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SCT's don't make good planetary scopes.

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#76 Ken Hutchinson

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Posted 14 March 2008 - 10:20 AM

Strehl is independent of aperture. It's the other way around -- the "adjusted Strehl" that removes the effects of the central obstruction is misleading. One can remove the central obstruction from the Strehl calculation, but pretending the co doesn't exist doesn't eliminate its effects on the image. The 35% obstructed scope still has a reduction of brightness of the airy disc from 84% to 64% with a resulting increase in brightness of the first and subsequent diffraction rings. There should be two terms: "True Strehl" where 1.0 puts 84% of the light in the central airy disc and can be used to compare scopes of any design, and "Adjusted Strehl" that removes the effects of the co and is really only valuable in comparing one scope with another of the same size co.


Since you seem to be contradicting yourself in this paragraph I don't quite know how to respond. As it is usually defined in professional optical engineering texts the Strehl ratio is independent of both aperture and central obstruction. It is intended to tell you how close your design approaches the limit set by your aperture and central obstruction. It is also intended to tell you how close an actual telescope that you have made approaches that limit. When we finally decide on a telescope and go into the store to buy one what we really want to know is does the one I bought live up to its full potential and if not how close does it come? Only the pure, unadulterated Strehl ratio can tell you that.

In essence the Strehl ratio is like a dog show judge. You ought to be able to judge a group of Beagles and determine which one of them is the best but how can you possibly fairly compare a Greyhound to an Australian Shepherd? The only way to do this is to set a standard for each breed and judge each dog on how close it comes to its own breed's standard, which is what the Strehl ratio does for telescope designs. The overall winner is, in theory, the dog that comes closest to its breed's standard. In practice it is all too often one of the three sizes of Poodle. The same thing happens in astronomy, even though the Strehl ratio gives you an objective measure that does not depend even slightly on the preferences of a human judge the conclusion all too often is that the refractor is best, no matter how large or small. And that is because unlike a dog show judge the Strehl ratio is incapable of comparing an SCT to a refractor.

The correct use of the Strehl ratio is to tell you if this SCT is the best SCT it could be, is this Newtonian the best Newtonian it could be, and so on. The question of which dog to buy is another matter entirely. Buying a Beagle because one won the Westminster Dog Show this year is a silly thing to do if you want to herd sheep or win at the local dog track. Yes, central obstruction is always an important factor and yes, image quality is always improved by choosing the smallest one available. The question you have to ask yourself is whether or not concentrating on one factor alone will get you the telescope that will deliver the most value to you? If you are exclusively a planetary observer in an area with excellent seeing then central obstruction should have enormous weight in your telescope purchase decisions. Otherwise there are many, many factors that go into deciding on the telescope that will deliver the most value to you.

I could not agree with you less on the subject of open and globular clusters. If it fits in the FOV any star cluster gets better and better as the aperture increases. M13 looks like a pale ghost of itself in a 150 mm anything. In a C11 and a C14 it looks truly grand. Someday I am going to have to get an 18 inch Obsession. Yeah, my Megrez 110 gives me a nicely framed view of the Double Cluster that I simply cannot get in either of my SCTs. The trouble is that in the small aperture they barely look like clusters, more like two chance groupings of field stars. In either SCT I can see only one cluster at a time or a view that includes part of both. In the SCT they actually look like star clusters however. I buck the trend of SCT owners who are enthralled with their 80 mm or even 66 mm piggyback scopes. In my opinion they give you a wide angle view of nothing because their light grasp is so much less than the main instrument. At 110 mm I am getting barely enough depth to say that the hassle of dealing with the piggyback is paid back by the view through the eyepiece. Relatively few star clusters have members that are bright enough to show diffraction rings at all and are largely composed of dim members that are as pinpoint in appearance as in any refractor. Star clusters look just glorious in a large SCT.

