212 replies to this topic

[simpleisbetter]

Peach uses a CCD camera to image. It's transfer function is quite different than that of the human eye.

Hi John,
I'm not disputing the transfer functions aren't different between camera/ccd and what we can see visually. And I'm willing to admit Peach also uses a C14, not an 11. But IIRC the other imager I'm trying to think of is using a C11 and getting results rivalling NASA.

But, what I maintain as true is that even if the transfer functions are different, the same laws of physics still apply to ccds and cameras. That law being the larger the lens in front of that chip or film, the larger the image plane, the larger the sight picture, and the better the resolution and detail observed. That's why I made that statement; I've never seen images of the planets taken by 6-8" APO's as having the same amount of detail or the same size as from those coming from the larger SCT's.

My personal experience with my C9.25 was that on good nights when the scope was cooled down, I could easily get 475x out of it and view without feeling like I was pushing the scope hard. I never got that regularly out of any 6" I've looked through or used over the years...Ever. And even when I was able to get that much magnification out of a refractor, I always felt like I was pushing it to or beyond its limit, and never could get the same level of detail resolved. Of course, the C9.25 still couldn't beat my humble 10" dob on contrast, like low surface brightness galaxies. So aperture wins, no big surprise.

[Cotts]

C.O.'s typically range from 15% or so (my 6" f/*8 Mak Newt has a c.o. of 16%) to around 40% in some Ritchey Chretien designs. Even at 40% there is much more light in the central spurious disc of the diffraction pattern than there is in the rings so resolution is the same as the unobstructed scope but the contrast will be quite noticeably less.

I doubt that there is any telescope made for use by amateurs that has a c.o. of more than 40 %. 95%?---why would anyone do that?

some stuff to read... http://www.beugungsb...iffraction.html

http://www.telescope...obstruction.htm

http://www.brayebroo...orum/c-o's.html

Dave

[Deep13]

I agree with this. there is theory and practice. And cost. I would keep the 5 inch for the wider view it provides.

It's not much wider at f/12. At a dark site it is okay on clusters, but the 12.5" is what really knocks ones socks off.

[Deep13]

Its always scope size, seeing, and collimation that's king!! I know a lot of people love refractors, and I do as well, however the image scale is too small in a smaller scope. This is why the larger scope always shows the most detail.

Yeah, Mars is tiny in my 5" refractor and Uranus barely shows a disk in my 8" Newt.

[Deep13]

The bottom line is that you gonna have to go to a star party and check for yourself.

I don't think I have been to a star party near my home in five years that wasn't clouded over. When I do and there are SCT users there, they are usually shooting pics of DSOs.

[JJK]

Peach uses a CCD camera to image. It's transfer function is quite different than that of the human eye.

Hi John,
I'm not disputing the transfer functions aren't different between camera/ccd and what we can see visually. And I'm willing to admit Peach also uses a C14, not an 11. But IIRC the other imager I'm trying to think of is using a C11 and getting results rivalling NASA.

But, what I maintain as true is that even if the transfer functions are different, the same laws of physics still apply to ccds and cameras. That law being the larger the lens in front of that chip or film, the larger the image plane, the larger the sight picture, and the better the resolution and detail observed. That's why I made that statement; I've never seen images of the planets taken by 6-8" APO's as having the same amount of detail or the same size as from those coming from the larger SCT's.

My personal experience with my C9.25 was that on good nights when the scope was cooled down, I could easily get 475x out of it and view without feeling like I was pushing the scope hard. I never got that regularly out of any 6" I've looked through or used over the years...Ever. And even when I was able to get that much magnification out of a refractor, I always felt like I was pushing it to or beyond its limit, and never could get the same level of detail resolved. Of course, the C9.25 still couldn't beat my humble 10" dob on contrast, like low surface brightness galaxies. So aperture wins, no big surprise.

Steve,

Damian's planetary work is absolutely phenomenal, but I think NASA's flyby missions and the Hubble had a slight advantage and edge.

