Why are Maks so good on planets ?

[Vox45]

I was attending a star party at a new astroclub I just joined and was the only one using a Maksutov-Cassegrain 150. The general public was quite happy with the view of jupiter and saturn in my scope and soon members of the club started to take interest in my small OTA compared to their big Newton OTA.

I had the best view of all scopes that night and even the club's MEADE 14'' was put to shame in term of contrast... This was very shoking to me. I knew that Maks were "planet killer" but I never had the chance to put one in direct competition with other scope under the same conditions... I assume that the MEADE was maybe not used to its full potential as this is a new acquisition by the club. Also, the EP might have played a role as well. But, compared to all other scopes present (mostly Newton scopes) the Mak was the only one to resolve Cassini's division and see 3 bands on Jupiter on a night with a so-so sky.

So I started to wonder what was making Maks so good at planetary observation ?  Focal lenght ? I read that on visual observation FL does not have a lot of importance... Diameter ? 180mm is less than the Newtons present (200mm) so resolution is lower than a Newton... Even my old C6 with the same aperture did not give me as good a view, so the cassegrain part is not in play (?)

Anyone can weight in on this subject ?

 

[John]

I get a similar reaction at my society get togethers regarding the planetary views with my ED120 refractor. Most of the members have SCT's with the odd newtonian thrown in.

With the refractor I guess it's the unobsetructed aperture that gives the contrasty, crisp views. I'd be interested to know that makes the mak-cassegrain design "tick" as well. They do have a secondary of course but no support vanes and an optical system that is enclosed by the meniscus lens. I'm sure those factors play a role but I'm not sure to what extent ?

[Stu]

I suspect a few things at play here. SCTs are very susceptible to collimation errors for higher power views so perhaps this one was not properly set up. Also larger scopes are often struggle much more with poor seeing conditions whilst your 150 would cut through these quite nicely.

The smaller central obstruction, lack of spider vanes and also the more 'solid' collimation on Maks will also have played a role.

I've had a similar experience with my 8" Mak some years ago, everyone was queuing to look through the biggest dob on the field, but those that looked through my scope said the view was much better!

[Vox45]

- smaller central obstruction

- lack of spider vanes

- 'solid' collimation

I can see all this playing a big role. thanks for your remarks

21 hours ago, Stu said:

larger scopes are often struggle much more with poor seeing conditions whilst your 150 would cut through these quite nicely.

Ha, I did not think of this ... I will have to compare under better conditions, It was quite shocking to me to see in the 14'' that, although Saturn was bigger, the view was less contrasted and detailled ... 

[Ben the Ignorant]

No spider vanes, almost no tube currents thanks to the short length and the corrector that shuts the tube. All-spherical surfaces that are more likely to be shaped and polished well, also the moderate diameter makes it more probable that the lens and mirrors will be executed well. And your personal scope is probably a lucky combination of above-average optical components and above-average mechanical assembly.

And lastly, your diagonal and eyepieces don't ruin what the scope does.

[jetstream]

There are no magic telescopes but there are ones more suited to given seeing conditions and thermal equalization becomes a big factor. Given equal optics and assuming thermal equalization a larger aperture will show more just as sharp as any scope in excellent seeing.

Edited July 22, 2019 by jetstream

[johninderby]

It has a lot to do with how rarely is seeing good enough to take advantage of a perfectly collimated large scope. Most of the time the mak will give great lunar / planetary views despite the seeing not being that good and collimation is not a problem with most maks. It’s potential vs real world conditions particularily in the UK.

A big dob can produce fantasrtic views on those rare occasions when everything is just right though. 👍🏻

[dweller25]

I have a really nice 7”  Intes Micro Maksutov that gives great planetary views.

But a standard 10” Skywatcher Dobsonian was better.

[FLO]

5 hours ago, johninderby said:

It has a lot to do with how rarely is seeing good enough to take advantage of a perfectly collimated large scope. Most of the time the mak will give great lunar / planetary views despite the seeing not being that good and collimation is not a problem with most maks. It’s potential vs real world conditions particularily in the UK.

It certainly is! 

My best view to date of Jupiter was achieved one night at Kelling, a few years ago. We had decided the Skymax 180 would be FLO’s ‘planet killer’ but we had arrived late so we’re still unpacking when Jupiter appeared between rain showers. The Mak’ hadn’t had long enough to cool so we quickly setup our old and battered Sky-Watcher 100ED doublet. The view was astounding but it was more a result of unusually good (even for Kelling) seeing conditions than telescope optics. Unfortunately when the Mak was ready to go seeing had deteriorated. 

Steve 

[John]

I had some of my best ever views of Saturn a few years back with a Celestron C8 SCT. The seeing was just perfect and the planet looked like the Voyager images at 250x. Absolutely stunning.

Saturn was really high in the sky back then.

 

[Ben the Ignorant]

10 minutes ago, FLO said:

We had decided the Skymax 180 would be FLO’s ‘planet killer’

It could very well have been. Even Takahashi has trouble making great aspherical surfaces so Sky-Watcher outperformed them with all-spherical ones of the same diameter..

Link to the complete article:

http://r2.astro-foren.com/index.php/de/12-beitraege/04-zweispiegel-systeme-astrofotografie/789-d069b-vergleich-180-mewlon-dall-kirkham-und-180-skywatcher-maksutov

[John]

Are the latest versions of the Skywatcher mak-cassegrains working at full aperture now ?

