INTRODUCTION
REMOVING THE COATING
REPLACING THE LENS
FIXING THE BRIGHTNESS
TESTING
CHANGING THE AIMING SCREWS
MOUNTING

Cut through the plastic retaining ring and twist it out.

After the lens is removed.

Getting ready to attach the lens to the rod.

Melting the pitch onto the Delrin rod.

The now attached lens--coated side out

Pressing the polishing pad--no, it is not as dangerous as it looks!

The mounted potentionmeter.

Soldering the Potentiometer

Use of the 'zero power' reflex sight was first popularized by the late Steve Kufeld with his now-famous TelRad reflex sight. While it has become standard equipment on most medium to large telescopes, its size, weight, and cost make it inappropriate for some.

A few years back, Daisy introduced a reflex sight for use on their BB guns. With only cosmetic changes, these same sights are being sold for use on amateur telescopes. (Orion, for example, markets a version of this sight for about the same price as a TelRad.)

The Daisy sight can be found at most Wal-Mart stores for about $13. Be sure you get the most recent version of the sight with gold lettering on the side and a 3/4" diameter lens. The older version, with orange lettering, has a 1/2" lens and is more difficult to modify and use.

Unfortunately, as it comes out of the box, the Daisy sight is inappropriate for astronomical use. The LED light is bright enough to use in the daytime and is therefore much too bright for night use. Also, the lens has an anti-reflection coating which dims the night sky by a full magnitude. Finally, altitude and azimuth adjustments require a small screwdriver, which is difficult to use in darkness. In the following paragraphs, we will describe our fixes for these problems. With these modifications, we will have a sight that is better than any that can be purchased commercially; at a fraction of the cost.

REMOVING THE COATING

Our first challenge is to remove the anti-reflection coating from the inside surface of the lens. Getting the lens out is no easy task. It is held in place by a retaining ring at the front of the housing that is cemented in place. Take an Exacto knife and cut straight down through the retaining ring until it has been entirely cut through; taking care not to scratch the lens. With the blade still embedded in the retaining ring, twist the knife so that you expose an edge of the now-cut retaining ring. After modifying seven of these Daisy sights, we have found that some of the retaining rings are glued all the way around, while others only have a spot of glue in a couple of places. If you are lucky, and yours doesn't have much glue on it, you can gently pull the ring out with a pair of needle-nosed pliers. If instead you have a lot of glue, use the knife to scribe between the ring and the housing while applying outward pressure (away from the lens) until the ring is entirely separated. If you are lucky, it will detach in one piece, and will be easy to get back in later (we got this to happen about 50% of the time). If not, don’t lose the pieces of the retaining ring, as you will eventually have to glue them back in place!

Now, press the lens out with your finger. Don't worry too much about scratching it, you are going to polish that side anyway.

There has been some talk on the Internet about using muriatic acid (HCl) to remove the coating. Don't bother trying this - it doesn't work! (Note: We have gotten feedback from at least one person who had success with Muriatic acid, and who pointed out that our method is way too much work. We have found the acid method too frustrating and not very successful. At least one person has been happy with acid, while all our experiences were bummers....(due to this last comment, your parental control software will probably block you from this website). To each his own, and keep the comments coming.) We mounted the lens on the end of a 3/4" diameter by 3" long piece of Delrin rod with a few drops of melted pitch. Remember that the concave side of the lens (facing the LED) is the side with the coating on it. It is thus facing out. When mounting the lens, be careful that it is squarely mounted, and not tilted to one side. You don't have to use Delrin, as any short rod will work. You also don't have to use a 3/4" diameter rod. It does, however, make it easier to center the lens on the axis of rotation if the rod is the same diameter as the lens.

Next, we mounted the rod in the chuck of a small lathe and spun it at medium speed. (This could also be done with a drill press, or even with a hand drill that was securely clamped in a vise.) We attached a small piece of optical polishing pad (available through Salem Distributing pH 800- 456-8468) to a fingertip with double-stick tape, and polished the lens with cerium oxide and water. The fingertip provided a built in temperature and pressure gauge. If you don’t want to purchase the polishing pads, you could use a small piece of felt in its place.

