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
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.
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.
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.
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.
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.
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?
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).
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?
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.
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.
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!
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.
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.
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.
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.