecting prism and 40mm Meade 4000 eyepiece, and a Baader finderscope bracket, I have a fine 60mm RACI finder with over 4º FOV, but I would like some sort of indication where the centre of the field is.

I have tried using a screw in crosshair, but I think it was designed for a smaller eyepiece, as it was much narrower than the field lens and so vignetted the field of view rather badly.

Alternatively, has anybody any tips about how to add cross hairs to my existing eye piece?



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You could use a piece of plastic material. If you can find a clear one, sometimes you can find them in supermarkets, but don't buy a salad pack unless it will be eaten.

You can use a piece of it in the form of a circle cut to the correct diameter. When handling, make sure it is kept free from scratches.

You can use a steel rule to carefully scribe a cross across it's two diameters. You need not apply too much pressure whilst using a very sharp modelling knife for this job. A razor blade will do too. Once the lines have been scribed onto it, there will be no need to fill the lines with paint, they will stand out OK. If you want to darken them, just rub your finger across them, and the line will darken from the natural deposits on your finger. If you are carefull, you could scribe parallel lines, as close together as you wish. This will result in a small box in the centre.

I used this method years ago when I could not afford an illuminated reticule. I did what I have just described, and placed a small torch bulb beside the objective of the refractor I was using as a guidescope. This illuminated the field slightly, and made the cross stand out. A guide star was easy to pick out. You could use a piece out of a CD case if you can find a pristine one. The are easily marked, so be carefull.

You will need to find a way to place and hold the engraved disc at the eyepiece focal point.

Sound a bit of a task, but it will work.

Ron. :wink:

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I did try one of these, but it vignetted anything larger than a 20mm plossl, which is not something they advertise, so wide fields of view are out. I assume it's not a problem if you want to use one in a guidescope. I use it for training my ETX drives, so not a total waste of money!

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