You might have a better than average C9.25 and your seeing might be better than mine (on average). In addition, your eyes might prefer the brighter image afforded by the larger aperture scope (about 1.6- to 1.7-fold greater effective area compared to an AP155, correcting for the CO and scattering off the primary and secondary). Some days, that's the case for me. Other days, my eyes prefer views that are less bright, but have other good attributes. Example: the Moon and Mars often look better in my Zeiss 130 mm f/7.7 APQ compared to my AP155 f/7 EDF (to my eye, the blacks in the Zeiss are blacker). For my style of observing and location, aperture doesn't always win.

You don't need to convince me that SCTs are useful (although I have an AP 10" Mak-Cass and a Tak Mewlon 300, I can still see getting a cherry C-14 someday). The AP Mak-Cass is as good as others have noted. In addition to providing high quality views on the Moon and planets, I've used it to see the central star in M57 (directly overhead and at very high magnification ~ 950x both at home and more recently at the Black Forest Star Party).


Best,
John

[BKBrown]

Peach uses a CCD camera to image. It's transfer function is quite different than that of the human eye.

Hi John,
I'm not disputing the transfer functions aren't different between camera/ccd and what we can see visually. And I'm willing to admit Peach also uses a C14, not an 11. But IIRC the other imager I'm trying to think of is using a C11 and getting results rivalling NASA.

But, what I maintain as true is that even if the transfer functions are different, the same laws of physics still apply to ccds and cameras. That law being the larger the lens in front of that chip or film, the larger the image plane, the larger the sight picture, and the better the resolution and detail observed. That's why I made that statement; I've never seen images of the planets taken by 6-8" APO's as having the same amount of detail or the same size as from those coming from the larger SCT's.

My personal experience with my C9.25 was that on good nights when the scope was cooled down, I could easily get 475x out of it and view without feeling like I was pushing the scope hard. I never got that regularly out of any 6" I've looked through or used over the years...Ever. And even when I was able to get that much magnification out of a refractor, I always felt like I was pushing it to or beyond its limit, and never could get the same level of detail resolved. Of course, the C9.25 still couldn't beat my humble 10" dob on contrast, like low surface brightness galaxies. So aperture wins, no big surprise.

The human eye/CCD camera comparison is an apples and oranges proposition. A camera will record detail not detectable by the human eye, Eddgie gave a very good explanation why earlier in this thread. I also agree that images from a 6"-8" Apo will not give up as much detail as a larger SCT can detect, I've done similar kinds of comparisons and there are threads on those efforts in the Refractor forum. But the camera does not make subjective judgements about image quality and what a brain thinks it is seeing, it records what it can within the electronic and optical limitations of its design. An image is what it is, and as close to truth as we can get today.
As for Damien Peach, he has numerous published images taken through different scopes including a C925, C11, and C14. Want to be amazed? Just look at this C925 thread off of Damien's web page: http://www.damianpea.../barbados05.htm, especially the Saturn shots. That's what a really skillful operator can do under excellent seeing conditions!

Clear Skies,
Brian

[JJK]

C.O.'s typically range from 15% or so (my 6" f/*8 Mak Newt has a c.o. of 16%) to around 40% in some Ritchey Chretien designs. Even at 40% there is much more light in the central spurious disc of the diffraction pattern than there is in the rings so resolution is the same as the unobstructed scope but the contrast will be quite noticeably less.

I doubt that there is any telescope made for use by amateurs that has a c.o. of more than 40 %. 95%?---why would anyone do that?

some stuff to read... http://www.beugungsb...iffraction.html

http://www.telescope...obstruction.htm

http://www.brayebroo...orum/c-o's.html

Dave

I didn't say anyone would want to make a scope with a 95% CO. I just wanted to open the discussion to learn about the resolving power of a SCT. I never bothered to think about it because most of my scopes are refractors. The 10" Cat I have has a 23% CO (and produces views that have exceeded commercial 11" SCTs), and I use my 25" Obsession almost exclusively for viewing DSOs.

Thanks for the links. I'll study the info on the sites.

[JJK]

Its always scope size, seeing, and collimation that's king!! I know a lot of people love refractors, and I do as well, however the image scale is too small in a smaller scope. This is why the larger scope always shows the most detail.

Yeah, Mars is tiny in my 5" refractor and Uranus barely shows a disk in my 8" Newt.

Mars has looked absolutely stunning in my Zeiss 130 APQ.

