Do I need to use a diagonal with a Mak?

[PatrickO]

I am using a maksutov telescope.

Presumably even the best diagonal with reduce image quality to some extent, and a mass produced one that comes with the telescope is worse.

Do I NEED to use a diagonal at all or can I just attach the eyepiece to the telescope and put up with an awkward viewing position.

Thanks

[vlaiv]

You don't NEED it. You will probably need to put some kind of extender to reach focus (although catadioptrics with moving primary have large focus range).

But that said, you will want to use diagonal - gains in optical quality over decent diagonal (don't need to go for premium ones for this), as you will see, is just not enough to sacrifice observing comfort. Depending on comfort you will actually see less in certain circumstances! One part of observing apparatus is the brain, and if brain is busy "handling discomfort" it will be less able to see. For best observing one needs to be relaxed among other things.

[jambouk]

You don't need a diagonal. If you want one then it won't degrade the image much at all unless it was one out of a Christmas cracker

James

 

[Putaendo Patrick]

I  don't know if this is actually true, but I've often heard that Japanese astronomers preferred not to use diagonals with refractor telescopes - I'm guessing these would be amateur telescopes from the 1960s and 70s. So perhaps it's just a question of getting accustomed - but my neck pretty well insists on a diagonal!

In general, most diagonals which come with Maks are acceptable for most purposes (although a decent dialectic may be an improvement), so if PatrickO is finding the viewing is seriously degraded, there may be a problem, for example with mirror alignment?

[Cosmic Geoff]

You should try it without the diagonal and find out for yourself.  If you can see a difference, consider buying a premium diagonal. 

After buying a couple of premium quality eyepieces, I could see nothing wrong at all with the image quality of my 127mm Mak, using the original diagonal. I could, on the other hand, see some quality loss when trying a 45 deg terrestrial prism diagonal.  

[Louis D]

I picked up a used 1.25" WO dielectric diagonal for $50 two years ago and have been quite happy with it.  It seems to perform as well as my 2" GSO dielectric diagonal, just smaller.  Both are way better than my 1.25" Celestron diagonal for not a lot more money (used vs new).

Watch the classifieds to see what you can pick up for cheap to improve your views.

[Alan64]

A quality 90° prism star-diagonal is best for slow telescopes, like a Maksutov.  In addition, a Maksutov already contains two mirrors, consequently there's no reason to increase the incidence of light-scattering by adding a third in the form of a diagonal.

 

[John]

Most folks do use a diagonal with a maksutov or schmidt cassegrain, when using the scope for astronomy purposes. All the scopes of this design that I've owned (about 6 or 7 over the years) have been used with a 90 degree mirror diagonal.

 

Edited March 17, 2017 by John

[Louis D]

2 hours ago, Alan64 said:

A quality 90° prism star-diagonal is best for slow telescopes, like a Maksutov.  In addition, a Maksutov already contains two mirrors, consequently there's no reason to increase the incidence of light-scattering by adding a third in the form of a diagonal.

 

What quality 1.25" 90° prism star-diagonal do you recommend for Maks without a 2" visual back?  The only two I know of are the Takahashi TAKSD1 and the Baader T-2 90º Prism Star Diagonal with 1.25" Nosepiece.  Which of these two is better?

[Dave In Vermont]

I agree that, while not a 'mandatory' acquisition, a decent diagonal will prevent diminishing the quality of the view by making your mind busy dealing~compensating with pain - instead of relaxing and allowing your mind to tease-out elusive  and complex material in the image coming through your eyepiece.

I'm partial to using 2" William Optics 90° Dielectric diagonals, but that's my personal taste at play. Experience will be your guide!

Enjoy the wonders -

Dave

P.S. - Louis D - I'm sorry I missed your question on quality 1.25" 90° diagonals earlier, but I've got a good one for you. GSO makes a really nice & affordable 1.25" Dielectric one for not much more than a prism-model 90° one. I have one of these for the times I need to swap-out my usual 2" WO ones I usually deploy. Very clear & sharp optics usually found in more expensive models. I'm very pleased indeed with mine. Have a link:

http://agenaastro.com/gso-1-25-90-deg-99-dielectric-mirror-diagonal-compression-ring.html

I believe you'd be quite surprised and impressed. The dielectric-mirror doesn't cause me any scatter-effects in my Maksutov. I'm amazed the price has actually dropped on these, too! I paid $75.00 about a year and a half ago. GSO has pulled-out all the stops to get their products out in the forefront after many years of mediocrity and slip-shod quality-control. Now their products tend to be excellent.

 

Edited March 18, 2017 by Dave In Vermont
Sp.

[Alan64]

13 hours ago, Louis D said:

What quality 1.25" 90° prism star-diagonal do you recommend for Maks without a 2" visual back?  The only two I know of are the Takahashi TAKSD1 and the Baader T-2 90º Prism Star Diagonal with 1.25" Nosepiece.  Which of these two is better?

