Cadioptrics?

[Alien 13]

22 minutes ago, Louis D said:

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.

I agree its all a compromise or as I would prefer a design choice, I agree fast scopes can do high magnification and wide field but the cost of those eyepieces to achieve this in a fast scope is eye watering for me at least.

Alan

Edited December 16, 2021 by Alien 13

[Louis D]

I would like to see someone commercialize Mel Bartel's ZipDob design.  A 13.2" f/3 Dob with a ~1000mm focal length that collapses into a sub-25 pound box via a few folds sounds great.  Of course, eyepieces that perform well at f/3 tend to be expensive.

[dan_adi]

22 hours ago, cajen2 said:

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!😄

Japanese triplet refractors are in a different quality "league". Better build quality, better optics, wonderful astrographs all around. If you plan on imaging, quality refractors are the way to go, in my opinion. For visual observations only, you need as much aperture as you can get, so  a big Dob/newt is the usual choice, preferably with quality optics. There are big refractors that can match the performance of bigger mirrored scopes, up to a point at least, mostly because, again, high build quality. The downside is price ...

Where does the SCT fit in? Well, besides the 'grab and go' attribute, I failed to see their appeal. I have used a 12 inch SCT in the past, and it was kind of a disappointment visually and specially in imaging.  Visually I was more impressed with a newt/dob, until I got a 8 inch refractor :). 

So, after using the newt design, SCT design and the refractor design, I prefer my 8 inch apo overall. It doesn't have enormous aperture, but for my seeing, 8 inches is enough for imaging. Visually it doesn't disappoint either, but it is not a light bucket, that's for sure.

The questions you are having are normal for a newbie, I was in the same conundrum few years back, maybe most of us have been there at some point. My best advice is, use what you have, enjoy it, try to figure out what you like (visual?, imaging?, planets? DSO?) and after you know what you want, build your setup with the highest quality you can afford.  

PS: There is no scope that can do it all. Most long term asto fans, end up using both refractors and reflectors, for different targets.

Clear skies!

[cajen2]

Cheers, Dan, though I think you've slightly missed the point of my question.

I'm not looking to buy a cadioptric, I was just reading new scope listings (doesn't everyone?😄) and wondering why they didn't seem objects of lust to the assembled great minds of SGL. I wanted to be educated as to their advantages and pitfalls.

I'm very happy with my Flextube dob, but like a newlywed husband, it doesn't stop me looking....🤭

 

[dan_adi]

8 minutes ago, cajen2 said:

Cheers, Dan, though I think you've slightly missed the point of my question.

I'm not looking to buy a cadioptric, I was just reading new scope listings (doesn't everyone?😄) and wondering why they didn't seem objects of lust to the assembled great minds of SGL. I wanted to be educated as to their advantages and pitfalls.

I'm very happy with my Flextube dob, but like a newlywed husband, it doesn't stop me looking....🤭

 

I can speak from an imaging perspective, in my experience, with my former SCT:

• focusing by moving the primary mirror is a  disadvantage, one needs to buy a separate external focuser
• thermal management issues
• slight mirror flop, not all models have mirror locks
• no options with regard to higher quality optics
• average build quality overall, they are made to a price point 
• longish focal length, demands good quality mount 
• advertised as flat field/coma free, but that is not always 100% correction
• more prone to temperature induced focus changes than a refractor
• usually in f 8 or f 10 variant, they are "slow"
• no optical test report if you really want to know what you are getting for the money

Maybe I am missing something :)) but those are important disadvantages for imaging. If I recall correctly, there was a presentation about planetary imaging by D.Peach I saw some time ago, and I remember him saying something about having to check/adjust his collimation on the SCT every half hour or so ... that is a big no no in my book. I mean I applaud him for the dedication and the incredible images, but I don't see myself doing that all night long. Maybe that is all he had to work with, and that is OK!, but there are other options, although more expensive ones ... 

