Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Cat for planets

This topic has been archived. This means that you cannot reply to this topic.
212 replies to this topic

#1 Deep13

Deep13

    Skylab

  • *****
  • topic starter
  • Posts: 4444
  • Joined: 25 Jan 2005

Posted 08 December 2011 - 08:45 PM

I'm thinking of replacing my 12.5" f/5 Discovery TD with a Cat. There are a lot of reasons including my frustration with the intermittent-functioning Roundtable tracking platform. Part of the reason for getting the 12.5" was to get good, high magnification views of planets. No tracking = no planets. I'd also like a more convenient eyepiece location.

I'm also thinking of parting with a 5" f/12 custom achro w/ D&G lenses because I also have a TV101 and the 5" is huge for its aperture.

I'm thinking of a C11. How are they on planets? How is the optical quality? How much light grab does it really lose to a 12.5" Newt? How much of a difference does that big secondary really make? I use the 12.5" for DSOs too, but I hardly ever get out to a dark site anymore.

Would I be better of with one of those Synta 7" MCTs? It's smaller, but I wonder if the smaller CO would give it better contrast and detail. Only thing is, I already have a good 8" f/6 newt that I can mount on my EQ6.

Thanks. I'm keeping the 8" and the TV101.

#2 Ed Wiley

Ed Wiley

    Surveyor 1

  • *****
  • Posts: 1988
  • Joined: 18 May 2005

Posted 08 December 2011 - 09:07 PM

Really want to go all out? Look at the Royce DK. F22.5, no compromises, easy collimation, easy cool-down, 25% secondary; mine is as close to an 8" refractor as anything around. DKs have a narrow useable field you say? Sure, but so what? Its for planets and double stars (and PNs, and small DSOs and...). But..you pay a premium price for a premium scope with premium optics and truly solid design.

Clear skies, Ed

#3 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 24979
  • Joined: 01 Feb 2006

Posted 09 December 2011 - 09:15 AM

A C11 should give performance on par with a 6" refractor, though you will have to be mindful of the cooling requirments to get the most out of it.

The problem of course is that a 6" APO to me personally doesn't give the best planetary performance you can get by a longshot.

The problem with a bigger MCT is that they get very heavy and very expensive.



If your current 8" f/6 scope can be mounted on a ground hugging GEM, I would get some rotating rings and use your Atlas mount. Put shorter legs on the Atlas, slap the rings on a solid tube, and you are going to wind up with as good a plantery scope than a C11.

You can make some stubby legs out or 2x4. Cut them JUST long enough for the counterweights or the end of the tube to clear the ground. It is actually possible to get the OTA LOWER to the ground on a GEM than on a Dob mount because you don't have the ground board and box clearence to worry about.



#4 dscarpa

dscarpa

    Skylab

  • *****
  • Posts: 4022
  • Joined: 15 Mar 2008

Posted 09 December 2011 - 11:12 AM

I prefer my IM715D to my C9.25 for lunar-planetary far more often than not. The C9.25 equals the mak with very good seeing and shows more with excellent. The problem is the SCT is too seeing sensitive. With good seeing the mak shows a sharp image most of the time while with the SCT I have to wait for the moments when the air is steady. With fair and below conditions the mak is much better. If the SCT is more seeing sensitive due to a bigger CO and or all that extra light to blur I can't say. David

#5 Patrick

Patrick

    Voyager 1

  • *****
  • Posts: 12155
  • Joined: 15 May 2003

Posted 09 December 2011 - 11:41 AM

I'm thinking of a C11. How are they on planets?



I have to respectfully disagree that a C11 is only equal to a 6" refractor (this IS the CAT forum isn't it? :grin: ). The resolution of a C11 is equal to a 11" aperture, and the contrast is approximately equal to that of a 7" unobstructed scope. Also in the C11's favor is the fact that extremely short focal length eyepieces or barlows are not required to boost magnification.

As far as cooling the scope, a Lymax cat cooler will quickly bring the scope down to ambient temps.

Comparing a 12.5" f/5 Newt with a C11 is probably going to be a step down optically, but a step up ergonomically. Of course the 12.5" Newt will gather more light and be slightly better on planets. Cool down will still be an issue with the Newt. Ergonomically, I have to ask the question...which C11 variant are we talking about here? I've found that the fork mounted C11 (ie CPC1100) is one of the most user friendly setups out there. Yes, you can mount a C11 on an EQ6, but you're still dealing with an GEM mount and there are not as ergonomic as a fork mount. Fork mounts are just plain easy and fun to use. The hardest part is mounting them on the tripod, but the CPC1100 is very easy to handle, IMO (and I have a bad back).

