Finderscope
A small auxiliary telescope for aiming a larger telescope.
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A finderscope is a small auxiliary sighting device used in astronomy, typically a low-power refracting telescope or monocular mounted parallel to a larger telescope so they share the same line of sight. Because its magnification is much lower than the main telescope’s, it offers a wider field of view. This makes it easier to manually point—or “slew”—the main telescope in roughly the right direction, so a desired celestial object comes into view when the observer zooms in.
Some finderscopes include sophisticated reticles for more precise aiming or even for making stadiametric measurements. Finderscopes have mechanisms to align them with the main telescope’s line of sight. On amateur telescopes, this alignment is usually done using three or six adjustment screws. Finderscopes are often labeled with a format like A×B, where A is the magnification and B is the aperture of the objective lens in millimeters (for example, a 6×30 finderscope has a 30 mm objective and 6× magnification).
This is the same format used for binoculars. A 6×30 finderscope is generally considered the minimum useful size for a magnifying finder on an amateur telescope; an 8×50 or larger finderscope is preferred because it allows sighting of fainter objects. Most finderscopes offer one of three viewing orientations. Another common type of finder on amateur telescopes is the reflector (reflex) sight.
Reflex sights
This non-magnifying sight—technically not a “scope”—uses a beam splitter to reflect a reticle, generated by collimating optics, into the user’s field of view. The observer sees the sky as with the naked eye, with an illuminated crosshair or dot that appears to float at infinity. The crosshairs are usually lit by a small LED. Reflex sights are useful for locating bright objects visible to the naked eye, like stars and planets.
Because the sight uses a beam-splitter window rather than an optical telescope that gathers light, objects dimmer than the naked-eye limit cannot be seen through it. To find dim objects, the observer uses the object’s known position relative to brighter ones as a reference, then slews a known angular distance—a technique called “star hopping”—from the bright object to the target. Many reflex sights have circles marked with specific angular dimensions to help with this.
Quick Facts
- Designation format
- A×B, where A is magnification and B is aperture in millimeters
- Example designation
- 6×30 (30 mm objective, 6× magnification)
- Minimum useful size
- 6×30
- Preferred size
- 8×50 or larger
- Adjustment screws
- three or six
- Viewing orientations
- three
Facts from the source article.
Purpose and Core Function
A finderscope is a small auxiliary refracting telescope or monocular that sits parallel to a larger astronomical telescope, sharing the same line of sight. Its primary job is to make the tedious process of locating a target object manageable. Because it operates at a much lower magnification than the main instrument, it delivers a far wider field of view, letting the observer slew the larger telescope into roughly the right direction before committing to a high-power zoom.
Without this preliminary aiming step, finding a specific star, planet, or deep-sky object through a narrow eyepiece would be nearly impossible. Beyond simple pointing, some finderscopes incorporate sophisticated reticles that allow more precise aiming or even stadiametric measurements, turning a basic alignment tool into a modestly capable measuring instrument. In essence, the finderscope bridges the gap between the vastness of the night sky and the narrow corridor of a high-magnification optical path.
Specifications, Sizing, and Alignment
Finderscopes follow a familiar naming convention borrowed from the binocular world: a designation like A×B, where A represents the magnification and B the objective lens aperture in millimeters. A 6×30 finderscope, for instance, pairs a 30 mm objective with six-power magnification and is generally regarded as the smallest size that remains practically useful on an amateur telescope. For those wishing to spot fainter celestial targets, an 8×50 or larger unit is preferred, as the bigger aperture gathers more light.
Proper alignment of the finderscope to the main telescope's optical axis is critical, and the method depends on the particular design and mount. On most amateur instruments, this is accomplished through three or six small adjustment screws that the user tweaks until the reticle in the finder matches the crosshair in the main eyepiece. Most finderscopes also come in one of three distinct viewing orientations, giving observers a choice in how the image appears through the small scope.
Reflex Sights – A Different Approach
Not every finder on an amateur telescope is a true optical scope. The reflector, or reflex, sight takes an entirely different approach: it is a non-magnifying device that uses a beam-splitter window to project an illuminated reticle into the observer's field of view. A small LED powers the crosshair or dot, which appears to float at infinity against the naked-eye view of the sky. Because there is no objective lens gathering light, the reflex sight cannot reveal objects dimmer than the unaided eye can see.
To locate fainter targets, the observer must rely on star hopping—using a bright, visible object as a reference point and then slewing a known angular distance toward the desired target. Many reflex sights include printed circles of a specific angular size to make this estimation easier. However, in heavily light-polluted areas, even the reference stars may vanish, rendering the reflex sight far less effective than a magnifying finderscope.
The Observer's Workflow and Practical Trade-offs
In practice, the finderscope transforms what would be a frustrating search into a structured two-step process. The observer first sweeps the sky through the low-magnification finder, using its generous field of view to identify the general region of the target. Once the object sits comfortably within the finder's reticle, the main telescope is already pointed close enough that a modest adjustment places the target squarely in the high-power eyepiece.
This workflow depends heavily on the quality of alignment between the two instruments; if the finder's line of sight drifts from the main scope's axis, the object may appear in the finder yet remain stubbornly out of reach at high magnification. The choice between a magnifying finderscope and a reflex sight also shapes the observing experience: the former can reveal faint objects directly, while the latter demands the observer mentally map angular distances between stars, a skill that becomes harder to apply under bright urban skies where the guiding stars themselves fade from view.
Frequently Asked Questions
What are the common Finderscope designations and sizes?
Finderscopes are labeled with an A×B format where A is the magnification and B is the objective diameter in millimeters, with 6×30 being the smallest practically useful configuration. Most enthusiasts prefer an 8×50 or larger for a steadier, brighter image.
How is a Finderscope physically adjusted?
The device is secured to the main tube with either three or six adjustment screws that let you fine-tune its alignment so both scopes point at exactly the same spot. There are also three standard viewing orientations the finderscope can be set to, accommodating different mounting positions on the telescope body.
Can a Finderscope do more than just help you aim?
Yes—some models carry engraved reticles that allow the observer to make stadiametric measurements or perform more precise targeting. In that sense, the Finderscope doubles as a small measurement instrument, not merely a pointing aid.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Finderscope (CC BY-SA 4.0).
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