This is just my experience, not trying to bash or upset the SCT fans. I tried it for some time and I found myself wasting clear nights, adjusting, tinkering, doing all this stuff, and getting no or very few images in return. It was a learning experience after all, and since then I have moved away to another design that I am more comfortable with.

[Alien 13]

I think the new breed of CATS like the RA, RC, RASA, CC etc will take over the world especially for imaging with the newer CMOS cameras and there smaller pixel sizes, A RASA, CMOS colour camera with dual or tri band filter is a match made in heaven.

Alan

Edited December 17, 2021 by Alien 13

[ollypenrice]

7 hours ago, cajen2 said:

Cheers, Dan, though I think you've slightly missed the point of my question.

I'm not looking to buy a cadioptric, I was just reading new scope listings (doesn't everyone?😄) and wondering why they didn't seem objects of lust to the assembled great minds of SGL. I wanted to be educated as to their advantages and pitfalls.

I'm very happy with my Flextube dob, but like a newlywed husband, it doesn't stop me looking....🤭

 

If your question is partly about the reaction of amateur astronomers to SCTs and Maks, I'd make these observations:

- Cats used to be relatively inexpensive because other designs, in the past, were much more expensive. They have spherical mirrors which are easy to make. They were also given a boost by well-funded advertizing from Meade and Celestron. Then fast Newtonians and good refractors became vastly less expensive when the Chinese started making decent ones. This leveling of the price playing field opened up more choices to us all.

- The rise of digital deep sky imaging favours other designs. Large apertures are no longer needed and long focal lengths are often a limitation. As pixels get smaller, focal lengths can and will follow. Also, even those not interested in deep sky imaging will have an eye on the potential re-sale values of their purchases and notice that the imaging-unfriendly SCTs and Maks take very big hits on the used market.

Olly

[johninderby]

The CC should do very well for lunar and planetary imaging due to it’s excellent resolution and contrast.for the aperture but not suited to DSOs due to it’s focal length and narrow fov.

Bit it is nice to see  a competatively priced alternative to the mak and SCT. The more choice the better.

Edited December 17, 2021 by johninderby

[Louis D]

On 15/12/2021 at 18:44, 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 type focuser.

Perhaps you can explain why the 8" CC at 18 lbs is about the same weight as the 180 Mak at 19 lbs when the latter has both a thick meniscus corrector and primary mirror moving hardware for focusing of which the former has neither?  I can't imaging moving from 50 sq. in. of mirror to 64 sq. in. of mirror adds as much weight as a 180mm+ meniscus corrector and mirror moving hardware.  Is the CC focuser incredibly heavy?  Are those tube baffles incredibly heavy?  Is the secondary support incredibly heavy?  Do all the differences simply add up to the same weight?  It just seems like the CC should be sooo much lighter, not just a single pound lighter, than the Mak.

[Nik271]

I don't have the CC but just looking at the photos the two full length dovetails bars will add some extra weight. The Losmandy one is quite massive. The Mak has just a single Vixen bar. Also the CC has a 2 inch focuser which the Mak does not have. I think these add up in the end.

The lightest in the 8 inch class is the Celestron SCT, just 6 kilos I believe.

[johninderby]

The CC is very solidly built with heavier gauge materials than the mak. The primary mirror looks thicker as well. Also the top and bottom dovetails are pretty heavy being thick CNC made ones and the Losmandy plate is a hefty chunk of metal on it’s own. The build quality of the CC is what most first notice about the CC.

[Stephenstargazer]

Reverting to @cajen2 original enquiry I would be very happy to list a Mewlon for my large aperture but practical visual scope of choice. Beyond 150mm refractors get to be quite a handful and a catadioptric or cassegrain with well designed optics and mechanics could be a joy. Others might go the Dob route, great value but less portable.

Edited December 19, 2021 by Stephenstargazer

[Louis D]

8 hours ago, Stephenstargazer said:

Others might go the Dob route, great value but less portable.

I don't know about that.  I'd rather transport an 18" Obsession Ultra Compact any day over a 16" Meade SCT:

That's 90 pounds vs 318 pounds, not to mention how compact the Dob stores.  If you need tracking, there are lightweight EQ platforms for the Dob available.

