Star diagonal

[dc240969]

With my Skymax 127 Mak l have a 90 degree star diagonal with 1.25". A couple of questions related to this.

1/ Are all diagonals created equally, and is there any benefit of replacing?

2/ As its 1.25" it accepts EPs of that size. If possible would a 2" diagonal be of benefit?

[johninderby]

Are you using the cheap plastic diagonal that comes with the scope? If so then yes fitting a 99% reflective dielectric diagonal is strongly recommended not just from the improved performance but the build quality. Also as most of the better diagonals use a compression clamp it diesn’t chew up the barrel of your eyepieces.  As to the diference between cheapish and expensive dielectric diagonal well there’s lots of discusion and arguments on that point. 

Yes you can use a 2” diagonal by using an SCT adaptor on your scope so that any SCT fit diagonal will fit or use a 2” visual back so any push fit diagonal will fit. Using the right 2” eyepice can give you a slightly wider field of view but may not be worth the extra expense. A 30mm eyepiece is about the widest that works. And before anyone points out that the small opening in the rear of the OTA makes using 2” eyepieces a waste of time well that isn’t so. I’ve tested this myself and it is indeed possible to get a wider fov using the right 2” eyepiece. Do some research to understand the technical reasons this is possible.

[Alan64]

One of, if not the, problem with using 2" accessories with a Maksutov is the hole in the center of the telescope's primary-mirror, not to mention the baffle that juts outward from the mirror...

That's when and where someone was flocking their own, and for improved contrast.  The hole of the mirror and the baffle are simply not 2" in diameter, therefore the images would be cut off, vignetted, when viewed through a 2" eyepiece.

Many gravitate towards Maksutovs for their short, compact tubes, and oft combined with go-to mounts, understandably given their long focal-lengths.  The Maksutov was designed for medium-to-high powers, and for which it is most capable, and with 1.25" eyepieces.  It is not a low-power instrument.  Quite a few who acquire them find that out after the sale, yet some will attempt to move heaven and earth to make it more versatile, for low-power observations in addition.  Now, there is a Sky-Watcher 127mm Maksutov, sold in the U.S., that sports a  2" visual-back, diagonal, and eyepiece...

https://www.skywatcherusa.com/products/sky-watcher-skymax-127

But I don't think it can make full use of the 2" format; for example, the full view afforded by a 2" 38mm 70° eyepiece, and certainly not Unk Al's 2" 82° offerings.

However, you do have the option of attaching a fast(f/5), short, 80mm achromat with a 2" focusser to your existing mount, and for low-power views, although you would have to fit a 2" focusser onto said achromat first.  For example...

https://www.firstlightoptics.com/startravel/skywatcher-startravel-80-ota.html

Naturally, only one telescope at a time.

My 80mm f/6 achromat, a little longer than that one listed above, came with a 2" focusser, and from Canada, but they're no longer available...

But don't point one at anything bright, as you'll see "rainbows", and forget about high-powered observing.  However that's precisely why one would complement your Maksutov beautifully.  One is the antithesis of the other, and in that perfectly matched.

 

[dc240969]

Thanks for the informed replies. Overall l cant fault the scope, but l just dont get why they have bundled lower quality accessories along with it. Like supplying a Ferrari with tractor tyres.

[Astro Imp]

14 minutes ago, dc240969 said:

but l just dont get why they have bundled lower quality accessories along with it.

It's all about keeping the cost competitive with rivals. Also bear in mind many purchases already have other scopes with decent accessories and wouldn't be too happy paying the higher price for kit they may already have. The accessories that are bundled with new gear are really just to get you started.
Enjoy your scope and over time you will find what you need/want to satisfy yourself.

[dc240969]

I stopped myself going for the Skymax pro 180 as l wanted to ease myself into this and now realise that most of what l now buy and keep will stay with me if and when l move to another scope.

[Alan64]

Now, about the diagonal that came with your kit.  Is it a mirror-type, or a prism?  Here's the difference illustrated...

In any event, a Maksutov already contains two mirrors.  I don't see the logic in adding a third mirror in the form of a diagonal, as mirrors scatter light in the first place.  Here, light-scattering illustrated, there on the left...

Now, that's not to say that the detriment would be to that extent, but why give it a chance in the first place.  Get a good star-prism diagonal, but make certain that it's not an Amici, an erect-image, as those are for daytime/terrestrial viewing.  For examples... 

https://www.rothervalleyoptics.co.uk/celestron-90-degree-star-diagonal-125.html

That one is mass-produced on a ten-pence, but I lucked out and got a good one; quite good, but not everyone does, so I've read.  It's like playing roulette, at that price.

https://www.firstlightoptics.com/diagonals/takahashi-125-diagonal.html

Yes, that's a pricey one, but it's a Takahashi.  I would suggest the Celestron star-prism diagonal if on a budget.

[johninderby]

Worth getting decent accessories such as diagonals that can be used with other scopes later on. ?

Better a decent mirror diagonal than a cheap prism. Would you actually notice the difference between a good prism and a mirror diagonal? Debateable. Some prefer mirror and some prisms. Loads of arguments on this point on CN. ?

[dc240969]

I couldnt tell you if its mirror or prisim at the moment as l am at work. Will check once home in April.

