diagonals

[morls]

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

[Wavytone]

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/optical-accessories/stardiagonal-mirror--prism/apm-2-inch-erect-image-prism-fast-lock-ultra-broadband-coating.html The Baader "Clicklock" is similar and others offer the same mechanism in various forms.

[iborg]

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

[Peter Ward]

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.

[GUS.K]

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.

[morls]

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.

[OzEclipse]

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

[morls]

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.

[morls]

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?

[Steffen]

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).

[morls]

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?

[Wavytone]

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.

[morls]

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.

[Wavytone]

Purely a matter of how much junk you want to drag around :rofl:

[morls]

maybe something like this? Have a massage while observing...

[MortonH]

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!

[astro744]

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_page.asp?id=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.

[morls]

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-bbhs-mirror-diagonal-clicklock.html
Televue everbrite https://www.bintel.com.au/product/tele-vue-star-diagonal-everbrite-2-inch-satin/ $495
Bintel quartz https://www.bintel.com.au/product/bintel-star-diagonal-2-inch-quartz/ $199
Bintel dielectric https://www.bintel.com.au/product/bintel-star-diagonal-2-inch-dielectric/ $179
Bintel https://www.bintel.com.au/product/bintel-star-diagonal-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/optical-accessories/stardiagonal-mirror--prism/apm-2-inch-erect-image-prism-fast-lock-ultra-broadband-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

[Merlin66]

Nick, Stephen et al,
When used on an f10 system there is no perceptible chromatic aberrations from a prism diagonal.

[Steffen]

What is the diameter of your Mak's secondary baffle? Your diagonal doesn't need to be larger than that.

[morls]

I'm trying to find out. I've just called skywatcher, who couldn't tell me immediately, but they're going to call back.

It's a hard spec to track down, so I'm very interested to know exactly what the diameter is.

[Wavytone]

Stephen, sorry I forgot - Steffen is half correct - the aperture limiting the exit beam is the diameter of the hole at the backplate, you can measure that for yourself with a rule.

[morls]

Well, I just measured it and the results are very interesting...

The diameter of the backplate exit is 1.25". I can see the diameter of the inner baffle from the secondary is also 1.25"

So it seems it doesn't have a 2" visual back as I'd come to believe after reading others state this as so. It's a 1.25" system.

Does this mean a 2" diagonal and 2" eyepieces are essentially a waste of money?

[morls]

Thinking some more about this now...if the practical field stop of this scope is 1.25", let's say 32mm, then I assume this is the largest eyepiece field stop I can effectively use.

I've made up my mind to get a televue for wide views (at least as wide as the scope can provide). I think the 35mm Panoptic is out because of the 38.7mm field stop. This leaves me with a choice between:

27mm Panoptic with 30.5mm field stop, x100 mag, 1.8mm exit pupil and 0.65 degrees true field
22mm Nagler with 31.1mm field stop, x123 mag, 1.5mm exit pupil and 0.66 degrees true field

I have a couple more questions:

The 22Nag isn't listed on the Bintel site - has this been discontinued?

Would a 1.25" diagonal with 2" eyepiece adapter suffice, given the above eyepiece field stops?

[Wavytone]

No, not at all. Personal experience is that it will usefully fill the field of 2" low power eyepieces with larger fields than what is possible in 1.25" barrels. As I indicated below your eye won't notice much until the vignetting exceeds 50% and that occurs when the field reaches about 40mm diameter.

For example the ProStar 38mm 70 degree UWA and Vixen LV50mm are both excellent in this scope.

But you are intent on using 2" eyepieces with no field stop at all, you will find the image is cut off by vignetting just inside the field stop. And example is the TMB Paragon 40mm which is also rebranded as various clone such as Titan II etc.

The same occurs in many newtonian telescopes - the size of the secondary mirror is often a compromise between the desire to deliver the full light cone over a reasonable field of view, vs the size of the central obstruction caused. The result is many opt for a diagonal that will cover a 20mm field but beyond that it starts to vignette.

[morls]

Thanks Nick, I really appreciate the help. I'm on a bit of a learning curve here...:confuse2:

[Wavytone]

Here's an analogy.

