A question about Maks

[mik]

Hello can anyone tell me the difference regarding Muksutov cass & Schmidt cass

also what is regarded as the better design.

thanks mick.

[Macavity]

I think old... Wikipedia says most of it re. construction etc.

http://en.wikipedia.org/wiki/Catadioptric (And several other linked pages)

In fact, I reckon Wikipedia ain't BAD re. optical topics?

[mik]

Thanks Chris that is a good link i will have a read.

Mick

[John]

As for which is the better design, well both have their strengths and weaknesses. Maksutov-Cassegrains (Mak's) tend to be known for being planetary scopes because of their longer focal ratios and smaller secondary obstructions wheras Schmidt-Cassegrains (SCT's) are slightly more all round scopes. Both are popular for their compactness of course

John

[Macavity]

Yes - I was a bit terse - Sorry.

Indeed, the above... and the "cooldown" time. Here MAKs have thicker correcting plates than equivalent SCT's and take longer to reach optimal performance due to internal (air) "tube currents", as thermal equilibrium is established. Personally, I've never felt "limited" with a MAK127 (Here, it probably corresponds to my general "faffing around" time?)! But larger ones e.g. a MAK180 are reputed to take ~2hrs to cool down properly...

[Ajohn]

It's not really possible to answer that question. It depends on the design. What can be said is that it is possible to produce a maksutov that will out perform a meade/celestron schmidt. Basically because the maksutov can be corrected for more aberrations. Does anybody do one? I don't think so as the point would be made in the adverts. A better schmidt type can be produced too.

There isn't any basic reason why a maksutov should have a longer focal length than a schmidt. The main advantage of the maksutov arrangement is that it can provide lower aberrations and still use all spherical surfaces. It's supposed to be harder to make a schmidt of equal performance.

The size of the central obstruction isn't straight forwards. There is no reason why with the same F ratio primary mirror and field of view that one type should have a smaller secondary than the other. Maximum eyepiece size has more impact on this as that sets the size of the baffles that keep the light out of the eyepiece end and then the size of the secondary mirror. If both scopes are otherwise the same a smaller eyepiece holder will give a smaller field of view so the 2ndry can be smaller. Either scope designed purely for 1 1/4 eyepieces might get down to the magic 20% when it's say 8ins dia or more likely bigger.

Out of interest maksutovs own designs that had low aberrations used a very very thick corrector. The telescope is made achromatic by the relationship of the index of the glass used and the relationship between the 2 curves that make it's surface. These vary with the thickness of the corrector plate as well. Certain arrangements will give the lowest aberrations. Notice I used the word achromatic too - not apochromatic. Not that this should put you off.

John

[mik]

John/John/Chris.

Thanks for those replys along with the thread from Chris i am understanding these things a little better as for achromatic/apochromatic that wont put me off john i don't worry about things like that ive looked through a ED100 but i still find the views through a Tal 100rs stunning the best budget planetry scope for the dosh i think. I am looking at getting a scope for planets you see so i need to look at all the variables.

Mick.

[Ajohn]

If you don't mind the size the 6ins skywatcher refractor might be a good bet but it does over stretch the technology used a bit. I'm surprised more people don't go for this

http://www.obm.co.uk/products/db/euro200f8d.htm

You can realistically look at anything you like with an instrument like that. The main problems with newtonians is that must have short length but personally I would avoid F5 like the plague maybe F6 but F8 has an awful lot going for it. Comes on a good mount too but you should be able to get the tube on it's own. The gp go to and drive bits may be a bit expensive.

In comparison to the other things you may be thinking of - on a budget - it's no contest. I do like celestron C8's on a fork mount though. Maks pass I did buy one once but sent it back as it hadn't been collimated. Doing that on a mak isn't as simple as cloudy nights would have you believe. The mirror to corrector distance is important as well as tilt. It pays to remember that all scopes are a compromise. Newts especially longer ones and refractors have a lot going for them. C8's are extremely convenient but will suffer because of the largish central obstruction.

John

[John]

You can realistically look at anything you like with an instrument like that. The main problems with newtonians is that must have short length but personally I would avoid F5 like the plague maybe F6 but F8 has an awful lot going for it.

I agree wholeheartedly with that - I'm very impressed with the performance that my 8 inch F/6 Skywatcher dobsonian delivers - just as good as the old C8 I owned a while back.

John

[mik]

Hi i was thinking about the 6 inch evostar but i would need at least a HEQ-5 and to be honest i think that is pushing the mount for high mag focusing and very hard on my wallet,as you can see from my sig ive got a xlt150 but to be honest im not keen on reflectors ( i purchased this for the mount) im not that keen on maks either oh dear,

I am a big fan of the chinese refractors the xlt 120 is getting good reviews but i think in the not to distant future i will get a Tal 100rs that will fit on my CG-4 nice.