At first we were very conservative, using a slow speed and light pressure. When this produced no discernible results after 15 minutes we decided to get more aggressive. We set the lathe to medium speed and applied medium (about 1lb) of pressure. This produced the desired result in 5 to 7 minutes. You will probably remove the outermost portion of the coating first; don’t worry; the rest will polish out with a few more minutes’ work.

If you apply too much pressure to one side of the lens, you will change its figure and the two curves will no longer share the same optical axis. This will result in a double LED reflection; embarrassing, but not fatal!

As you polish the lens, it will tend to heat up. Don't let it get too hot or the pitch will soften and the lens will shift on the rod. (As a service to others, we tried to make every mistake that was possible!)

REPLACING THE LENS

When you are finished polishing, press the lens back into the housing with the concave side facing the LED. (Be sure to tell your friends that the little spot of coating remaining in the exact center of the lens was deliberately left so that there will be something for the LED light to bounce off. Those who know little about optics will be impressed; everyone else will call your bluff!)

Glue the retaining ring back in using two small drops of model airplane glue, or equivalent. Start with the side that his been split so that the ring is pressed together when you force it in.

FIXING THE BRIGHTNESS

The next step is to dim the LED so that it is useable at night. This is accomplished with a 10K PCB mount potentiometer (Radio Shack #271-282). The dial on this potentiometer is 1/4" in diameter. Take the sight apart and drill a hole 9/32" in diameter just behind the lens. From the underside, this places the hole halfway between the on/off switch and the hole where the adjustment screw passes. Then, press the potentiometer into place and apply a couple drops of the model airplane glue around the sides; being very careful not to get any on the moving dial itself. Let the glue set up for an hour and test the potentiometer to make sure it turns. If you have gummed it up, you can still pry it out and mount another (an experiment we tried several items with different glues).

After the glue has dried overnight, bend the potentiometer leads down flush with the housing so that they are out of the way. Now it’s time to get out your $10 soldering iron. There is a wire running from the negative pole of the battery all the way back to the LED (did we mention not to get glue all over this wire?). Cut this wire and solder it to one side of the potentiometer. We were warned by somebody who knew something about electronics that the potentiometer could be burned out unless we mounted a 1K resister in series with it (Radio Shack # 271-1321). Trim the excess wire off the LED and solder the resister straight to the LED. Solder the other side of the resister to the other side of the potentiometer.

TESTING

Now it is time to test the unit. Dim the lights and turn on the unit (it works better if you remove the paper protector from the battery). Adjust the potentiometer with a small screwdriver to obtain a suitable brightness. If things don't work and the reason isn't obvious, check the battery to make sure it isn't dead. (So far, 1 in 10 have been dead.)

CHANGING THE ADJUSTING SCREWS

So far so good, the unit is completely operational. However, it is still none too easy using a small screwdriver on those tiny adjustment screws in the dark. Therefore, we will swap out the screws for nylon knurled thumb screws. Unfortunately, the present screws have a thread size unknown in this quarter of the galaxy (it may be something called 'metric'). Therefore, we must first drill out the existing holes and thread them for 8-32 nylon screws. A 1" screw fits perfectly for the front hole. A little of the screw’s length will have to be trimmed off for the back screw. Cap these screws with a nylon acorn nut, or a lock nut. (We found nylon acorn nuts in the Small Parts catalog pH 800-220-4242.)

Now you have Alt/Az adjustments that can be done by hand while looking through the sight. If you find you don't have enough adjustment, you can trim the thickness of the internal mounting post with a mill or Dremel Mototool.

MOUNTING THE SIGHT

You can get a dovetail mounting plate to fit your sight at most gun stores, or you can order one from Orion Telescopes for $7.95. For most scopes, you will want to elevate the sight on a 1 to 2" block of wood so that your eye will be in a convenient location during use. The metal dovetail plate can be screwed or glued to this wood block.

Have fun constructing and using this sight!