Cadioptrics?

[cajen2]

Just a general question. As a newbie, while I have used refractors and own a reflector, I know very little about Maks, SCTs, etc. I note that in the recent 'dream scope' thread, most of those mentioned were either Japanese triplet refractors or enormous dobsonions. I hear very little of cadioptric scopes on this forum, apart from the odd 'grab and go' Mak. Surely they have other strengths apart from portability? I was looking at some on FLO (as you do☺️) and I imagined they'd have the light-gathering capacity of a decent dob with the potential high mags of a refractor. So where's the catch?

I imagine I'll have hundreds of fans now telling me that they're wonderful!😄

Edited December 16, 2021 by cajen2

[Sunshine]

Rest assured that "dream scopes" and I just quoted myself 😂 are most certainly not restricted to bank busting triplets and gargantuan dobsonians. Having said that, I can speak of Mak's as I have owned one in the past few years and loved the planetary and lunar views they produce. One drawback which was a dealbreaker for me was their long cool down time and relatively narrow field of view comparing to a refractor. Unfortunately I do not have time to let a scope cool for an hour or longer, this combined with my yearning to have a quality refractor led to my selling my mak, I do miss it, though. There are some who wrap their Mak's in insulative material which they say helps greatly with the cooling issue, if you love planetary and lunar combined with great portability then a Mak may be the scope for you. There are many tips and tricks on Mak's here on SGL for Mak owners, they are planetary/lunar killers.

 

Edited December 16, 2021 by Sunshine

[cajen2]

5 minutes ago, Sunshine said:

Rest assured that "dream scopes" and I just quoted myself 😂 are most certainly not restricted to bank busting triplets and gargantuan dobsonians. Having said that, I can speak of Mak's as I have owned one in the past few years and loved the planetary and lunar views they produce. One drawback which was a dealbreaker for me was their long cool down time and relatively narrow field of view comparing to a refractor. Unfortunately I do not have time to let a scope cool for an hour or longer, this combined with my yearning to have a quality refractor led to my selling my mak, I do miss it, though. There are some who wrap their Mak's in insulative material which they say helps greatly with the cooling issue, if you love planetary and lunar combined with great portability then a Man may be the scope for you. There are many tips and tricks on Mak's here on SGL for May owners, they are planetary/lunar killers.

Interesting. Do their internal design make a cooling fan difficult or impossible to incorporate?

[Sunshine]

11 minutes ago, cajen2 said:

Interesting. Do their internal design make a cooling fan difficult or impossible to incorporate?

Honestly, I have never heard of a cooling system for a mak, maybe there are some models I am not aware of that are cooled. Don’t let my experience stop you, as mentioned there are many here on SGL who love their maks and have gotten around their weaknesses with inexpensive and nifty solutions. Maybe have a search in Mak related threads here on SGL, you’ll have a better understanding of what they offer vs what you want. Would I buy another Mak? YES! now that I am aware of some tricks others have used to make the experience easier I would gladly have another, one look at Jupiter through a Mak and you’d be sold.

Edited December 16, 2021 by Sunshine

[johninderby]

Maks and SCT have corrector plates on the front so you can’t use a simple fan. You can use a specialist cooler  that is inserted into the scope through the focuser and circulates air through the scope and blows it back out. 

https://www.firstlightoptics.com/misc/asterion-cooler-cat-for-sct-maksutov-telescopes.html

I’ve had both maks and SCTs but have now moved on to a Classical Cassegrain which is open at the front so cools down faster and no dewing problems. Also uses a proper crayford focuser and fixed primary instead of the mak and SCTs moving mirror type focuser.

Edited December 16, 2021 by johninderby

[Sunshine]

1 minute ago, johninderby said:

Maks and SCT have corrector plates on the front so you can’t use a simple fan. You can use a specialist cooler  that is inserted into the scope through the focuser and circulates air through the scope and blows it back out. 

https://www.firstlightoptics.com/misc/asterion-cooler-cat-for-sct-maksutov-telescopes.html

I’ve had both maks and SCTs but have now moved on to a Classical Cassegrain which is open at the front so cools down faster and no dewing problems. Also uses a proper crayford focuser and fixed primary instead of the mak and SCTs moving mirror tpye focuser.

 

Have to say that is a beautiful looking setup.

[cajen2]

Now that looks a serious piece of kit, John. Are you happy with it?

[johninderby]

Yes loving the scope. As mentioned have had similar sized maks and SCTs but find the Classical Cassegrain just fits my needs better. Not that maks or SCTs aren't good scopes but just my personal preferance. 🙂

Reasonable priced Classical Cassegrains have only very recently become available so not as many around……yet. 🙂

Edited December 16, 2021 by johninderby

[johninderby]

The CC8 plus a 125mm APO make a nice combination. 🙂🙂🙂

[jetstream]

46 minutes ago, johninderby said:

Classical Cassegrain which is open at the front

I sure like those baffles... is it easy to collimate if need be?