Mick

[FLO]

If you don't mind the size the 6ins skywatcher refractor might be a good bet but it does over stretch the technology used a bit. I'm surprised more people don't go for this

http://www.obm.co.uk/products/db/euro200f8d.htm

... personally I would avoid F5 like the plague maybe F6 but F8 has an awful lot going for it. Comes on a good mount too but you should be able to get the tube on it's own. The gp go to and drive bits may be a bit expensive.

AJohn, I don't think the scope shown in the pic is an f8, it looks more like an f4-5(?). Not sure I agree with avoiding f5 'like the plague' but I do understand where you are coming from

[Whippy]

Mik, that TAL100RS will go nice on a CG4 but you might want to factor in a fringe killer filter for those bright planets. If anything is going to show if you've got CA, then it'll be the Moon and Saturn. Superb scope nonetheless .

WRT Maks, my understanding is that it's easier to make a good quality Mak-Cass than it is to make a SCT hence the myth that Mak-Casses are better but I would in the real world there's probably not a whole lot of difference between the two designs of equal optical quality. As a side note, there's two types of Mak-Cass easily available. There's the Gregory design manufacured by Synta and Orion Optics' OMC 140 where the secondary is a silvered spot on the Meniscus and if it's not collimated, then it's back to the manufacurer with it.. Then there's the Rumak design manufacured by Intes Micro (and I think the bigger Orion ones are too) where the secondary is a seperate assembly on the Mensicus and is collimateable. I bought my Intes MK66 (6") knowing that the previous owner fitted some Bob's knobs and hadn't bothered to collimate it. I sorted that out in about 30 mins and it gave excellent start tests (still does ). Also the larger Intes models have a fan fitted so cooldown (the one serious drawback to a Mak-Cass) isn't such an issue.

Mik, if you see a 5" or 6" Intes or Intes Micro Mak-Cass around second hand, give it some serious consideration as they really are excellent quality scopes.

Tony..

[Kaptain Klevtsov]

WRT the thick corrector plate and cool down, my current Tal200K has the "corrector plate" as a meniscus lens mounted close to the secondary mirror. This is the modification to the Maksutov design that makes it a Maksutov-Klevtsov design. This means that a much smaller piece of glass can be used as its much smaller than the 'scope aperture. Its also a "non-aspheric" design, which means that there aren't any expensive shaped pieces of glass in it, all the surfaces are spherical (as in cheap and easy to make to a high degree of precision).

I didn't realise that Maks aren't Apochromatic and I certainly haven't noticed anything like colour fringing with my Tal or the OMC140 that I had. I must make a point of having a look for colour fringing as I didn't know to look for any. You learn something new every day here, don't you?

I would agree that an f/8 Newt. would be a planet killer because of the small central obstruction and collimation tolerance, but I'd never get rid of my 200mm f/5 Newt. For imaging DSOs, they are awesome things. Fast enough and with a decent focal length. For planetary observing, I'd do the Maksutov route anytime over a similar aperture Newtonian as they are much more compact and you get a longer native focal length, but I still wouldn't rule out the f/5 Newt. Cheap as chips compared to an 8" SCT or Mak. and it'll do other things too.

8" f/8 Newt? That's going to be a big beasty, much longer than the one in the picture.

HTH

Kaptain Klevtsov

[mik]

Thanks for the replys gosh there is a fair bit to consider i am in the position as most are that i have to pack my gear away after each session due to this i also wish to stay away from collimation so i think a refractor is the way to go for me due to bad back i struggle with the xlt150 the tal 100rs i think i can manage ok, i have to carry bits out and put it all together on the patio ie tripod then head then weights etc etc.

Ive got a william optics VR1 filter that i got to go with the zs66 but never needed it but its there a bit o colour never bothers me to much and the Tal as far as i am aware has got somekind of yellow tint to the optics that seems to help correct the violet tinge'the xlt120 i feel may be a bit heavy for me but the 102 will be ok.

Thanks Mick.

[Whippy]

Out of interest maksutovs own designs that had low aberrations used a very very thick corrector. The telescope is made achromatic by the relationship of the index of the glass used and the relationship between the 2 curves that make it's surface. These vary with the thickness of the corrector plate as well. Certain arrangements will give the lowest aberrations. Notice I used the word achromatic too - not apochromatic. Not that this should put you off.

I'm curious, this is the first I've heard that Mak-Cass' are achromats too. If the glass isn't bringing light to focus then why would it introduce false colour? AFAIK, I've never heard any imagers using one having to add any colour correction to either the optical train during image capture or processing.

Tony..

[Macavity]

Yeah - I'd've thought, if it were a true menisus, i.e. the two surfaces were "parallel", white light would indeed split at the first surface, but then recombine "by the same amount" at the second - To re-emerge as white light! (Thinking cowed little boys, a sadistic PHYSICS master, "apparent forward displacement" 'n' all that good stuff)! :mrgreen:

[mik]

Out of interest maksutovs own designs that had low aberrations used a very very thick corrector. The telescope is made achromatic by the relationship of the index of the glass used and the relationship between the 2 curves that make it's surface. These vary with the thickness of the corrector plate as well. Certain arrangements will give the lowest aberrations. Notice I used the word achromatic too - not apochromatic. Not that this should put you off.

I'm curious, this is the first I've heard that Mak-Cass' are achromats too. If the glass isn't bringing light to focus then why would it introduce false colour? AFAIK, I've never heard any imagers using one having to add any colour correction to either the optical train during image capture or processing.

Tony..

Tony is the scope not designed as a achromat and because it was a flawed design a corrector plate was added and is this why imagers don't have to correct colour because it has already been done that is how i read the link on the second post. but what do i know

[mik]

Think i will get a Tal 100rs

[Whippy]

Mik, I think you're reading it wrong mate.

Mirrors do not display any Chromatic Abberation, a corrector plate does not bring any light to focus. Therefore why would Mak-Casses be classed as Achromats? The reason why Maks and SCT's have corrector plates is to correct other image abberations such as sphercial abberation due to the fact that the mirrors on their optical systems have to be much faster (somewhere around f2 or f3) than their Newtonian counterparts (around f5 and up) which increases such issues. It's not that the Mak-Cass (or indeed any catadioptric like the SCT) is a flawed system, it's the fact that there has to be some kind of corrector there to make the whole system work.

Stick the TAL100 Mik, you won't go wrong with one .

Tony..

[Macavity]

"Oh zose Russians"...

(Sorry - Couldn't resist)

[Whippy]

I was completely unware of that rather good 'tribal' drumming intro, then it went pear shaped :shock: . Especially with the fake beard. The 70's eh? What larks they had!!

Tony..

[mik]

Tony you say mirrors don't display CA i am not sure that anyone refered to that i think the ref was to sphercial abberation in johns post thats how i read it.

ok on the Tal i can understand that.

Cheers mick

[GazOC]

Neither SCTs or Maks are truely apochromatic, as soon as you pass light through the corrector plate (a weak lens) the colours will split to a certain extent. I find the effect somewhere in between neglidgeable and undetectable in either type of scope for visual use though.

[Kaptain Klevtsov]

The stuff I read about the Klevtsov design before I bought my Makski suggested that as the meniscus is a two pass arrangement, the light going one way gets messed up with the prism effect, but gets put back together almost perfectly on the way out again. If you do the sums properly you can, apparently, arrange for the effect to cancel itself out so that any chromatic abberation disappears to all but the very slightest of degrees.

Kaptain Klevtsov

[Ajohn]

Doh - I should have typed a bit more.

The corrector plate is there to bend light otherwise it wouldn't do anything. The term achromat means that 2 colours of light are bought to the same focus. It's 3 on an apochromat. The residual colour error on a well designed schmidt or mak is very small but can sometimes be just seen under very high magnification just like on some apo's. From memory the mak is superior to the schmidt cas in this respect but there isn't much in it. Both types use one glass for the corrector. There are types that use 2 to achieve even better colour correction.

The mak can be corrected for spherical aberration and coma but this needs hand figuring (spherical) and a very thick corrector (coma). The colour correction can be achieved with almost any thickness of corrector. The schmidt design originated with a camera. The tube length is twice the focal length and the focal plain is curved and in a place where it can't be viewed - otherwise it's probably the nearest thing to perfection available. There are other catches too but I wont go into those, there are compromises involved in reducing the length of the tube on a sct also on a mak. Maybe it's just best to know nothing about them but I think it pays to know a little so that people stand the best chance of buying the best compromise that they can afford.

Some one recent;y marketed a schmidt camera with a ccd camera and a field flattener built into it. The flattener is a simple lens. I suspect that most manufacturers will be offering them in the not too distant future.

On Kaptains comment - if he looked at deep sky objects with a good 8ins F8 mirror he might well sell his F5. The catch with an F5 newt is off axis coma. There is a lot less in an F8. Against that imaging will take longer but taking sky glow into account etc I personally don't think that really matters. It just means more exposures. Might even find that the image scale is better too.

On compound telescopes in general I'm inclined to think that they come into their own in situations where the user just can't accept the size of the scope. Shortness is great for the shippers but it does come at an optical cost. Telescopes are terrible things just about every aspect is a compromise of one good quality against another.

John