SCT vs DOB on planetary viewing??

[robz]

Hi everyone. Would like to throw a question out there for you guys on planetary viewing and telescope suitability.

I own an 12 inch LX200 ACF SCT which is well collimated and has good optics from what I have seen. I have observed Jupiter and Saturn on many occasions. 380X seems to be the sweet spot here in Perth where seeing is good enough to crank up the power to get in close on your object.

Saturn has always produced a real wow factor without any trouble.
Jupiter, however, has been either just o.k. or disappointing....... even after sufficient scope cool down time and average to good seeing and transparency.
For some reason, I have yet to see the small white ovals and other fine details that people report on the forums.:shrug:

My question is................should I be considering a 20 inch GSO dob(coming soon) or even a 16 inch as a better option to obtain higher resolution through larger aperature/smaller central obstruction?

How do the 16 inch Skywatcher and lightbridge perform at higher power on planets?.............or are the optics only good for DSO's.

Would love to hear your thoughts:)

[AstroJunk]

I would dare to say that in my experience, seeing plays the greatest part in resolution. I have been fortunate in enjoying a view of Jupiter nearly as lovely as the images posted here, but that was once in 5 years from my site! Saturn always looks great because there are few details to blur...

There is plenty of talk of why a refractor is better than a newt, and why a newt is better than an SCT, all of which has good evidence in theory to back it up. But nothing less than perfect sky will allow you to resolve the details you want!

(But ultimately, I recon that a large Newtonian makes the best planetary scope visually)

[Allan]

As Jonathon says, medium/large newtonians hit the sweet spot for resolving planetary detail. But, only if they have high quality optics on board. Merely upgrading to a larger aperture will achieve nothing or worse, if the optics are poor. There is a lot of information out there that indicates the quality of the mass produced mirrors deteriorates as the aperture increases.

If deep sky is your passion then the 20"GSO would be worth a look when they come out. But for lunar and planetary, the best setup is a premium mirror in a Dob platform that tracks for easy high power observing.

[robz]

Yes, gents I realize that seeing is the key to a blissful night of high resolution viewing. However those nights are rare in the case of Jupiter in particular for some reason.

I agree that a large dob will do nothing if the optics are below par. At present it seems to be a lottery at best and this is at the risk of the buyer.

I guess that that one night in 5 years is quite possibly the ratio to finally seeing what you hope to see on that massive gas giant. :(

I have owned a large newtonian during the Hayleys comet of 86 (?) mania and it did indeed show great DSO's, but for planetary it was a total abomination :mad2: !!!!

It seems a shame that going up in aperture withe the various brands available at present is o.k. for the faint fuzzies, but not a guarantee for good planetary viewing also :rolleyes:

[Camelopardalis]

You'll see more in almost any scope with good seeing, but since we can't buy that we're left debating your scope is better than mine :lol:

Jupiter has low altitude this time around which - from Sydney at least - doesn't help the viewing :sadeyes:

But if you want a quality scope be prepared to pay for it no matter what :D

There's anecdotal evidence suggesting some of the mainstream manufacturers, at least Synta brands (Skywatcher, Celestron), are upping their game a bit in recent years and putting out scopes with more consistency, but the jury is still out whether they're consistently better :lol:

Each design and it's merits and weaknesses, but you get what you pay for regardless of design, and probably only by winning the lottery will most of us get a chance to compare and contrast all the possibilities here...or see what selection shows up at IISAC ;)

I've seen arguments for and against central obstructions, how aperture affects resolution, contrast, etc... in the end the important thing is to enjoy our time out under the stars and planets.

[pdalek]

The best Jupiter I have seen was with a 16" f/16 classical Cassegrain.
Also could get really bad images with the same scope.