[Bob Abraham]

Paul: no need for a flame suit, that is a really great description of the nature of atmospheric seeing. About the only thing I'd suggest is incorrect in your description is that big telescopes are diffraction limited without AO at 10 microns... in practise some basic very low-order AO is still needed to hit the diffraction limit in the mid-IR with the large telescopes I'm familiar with.

About the only thing I'd add to your nice description is a bit of emphasis about the statistical nature of the seeing. The abberated wavefront's structure is described by decomposing it into Zernike polynomials, the first few of which correspond to the familiar Seidel aberrations (coma, astigmatism, etc). The amplitudes of these aberrations is a random variable. So you can think of the action of seeing as adding a random mixture of all these usual aberrations to your optic on short timescales (tens of milliseconds).

For amateur-sized telescopes every once in a while you're going to get lucky and the dice roll of the aberrations is such that you wind up with a nice planar wavefront. (This is of course the basis for "lucky imaging", which is the technique used by webcam planet imagers). How often you get lucky depends on the intrinsic seeing (if seeing is good you get lucky most of the time) and on the size of your aperture (the bigger the telescope the less likely you are to get lucky). Once you get beyond about a 2m aperture you, in effect, never get lucky, even on Mauna Kea, and you really need Adaptive Optics to hit the diffraction limit even for very short exposures of a few ms in length.

Bob

[simpleisbetter]

Steve,

Damian's planetary work is absolutely phenomenal, but I think NASA's flyby missions and the Hubble had a slight advantage and edge.

You might have a better than average C9.25 and your seeing might be better than mine (on average). In addition, your eyes might prefer the brighter image afforded by the larger aperture scope (about 1.6- to 1.7-fold greater effective area compared to an AP155, correcting for the CO and scattering off the primary and secondary). Some days, that's the case for me. Other days, my eyes prefer views that are less bright, but have other good attributes. Example: the Moon and Mars often look better in my Zeiss 130 mm f/7.7 APQ compared to my AP155 f/7 EDF (to my eye, the blacks in the Zeiss are blacker). For my style of observing and location, aperture doesn't always win.

You don't need to convince me that SCTs are useful (although I have an AP 10" Mak-Cass and a Tak Mewlon 300, I can still see getting a cherry C-14 someday). The AP Mak-Cass is as good as others have noted. In addition to providing high quality views on the Moon and planets, I've used it to see the central star in M57 (directly overhead and at very high magnification ~ 950x both at home and more recently at the Black Forest Star Party).


Best,
John

LOL, yeah...comparing Damien's work with the NASA flyby's is an exaggeration; but some of his and other's work is getting awful close to what one would expect from some of the observatories.

I'd love to have one of those AP Mak's, or at least get to use one someday; haven't been around one yet to have the chance, but always wanted to try. I hear they're really good, even better than Mewlons. I'll bet you get some outstanding views from your scopes.

I'll agree with you on the higher contrast of the refractor, my 4" does quite nicely in that department too. That's one of the things that I really like about them. Even as a 4" I'm continually amazed how well it can find and pull in dim galaxies, and I attribute that to the contrast.

The more I think about it, you could have a point about people's preferences for contrast. I was out over the weekend on the Moon and Jupiter, with high shelf clouds moving through messing transparency up, while giving excellent P8 seeing. The slight decrease in contrast from the clouds actually boosted the detail I was seeing. On Jupiter the effect was like my scope had gone from a 4" to a 5" refractor with a polarizing filter. I remember that same thing back in '09 on Mars. For the Moon & planets, it seems the decrease in contrast can help to bring out detail. Other times, I want all the contrast I can get and an SCT doesn't do well on that IMO - that was my one complaint with the 9.25, that it didn't do as well visually on DSO's as I'd hoped. Up to that point I'd owned newts for about 20 years.

BTW - the 4" is all I currently own, hoping to either find the room here for another dob. If not, then I'll pickup a C8 or 9.25 to ride on my CG-5, though I have been considering a C6R or ES-150 OTA for DSO work. So I hope I've been able to keep my comments unbiased, since I really don't tie myself down to one type of scope.

Anyway, best to you here, I'm out for a while, maybe the whole night.

[maknewtnut]

I agree with bobhen, who stated jgraham made a great point in quoting Wesling.

