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90 degree Star Diagonal or 90 degree Dielectric Diagonal

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#1 drumjpic

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Posted 15 January 2022 - 09:33 AM

My Orion ShortTube 80 came with a 90 degree star diagonal. I'm considering purchasing an Orion 90 degree Dielectric Diagonal in hopes of improving images. I use Orion EF Widefield 27mm and 12mm eyepieces (and Orion Shorty x2 Barlow) Is it worth the investment ?
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#2 LDW47

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Posted 15 January 2022 - 10:14 AM

My Orion ShortTube 80 came with a 90 degree star diagonal. I'm considering purchasing an Orion 90 degree Dielectric Diagonal in hopes of improving images. I use Orion EF Widefield 27mm and 12mm eyepieces (and Orion Shorty x2 Barlow) Is it worth the investment ?

Are dielectric diagonals not still a star diagonal in many cases ? Dielectric just identifies how the mirror is treated to provide a higher level of protection and brighthess, right ? I have a dozen of them between 1.25"-2".


Edited by LDW47, 15 January 2022 - 10:15 AM.


#3 drumjpic

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Posted 15 January 2022 - 10:41 AM

According to what I've read, the star diagonal I currently have has 90% reflectivity. The dielectric diagonal has 99% reflectivity. Which (I assume) means more light at the exit puple of the eyepiece. With a short focal length telescope (400mm), getting more light would be an improvement.

#4 LDW47

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Posted 15 January 2022 - 10:44 AM

According to what I've read, the star diagonal I currently have has 90% reflectivity. The dielectric diagonal has 99% reflectivity. Which (I assume) means more light at the exit puple of the eyepiece. With a short focal length telescope (400mm), getting more light would be an improvement.

You won't notice any difference to your eye, the views but the glass finish will be quite a bit more resistant to wear, minor scratching etc. But if you feel you would like to try one go for it, they aren't a big cost item unless you want to pay for a name. I have both, no different.


Edited by LDW47, 15 January 2022 - 10:47 AM.

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#5 LDW47

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Posted 15 January 2022 - 10:50 AM

My suggestion is, put a 2" focuser on your scope before changing diagonals, if you can find a 2" Crayford right now. That will give you more capabilities than just changing diagonals, a lot more. It will put you into a different viewing realm.



#6 drumjpic

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Posted 15 January 2022 - 11:51 AM

Thanks for the advice !

#7 GGK

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Posted 15 January 2022 - 12:14 PM

My Orion ShortTube 80 came with a 90 degree star diagonal. I'm considering purchasing an Orion 90 degree Dielectric Diagonal in hopes of improving images. I use Orion EF Widefield 27mm and 12mm eyepieces (and Orion Shorty x2 Barlow) Is it worth the investment ?

Can you describe what’s wrong with the image?  At f/5, the ST-80 gives bright wide-field views already.



#8 drumjpic

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Posted 15 January 2022 - 12:27 PM

Nothing wrong with the image with what I'm using. Just looking to get the most out of this telescope.

#9 Sketcher

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Posted 15 January 2022 - 12:53 PM

My opinion:

 

You're unlikely to notice any difference in terms of image improvement.

 

That being said, I have an Orion ST-80 that I've made extensive use of -- with a standard (mine was "made in Japan") 90-degree mirror diagonal.  But I don't recall if it's the same diagonal that came with the scope many years ago.  I suspect not.  Similarly, I don't know if the diagonal that Orion supplies with their current ST-80s is the same, different, better, or worse than the ones supplied in years past.  So I really don't know anything about the quality of your current diagonal.

 

I also have a 1.25-inch, 90-degree dielectric diagonal (from Orion).  But I've chosen not to use it with my ST-80.  My dielectric diagonal has a smaller clear aperture than the above mentioned "standard" diagonal.  For me, that was of greater significance than the dielectric's greater light transmission.  My 1.25-inch dielectric is now used with a modified (for diagonal use) 50mm Galileoscope.

 

As far as image quality and brightness goes (ignoring any possible vignetting with my dielectric), my eyes don't really see any difference between these two diagonals.

 

As others have mentioned, dielectric coatings are much harder and much more durable than non-dielectric coatings.  This can be important for some people and/or in certain environments; but for me, even that difference is of no real concern.

 

That being said, I use 2-inch dielectric 90-degree diagonals with my larger (5 and 6-inch) refractors.

 

Different diagonals will have different quality differences.  A dielectric diagonal will not always out-perform a "standard" mirror diagonal.  Besides reflectivity, flatness is also of importance.  Back in the early days of dielectric coatings, I've seen mention of (if I recall correctly) substrate (the glass) warping occurring (with some manufacturers) during the (dielectric) coating process.  Does this still occur?  I don't know.

 

As for getting the most out of an ST-80 goes -- I see that as more of a user issue than an issue with accessory upgrades, not that higher quality accessories don't also have their place.