I can remember reading what appeared to be the first discussion of central obstruction in the pages of Sky and Telescope years and years ago. The purpose of the article was not to convince amateur astronomers that central obstructions were the spawn of the devil. Rather it was to point out to Newtonian owners, who have a relatively free choice in the matter, that the vignetting produced by a small secondary was relatively minor compared to the contrast gain that it would provide for planetary work. A pretty modest and sound engineering proposal. From that beginning has grown a movement that lately threatens to become an all out war on central obstructions. The leaders of this movement would have us SCT owners believe we should be like the children of the 1950's who were afraid of the Communists hiding under their beds. Instead, I would say that everyone who wants to make an informed telescope purchase should be aware of the tradeoffs. In return for a larger central obstruction the SCT gives you a telescope that is remarkably compact and can usually be had on a tracking or goto mount for less money than any equivalent telescope. It will not excel at anything, yet it will be able to do everything you ask of it. It is not the ideal telescope for everyone. It might be the ideal telescope for you. If you want to know what a telescope's central obstruction will do to you, study MTF curves. If you want to know what its aperture will do for you correctly scaled MTF curves and standard light grasp equations will tell you that. If you want to know how closely it approaches the performance the MTF promises the Strehl ratio will tell you.

Ken

#77 Eddgie

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Posted 14 March 2008 - 10:36 AM

This just about EXACTLY matched my side by side comparison beteen my Meade 152ED and a NexStar 11 a few years ago.

On nights of really excellent seeing, the NexStar 11 was able to VERY VERY SLIGHTY edge out the 6" ED scope. Now I KNOW that the C11 didn't have perfect optics, though I thought that they were quite good. The ED scope I don't think had PERFECT optics, but I think that they were close to being perfect than the SCTs were.

The MTF suggests that the SCT should have had a slight edge, and many many nights of side by side testing makes me agree. But I had to be VERY pateint to see the difference. On many nights, the 6" ED scope seemed to be performing as well or even slighly BETTER than the 11" SCT. But if I was REALLY patient, sometimes, I could glimps detail in the 11 that was just not quite showing up in the 6".

But I will say this. The detail that often DID pop out was NOT faint low contrast detail. IN almost ever case, when I spotted something in the C11 that was not showing in the 6", it was often HIGH CONTRAST detail. For example, every now and then, I would spot a white oval in the SCT that didn't quite pop out in the 6".

I atrributed this more to the RESOLVING power of the big SCT, and not to any contrast advantage. I always felt that for the really subtle shadings, the refractor was perhaps a bit better.

So, it is complex to make the comparison.

But in the end, I still detected more detail in the SCT.

Same with my C8 vs the 4" APO I used to own, though in that case, I think that the difference was a bit more apparent than with the 6" vs the 11". My C8 has really excellent optics, and there weren't any nights were I had to struggle to see more detail in the 8" SCT. The 8" handled seeing better and just about any time I pointed it at Jupiter or Saturn, I simply felt that there was no contest. The C8 made it easier to see detail, and almost never failed to provide slighly more detail than the 4" refractor.

Now considering that I paid $375 for the C8, and I paid several TIMES that much for the 4" APO, the deal becomes even more lopsided.

But I had a MN61 for a while that easily matched the C8 on Jupiter.

SCTs good.. Newts Better... Refractors best. Size and quality being equal.

Othewise, these comparisons are meaningless.

#78 sixela

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Posted 14 March 2008 - 10:39 AM

[quote name="Rick Woods"]Yeah, right!
Who started that idea, anyway?
[/quote]
In my neck of the woods, usually SCT users too lazy to do anything about their scopes ;). Favourite other opinions of these people: collimation of SCTs is stable and was set correctly at the factory, and cooling isn't that important (or takes just ten minutes).



With my SCT, I'm still following Mars at ~600x, and getting fairly detailed drawings despite the fact that it's barely 8" in diameter. I have a 5mm ortho on the way, so soon we'll see how it does at ~700x. I expect it to perform well.
IMO, any telescope that will provide a sharp view of Mars with good detail and contrast at 600x deserves some respect as a planetary scope. [/quote]

#79 Jon Isaacs

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Posted 14 March 2008 - 11:36 AM

Trying to capture the performance of a complex system in a single number is bound to be an exercise in frustration. Using the total area under the MTF curve would be a better choice than adjusting the Strehl ratios for only the central obstruction. However for planetary observing some sources claim that this low contrast detail that everyone is chasing is mostly in the lower frequencies. If that is true then the refractor truly is as good as the C11 because the frequency range where the C11 beats it doesn't come into play. The problem is that your "system Strehl" predicts a result that is not true.

Ken



I say this:

The adjusted Strehl allows for a better comparison than the non-adjusted Strehl. The Strehl is not appropriate for comparing telescopes of different aperture, you pointed this out and it's reasonable.