Patrick

#6 rmollise

rmollise

    ISS

  • *****
  • Posts: 22879
  • Joined: 06 Jul 2007

Posted 09 December 2011 - 12:27 PM

I have to respectfully disagree that a C11 is only equal to a 6" refractor (this IS the CAT forum isn't it? :grin: ). The resolution of a C11 is equal to a 11" aperture


:goodjob:

#7 Mirzam

Mirzam

    Aurora

  • *****
  • Posts: 4801
  • Joined: 01 Apr 2008

Posted 09 December 2011 - 01:07 PM

Why not get a better tracking platform for the newt? This sounds like a case of "grass is always greener" to me. A properly functioning 12.5 inch newt will be superior to the C-11 wrt. resolution, contrast, and cool down. :penny: :penny:

JimC

#8 Arizona-Ken

Arizona-Ken

    Surveyor 1

  • *****
  • Posts: 1875
  • Joined: 31 Aug 2008

Posted 09 December 2011 - 01:15 PM

I have to respectfully disagree that a C11 is only equal to a 6" refractor (this IS the CAT forum isn't it? :grin: ). The resolution of a C11 is equal to a 11" aperture


:goodjob:



:goodjob:


Arizona Ken

#9 ahlberto

ahlberto

    Apollo

  • *****
  • Posts: 1350
  • Joined: 06 Sep 2008

Posted 09 December 2011 - 01:38 PM

I have to respectfully disagree that a C11 is only equal to a 6" refractor (this IS the CAT forum isn't it? :grin: ). The resolution of a C11 is equal to a 11" aperture


:goodjob:



:goodjob:


Arizona Ken

Indeed :bow: Mi C8 at least is a well respected contender in the batlefield and rarely a scope of equivalent aperture beat him in planets and deep space and it deals in most nigth with mi 6.7mm meade sp with ease with tack sharp image and thats says allot...Mi SW Newton 8" f4.9 does equaly very,very well on all targets,-(( providing of course its well collimated...Hate newton scopes...just hate them...because i am very picky with collimation and somethimes i spent hours in that thing...but..i love them too,-its a love and hate relation and wen its all in place it delivers so brutal mags on planets thats it almost comic)))
:p :p :p

#10 Al Miller

Al Miller

    Apollo

  • *****
  • Posts: 1314
  • Joined: 25 Aug 2008

Posted 09 December 2011 - 02:16 PM

I have to respectfully disagree that a C11 is only equal to a 6" refractor (this IS the CAT forum isn't it? ). The resolution of a C11 is equal to a 11" aperture, and the contrast is approximately equal to that of a 7" unobstructed scope.



:applause:

Thank you Patrick, you took the words right out of my mouth.

#11 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 24979
  • Joined: 01 Feb 2006

Posted 09 December 2011 - 02:17 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.

Having owed both, I feel that my own experience matches the physics rather perfectly.

Here is an MTF plot that compares the contrast performance of a 6 inch APO to a C11 type aperture.

I have graced the C11 with only 1/8t wave of spherical abberation, though it is hard to find one this perfect in serial production.

I have ignored the outer zone that appears frequently in C11s and C14s, and I have ignored the slight surface rougness that also appears in these scopes.

In other words, you will NEVER see a C11 as good as the one dipicted here. The Strehl would have to be .98 to be this good, and this is almost unheard of in commercial SCTs.

The 6" APO is represented as perfect because in the realm of 6" APOs, most are near perfect.

The MTF chart shows that it the low and mid frequenceis, the 6" APETURE has the same contrast as the 11" obstructed scope.

Seeing usually causes the high frequency deatail (the smallest detail and the detail to the right of the MTF plots) to be lost to the visual observer anyway.

Again, I have owned two 6" refractors... A Meade 152ED and an Astro-Physics 6" APO. I also owned a C11 at the same time as the Meade 152 and did numerous side by side comparisons.

In direct comparison to the Meade 152ED, I had to struggle hard to see detail in the C11 that was not visible in the 6" Meade 152ED even when seeing was very good.

