HCR32
sage
Reged: 08/27/10
Loc: Australia
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C14. Refractor Equal
#5292763 - 06/28/12 06:49 AM
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What size refractor would equal the performance of a C14 visually?
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t.r.
Post Laureate
Reged: 02/14/08
Loc: Upstate NY
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Re: C14. Refractor Equal
[Re: HCR32]
#5292801 - 06/28/12 07:48 AM
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There were some MTF graphs done on this awhile back, and if memory serves, it was in the 8" to 9" range, all things being equal, which seldom are. Some have said that for planetary detail, a 7" will get you there.
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Eddgie
Postmaster
Reged: 02/01/06
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Re: C14. Refractor Equal
[Re: HCR32]
#5292880 - 06/28/12 09:07 AM
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Assuming perfect optics (which of course is not a reasonable assumption), A C14 would have contrast transfer that would be better than a 9" unobstructed aperture.
Of course it would be hard to find a perfect C14, though very good ones are out there. My own C14 has very good quality optics, bordering perhaps on excellent.
Given very good optical quality, a C14 should easily equal an 8" unobstructed scope for contrast transfer for larger detail and far exceed it for the finest detail, but for fine detail, conditions would have to be perfect to take advantage of this.
I have made visual observations many times of details that were reported as being imaged in an 8" APO.
Early this year, an 8" APO owner took pictures of Galilee Regio and Osiris on Ganymede, and I saw those features visually on the same night.
If the C14 is only fair optically I would think that it would be equivilent to perhaps a 7" unobstructed scope. Here is a link to a review where a C14 was compared to a 180mm refractor. Both owners seemed to think that their own scope did best, so I would call it a tie, but I don't know the quality of the C14. It was an orange tube though, and sadly, I think the quality on these varies.
Review of C14 compared ot a 180mm AP
Here is a link to a picture of Ganymede showing Galilee Regio and Osiris. This feature is visible in both an 8" unobstructed scope and a C14. The Regio is the "C" shaped mare-like feature on the upper right, and Osiris is the bright feature inside the limb on the lower left.
I did not see these as "Albedo" features.. I resolved them. This means that I saw them as very well defined shapes, and not just indistinct shadings.
The 8" APO scope capetured them as well using a camera.
Galilee Regio and Osiris on Ganymede
Anyway, based on my own observations and my own 6" APO, I would guess that the C14 is working at the minimum of an 8" unobstructed aperture.
Hope this helps.
Edited by Eddgie (06/28/12 09:13 AM)
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Mark Costello
Carpal Tunnel
Reged: 03/08/05
Loc: Matthews, NC, USA
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Re: C14. Refractor Equal
[Re: HCR32]
#5292945 - 06/28/12 10:07 AM
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What size refractor would equal the performance of a C14 visually?
It'd be a lot bigger than what I could handle.  
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AlBoning
sage
Reged: 03/06/11
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Re: C14. Refractor Equal
[Re: Mark Costello]
#5292955 - 06/28/12 10:23 AM
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I was reading just yesterday, was it in S&T, that it can be approximated by subtracted the diameter of the central obstruction from the aperture. What would that be? Somewhere in the neighborhood of 9"?
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Muffin Research
Carpal Tunnel
Reged: 09/28/07
Loc: Belgium
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Re: C14. Refractor Equal
[Re: AlBoning]
#5293079 - 06/28/12 11:51 AM
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The answer is probably something a lot more expensive than a C14
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Eddgie
Postmaster
Reged: 02/01/06
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Re: C14. Refractor Equal
[Re: HCR32]
#5293107 - 06/28/12 12:10 PM Attachment (91 downloads)
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Here is a followup to my pervious post on the subject.
The MTF chart I am providing shows the MTF of a C14 with 1/8th wave spherical abberation but no other error. This would be an excellent C14. Mine is not quite this good but I would say close.
