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Review of four 65 ED spotting scopes

16 April 2013
Arkadiusz Olech

2. Acuter ED 16-48x65 – spotting scope review

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review


Design and build quality

The Acuter brand name includes spotting scopes produced by Sky Watcher, a company well-known on the astronomical market. The series consists of models with the following objective diameter: 65 mm, 80 mm and 100 mm. All of them are offered in straight or 45-degree angled version. No matter what objective diameter, you can choose from two models: with a normal achromatic doublet objective lens or a better one, featuring low dispersion ED glass.

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The producer doesn’t provide any precise information concerning the optical construction of the offered equipment. One glance inside the spotting scopes allows you to find out that most probably we deal here with a doublet objective lens in which one element was made of low dispersion glass. Inside the tube there is a focusing lens (perhaps an achromatic doublet as well). Then we have Porro prisms and behind them, in the angled version, there is a prism which directs beam of light at an angle of 45 degrees toward the axis of the objective. Behind it there is one more flat protective element.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

The spotting scope features a focal length of 386 mm which, with the 65 mm objective lens, gives us the aperture fastness of f/5.94. As you see we deal here with a fast device in which the chromatic aberration makes itself felt rather often. The presence of low dispersion glass is more than recommended then. A chart below includes exact parameters of the tested spotting scopes.

Objective diameter:
65 mm
Focal length:
386 mm
Aperture :
f/5.94
Prisms:
BaK-4/Porro
Optical construction:
2-element objective lens with ED glass
Dimensions (length × width):
325 mm × 80 mm
Weight:
885 grams
Price:
1099 PLN / 250 Euro


The body of the scope was made of light aluminum which, in the rear end, was additionally padded with rubber. On some joints the rubber doesn’t stick to other elements evenly so you get an impression of slightly shoddy workmanship. The instrument is waterproof and nitrogen-filled. It is worth emphasizing that the Acuter, weighing just 885 grams, is physically the lightest 65ED spotting scope in our test.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

The spotting scopes unfortunately don’t feature any dew shield so when you observe something in conditions of high humidity you certainly can expect droplets of dew on the objective lens. A ribbed focusing roller is well-damped and it works smoothly, without any squelching. Unfortunately it boasts only one focusing gear – it is a pity there isn’t a second one, slower, which would allow you to focus more precisely when needed.

The spotting scope is equipped with a tripod adapter with two holes of different diameters. It allows you to put the device on a standard photographic lens head.

At the end of the spotting scope you can find an eyepiece mount, with a 34 mm thread, outside diameter. Inside the mount is 31 mm in diameter so a bit smaller than the standard 1.25 inches, so the eyepieces of such diameter can’t be used here.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

The producer emphasizes the fact that all optical elements were covered by antireflection multilayer coatings. Indeed they can be noticed both from the side of the objective lens and the side of the eyepiece. They are light greenish, a characteristic colour indicating a distinct dip in the centre of the transmission curve. Such coatings can be found in a big number of spotting scopes and binoculars, produced in China nowadays. Unfortunately their intensity is significant and it bodes ill for transmission results.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

A minute inspection of the inside shows that the inner tubes of the spotting scope are painted black but unfortunately the paint is quite shiny so it doesn’t create favourable conditions for dampening inner flares. The bottom and prisms housings are black or grey-matt so here we don’t have any reservations. The cleanliness inside the tubes should be praised as well – we haven’t noticed any specks of dust, hairs, scratches or any other snippets.


Eyepiece

The Acuter spotting scopes are sold with a zoom eyepiece with the focal range from 8 to 24 mm. In the case of the 65ED model it translates into the magnification range from 16.1 to 48.2 times and a size of exit pupil changing from 4.0 to 1.3 mm. The eyepiece has its own field of view, ranging from 40 degrees at 24 mm to 60 degrees at 8 mm – that’s why the fields of view, offered by this spotting scope alter from 2.49 to 1.24 degrees. These 40 degrees at wide angle aren’t especially impressive. Using such an eyepiece is like looking through a tunnel or a keyhole. Fortunately that problem disappears when you pass to shorter focal lengths because 60 degrees don’t create you such effects any more.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

The eyepiece also provides quite comfortable eye relief – from 18 to 15 mm. Even people wearing glasses shouldn’t have any problems with observations. Actually the eyepiece you get in the box with the Acuter is the smallest and physically the lightest 8-24 mm device in our test. Its complete length is 108 mm, it weighs 210 grams and it is 43 mm in diameter the widest place. Its basic parameters are presented in a chart below.

Focal lengths range:
8–24 mm
Fields of view range:
60–40 deg
Eye relief:
15–18 mm
Optical construction:
no data
Dimensions (length × diameter):
108 mm × 43 mm
Weight:
320 grams
Price:
added in the set


A photo below shows a comparison between different 8-24 mm eyepieces. From the left you see devices added to the Pentax, the Delta Optical Titanium, the Celestron Regal and the Acuter.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

Looking from the side where you put your eye there is a rubber eyecup, over one centimeter high. If you wear glasses you can fold it outside or take it off completely. The lens inside the eyecup is 22 mm in diameter and covered by characteristic greenish antireflection coatings.

