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Diagonal Dilemma – Part 1 - Updated !

John

By John
in Member Equipment Reviews

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John

John

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One question that seems to get asked on this and other forums quite frequently is “should I upgrade my diagonal ?”. This is probably prompted by the rather mediocre reports in reviews of the diagonals that are commonly provided with scopes as standard and, in that light, it’s an understandable dilemma – having invested in a nice scope and nice eyepieces why spoil things with a potentially weak link in the optical system ?.

Finding myself currently owning both an economical Antares 1.25 inch mirror diagonal and a premium Tele Vue mirror diagonal in the same size, I thought it might be interesting to make some comparisons between the 2 both mechanically (this report) and in actual use (part 2 – clouds allowing !).

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The Antares 1.25 inch Mirror Diagonal

The Antares diagonal uses a mirror which claims have a reflectivity of around 90% and it uses standard mirror coatings. The chrome barrel and eyepiece drawtube are metal and both screw into the body of the diagonal which is plastic. The mirror is fixed to a plastic plate which is held to the body by 4 screws.

Eyepieces are held in place by a single chrome set-screw. The chrome barrel has a safety recess machined into it to help stop it slipping out of the telescope drawtube. The interior of the chrome barrel is ribbed and blackened to stop stray light and the barrel is threaded to fit normal 1.25 fit filters. The cost of the Antares diagonals is around £20 new or about £10-£15 used.

Overall the Antares unit feels like it is built down to a price (which it is of course !) but seems to fit together reasonably well and feels solid, if a little basic. My assessment is that these are a little better made than the standard Sky-Watcher / Celestron / Meade offerings.

Issues which I think could be improved on the Antares diagonal are:

- The blackening inside the chrome barrel – you can see a little chrome showing through the threads so it is not as black as it could be.

- The interior of the plastic body, around where the mirror sits, is also quite shiny so there is a chance of stray reflections there.

- The end of the single set-screw is not rounded and could easily cause damage to eyepiece barrels, especially larger and heavier ones.

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The Tele Vue “Everbright” 1.25 Mirror Diagonal

The TeleVue diagonal uses a mirror which claims a reflectivity of 99% and it uses dielectric mirror coatings which are reputed to be harder wearing, more reflective and optically superior to standard coatings.

The construction of the Tele Vue diagonal is entirely different to the Antares unit with the body, drawtube and barrel being milled out of a single piece of metal – presumably a toughened alloy of some sort. Quite a feat of engineering - at least to a non-engineer like myself !.

The mirror is fixed to a thick precision machined alloy plate which is held to the body very precisely by 4 countersunk allen bolts. The drawtube and barrel of the Tele Vue diagonal are both around 5mm longer than on the Antares diagonal. Eyepieces are held in place by a brass compression ring which contracts under pressure from an alloy set-screw.

The barrel has a safety recess machined into it as the Antares did and again the interior of the barrel is ribbed and threaded to fit normal 1.25 fit filters. This ribbing is carried on into the body of the diagonal and the area around the mirror. The cost of the Tele Vue Everbright 1.25 inch diagonal is around £150 new or about £60-£70 used. Considerably more expensive than the Antares !.

Overall the Tele Vue Everbright unit feels like top quality and precisely engineered item. The exterior is beautifully finished and anodised to a very high standard – the finish is silky smooth to the touch and the whole diagonal feels extremely solid – which it is of course, being machined from a single piece of metal !.

Issues which I think could be improved on the Tele Vue diagonal are:

- The threading of barrel could be matt blackened more – you can still see some light shining off the threads at certain angles.

- Some people find that the compression ring eyepiece fitting can catch on the safety groves cut into the eyepiece barrel when they insert or remove eyepieces.

- The set-screw on the Tele Vue projects out much further than on the Antares unit – it could possibly snag on clothing etc.

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Summary of mechanical assessment:

These diagonals are both designed to do the same job and superficially look the same. On closer examination, even someone with no interest in astro equipment (eg: my wife !) can tell that the Tele Vue appears to be made to much higher standards and starts to ask questions about how much it cost (!!). No such interrogation follows when she looks at the Antares unit. I’ll leave it for the reader to decide if that is a “plus” or a “minus” for the Antares Unit …..

So what are you getting for the extra money you spent on the Tele Vue ?:

- A dielectric-coated mirror with 9% additional reflectivity.