The MTF suggests that this scope would perform similarly to an unobstructed 214mm perfect aperture at the low and mid frequencies, giving contrast transfer within about 2% down to about the .23 of max spatial frequency of the C14 (the max spatial frequency of the 214mm unobstructed aperture has a max spatial frequency of .6 of the C14, meaning that is does not have the ability to resolve detail smaller than than indicated by the .6 mark on the bottom axis.
Past the C14s .33 or .34 max spacial frequency, the C14 pulls smoothly ahead.
The problem though is that .33 or .34 of the max spatial frequency for the C14 represents detail only a couple of arc seconds across. Detail smaller than about 1.4 Airy Disks is often obserured by seeing meaning that while the C14 could indeed resolve much smaller and lower contrast detail, it would take perfect conditions for it to do so.
But for general observing on nights of decent seeing, this MTF chart suggests that a very good C14 would give contrast transfer that is similar to the detail that could be preceived in an 8" unobstructed scope.
Not bad for $2800 on the used market.
The C14 has been the best planetary scope I have ever used. I feel pretty confident in my estimate that it would match a good 8" refractor.
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jmiele
Patron Saint?
Reged: 12/04/10
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Re: C14. Refractor Equal
[Re: Eddgie]
#5293156 - 06/28/12 12:36 PM
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Eddgie,
Starting with theoretically perfect on a c-14 requires the SCT "system" to be perfect. Not as with a lens cell assembly on a refractor. With the SCT's more complex system, the primary, secondary and corrector must all be taken into account, correct? Do you account for such in your estimates? I'm interested even if the resolution celling ends up being our local seeing in most cases.
I will say that my own experience when comparing the 7" 180mm TEC to my 12" Mewlon 300, the Mewlon does toss out a beating quite often in planetary detail. Prior to my side by sides I would NOT have thought that would be the case. I believe some of it has to done with the "total system" performance of the Mewlon. I've have C-14's and while I've not performed same night side by sides, I didn't recall the difference being as dramatic. Saturn's crepe, for instance has never been as readily viewable for me on many scopes until the Mewlon 300. Contrast plays a huge part in its visibility and not even the 180FL shows it as often in my poor seeing and darkness.
Some of this is why I've previously posted I felt the 180FL over the 160EDF required better seeing and darkness to fully exploit it's increased aperture. I feel I was dismissed in those posts, however that is what my side by side (all things being equal) have shown - at least to my eye. Because the MTF difference is even greater between the 180 TEC and the 300 Mewlon (as compared to the 180 and 160), the difference is more apparent,. Once again, at least IME. Any thoughts?
Best, Joe
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Eddgie
Postmaster
Reged: 02/01/06
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Re: C14. Refractor Equal
[Re: jmiele]
#5293179 - 06/28/12 12:47 PM
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It doesn't matter what errors exist on individual elements as long as the total error is small.
Many SCTs have a Strehl ratio above .95 which is considered excellent. My own C14 I believe is very close to this level of quality, and my EdgeHD is near perfect.
The Strehl ratio of the instrument defines how close to perfect it is, and ignores the central obstruction for Strehl calculation, however the MTF chart provided above shows a C14 that with 1/8th wave central obstruction (which would mean the the Strehl would be a bit better than .95) and includes the contrast loss caused by the obstruction.
The solid red line shows how a perfect 14" aperture would perform vs both instruments.
I have used a great number of telescopes, and as long as optical quality is reasonably good, the more clear aperture, the better the scope has performed on planets, regardless of design.
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Eddgie
Postmaster
Reged: 02/01/06
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Re: C14. Refractor Equal
[Re: Eddgie]
#5293222 - 06/28/12 01:12 PM
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I might ad this as well.
Even if two scopes have similar contrast, when used visually, the larger scope will have an edge.
This is because when contrast is very low, the illumination of the target can play a role.
For a given exit pupil, the illumination in a much larger scope will be much higher due to the larger exit pupil fot that magnification when using the larger scope.