However when comparing scopes of identical aperture, the adjusted Strehl is the more appropriate tool because it includes not only the quality of the optics but the limitations inherent in the design. The Strehl of the optics is a measure of how closely the optics meet the design criteria, the adjusted Strehl is a measure of how telescope compares to a perfect telescope of the identical aperture.

So, if one is comparing an 11 inch Newtonian to an 11 inch SCT, the advantages/disadvantages of the design need to be included in that comparison. The Strehl is a complex number but adjusting it for design limitations makes it more robust.

Jon

#80 Steve D.

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Posted 14 March 2008 - 02:01 PM

Can someone explain the MTF chart above or tell me where I can read more info on this topic. Thanks.

#81 Make-it Better

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Posted 14 March 2008 - 02:58 PM

The MTF (Mirage Transformation Formula) chart is unique in that it proves two people on opposite sides of an issue are both correct. Seriously, I like it explained to me as well.

Cheers,

#82 KBP

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Posted 14 March 2008 - 03:38 PM

I had a good friend explain to me that there were three sides to every dispute! There was the one side, the other side, and there was the third side- the truth! So most people skew their facts to prove their preferences. I do know that a few years back I observed comet Shoemaker- Levy hit Jupiter and could easily see two of the black impacts on the surface with my little Celestron Ultima 8 scope! Not to bad for a SCT scope! As noted in other posts, there are many varibles that affect good viewing, imp the type of scope is not one of the major factors for most of us amateurs with small scopes.

#83 Eddgie

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Posted 14 March 2008 - 04:21 PM

The MTF (Modulation Transfer Function) is explained here:
http://www.normankor...orials/MTF.html

The MTF chart is usually shown for a SINGLE scope, with curves that show how the contrast would be damaged by different problems (Spherical Abberation for example) or design parameters (larger or smaller central obstruction).. The line on the right shows how a PERFECT, UNOBSTRUCTED telescope of a given aperture would behave. As you start introducing abberations or obstructions, the line "Sags" which means that difference frequency detail will loose contrast (though some detail might IMPROVE in contrast).

What people don't usually realize is that the MTF can be used to COMPARE two scopes DIRECTLY. The diagram included in the post is doing exactly that. It is showing two different telescopes would perform on contrast.

The CLOSER to the line on the right, the better the contrast would be.

A PERFECT 11 inch refractor would have a line that matches the line on the right most closely,and consequently would provide the best contrast. But a 5.5" perfect refractor would have a line that looks more or less identical, but would fall half way between the 11" and "0" inch. at the bottom of the table.

It is a very useful tool for presenting the relative performance of two telescopes if you know the quality and the design parameters for each one.

So, according to the MTF chart presented, even an 11 SCT with average optics SHOULD out-perform a 6" refractor in terms of contrast. But the 17% obstruced Newtonian would give the BEST contrast.

Hope this is helpful.

#84 Eddgie

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Posted 14 March 2008 - 04:24 PM

My mistake.. Upon looking at the chart again, the line on the right is NOT of a perfect aperture. It is for the perfect 17% obstructed scope. But the curve would be VERY close to the curve for a perfect refractor, which means that it would look like the line for the 6" scope, but just stretched further to the 2.8 line (which is the aperture scale).

#85 Ken Hutchinson

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Posted 14 March 2008 - 04:26 PM

Can someone explain the MTF chart above or tell me where I can read more info on this topic. Thanks.


MTF stands for modulation transfer function. It represents how the contrast of a telescope (or any optical system) varies with the size of the details you are looking at. So in the chart the vertical axis is the contrast ratio, the horizontal axis is "spatial frequency". Small objects or details have a high spatial freqency, large objects or details have a low spatial frequency.

Two popular books that discuss this in some detail are Suiter's book on star testing telescopes and Rutten and van Venrooij's book on telescope optics.

You may be familiar with how audio systems are tested. You inject a sine wave audio oscillator into an amplifier, for example, and vary the frequency of the sine wave from 10 to 40,000 Hz for example. While you do this you keep the input amplitude constant and you measure the amplitude of the output at each frequency. In an audio system you expect to see the output amplitude constant (flat) from around 20 Hz to around 20,000 Hz and then can fall off outside that range. A system that is flat from 300 to 3,000 Hz will reproduce human speech well but will not reproduce music well.

