Review – Takahashi 7x50mm Finderscope

The Takahashi 7x50mm Finderscope is a doublet refractor.

Specifications

Aperture:  2.0 in. [50 mm]
Focal Length:  12.1 in. [309mm]
Focal Ratio @ Prime Focus:  f/6.2
Field of View:  6.3°
Limiting Magnitude:  ~ 9.5 urban / ~ 11.0 rural
Resolving Power:  2.3 arc seconds
Tube Diameter:   in. [mm]
Tube Length:   in. [mm]
Weight:   .86 lb. [.39 kg]

Introduction

Design

Yes, a finderscope is a real telescope – some are just better than others.  Takahashi makes some of the better ones; baffled, with machined aluminum and quality fit just like their OTAs.  There are three finderscopes to choose from: the 6x30mm, 7x50mm, and 11x70mm.  I chose the 7x50mm primarily because the magnification [at 7x], aperture [50mm], and fov [6°] were close to a “threshold” minimum to detect the brighter deep-sky objects [there are some general finderscope requirements involved that depend on your main scope and observing location – i.e. you don’t need a finderscope as large as your primary scope and light pollution limits the effectiveness of smaller aperture finders].

If you star-hop, you need a finderscope to help fine align your telescope after a rough positioning on targets.  Since a powered finderscope is intended to be used to micro-guide you to a target object, an optional reticle illuminator comes in handy by lighting the crosshairs towards the fov center [this is most helpful when centering objects with small angular sizes like planets and double stars].

Observing

Another useful application would be to roughly test the night sky’s limiting magnitude and seeing conditions by viewing familiar objects that you’ve committed to memory like M 7, The Double Cluster, M 35, and Mel 111 through the finderscope because its smaller aperture is more sensitive to atmospheric changes [at least in the same way your eye or any telescope is, knowing the limits of each instrument is the key].  Using M 7 as an example: On the very best nights here in suburban Austin, the cluster can first be seen as a faint smudge visually [looking out the window from my house] indicating easy naked eye stars to magnitude 4.5 and greater.  Next, the 7x50mm is used to confirm sky transparency by attempting to view the faint outer cluster members [I need to make a rough star count to show the difference between mediocre and exceptional nights; a bad night only shows the brightest stars, a great night shows many faint stars across the fov].  If the former is the case, only the Moon, planets, and double stars are viable telescope targets.  If the latter is the case, observing planetary nebulae, open and globular star clusters, and bright galaxies can be done successfully.

Visually, the 7x50mm shows stars across a fairly flat field with nice colors; in fact, if it weren’t for the giveaway crosshairs, I’d swear I was looking through a larger OTA!  Yet some objects are at their best in a finderscope simply because of the large fov needed to frame them; for instance, The Pleiades – M 45 looks great when the outermost radiating stars can be seen along with the cluster’s brighter members.  The same effect can be had with other large clusters although you shouldn’t expect a whole lot of detail with only 7x magnification.  The globular star cluster M 15 looks distinctly nonstellar, and shows an off-white glow around its condensed core.  On one memorable occasion in Big Bend, the constellation Lyra was remarkably framed with dimmer stars shining throughout as a kind of sugary background – not exactly what you might expect from such a small instrument.

All-in-all, the Takahashi 7x50mm Finderscope is a fantastic companion to any quality telescope.

Priced ~ $470 each w/ finder bracket & reticle illuminator.

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