Keeping everything else constant, in other words, even if both scopes have the same contrast level for a given detail size, the higher illumination in the larger scope will afford the observer a better chance of seeing the feature.
This is not a factor when imaging, but for visual use, it is a very importat consideration. We often talk about MTF in a vacuum and do not connect it to the contrast sensitivity of the human eye, but that is a mistake (in my own opinion).
For example, if I were to use 300x (many people suggest 50x inch of aperture, but I think that is way to high) in my 6" APO, the exit pupil would be about .5mm. My view is very dim. Plasing, but dim.
Using the C14 at 300x, the exit pupil is over twice as large, and the image is more than 3 times as bright.
It is this added "illumination" of the target that makes it possible to detect detail that has become to faint in the 6" APO to be easily resolved.
You may have experienced something similar in your Mewlon. The added brigntess could have simply made it easier for your eye to detect detail that may have been present in the smaller scope.
I see Crepe easily in my C14. very bright and distinct.
I have resolved features on Ganymede in the C14, and have never done that in my 6" APO.
The best planetary views I have ever had with an amateur instrument was a boundry layer fan scrubbed 12" dob with 20% obstruction. Better than my C14. I keep wanting to build one like it one day.
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gnowellsct
Postmaster
Reged: 06/24/09
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Re: C14. Refractor Equal
[Re: Eddgie]
#5293235 - 06/28/12 01:19 PM
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I have looked through a nine inch apo side by side with my C14 and I would say the theory was confirmed. Planetary views are approximately equivalent in terms of jigs and jags and curls. Still, there are just some things that don't get discussed. The C14 delivers a nice palette of color on Jupiter, for example, whereas a nine inch apo is more like a c8 or a 9.25 color-wise. You just don't get the same palette with a smaller aperture of any type, and I'm surprised that this is not discussed much. The apo of course rules when it comes to having more field of view. The C14 is more cost effective to mount. A nine inch apo needs an AP1200 to get about the stability performance that a C14 on an AP900 has. In a C14 in a good sky you'll be going about one magnitude deeper. You'll get more pop out of globs. Even if I hit the $100m lottery, I wouldn't buy a nine inch apo. Not sure if I would get something else to take the place of the c14, but whatever that might be, it would definitely not be a nine inch apo.
Maybe a 20" RCOS and use the C14 as a finder.
I would probably get a six inch apo, though, no problem there.
Greg N
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coz
super member
Reged: 08/25/10
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Re: C14. Refractor Equal
[Re: Eddgie]
#5293249 - 06/28/12 01:24 PM
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Very interesting thanks. So Eddgie would you say your C14 is the most used scope because of this performance? Do you use the 6" refractor because of easier setup or some other reasons?
Joe, with your M300 do you have to travel to a dark site to use it? I thought I read once that your home seeing conditions were average.
I picked up the 160edf this year and it's amazing. I'm trying to determine if I should pair it with something larger like a C14. I'm in poor seeing conditions unfortunately.
Regards
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jrbarnett
Eyepiece Hooligan
Reged: 02/28/06
Loc: Petaluma, CA
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Re: C14. Refractor Equal
[Re: HCR32]
#5293301 - 06/28/12 01:45 PM
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I'm not sure what you mean by "equal". A 60mm achromat will not just equal, but will massively exceed the C14 in image stability in less than perfect seeing, for example. Nothing but a 14" refractor will equal any other 14" objective in resolving power, though. So I think the concept of "equal" is illusory here. Smaler refarctors will do some things better than larger SCTs and other things worse.