And the Meade was NOT as good as my current 6" APO. I would say that it was decent optically, but just not quite as good as the Astro-Physics 6".

So, my own experience is that a C11 does not make a BETTER planetary scope than high quality 6" APO. As good? If condistions are favorable, yes.

If the C11 were PERFECT, I think it could actually slightly outperform a good 6" APO, but no such C11 exists to my knowledge.

I am eager of course to hear of your own comparisons with these scopes. What 6" APO did you use? What was your assessment of the optical quality of the 6" APO that you used and the C11 that you used?

If you can provide the assesments, I will work up an MTF chart based on them and we can compare it to the one here.

Attached Thumbnails

  • 4958325-MFT C11 vs 6 inch APO.jpg


#12 tim53

tim53

    Voyager 1

  • *****
  • Posts: 14483
  • Joined: 17 Dec 2004

Posted 09 December 2011 - 02:18 PM

A good classical Cass or DK would be my first choice. But a 12.5" would probably overwhelm an Atlas mount. And there are few commercially available Casses.

C-11's are plentiful, which makes it relatively easy to find one for a good price with good optics.

You can overcome a lot of the shortcomings of the closed tube for planets (like using lymax coolers and dew heaters,... ...which sounds Rube Goldberg because it is), as can be seen by perusing the solar system imaging forum.

SCTs are good planetary scopes. There, I said it.

(...but Newts and Casses are better! ...there, I said THAT, too! :grin: :tonofbricks:)

...but it is the cats and CASSES forum, doncha know, or doncha? ;))

#13 rmollise

rmollise

    ISS

  • *****
  • Posts: 22879
  • Joined: 06 Jul 2007

Posted 09 December 2011 - 02:22 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. ;)

#14 Ed Holland

Ed Holland

    Cosmos

  • *****
  • Moderators
  • Posts: 9659
  • Joined: 16 Jun 2010

Posted 09 December 2011 - 02:32 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

#15 Erik Bakker

Erik Bakker

    Cosmos

  • *****
  • Moderators
  • Posts: 8135
  • Joined: 10 Aug 2006

Posted 09 December 2011 - 02:35 PM

Love the views and ergonomics of my undriven dob. Easily outclasses my Questar 7 in the ergonomics and fun department. And is in another class with it's views. I don't see yet how you would win with a 11" SCT. If an SCT is what you like, consider a C14, otherwise further exploit the higher-end Newt route.

#16 tim53

tim53

    Voyager 1

  • *****
  • Posts: 14483
  • Joined: 17 Dec 2004

Posted 09 December 2011 - 02:40 PM

Relevant to this discussion:

I had my 1965 Cave 10" DK (with Tinsley optics, original to the scope) with me at Mt Pinos for the classics star party last August. Many have said that the Cave DK optics are poor. I've only looked through 3 of these - mine, Ron's 12.5", also at Mt Pinos this summer, and an 8" that Ed Beck had (that I saw about 20 years ago).

Mine's optics are pretty good, but I need to add a "real" fixed primary baffle to the OTA to improve contrast. Ron's seemed decent, though we didn't put any magnification to it (the mount never stopped shaking, so it would have been futile to try). Ed's 8" was superb, though he may have made it while he was at Cave or he may have refigured it later.

Anyway, I had the chance to compare my views on Jupiter and Saturn with those through Clint's 9" AP, Robert Provin's 6" AP, and Steve Miller's Quantum 6.

...they all beat the snot out of the Cave, but I think I'll blame that on the light leak around the "baffle" (really, the drawtube of the Unitron focuser, there was no primary baffle to speak of).

I've had very nice views at home with the Cave, except for the reduction in contrast due to the poor baffling, so I know it's not a bad one. I can only wonder what a Royce DK might do to an unsuspecting planet, though!

Finally, as I mentioned, I have a 12.5" Classical Cass, optics by Ed Beck. Planets through it are superb, though I didn't do a serious real-time comparison with the other scopes at Mt Pinos when I brought it up for the 2010 star party. We were pretty busy showing stuff to tourists that weekend.

I've had a handful of SCTs myself. All have been pretty decent. The 1975 C8 I purchased a couple months back in Vegas is probably the best I've had thus far, possibly with the exception of my NS 925. The Meade 2120 I haven't had on a driven mount yet to know how it'll compare to the others.