Telescopes can be tested in a similar way. Your input signal in this case is a test chart of bars that vary sinusoidally in darkness from white to some shade of gray (or black). You need many different such charts to do a complete test, the spacing between adjacent bars in the various charts will vary from large (low spatial frequency) to small (high spatial frequency). If you measure the contrast of the image of your test chart and divide it by the contrast of the test chart you have the contrast transfer ratio for that spatial frequency.

Either the light blue or dark blue lines in the chart above show the results expected for an essentially perfect telescope (the Newt with a 17% obstruction does not depart from perfection by very much). Instead of being flat like a good audio system, even perfect telescope decrease in contrast as the frequency increases. Note also that the larger Newt delivers contrast greater than zero at spatial frequencies well above the point where the smaller refractor has fallen to zero contrast. This reflects the higher resolution of the larger aperture.

The perfect C11 (green) curve shows the dreaded effects of the insidious central obstruction. Instead of matching the Newt, which has the same aperture, it falls short of perfection and delivers lower contrast for the low and middle spatial frequencies. In fact if its Strehl ratio is low enough it can barely keep up with the smaller refractor. But at the higher frequencies it equals the performance of the same sized Newt.

Hopefully that helps, my train is coming so it is all I have time for now.

Ken

#86 Jeff Lee

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Posted 14 March 2008 - 07:32 PM

My opinion - "Love the one your with:)"

#87 Ken Hutchinson

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Posted 14 March 2008 - 10:45 PM


I say this:

The adjusted Strehl allows for a better comparison than the non-adjusted Strehl. The Strehl is not appropriate for comparing telescopes of different aperture, you pointed this out and it's reasonable.


To be sure an adjusted Strehl has some validity if it is used to compare telescopes of the same aperture. However, what value is it to a C11 owner? There are no other 11 inch telescopes to compare to. An even more insidious problem is that if you quantify your telescope with a number other people are going to compare your number to theirs, no matter what. Using a number that ignores aperture is pointless, using a number that ignores both aperture and obstruction is very valuable.

If you want a single number that characterizes a telescope's true merit that has more than a very minor utility and which cannot be misinterpreted by those who do not understand what it means then you have to correct for both aperture and central obstruction. You don't have to be able to measure the MTF to do this. You can take a Strehl ratio from a Roddier test (or just assign it a value of 1.00 if you want to compare the theoretical performance of two telescopes) and then adjust that for Strehl using whatever method people use to do this. I can't comment on that because I am not familiar with it although it is obvious that some fair method does exist. Now you still need to correct for aperture. The easiest way to do this is to pick some aperture and arbitrarily assign it a value of 1.00 and then if your telescope is larger or smaller than this standard aperture you multiply your adjusted Strehl by the ratio of your aperture to the standard.

Any aperture could be used as the standard, the resulting number is going to be arbitrary in absolute value but useful for its relative value. An aperture that makes the math easy would be a good choice so since it was a traditional aperture for small refractors let's pick 100 mm for argument's sake. Then if you have an 80 mm telescope you multiply your adjusted Strehl by 0.8 and if you have a 280 mm telescope (a C11) you multiply it by 2.8 . This gives you a figure of merit (FOM) that can be used to compare any telescope to any other telescope with some degree of rigor.

It is important to keep in mind that this technique has limitations. That is the nature of a single number FOM. Two hot rodders are out to pick engines to use against each other in a drag race. They both know that acceleration is driven by torque and driver A picks an engine with a peak torque of 350 ft-lbs, while driver B picks an engine with a peak torque of 330 ft-lbs. Driver A is laughing to himself for days and days, right up to the time on race day when driver B blows by him with 200 yards to go before the finish line. How could this happen? Well, driver A's engine has the higher peak torque at one rpm but its torque falls off sharply on either side of the peak. The engine driver B chose has a lower peak but its torque is nearly flat from 1000 rpm to the red line. Peak torque is not the whole story.

An adjusted Strehl does not necessarily do any better. Look at the curves for the C11 and the 155 mm refractor again. Adjusting the Strehl along the lines I propose will give the C11 a higher FOM. If your observing task excercises the frequency range at the high end it should deliver for you. If instead it uses the low end then the C11 will be no better than the refractor even though its FOM is higher. There is only so much that a single number can tell you about how your telescope will perform.

