Setting Up The Finder

Introduction

One of the most common complaints I hear when helping beginners is “the finder sight on my telescope doesn’t work”. Many don’t understand (and the instructions with entry-level telescopes are not clear) that the finder must be carefully aligned after it is installed — it will not automatically point your telescope to things in the sight without this calibration.

Furthermore, you will likely need to re-align your finder regularly, so it’s important that you become familiar with how to do it quickly. How you align your finder depends on what kind of finder you have. You might find it useful to review this article on the types of finder scopes before continuing here.

When to Align the Finder

Your finder is a precision instrument that is removable, and can be precisely moved in two directions. All this ability to move means you can’t expect it to remain rigidly pointed in precisely the desired direction forever. You will need to align your finder:

• When you first assemble your telescope; and
• Any time you remove the finder from the telescope; and
• After any shocks or jolts to the telescope or finder; and
• With some telescopes, if the telescope or finder are rotated; and
• At various other times.

The other consideration on when to align your finder is that it’s better to do it in daylight, if possible, or at least not in the dead of night. You’ll be better able to see what you are doing, and you won’t be cutting into your precious observing time. Finders with cross-hairs can be aligned in daylight. “Red dot” finders must be aligned when it is dark enough to see the dot against the sky, but this can often be dusk rather than full darkness.

General Approach

The details of how to adjust your finder will vary with the specific model, but the general approach you will follow is as follows:

1.

Securely attach the finder to the telescope, pointing as straight as possible along the tube of the telescope.

2.

Snugly tighten whatever adjustments the finder has, so it is stable.

3.

Point your telescope at an easy-to-find, non-moving object as far away as possible.

• If you are aligning a cross-hair finder in daylight, use a distant tower, chimney, telephone pole, etc.
• Avoid moving objects such as trees swaying in the breeze.
• If you are aligning at night, use the North Star if at all possible — it doesn’t move.
• As a last resort, use a bright star, but be prepared to have to repeat the next steps several times to accommodate the star’s motion. The advantage of using a piece of a distant building or other object on the ground is that you can find it by “walking the scope” along the horizon and up the side of the building or object.

4.

Get the distant object centred in the scope at low or medium magnification. Remember it may be upside down or reversed left-right depending on the optics of your telescope.

5.

Now, without moving the telescope, adjust the finder’s aim controls (see below) to centre the cross-hair, red dot, or reticle over the distant target. Again, remember that some finder telescopes may reverse or invert the image. (The image shown here is through a “correct image” finder, which does neither.)

6.

Repeat at an increased magnification.

7.

Test by using the finder to centre a different distant object and confirm it is visible in the telescope.

Adjusting a Unit-Power Finder

Unit-power finders are generally very easy to adjust. Most have simple dials for up-down and left-right. Some use a 3-point adjustment system or require tools. On every unit I’m familiar with, adjustment can be done with one hand, by adjusting one knob or screw at a time, so there are no complex motions to master.

Here are a few examples:

This Stellarvue “red dot” finder has simple knobs for adjusting the up-down and left-right location of the red dot. The brightness control is also visible here, underneath the front window on the right in this photo.

Most other small red-dot finders have similar controls.

This Stellarvue multi-reticle finder is one of my favourite unit-power finders. Despite its convenience and high build quality, it has an annoying disadvantage: it requires a tool to align it. I found the supplied bent hex key awkward to use, and often dropped it in the dark, so I now use this long T-handle hex driver to adjust the marked alignment screws.

The Telrad finder’s adjustments are slightly different. There are three adjustment knobs, each moving the reticule toward or away from the centre of an imaginary triangle. The directions of movement are up-down, "up-left, down-right", and "up-right, down-left".

While it may take you one or two minutes to become used to these directions, I find the Telrad quickly becomes the easiest of the unit-power finders to align. The large, well-spaced knobs produce a predictable and smooth motion of the large concentric-rings reticle, making fine-tuning so easy that I always touch up the alignment before every observing session.

Adjusting a Magnifying Finder

Adjusting a magnifying finder scope is similar in principle, but the actual technique is slightly different because of the way most finder scopes are mounted. Finder scopes are usually mounted in a single or double set of rings which are, themselves, mounted to the main telescope. The telescope-to-rings mount is fixed and non-adjustable; what you adjust is the system that holds the finder scope inside the rings.

To be steady yet adjustable, the finder scope must be held by pressure at two different points along its length. Most do this by using two separate rings, each of which has 3 screws that press against, and support, the finder scope.

The finder should be securely mounted on your telescope (this one is removed to facilitate the photo). Locate the 3 or 6 adjustment screws on your finder scope and tighten them all just snugly (holding without any “flop”, but not so tight they deform the screws or the finder scope). On first setup, try to tighten them in such a way that the finder scope is roughly centred in the ring system — with each of the screws screwed in about the same distance.

To move the finder a small controlled distance, you will need two hands. Hold two adjusting screws, one in each hand’s thumb and forefinger. While looking through the finder, smoothly and gently loosen one screw a small amount and tighten the other screw at the same time, so the combination remains snug. You will see the view in the finder shift slightly. Note the direction of shift and continue, reverse, or pick a different pair of adjustment screws, and continue until you have the target centred. Because you have adjusted two screws all along, the screws should still be snug when you have the finder perfectly pointed, so no final tightening will be needed.