- Jim
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drollere
Pooh-Bah
Reged: 02/02/10
Loc: sebastopol, california
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Re: C14. Refractor Equal
[Re: jrbarnett]
#5293372 - 06/28/12 02:17 PM
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jim hit the main point. astronomical optics are always a *system*, which at minimum includes the telescope, eyepiece and eye, or telescope and CCD sensor. all the systems are affected by light pollution, atmospheric seeing, and atmospheric dispersion. you have to evaluate the *system* performance as a whole and in a physical environment.
optical systems are used for different tasks, and this makes "abstract" or "objective" optical tests a little pointless. or rather, they only suggest a worst case evaluation.
in the case of an MTF posed above, for example, you notice a little bump *above* the "ideal" aperture performance at the left of the graph, and a sag below the ideal aperture performance at the right of the graph. the angular separation indicated at the left of the graph is approximately equal to the rayleigh criterion (theoretical minimum double star resolution) of the optics. the separation at the right is roughly equal to fine planetary or lunar structure. net conclusion: an obstructed aperture will actually perform better than an unobstructed aperture in double star work, but will perform somewhat worse in planetary observation.
even those tentative generalizations need to be anchored in the entire system configuration and use in specific locations.
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jmiele
Patron Saint?
Reged: 12/04/10
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Re: C14. Refractor Equal
[Re: coz]
#5293440 - 06/28/12 03:11 PM
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Quote:
Joe, with your M300 do you have to travel to a dark site to use it? I thought I read once that your home seeing conditions were average.
I picked up the 160edf this year and it's amazing. I'm trying to determine if I should pair it with something larger like a C14. I'm in poor seeing conditions unfortunately.
Regards
Coz, No I'm a cripple with MS so moving around to dark sites is no longer possible for me. But yes, my local conditions are not ideal. I still enjoy the solar system, doubles and luna. I'd also like to get move involved is Solar observing. Imaging is also no long possible for me.
Yes, the AP160EDF is an amazing instrument. One of the prettiest in terms of the image displayed. Very pleasing to observe through.
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jmiele
Patron Saint?
Reged: 12/04/10
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Re: C14. Refractor Equal
[Re: Eddgie]
#5293442 - 06/28/12 03:11 PM
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Thanks Eddgie. Joe
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saemark30
scholastic sledgehammer
Reged: 02/21/12
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Re: C14. Refractor Equal
[Re: jmiele]
#5293566 - 06/28/12 04:48 PM
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I find it very difficult to believe something as complex as a C14 has a strehl of 0.95. Not all commercial Maks or APO refractors do, and add in the secondary obstruction and the surface accuracy needs to be at least twice that of a refractor surface makes than very unlikely.
I would think a mirror of f/2 would be rough, as the Schmidt plate and secondary that is "hand figured" by Celestron.
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jmiele
Patron Saint?
Reged: 12/04/10
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Re: C14. Refractor Equal
[Re: saemark30]
#5293569 - 06/28/12 04:49 PM
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Are you addressing me? I think that was Eddgie's graph.. That's why I asked the question I did..
Joe
Edited by jmiele (06/28/12 04:50 PM)
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Jon Isaacs
Postmaster
Reged: 06/16/04
Loc: San Diego and Boulevard, CA
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Re: C14. Refractor Equal
[Re: jrbarnett]
#5293677 - 06/28/12 06:12 PM
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I'm not sure what you mean by "equal"
It seems that discussions like this always end up focusing on one thing: Planetary performance comparing the two scopes with the seeing is excellent, the SCT or Newtonian thermally stablized and with excellent optics. Such comparisons do not represent what the vast majority of us experience on a nightly basis.
The planets are just one small corner of the sky and there are many other aspects and interests to consider.
Jon
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Rustynuts
super member
Reged: 07/25/11
Loc: East Tennessee
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Re: C14. Refractor Equal
[Re: Jon Isaacs]
#5293952 - 06/28/12 09:15 PM
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from a shootout conducted earlier my 9 vs a 9.25 the APO is around 40 % brighter and shows much more contrast than a 9.25, not to mention the lack of light scatter in the refractor . Now when the observatory is operational, I should mount a local friends c14, next to my 9 APO on my large gem and do a true side by side comparison. Jon
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