Take the cardboard tube from a roll of paper towels or gladwrap.

Hold it up about a foot from your eye and look though it.

Move your eye side to side. You will notice the light passing through isn't cut off immediately your eye is in line with one side (or the other). In fact your eye can be significantly beyond the edge of the tube before you see nothing through the tube. So in other words the useable field of view is significantly wider than just the diameter of the tube.

You're not a bit OCD by any chance ?

[morls]

maybe a little...:whistle:

[morls]

I've found the info I need, so thanks to everyone for trying to help. My questions centred around whether a 35mm Panoptic and 2" diagonal would work, which a lot of people have already commented on :thanx:.

It was a post from user Tanglebones (Stu) (in stargazerslounge.com back in April 2016) that finally convinced me, so I'll copy it in here just to finish this thread.

Thanks to all who helped. and apologies for the barrage of posts!

Stephen

In one of my last posts I mentioned wanting to try the Panoptic 35 (P35) on M42 and I had my chance a few weeks ago out at a darkish-sky site. First though, here is an entry from my observing log taken on the 27th of September 2014, which is the first time my eyes ever looked at M42 through a telescope - "I have to say, M42 was a bit underwhelming and I’m not sure why.” Contrast this with my entry on Friday, 11th of March 2016, which is when I went out to the dark sky site::

Tonight, there is only one word I can use to describe M42. Two words. Absolutely stunning. Oh, how my comment of 2014-270/21 comes back to haunt me ("I have to say, M42 was a bit underwhelming and I’m not sure why.”). I have never seen M42 looking so beautiful and I think I am just a little bit in love. And not just with M42, the TV35P was pretty much my only eyepiece for the majority of the evening. Using it, I saw the Trapezium as clearly as if I was there. Pin-sharp and very bright, surrounded by fold after fold of billowing nebulosity. That nebulosity seemed to take on a 3D perspective, even though I knew I wasn’t seeing it as such. It looked like it had depth to it, though, like it would look as I approached it.

The fish mouth was dark, far darker than I think I’ve seen before in other scopes. As too was the dark nebulosity that separates M42 from M43. I also saw the swept-back ‘wings’ for the first time, matching what I see in people’s images. And those three distinct stars all lined up underneath the starboard wing. And all this was before trying the UHC and OIII filters. Under the UHC filter, the Trapezium stars were a bit harder to see but I attribute this to the extra nebulosity I was observing, which only enhanced the 3D-like experience. The stars all had a blue-green tint to them, but strangely this didn’t detract from the view. It gave it an unearthly, surreal perspective. I then switched to the OIII filter and there were less stars again and I noted the nebulosity had a different shape to it.

It amazes me that this hauntingly beautiful sight was right there for me to view my entire life, and until just over a year ago I never took the time. I’m so glad I finally did.
The views through the two DeLites are *almost* as good, but not quite, which I attribute to illumination of field. They are just as sharp and clear as the P35, but not quite as well lit. That Panoptic 35 is now my go-to eyepiece and it is always the first one I reach for. I haven't noticed any vignetting at all. If it is present then it is very subtle. So if any others find themselves wondering which eyepiece will give them the widest possible field of view in a Sky-Watcher Mak 180, with a minimum of distraction, then I highly recommend they look at Tele Vue's Panoptic 35. In this particular telescope it gives some lovely, lovely views.

[Wavytone]

I'd also suggest you refrain from cross-posting here on IIS and on CN. One or other will achieve what you want as many read both sites.

[morls]

Sorry, my mistake. I've just emailed CN mods to get that post removed.

[Kunama]

The actual visible differences in real life are very small, often not visible at all.
The only exceptions I have found are the Baader BBHS coated prisms and mirrors.

The Baader ones also have their ClickLock eyepiece holders which are superb.

The views through the various dielectric ones are very very very similar...

I did not see any difference between the AP dielectric and the Baader dielectric when testing on my TOA130....

[LewisM]

Maybe - just maybe - that's because Baader makes the MaxBright for AP :lol: (just buy the Baader badged one and save about $50)