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emadmoussa

Diagonals - can you really see the difference?

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I had the Skywatcher 2" diagonal with 90-95% reflectivity and two other diagonals with 99% reflectivity, but I couldn't see any difference in the image quality. Is this normal, or I need to check my eyesight? 

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The title should read 'DiagonalS' ...plural ... :( 

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Posted (edited)

The difference is mainly in the durability of the mirror surface and the mechanical construction of the diagonal. A new ordinary £30.00 mirror one is going to perform pretty well. With the more expensive diagonals the mirror is going to be more accurately located and not likely to shift position. 

I have noticed some diagonals do give a slightly brighter image due to the higher reflectivity but you need good seeing to,notice. 

 

Edited by johninderby
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Posted (edited)

The leap from stock diagonals which come with SCTs and refractors (I've had a few Celestron and Skywatcher units that have been very poor) to a decent upgrade (for e.g. the various dielectric diagonals - which in my experience are fairly similar - and excellent) is enormous - the subsequent gains when you trade up to the various pricey Baader options are less dramatic. The best value diagonals are the 2" SW/Revelation dielectrics or the 1.25" Takahashi in my experience.

Edited by Highburymark

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The biggest gain over cheap stock diagonals with plastic cases to dielectrics with metal cases was the increase in rigidity.  My binoviewers at 2+ pounds when loaded with eyepieces was just too much for the plastic diagonal to hold without flexing.

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I've always found the difference between 'reasonable' and 'posh' diagonals to be deeply underwhelming.

Olly

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I'm not asking about the difference between a stock diagonal and a high quality one. As you say, the difference is readily discernible. 

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I tend to go for the best quality that I can get for the peace of mind as much as anything. One link less in the optical chain to worry about !

But in truth, once you get to the mid-priced dieletrics such as William Optics and GSO / Revelation, the differences offered by the next step up are more about build quality than any regularly available performance gains I think.

 

 

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45 minutes ago, John said:

I tend to go for the best quality that I can get for the peace of mind as much as anything. One link less in the optical chain to worry about !

But in truth, once you get to the mid-priced dieletrics such as William Optics and GSO / Revelation, the differences offered by the next step up are more about build quality than any regularly available performance gains I think.

 

 

My impression too actually. 

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I think that I noticed slightly more clarity and contrast in my Baader Zeiss T2 prism, relative to my 2” GSO 99 percent reflective Quartz diagonal.

Anders

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I agree build quality is the difference in my experience. I use the Baader T2 system which offers a lot of flexibility.

That said if you use 2" eyepieces with large field stops you'll need a more traditional design.

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I think it comes down to things like light scatter and detail at high power. I saw a small but noticeable improvement with my Baader Zeiss prism over a standard diagonals.

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Posted (edited)

Ive never noticed a difference between anY diagonal that comes with the scopes i own.The best diagonal/prism i have came with the Celestron 70mm travelscope (IIRC). Its a 45 degree erect prism. The diag that came with my 8SE was pretty standard. Dissapointing on a scope that cost nearly 2k.

Edited by LukeSkywatcher

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@emadmoussa You've probably already read Bill Paolini's Mirror vs. Dielectric vs. Prism Diagonal Comparison and THE BAADER BBHS-SITALL SILVER DIAGONAL articles, but they mainly point out that surface roughness is what separates high end diagonals from each other.  The smoother, the better to reduce scatter and increase contrast and fine detail.  If you're not looking at low contrast objects, you might never notice the difference between them optically.

I've read that some Japanese observers are willing to sacrifice comfort to achieve the best refractor views possible by dispensing with the diagonal altogether and viewing straight through instead.

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On 07/08/2018 at 08:11, Stu said:

I think it comes down to things like light scatter and detail at high power. I saw a small but noticeable improvement with my Baader Zeiss prism over a standard diagonals.

£400 for a diagonal...?! WOW!! 

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On 06/08/2018 at 22:49, John said:

I tend to go for the best quality that I can get for the peace of mind as much as anything. One link less in the optical chain to worry about !

I agree with that entirely. The reality is that in any set-up the light goes in one end of the optical chain and comes out the other end into your eye. The quality of what you see is much more likely to be influenced by the weakest link in that chain than any other component. Eliminate weak links tends to be my philosophy when selecting bits of glass to chain together.

Is it worth sticking a £400 diagonal in a £70 refractor? No. Is it worth sticking one in a refractor that costs 2, 3, 4 or 10 times that price along with equivalent quality eyepieces, yes 😉

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On 08/08/2018 at 13:39, emadmoussa said:

£400 for a diagonal...?! WOW!! 

I had to look at that 3 times. I thought my eyes were playing mind games with me.

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Posted (edited)

I haven’t really noticed any difference unless they are really cheap ...... but quality construction does give some other assurances, ie centering, holding the eyepiece securely, squareness etc.... and in some instances they are made from a single piece of aluminium, thus the barrel won’t suddenly spin when using heavier eyepieces. Which makes your heart skip a few beats 😀

Edited by Pig

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38 minutes ago, Pig said:

I haven’t really noticed any difference unless they are really cheap ...... but quality construction does give some other assurances, ie centering, holding the eyepiece securely, squareness etc.... and in some instances they are made from a single piece of aluminium, thus the barrel won’t suddenly spin when using heavier eyepieces. Which makes your heart skip a few beats 😀

Sometimes in astornomy, it feels that quality is about how an item feels in your hand. If a diagonal is likely to tip the scope's balance, then, hooray...high quality! 

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Ive never noticed a difference personally, and its not surprising.

In terms of durability, a high quality prism or dialectric should outlast an aluminium mirror.

For brightness, bear in mind the logarithmic response of the human eye. A move from 95% to 99% reflectivity nets you a whopping 0.044 magnitude gain. About the same as moving from a 100mm to a 102mm objective.

If you have 2 stars right next to one another an experienced variable observer will be able to tell which is brighter more times than not. Maybe not enough to be "sure" but if you average a bunch of their observations that kind of difference is certainly detectable.

For more general observation where you are not directly comparing two objects, it's not going to make a noticeable difference in brightness.

That leaves light scatter, which with well made examples should be approximately equal at first, but if we assume that the aluminium mirror is less durable they may diverge over time, with advantage to the dielectrics.

Billy.

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