[John]

The stock Skywatcher diagonal is a mirror one usually.

Good quality (really good) prism diagonals can work well in refractors of F/7 and slower but I'm not sure if they are suited to mak-cassegrains ?

Personally I'd stick to a dielectric coated mirror diagonal such as the William Optics, Revelation and other clone brandings. Something like this:

https://www.rothervalleyoptics.co.uk/altair-125-lightwave-premium-dielectric-diagonal.html

It's interesting that Alan64 and johninderby have different opinions on the extent to which a move to a 2 inch diagonal is worthwhile with the 127 mak ?

 

 

 

[Alan64]

To make it clear about mirrored-diagonals:  they're cheaper to manufacture.  That's their only advantage over star-prisms, that and the zero addition of false-colour to fast-achromats.  But I'd rather have a wee bit of false-colour than of light-scattering any day, or night.

You can read all about the different types and brands of diagonals within this test that was conducted a while back...

http://baader-planetarium.de/news/mirror-Prism-Dielectric-Diagonal-Comparison_2014-03-06_v2.pdf

Incidentally, I have this Baader T2 Zeiss-prism, and that's mentioned within that article...

I had gotten that one back in 2007 or so; perhaps earlier than that, I don't remember exactly.  I use it with this telescope only...

Know this: there is absolutely no optical advantage in using a diagonal.  Observing straight-through, without a diagonal in place, is always going to be best, but diagonals have become, and will remain, a most necessary evil.

[John]

2 minutes ago, Alan64 said:

...Incidentally, I have this Baader T2 Zeiss-prism, and that's mentioned within that article...

I have one and use it with my Tak F/9 fluorite refractor. It's excellent in that sort of scope but I'm not sure that it's as effective in a mak-cassegrain that the OP has.

Also the Baader T2 zeiss prism together with the 1.25" barrel and eyepiece fittings costs somewhat north of £200 in the UK.

[Alan64]

1 minute ago, John said:

I have one and use it with my Tak F/9 fluorite refractor. It's excellent in that sort of scope but I'm not sure that it's as effective in a mak-cassegrain that the OP has.

Also the Baader T2 zeiss prism together with the 1.25" barrel and eyepiece fittings costs somewhat north of £200 in the UK.

Any star-prism diagonal will perform better with a Maksutov than a mirrored one.  Maksutovs are known for their superior contrast among reflector telescopes, and a prism will help preserve that very thing.  Honestly, three mirrors within a single optical path?  Leave that configuration to the Tri-Schiefspiegler.

[johninderby]

I think using a 2” diagonal on the 127 mak isn’t really worth it unless you already have the parts. I did some comparison tests using 1.25” and 2” diagonals and found you could indeed get a wider fov with the right 2” eyepiece but while a definite increase it wasn’t a huge amount.

It’s funny how some love their prism diagonals and some just don’t find any real difference. Maybe down to differences in individuals eyes. ?

 

[Alien 13]

I actually use a WO 90 degree correct image diagonal with my Mak and its pretty good for astro with the added bonus of being able to use the scope for birding etc.

https://www.firstlightoptics.com/diagonals/william-optics-125-90-degree-erecting-prism.html

Alan

[John]

24 minutes ago, Alan64 said:

Any star-prism diagonal will perform better with a Maksutov than a mirrored one.  Maksutovs are known for their superior contrast among reflector telescopes, and a prism will help preserve that very thing.  Honestly, three mirrors within a single optical path?  Leave that configuration to the Tri-Schiefspiegler.

Well there are very many mak-cassegrain owners worldwide who are using the wrong sort of diagonal then, the vast majority probably.

[Alan64]

22 minutes ago, John said:

Well there are very many mak-cassegrain owners worldwide who are using the wrong sort of diagonal then, the vast majority probably.

Just which type of diagonal is predominant within the astronomical marketplace...oh, and vastly so, and why?  

[Saganite]

A decent quality mirror diagonal, such as a revelation dialectric 2" can be bought second hand ,in excellent condition for about £50, and has already been suggested. One of these could easily be the lifetime diagonal, and will certainly be good enough to allow you the time to get into the hobby and decide what is best for you. 

Enjoy your telescope ! 

[pregulla]

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

[John]

47 minutes ago, pregulla said:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

This is the same review that Alan64 links to in his earlier post except the hosting is by Baader in the 1st one and by the Cloudynights forum on the one above

[Stu]

My understanding is that a prism diagonal would work fine in a Mak because they have long/slow focal ratios, so are less likely to suffer any additional CA when passing through the prism. The light rays hit the prism surface at closer to perpendicular so suffer less separation of colour.

A prism reflects off its internal surface, which by definition can't get dirty and is, it think, easier to polish. Having said that, I've had good results with both mirror and prism diagonals. I tend to stick to decent quality items so I'm not reducing the effectiveness of my scopes.

That said, I tend to stick to prisms in the slow scopes and mirrors in the faster ones.

I do agree with Alan that Maks are basically high power, narrow field of view scopes, and it is better to stick to their strengths rather than struggling to overcome them. That said, I have had some surprising success using 2" eyepieces in my OMC140 which showed comparatively little vignetting even with quite long focal length eyepieces. Even with a 56mm plossl, the benefits of a larger exit pupil often outweigh the drawbacks of the slightly vignetted view.