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#101 lightfever

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Posted 12 December 2011 - 01:04 PM

You can disagree if you like, but the contrast transfer of a C11 is only about as good as a 6" APO.

Angular Resolution and Contrast transfer are two totally different things and the central obstruction of the SCT, and usually slightly less than excellent optical quality mean that it can only really match a 6" APO in planetary performance.


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. ;)


Agree!

#102 Eddgie

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Posted 12 December 2011 - 02:18 PM

Oh, we agree that the 11 can best a smaller scope.

The only bone of contention how much smaller.

My own experience was that when used visually on planets, my C11 performed about as well as a high quality 6" APO.

Dozens and dozens of hours of direct comparison. Good seeing, and well cooled C11.

VERY difficult to see more detail visuallly in the Cll than in a high quality 6" APO.

Both to me are entrly level planetary scopes. Far better than a 4" APO because of the superrior contrast they offer as opposed to even a perfect 4" apereture, but not up to what one can do with a good 10" f/6 Newt.

What telescope did you own to do the comparisons if I may ask?

#103 George Methvin

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Posted 12 December 2011 - 02:26 PM

Best view I have every seen of Saturn was through a 8 inch Refracter best view of Jupiter was through a old 1970 C-8. That was over 35 years ago and to this day I have not look through a scope that has bested those views. Those were very standy clear nights. The older I get the more my vision fads but my memoirs of the past get clearer...Thank you Jack Daniel.....LOL

#104 rmollise

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Posted 12 December 2011 - 02:27 PM

Oh, we agree that the 11 can best a smaller scope.

The only bone of contention how much smaller.

My own experience was that when used visually on planets, my C11 performed about as well as a high quality 6" APO.

Dozens and dozens of hours of direct comparison. Good seeing, and well cooled C11.

VERY difficult to see more detail visuallly in the Cll than in a high quality 6" APO.

Both to me are entrly level planetary scopes. Far better than a 4" APO because of the superrior contrast they offer as opposed to even a perfect 4" apereture, but not up to what one can do with a good 10" f/6 Newt.

What telescope did you own to do the comparisons if I may ask?


Appropos this, a 155mm AP. ;)

#105 Eddgie

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Posted 12 December 2011 - 02:34 PM

And was it yours?

If not, did the owner also make the comparisons?

Who was that, and what was the owner's opinion?

#106 esldude

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Posted 12 December 2011 - 03:22 PM

Not very experienced with too many scopes other than small and affordable models. Mostly long achro's at that. Seems to me a good 6 " APO costs lots more money. A better question in my mind is what size refractor scope equals say a C11 at the same approximate price of a C11? Does any refracting scope of the same price equal it for planetary viewing?

#107 TG

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Posted 12 December 2011 - 03:31 PM

The eye simply responds better to low contrast detail when it is MUCH larger than the one arc minute commonly quoted and being at the LIMIT of human vision.


I don't think it's true. Counterexample: Jupiter. It's always much easier to make out features at lower magnifications. Pumping up power only makes it more difficult to see e.g., ovals or festoons.

Tanveer.

#108 orion61

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Posted 12 December 2011 - 03:45 PM

This is an interesting thread. When I get my C8 off the ground, it will be interesting to compare planetary views with those from my my 5" Mak and 5' refractor. Of the two 5"ers, it's hard, from recent memories, to put much distance between them on Jupiter or Saturn if one ignores the CA from the refractor (which I'm happy to do). The C8 will be interesting, though my real motivation to aquire it was to have increased light gathering for feinter stuff in the sky.

Cheers,

Ed

I agree with Rod I have owned both and as long as the seeing hold out my 11 outperforms the 6.
This is why I no longer have the 6,
and have even moved up to the LX200 Meade, but if you have an underperforming CAT it will look like *BLEEP*.
I have used an off axis mask on a 12.5 and found that to be as rewarding as the refractor.

#109 Paul G

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Posted 12 December 2011 - 04:55 PM

When you bring price into the mix neither the SCT nor the refractor can compete with a dob.

#110 Ed Holland

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Posted 12 December 2011 - 05:06 PM

This is an interesting thread. When I get my C8 off the ground, it will be interesting to compare planetary views with those from my my 5" Mak and 5' refractor. Of the two 5"ers, it's hard, from recent memories, to put much distance between them on Jupiter or Saturn if one ignores the CA from the refractor (which

I'm happy to do). The C8 will be interesting, though my real motivation to aquire it was to have increased light gathering for feinter stuff in the sky.

Cheers,

Ed

I agree with Rod I have owned both and as long as the seeing hold out my 11 outperforms the 6.
This is why I no longer have the 6,
and have even moved up to the LX200 Meade, but if you have an underperforming CAT it will look like *BLEEP*.
I have used an off axis mask on a 12.5 and found that to be as rewarding as the refractor.



