I gave it an attempt last night, although Saturn wasn’t moving a lot i was struggling to get a clear and defined Cassini let alone an Encke! The Mewlon had had a solid 5 hours of cooling by that stage but it was horribly dewy.
I had the Intes out on Monday night here in Cairns. Despite it being a wonderfully cool dry season night, the dew was horrific & the seeing even worse...
Best views of Saturn were with a 17mm Vixen LVW but, even with that I could not resolve Cassini division... seeing was just horrible... twinkly stars, both Saturn & Jupiter were fluttering in & out of focus. Any higher power just resulted in a homogenous blob...
Four conditions need to be met to see it:
* Excellent optics
* Outstanding seeing conditions
* Sufficient aperture
* Acute vision
And it is not so clear cut as these simple statements either. You may need to try different things such as using filters if using large aperture in order to improve the noise to signal ratio - in other words, tone down the glare from the rings. It's a question of contrast. Collimation must also be so tight that the optics are squealing. If your scope shows mirror shift, you must know how to work with it. When using a reflector (of whatever flavour, Newt, Mak, SCT, etc), use a very small doughnut for final star testing tweaks as a large one will spread out any colllimation error and not provide the final quality control. Don't rely just on your laser - verify it with star testing. Even learn how to identify different aberrations (such as astigmatism, chromatic aberration & others) to help sort out your gear.
There are also other features that are in the same realm of the Encke Division. On the Moon there are the two wee riles that run down the centre of the Vallis Alpes & Vallis Schroteri. The angular size of each wee rile is the same as that of the Encke Division. Yet seeing or photographing these two features is neither questioned or denied. To see either one of these through a large aperture, the image may needed to be attenuated or the glare of the Moon as a whole will overwhelm the faint signal that is these two riles. An 82A filter is a good start - I use one now with my 9" Mak to pull detail that is overwhelmed by the mass of light 9" of aperture pulls in. I use it along with no filter together.
To spot either one of these two lunar features, all four of the above conditions must also be met.
To argue that it took a 32" refractor to first show the Encke Division is a mute point. Refractors of that time were all achromats with questionable eyepieces (all excellent for their time though) and it wasn't until the that particular scope that an instrument of sufficient quality was available to make out the Encke. We also do not know how the scope was being used to show it - ie filter?
I have seen it on three separate occasions through four different scopes. I have not been able to see it again since with any of those scopes and I know seeing conditions have just not matched those nights. Of my 12 scopes I know only one is capable of doing so, and one other I haven't tried. The others no chance including my 17.5" (It's astigmatic and I am honest to say this).
No one quibbles when the Cassini Division is seen through a high quality 50mm scope. If one still insists on believing the Rayleigh and Dawe's limits are the smallest detail that a given apertre can show, your 8" scope shouldn't show you the Cassini Division either. So think about it.
I am not just only saying I've seen it. I am also giving tips on how to improve your chances.
The Encke Division is a very illusive feature. Not all scopes can show it. Not all eyes can see it. There are also lunar features that are the same angular size that provide a parallel test.
You can only try, and try often. If you don't, then one or more of the above four conditions are not being met. Seeing conditions for me this year have not been good enough. But I will keep trying too.
Last edited by mental4astro; 16-07-2020 at 11:17 AM.
I think there's some pretty bold statements made here about what's possible and not. @Spacecat (& Les) - it sounds as though you have some misunderstandings about planetary image processing, not helped by 101 statements such as "over -sharpening can introduce artefacts", well yes of course! We all try rather hard to minimise that, and methods have come on quite a lot since early image processing programs.
First of all, the key relevant issue that we encounter with imaging planets is a diffraction rind effect on very hard high-contrast edges - the issue is most pronounced off some higher surface brightness objects such as the limbs of Venus and Mars, lunar craters, Mars' polar cap. The issue is wavelength-dependent and can create unnatural light/dark banding in some areas of some images, perhaps best discussed by Martin Lewis here. The diffraction effect is present in raw data, and is (importantly) not a processing artefact. Well-processed images these days show few true "processing artefacts", though this can be very hard to do!