Ken

#88 Jon Isaacs

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Posted 15 March 2008 - 05:06 AM

To be sure an adjusted Strehl has some validity if it is used to compare telescopes of the same aperture. However, what value is it to a C11 owner? There are no other 11 inch telescopes to compare to.



Ken: Here's my thinking...

The value in any measure of optics is to allow one to make a reasonable comparison as well as to help one understand what one is seeing. If someone is considering buying either an 11 inch SCT or an 11 inch Starmaster with a Carl Zambuto mirror and say 20% CO, the adjusted Strehl tells the story, the Strehl for just the optics does not. If someone has a C-11 and wonders whether a 11 inch Newt or a 12 inch Newt might give better views, the adjusted Strehl gives the better answer.

Here's a few thoughts:

- Whatever faults the adjusted Strehl has for making relative comparions, the Strehl for the optics alone is even worse. All it allows one to do is compare scopes of the same design and the same aperture whereas the adjusted Strehl allows one to compare scopes of all designs and the same aperture.

- It is true that there is only so much a single number can tell. When you combine two numbers, in this case the Adjusted Strehl and the aperture, one should be able to get a good idea of the relative performance.

- I am not the guy who claims the 155mm APO is going to give views comparable to a good 11 inch telescope, you are barking up the wrong tree by discussing that issue, that's really a non-issue. I am the guy who thinks a good quality Newtonian of similar aperture will be a superior planetary scope to an SCT.

- Personally I don't really care about the Strehl values of my optics, I just go out and use 'em. I am highly doubtful that the measured values have any validity for larger scopes simply because of mechanical and thermal issues.

- In the context of this thread, the issue of the whether SCT's make good planetary scopes, well, just the Strehl for the optics says little about the relative performance of the design,with the Aperture and the adjusted Strehl, one can make a reasonable comparison. This is why I brought up the issue in response to your posting the various Strehl's.

- Myself, I think any telescope that someone is using to look at a planet with is a good planetary scope. One can get good planetary views with most any telescope, whether it's a 60mm with a near perfect optic or a 12 inch with some design and manufacturing issues, if someone is looking through it at a planet, it's a good planetary scope. This is a hobby, we are not doing science, getting out an looking is the most important thing.

Jon

#89 payner

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Posted 15 March 2008 - 10:46 AM

I am thrilled with my two scopes and enjoy them both. Most of the time the consistent good viewing I get of planets is with the MCT, simple beautiful, clean, high resolution views. For deep sky the detail I can see in the CGE1400 is marvelous, most exciting to me is the detail I can see in planetary nebulae.


All scopes are a compromise, even the professional instruments. I think Jeff hit it on the head and I am quoting: "My opinion - "Love the one your with:)"

Clear, steady skies to all,
Randy

#90 CHASLX200

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Posted 15 March 2008 - 02:38 PM

I say a Newt will beat out a SCT on the planets. But a SCT can do a very well job on the planets as well, i had a older black C8 on a AP-400 mount and it would do very well on the planets even at 500x. But my 14.5" Starmaster would eat alive my orange C14 anyday of the week.

Chas

#91 serambin

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Posted 15 March 2008 - 03:35 PM

To learn about MTF and the strengths and weaknesses of the various designs (believe me, errors exist in this thread), look here: (but if you want to really read it, allow 3-4 hours)

web page

Thanks,
Stan

#92 Ken Hutchinson

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Posted 15 March 2008 - 09:03 PM

If someone has a C-11 and wonders whether a 11 inch Newt or a 12 inch Newt might give better views, the adjusted Strehl gives the better answer.

Here's a few thoughts:

- Whatever faults the adjusted Strehl has for making relative comparions, the Strehl for the optics alone is even worse.


I did not know that Zambuto was making 11 inch mirrors. If you want to compare a Celestron to a Zambuto the unadjusted Strehl tells you everything you want to know. Generally people who buy Zambuto mirrors are looking for optical perfection. Zambuto claims to make mirrors with Strehls north of 0.95. Celestron claims to make telescopes with Strehls of more than 0.8 and experience says they top out about 0.93. What more do you need to know? If you are looking for a Zambuto you are not going to settle for a Celestron and I think you knew that very well before you even asked the question.