Thanks for quoting me. For those interested, it was a typing slip (only just noticed) I do not own a 5' refractor :o. It would be a bit unwieldy, and the CA objectionable on planets :lol:

That the thread turns to the interaction of human visual perception with the differing physical performance of telescope sizes and designs is very good food for thought. So often I've been working on a house or DIY project as the light fades, not realising just how much visual acuity has slipped away because of the gradual shift in lighting conditions. Then I put the lights on and all the little details are rendered visible again.

Interesting thread...

#111 rmollise

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Posted 12 December 2011 - 05:21 PM

And was it yours?

If not, did the owner also make the comparisons?

Who was that, and what was the owner's opinion?


No, it wasn't mine. The owner also agreed more detail (Jupiter) was visible in the C11.

Please keep in mind that our seeing is OFTEN rock solid. In an area that does not have good seeing, a small scope will often present an aesthetically more pleasing image. I will say that if I continue to observe with the larger scope, I'll generally have seen more by the end of the evening...just not all at once. ;)

#112 MrJones

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Posted 12 December 2011 - 05:27 PM

Kodak and many other imaging companies have used SQF rather than straight MTF since the 70's for evaluation of optical systems. SQF combines human eye contrast sensitivity with MTF. An overview: http://www.imatest.com/docs/sqf/

#113 Paul G

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Posted 12 December 2011 - 05:31 PM

Kodak and many other imaging companies have used SQF rather than straight MTF since the 70's for evaluation of optical systems. SQF combines human eye contrast sensitivity with MTF. An overview: http://www.imatest.com/docs/sqf/


Interesting!

#114 BKBrown

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Posted 12 December 2011 - 05:44 PM

There is another affect that is never mentioned in these posts, or rather rarely mentioned, because I myself do mention it from time to time.

There is the contrast sensitivity of the human eye.
This is a very distinct profile of contrast sensitivity to the human eye that cannot be ignored...


Thank you for explaining this Eddgie...again :grin: I have tried to make this point from my perspective as a planetary imager, generally fruitlessly, because I think few people really understand the concept. Some just keep coming back relentlessly saying that they can see these details (that only a camera can capture), it just takes practice. Thanks again for the effort!

Clear Skies,
Brian :waytogo:

#115 barasits

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Posted 12 December 2011 - 06:48 PM

There is another affect that is never mentioned in these posts, or rather rarely mentioned, because I myself do mention it from time to time.

There is the contrast sensitivity of the human eye.
This is a very distinct profile of contrast sensitivity to the human eye that cannot be ignored...


Thank you for explaining this Eddgie...again :grin: I have tried to make this point from my perspective as a planetary imager, generally fruitlessly, because I think few people really understand the concept. Some just keep coming back relentlessly saying that they can see these details (that only a camera can capture), it just takes practice. Thanks again for the effort!

Clear Skies,
Brian :waytogo:


Comments frequently refer to "the human eye" as if there were no variability in human visual performance. There is, in fact, substantial variation, and I've met people who have truly amazing eyesight (alas, it changes with age), so I caution against being too quick to discount an observer's claim.

Geoff

#116 BKBrown

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Posted 12 December 2011 - 07:20 PM

I understand Geoff, and agree that some folks have remarkable eyesight...I have known many of them, particularly during my time in the military. My point is that some things are beyond the threshold of detection by the human eye, any human eye, and no amount of experience will reveal them. My hat's off to that cadre of super experienced observers who really know how to make the most of an observing opportunity...but humans have limits, as do machines.

Clear Skies,
Brian

#117 mistyridge

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Posted 12 December 2011 - 07:44 PM

Yes indeed the human eye does change as we age. I use to have 20/20(corrected) now as I pass 70 it is now 20/30. I was at a public astro set up a while ago. I had my scope (a 5" APO) set up to look at Saturn I could clearly see Titan, but a young lady in her late teens or early 20s said look at all those moons....I could not see them. So the human eye is quite a variable when you compare scopes in a planetary shoot out.

#118 Deep13

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Posted 12 December 2011 - 08:04 PM

Seeing does not "limit" SCTs any more than other scopes.


I assume you meant for identical aperture scopes.

However, that has not been the case, in my experience. The AP Mak Cass seems to deal with seeing better than 11" SCTs. That is most likely due to how the AP handles the thermal equilibration issue, and thus the atmospheric turbulence adjacent to the primary and the baffle tube (i.e., highly local seeing).

IIRC, Roland Christen had suggested that OTAs with better optics can handle seeing better than those with lesser quality optics. Unfortunately, I don't recall the points of his argument.


