Finder Alignment

A go-to scope will have to be pointed at alignment stars before it will find anything. To do that, the telescope's finder will have to be properly aligned. If the scope is a non-go-to model, accurate finder alignment is even more vital since the finder will be used to locate all objects for viewing. In an aligned finder, what is in the crosshairs of an optical finder or under the red dot of a zero-power sight is also in the center of the main telescope's field.

There are two ways to adjust a finder's alignment: by using a distant terrestrial object or by using Polaris or another bright star. A star is generally best since a terrestrial target must be far enough away that it will come to focus in the main scope and also far enough away that parallax is not a problem. If an object is too close, the physical separation between the main scope and the finder will cause targets centered in the finder to be "off" in the main scope, no matter how carefully the finder was aligned. A Polaris alignment will probably be required for a red-dot sight since the dot will likely not be visible in daylight. Why Polaris? For all intents and purposes, it does not move, which makes it a great alignment "tool."

To align the finder, insert the lowest-power eyepiece available in the main scope and remove the dust cap from the corrector. Uncap the finder objective and eyepiece or switch on a red-dot model. Unless the scope is one of the few that cannot be moved except with the hand control, it does not have to be powered up for finder alignment; just unlock the mount's locks and slowly move in the direction of Polaris. As the star is approached, look through the finder and continue to move the OTA until the star is in the crosshairs or under the red dot. When it is centered in the finder, lock both telescope axes.

Take a look through the main telescope. If all that is visible is something that looks like a donut, a round blob with a dark center (the shadow of the telescope's secondary mirror), turn the CAT's focus control until the star (that is what the donut is) becomes as small and sharp as possible. If it gets bigger, turn the control in the opposite direction. What if nothing at all is visible? It may be that focus is so far off that the star is a huge, invisible blur. Turn the knob a few turns in either direction experimentally and see if anything appears. Still nothing? It is likely the finder is so badly misaligned that Polaris is outside the field of even the lowest-power eyepiece. Move the scope slowly in all directions using the mount's slow-motion controls if it has them (and assuming it is okay to use them with the power off—check the manual) and sighting along the side of the tube if necessary until Polaris is in the eyepiece's field. If that does not help, move the scope to the Moon if it is in the sky or a streetlight. Neither of these is an ideal finder alignment target, but they can be used to get the finder "in the neighborhood."

When the star or other object is in the field of the main scope, tighten the mount's locks and look through the finder again to see if Polaris is still centered. It probably is not. Adjust the finder's aim until Polaris is back in the crosshairs by means of the adjustment screws in its ring mounts or, for a zero-power finder, by tweaking two little knobs or screws, one for left/right and one for up/down. When it is centered, look through the main scope again. Is it still in the middle? If not, adjust the mount until it is and go back to the finder and readjust that. Keep going, maybe changing to at least a medium-power eyepiece in the main scope, until whatever is put in the finder crosshairs or under a red dot is reliably centered in the main scope's eyepiece.

What now? Take another look at lovely yellow Polaris in the main scope. Can you see the tiny spark that is this double star's "little" companion? Whether you can or not, just enjoy the sight of the new CAT's first star for a moment. Savor the wonderful feeling that comes with taking first light with a new telescope and do not feel embarrassed if you find yourself yelling for your family to, "Come look at the beautiful star!"

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