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So why exactly are Cats closed tubes?

Dave_D

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Stu

Stu

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I shall await responses to this with interest Dave. I've often wondered why it is not possible to open up the tubes on Maks and Cats more to speed up cooling and thermal stabilisation.

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fatwoul

fatwoul

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My guess is that an open cat would lose one of the benefits of a closed design - less dust falling on the mirrors, and on the rear side of the front glass.  That was certainly compensation for me with my Mak-Newt, which took a while to cool down but has remained fairly clean.  I wonder if their might also be glare issues with lenses that are less present with mirrors.  That would explain why open refractor designs aren't used, although there is probably a far better reason that I am unaware of.

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Dave_D

Dave_D

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yeah agreed on the dust issue, but i mean, if multithousand pound/dollar/yen/shekel/whatever planewaves are open, then surely it's basically a matter of engineering and not optical issues? and with modern cnc manufacturing, even that isn't an issue?

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Stu

Stu

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(BTW - some cats are vented, are they not? Celestron SCTs have vent holes at the back that can be fan-assisted.)

Yes, most of them are, but it still takes a long time to cool, and I used to find my mak would must up internally if conditions were bad and that takes ages to clear

EDIT: Just to add to this, the closed tube will also make clearing the boundary layer infront of the mirror. The fans cool the mirror from the back, but I reckon cooling fans blowing across the face of the mirror would be quite beneficial. Not sure if anyone has tried this

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ronin

ronin

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Isn't the front plate on an SCT a corrector?

Meaning that the whole plate is shaped and not to just there to support the mirror bit. So if open light would possibly be able to get to the mirror that is uncorrected.

On a Mak I think it is flat so an open design on that may be possible, allowing for stray light.

I suppose you could ask why no "newtonian" on the same design?

Drill hole in the mirror, flat mirror perpendicular at the secondary and view through the rear end. Pretty close to a SCT/Mak anyway. Secondary would be relatively big I suppose as it would be located at F/2 more or less.

Sure I have seen an open SCT somewhere,

Would the RC meet the open SCT criteria?

RC Scope

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JamesF

JamesF

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Given that there are open truss newts and open truss RCs I assume that one of the reasons we don't tend to see open truss Maks and SCTs is the expense of mounting the corrector and keeping it stable.  That's probably far more easily done with a tube.  The other possible objection to an open truss design that springs to mind is that the baffle tube is often covered in lubricant that would collect dust and other particles over time.

James

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Stu

Stu

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Given that there are open truss newts and open truss RCs I assume that one of the reasons we don't tend to see open truss Maks and SCTs is the expense of mounting the corrector and keeping it stable. That's probably far more easily done with a tube. The other possible objection to an open truss design that springs to mind is that the baffle tube is often covered in lubricant that would collect dust and other particles over time.

James

That makes a lot of sense James. I guess I'd seen it more along the lines of a ventilated tube rather than a full open truss. That way you would get the airflow and cooling whilst maintaining rigidity.

Ronin, the front plate on a Mak is still a corrector but much thinner/with less correction. Than an SCT. I don't think stray light would be any more of an issue than open tube newts, and could still be shrouded.

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Zakalwe

Zakalwe

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Its also a darn sight cheaper to simply roll a thin steel tube than to start making all the parts that a truss scope "tube" would need.

^^This^^

Why add to the cost of manufacturing (and hence the end price) for little or no gain.

An open tube would mean that the mirrors could get damaged much more easily. Dewing would become a problem. The overall weight would go up (there's no way that a simple tube will weigh more than trusses, connectors and so on).

Given that there are open truss newts and open truss RCs I assume that one of the reasons we don't tend to see open truss Maks and SCTs is the expense of mounting the corrector and keeping it stable.  That's probably far more easily done with a tube.  The other possible objection to an open truss design that springs to mind is that the baffle tube is often covered in lubricant that would collect dust and other particles over time.

James

Great point. SCTs slide the mirror on a greased baffle tube. That'd definitely get contaminated.

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ollypenrice

ollypenrice

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Tubes may be cheap but they are not particularly stable. I think most catadioptrics in large sizes would be better off with truss construction. This is certainly the case with RCs. Why not with all of them? On the other hand it does seem that the professional Maks are closed tube so there may indeed be more to it.