Sorry for the ramble. Get what you can find, what you can afford, and what you're comfortable using.

-Tim.

#17 Al Miller

Al Miller

    Apollo

  • *****
  • Posts: 1314
  • Joined: 25 Aug 2008

Posted 09 December 2011 - 02:42 PM

I'm thinking of a C11. How are they on planets? How is the optical quality?



I'm thinking pretty darn good overall. I suggest you go to the solar system observing forum and see what the folks there are seeing with their SCT's. Then, you might go look at the planet imaging forum to see the large number of high quality images taken with SCT's. After that, check the works of Damian Peach, and Christopher Go. Both used C11's in the past. Mr. Peach has a review in the user review section where he compares the C11 to a Meade 12. Check that out too. Why these same arguments come out everytime a question such as this is asked is beyond me. The C11 has been and will continue to be one of the most used planet observing and imaging scopes around. It has the aperture to bring in the details, and the brightness to show them well. Yeah, contrast suffers a bit but, who, really, can see that difference? Does it even matter? Besides all that, the cost won't have you taking out a mortgage to get one.

#18 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 24979
  • Joined: 01 Feb 2006

Posted 09 December 2011 - 02:54 PM

Here is a review someone else wrote a while ago of a C14 vs a 7" APO.

If a C11 can match a 7" APO, why did this C14 not do better than the 7" APO????

Notice that the author siad that he felt that is own scope was giving a slightly better view than the 7" APO, but that the 7" APO ownwer felt his scope was giving a slightly better view. I called it a draw because it seems that neither scope proved clearly superrior to the other.

http://www.cloudynig...php?item_id=909

What 6 inch APOs did you use to make YOUR compraisons?

I had to go to a C14 to see a distinct improvement over the 6" APO.

And what part of the MTF chart do you disagree with? It was plotted for a C11 that does not actually exist. I have NEVER seen a commmercial SCT with the quality of optics dipicted in the MTF plot I provided.

And in NUMEROUS side by side comparison, I came to the conclusion that a C11 is really only equivilent to a good 6" APO for planets.

So, my own actual ownership of these instruments, and my own side by side compraisons made over a great number of evenings and a great many comparisons line up rater perfectly with the physics of contrast transfer as depcited by the MTF plots I provided.

I stand 100% behind what I said earlier. A typical C11 will only give planetary performance similar to a good 6" APO.

The physics depcited above pretty much support my claim, and my actual ownership and countless side by side comparisons back this up. Heck, the Meade 152ED was not even close to perfect (not terrible) and the C11 STRUGGLED to so more than it did.

I don't think I could ever see anything in a typical C11 taht can't be seen in my 6" AP.

I don't know why people would be sad about this considering that you can pick up a C11 for half the price of a 6" APO.

To say that it can MATCH the performacne to me deserves a big round of appause.

If you were doing this on the REFRACTOR forum, people would be telling you that a C11 can't come CLOSE to a top qualityy 6" APO.

Me? I consider myself impartial. I call them like I see them. I own both large SCT (A C14, that is indeed a better planetary scope than a 6" APO) and a wonderful classic 6" refractor.


SO now we have a C14 owner that attests that his scope that in a shootout, he was unable to estabish a clear superriority over a 7" APO, and if a C14 can only struggle to a draw with a 7" APO, and I my own experience says that the C11 can only struggle to a draw with an excellent 6" APO, it sounds once again like it is alll lining up with the physics of the MTF chart I provided earlier.

#19 Rick Woods

Rick Woods

    In Memoriam

  • *****
  • In Memoriam
  • Posts: 20656
  • Joined: 27 Jan 2005

Posted 09 December 2011 - 03:04 PM

Al,

While I agree that SCTs do a fine job on the planets, you can't judge visual performance by the results a good imager like Go or Peach get. You won't see that in the eyepiece. So that's a little misleading.

The low-contrast detail loss is really the only visual issue I know of; and about all you can do about that is either suck it up and don't worry about it, or get a refractor or long-focus Newt.

Maybe someday. But when I look at Mars on a calm night in my SCT, somehow it doesn't really seem important. The wealth of detail I can see is enough to make up for whatever bit I can't see.

