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Old 03-08-2018, 02:38 PM
morls (Stephen)
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diagonals

Hi,
I'm really enjoying the views through my 180 Mak, they are just stunning. I can't leave well enough alone though, and so want to make sure the great optics of the scope aren't compromised by lesser components further down the chain.
In particular, I wonder what people's thoughts are regarding diagonals - the stock skywatcher certainly does the job, but is there much to be gained by upgrading to something high quality? If so, what are the important parameters to consider?
Thanks
Stephen
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Old 03-08-2018, 03:58 PM
Wavytone
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The optimum is a 99% dielectric one (see Bintel for example). There's no point in quartz or prisms - the difference between that and 100% is impossible to spot. Prisms use total internal reflection (100%) but suffer minor losses at each air-glass surface and add spherical aberration - which you don't need.

The point about the dielectric coatings is that they're tough, will survive cleaning and last you a lifetime.

Another issue is how these hold the eyepiece - in my experience the ones with compression rings do not play nice with eyepieces with undercuts on the barrel - the usual result being the eyepiece is tilted off-axis, or worse, hard to extract if the edge of the undercut catches on the compression ring.

I've just ordered a new one from APM with a fastlock which is a different mechanism to hold the eyepiece in position. see https://www.apm-telescopes.de/en/opt...d-coating.html The Baader "Clicklock" is similar and others offer the same mechanism in various forms.
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Old 03-08-2018, 05:30 PM
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iborg (Philip)
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Hi


I had the same thougts myself. Using the same eyepiece, I compared looking at a star through the Meade stock diagonal, and then straight through the back of the scope.


I thought that straight through was clearly brighter, so I have now have a dielectric diagonal.


I really should repeat the test, comparing striaght through and both diagonals.


Philip
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Old 03-08-2018, 05:48 PM
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Peter Ward
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Best on the planet is tihe Astro-physics dielectric. I and another very experienced planetary nutter extensively tested it with a William Optics and TeleVue dielectrics.

Going to insane magnifications with an AP155 both the WO and TV showed a smidge of distortion in the airy disk.

The AP remained perfect.
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Old 03-08-2018, 06:07 PM
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GUS.K (Ivan)
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Stephen, I use a Tak 1.25 in prism diagonal for Lunar and planetary on my SW180 and I can't fault it, and combined with my Delos 6mm, provides some of the best lunar views I have seen. I also have a TV everbright 2 in diagonal( also excellent), but like the smaller form factor and lighter weight of the Tak prism.
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Old 03-08-2018, 06:37 PM
morls (Stephen)
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Great info here, thanks everyone.

Peter, that's an interesting comparison between the AP, WO and Televue.

I think I'll be getting a 2" as I want to use 2" eyepieces as well.
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Old 03-08-2018, 07:24 PM
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OzEclipse (Joe Cali)
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A mirror diagonal or the secondary mirror on a newt need to be more accurate. At a 45 degree angle of incidence and reflection, the wavefront error induced by the surface is doubled. So a 1/10 wave diagonal is 1/10 wave for 90 deg angle of incidence & will propagate the wavefront with a 1/5 wave error.

It is perfectly sensible to invest in a higher quality diagonal for possible future high quality optics, however a diagonal really only needs to be twice the precision of the primary optic it is used with to meet the Rayleigh diffraction limit conditions.

Joe
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Old 03-08-2018, 08:27 PM
morls (Stephen)
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Thanks Joe, that makes sense and suggests it would be a worthwhile upgrade.

I'm not sure of the precision of the primary in my Skywatcher 180 Mak, but I suspect it's of reasonable quality.
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Old 03-08-2018, 09:43 PM
morls (Stephen)
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I've just found this on CN, which suggests I should be thinking about getting a prism diagonal (my scope is f15):

If you notice, every manufacturer of quality scopes with fast focal ratios uses mirror diagonals. Every manufacturer that sells quality scopes with long focal ratios sells prism diagonals. It’s not an accident. You get the diagonal that matches the scope.

Nick, I noticed the diagonal you linked to is a prism. Could you tell me why you chose this and not a dialectric?

Last edited by morls; 03-08-2018 at 10:03 PM.
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Old 03-08-2018, 11:13 PM
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For a slow scope I'd pick a quality prism, like the Zeiss T2 from Baader. A shoot-out on CN a while back showed that the prisms have less scatter with high-mag, high contrast views (like planets).

Prisms also have the advantage that they seal the OTA, so you don't introduce tube currents when inserting a warm eyepiece (out of your pocket).
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Old 04-08-2018, 07:24 AM
morls (Stephen)
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Thanks Steffen,
I notice the specs for the Baader T-2 state it has "full 34mm clear aperture". I've just ordered a 35mm Panoptic, with a field stop of 38.7mm, so would there be a restriction of FOV?
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Old 04-08-2018, 12:18 PM
Wavytone
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Stephen technically yes, but not visibly. Vignetting has to be pretty bad - like 50% - before it’s noticeable.