[glend]

In reading the article "Get the Right Telescope" by Rod Mollise in April issue of Aus Sky and Telescope (page 44), I ran across some comparative statements related to newts (ie Dobs) and SCTs:

SCTs: Fairly high cost per inch of aperture and longest cool down time of any design. In their favour most are sold on motorised mounts

Newt (dob): Lowest cost per inch of aperture,and obviously Dobs don't require mounts like the SCTs do.

The article is worth a read if your in the market.

[Shiraz]

stick with the SCT if it works on other objects. Jupiter is so low currently that you will probably only see a disk and a couple of bands through the atmospheric turbulence. The people reporting good viewing on other forums are from the northern hemisphere?

[firstlight]

Hi Robert, it's funny to hear what you said about Saturn looking stunning and the other planets not so hot. Jeff Ryder, former curator of the Sir Thomas Brisbane Planetarium (AKA the Skydome) commented that Saturn seems to be just the right apparent size in a scope that seems to almost always have great results.

BTW, a refractor will give you the best planetary views.

[John K]

The point about Saturn looking better then Jove is a good one - I think even showing someone Saturn with an 80mm refractor who has not seen Saturn before and seeing the rings etc always creates an impression.

Jupiter will always be a tougher object - have you tried using some colour filters? Also remember, that for several years to come, Jove will be low for us and Saturn will be high.

My view is that you will nearly always get sharper views in a Newtonian vs an SCT. I don't however know about the optical quality of your SCT.

The issue you will also have with a Dob especially something as large as a 16" or 20" is tracking. At high magnifications, no matter how great the view, it will get annoying after a while trying to track at high magnifications - gather you have observed through large Dob's? Perhaps an equatorial platform is the way to also go for you.

Down the track are you also interested in doing some planetary astrophotography?

So to answer your question, optically speaking, a well made Newtonian mirror should give you way better views in my mind vs an SCT. But Jove will be a tougher object as it will be low down.

My money would go towards a slightly smaller Newtonian (say 12" to 14") with maybe a new EQ8 type mount, or if you want to go Dobsonian, then build in the cost of an equatorial platform.

But, if you simply want more light gathering power to also do deep sky stuff that doubles up as a better visual scope for planets, then a 16" to 20" Dob with a well made mirror and good tracking will satisfy you - not sure about the optical quality of some of the Chinese mirrors - also consider that cooling ect will come into play more and more with an increase in aperture - many years ago when I had my 10" Dob with a well made Parks mirror, I remember being at the ASV's Parer St observatory in Melbourne and comparing the view with the society's 20" fork mounted reflector which had been heating up all day in the observatory - the 10" won hands down! It was a sharp reminder on the other things that come into play with larger telescopes - more complexities to deal with - so larger is not necessarily better.

Good luck with your decision.

John K.

[sn1987a]

I get great views of the planets in my dobs when the seeing is good even in my suburban backyard. The bigger the Dob the better the view. I'd be interested to compare with a top shelf refractor side by side they must be awesome to beat a good reflector optic on a good night.

[icytailmark]

SCT's are the closest to a perfect scope. They are compact,high focal length(great for planets),and hold collimation very well. The only bad thing is they are expensive. You get what you pay for!!!

[Allan]

These discussions are always interesting. In just a few posts we have Newtonians, Refractors and SCT's all claimed as the best planetary scopes.

Any comparison is like comparing apples to oranges unless the telescopes have top quality optics. A 4" APO sure will show more detail than a large Dob if the Dob mirror is a poor one. Spend a couple of thousand dollars on a premium Zambuto mirror and the comparison ends. An 8" or 10" APO would level the playing field again, but at 10 times the cost though.

[AstroJunk]

This is the internet, surely you wouldn't expect anyone to agree with one-another would you? :rofl:

[Allan]

Cloudy Nights separates the Reflector, Refractor and SCT's forums so everyone can play together nicely. A little debate always makes life more interesting though in my opinion.