The problem with adjusted Strehls is exactly what I said is was. They are not used to compare telescopes of equal apertures they are used to compare any telescope to any other telescope. Say what you will I see people doing exactly that, all the time. I meet way too many people who think that a 100 mm refractor with an adjusted Strehl of 0.95 (of course there is no adjustment for a refractor) is superior to a C11 with an adjusted Strehl of 0.6 or whatever it comes out to be. Look at the MTF curve, an absolutely perfect 100 mm refractor has less contrast than the 155 mm.

If you want to compare a hand figured Newt to an SCT of equal aperture just use the MTF curves. That tells you everything you need to know and it gives you far more detail than any single number ever could. Study the curve and you can learn a lot. A number reveals no more after an hour of study than it does at a glance. A single number can tell a lie, the full MTF curve cannot. People making same aperture comparisons can find the MTF curves they need in any number of publications, any number of web sites, and they can download free versions of the tools needed to make comparisons to their heart's content.

The thing is they don't even need to bother to do that. This is not breaking news, the results are well known and well reported. At a single wavelength a refractor is king, it is unsurpassed. A Newt with a 20% obstruction or less is so close to it as to be indistinguishable and when you consider chromatic aberrations it has its own claim to the throne. Above 25% obstruction the differences start to become apparent. Now if you want to quantify exactly what a given level obstruction will mean to you an adjusted Strehl will do nothing for you, the MTF curve will, with a lot of effort.

I don't think the point of this thread was to claim that SCTs were the best possible planetary telescope for a given aperture. We know that isn't true. We also know that SCTs give excellent planetary images even if they are not the best possible for a given aperture. The point of this thread was to counter a perception that is driven by many things and this adjusted Strehl nonsense is one factor that is starting to assume major proportions. People are misusing it because it is easy to misuse and it is difficult for most people to understand why it is misleading. If you are going to advocate a single number that people can use to compare telescopes then advocate one that cannot be misused. Adjusted Strehls can and are being misuesd every day.

Ken

#93 Patrick

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Posted 15 March 2008 - 09:08 PM

Before everyone gets too carried away with the 280mm reflector with the 17% CO, it should be pointed out that to get that 17%, the scope will have to be VERY, VERY long, which makes it particularly impractical if not in an observatory. It's easy to talk about such scopes, but quite another thing to bring them to reality for most people, particularly beginners.

I suspect the intent of the original poster was to simply say that he's getting very satisfying views of the planets with his SCT without regard to other instruments being capable of getting better views, and in his mind it is a 'planetary' scope. Who's to say it isn't since he's actually getting good views of the planets with it?

I think Shakespeare said something like that when he wrote about roses didn't he?

Patrick ;)

#94 jgraham

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Posted 15 March 2008 - 09:20 PM

heh, heh, yeah the title of the thread said "good planetary scopes" not "best planetary scopes". Personally, by far the best planetary scopes I've ever looked through were Scheifspeiglers (sp?) built by Dick Wessling and Ed Jones. Next to those was a wonderful (and huge) classic 12.5" f/11 Newtonian, an 8" f/10 Newtonian, and finally my own lowly Meade LXD75 SC8. Is my SC8 the best? No. Is it good? You bet.

Have fun!

#95 Jon Isaacs

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Posted 15 March 2008 - 11:07 PM

The problem with adjusted Strehls is exactly what I said is was. They are not used to compare telescopes of equal apertures they are used to compare any telescope to any other telescope.



- That may be a problem, it is not a mistake I make. The simple fact remains that the adjusted Strehl is a better measure for comparing scopes of similar aperture than the non-adjusted Strehl. The non-adjusted Strehl looks better if the CO is large but does not accurately capture the full picture.

Comparing an 11 inch hand crafted mirror is not a good example of the advantage of the adjusted Strehl, a better use would be for comparing 8, 10, 12 inch commercial SCTs with the respective 8, 10, 12 inch Commercial Newtonians.

- As far as Patrick's suggestion that a Newtonian needs to be long (ie slow focal ratio) to use a small CO, it helps but with a low profile focuser such as the Kineoptics Helical Crayford an 11 inch F/5 Newtonian can still provide a fully illuminated TFoV of greater than half a degree with an 18% CO. With a more standard low profile focuser, one can go to slightly larger 20% CO to retain the greater than half degree fully illuminated FOV.

So, I say this:

To the original question: Yes, SCT's can provide very satisfying planetary views... Take some time and make the effort to collimate and cool the scope, add in some good seeing and there's some good stuff to see.