I had received said Takahashi one day back in 2003, then packed it back up the same day without even trying it out.  The prism is undoubtedly of very good quality, excellent even, but housed in plastic.  I then got a Baader T2 Zeiss.

There's a considerable jump in pricing among 1.25" prism-diagonals, from this...

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

...to this...

http://www.rothervalleyoptics.co.uk/baader-prism-diagonal-t-2-90-degree-32mm.html

http://www.rothervalleyoptics.co.uk/baader-nosepiece-125-t2.html

http://www.rothervalleyoptics.co.uk/baader-125-helical-focusing-eyepiece-holder-t2.html

...and all three of those components required to complete the diagonal.  I would choose that one over the Takahashi.

I have the inexpensive Celestron also, and it's not bad, not at all, and worth a try at that price.

There are a few reasons why a mirrored diagonal is chosen for a slower refractor or a Schmidt or Maksutov Cassegrain...

1. The bundled diagonal requires upgrading, but with the user not wanting to invest too terribly much in a replacement.

2. To those first starting out, oft a diagonal is a diagonal is a diagonal.  Incidentally, they are many out there now, and using Amici diagonals at night, and completely unaware of the far more suitable star-type.  These are users of the entry-level refractor kits, and that generally come equipped with the Amici type, which is only suitable for less-demanding daytime/terrestrial observations.

3.  Mirrrored diagonals are the most plentiful on the market, and economical, yet were originally intended for fast refractors(f/6 and faster) only.  Many users who own fast refractors think nothing of inserting said refractor's mirrored diagonal into their slower telescopes.  They then recommend the practice to others; bad practice, that.

I use a mirrored diagonal with this 80mm f/6 achromat, despite my misgivings in converting a refractor into a catadioptric...

For these, however, at f/8 and f/14, I use a prism only...

...the right tool for the right instrument.

It can mean the difference between seeing this, or that...

Of course, such would depend upon the smoothness of the mirror's coating, and such will vary from diagonal to diagonal.  In any event, a prism is going to exhibit the very least light-scattering...

http://www.doctordreviews.com/mirror-prism-dielectric-diagonal-comparison-v10-9-2014-wp.html

The nice thing about acquiring better-quality eyepieces, barlows and prism-diagonals, is that, unless they're lost or spirited away, they will last a lifetime, and beyond.  In addition, they can be used with telescope to telescope to telescope within one's own collection.   

In the end, however, optically, straight-through viewing is always best, ergonomics notwithstanding; and then with the insertion of a better-quality prism heralding the very beginning of a progressive optical regression, albeit subtle and taking a trained eye to see the difference.

Edited March 18, 2017 by Alan64

[Stu]

I had always been under the impression that using a Prism diagonal in fast refractors risked showing up chromatic abberation, but that using a decent mirror diagonal in a slow scope was perfectly sensible and acceptable practise?

There are quite a few very good quality mirror diagonals around such as the Televue Everbrite and and Baader BBHS Sitall which give excellent results.

I have found that a Baader Zeiss Prism will show better results at high power, the image just seems to hang together at higher mag before it starts to degrade. I would not say though that this is a reason to say using a good mirror diagonal in slow scopes is bad practise.

I use a Takahashi Prism in my 1.25" travel kit because it is excellent quality optically, much lighter than the Baader and very good value for money.

[mikeDnight]

The Takahashi Prism diagonal is top class! Though Tak use a part plastic construction the diagonal is truly solid, very strong and optically outstanding. My Tak prism diagonal carries the weight of my binoviewer without any issue. It is far superior to the cheap and often nasty prisms offered by Celestron and other mediocre manufacturers, and will deliver super high contrast and well defined images. The Tak prism will also keep pace with the very best mirror diagonals. It's only drawback is that it is only 1.25" fit. There isn't a better prism diagonal offered anywhere that is in the same price bracket as far as I'm aware.

A good diagonal, prism or mirror, will not degrade the image in your telescope!

Mike

[Alan64]

"I had always been under the impression that using a Prism diagonal in fast refractors risked showing up chromatic abberation..."

In so far as the faster refractors and prisms, I unearthed this... 

https://www.cloudynights.com/articles/cat/user-reviews/accessories/star-diagonals/improving-refractor-performance-with-a-prism-diagonal-r1788

...a bit dated, but interesting nonetheless.

EDIT: In the case of a f/15 achromat, or an f/6 apochromat, false-colour induced by a prism should be of very little if any concern, with less light-scattering guaranteed, and with Cassegrains especially.

"...but that using a decent mirror diagonal in a slow scope was perfectly sensible and acceptable practise?"

I ask myself the very same when encountering recommendations of mirrored diagonals for slower telescopes. 