We're likely to keep hearing the same old argument about an SCT and it's aperture advantage being THE way to obtain more planetary detail, and/or those that might even push that aperture envelope further to include even larger Newtonians, and yet.....

Those that are planetary observing afficinados (which are almost always extremely experienced amateur astronomers as well), along the the optics gurus, and even equipment junkies that have owned numerous telescopes often tend to concur on the side of optical quality, minimized CO, effective cooling, recognition of variables such as seeing or ambient conditions, and matching aperture to a given location based on those factors.

In between the endless debate, the relative lack of baselines when comparisons are made, and my years of firsthand experience looking after many say, "Come take a look. This is a great view of Jupiter!"....I've come to the conclusion that it boils down to a huge disparity in how one might define "great" as compared to the next.

[WAVT]

The abberated wavefront's structure is described by decomposing it into Zernike polynomials, the first few of which correspond to the familiar Seidel aberrations

Can't argue with that!

[JJK]

So, we have an 11" SCT and a 6"APO. The SCT has MUCH better resolution, almost double that of the APO. The APO has SLIGHTLY better (but noticeable to most eyes) contrast than the SCT.

The SCT will resolve details that the 6" is incapable of using its better contrast on.

The fact that these extra details will be seen with the SCT's poorer contrast is moot. The SCT will see them! And the APO will be blissfully unaware of them.

"There is no substitute for aperture" - Peter Ceravolo

Dave

One of the websites you suggested later in this thread states
"the resolving power of a telescope increases as the c - o ratio increases. For example a 33% c - o reduces the diameter of the Airy disc by roughly 10%. However this enhancement in the resolving power is only realised on equal or nearly equal doubles. Light removed from the Airy disc is dumped into the ring system, and causes a reduction in the contrast of detail in extended images, and a concommitent reduction in the resolving power of the same telescope on markedly unequal doubles. (Dall 1937 - Paul 1965)."

[Cotts]

So, we have an 11" SCT and a 6"APO. The SCT has MUCH better resolution, almost double that of the APO. The APO has SLIGHTLY better (but noticeable to most eyes) contrast than the SCT.

The SCT will resolve details that the 6" is incapable of using its better contrast on.

The fact that these extra details will be seen with the SCT's poorer contrast is moot. The SCT will see them! And the APO will be blissfully unaware of them.

"There is no substitute for aperture" - Peter Ceravolo

Dave

One of the websites you suggested later in this thread states
"the resolving power of a telescope increases as the c - o ratio increases. For example a 33% c - o reduces the diameter of the Airy disc by roughly 10%. However this enhancement in the resolving power is only realised on equal or nearly equal doubles. Light removed from the Airy disc is dumped into the ring system, and causes a reduction in the contrast of detail in extended images, and a concommitent reduction in the resolving power of the same telescope on markedly unequal doubles. (Dall 1937 - Paul 1965)."

The above quote parallels what I said above that. The larger aperture will have the smaller central disc to begin with and the central obstruction makes it even smaller (not much and at the expense of poorer contrast).

I've read all the posts about the seeing being the deciding factor and I agree. The seeing that would allow an 11-inch scope of any design to reach its resolving limit is extremely rare where most of us live but it does happen - 'lucky observing' to steal Bob's phrase. As I said before, visual planetary observers patiently wait for these moments to take advantage of their large apertures. Or they move to Florida....

Dave

[John Kasianowicz]

So, we have an 11" SCT and a 6"APO. The SCT has MUCH better resolution, almost double that of the APO. The APO has SLIGHTLY better (but noticeable to most eyes) contrast than the SCT.

The SCT will resolve details that the 6" is incapable of using its better contrast on.

The fact that these extra details will be seen with the SCT's poorer contrast is moot. The SCT will see them! And the APO will be blissfully unaware of them.

"There is no substitute for aperture" - Peter Ceravolo

Dave

One of the websites you suggested later in this thread states
"the resolving power of a telescope increases as the c - o ratio increases. For example a 33% c - o reduces the diameter of the Airy disc by roughly 10%. However this enhancement in the resolving power is only realised on equal or nearly equal doubles. Light removed from the Airy disc is dumped into the ring system, and causes a reduction in the contrast of detail in extended images, and a concommitent reduction in the resolving power of the same telescope on markedly unequal doubles. (Dall 1937 - Paul 1965)."

