Six of the best 2-inch dielectric diagonals: £99-£200

Our experts review six 2-inch dielectric diagonals, suitable for a range of budgets and requirements.

resized_William Optics 2-inch Dura Bright

Sky-Watcher Deluxe Dielectric Coasted 90° Star Diagonal

Price: £109 
Surface reflection: 99% 
Weight: 470gm 
Extras: 2 dust caps, 1.25-inch adaptor, barrel filter threaded for 2-inch filters 
Supplier: Optical Vision Ltd 
Tel: 01359 244200 
www.opticalvision.co.uk

resized_Sky-Watcher Di-electric
www.thesecretstudio.net

The Sky-Watcher Deluxe is a straightforward diagonal – designed to put eyepieces at a comfortable angle to look through – that uses dielectric coating to give the maximum light transmission of 99 per cent.

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In all our tests it gave brighter views compared with our basic test 2-inch diagonal.

The internal baffling was good, with little in the way of internal reflections when we viewed the Moon.

The stars remained crisp, but, when compared with some of the others on test, we had an impression of slightly less contrast in deep sky objects such as the Great Globular Cluster, M13, and the galaxy pair M81 and 82.

As we slotted our 2-inch eyepiece in and out, we noticed how the Sky-Watcher diagonal showed good quality-machining, combining a sleek design with smooth operation.

The diagonal can take 2-inch filters in the threaded barrel, which is useful if you intend on using a light pollution filter.

Packaged in a basic black box, the diagonal does its job well.

Verdict

A good overall performer with no frills

For: Simple and easy to use

Against: Slightly less colour contrast in stars

4.0 out of 5 star rating

Altair 2-inch Positive Lock 90° Prism Diagonal

Price: £125
Surface reflection: 99%
Weight: 646g 
Extras: 2 dust caps, 1.25-inch adaptor, positive-Lock, barrel filter threaded for 2-inch filters
Supplier: Altair Astro Ltd 
Tel: 01263 731505
www.altairastro.com

resized_Altair 2-inch Positive Lock
www.thesecretstudio.net

The Altair Positive Lock was the white sheep of the pack, as its body stood out among the other black-coated diagonals.

It will certainly be easy to spot in the dark and you won’t bump into it with your head.

It’s the heaviest diagonal, at 646g, and it felt like a good sturdy piece of kit.

In our practical tests the Altair diagonal gave the best colour rendition of Arcturus, the brightest star in the constellation of Boötes.

The internal baffle’s matte coating did its job: we had good contrast in our views of the deep sky and the Moon showed no internal reflections.

A real plus, is the Positive Lock knurled ring which, when turned, locks the eyepiece firmly in position. It was so efficient that we felt the supplied thumbscrew was almost unnecessary.

It’s one of those simple ideas that deserves to catch on and be incorporated as a standard feature on all star diagonals.

A nice touch, the ring held our large, heavy eyepiece perfectly.

Verdict

Excellent performer overall despite the weight

For: Positive Lock held eyepieces securely

Against: Heaviest diagonal of the six

5.0 out of 5 star rating

Explore Scientific Diagonal Mirror 2-inch

Price: £142
Surface reflection: 99%
Weight: 466g
Extras: 2 dust caps, 1.25-inch adaptor, barrel filter threaded for 2-inch filters 
Supplier: Telescope House
Tel: 01342 837098
www.telescopehouse.com

resized_Explore Scientific
www.thesecretstudio.net

Like all the test diagonals, the Explore Scientific boasts a reflectivity of 99 per cent due to the dielectric, coated mirror.

It weighs in as the lightest model, at 466g, yet its sturdy and could easily handle our 21mm Ethos eyepiece, which itself weighs just over 1kg.

Unlike the other review models, it uses two thumbscrews to apply pressure to the brass compression rings that held the 2-inch fit eyepiece firmly in place.

Just like the rest, it uses a single thumbscrew to hold 1.25-inch fit eyepieces via a compression ring on the adaptor.

The contrast on our various test subjects was good and showed how well the matte coating inside cuts down on internal reflections.

The only surprise was that the mirror juts out a little into the end barrel clipping the view, yet visually we didn’t notice any detrimental effects.

The Explore Scientific 2-inch diagonal is supplied in a very smart well-padded box that sets it apart from the rest.

