Due to the certain views most people have about optical design, i though a post soley for optical discussions be started.
RC's are NOT the best optical design, contrary to many peoples beliefs. But by placing a lens correcting setup, it puts it close to perfect!. But then the scope becomes a Cataioptric (Cat).
The Corrected Dall Kirkham (CDK) is corrected to remove many abberations.
To an RC, the CDK is a better optical design..
Further a mention was made on another post about spot diagrams " More importantly, again we see a manufacturer (PlaneWave Instruments) who compares their products to the RC optical design through spot diagrams, etc. just like Meade and Anssen Technologies have done in the past. It is the statements and figures conveyed that have the potential to fool the prospective buyer"
This is partly correct, this is because manufacturers tend to only toot the good points and leave out the drawbacks. Problem is ALL manufacturers do it. Yes even RC Optical and RCOS, so we cant just say Meade or Anssen have done it in the past. Just to nip it in the bud, Anssen has a manufacturing problem, and nothing to do with the spot diagram or design of the scope. The scope is designed correctly, just other forces producing bad images.
Further, the Spot diagram is the process of comparing telescope performance, and every telescope manufacturer designs their optical systems using Spot diagrams as its proof of performance.
Even the Vixen VC200L is a CDK type, so read about it.
Lastly, one question was asked about why do they prefer the RC over other designs.
Well, the RC only uses 2 mirrors, so you get NO loss or abberations introduced by passing light thru glass, plus ALL optical wavelenghts are also reflected. Not to mention the size and weight of the correcting lens's for a 2 meter or larger telescope.
This is why they dont make big Schmidt telescopes. The glass on the 48" Schmidt is hugeious enlargicus !!!!. So corrector plates are out of the question.
Heres a small cut and paste of the similar discussion about the Allira and the spot diagram on the homepage and the RC i was involved in many many months ago on cloudy nights.
Quote:
I do think their 1st spot diagram for the RC is suspect because it doesn't look like a spot digram for any scope I have ever seen, not to mention an RC and I think the corner image from my RC indicates the distortion isn't anywhere near what the spot diagram would indicate.
Dean,
I assure you that those spot diagrams are 100% accurate. What you may not notice immediately that RC diagram quoted on Anssen site is focused for an on-axis image, so astigmatism and field curvature become rather severe off axis. If you refocus for best overal images (approx 30% from centre) things will look considerably better (but still INFERIOR to Allira's). RC's own spot diagrams are in fact misleading too - triangular shape you noticed is from vignetting. The outermost parts of the image don't make it so astigmatic shape is truncated, hence non-symmetrical appearance. Even then, spot images are rather poor (notice scale of 200 microns !). One can argue that RCOS's diagrams are better represenatation of real world images, but nevertheless.
Allira seems to be a variant of Dall Kirkham + dedicated Ross corrector. These systems are capable of exceptionally good images across a very flat field. Definitely an interesting design that has been curiously neglected by ATMs.
All said, the quality of the REAL image in the end depends on many factors that are well outside the domain of Zemax. How well the particular specimen of the scope is made, how well it is focused, how well it is collimated, how well it KEEPS collimation and focus, how well the mount tracks, seeing conditions, you name it. Had you used a well made and collimated D-K+Ross (I can see why they try to distance from this by choosing a fancy name :-) on the same mount and guided by the same gear, I am sure you would easily come up with superior images. In fact your own analysis shows an out of square being the most significant aberration on Allira's image (most likely from focuser sag), while your RC was dead-on. Large FWHM was likely from poor tracking/guiding/seeing. Yes, I can hear "excuses!" but these are merely facts. The DESIGN is indeed capable. The optics/mechanics/mount/seeing/astrophotographer in question might not be.
Due to the certain views most people have about optical design, i though a post soley for optical discussions be started.
RC's are NOT the best optical design, contrary to many peoples beliefs. But by placing a lens correcting setup, it puts it close to perfect!. But then the scope becomes a Cataioptric (Cat).
The Corrected Dall Kirkham (CDK) is corrected to remove many abberations.
To an RC, the CDK is a better optical design..
Well, I can’t help starting without a diplomatic statement - the best optical design is one that meets your needs. I’ve done my fair share of studying optical statistics, ray-trace diagrams and the like. You begin to realise as you sift through the data that a perfect optical design does not exist. Sure, some are better than others for specific applications. There are differences between taking “pretty pictures” compared to scientific data acquisition.
Firstly, I would like to clarify I don’t have anything against corrector plate/lens designs. We should distinguish there is a difference between the two. A corrector plate resides in front of the primary mirror. A corrector lens is located after secondary mirror usually close to the focal plane. The latter is actually no different to an advanced focal reducer/field flattener such as those used in the Vixen VC200L and the Astro Systeme Austria (ASA) series.
Your table appears to be directly from Vixen’s info- http://vixenamerica.com/Pdf/Scopes/V..._SX_VC200L.pdf
Of course they are going to say it corrects everything, they’re trying to sell their product. It is disappointing when manufacturers make such optical design comparisons unless it’s an apples to apples case - such as a RC with Star-Instruments mirror sets vs. a RC with Astrosib mirror sets. In some cases, they’re just stating the stating the obvious.