[Stephenstargazer]

Point taken Louis ! there is an upper size at which only a truss Dobsonian remains 'practical' or transportable. There were quite a few represented in the 'dream scopes' thread, and that was mentioned by @cajen2. I believe there was also at at least one truss cassegrain chosen.

PS My first scope was a Rumak Maksutov and I still have one. Nice to look through, compact to mount , very long f/r can be a blessing and a limitation.

Edited December 24, 2021 by Stephenstargazer

[catburglar]

I love cats…  sealed tubes, no diffraction spikes, lots of back focus, more compact (although not necessarily lighter) than other designs of the same aperture make them easier to mount. Because of their compact nature the eyepiece position changes relatively little irrespective of where you point it, so it’s easier to get comfortable whilst observing.

I’ve only used mass produced models by skywatcher, Meade and celestron  but they’ve all produced good views…

I’d like to try a premium model at some point, but the limits of what I can observe are almost certainly set by my eyes, so it’s probably a bit wasteful…

 

[Stardaze]

On 16/12/2021 at 01:00, johninderby said:

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

I never tire of that picture John.... drool 😊

[SuburbanMak]

I am just coming up on a year with a Skywatcher Skymax 127 Mak which the family clubbed together to buy for my, ahem, "significant" birthday last year. 

In the research I did leading up to that decision the Mak very definitely became my "dream scope" - where I live is plagued by civic and security lighting so I really needed a compromise between budget, decent optics, enough aperture to see a wide range of objects and portability.   I can't speak for the larger Maks but the 127 cools in 15-20 minutes, mine lives in a backpack in a particularly draughty corner of the house so is permanently part-cooled.   

I've had a great first year as a telescopic observer with mine & been able to track down 75/110 Messier objects, develop a passion for double stars, watch shadow transits on Jupiter and been blown away by the views of Saturn and the incredible detail on the moon.  On a couple of occasions in the spring I got out to a good dark site (that portability again) and the effect was transformational on the Mak's performance on DSOs revealing for example a degree of form in the dim M61 and M99 galaxies. The brighter globular clusters, planetary nebulae and all but the biggest open clusters give really rewarding views.  

The FoV is just over a degree and initially I struggled a bit with finding objects but ditching the supplied RDF and replacing with both a Telrad and a 9X50 RACI finder plus, for the more obscure targets, rehearsing star-hops in Stellarium largely solved this problem. 

Along the way I've picked up an ST80 which I've converted to 2inch focuser for really wide views and developed a serious eBay long focus refractor habit, dream scopes for me are to upgrade the ST80 to a Tak 76 DCU and to get something "big" for inter-galactic travel (weighing up pros and cons of a C8/9.25, CC8 or just a basic 10inch Dob for the few occasions when I get the combination of dark, clear moonless nights and the time flexibility to schlep out to a dark site, realistically based on this year that's only going to get a handful of outings each year).   

But the 127 Mak is certainly delivering on the telescopic dream I had as a boyhood observer back in the 80s and is cemented in my arsenal as the 4-5inch choice.

 

 

 

[Peter_D]

It's often said that an 8 inch SCT hits a sweet spot: you have a relatively cheap (esp if bought used), light OTA that can be mounted even on a EQ5 equivalent mount.

More generally SCTs are compact so a short movement arm. Easy to store and the smaller ones are easy to move around. As mentioned before, a external focuser can be fitted to negate focus shift.  You can also get a 0.63 reducer that increases (and flattens) the fov somewhat and speeds up astrophotography. More advanced optics called Fastar can be added to make the system an F2 instrument.

For Planetary imaging, apperature is key. The comparatively large central obstruction  of an SCT will give a slightly less bright and contrasty image than a refractor or Mak but the larger apperature of a C11 or C14 should allow more resolution in the planetary image. Of course, this is not always the case because of seeing.

Last point: lower cost eyepieces like Plössls work well in Cats.

Edited December 22, 2021 by Peter_D