[Gem]

As an owner of a 9.25" SCT and 16" dob, I would say I prefer the dob. However, the difference varies. You see I can put the cooling fan on my dob several hours in advance. My SCT is a standard Celestron, not an EdgeHD that has quicker cooling. So, if I am patient and use my SCT around midnight - the results are quite good.
My SCT is more expensive than my dob, and in terms of bang for buck - the dob wins.
To those who prefer refractors - yes I agree they are wonderful. But comparing my 16" dob to a similarly priced 110mm refractor, and I still prefer my dob!
That's my opinion anyway. :)

[Gem]

It is also worth remembering that half of the optics in a dob, and nearly half in the others, is your eyepiece. A very good EP goes a long way to maximising the potential of any scope.

[BlackWidow]

I own an SCT and love it! I have also owned a Dob in the past an liked that also. Sky condition make a large diff, but I have also found it depends on whatt Eye I use. I seem to get better results out of my left eye than my right eye.

I am not sure if I have two different brands of eye, or if one is from my Father and the other from my Mother.. Dad has bad eyes so it might be his fault.

So if you are going to compare make sure you have good optics attached to your scope and always keep and eye out..... Well at least the bad eye.

So many variables :screwy:

[robz]

Thanks to all for your thoughts and input.
I agree, and realise that yes................Jove is very low compared to previous years so that may be a huge factor towards bad seeing(which is obviously the case here).
Secondly, I use an 8mm 72 degree Skywatcher E.P.(2 inch barrel) which has noticeable coma anywhere past the centre of the image. As the scope is an ACF, I know it's not the scope.:D
I haven't tried any other E.P.'s because I don't have any..... apart from some cheap super Plossl's that came with my other 6 inch achromat refractor.

Anyhow,for the moment, I will take a peep now and then at Jupiter, but I don't expect that magic night of great seeing this year around ? :rolleyes:

[Camelopardalis]

Just a suggestion Robert...how about trying a longer magnification eyepiece? 380x might be a sweet spot on a good night, but on a not so good night I feel it's really high.

I've had many of my best planetary views at under 200x...to me, it's not the size that matters, it's the level of detail and crispness, vividness of colours, etc that are more important to me.

I'd also suggest something more in the direction of a premium eyepiece - some will disagree and say that slow scopes are more tolerant of poor eyepieces - but IMO _every_ scope benefits from better eyepieces.

Oh and Jupiter seems to have bit its low point and will climb by about 10 degrees in altitude for the next few oppositions :)

[ausastronomer]

That depends on how discerning a visual observer you are. If you want one scope for both visual and imaging they are a great choice and probably the best choice.

If you are a specialist visual observer their optical performance level is below that of a premium grade Newtonian IMO. As I indicated in another recent thread physics supports my experience having looked through in excess of 100 different SCT's over the past 30 years from both Meade and Celestron ranging in aperture from 5" to 16". I am yet to look through one that can equal a top grade Newtonian of equal aperture as a visual planetary telescope. They do a very good job of planetary imaging because unlike for visual use the larger central obstruction and increased number of reflective and refractive surfaces does not affect their performance and their long focal length becomes an advantage due to the larger image scale.

I would happily put my money and my 14"/F4.5 Zambuto powered SDM up against any SCT, currently in Australia, in a visual shootout on the Moon or planets. In fact I would happily put my money on Rick Petrie's 14" Skywatcher against any SCT that anyone cares to throw in to the mix. Rick's 14" Skywatcher has a better mirror in it than any mass produced budget telescope has a right to have, but it is what it is. The downside of mass produced scopes, of course, is that you take the good with the bad and not everyone gets a mirror as good as Rick's.