To the issue of the Strehl: A list of Strehl values says that optics themselves can be reasonable/decent but adjusting that number for CO allows one to make direct comparisons with other designs and addresses the question of how well the scope itself performs in relation to a perfect scope of identical aperture. I believe this is more useful in gauging the relative merit of a telescope.

Jon Isaacs

#96 Rick Woods

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Posted 15 March 2008 - 11:16 PM

Before everyone gets too carried away with the 280mm reflector with the 17% CO, it should be pointed out that to get that 17%, the scope will have to be VERY, VERY long, which makes it particularly impractical if not in an observatory. It's easy to talk about such scopes, but quite another thing to bring them to reality for most people, particularly beginners.

I suspect the intent of the original poster was to simply say that he's getting very satisfying views of the planets with his SCT irregardless as to whether another instrument can get better views or not, and in his mind it is a 'planetary' scope. Who's to say it isn't since he's actually getting good views of the planets with it?

I think Shakespeare said something like that when he wrote about roses didn't he?

Patrick ;)

Thank you, Patrick.
Yes, my intent was to deflate the myth that for some reasons, SCTs aren't good for planetary viewing. They sure are! I'll gladly admit that a 8-10" refractor will probably beat it at almost everything. But that doesn't mean it's not good. It is - very, very good. Eddgie may have hit the nail on the head - you can get such a large SCT that by brute force you get the resolution you need.

But one factor not often considered is that you have to use the right eyepieces. I was using Naglers, S4000 UWA's, and plossls with and without a barlow. WRONG! For planetary viewing, I have finally arrived at a fundamental truth: No barlows, and ultra-high quality EPs with few elements. Less glass. No weak links. This has made my SCT perform at an entirely new, much higher level.

#97 blackhaz

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Posted 16 March 2008 - 03:25 AM

Nice thread. Let me add. :)

After selling C11, past yesterday I just bought a C11 once again. There's just simply nothing out there on the market to compete with C11. The same size Newt is out of question for me - takes too much space, not comfortable viewing position. Smaller refractors would be far behind on DSO, especially globulars. Maks would be too heavy and have a narrower field (if speaking about true Maks with good CO.) I can continue forever. I need something I can setup in few minutes and start observing everything up there that's interesting for me. Yep, I'm loosing detail on planets if compared to premium planetary telescopes, but the size factor of C11 would make it getting used more often. It's a hobby at the end. I am very happy with the C11 design and think about it as the most universal telescopes ever produced.

#98 firestar

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Posted 16 March 2008 - 04:46 AM

Most production SCT's have primaries of 1/4 wave at best (if you are lucky!), probably the same as production Maks but inferior to mass produced Newts.


Yes. If you take the time to look at tests posted on mass production sct's they are a bit troubling. To some.

I wonder, how much would be added to the cost of an sct if the optics were more in line with a Santel or Intes-Micro Mak Cass for example. I'd love a 10" IntesMicro MakCass :grin: But in my case i'm looking at $2200 for the C11 vs. ~ $8,000 for the MakCass.

To get back on topic, that $8,000 10" MakCass is going to outperform any equivalent SCT on planets, or any other target. Superior optics will guarantee that.

But to my eyes the SCT has been a very good design, once you accept it's limitations.


How about comparing your $8000 mak to an equally affordable rcos telescope?
Now we are talking superior optics.

:lol: :winky:

#99 Steve D.

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Posted 16 March 2008 - 08:48 AM

Slightly off topic, is there a MTF chart that compares normally available telescopes. (The scopes used in the chart in this post are outside the ability of many to purchase.) It would be useful to start with a 4 inch refractor, since it is the most common size refractor, compared to the minimum size aperature of other telescope types needed to achieve the same level of performance. Ideally, the chart would use the closest aperatures that are actually produced. So if it is determined that it takes a 5 3/4 inch MCT to match a 4 inch refractor then the chart should include a 6 inch MCT, since the 5 3/4 size isn't avaiable. I think this would be a great tool for new scope purchasers to help them make a decision regarding relative performance of different scopes.

#100 OldDeadOne

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Posted 16 March 2008 - 09:54 AM

I confess since I've had my 10" classic I've never collimated it and have been a little leery doing so,since I've never done it and I don't know what to look for(I have been calling for a video of the process in the past and a webpage with pictures really don't work with me,anyone up for a video of a SCT collimation procedure?),don't have anyone around to help me with that process.


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