Introduction: Add a Reticule to a Telescope Eyepiece
A reticule (or reticle, or graticule, or whatever you want to call it), is the cross hair you typically see in a telescope's finder scope. In order to ensure the finder scope and main scope are aligned it's handy to have an eyepiece with a reticule. However, if you're like me, and don't have a heap of money to spend on an eyepiece with a reticule, then it's fairly simple to add one to an existing eyepiece.
The way a reticule works is for the cross hair to be placed on the focal plane of the eyepiece. I found a reference on an astronomy forum to someone adding a reticule by mounting the cross hair on a ring made from brass sheet, and sliding that into the rear of the eyepiece. Looking at a spare eyepiece I had, It seemed like this would be a good, non-permanent, solution for me.
Step 1: Materials and Tools
There are a few things needed to make the reticule:
- An eyepiece (I used an old eyepiece that I no longer use any more)
- Thin copper or brass sheet (I used 0.2 mm copper sheet, because I had plenty lying around)
- Thin thread* for the cross hair, the thinner the better
- Electrical tape
- Cyanoacrylate glue
The tools you'll need are:
- Metal ruler
- Vernier calipers
- Hobby knife/box cutter or similar
- Something round to form the copper sheet around
- Sand paper
* I had initially planned on using cotton or similar, but after some research I decided it would be too thick. Reading through a few different websites, I learned that originally spider's silk was used to make the cross hair. I found a reference to someone making a reticule using hair, so I decided to use that. My hair is nowhere near long enough, so I grabbed a couple of loose hairs from my wife's hairbrush.
Step 2: Measuring and Cutting the Copper Sheet
The first thing to do is measure the internal diameter of the eyepiece. This will allow you to calculate the required length of brass/copper sheet to cut (multiply the diameter by 3.14159). I found that my sheet of copper was about 5mm wider than what I needed, so I simply measured a 5mm wide length to cut off.
I've found from past experience that trying to cut copper sheet with tin snips is a recipe for disaster. Every time I've tried it I've ended up with a strip wider at one end, and with a pronounced curve, too. So, to avoid this, I clamped my metal ruler on the line I measured, and scored the copper with a hobby knife (which pretty much destroyed the blade, but it made life easier). Once I'd scored the copper I could bend it back and forth to complete the cut.
A gentle tap with a hammer soon smoothed out the spots where the strip was warped. I measured the length of the strip, and cut the excess off. Don't lose the excess (if you have it), it'll come in handy later
Step 3: Forming the Strip Into a Ring
To get the strip into a ring shape I bent it around the head of my hammer. This was much smaller than the diameter I needed, but I found that the copper ended up very close to the right diameter. I did the final forming around a coin which coincidentally happened to be exactly the right diameter.
The next step is to stop the ring from springing apart. Normally I'd solder this, but my soldering iron is packed away, so I decided to glue the excess I trimmed off earlier over the join with cyanoacrylate glue. To stop the ring coming apart while I did this I wrapped thin strips of electrical tape around it. I finished off by smoothing the edges of the ring with wet and dry sandpaper
Step 4: Adding the Cross Hairs
To ensure that my cross hairs were as close to 90° as possible I grabbed a piece of scrap wood and a compass. I marked a straight line, and marked a circle the same diameter as my ring. I placed 4 marks on the circle 90° apart. I could then line the copper ring with the pencil circle and transfer the marks to the ring.
I used my hobby knife to make a small nick at each mark to hold the cross hairs in place. I used cyanoacrylate to hold the hairs in place. I found it easiest to glue one end first, wait until it was set, and then glue the other end, in order to keep the cross hairs tight.
Step 5: Installing the Reticule
Now that the cross hairs are glued onto the ring, it's time to install it in the eyepiece. The cross hairs need to be as close to the focal plane of the eyepiece as possible, so make sure that the side of the ring the cross hairs are glued to is inserted first. After pushing the ring in I used a wooden dowel to push the ring all the way into the eyepiece. It pays to be careful when doing this, because if the dowel slips off the ring it could easily break a cross hair. I found that it was quite hard to push the ring past the filter thread, but once past that it slid into position quite easily.
Step 6: Testing the Eyepiece
Having installed the reticule, I put the eyepiece in my telescope and tested it out. The result, while not perfect, was pretty good. The first thing that I noticed was that I did not have the reticule close enough to the focal plane of the eyepiece, so it was out of focus when focusing on an image in the telescope. The second thing I noticed was that one of the cross hairs was not straight, I assume because I did not get it tight enough when I glued it in place.
Having looked closer at the eyepiece, I discovered that there is a gap between where I can push the ring to, and where the glass elements of the eyepiece are, which would account for the discrepancy in placement. It should be possible to put a smaller ring in this section, but it would be very difficult to measure and install without damaging the eyepiece.
In practice, though, the eyepiece worked just fine, and I was able to use it to get my finder scope aligned much quicker than in the past.
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