No, primaries on Dobs don't usually dew over. If they do, it is because they are picking it up from the ground. Maybe my remark was misleading because I used the word "dew," so sorry about that. If the ground is wet, like pretty much any time before August, the primary will get wet too. The mirror box is open on the bottom and very close to the turf.

My 8" f/6 Discovery (Terry O. mirror) is a nice scope and once cooled it delivers more detail than my 5" f/12 refractor. (As I noted in my OP, the thing is gigantic for its aperture and somewhat redundant since I have a TV101 now.) My only real complaint is that its aperture limits its ability to find details on Mars or the possibility of details on Uranus. I have a dew chaser on my secondary and am resigned to getting one for a CAT if I go that direction. The solid tube 8" doesn't need one.

I know that all else being equal a good newt will outperform a similar size SCT and that a good refractor of that size will outperform any of them. The thing is usability. I can put a fork mounted SCT on wheeled thing in the garage and roll it out ready to go. I can't do that for an EQ-platform truss dob. It would be too shaky and the position of the base of the EQ platform is too critical. Plus, as I said, mine stopped working. And there's the need to sit on the ground to view Sagittarius and the inability to see over the neighbor's hedges if the planet is low in the sky, which it often is.

I know I did not mention this before, but I am also looking down the line to the day I can build a back yard observatory, and the short, tripod mounted SCT is just more observatory friendly than a Dob.

#119 Deep13

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Posted 12 December 2011 - 08:09 PM

Its more to do with seeing conditions provided the optics are good enough and the scope is VERY WELL colimated.


Is there a way to make sure the optics are very well collimated? I know the secondaries have adjustment screws, but what about the primaries? Don't they move when focusing? How good can that be for collimation? I'm a bit of a novice on CATS, though I've spend years practicing collimation on Dobs.

#120 Deep13

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Posted 12 December 2011 - 08:15 PM

If, as in most backyards, the seeing is 1 arc second then both scopes have effectively been limited by the seeing to the resolving power of a 5-inch scope. If we were to compare a 5” ¼ wave SCT with a 1/10 wave 5” Apo would anyone doubt the Apo would produce a better image with more planetary detail? This is why A 6-inch Apo can out-perform an 11-inch SCT under most seeing conditions. And even if the seeing gets to be really great at .5 arc seconds - allowing the SCT to theoretically out-resolve the 6” Apo, you would still have to contend with the SCT’s obstruction, and lesser optical quality. This is why people that own both an 11” SCT and a 6” Apo say that the 2 scopes are close in rare excellent seeing but the Apo delivers better planetary detail and sharpness in good and average seeing.


That seems to argue in favor of forgetting this whole thing and keeping my 5" refractor.

#121 Deep13

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Posted 12 December 2011 - 08:18 PM

Oh, we agree that the 11 can best a smaller scope.

The only bone of contention how much smaller.

My own experience was that when used visually on planets, my C11 performed about as well as a high quality 6" APO.


Well, I haven't got a 6" apo, nor am I likely to get one anytime soon. I have a 4" apo, a 5" f/12 achro, an 8" Newt/Dob and a 12.5" Dob with no tracking.

#122 Deep13

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Posted 12 December 2011 - 08:22 PM

When you bring price into the mix neither the SCT nor the refractor can compete with a dob.


True, but that is partly because Dobs do not have separate mounts. Once the big Dob is servocat-enabled or the small one put one a suitable GEM, the price difference while still present is not as great.

#123 Deep13

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Posted 12 December 2011 - 08:24 PM

I think all this talk about the limits of the human eye is off-topic. There is no way to improve ones own eyeballs, so it is not a relevant consideration.

#124 JJK

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Posted 12 December 2011 - 10:51 PM

This is a great discussion.

Bang for your buck, you can't beat a good C11 or 12" Meade SCT.

That said - There obviously is a reason people pay $8,000 for some refractors.

My thoughts? There are also other planets besides Jupiter for planetary observing. Take a look at Neptune through that 6" refractor, then look at it through the 11" or 10" sct. No comparison - 11" beats 6" hands down.

After that, go look at any deep space object -

Light gathering is not D - d, it is the total inches of unobstructed glass.

Total size in inches isn't diameter, it is 3.14 x (Radius x Radius)


It's more than just the area (based on subtracting the obscured by the CO). You need to factor in losses due to: scattering off two rough surfaces (SCT primary and secondary) vs. smooth (high end apo) optics, and coatings (which used to be worse on SCTs, but I haven't checked them lately).

#125 JJK

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Posted 12 December 2011 - 11:06 PM

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


How did you calculate the resolving power of an 11" SCT?


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