 

Frequent use (of the ST-80), taking more time to actively look for more detail, etc. will likely reap greater benefits than upgrading the diagonal.  A telescope does what it does; but it's up to the observer to learn how to see all that the telescope, diagonal, and eyepiece is putting on the observer's retina.  And that takes time and practice.

 

The equipment is easy.  Learning to see . . .  That's a bit harder and more time-consuming.  But it's also more rewarding and satisfying.

 

Buying more and buying better certainly can help; but don't discount what can be accomplished with what you have.


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#10 drumjpic

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Posted 15 January 2022 - 01:27 PM

I agree with viewing properly (averted view, ect) I'm a new-b and continue to learn (asking questions on forums,Youtube videos, *Ed Ting and others) Thanks for your experienced opinion and information !

#11 SeattleScott

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Posted 15 January 2022 - 01:35 PM

An ST80 goes so wide already, I don't know that there is a big benefit to going 2". And it would likely cost as much as the scope. 

 

Technically a dielectric should transmit a bit more light. Most aluminum diagonals are enhanced aluminum with around 95% reflectivity. I suppose a really cheap stock diagonal might be standard aluminum around 90%. Generally it takes a 10% difference to notice the difference visually, so there could be a slight benefit to dielectric. Dielectric is also easier to maintain; should in theory last longer. It might be flatter and therefore provide sharper views, but this is more an issue at high power where ST80 doesn't excel anyway. From a value perspective, with your scope, it isn't great. Basically costs maybe 30-40% of what your scope costs to increase light transmission about 9%. If you had a $2,000 scope it would look like a great value. But ultimately value is in the eye of the beholder. However if you start spending significant sums for very modest improvement, at some point you have to look at it from a different perspective. Like instead of spending $70 for 9% brighter views, one could spend $350 on a 6" Dob and get immensely brighter views. Obviously a different scope has different drawbacks. A 6" Dob will need periodic collimation, it won't go four degrees wide, and you can't mount it on a photo tripod. But in terms of increasing brightness of the view per dollar spent, it annihilates upgrading the diagonal.

 

Scott



#12 hlee

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Posted 15 January 2022 - 01:39 PM

Ed Ting (YouTube) suggests the following test:

 

Look at something using the diagonal and then look at it again with the diagonal removed (i.e., just the eyepiece).  If you do not see any degradation in the image, then your diagonal has a neutral effect and you do not need to upgrade it.

 

To get the most out of your telescope, you should investigate whether your eyepieces fit your observing style and whether other ones might be better for you.


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#13 drumjpic

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Posted 15 January 2022 - 01:45 PM

I understand the limitations of the Orion ShortTube 80. I'm happy with the views and I only using it for viewing. (If something looks good enough I'll take a photo using my android phone). I may buy a cassegrain in the future. I want to stay with portability and ease of use.
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#14 LDW47

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Posted 15 January 2022 - 02:34 PM

50% of the reason to upgrade a diagonal, to upgrade to a 2" focuser is because the astronomer, a fellow astronomer just plain wants to for the benefits large or small. Its not the cost vs the benefit they just want to, period. Many don't care if the add ons cost as much or more than the scope, they just plain love the ST80 for what it can do. For most its not an only scope, its a complement to scope and you will see, will learn all of that in time and space. It can be a lifetime keeper !



#15 sevenofnine

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Posted 15 January 2022 - 03:03 PM

Consider building an eyepiece collection that will be good for your ST-80 and future scopes. By good, I mean ones that most members agree are a definite step up from the supplied plossls. Our sponsor sells the AT Paradigm Dual ED's for a very reasonable $65 ea. They are sharp, well made and have a 60 degree FOV as opposed to 52. Get a Pelican clone case and you will be all set. This will show you more of an improvement than any star diagonal IMO. Best of luck to you and your decisions! waytogo.gif


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#16 drumjpic

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Posted 15 January 2022 - 03:52 PM

A "collection " of eyepieces is (for me) a waste of money. With (my 4 eyepieces and my x2 barlow. Nice & clear views) I don't need anything else with this focal length telescope. I (may) consider buying one additional *quality flat field 5mm in the future (=80× without barlow. 160×with), but since max magnification is around 100x (theatrically 160x) for this telescope, I'd rather be under magnified than over. Thank you for your input.

#17 sevenofnine

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Posted 15 January 2022 - 04:33 PM

drumjpic...I have an 80mm refractor too. Improved eyepieces are not a waste of money in this or any scope, trust me. I resisted the expense for many years. My go-to eyepieces were zooms. So what's the improvement? Wider field of view throughout the collection is the main one. The other big one is that all of the oculars are much larger and easier to look through. This makes a huge difference especially at higher powers and when using a barlow. Just something to consider from someone who's been doing this for decades. flowerred.gif


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#18 LDW47

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Posted 15 January 2022 - 04:37 PM

A "collection " of eyepieces is (for me) a waste of money. With (my 4 eyepieces and my x2 barlow. Nice & clear views) I don't need anything else with this focal length telescope. I (may) consider buying one additional *quality flat field 5mm in the future (=80× without barlow. 160×with), but since max magnification is around 100x (theatrically 160x) for this telescope, I'd rather be under magnified than over. Thank you for your input.

The 5mm may be a little too high in power regardless of any theoretical numbers.



#19 drumjpic

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Posted 15 January 2022 - 11:38 PM

(*5mm) Thanks for the heads up

#20 GDG

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Posted 16 January 2022 - 02:00 PM

I have an ST80 clone and find that a 5mm eyepiece gives nice, sharp views with it, using an Agena Starguider dual ED. 

I collimated my ST80, that made a big difference; this telescope can used it up to about 125x and get pleasing, useful views with the moon, for example.



#21 vtornado

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Posted 16 January 2022 - 02:05 PM

Excuse me if I am wrong, but I believe "star" diagonal refers to the diagonal being 90 degrees, as opposed to

45 degrees which is commonly used for terrestrial viewing.

 

So a star diagonal could be a prism, aluminized mirror, or dielectric mirror.

What is yours?  It might be hard to discern the difference between an aluminum and dielectric mirror.

 

Your scope at f/5.  A prism will inject a small amount of chromatic abberation to the image.  This occurs at high power.

The ST80 has so much CA that the additional amount added by the prism may not be noticible.   It is noticable with a

fast ED refractor.

 

As others have said, it is hard for the human eye to discern a 10% difference in illumination.

 

Things to check about your diagonal that do make a difference.

Make sure it is collimated.  (might be hard to do without a laser collimator)

Check the clear aperture.

As others have said it is easier to clean  dielectric mirror because it is harder than aluminum.

 

Despite the suggestions to add a two inch focuser, it really does change the nature of the scope.

These scopes are cheap an light, they can be mounted on a medium photo tripod.

Going to 2 inches does give a much larger field of view, but ... the focuser is just as much or more than the scope,

and with the large focuser, 2 inch diagonal and 2 inch eyepiece it is no longer a cheap lightweight scope.

I'm not saying this is bad, it just changes the nature of the scope.

Also at f/5 you will have to spend $200+ for a well corrected eyepiece.

 

I down graded my ST80 from 2 back to 1.25 inches to recapture the small size.

I have an 80 ED that I have setup for 2 inch.


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#22 GGK

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Posted 16 January 2022 - 02:25 PM

A "collection " of eyepieces is (for me) a waste of money. With (my 4 eyepieces and my x2 barlow. Nice & clear views) I don't need anything else with this focal length telescope. I (may) consider buying one additional *quality flat field 5mm in the future (=80× without barlow. 160×with), but since max magnification is around 100x (theatrically 160x) for this telescope, I'd rather be under magnified than over. Thank you for your input.

If the ST-80 is the only telescope, then you’re already set up pretty good. If your present diagonal is in good shape, then I doubt you would see any value in upgrading. The ST-80’s strength is bright wide views. Your 27mm eyepiece is already hitting 3.6 degrees, which is enough to show most objects. If you didn’t have that, I would suggest a 24mm UFF for the widest field of view. 
 

Regarding the 5mm, my experience is that 80x is easy for the ST-80, but won’t show the sharp detail of a better OTA (and more expensive). However, it’s detail that nice to see and is good enough, especially on the moon, So I think it’s worthwhile. 

 

I found a UHC filter was very useful in the ST-80 for brighter nebula, especially M42.

 

Gary


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#23 LDW47

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Posted 16 January 2022 - 02:54 PM

If you want to do some math, the optimum for that scope on the high side is 40x using a 10mm ep. Therefore a 5mm ep puts out a much to high power to get optimum performance so you are better to get a 10mm ep and try a 5mm performance using a 2x barlow lens, that will likely save a lot of disapointment and you can still use the barlow for other eyepieces.


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#24 LDW47

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Posted 16 January 2022 - 03:33 PM

And the other issue is that to use an ep greater than 10mm you will need almost perfect sky condtions which doesn't come around too often.



#25 drumjpic

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Posted 16 January 2022 - 04:01 PM

Once again, I do appreciate everyone's advice. Better decisions are made when asking those who have the most experience. Just to clarify, I am using Orion EF 27mm and 12mm eyepieces (in addition to the 10mm and 25mm Plossls that came with the telescope). Also an Orion x2 Shorty barlow.
I have read where some have been able to obtain good images at 100x magnification with this scope. I completely understand it all subjective. That is why I had thoughts of purchasing a 5mm wideview. If a 5mm would provide too much magnification and poor images, how high of power would be the limit (without using a x2-x1.5 barlow ?) I understand using the barlow with my 10mm Plossl I can reach 5mm. But the Plossls don't have the field of view as the EF Widefields.


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