Already on the casing of the eyepiece you can find a very big focus ring, almost 5 cm wide. It is padded with rubber with 37 mm ribbing. The part without the padding has magnification marks for values: 16x, 24x, 32x and 48x. The ring itself moves stiffly and its movement doesn’t change the position of the lens near your eye but it changes the front element set, hidden in the eyepiece extension tube. During the focusing you can hear very distinct shuffling of the moving parts.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

Further on, you see a metal ring, 7 mm wide with an inner thread which is used to fix the eyepiece to the body of the spotting scopes. A bushing, 26 mm in diameter, sticks out of it with the front element set inside. The set is also multi coated with greenish antireflection layers.


Vignetting

The light beam, coming from the prism set, is not directed perfectly well toward the central axis of the extension tube. It makes the exit pupil noticeably truncated and you can observe it in several places. It is visible when you take a photo of the proper exit pupil (it is noticeably truncated in it) or a photo of a ring of light coming from the prisms, taken perfectly within the axis of the extension tube. What’s more, very defocused images of stars are not round but rather reflect the shape of the truncated pupil.

Exit pupil Eyepiece tube
Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

Photos above show how the exit pupil looks for minimum magnification and the shape of the prisms photographed along the axis of the eyepiece. In this case the vignetting amounts to 4.6% and exactly so much light, diligently gathered by the objective lens, you lose at the very beginning.


Inner flares

The photo presented below show the area next to exit pupils at maximum and minimum magnification ratio.

Power 16x Power 48x
Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review

Around the pupil you can notice lighter, concentric rings of light and additionally, near the pupil, to the left and below, there is a distinct flare from the prisms. Despite these shortcomings the Acuter doesn’t fare badly in this category – the competitors can perform a lot worse.


Chromatic aberration

During a sunny day we pointed the spotting scopes at a chimney positioned against a cloudless sky. At 16x the Acuter showed us a very nice image, practically without any chromatic aberration in the centre. You could only perceive a slight yellow rim near contrasting edges. Closer the border of the frame yellow becomes more visible but it is an interesting fact that the Acuter fares better than other spotting scopes, taking part in this test.

The maximum magnification is a much worse matter. In the centre you can notice distinct purple-blue rims and very pronounced yellow on the edge of the frame. Around the eyepiece diaphragm there is a very light, purple rim which is quite annoying.

The observation of bright Venus was a very painful test for the Acuter. With the magnification of 48x we should be able to see a disc of the planet very well. Every other spotting scopes managed to show it nicely apart from the Acuter. In its case the combination of chromatic aberration and huge astigmatism turned Venus into a blurred purple-yellow-red ball.

We also pointed the spotting scopes at the Moon near its quarter. The chromatic aberration levels were here the same as in the case of the chimney so a very nice performance at 16x and noticeably weaker results at 48x.


Astigmatism

The astigmatism correction is a huge problem for the Acuter. We’ve already mentioned Venus observations – also because of a high level of astigmatism we couldn’t manage to see the disc of that planet. At minimum magnification the astigmatism of the Acuter is also very distinct and in this category it fares the worst of all spotting scopes which took part in our test.


Coma

In the case of 16x magnification the coma appears in the distance of about 65% from the field of view centre and it is medium on the very edge. Better results you get at 48x, where the coma appears in the distance of more or less 80% from the centre and on the edge it borders low and medium values. The shapes of stars turn into noticeable commas.


Distortion

At 16x you can notice a slight pincushion distortion which decreases as the magnification rises. At 48x that aberration becomes practically invisible.


Blur on the edge of the field

The eyepiece which you get in the box doesn’t feature especially wide fields of view so in its case you can be pretty demanding when it comes to the image resolution on the edge of the field. Using 16x magnification you have to take into account the fact that significant blur might appear even in the distance of about 70% from the centre. The situation is a bit better at the minimum focal length (at 48x magnification) because 85% of the field of view is sharp there.


Accessory kit

Buyers get a soft protective case for the spotting scopes and a small case for the eyepiece. The latter is additionally put into an oval container made of plastics.

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review


Summary

Pros:

  • relatively low weight,
  • the cheapest 65 mm class ED glass spotting scopes,
  • decent price/quality ratio,
  • not very bothersome distortion,
  • exit pupil visible on a quite dark background.

Cons:
  • high intensity of antireflection coatings,
  • truncated exit pupil,
  • high astigmatism,
  • noticeable chromatic aberration at maximum magnification.

The Acuter ED 16–48×65 is the cheapest 65ED spotting scopes which took part in our mini test. Unfortunately its cheapness made itself felt many times. The optical properties are better than in the case of typical spotting scopes without low dispersion glass, especially when it comes to the chromatic aberration correction; still it is too little to compete successfully with other ED spotting scopes. However when the price is your main criterion the Acuter might be actually a good choice. It is not expensive, it provides decent images (even good ones at smaller magnifications), it is handy and physically light so its price/quality ratio remains decent. There is still one snag – it is called the Delta Optical Titanium 65ED spotting scope. That device, only 100 PLN more expensive, is optically better and it comes with better accessory kit at the same time...

Review of four 65 ED spotting scopes - Acuter ED 16-48x65 – spotting scope review