- A one-piece precision machined body.

- A compression-ring eyepiece fitting.

What these extra features stack up to in actual use will be examined in the 2nd part of this review – if the clouds ever lift of course !.

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John

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MartinB

MartinB

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Will be very interested in your findings Andrew. I recently had a 2" Antares alongside a 2" Dielectric WO anc couldn't make out any difference. I think the dielectric is worthwhile though because of it's durability. You want to keep a diagonal nice and clean.

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barkis

barkis

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Manufacturers are always keen to stress the reflective capabilities of their Dielectric Diagonals, but I can't say I've seen one yet that states to what accuracy the mirror is flat. Surely a far more important aspect than how shiny it is. When those bundle of rays hit that surface, they are pretty compressed, and any deviation in the plane surface of the mirror, could reduce the whole optical train, to one of mediocrity.

A large portion of the price of one of these devices, is surely tied up in producing a very accurate plane surface, not an easy thing to achieve, possibly as difficult as producing a paraboloidal mirror. Certainly true in the production of a large flat for null testing.

Ron.

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John

John

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Manufacturers are always keen to stress the reflective capabilities of their Dielectric Diagonals, but I can't say I've seen one yet that states to what accuracy the mirror is flat. Surely a far more important aspect than how shiny it is. When those bundle of rays hit that surface, they are pretty compressed, and any deviation in the plane surface of the mirror, could reduce the whole optical train, to one of mediocrity.

A large portion of the price of one of these devices, is surely tied up in producing a very accurate plane surface, not an easy thing to achieve, possibly as difficult as producing a paraboloidal mirror. Certainly true in the production of a large flat for null testing.

Ron.

Tele Vue claim "1/10 wave flatness" for the mirror but I don't claim to know the significance of that :(

I'd almost like to do the optical testing "blind" so I would not know which diagonal was in use. Trouble is I'd have to trust my wife or one of my kids to swap the diagonals and my Nagler's in the dark :shock:

I'll have to just try and be impartial I think !!.

John

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  • 2 weeks later...
John

John

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Part 2 - have do they compare in use ?

I'll keep this short as there is not a lot to say !

With all the cloud about lately my comparison was limited to a few high power views of the Moon, Mars and some double stars.

Rather than waste time giving a blow by blow account of how the 2 diagonals performed object by object I can summarise by saying it's more or less as Gaz intimated above - I could not discern any real differences in performance between the two, at least on the objects I looked at using my Vixen ED102 F6.5 refractor as a test scope and Nagler eyepieces. At one point I suspected that the fainter star in the triple Iota Casseopea was slightly easier to pick out in the Tele Vue Everbright but that was marginal at best. The same went the amount of light scatter and glare observed (minimal) showing that the internal blackening of both diagonals was up to scratch.

I'm not entirely suprised at this result - when the subject of upgrading diagonals crops up the more experienced observers usually recommend that the money is better spent on an additional eyepiece or another accessory.

One thing that I would say in favour of the Tele Vue Everbright is that it's one-piece construction and compression ring eyepiece fitting make it the more reassuring choice if you are going to be using heavy and expensive eyepieces - the barrels of cheaper diagonals have been known to unscrew which can be a bit heartstopping if you have a £200 eyepiece hanging a few feet over a concrete floor :shock:

Optical performance-wise though there is nothing to pick between these two diagonals IMHO.

John

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asteria

asteria

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Hi - an interesting review and a thought-provoking conclusion. I guess I'll be hanging on to the standard issue diagonals for a while. Thanks for taking the time to do this. It's a helpful item.

Best wishes.

Ed

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John

John

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The interesting thing is that I have read opinions on other forums that are quite the opposite of what I found and they swear that there is a substantial and noticable difference between a reasonable economy diagonal and a premium dielectric.

As our US colleagues often put at the bottom of their equipment reports "Your Mileage May Vary" :icon_jokercolor:

John

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John

John

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Hi John I have a Tele Vue 2" diagonal I changed it from a Mead the only differance we found was on deep field objects we could poss see just a little clearer

Stephen

Easbourne AS East Sussex

That's interesting Stephen. I've not tried a direct comparison on DSO's yet but according to the specifications the Tele Vue has a slightly higher reflectivity so that might show itself when viewing faint objects.

John

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