As for cooling, there is a curious reluctance on the part of the manufacturers to do what is really necessary and blow air across the top of the secondary to break the damaging boundary layer. Huffing and puffing up and down the tube is extremely inefficient. The mirror itself prevents these currents from getting to the damaging bit of warm air. Why not a filtered air intake on one side and a filtered outlet on the other with both fans blowing in the same direction, so one an entry fan and the other an exit? I think research has shown this to be the most effective yet it is rarely seen.

Olly

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ronin

ronin

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Stu, I knew one was a corrector and just wasn't sure about the other, and too lazy to go search the design details out.

Like most aspects it will likely be the cost, at the smaller sizes a solid tube is easier and less cost. As SCT's and Mak's seem to have spherical mirrors so requiring a corrector for whatever aberration I wonder what the situation would be if they produced them with parabolic mirrors. However cost for the mirror is higher and likely the truss mechanism also so again cost would prevent it as a viable option.

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JamesF

JamesF

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As SCT's and Mak's seem to have spherical mirrors so requiring a corrector for whatever aberration I wonder what the situation would be if they produced them with parabolic mirrors.

Then I think you'd have a standard Cassegrain.  It uses no corrector, but still has to support the secondary mirror somehow.

James

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nightfisher

nightfisher

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I have a VMC110 on its way, its sold as a mak/cass, but has to be a small Klevtsov like the TAL 200k, its open fronted having the corrector on a curved spider vane but other than that it works as a mak

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Zakalwe

Zakalwe

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Tubes may be cheap but they are not particularly stable. I think most catadioptrics in large sizes would be better off with truss construction. This is certainly the case with RCs. Why not with all of them? On the other hand it does seem that the professional Maks are closed tube so there may indeed be more to it.

I guess there will be a tipping point where larger diameter tubes start to weigh more than a truss. Plus the tube wall would need to get thicker to retain strength.

RCs also use a different focus method than something like a SCT. SCTs need to slide the mirror up and down a greased baffle, which as JamesF pointed out will get contaminated. Then again, SCTs can only go so big before the corrector gets too big to carry it's own weight.

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ollypenrice

ollypenrice

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The key thing about the truss is that it deforms in a particular way. All structures will deform, but Serrurier's truss (how personal are we becoming??? :eek: ) deforms by sliding in the same plane as that of the primary mirror, which is less damaging than allowing the plane of the secondary to get out of parallel with that of the primary.

Olly

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Stu

Stu

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I guess the larger Orion Optics maks (250 and 300) were along these lines with a sub aperture corrector plate and open front aperture. They also had 4 vanes supporting the corrector and secondary so would have diffraction spikes. I think they are discontinued now

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HD-AP

HD-AP

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No reason why cats could not have semi open or truss tube designs.

Q Wonder why the major manufacturers haven't offered this as an option .

Cost and ease of construction come into play .And dirt and dust and dewing of the optics when in use also comes into play as dewing on the front and back of the corrector plate main and secondary mirrors is all a possability which won't happen with the closed tube .

The repeated cycles of contamination and dewing lead to coating deterioration over time .

Some celestron sct's 30-40 years old are still on origional mirror coatings and are still working well, can't be many newtonian's .cassesgrains with mirror coatings this old still working as well ?

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Stu

Stu

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No reason why cats could not have semi open or truss tube designs.

Q Wonder why the major manufacturers haven't offered this as an option .

Cost and ease of construction come into play .And dirt and dust and dewing of the optics when in use also comes into play as dewing on the front and back of the corrector plate main and secondary mirrors is all a possability which won't happen with the closed tube .

The repeated cycles of contamination and dewing lead to coating deterioration over time .

Some celestron sct's 30-40 years old are still on origional mirror coatings and are still working well, can't be many newtonian's .cassesgrains with mirror coatings this old still working as well ?

The only piece of this I disagree with, and it is from experience, is that the mirror and corrector won't dew up inside. The issue is, I'm order to cool the scope you have to run the fans. This can draw moist air into the otherwise closed tube so the mirror and corrector can dew up inside.

It happened to my mak quite dramatically at PSP. Mind you, we were trying to observe from inside a cloud at the time!! ;-)

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