Cheers,
John B

[Camelopardalis]

I'd love a look through your scope, John, should you wish to let me if our paths ever crossed, seriously curious to see what I'm missing, but then there's the inevitable wanting for a cherry-picked, unique, or at least very expensive scope :sadeyes:

The crux of the issue is summed up by your last sentence...for the rest of us, unless we dig deeply we're all at the mercy of whatever Skywatchers, Meade, GSO or Celestron churns out. So unless we win the lottery, it's a lottery ;)

[AG Hybrid]

To be fair. SKywatcher/synta mirrors consistently perform well. You rarely hear a story about anyone who has bought one recently that has optics that needs to be refigured. Meade/GSO mirrors on the other hand. You hear stores of getting truly exceptional optics and sometimes complete dogs.

If I had to choose between either Synta or GSO. I'd choose Synta every time.

That being said. I thoroughly enjoyed the views I saw in Dunk's C11HD of Jupiter. The color contrast on the bands of Jupiter were excellent. The upside of the Celestron HD series scopes is they are made in the US and quality control is better. The standard models are built in Taiwan/China

[MortonH]

It also depends where you do most of your observing from. I'm stuck on a small balcony most of the time, so an 8" SCT would fit, but even my 8" f/5 Newtonian is too big. A 14" Dob is definitely out of the question! :lol:

[Shano592]

Robert, if you get the chance, try to borrow a Televue, or Pentax, or other high-end eyepiece off of someone nearby.

As has been mentioned previously in this thread, the telescope is only half of the optics. Generally, the glass in the higher-end eyepieces is magnificent, and can really make your night.

I have the 10" LX200, and view Jupiter with a 10mm Ethos. Aside from being visually very bright, and quite hazy due to the low altitude, Jupiter generally resolves quite nicely... although a moon filter or some other dimming filter of the like wouldn't go astray.

Would a Barlow help to dim the view? (A genuine question from me, as I haven't used one.) I know they extend the focal ratio, and in my head, that makes the image "slower," therefore dimmer.

[Gem]

Yes it would. Its like halving your EP focal length (if a X2 barlow). But you only want to turn up the magnification if the seeing conditions are good enough. My Televue 4.8mm Nagler rarely gets used since the conditions never suit that magnification.

[Amaranthus]

The dimness is nothing to do with slow/fast f/ratio, visually. It is simply the the Barlow will yield a longer FL/magnification.

[bigjoe]

Hi Rob.

I find that No.1 Altitude and seeing

Then. No.2.Central obstruction size

Then. No.3 optical quality of eps and scope and scope type etc.

All DO MATTER greatly esp on Jupiter if it is low in the sky, as its features are low in contrast. A good dob should beat an Sct.
PS: use a light/ mid blue filter. It DOES make a difference.
Maybe this is why my 7" bd MAKSUTOV. slays my 10"SCT ,6"SCT, 80MM REFR
ETC on Jupiter, in fact all the planets and some Dsos.
Cheers bigjoe.:)

[bratislav]

Disagree on all accounts. Visually SCTs suffer from huge central obstruction. There is no way around that - you can have a perfect SCT optically (and they are nowhere near that) but damage to contrast transfer has already been done. Not really that objectionable on Moon or Saturn (rings), but any ultra low contrast feature (all of Jupiter, detail in Saturn belts and fine features of Mars) will be hard hit.
Imaging wise, SCTs are hard work because they are dreadful in keeping collimation (I have to check and usually recollimate every time I change elevation more than about 20 degrees with my C11); but main issue is enclosed primary which lags in temperature and causes internal heat plumes. Short of actively tackling that via pelitiers and internal fans, you are at the mercy of local temperature gradient. Quite often SCT users will report "bad seeing" while Newtonian and Cassegrain counterparts will work close to their maximum resolution - same place, same time.

The only real advantage of SCT is comfortable observing. Using binoviewer (compulsory for people with floaters like yours truly) and having comfy chair is a real game changer. I know you can use bino in a Newtonian, but they often end up in all sorts of neckbreaking angles and it is impossible to seat back and relax. Planetary observing is a waiting game, being comfortable is a definite advantage.