[johninderby]

Had it two years and not needed to collimate it yet. Not too hard to collimate though according to the instructions. Lot easier than an RC. Seems to hold collimation really well. 👍🏻

Better look at the baffles. 

Edited December 16, 2021 by johninderby

[jetstream]

10 minutes ago, johninderby said:

Better look at the baffles. 

Thats a nice looking scope.

[Louis D]

CATs are compromise scopes by design.  To achieve a compact form factor, they necessarily have long focal lengths and large central obstructions relative to reflectors of similar aperture due to their folded optical path.  Of course, refractors have no obstruction at all.  Large central obstructions reduce contrast.

CATs also have the issue of having to use baffles to prevent light from going straight into the eyepiece bypassing the optics because the eyepiece is looking along the optical axis instead of at a 90 degree angle.  These baffles reduce the maximum true, unvignetted field of view in smaller CATs relative to their reflector and refractor equivalents.  This compounds the narrow field of view caused by the long focal length previously mentioned.  This is not an issue with 14" and larger CATs due to their large rear ports when using 2" eyepieces.

The long focal length is caused by their slow/long focal ratios.  This leads to making it difficult to achieve maximum exit pupil size for narrowband filter usage (think OIII nebula filters).  Thus, filtered nebula images can tend to be somewhat dim.

Maks don't scale well beyond 10" for amateurs (20" for professionals) because the meniscus corrector becomes quite large and heavy.  There's no way to reduce its bulk.  It's similar to why no refractor was ever made over 40".

SCTs have really thin correctors, so they do scale well up to about 16" to 20" for amateurs and 54" in a professional setting (at least as a Schmidt camera).

Cassegrains (classic, RC, DK, etc.) scale very well since they have no front corrector.  The Hubble ST is an RC Cassegrain, and it is 94.5" aperture.  Pretty much all professional observatory class telescopes are Cassegrains of one design or another.  The Cassegrain design places the focal point in a convenient location for instrumentation.  Typically, there are tertiary mirrors and relay optics to bring the image out to the side along the altitude axis so the instruments can simply ride along on the azimuth platform.  Derotators take care of image rotation during tracking.

Edited December 16, 2021 by Louis D

[Alien 13]

There are compromises with a MAK but I dont see the narrow field of view one of them after all traditionally refractors were long and thin with a narrow field too before the advent of ED glass etc. I do agree that they dont scale well but the smaller ones are great for grab and go with little or no cool down time. The biggest advantage of a MAK is its huge back focus range so its very easy to add cameras/bino viewers etc and also the ability to hang large loads off the focusser which no other scope type could handle. I personally like the internal focusing that a MAK offers, cant abide a focusser that moves in and out...its just wrong.

Alan

[johninderby]

Unfortunately the moving primary mirror in a mak or SCT also means mirror flop and also the scopes focal length keeps changing as you focus. Having a proper focuser with it’s fine focusing is a huge advantage of the CC for me.

As to the weight a focuser can handle those using refractors seem to manage just fine.🙂

Any way lets not get into my scope is better than yours discussion. It’s all about choosing the scope that suits you whatever it may be. 

Edited December 16, 2021 by johninderby

[ollypenrice]

What's the catch with catadiptics?  The focal length. Dead simple and just that. For the rest, they have a lot going for them (apart from the dewing issue which a dew heater and camping mat extender will fix.)  I don't think the smaller SCTs give the tightest stars but my current one, a 14 inch, actually gives great little stars, nicely coloured. 

However, there are objects which are very disappointing in the 14 inch, as compared with a faster Newtonian. The Veil, for instance, is far too heavily cropped and lacks apparent brightness. Indeed all the extended objects suffer at the enormous focal length. Even a forgiving F6 Newt of the same aperture would make a huge difference.

These are comments about visual observing, by the way. I don't image with an SCT largely because of the F ratio in an age of cameras with small pixels.

Olly

[badhex]

First scope I purchased was a Skymax 102 maksutov, which I still have but has been inaccessible for some years (in storage in another country). I absolutely loved that scope, and would highly recommend it to anyone who would like reasonable power in a compact package.

As mentioned, the narrow FOV as mentioned is really the main downside with Maks/SCTs but the cool down time is less of a problem with the smaller options like the 102. I have since bought a Celestron C5 but don't seem to enjoy it quite so much despite the larger aperture. 

[Zermelo]

Some good points made already.

If I were allowed only one scope it wouldn't be my Mak, but it does get used a lot. If I know that the moon will be up (and if I don't feel like looking at it) and the fainter stuff will be more difficult, I'll often put out the Mak and concentrate on doubles, planetary nebulae, and globulars and planets if there are any about. All of these are OK with the Mak's smaller FOV (and I use a tracking mount, so no need for frequent manual adjustments). Actually the Skymax design is pushed a bit, so that the focal length is a bit shorter than most - F12-F13 depending on how it's set up. I've found that to be long enough to give me decent magnifications for smaller objects without trying too hard.  Yes, the larger central obstruction will reduce contrast. I also upgraded the back to 2" to increase the FOV (accepting the vignetting that causes).

