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Finder scopes esp. 50mm right angle units

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About this observation
Observer
Peter Anderson
Time of observation
13/10/2019 - 03:50
Object
102mm F4.9 refractor with 9X50 right angle finder
Observing location
Brisbane Australia
Equipment
Standard camera
Exposure
Not applicable
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The 'straight through' 6X30 finder has been replaced by the right angled unit for ease of operation. (Article follows)

Finder telescopes including the common 8X50mm.                             by Peter Anderson 10/19

In this modern age many ‘red dot’ finders are marketed but these suffer from some drawbacks, namely being limited by the acuity of the observer’s eye, the occasional electronic issues, and flat batteries. The same applies to laser pointer finders mounted on telescopes.

Optical finderscopes have endured by their ability to show fainter objects and also their reliability. Their good name was threatened some decades ago when the manufacturers of cheap telescopes inserted a single element objective lens into their already barely adequate 5X24 size and then placed an aperture stop about 8mm in diameter so the image remained passable. The resulting faint images were only usable for finding brighter objects. Even now some contemporary 5 X 24 units on sale are stopped down to around 12mm (5X12), but are cheap and can be purchased with mounting for around AUD$20 or less.

A step forward is the 6X30 finder, (but full aperture this time), from $29 and this is the common and very useful size attached to many smaller telescopes.

The next popular size, and in use with virtually all larger telescopes is the 8X50mm. Whilst still producing a very useful field of a little over five degrees, it has sufficient light gathering power to locate many fainter objects. These sell (complete with mount) from $69.

Contemporary finder telescopes have two alternate types of focussing. The more expensive units have focussing at the eyepiece enabling easy and convenient adjustment. The remainder have a locking ring at the objective end.  The objective is screwed forward or backward until the desired focus is achieved, and then locked in place. Thus any further adjustment to focus is not simply a matter of turning an easily accessible thread. The 30mm and 50mm finders described herein employ this latter system.  The so titled ‘5X24’ is screwed at the eyepiece.

Many observers find that access to the eyepiece of a traditional ‘straight through’ finder telescope becomes very difficult at certain attitudes, and at higher altitudes. I have a small prism, and a dentist’s examination mirror each mounted in a small tube to slip over an eyepiece tube and give me a convenient right angle view. However the field is restricted and generally unsatisfactory.  

Right angle finders telescopes largely address this problem. There are 6X30 units but the most popular seems to be the right angle 8X50’s. Unlike the classic inverted images of the straight-through finders, the prisms in these right angled finders produce an upright, left to right correct image, making it very convenient for referencing the sky when aligning the telescope. 8X50prices range from $99.

I own two such two right angle finders. One is a Saxon 9X50 and the other a GSO 8X50. They are lightweight, the Saxon including mount and caps weighing just 478grams.  The differences between the two are not significant and for the purposes of this article, I have described the type generically as ‘8X50’.

A ‘straight through’ finder must be approached and used from behind at any angle but always facing the object to be viewed. A right angle unit must be approached from the same right angle, and this can also be inconvenient because there is no facility on these described units to rotate the eyepiece sideways such as is possible when adjusting the eyepiece prism on your refractor or SCT.  This fixed eyepiece position detracts considerably from the unit’s usefulness when attached to equatorial mounts. (Using more than one finder, as always, may alleviate some difficulties.)

However the right angle finders are a boon and very convenient when using alt-azimuth mounts, when the movement is simply up and down, though there can also be difficulties. On my alt-azimuth CPC11 the finder is mounted on top and so becomes too high at very low altitudes. I have adjusted it, loosening the eyepiece adaptor tube to rotate sideways, with a physical stop after 90 degrees of rotation. This arrangement serves well for objects near the horizon.

An alternative: Finder telescopes exist that have, or can be fitted with rotatable eyepiece prisms.  These include the Orion 70mm F3.9, but the exercise then becomes considerably more expensive. This concept can be extended to mounting an 80mm F5 short focus refractor optical tube. I mounted one atop my 150mm refractor, but discovered when using it as a finder that some cheaper eyepieces are not properly optically centred and in slewing the angle of the eyepiece prism around the target object also moved somewhat in the field. This is not good if the unit is intended as a precision finder. Nowhere is this effect more obvious than when using bino-viewers where the image to the eyes may need to be aligned by rotating the eyepieces. In my experience, good quality eyepieces do not have this problem.

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