Bratislav

[Gem]

While some of this is subjective - I find it hard to see how you think a dob is better than a SCT for imaging. Unless you are comparing a GEM Newt (rather than a dob) to a GEM SCT, the SCT will always be a better imager. If you are comparing GEM scopes, then the newt's size and weight means you will need a massive mount to have anything over 10". A similar aperture of SCT will be lighter and easier to get good tracking than a newt on the same mount.

I take you're point on everything you say about optics. I always choose my dob over my SCT for visual observing (partly since it is choosing 16" over 9.25") - but my SCT will always be the best compromise scope for size/portability/ability to image/visual observe. Jack of all trades, but master of none. :)

[Don Pensack]

An SCT is hampered in planetary observing by 3 major factors:
1) a large secondary obstruction. This will prevent seeing the smallest details because of insufficient contrast. I would want to note, however, I'd rather have an SCT in superb seeing than a dobsonian of the same size in mediocre seeing, if planetary observing is the aim.
2) Cooling. Simply put, the night is not long enough for a 12" SCT to achieve thermal equilibrium. Some form of active cooling is essential (Lymax SCT Cooler, which can be used in an ACF, but not in an Edge HD; or a home-made equivalent) or the mirror will never cool down and allow resolution the aperture is capable of.
3) contrast robbing scattered light. Now, a dob can have this too, but it is easier for extraneous light to get into the scope and bounce around in the typical SCT design. What can be done? Flock the inside of the dewshield (and always use one), the inside of the tube, the outside of the primary baffle, the inside of the secondary baffle, and the inside of the primary baffle. Make sure the inside of the star diagonal is really non-reflective. I did all this to an 8" SCT and the difference was profound. It yielded superior contrast at all powers and allowed a couple tenths fainter stars to be seen on a regular basis at high powers.

Dobs have smaller secondaries and typically have fans to cool the optics. Other than that, there is no particular reason an SCT shouldn't give good planetary images so long as the optics are good. It may be easier to get 2 excellent surfaces in a dob than 5 excellent optical surfaces in an SCT. I note that optical testers find excellent optics more frequently (though not a high percentage of the time) in dobs than in SCTs. It's hard to tell about SCTs, though. 95% of the ones I've looked through aren't anywhere near cooled down.

[bratislav]

One word - equatorial platform.
Sorry that is actually two words :lol:

[Amaranthus]

Wow, I find that staggering. What criteria are you using to judge collimation accuracy? I personally have never needed to collimate my SCT -- it holds it beautifully -- let alone feeling the need to do it every time one raises the OTA's altitude axis!

[bratislav]

I normally use high power extrafocal images (500x or more).
In this case I actually use CCD chip (QHY5L II) at focal length of over 7m.
If we think of that chip as a shortish eyepiece (chip is only 5mm across; a typical 5mm eyepiece would have itself ~5mm field stop) we are talking about 1500 times effectively to check collimation when imaging planets. And believe me it is necessary if you want a last ounce of resolution SCT can offer.

Try it yourself. I haven't really seen through too many SCTs, but of couple of dozens or so that I did, they ALL had the same problem.

If you don't believe me, check what Thierry Legault has to say about SCTs and collimation.

"People who think that a SCT does not need to be collimated very often probably do not realize the level of precision required for this type of instrument. The constraints are so high that a simple car trip always changes the alignment in small amounts, and sometimes in large amounts. Collimation can even vary according to the orientation of the optical tube (with a German mount, an interesting experiment is to point at the same star at the meridian successively from the left and the right of the mount, to observe the modification of collimation due to the reversal of the tube). This is the reason why it is advised to choose a star in the same area as the object of interest (planet or Moon). If a small misalignment can be tolerated in deep-sky observation, it is taking a big risk not checking the collimation before a planetary session. The idea is that the alignment becomes as familiar as checking the oil level or the pressure of the tires in a car before a trip !"

http://legault.perso.sfr.fr/collim.html

[ausastronomer]

Bratislav,

I don't think you actually took the time to correctly read and comprehend what I was implying. At no time did I imply that a SCT was the best at anything, either visual or imaging. Truth is I wouldn't own a SCT if someone gave it to me.



Isn't that exactly what I said in my post above, excepting that I expanded on it somewhat ?



If you read some of the posts I made in this recent thread (http://www.iceinspace.com.au/forum/showthread.php?t=117484)you will see exactly what I think of SCT's as visual instruments. Here are some of my posts replicated below





Here is the table I posted in the other thread.

Now, if I don't really like SCT's, particularly as a visual instrument, why would I make the comment "If you want one scope for both visual and imaging they are a great choice and probably the best choice."

They are "reasonably" portable, transportable and storable. I live on an acre of land with a 3 car garage as a storage shed. I have no problems storing 18", 14" and 10" dobs. If I lived in a 2 bedroom unit in Sydney I doubt that would be the case. SCT's do a "reasonable" job of both planetary and deep sky visual astronomy and planetary and deep sky imaging. While not the best at anything they can do a reasonable job on all fronts. Because of their short tube length and thus moment arm they can be used for imaging on a medium sized EQ mount. Dobsonians make easily the best visual instruments IMO, for both planetary and deep sky, but they are a poor choice for imaging. IMO an EQ table doesn't help. Just ask Paul Haese who spent about 20K on an 18" SDM dob for planetary imaging and sold it 6 months later and bought a SCT. Refractors give aesthetically nice images but because of their limited aperture they cannot match a medium / large aperture high quality Newtonian as a visual instrument. Ant that is how I came to make the comment I did, nothwithstanding that I would never own a SCT.

Cheers,
John B

[AG Hybrid]

Interesting read John. I also agree with a 10" goto dob over an 8" sct for value and performance.

On a side note:

Isn't it your birthday? Why are you trippin about SCT's on IIS instead of chilling out and enjoying some cold ones?

[ausastronomer]

Hi Adrian,

Because at the moment I am actually at work pouring all the cold ones for my patrons to enjoy :)

I made a decision yesterday afternoon, when the weather forecast was good, to go fishing this morning and work tonight instead of working during the day today.

Cheers,
John B

[Amaranthus]

Thanks Bratislav, I'd read that article by Legault quite a while ago so it was good to revisit it. But I think we need to be clear here about expectations! I use my SCT as a flexible visual workhorse, from DSOs through to planetary. For me, high magnification is 350x (and I rarely go above this) and I typically work in the 80-140 magnification range. So my absolute tolerance for slight miscollimation is moderate, and I rarely see my 8SE requiring an stage-3 'in-focus' collimation. I check this maybe once per month and may need to do slight adjustments a few times a year. I call that robust!

It is, like anything, a matter of trade-offs, and I appreciate that if you are willing to spend a LOT of time CONSTANTLY collimating, then you might just squeeze a little extra resolution out of your SCT. Or, you can take the attitude that 'near enough is good enough' and keep collimation at a reasonable 'fit' (based on diminishing returns), and so have more time for observing rather than fiddling. Or, buy a larger aperture :P

I guess I follow Rob Mollise's philosophy on this, rather that Legault's perfectionism: http://skywatch.brainiac.com/collimation.pdf ... But each to their own.

[ausastronomer]

Hi Dunk,

This is the scope I use almost exclusively for the outreach work I do with 3RF. It's ideal for this as the eyepiece height is perfect for all adults and kids only need a single step.

I have always made this scope available for anyone to look through.

Last year I had it at a new outreach event called "Stars on The Fly" which was an event that I put together with the management of the Illawarra Fly Treetop Walk. Sadly, their Marketing Manager, who I coordinated the event with left at Xmas; and the event will not run this year. I have also had it up at Lostock (IISAC 2010) and Border Stargaze (2009) and a big public event with about 500 people at Armidale in 2010. I will probably take it to the Parramatta Park Public event which will be held in September or October this year.

I am hoping to be able to attend the Pony Club (Mangrove Mountain) observing session for the new moon in May. I will be taking the scope to that event if I can attend. If I attend you are more than welcome to come and have a look through it.

Word of fair Warning :)

A couple of people have subsequently ordered an SDM telescope after looking through this scope. One US visitor to our annual 3RF Coonabarabran trip ordered an SDM telescope about 15 minutes after first using my scope. He could have ordered a scope from any one of a number of US manufacturers when he returned home, but he wanted one " just like that only bigger".

Cheers,
John B

[bratislav]

Is this your quote then ?

"If you want one scope for both visual and imaging they are a great choice and probably the best choice." (my italics)

That was the only statement I argued with. SCTs are not 'great' at visual, nor imaging, nor they are the best choice.
They are convenient (for both application), but that is all.

"IMO an EQ table doesn't help. Just ask Paul Haese who spent about 20K on an 18" SDM dob for planetary imaging and sold it 6 months later and bought a SCT."

As you know, SDMs are not equatorials, but AltAz scopes. They suffer from field rotation, which is hard to combat, but they could still be reasonable imaging scopes. I am not sure why Paul went Dob -> SCT route. I am in the process of building a platform for my 16" Dob, I guess I will find out soon enough.

[bratislav]

Point is, it takes few minutes max to collimate (once you are adept at it - which you should be!) and it costs nothing. Even at 140x you will notice that collimated SCT has "better seeing" (it stays sharp for longer periods) on planets. At 350x, or for imaging at long f.l., there is no question about it.
As Legault points out "So, since a simple fraction of a turn on a collimation screw can suppress it, why go without this easy gain" ?
And I'd collimate larger aperture too! What's the point of going for extra $$$ and throw it down the drain? SCTs are already pretty "wobbly stack" as Suiter calls it, obstruction, thermals, roughness, you name it. You simply can't afford to not be perfectly collimated on top of all that.

[Gem]

Ahh - ok. To me an equatorial platform is an equatorial mount rather than a dobsonian mount.

[Camelopardalis]

Cheers John! I look forward to it...if not May, at some future opportunity :thumbsup:

[pdalek]

Given their poor off-axis performance, sagging MTF and chromatic aberration, why use eyes?

[Satchmo]

These graphs are a not really relevant as a comparison to an imaging instrument - you are only ever directly aware of the quality of the center of your vision and your eye scans around as you look at the view.

[pdalek]

I was half joking - all these visual observers with their 2% QE detectors. Mark is right that the graphs refer intrinsic aberration about central axis.
However, visual acuity is altered significantly as the eye is turned. I have had sensors glued - literally - to my eyes in experiments on this.

How well a scope performs visually depends on the whole optical train and how it is used. Just considering just OTAs relative merits is bit too restrictive.

If we take the example of a 16" F4.5 undriven Dob, pick an eyepiece with a good exit pupil and a fov which keeps the planet in view for, say, 30s.
The off-axis performance of the parabolic mirror will give a horrible MTF for most of the view time.
Fix by adding a corrector, fancier eyepieces to cope with f4.5, and soon you have two dozen extra optical surfaces and far less cash.

[sn1987a]

Invested in a Suchting 16" f 4.5 mirror, a Protostar secondary, a Feathertouch focuser, a Telvue Paracorr and a swag of Televue Ethos/Naglers. Every time I view with it I'm glad I went hard and invested in the good gear that will last me the rest of my life. When the seeing is playing the game, the mirror is cool and the collimation is perfect.... I tell you man, you don't give a rats about the money.

[Gem]

While the rest of us get a SCT or a cheaper dob to try to cover the most bases for our limited $. Unfortunately budgets are a reality for most of us.

[sn1987a]

Didn't buy it all at once cobber. Takes a year just to get a mirror made minimum. Starts with a 50 percent deposit. Journey of a thousand miles begins with first step.:P

[clive milne]

Barry... did you get around to changing the secondary support attachment on that scope?

[sn1987a]

I just re siliconed it with the blobs a bit thicker, still the original Meade stalk. You know how it is if it ain't broke don't fix it :D

[pgc hunter]

I'm using a premium 8" dob for my lunar/planetary viewing. The contrast is startling and wouldn't trade it for anything unless something bigger of the same quality came along. Every time I use this scope, I just..... just smile. Plus it is relatively light and sets up in 5 minutes. I'm planning on upgrading it with a smaller secondary as the current CO is 23% and there is plenty of room to reduce that.

[clive milne]

Yep... that's all it needed. I imagine it is an absolute belter of a scope now.

btw, probably heading up to the property next new moon if you are interested.

[sn1987a]

I've got it out on Jupiter right now with red spot dead center and moon shadow I can see detail in the swirling festoons and white snake through the light cloud cover we have at the moment. Sharp outline of the red spot, sun has just set.

[ausastronomer]

I just got all that in my 10"/F5.3 Suchting powered SDM :) Plus 6 craterlets in Plato and the rille in the floor of the Alpine Valley. I didn't think conditions would favour using the 14" or 18" scopes, as it was a bit windy, but things held together nicely at 260x in the 10".

Cheers
John B

[sn1987a]

Good stuff John glad someone is getting a clear sky. It was surprisingly good here as the sun was setting even with the light cloud but its gone to .... now! :sadeyes:

[Satchmo]

Or just add a servo cat for $1500 and save on all that extra glass :)

[sn1987a]

Fantastic views of Mars in the 18" f4.2 Plettstone with 6mm Ethos last night similar to the photographs being published on IIS. The white cloudy areas really stood out against the Peach coloured planet, particularly the small cloud over O Mons. I'm hoping for similar tonight.

[Don Pensack]

The dob does have the theoretical advantage of having a smaller secondary obstruction, which tends to improve contrast in small details.
However, a 12" SCT is no slouch, so here are the things I suspect:
1) collimation. Are you familiar with it and have you done it at high power to get the best star images? A tiny bit of mis-collimation will fuzz out small details
2) cooling. Here is the truth: without active cooling of some sort, a 12" SCT will never get anywhere near ambient temperature. If the temperature of the night is falling, the mirror will never get to ambient or even a couple degrees away from ambient. An active cooling device, like the Lymax SCT Cooler or equivalent, is essential to remove the heat from the tube. It definitely helps to leave the scope outside for a couple hours before viewing (with rear cell pointing at the sky), but that's not enough.
A small warm layer in front of the mirror will forever soften the images of small details. Any reflector of this size would have fans. Does your SCT?
3) conditions. Local seeing can be improved by not looking below 30 degrees off the horizon, over a roof (where heat rises into the image), and staying away from the sides of hills (where air is either rising or falling). The centers of valleys are usually more stable than the edges. Regional seeing can be improved by looking at the pressure isobars on weather maps--where they're close together, the air is moving a lot. You want them to be far apart and the air a little stagnant. Look also for the jet stream--when it's overhead, your seeing will suffer.
I observe on a mounting at 2550m, and I have seen seeing so bad every star looked like a planetary nebula. And, I have seen seeing so spectacular your 12" would have resolved to its theoretical limit. Both conditions are equally rare. But on the latter, I used 456-608X on Jupiter and got an almost 3D technicolor image with white swirls in a salmon colored GRS, surrounded by small gray and white storms.
I don't know if your optics will support that view, honestly, but you'll never know if the three "C"s aren't right. I suspect an absence of cooled optics and poor seeing conditions.

[jetsteam]

In my 10" dob with good optics the GRS in the pocket,big white oval,small white ovals polar shading etc is easily visible when seeing allows.Best for me @250x-300x.Scope will cool in about 45 min with the fan on,from 20c down to -20c