Steffen you’re right - contrast/scatter may be better with a prism.

But better still is no diagonal. Back in the days when used the Oddie at Stromlo there was an “observers chair” - more like a leather banana-bed - you could lie on it to get right under the big refractor and look up directly without a diagonal.

I’m think of trying same with a Steinheil monocentric eyepiece, having found a source.
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Old 04-08-2018, 01:03 PM
morls (Stephen)
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Ok, that's good about the vignetting.

You've got me thinking now...if a Baader prism is going to set me back hundreds of dollars, would it be better to get a nice recliner so I can view 'straight through' in comfort?

Not sure how practical this would be with the mak though...there'd need to be a way of adjusting vertically and horizontally...

I need to see some pictures of what others have done I think.
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Old 05-08-2018, 02:17 PM
Wavytone
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Purely a matter of how much junk you want to drag around
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Old 05-08-2018, 03:07 PM
morls (Stephen)
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maybe something like this? Have a massage while observing...
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Old 05-08-2018, 04:28 PM
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Quote:
Originally Posted by morls View Post
Ok, that's good about the vignetting.

You've got me thinking now...if a Baader prism is going to set me back hundreds of dollars, would it be better to get a nice recliner so I can view 'straight through' in comfort?

Not sure how practical this would be with the mak though...there'd need to be a way of adjusting vertically and horizontally...

I need to see some pictures of what others have done I think.

By all means replace the diagonal that came with the scope but a standard GSO/Bintel dielectric 2" diagonal will do just fine.

Remember that 99.99% of the time the amount of detail that's visible will be dictated by the seeing and and whether your Mak is in thermal equilibrium. No amount of expensive accessories will change that!
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Old 06-08-2018, 04:17 AM
astro744
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Note the Tele Vue Everbrite diagonals are a one piece construction to ensure alignment and won't rotate with heavy accessories attached.

See http://www.televue.com/engine/TV3b_p...d=62&Tab=_90EB

The Tele Vue 2" Everbrite will accommodate the 46mm field stop of the 41mm Panoptic so you will have no trouble with the 38.7mm FSD of the 35mm Panoptic or the 42mm FSD of the 31mm Nagler.

You may not notice vignetting with a smaller clear aperture but it will be there and if you are doing critical work like estimating variable star brightness then you need zero vignetting (and of course other aspects of the telescope design require you to have a 100% fully illuminated field for the eyepiece you intend on using). Vignetting is stronger closer to the field stop so if at the front of the 2" nosepiece of the diagonal you may not notice it but if at the bottom of where you inset the eyepiece it will be noticeable and this is where it is more likely to be in a poorly designed diagonal.
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Old 06-08-2018, 08:01 AM
morls (Stephen)
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Thanks Astro, that's very useful.

At the moment the issue for me is understanding the current product range, and where each sits with regards performance. After that comes price. I feel that the top tier will give exceptional performance at a premium price, while the lower spec'd products will have shortcomings.

If it's anything like pro audio, the top 5% of performance is the most expensive to achieve, so I'm looking for something that will give me excellent performance with no vignetting, but not the best in class.

Here's a quick list of different diagonals:

2" mirror
Baader #01A T-2 Maxbright Mirror Diagonal (Sital-ceramic mirror/140 coating layers), w.male/female T-2 thread / solid metal body $403
Baader bbhs mirror https://www.ozscopes.com.au/baader-2...clicklock.html
Televue everbrite https://www.bintel.com.au/product/te...-2-inch-satin/ $495
Bintel quartz https://www.bintel.com.au/product/bi...2-inch-quartz/ $199
Bintel dielectric https://www.bintel.com.au/product/bi...ch-dielectric/ $179
Bintel https://www.bintel.com.au/product/bi...agonal-2-inch/ $109

I'm going to assume the Bintel products are the same as GSO and other generic diagonals around each price point...

2" prism
Baader #01B T-2 Stardiagonal Prism (Zeiss Prism), w.male/female T-2 thread $348 (need accessories to connect)
APM https://www.apm-telescopes.de/en/opt...adband-coating $404 + shipping

At the moment I'll leave it there (work drags me away), but my thoughts at the moment are that the Baader T-2 zeiss prism looks good. If I'm going to spend at least $200 for a decent mirror diagonal, then the step up to zeiss prism might just be worth $150, although accessories will be needed. I'm sure quality would be excellent though.

Cheers

Stephen
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Old 06-08-2018, 08:27 AM
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Merlin66 (Ken)
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Nick, Stephen et al,
When used on an f10 system there is no perceptible chromatic aberrations from a prism diagonal.
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Old 06-08-2018, 10:37 AM
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Quote:
Originally Posted by morls View Post
Thanks Steffen,
I notice the specs for the Baader T-2 state it has "full 34mm clear aperture". I've just ordered a 35mm Panoptic, with a field stop of 38.7mm, so would there be a restriction of FOV?
What is the diameter of your Mak's secondary baffle? Your diagonal doesn't need to be larger than that.
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