"I would not say though that this is a reason to say using a good mirror diagonal in slow scopes is bad practise."

The recommendations are, rather.  Then, when a user of a slower telescope has access to a mirrored diagonal only, whether due to economy, or simply in being oblivious to a prism's existence and advantage, or without a prism to compare, such is the only practice.

In the case of a slow Newtonian, the secondary mirror is the telescope's diagonal, and there will be no getting around that; but then, it's only two mirrors, total, and after all.

Edited March 18, 2017 by Alan64

[Dave In Vermont]

mikeDnight has brought up a oft misunderstood point: some prism diagonals are top-line and up there with, or exceeding even, good-quality mirror diagonals. Though I've never had the chance to use the Takahashi entry here, I'd imagine it would be of the class of prism-diagonals that are at least equal to a good dielectric mirror one. Some of the top-pier prisms are fantastic! But as money is also at play - I'd personally stand by my suggestion previously - take a look as a GSO 1.25" (Louis D - if 1.25" is what you wanted? I thought this was the case?).

This hasn't tossed me any undue false colour and/or CA or reflections in my Maksutov (SW 150mm F/12) - or my ST80 80mm F/5 achromatic refractor either, for that matter. It's a nice, solid non-petroleum-distillate (plastic) entity that looks nice and works very well indeed!

Now I've completed my task here, and we can return to the scheduled return to the Monty Python 'Fish-Slapping Dance' or something else more on-topic.....

Dave?

 

Edited March 18, 2017 by Dave In Vermont

[Cosmic Geoff]

Here's a review for those who want to get into the details of this subject.

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

[Stu]

40 minutes ago, Alan64 said:

In the case of a long-focus refractor, or an apochromat of any focal-length, false-colour induced by a prism should be of very little if any concern, with less light-scattering guaranteed, and with all types of telescopes that use diagonals at that.

Focal length has little to do with it Alan, focal ratio is what is important. A fast apo would not be a good candidate for use with a Prism diagonal as it does risk showing CA due to the steeper light cone going through the prism. In this instance, a top notch mirror diagonal would be preferable I think.

41 minutes ago, Alan64 said:

The recommendations are, rather.  Then, when a user of a slower telescope has access to a mirrored diagonal only, whether due to economy, or simply in being oblivious to a prism's existence and advantage, or without a prism to compare, such is the only practice.

Can you explain this paragraph, I'm afraid I don't quite understand it. What I am trying to say is that it cannot be considered bad practise to use a good mirror diagonal in a long focal ratio refractor as it should perform just fine. A Prism per se is not necessarily always better than a mirror diagonal, it needs to be high quality such as the Tak or the Baader Zeiss to make a real difference. If you need a 2" diagonal then the Baader Zeiss Prism can be prohibitively expensive so it is still good practise to use a good mirror diagonal.

[Louis D]

56 minutes ago, Dave In Vermont said:

take a look as a GSO 1.25" (Louis D - if 1.25" is what you wanted? I thought this was the case?)

I have the WO 1.25" dielectic diagonal right now, and it works just as nicely as my 2" GSO dielectic.  I bought it to make it easier to bring my binoviewers to focus.

[Louis D]

Well Alan64, now you've got me thinking about getting the Baader T2 someday.  I reread Bill's nice diagonal review on your suggestion, and am thinking that diagonal might be worth a try instead of yet another premium eyepiece when Santa comes calling.

[Alan64]

11 hours ago, Stu said:

Focal length has little to do with it Alan, focal ratio is what is important.

The post has been edited to reflect that.

11 hours ago, Stu said:

What I am trying to say is that it cannot be considered bad practise to use a good mirror diagonal in a long focal ratio refractor as it should perform just fine.

With a slower telescope, if a prism is an option, then choosing a mirror instead is a lesser, and therefore an ill-advised, choice.  

The OP has a Sky-Watcher 127mm Maksutov.  We do not know the exact nature of their present diagonal, whether a mirror or a prism, but whichever it is its quality is responsible for the OP's consideration of abandoning a diagonal altogether, and for a straight-through observance.   

I suppose that we should then do our duty, and recommend a mirrored diagonal.

Mirrors scatter light, and to a greater extent than optical-glass.  A Cassegrain already contains two mirrors, and with both already contributing their share of said scattering before a diagonal is even integrated.  Just how sensible is it, then, to add a third mirror in the form of a diagonal?  

I have two Newtonians.  I flocked and blackened the 150mm f/5, and I'm going to need to do the same for the 100mm f/4, and why?

...light-scattering, and greyish-black sky backgrounds.  The 150mm f/5 now exhibits the backgrounds as very close to jet-black.  The light-scattering seems to have been eliminated...

But then, the camera revealed the true nature of a mirror, and when the angle was just right...

  

Both afocal photographs were taken within seconds of one another.  Imagine my surprise.