I have the 127mm size, which just about cools fast enough for an impromptu session, and was the heaviest that my inexpensive mount would take. My tripod isn't the sturdiest, but the short physical length of the Mak helps stability and it settles in half the time of a Newtonian of similar weight.

So yes, compromises, but I think it's a good complement to a reflector or refractor with a wider field.

 

[Andrew_B]

Insulating the tube apparently goes a long way to solving the thermal issues with Cats.

As to their reputation, I think it's suffered in part because high-end models (Intes Micro, A-P, TEC) have been very rare instruments that few will have had a chance to use while the opinions of most users have been  informed by their experiences with mass-market SCTs. Although modern ones from Celestron in particular seem to be pretty good, the ones being built in the 70s/80s/90s that cemented the reputation of the SCT were a lot more hit and miss due to poorer quality control that let far too many lemons slip out of the factory.

[Pixies]

I think the Maks and SCTs  are excellent 'real-world' scopes. They have compromises (as do all) but try to combine the best parts of refractors and reflectors, albeit with a long FL.

An excellent scope for many - but a 'dream' scope? Maks and SCTs won't have the contrast and optics of a high-end refractor, nor the comparable simple light-bucket aperture of a big dob. Now, there are more esoteric cats, with much more complicated optics, but I know very little about them. I've had a Mak and used an SCT plenty of times, though - and the above comments are regarding visual only.

[Nik271]

I have two Maks: the  127 and 180 Skymax. 

I agree with all views expressed above both pro and against. Catadioptics are a compromise of various features. As a happy Mak owner I'm of course somewhat biased and here are some more pro points:

Maks are very cost effective, in cost/aperture they deliver lots for your money (but note that simple Newtonians deliver more, and the new StellaLyra CCs from FLO are comparable)

One big advantage for me is the total lack of the need to collimate, the secondary is just painted on the front corrector and will never move. In principle you can collimate them by moving the primary but I never had to do it and never heard of a Mak that has arrived out of collimation.

For me the sealed tube is a plus not a negative, sure it needs more times to cool and a dew shield but the mirror surfaces stay dust free for a long long time. If you are careful you'll probably never need to clean them until the coating fades (which is does eventually with all mirror scopes).

For this reason the Maks up to size 127mm make excellent travel scopes. More than 127mm they are specialized for small objects (planetaries, globulars, double starts and of course planets)

I say if your sky is dark go for a  big mirror scope. If you have significant light pollution then a catadioptric and/or a refractor is a maybe a better choice. 

[Captain Scarlet]

I have 3 Maks, a skymax 180, a skymax 150 and an Intes M603 (150mm).

Whilst I agree with @Nik271 that once collimated, the skymax line are unlikely to need collimating again, if one does need collimating, the mechanism skywatcher provide for these scopes is crude and not especially easy: 3 pairs of “push-pull” threaded bolts at the back for the primary only.

Other Maks, such as my Intes (which is a Rumak-Mak) can be much more difficult owing to adjustable primary AND secondary. In certain circumstances they can be impossible to collimate, where for instance the primary has been seated tilted relative to the focuser, which might also be the case in a bad example of an SCT or CC with unadjustable fixed primary mirrors.

If that focus-tube-vs-primary-axis tilt is not adjustable, which most small ones are not as I understand it, and it's out, you’re in trouble from the start. With mine, that was indeed “out” when I got it, there was no native adjustment mechanism, so I had to shim the primary on its seat around the baffle-tube before even starting on the 6 collimation-adjustments proper.

So collimation can be a problem with Cats, and can get horrible sometimes. A situation you’ll rarely get with refractors.

Cheers, Magnus

 

Edited December 16, 2021 by Captain Magenta

[Alien 13]

6 minutes ago, Captain Magenta said:

So collimation can be a problem with Cats, and can get horrible sometimes. A situation you’ll rarely get with refractors.

Cheers, Magnus

 

I am not so sure an ED triplet could even be colimated without specialized equipment and an optical bench and of course a delivery driver to send it back to the supplier 😃

Alan

[johninderby]

The CC does  have primary mirror adjustment. You can also fit the optional tilt adjuster for the focuser  if wanted. 

Edited December 16, 2021 by johninderby

[Louis D]

12 hours ago, Alien 13 said:

There are compromises with a MAK but I dont see the narrow field of view one of them

It is if you want an all around scope that can go from wide fields to high power.  A fast/short f-ratio reflector with an extremely well figured primary and coma corrector can do both.  However, it will be bulkier than an equivalent aperture CAT unless a minimal truss design is employed.  It will still require some extension/assembly before use, so the CAT is more user friendly in that regard.  Again, compromises.  No one scope is good at everything.