The above quote parallels what I said above that. The larger aperture will have the smaller central disc to begin with and the central obstruction makes it even smaller (not much and at the expense of poorer contrast).

I've read all the posts about the seeing being the deciding factor and I agree. The seeing that would allow an 11-inch scope of any design to reach its resolving limit is extremely rare where most of us live but it does happen - 'lucky observing' to steal Bob's phrase. As I said before, visual planetary observers patiently wait for these moments to take advantage of their large apertures. Or they move to Florida....

Dave

Given your earlier statement "The SCT will resolve details that the 6" is incapable of using its better contrast on.", I wouldn't say the description parallels your comment. If the information in that website is correct (and it agrees with my intuition), then the advantage the CO offers in enhancing resolution is restricted to double stars of nearly equal magnitude and doesn't apply to visual planetary observations. There are no details to observe on double stars (other than their separation). There are on planets.

[TPMack]

An excellent post by Damian Peach. Been around awhile but never too late to resurrect:

http://www.damianpea.../simulation.htm

Tom Mack

[Deep13]

An excellent post by Damian Peach. Been around awhile but never too late to resurrect:

http://www.damianpea.../simulation.htm

Tom Mack

Interesting, especially that last set of pictures.

[CounterWeight]

I sold my C11 a few years ago after comparing to refractors (a first for me as I'd always had Cat's, Mak/Cass or a Newt).

A lot of great posts in this thread and not sure mine will live up to them, or at least not muddy the waters.

Just want to agree that IME better made or figured optics 'can' normally do better than those of a lesser quality, so quality of optics is always important. I say that discounting all other parameters.

Exit pupil used to derive desired detail, also worth considering. As was posted a ways back - not limiting to the MTF / PSF there is also a measure of what relative 'stress' a system is at wrt the components - at a given resolution / magnification (with wavefron figure 'm')at the human eye (or other detector). As for human eye, not as simple a scope of a type at aperture x and scope of type y -at mag x, if seeing discounted.

Including seeing, it's far from simple and IMO seeing not quite a true variable it's more Gaussian. Different median and mean but still there are point sources and gradients possible in planetary. So again if you compare the x to the y, you could introduce step functions and boundry layer conditions at each wavelength.

In answering in the refractor forum about this I tried to state that within reason the 11" scope wouldn't be terrible compared to a 6" APO under whatever the viewing normal is. Obvious planet, obvious banding or rings, obvious moons... Here I also want to inject that all planets are not equal. Mars, Jupiter, Saturn each have a certain set of IMO what makes them interesting. And I feel for the price a C11 can bring a lot of meaningful enjoyment to the table. With 11" aperture the planets are bright! I personally don't enjoy low mag as they seem to be burning my very brain. Increasing magnification increases my ability to enjoy them proportionally... but will the sky hold that magnification and exit pupil in a beneficial way?

My own experience was that far more often the refractor was/is much sharper and crisper across all the seeing variability where I live 'at pleasure point x' for my eyes. Because of that I do feel I am seeing more because I am seeing it better. Rings, moons... transits and shadow transits, banding, gradient shading, subtle colors and transitions between them. That improvement came at what to some including me a huge increase in $. So as I commented in the other forum it is a very expensive opinion to arrive at, and I don't expect there would be a lot of them with long period exposure. This small improvement was true using my volcano-top orthos, Pentax XW, ZAOII, Brandon, Meade Plossl, TV Naglers - so the scope performance was eyepiece independent. But then I used an entire different set of ep's for the ~6" refractor than I did for the 11" Cat. That means different exit pupils.

I tend to put together something I enjoy and stick with it until I decide to move in another direction. Sticking to planets alone, I think folks should be overjoyed a relatively easy to obtain and mount C11 could compete with a hand figured 6" apo that is also $ to mount properly.

Tried to keep this as narrow as possible and only about the planets. My own experience with my seeing and my TEC apo160ed and my Celestron C11, sold some time ago. You don't need to wander to far afield into nebula, planetary nebula, globulars, doubles, ..? for other designs that lend themselvs to aperture increase and remain relatively affordable to truly strut their stuff all over my 160 refractor.