The diffraction issues are not generally so much of an issue for Saturn's rings though, otherwise we would expect to see it all the way around the rings. I think this is because Saturn's surface brightness (even the rings) is lower than surfaces where we commonly see the edge-rinds. But is it of course plausible that this explains some of the images with Encke-located brightness minima, but I'd argue that this is not typically the case, especially where we don't see such a deep brightness minimum from nearby harder edges, such as the Cassini Division, or on ring edges away from the ansae. Ultra-minimally processed stacks of Mars and Saturn are attached (so high noise), with the bright limb of Mars showing the diffraction effect, Saturn's rings show it a little on the highest contrast outer B ring, but not elsewhere.
The second is that what we see at the ring ansae is very consistent with what is seen in larger scopes - see the attached example with Hubble, Pic du Midi, Tiziano Olivetti, Damian Peach and myself. It's obvious that Hubble resolves it, it's pretty clearly visible in the 1m (Peach processed same technique as C14 images) and 505mm images, but likely not truly resolved in either. The darkening appears consistently in C14 images such as mine and Peach, consistently in the right regions to not be a processing artefact. I would draw your attention to my ultra-soft processing sample of a single 1min red stack which, with the barest of processing immediately shows the minimum brightness at the correct region for Encke, and not elsewhere. This demonstrates that it is a real feature of the data and not an artefact by overenthusiastic image processors!! That it only appears at the ansae, at the correct location where we would most expect to observe Encke, and not from the brightest hard edges in the image strongly suggests it's not a diffraction effect (one of those is faintly visible in my raw frame on the outer edge of the B ring).
Should it be visible - well, yes! It is not truly being resolved in our images, probably not even in the Pic du Midi image. But it is helping to darken the pixels that otherwise would be smooth equal brightness across the outer A ring. So it is increasingly prominent as this darkness becomes a large fraction of the resolved pixels as imaging resolution increases. The example I've also put in is the Hubble image, resampled to 15% (about 3.5px/", and close to a decent amateur scope resolution), where the original Encke would be <1/3 a pixel in width. You see very similar ring features in the resampled Hubble, Damian's, Tiziano's and my images, including an Encke minimum. And our wavelet processing will bring out this minimum in brightness. So we're not "resolving" it, but we are genuinely imaging it, rather than a pure artefact. And it's fair to say, if we're not imaging it, then visual observers definitely are not seeing it!!
[worth noting that Chris' processing is stronger than many imagers, as he is aiming to bring out low contrast cloud features on the globe, and is distinctly overprocessing the rings in his example posted, he's not all that bothered by the rings]
That is a very good write up and explanation of what is going on in the images showing or possibly showing the Encke Gap. With a lot of images that I have looked at, including yours, they definitely show a darkening in the right spot in the A Ring to coincide with where the Encke Gap would be.
The resolution of the scopes and cameras and the quality of the seeing and possibly the processing, may not be good enough to resolve the gap in sharp definition but as a lot of the images are showing a dark band in the same area, then it seems to suggest that it is a real feature.
Still makes it a very real visual challenge to those with larger aperture scopes, waiting for the magical session when the seeing is at a once in a year, decade, lifetime, perfection.
Four conditions need to be met to see it:
* Excellent optics
* Outstanding seeing conditions
* Sufficient aperture
* Acute vision
.
Here mate we're in complete agreement
Quote:
Originally Posted by mental4astro
There are also other features that are in the same realm of the Encke Division. On the Moon there are the two wee riles that run down the centre of the Vallis Alpes & Vallis Schroteri. The angular size of each wee rile is the same as that of the Encke Division. Yet seeing or photographing these two features is neither questioned or denied.
This is an important statement -- and I'm sorry, it's not correct. The angular width of these features are not in the same "realm". Here are the angular widths given Saturn at opposition at 10AU distance from Earth and given the Moon's mean orbital distance of approx 380,000km and the formula:
Angular Diameter in arc-seconds = 206265 X (Actual diameter (km) / Distance km).
Cassini Division (4,800km): 0.67 arc-seconds
Vallis Schröteri rille and Valles Alpes rille: (~600m): 0.325 arc-seconds
Encke Division: (325km): 0.045 arc-seconds.
Dawes limit various apertures:
Dawes limit (approximation) formula 11.6/D (where D is the aperture in cm)
Dawes limit 50mm: 2.32”
Dawes limit 15cm: 0.77”
Dawes limit 18cm: 0.64”
Dawes limit 23.5cm: 0.49”
Dawes limit 25cm: 0.46”
Dawes limit 30cm: 0.38”
Dawes Limit 46cm: 0.25”
Dawes Limit 63.5cm: 0.18”
I should also add that in one of the other threads, regarding these lunar features you wrote: "At this distance, that 500m detail is 0.0476" in size. Oh, and guess what, the Encke is 0.045"... Hmmm."
You didn't show any working for that calculation, but it appears to me to be incorrect.
Using the formula I cite above: 206,265 x (Actual diameter (km) / Distance km)
206265 x (0.5 / 363766) = 0.28 arc seconds -- ie more than six times the apparent angular width of the Encke division from 10A.U.
In other words, these two rilles are about half the apparent angular width of the Cassini Division but, they are about seven times the apparent angular width of the Encke Division. These two rilles are in the same realm as the Cassini Division, but not in the same realm as Encke.
Yes, (as I have pointed out myself several times during this debate over the past year or so via several threads), the Dawes Limit isn't the be-all and end-all in terms of visually detecting (though not "resolving") high contrast albedo features. Yes, certainly, high albedo contrast features somewhat smaller in angular width than the Dawes limit are detectable in high quality telescopes in superb seeing. I have little doubt that features less than half, maybe even down to a quarter or a fifth of the Dawes limit might be detectable. Beyond that ...? Hmmm ...
The observational circumstances on the Moon with rilles of this sort are extremely high albedo contrast difference. Absolutely black -v- near brilliant white (when the shadow is created by the sun-angle).
The contrast at the Cassini division is high, but not as good as a rille on the moon. Further, the "B" ring is the brightest of the Saturnian rings and is at its highest surface brightest at the inner edge of the Cassini division.
The Encke division contrast could be described as "good-ish" but not nearly as high as Cassini because the "A" ring is somewhat fainter than "B" and tends (generally) to fade toward its outer edge -- where the Encke division is located. It's a black -v- grey situation, not black -v- white.
What should be noted is that for a 7" (18cm) telescope, the Dawes limit is 0.64" and the angular width of the Encke division (0.045") more than fourteen times smaller. Even for a 9" (23.5cm), Encke is over eleven times smaller than the Dawes limit.
Adding complications are the significant, approaching large central obstructions (~30%) native to all Maksutov-Cassegrainian (and Schmidt-Cassegrainan -- even larger) telescopes further interfere with and detract from visual contrast compared to unobstructed telescopes or those with a genuinely small (<20%) obstruction.
Quote:
Originally Posted by mental4astro
To argue that it took a 32" refractor to first show the Encke Division is a mute (sic) point. Refractors of that time were all achromats with questionable eyepieces (all excellent for their time though) and it wasn't until the that particular scope that an instrument of sufficient quality was available to make out the Encke.
To be candid, this argument is absurd. Between the time of Encke observing with a 9.6" refractor at Berlin in the 1820s and the first observation of the division in 1888 using the 36" Lick refractor, there were 17 "great refractors" in excess of 30cm aperture built and commissioned around the world by makers such as the Clarks, Merz & Mahler, Zeiss and Cooke. Nearly all of them made by renowned telescope/lens makers and were (still are) of exceptional visual-use quality. Sure, they will have chromatic aberration but that will not affect their capacity to resolve or "see" fine detail. Telescopes and the technology behind making lenses was vastly higher than the days of Gallileo & Huygens etc. They not only were but still are pretty much state of the art visual-use telescopes. All of them, were long (if not very long) focal-length instruments that simple eyepiece designs worked perfectly well with. Most of the advances in eyepiece design (particularly in the last 50 years) has revolved around making eyepieces cope with optically much "faster" telescopes and to have wider, well-corrected fields (and better anti-reflection coatings). Almost irrelevant to planetary observing where all you need is "sharp" at the centre of the FOV -- the width of field is a much less important consideration.
Similarly, the astronomers that used them were no mugs behind the eyepiece either. Back in the 19th century, while photography was in its infancy and genuine astrophysics somewhat in the future, the career of a "professional astronomer" rose or failed on their visual acuity and skill at the eyepiece.
Quote:
Originally Posted by mental4astro
No one quibbles when the Cassini Division is seen through a high quality 50mm scope. If one still insists on believing the Rayleigh and Dawe's limits are the smallest detail that a given apertre can show, your 8" scope shouldn't show you the Cassini Division either. So think about it.
I have thought about it. Here is the simple arithmetic: The Cassini division is somewhat larger than 1/4th the Dawes limit for a 50mm telescope and is likely detectable (though not "resolvable") in good conditions. The Cassini division has somewhat better contrast/albedo features compared to Encke.
Seeing the Encke division is all together in another league -- and not in the same realm (to use your words). Using 18cm it is worse than 1/14th the Dawes limit for that aperture and even in 23.5cm is about 1/11th the Dawes limit for that aperture. Then there is the relative lack of contrast inherent in Maksutov-Cassegrainian telescopes with their comparatively large central obstructions added to the fact that out at the near edge of the visible ring Saturnian ring structures, the albedo contrast while good-ish, is not in the same league as Cassini and a considerable distance behind the rilles you cite (that in reality have considerably larger (seven times) angular widths than the Encke division).
Quote:
Originally Posted by mental4astro
I am not just only saying I've seen it. I am also giving tips on how to improve your chances.
Not for one moment do I doubt the sincerity of you claim Alex but I have to conclude your "detection" is a spurious observation. Many, many observers of very high repute in the past have made what are now termed "spurious" observations -- errors. I have very little doubt that what your telescope/eye/brain (interpretive) combination is "seeing" is the Encke minima that is somehow being interpreted as a narrow gap.
As for famous cases of "spurious observations" look no further than the Martian "cannali". Once "discovered" suddenly many observers "saw" them. After they were disproved, suddenly, no-one saw them anymore.
Also, look up the curious case of "Baxendell's unphotographable nebula" (NGC 7088).
Baxendell's Nebula (NGC 7088) is a nebula that apparently never was. The discovery of a large faint nebula near the globular cluster M2 in Aquarius was announced by the English amateur astronomer Joseph Baxendell in 1880 ("A New Nebula," MNRAS 41, 48). He found the object, designated as NGC 7088 by Dreyer, on September 28, 1880 at his private observatory in Birkdale, using a 6-inch refractor, and described it being of "irregular oval form, its longer axis lying in a nearly east and west direction". It was, he reported, 30' north of M2 and 75' × 52' in size. He writes "It seems to be similar in character to the large nebula near the Pleiades [found by Tempel], but is slightly less bright. NGC 7088 was visually seen by a number of other observers, including Dreyer (1885, 10-inch refractor), Bigourdan (1897, 12-inch refractor), Hagen (1915 and 1917, 16-inch refractor), Wolf (1927, 6-inch refractor), O'Connor (1929, 15-inch refractor), Becker (1930, 12-inch refractor) and Lehner (1930, 4-inch refractor). However, the object could never be photographed – hence its nickname, "Baxendell's Unphotographable Nebula". It was eventually concluded that the object wasn't real and that visual sightings of it were due to effects such as reflections of the nearby bright cluster M2.
This object, despite the "observations" of some great astronomers, simply does not exist.
People make mistakes. Our brains are natural, active interpreters of what our eyes apepar to be seeing at both conscious and sub-conscious levels. None of us are immune. We can do our best to have other people confirm sightings and trying our best to be objective, but it doesn't always work.
The Encke Division is a very illusive (sic) feature. Not all scopes can show it. Not all eyes can see it. There are also lunar features that are the same angular size that provide a parallel test.
The Encke division is an extremely severe test, of telescope, observer and seeing, I agree. Yes, it takes functionally perfect seeing. Good, very good or excellent seeing are generally not enough. Conditions of this sort are exceptionally rare. Near or at sea-level maybe only one in a thousand days. The lunar rilles are in no way a parallel test as I have demonstrated.
Not all 'scopes can show it -- certainly correct and I'd respectfully suggest the minimum aperture for a very high quality telescope with a small or no central obstruction in functionally perfect seeing is about 30cm. The fact that it eluded a great many highly credentialled observers, using giant (30-70cm), very high quality refractors sited (often) at high-quality observing sites for over 70 years after Encke's heyday, in fact speaks volumes for the difficulty this feature presents.
Quote:
Originally Posted by mental4astro
You can only try, and try often. If you don't, then one or more of the above four conditions are not being met. Seeing conditions for me this year have not been good enough. But I will keep trying too.
If you can see or image a human hair (100 microns average*) in the best conditions ,say on a high contrast background, at around 500 metres then your system is all set for the Encke Gap, notwithstanding the atmosphere. If not ...
If you can see or image a human hair (100 microns average*) in the best conditions ,say on a high contrast background, at around 500 metres then your system is all set for the Encke Gap, notwithstanding the atmosphere. If not ...
This is an important statement -- and I'm sorry, it's not correct. The angular width of these features are not in the same "realm". Here are the angular widths given Saturn at opposition at 10AU distance from Earth and given the Moon's mean orbital distance of approx 380,000km and the formula:
Angular Diameter in arc-seconds = 206265 X (Actual diameter (km) / Distance km).
Cassini Division (4,800km): 0.67 arc-seconds
Vallis Schröteri rille and Valles Alpes rille: (~600m): 0.325 arc-seconds
Encke Division: (325km): 0.045 arc-seconds.
Dawes limit various apertures:
Dawes limit (approximation) formula 11.6/D (where D is the aperture in cm)
Dawes limit 50mm: 2.32”
Dawes limit 15cm: 0.77”
Dawes limit 18cm: 0.64”
Dawes limit 23.5cm: 0.49”
Dawes limit 25cm: 0.46”
Dawes limit 30cm: 0.38”
Dawes Limit 46cm: 0.25”
Dawes Limit 63.5cm: 0.18”
I should also add that in one of the other threads, regarding these lunar features you wrote: "At this distance, that 500m detail is 0.0476" in size. Oh, and guess what, the Encke is 0.045"... Hmmm."
You didn't show any working for that calculation, but it appears to me to be incorrect.
Using the formula I cite above: 206,265 x (Actual diameter (km) / Distance km)
206265 x (0.5 / 363766) = 0.28 arc seconds -- ie more than six times the apparent angular width of the Encke division from 10A.U.
Well, I stand corrected!
I reviewed my working out and I did make a mistake!
Again, I never said the astronomers of the time were dumb. I mentioned that in the post you most kindly misrepresented.
As for "imagined seeing" of the Encke Division, when I first saw it I had no idea about it. I was observing Saturn when I saw that feature. No expectation, no prompting, but there it was. This has been noted many times by others too, no prompting, and using different scope designs. But all these people did see the same thing, and yet you offer not one single plausible explanation for what we all saw other than we were "imagining" it? Difficult to imagine something when you do not expect such a feature.
What is interesting is this following article discusses this very topic, using the notes of the astronomers tracing the history of the detection of the Encke Minima and Division. It also quotes none less than William Dawe's observation and sketch of the Encke Division from 1843 using a 9" refractor (page 5, fig 7 of the document):
When it comes to the Encke Minima vs the Encke Division, one is low contrast the other high contrast, respectively. What I have seen is the Divison - the feature's location was exactly where the Division is in the A ring. The same position of the feature that Dawe's himself noted. The Minima is a much broader feature within the A ring, something that I have seen too, and not the same thing as the Division.
Adding to this is the drawing by E. M. Antoniadi of Saturn from 1899 using the 10.25" refractor at Juvisy Observatory showing the Encke Division, in exactly the same position as I have (page 7 of that article).
A photometric diagram (page 9, fig. 12) of the intensity of the rings is on page 9. It labels the position of the Minima (g, h, i) and notes the position of the Division (E). Again, the location of E is where I have seen this feature.
Interesting to note that the apparition of the Encke Division in the 36" Lick refractor was only visible on just the one night, July 3 1899, adding to just how difficult it is to chance upon conditions favourable enough to see it.
So here we have instruments from 9" through to 36" showing it. All professional observations.