#20 Al Miller

Al Miller

    Apollo

  • *****
  • Posts: 1314
  • Joined: 25 Aug 2008

Posted 09 December 2011 - 03:20 PM

While I agree that SCTs do a fine job on the planets, you can't judge visual performance by the results a good imager like Go or Peach get. You won't see that in the eyepiece. So that's a little misleading.



Wasn't meant to Rick. Just to show that there is plenty of other fine things an SCT can do. Not suggesting at all that an image is what you see visually (although there will be plenty!).

#21 Al Miller

Al Miller

    Apollo

  • *****
  • Posts: 1314
  • Joined: 25 Aug 2008

Posted 09 December 2011 - 03:26 PM

SO now we have a C14 owner that attests that his scope that in a shootout, he was unable to estabish a clear superriority over a 7" APO, and if a C14 can only struggle to a draw with a 7" APO, and I my own experience says that the C11 can only struggle to a draw with an excellent 6" APO, it sounds once again like it is alll lining up with the physics of the MTF chart I provided earlier.



One or two experiences only. Much has to do with the observer's own eye, expectations, and pre-conceived notions of what each scope should show.

#22 coutleef

coutleef

    Aurora

  • *****
  • Posts: 4578
  • Joined: 21 Feb 2008

Posted 09 December 2011 - 03:40 PM

So if a C14 competes with a 7 apo, a C11 with a 6" apo, i suppose a C8 competes with a 5 inch apo?? I am not an expert but i always thought from discussions on CN that the increased aperture of a C8 would help show more details that would not be compensated by the better contrast of the 5" refractor??

#23 Patrick

Patrick

    Voyager 1

  • *****
  • Posts: 12155
  • Joined: 15 May 2003

Posted 09 December 2011 - 05:05 PM

Okay Eddgie...whatever.

I don't get to look through too many 6" APO's and certainly don't own any. The few I've looked through have been fantastic, of course, but I've never done any side by side with my C11. All I can tell you is the very best views I've ever had of Jupiter have been through my C11. If you'd like to let me borrow one of your 6" APO's I can do a comparison review! :jump:

Patrick

#24 Bob Abraham

Bob Abraham

    Messenger

  • -----
  • Posts: 407
  • Joined: 17 May 2005

Posted 09 December 2011 - 06:10 PM

Hi Eddgie,

While I think your point of view generally makes sense (and I've spent hours comparing my TEC 140 to a C-11) I think you're dismissing the C-11's capability a little too quickly. You state:

Seeing usually causes the high frequency deatail (the smallest detail and the detail to the right of the MTF plots) to be lost to the visual observer anyway.


Well, where I live (Toronto), the seeing is typically pretty dreadful but even here your statement is only true over longish timescales. Your eye/brain can operate at fairly high frame rates and process features seen only for a fraction of a second and even on a marginal night if one is really really patient one can find moments where a C-11 can exploit the full range of angular frequencies on your MTF plot. And also bear in mind a planet like Jupiter has enough features at high spatial frequencies to make such moments pretty worthwhile. So by your own argument the 6" apo and the unusually good C-11 are equivalent at low spatial frequencies but the good C-11 gives you occasional access to high spatial frequencies too so it's not unreasonable to claim it's better for planets.

That said, I personally preferred my 5.5" apo to the C-11 I used to own for planets (and I prefer my 8" TEC Mak to both). I prefer a more consistently very good view to a somewhat more unstable view with occasionally better moments (plus my C-11 wasn't 1/8 wave!). That's a personal value judgement thing. Anyway, I can easily see why somebody might prefer the view in a C-11, even though I don't.

Bob

P.S. I just came out of co-teaching a graduate class in adaptive optics where my students and I explored the MTF of the Keck telescope. Now THERE is an example of a scope where the atmosphere really fundamentally limits you to lowish spatial frequencies. A C-11... not so much.

#25 Ed Wiley

Ed Wiley

    Surveyor 1

  • *****
  • Posts: 1988
  • Joined: 18 May 2005

Posted 09 December 2011 - 07:01 PM

Hey Tim:

Views through my Royce 8" DK are all that one might expect through Royce optics. Years ago I have a Cave 8" Cass and I thought the optics were just fine, but it has been long enough since I had that scope that I cannot compare it to the Royce. However, I remember that the Cave was hard to collimate while the Royce is a snap, but that might have been my inexperience. My only problem with my Royce is that I want a bigger one!


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics