Iluminated Telescope Eyepiece Croshairs

As you spend time with your telescope you might find yourself needing to be pointing the telescope in a repeatable way (like when you are calibrating your setting circles for instance, or when trying to polar align a GEM). The way to add repeatability is to introduce a co-ordinate system into an eyepiece. It could be concentric circles, or a grid, or or a combination of the two, but at the very least a centre mark. That centre mark could be a dot or a pair of perpendicular lines that cross in the centre of the eyepiece field of view. This instructable is about how to create an eyepiece that has cross hairs in the centre of the field of view.

But, if you can not see the cross hairs in the dark what good are they? So you want to have them light up somehow. The way you do that depends on how you construct them. What I have done is to steal^H^H^H^H^Himplement something using:

1) inexpensive eyepiece that provides medium magnification in your telescope

2) dollar store laser pointer

3) card stock

4) torn earbud cable

To do this you will need:

Materials:

1) Eyepiece that provides the magnification that you want in your telescope. If you have not accumulated any eyepieces that you consider worthy of this treatment, this is what I used,

2) File folder or card stock that you can print or have printed.

3) double sided tape. I did not use that, just the dollar store variety (but I think I might just order some of that to have on hand.)

4) Dead earbud headphones.

Tools:

1) Sharp knife ( or two...)

2) Hot glue gun

3) Printer, or access to a printer (attached is a .dxf of what I used)

What follows is a gross oversimplification. If you want to know more there is, you know, the internet?!

When you are trying to place a cross hairs inside an optical system so that they are visible you need to place them in what is called the Field stop of the eyepiece. That way the eyepiece will render the cross hairs just as it renders the objective image. Wait, what?! In any optical telescope (or microscope, but reverse mostly everything that follows) you can divide the effect of the telescope into two categories; light gathering and magnification. As you can see from Figure 209 of the last link above, the role of the objective is to gather light (the objective is larger in diameter than the objective image), and the role of the eyepiece is to magnify the objective image. The aim of the act of focusing is to achieve a complete union of the objective image plane and the Field Stop pane of the eyepiece (in reality neither the objective image nor the field stop is a plane at all wavelengths of light so what we should expect is more of a happy menage that is mostly inclusive).

If that made no sense and was a bit annoying, you can find out where to place the cross hairs by using the eyepiece alone and trying to bring something into focus in the eyepiece. That point will be in the Field Stop plane. That plane ought to be perpendicular to the axis of the eyepiece. Try to get close to there with the cross hairs assembly.

In my case the field stop happened to be at approximately where the baffle meets the barrel of the eyepiece. (not a coincidence as a baffle at that point will optimize the reduction of stray light reaching the eye while not introducing cropping of the light cone).

I measured the inside diameter of my eyepiece at the point that I would place the cross hairs. I created a template that had the cross hairs I wanted superimposed on the circle of the eyepiece inside diameter, and included a concentric circle that was the same diameter as the field stop of the eyepiece.

Take your dead earbuds and strip off a few inches of cable. Cut one of the channels off, and untwist to reveal a very fine fibre. That is what will be used to make the cross hairs.

Cut out both copies of the template using a sharp knife. Apply the double sided tape so that it is oriented in alignment with the printed guide lines.

Using small pieces of double sided tape on one copy of the template attach the fibres in alignment with the template. Then add more tape to fill in the gaps between the axes. I use a scalpel blade to pick up the backing from side 2 of the double sided tape. Then place the second copy onto the first to create a sandwich with the cross hairs captured between the two layers of card stock.

This step is where you need to adapt to your situation. The goal is to illuminate the cross hairs while not flooding the view with light. There are many solutions to this problem as a quick search on 'DIY Illuminated Reticle' will produce. For my case I estimated the angle I would need to aim the light at, and drilled a hole in the side of the eyepice to accept the laser pointer. I also inserted a translucent material to disperse the light. The result should look something like the second picture. I also machined a sleeve to the OD of the laser pointer so that I could use a thumbscrew as a switch actuator, but a binder clip on the push button switch would also work, and if you are deft, pressing the button while aiming the scope will work as well.

And there you have it. A cheap but functional way to introduce consistency to your aiming. Note that the errors that this does not resolve:

1) inaccuracies in getting the exact centre of the eyepiece.

2) inaccuracies in the colinearity between the eyepiece barrel axis and the focuser axis.

This is entered into the Space competition. If you like it please vote. Thanks for looking.