This article comes as a complete revelation to me.
As for a 50mm high quality refractor showing the Cassini Division, you need to try this yourself - but only derision from you here. Myself I have seen the Cassini Division with a 53mm aperture - masked down an ED80 refractor. David Knisely posted on CN he saw it with the 50mm aperture at 176X
As for a 50mm high quality refractor showing the Cassini Division, you need to try this yourself - but only derision from you here. Myself I have seen the Cassini Division with a 53mm aperture - masked down an ED80 refractor. David Knisely posted on CN he saw it with the 50mm aperture at 176X ...
No, sorry, incorrect, no derision from me at all actually! Please re-read what I wrote above last night:
"I have thought about it. Here is the simple arithmetic: The Cassini division is somewhat larger than 1/4th the Dawes limit for a 50mm telescope and is likely detectable (though not "resolvable") in good conditions. The Cassini division has somewhat better contrast/albedo features compared to Encke."
I regard detection of Cassini using 50mm aperture as very likely credible, as it is better than 1/4 the Dawes limit for that aperture and that Cassini has somewhat better albedo contrast circumstances than the Encke division. Personally, I have seen it in an 80mm refractor but have not tried in lesser apertures. 1/14 the Dawes limit on a division (Encke) that has lesser albedo contrast, using 18cm (7") is an altogether different bucket of fish -- a very significant (much more than triple) increase in degree of difficulty over the 50mm/Cassini example.
As I've said before many times Alex, I don't doubt your sincerity, but I equally believe that people can be sincerely incorrect, or sincerely mistaken. Over my reading of the material here, there and elsewhere on the net, one thing that is abundantly clear is that people who make entirely credible claims of glimpsing the Encke division also simultaneously record seeing the Encke minima as well. The Encke minima is a more easily seen feature than the gap, yet so far as I have been able to make out, you see the gap but not the minima and this is at variance with nearly every recent, credible claim. It is for this reason I have to conclude that you have in fact seen the minima and somehow, via a combination of telescope/eye/brain/physiology/psychology mistaken it for the gap.
Your argument regarding the visibility of a few lunar rilles being a "parallel" situation is shown as clearly incorrect.
I think I've said everything that I can usefully contribute on the subject. I stand by my conclusion that the minimum aperture needed, in the absolute best possible circumstances is about 12" (30cm).
If people would like to prove that conclusion incorrect with physics/arithmetic/mathematics please feel free to do so -- I'm all ears.
Like Alex, I invite people to both try and try hard with whatever telescopes they have and report both positives and negatives here -- but be very, very wary of potential false positives.
Hate to break it to the nay-sayers, but planetary imaging is predicated on "lucky" moments of seeing where capture is on the order of 0.1-0.2" per pixel.
Dawes limit is a star-splitting measurement, not a measure of contrast detection on bright, relatively-nearby objects...the pixels don't lie
Les, all I am reading from you is "if I haven't seen it then no one else can". Encke and now Cassini. Your problem now. I'm too busy doing, not tearing down.
To everyone else, try for it. Lots of people have seen it and photographed it. Even better look at making out the all subtle details within the rings. A quick glance only robs you of their full majesty. A patient eye is your best tool.
I want to ask what is probably a dumb question regarding these resolution figures published below and the associated aperture. The question is, does the calculation allow for central obstruction affects, or is it assumed that the aperture is a purely open one (ie a refractor)?
As an example I will use a typical GSO 300mm f5 Dobsonian, which is advertised as having a resolution of 0.38 arc seconds, but apparently has a central obstruction of 25% (according to the TS website specs). If someone has a 12" GSO Dob, or any other 12" reflector, are they really getting 0.38 arc sec of resolution?
The question is, does the calculation allow for central obstruction affects, or is it assumed that the aperture is a purely open one (ie a refractor)?
Not a dumb question at all.
From a technical (mathematical?) point of view, it doesn't really change if the telescope is obstructed or not, or the extent of the obstruction, because this is a function of the Dawes limit. The Dawes limit really isn't an absolute rule of physics or mathematics but an approximation based on observation.
It is normally stated as: R = 116/D where D is the aperture in millimetres, R is the resultant "limit" in arc seconds. Therefore the Dawes limit (resolution) for a 12" 'scope is 0.38 arc-seconds. It was arrived at by Dawes as a statement of what he found to be the aperture needed, in perfect conditions to resolve a magnitude 6 double star of exactly equal magnitude with a given aperture. For all practical purposes, it describes the diameter of the central "dot" (pixel if you like) in an airy-disc pattern for a telescope of a given aperture. You will notice that as aperture increases, the size of that "dot" decreases.
The effect of a central obstruction is well known and is describable by physics.
In a perfect optic with unobstructed aperture, 84% of the light from a star will be placed in the central disc and the remainder distributed to the surrounding rings in the diffraction pattern with the first ring being the brightest and having (from memory) 7%, the second ring about 3% etc etc outward. This effect is caused by the circular nature of the aperture (entrance pupil) and the wave-nature of light. When you introduce a central obstruction (like a secondary mirror) diffraction effects from that obstruction shift light from the central dot into the surrounding rings. So the dot gets less intense and the surrounding rings brighter.
For central obstructions that are less than about 20%, the effect visually is almost negligible and only detectable in quite extraordinary seeing conditions. Once they rise beyond 20% they become increasingly noticeable. A lot of commercially available Newtonians (GSO, Skywatcher) nowadays are around 25% obstructed, commercial Maksutov-Cassegrainians are typically about 28-30% and nearly all commercially available Schmidt-Cassegrainians are in the 35-38% range.
By way of real-life examples, my 18" f/4.9 Newtonian is about 17% obstructed and the 25" f/5 is 14%.
Significant central obstructions do not affect the Dawes-limit resolution of the telescope, but they do affect visual contrast and make detection of detail (particularly low-contrast albedo detail on planetary surfaces) significantly more challenging in the eyepiece. The reason, put simply, is that an image of a planet in the eyepiece is a in effect a cheek-by-jowl mosaic of Airy-disc diffraction patterns with the rings overlaying the adjacent disc/rings. A bit like the resolution on a T.V or a monitor, the smaller the pixels, the more detail you see, the smaller the central obstruction (best of all, no central obstruction) the better the contrast between adjoining "pixels".
In terms of the telescope's capacity to visually reveal and display detail and contrast between adjacent albedo features, the nett effect (and this is a well known -- or at least used to be well known) rule of thumb: The aperture diameter minus the central obstruction diameter shows the relative unobstructed aperture for revealing planetary detail.
ie (And this is where I finally it the nail on the head), a 10" telescope with a 2" central obstruction is roughly equivalent to an 8" refractor in terms of showing low-contrast detail/albedo features. Again, I stress this is a rule-of-thumb, not an absolute rule.
Accordingly, your 12" Newtonian with a 25% obstruction is the approximate equivalent of a 9" unobstructed telescope when it comes to displaying fine, low-contrast albedo features on a planetary disc. That said, when it comes to deep sky observing and faint objects, your 12" will be gather more light and render objects brighter in the eyepiece compared to a 9" refractor.
What must be ultimately remembered is that all telescopes, without exception are compromised one way or another. There is no perfect telescope for all people and all uses. If there were only one perfect, one-size-fits-all design that's all that would be sold. They all have strengths and weaknesses whether it be cost per inch of aperture, portability, visual contrast, photographic utility/aberrations, light-gathering capacity, ease of use etc etc. It really comes down to which compromises you are willing to live with and which compromises you can't live with (or more frequently can't afford), what you want to use it for and what doesn't matter.
Hope that helps.
Best,
L.
Last edited by ngcles; 19-07-2020 at 08:57 PM.
Reason: Added links.
This is a luminance-only raw file from my capture last Saturday night, in case anyone is curious enough to try for themselves. The capture resolution was approximately 0.135"/pixel, using my Celestron EdgeHD 11" SCT, 1.6x Barlow and ZWO ASI290MM camera.
Every deconvolution algorithm I've tried (using PixInsight and Photoshop) has resulted in several contrast bands appearing in the rings.
I've also included my rendition. As you can see, there's not a lot of effort involved in precipitating these features. No other processing operation was performed on this data besides deconvolution (and increasing the canvas size).