In the case of f/7+ achromats and f/6 apochromats, I'd rather run the risk of increased false-colour rather than that of increased light-scattering, and use a prism diagonal instead.

Incidentally, APM is offering this relatively economical 2" prism...

http://www.apm-telescopes.de/en/optical-accessories/stardiagonal-mirror-prism/apm-2-inch-diagonal-prism-quick-lock-ultra-broadband-coating

[Alan64]

12 hours ago, Louis D said:

Well Alan64, now you've got me thinking about getting the Baader T2 someday.  I reread Bill's nice diagonal review on your suggestion, and am thinking that diagonal might be worth a try instead of yet another premium eyepiece when Santa comes calling.

Its choice would be dependent upon the telescope or telescopes, to an extent, for many simply choose a 2" mirrored diagonal and use it for both formats, 2" and 1.25".

When I got my Baader, the non-Zeiss version was not available, but no longer...

http://www.highpointscientific.com/baader-t2-star-diagonal-with-nosepiece-and-eyepiece-holder-prism1

...and fully tricked-out.  However, there is a helical visual-back that is sold separately...

http://www.highpointscientific.com/baader-planetarium-1-25-t-2-focusing-eyepiece-holder-eyehold-2

...and for finer focussing right there at the eyepiece.  I do not know if the full version is equipped with it or not.  That would require a call to the vendor, and before purchasing.

Given what I've read of that prism, its performance, I cannot really see the point in suggesting the Zeiss version instead.  Such a decision is then left to the buyer instead.

[Stu]

6 hours ago, Alan64 said:

The post has been edited to reflect that.

With a slower telescope, if a prism is an option, then choosing a mirror instead is a lesser, and therefore an ill-advised, choice.  

The OP has a Sky-Watcher 127mm Maksutov.  We do not know the exact nature of their present diagonal, whether a mirror or a prism, but whichever it is its quality is responsible for the OP's consideration of abandoning a diagonal altogether, and for a straight-through observance.   

I suppose that we should then do our duty, and recommend a mirrored diagonal.

Mirrors scatter light, and to a greater extent than optical-glass.  A Cassegrain already contains two mirrors, and with both already contributing their share of said scattering before a diagonal is even integrated.  Just how sensible is it, then, to add a third mirror in the form of a diagonal?  

I have two Newtonians.  I flocked and blackened the 150mm f/5, and I'm going to need to do the same for the 100mm f/4, and why?

...light-scattering, and greyish-black sky backgrounds.  The 150mm f/5 now exhibits the backgrounds as very close to jet-black.  The light-scattering seems to have been eliminated...

But then, the camera revealed the true nature of a mirror, and when the angle was just right...

  

Both afocal photographs were taken within seconds of one another.  Imagine my surprise.

In the case of f/7+ achromats and f/6 apochromats, I'd rather run the risk of increased false-colour rather than that of increased light-scattering, and use a prism diagonal instead.

Incidentally, APM is offering this relatively economical 2" prism...

http://www.apm-telescopes.de/en/optical-accessories/stardiagonal-mirror-prism/apm-2-inch-diagonal-prism-quick-lock-ultra-broadband-coating

I think we will just have to agree to disagree then. I've had all this kit and use it regularly. I use a Baader Zeiss Prism in my Tak FC100DC and in an OMC140 Mak and am aware of its qualities.

This is all about comparative quality though. A good quality mirror does not scatter light badly as you suggest. Try saying that to the dob boys with beautifully smooth mirrors which give amazing contrast. A poor quality Prism will give worse performance than a good quality mirror.

Your images tell a thousand stories, mainly about how misleading images can be. You cannot tell anything about comparative images without the exposure settings and angle of image being exactly the same.

The basic point is that I simply can't agree with calling it bad practise to use a good quality mirror diagonal, there is far more to it than that.

[Peter Drew]

I've never seen scatter as bad in any telescope as shown in the second photo unless the optics were dewed up.  

[jambouk]

24 minutes ago, Stu said:

Your images tell a thousand stories, mainly about how misleading images can be. You cannot tell anything about comparative images without the exposure settings and angle of image being exactly the same.

1

I was going to say the same thing. If the images above were standardised then this would be the best evidence I have EVER seen for flocking a telescope, however I think there are too many variables including:
- the two telescopes aperture (150mm vs 100mm)

- the two telescopes focal ratios (f5 vs f/4)

- the image method (afocal)

- the absence of any capture settings

- the absence of any information on processing methods

- the absence of information on sky conditions

- the absence of information on the quality of the optics...

James

[Mr Spock]

I've used many diagonals over the years and my current 2" Revelation Quartz Dielectric, in my view, is as good as anything out there. I see no difference in detail with or without it, nor is there any light scatter.

I certainly wouldn't use a refractor, Mak or SCT without one. The lack of comfort just isn't worth it!