 

[Alien 13]

The thing about Maks (and SCT) is the strength of the "focusser assembly" you could hang 10Kgs of camera or eyepiece off it and it wouldn't sag at all plus the spherical mirrors can achieve far greater accuracy than any parabolic one. Another factor is that a Mak will make even a cheap eyepiece perform like a Teleview, well almost...

Alan

Edited July 22, 2019 by Alien 13

[Vox45]

I think the problem is that I went in there with a lot of assumption on what focal lenght and diameter really meant. I was sure that I would get a better view with 1800 FL compared to 1000/1200 Newtons, but was shocked by the poor view of the 3550mm of the 14'' SCT !

So I spent quite some time this afternoon to research what a Mak is really about, how it came to be and what kind of optics it uses and learned quite a lot. Most of it was covered in the first answers here (thanks guys) and some more on a couple of in-depth explanation of meniscus corrector plate, achromatic corrector and spherical aberration on different websites...

What I take away from this experience is that pure numbers was not the only factors in play: size of the scope, thermal equilibrium, EP, mount, seeing, wind, collimation can all throw a wrench in the proverbial works ... I'll try to reassure my pal Pascal shown here with the MEADE 14'', unaware that he would be crying later, and the guys back at the club that they did not spend 10K€ for nothing   (they were a bit upset)

Edited July 22, 2019 by Vox45

[FLO]

1 hour ago, Ben the Ignorant said:

Sky-Watcher outperformed them with all-spherical ones of the same diameter..

You are not comparing like-for-like but it is true that the Maksuov’s success can be largely attributed to its long focal ratio and use of spherical surfaces (all optical surfaces in a Maksutov are spherical). This is not because spherical surfaces are ‘better’ but because they are easier to manufacture to a high surface accuracy, at an affordable price. 

Steve 

[FLO]

23 minutes ago, John said:

Are the latest versions of the Skywatcher mak-cassegrains working at full aperture now ?

I assume they are but I wasn’t aware they previously were not. Did the design include some kind of sub-aperture mask? 

Steve

[Paz]

The original post doesn't surprise me, I'm a big fan of maksutovs. The contrast and clarity you get for the cost and aperture of the scope is very good.

[johninderby]

I think the bottom line is the Skymax 180 simply works. Not perfect but capable of giving very satisfying lunar / planetary views. 

[John]

2 minutes ago, FLO said:

I assume they are but I wasn’t aware they previously were not. Did the design include some kind of sub-aperture mask? 

Steve

My understanding is that the maksutov-cassegrain design used for the Skywatcher models requires a slightly oversized primary (about 6-7% larger) to deliver the full claimed working aperture. I've not measured this myself but I have read reports from a number of owners (including Neil English) who have measured the effective aperture of their Synta mak-cassegrains and found it a few % less. The 127mm seems to be working at 118mm-120mm for example. Neil described his Skywatcher 180 as a 170mm scope in his report on it. I believe a Sky & Telescope test of the 127 a few years back (probably the Orion USA branded version) was of a similar opinion.

The design may have changed though so this might not apply to current models.

 

 

[johninderby]

As far as Iknow they haven’t changed the Skymax maks so assume they are are all slightly under aperture. 

Also have a Bresser 127 mak which is a full 127mm aperture and a longer focal length than the Skymax 127 and the OTA is bigger in diameter. Just that bit sharper and a bit more contrast.

Edited July 22, 2019 by johninderby

[FLO]

37 minutes ago, John said:

My understanding is that the maksutov-cassegrain design used for the Skywatcher models requires a slightly oversized primary (about 6-7% larger) to deliver the full claimed working aperture. I've not measured this myself but I have read reports from a number of owners (including Neil English) who have measured the effective aperture of their Synta mak-cassegrains and found it a few % less. The 127mm seems to be working at 118mm-120mm for example. Neil described his Skywatcher 180 as a 170mm scope in his report on it. I believe a Sky & Telescope test of the 127 a few years back (probably the Orion USA branded version) was of a similar opinion.

The design may have changed though so this might not apply to current models.

I was aware of the oversized primary mirrors but hadn’t heard the effective aperture was reduced. If you and Neil English think it true then so do I. Thank-you 🙂

Steve 

[Peter Drew]

The light path inboard of the Maksutov corrector lens actually diverges so if, for instance, you have a 6" corrector and a 6" primary, some of the peripheral light from the corrector misses the primary. Hence the need for an "oversize" primary to address this.    😀

Edited July 22, 2019 by Peter Drew
typo

[FLO]

30 minutes ago, johninderby said:

Also have a Bresser 127 mak which is a full 127mm aperture and a longer focal length than the Skymax 127 and the OTA is bigger in diameter. Just that bit sharper and a bit more contrast.

Is it this one? 

Steve 

[johninderby]

Just now, FLO said:

Is it this one? 

Steve 

Yes that’s the one. I find it has a small edge in performance over the Skymax but then it is more expensive.

[Paz]

7 hours ago, johninderby said:

Yes that’s the one. I find it has a small edge in performance over the Skymax but then it is more expensive.

I have one of these and went for this as it is f15 which means that compared.to faster focal ratio maksutovs it has a smaller central obstruction which helps with contrast,  its easier to make them to a high standard for the price,  and it's easier on eyepieces.

I'm sure I will have tested the effective aperture although I can't remember specifically,  I'll see if I wrote anything in my notes.

Edited July 23, 2019 by Paz