Verdict

Well packaged and lightest of the group yet still quite capable

For: Two thumbscrews to hold 2-inch eyepiece in place

Against: Bottom edge of mirror visible from barrel end

4.5 out of 5 star rating

Revelation Diagonal Dielectric 99% Quartz 2-inch

Price: £139
Surface reflection: 99% 
Weight: 530g 
Extras: 2 dust caps, recessed 1.25-inch adaptor, barrel filter threaded for 2-inch filters 
Supplier: Telescope House 
Tel: 01342 837098 
www.telescopehouse.com

resized_Revelation
www.thesecretstudio.net

The Revelation Diagonal Dielectric Quartz is solidly built with a practical design and it’s the only model featured with a silver end barrel – helping it to stand out.

It also features an unusual recessed 1.25-inch adaptor that allows eyepieces to have a lower profile, instead of jutting out like the rest of the diagonals on test.

The internal baffle coating surprised us as it appeared to be far shinier than the others in the review, suggesting the possibility of internal reflections when viewing the Moon.

However, we saw little detrimental effect when we tested it with our lunar target and bright stars.

The dielectric coating had the strongest green hue of the diagonals we were testing, yet the contrast when viewing deep sky targets – such as a wide-field view of the Orion’s Belt and Sword region – was very enjoyable.

Despite being midway in weight and not the lightest on test, the Revelation performed well visually.

The only concern was that the 1.25-inch adaptor felt a little tight when it was taken out and put back into the telescope focuser.

Verdict

Interesting design and good performance

For: Recessed 1.25-inch adaptor for eyepieces

Against: Adaptor a little awkward on insertion and removal

4.0 out of 5 star rating

Omegon Star Diagonal with 99% reflection, 2-inch

Price: £99 
Surface reflection: 99%
Weight: 470g
Extras: 1.25-inch adaptor, 2 dust caps, barrel filter threaded for 2-inch filters 
Supplier: Omegon 
Tel: 020 386 88042 
www.omegon.eu

resized_Omegon
www.thesexretstudio.net

The Omegon bears a strong resemblance to the Sky-Watcher Deluxe model and we found little to distinguish between them performance-wise.

Its overall look is stylish, with a sleek curved neck, and a nicely fitting and smooth-to-use 1.25-inch adaptor that uses brass compression rings to hold the eyepiece in place.

We found this did the job nicely, with no sign of slippage when our review scope was aimed at targets high in the sky.

The internal baffling was matte black and when we looked at the Moon we saw little in the way of internal reflections, showing the internal dark coating was doing its job.

Like the Sky-Watcher, we did suspect that the contrast on deep sky targets was a little less compared with the others on test, but we felt the effect was minimal and did not really detract from the views.

The dielectric-coated mirror was well positioned and did not protrude into the end barrel, allowing a clear view.

Verdict

A simple design that does the job without any fuss

For: Good internal blackout

Against: Slightly less contrast in deep-sky targets

4.0 out of 5 star rating

William Optics 2-inch Dura Bright Dielectric Carbon Fibre

Price: £135 
Surface reflection: 99% 
Weight: 540g 
Extras: 2 dust caps, 1.25-inch adaptor, barrel filter threaded for 2-inch filters, temperature gauge 
Supplier: Widescreen Centre 
Tel: 01353 776199 
www.widescreen-centre.co.uk

resized_William Optics 2-inch Dura Bright
www.thesecretstudio.net

William Optics has a reputation for manufacturing high quality telescopes and accessories and its Dura Bright 2-inch Dielectric Carbon Fibre diagonal lives up to this reputation.

It exudes quality, looking a bit like a cross between the Explore Scientific and Sky-Watcher diagonals, and it’s supplied in a well-padded plain black box.

The end barrel is threaded internally for 2-inch filters while the internal baffle coatings are matte black with little, if any, sign of reflections or loss of contrast when used on the Moon, bright stars or deep sky.

We found that eyepieces were easy to slot in and take out, regardless of either 2- or 1.25-inch fit, and it held them rigidly, especially considering the weight of our 21m Ethos eyepiece used in the review.

A useful addition is the thermometer on one side which is handy when using the diagonal for imaging, as you can control the camera temperature more accurately by referring to it.

Verdict

Stylish, well-made and a pleasure to use

For: Good colour contrast for the stars and bright views

Against: Second heaviest of the review

5.0 out of 5 star rating

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These reviews originally appeared in the June 2019 issue of BBC Sky at Night Magazine