So in summary I'd just say that the 6" refractor (again at at a HUGE price differential)gives me a sharper / crisper view rendering detail, color, and color gradients more obvious of the larger (to us here) planets far more often than the C11, but the C11 was a fine performer in it's own right when I could turn down the brightness with magnification and the seeing would allow.

[rmollise]

So in summary I'd just say that the 6" refractor (again at at a HUGE price differential)gives me a sharper / crisper view rendering detail, color, and color gradients more obvious of the larger (to us here) planets far more often than the C11, but the C11 was a fine performer in it's own right when I could turn down the brightness with magnification and the seeing would allow.

I don't think anybody would deny your scope can produce very attractive images. Show more detail than a C11? No.

[Sarkikos]

Uncle Rod,

Not my experience at all. A wonderful six inch refractor is still a six inch and simply cannot defeat five extra inches of aperture. If nothing else, the way superior light gathering power means you will see more on the planets--light is NOT just important on the deep sky.

A 6-inch refractor can produce a very aesthetically pleasing image, especially under poor seeing. Show as much detail? Uh-uh. Nosir buddy.

I've viewed Jupiter through an 8" f/15 Alvan Clark refractor and was not impressed by the detail. Of course, it wasn't an APO ... but f/15! I could see just as much through my 8" f/6 Newt and much more through my 10" f/4.8 Newt. The seeing was good, 4/5, during the evening with the 8" refractor.

I just don't buy the buzz about 6" refractors - even APOs - being a superior planetary scope in comparison to a good Newt or Cat of larger aperture.

FWIW, my 70mm f/12.9 achro shows a very pleasing image of Jupiter and the Moon, but for fine detail, I'll take out the 6" Mak or a bigger Newt.

Mike

[Mark Costello]

This thread and the related one on the Refactors forum is of great interest in me. I have no overriding concern as to whether an APOCHROMAT 6" is as good as a C11 on planets since I'll almost certainly never get an apochromat. My concern revolves around my 5" ACHROMAT and what would be the best complimentary telescope for it. My personal opinion is that a good complementary telescope for a refractor would be a long focus Cassegrain telescope like the SCT or MCT since the eyepieces for both are in the same position, allowing them to be mounted together and that a SCT or MCT would have less overlap with a refractor than, say a Newtonian of the same objective size as the Schmidt Cassegrain telescope (SCT)or Maksutov Cassegrain telescope (MCT). My concern goes like this:'

1) Due to my age (61) and physical capacity (fair to mediocre), I'll probably limit the size to 10" for the SCT (more like 8" or 9") or 7" or maybe 8" for the MCT.

2) The proposition discussed here and on earlier like threads is that the central obstruction (CO) limits the performance of a SCT or MCT to a refractor of diameter equal to that of the SCT/MCT minus the diameter of the CO - or worse since we're discussing a 6" apo being equivalent to a 11" SCT on planets.

3) My achro puts up good - not excellent due to the chromatic aberration - but good images of Jupiter and Saturn at 206X and very good to excellent images of the lunar features at 206X.

4) I want the complimentary telescope to do BETTER on the planets at least on good steady nights.

5) There is the issue of cool-down for SCTs and MCTs.

At some point when I get close to actually getting a complimentary telescope, I'll probably go to a star party or other outreach of our local astronomy group and beg for a good long look at Jupiter from a 8" - 10" SCT or 7" - 8" MCT.

About 10 years a thread like this appeared on Science Amateur Asro. It featured a planetary shoot-out between a 7" apo and 10" MCT conducted by the optician who made both excellent telescopes. His report was that the apo won the shoot-out at his less than ideal location. There was a long squabble about this.....

[Mitchell Duke]

There is only one way to truely solve this on going issue. Someone with a 6" or 7", and a 11" or 14" SCT needs to image Jupiter on the same night. This side by side comparison will show which scope works best. If anyone has not noticed yet look at the solar system imaging forum. There has to be some reason why everyone is imaging with a Newtonian, or a SCT. Sorry guys sometimes the truth hurts

[simpleisbetter]

Very nice suggestion Duke. And if one were to take that project on, and could do it side by side on the same mount simultaneously, they would be viewing through exactly the same air column, with the same ancillary equipment and at the same time, which would remove all doubt about variances entering in. No excuses or doubt would remain.

[Mitchell Duke]

Thanks Simple