Quote:
Originally Posted by Gama
Further a mention was made on another post about spot diagrams " More importantly, again we see a manufacturer (PlaneWave Instruments) who compares their products to the RC optical design through spot diagrams, etc. just like Meade and Anssen Technologies have done in the past. It is the statements and figures conveyed that have the potential to fool the prospective buyer"
This is partly correct, this is because manufacturers tend to only toot the good points and leave out the drawbacks. Problem is ALL manufacturers do it. Yes even RC Optical and RCOS, so we cant just say Meade or Anssen have done it in the past. Just to nip it in the bud, Anssen has a manufacturing problem, and nothing to do with the spot diagram or design of the scope. The scope is designed correctly, just other forces producing bad images. Further, the Spot diagram is the process of comparing telescope performance, and every telescope manufacturer designs their optical systems using Spot diagrams as its proof of performance.
Even the Vixen VC200L is a CDK type, so read about it.
That simply doesn't cut it. You buy a telescope of its optical quality/performance. It doesn't matter how good the design. If the manufacture can't deliver the indicated performance due to manufacturing defects the telescope is next to useless. Spot diagrams are only part of the optical testing equation.
What has happened to the Strehl ratio – the definite measurement in determining optical quality? Manufacturers are extremely shy in providing these details. Why is this so? Poor internal quality control, difficult to obtain consistency, all too hard and time consuming? Seriously when you’re entering into the high-end market, I’d be extremely sceptical in purchasing an OTA that didn’t provide this. It is not possible to purchase a new RC without receiving such information on your specific mirror set in the OTA you’ve purchased.
Quote:
Originally Posted by Gama
Lastly, one question was asked about why do they prefer the RC over other designs.
Well, the RC only uses 2 mirrors, so you get NO loss or abberations introduced by passing light thru glass, plus ALL optical wavelenghts are also reflected.
I think you nailed this one nicely – be wary of corrector/lens designs in particular the coatings they use. Try imaging at UV wavelengths for photometry with some SCT’s for example, it’s near impossible because of the glass/coating combination. Thus it comes back to application – “pretty pictures” or scientific data acquisition.
If we purely go on what is happening in the market with high-end optical purchases, I’d like to challenge why there are more RC’s are out there than CDK’s. If the CDK design is indeed superior, why aren’t there more people using them? The cost is certainly CDK favourable considering a 20” CDK - US$32,500 and the 20” RCOS is US$54,300. There has clearly got to be a reason why many advanced amateurs, government agencies, etc go the RC path. Admittedly, this is a basic viewpoint, but if there isn’t a market then these companies would surely fold.
best to have the pages side by side.
Have to say its pretty impressive .
Impressive indeed. I've seen this comparison before. What is not clear is how far off the optical axis each image is. I'm not trying to play this down, but one must consider such variables before drawing a conclusion.
Well, not being terribly up on all of this, seeing, focus and guiding often renders this sort of OTA ubertech comparison mute. The aformentioned variables are essential factors before spending megabucks on optics for the vast majority of Amature Astrophotographers.
Further, the Spot diagram is the process of comparing telescope performance, and every telescope manufacturer designs their optical systems using Spot diagrams as its proof of performance.
How would one interpret these spot diagrams? A Meade 14" RCX vs. a 14" RC using a Star Instruments mirror set...
I’m all for the advancement in optical designs regardless of its construction characteristics. I fully acknowledge the contribution Meade as made in producing an aplanatic SCT. This design has been well known for sometime. It has its own merits and is a significant improvement to the conventional SCT design.
If we were to make a theoretical performance vs. cost comparison, the aplanatic SCT such as the LX200R and RCX do have an advantage over the professional RC’s, though this is waning.
RCOS and OGS in conjunction with star instruments are presently releasing high-end RC optical scopes (known as RC astrographs) at a lower cost. The cost reduction is achieved by using different glass types and construction materials. Unlike the professional series RC’s where you get to choose your optical coating, focal ratio and ion-milled optics, the lower cost scopes are a little lean.
If any one is going to spend big bucks, then you would know what the specs say and do including the spot diagrams. Posting a Spot diagram from the 2 scopes will mean nothing to the normal person off the street, but to a university or professional, that is one of the main things that stick out the most.
This whole point about the RCX was not that the RCX is a better scope, just another optical system that performs well, not that its better than an RC..
It also doesnt matter were any data was obtained to descibe the pros and cons to the different optical methods, the point is its true. Which do you believe to be incorrect ?. To say that the RC's are the best optical scope is easy but i havent found any data that says its better than a CDK, if there is please let me know, as i havent been able to find much to say this..
The cheaper RC's which are being released, do not perform as good as the current RC's, and if you know RC's, then collimation will be a bigger problem with the cheaper versions.
Not much point in talking about the designs if you dont believe there is a better optical system. You didnt comment on the Quoted comment post, as that was also posted at another person who also believed that RC's were the the greates on the cloudynights forum, id like to know your views on it.
Something also you may not know, you dont need the corrector at the front of the SCT type scopes, it can actually be placed further down the optical train.
I agree with you the RC's are a magnificent scope, and it does have its pros and cons, as do all things on earth. But i looked form many months for my next scope and the RC was bumbed off by the CDK. Up until then it was on the top of my list. I am open to any data you have to show me a better optical scope, and possible save me (Or cost me) money.
One other optical system was also the Newt with a corrector, this is also high on my list of scopes.
I also gave one reason as to why there are RC's in large sizes over the CDK's, and again, the corrective len's would be too big and too lossy.
Heres some web links and quotes :
If any one is going to spend big bucks, then you would know what the specs say and do including the spot diagrams. Posting a Spot diagram from the 2 scopes will mean nothing to the normal person off the street, but to a university or professional, that is one of the main things that stick out the most.
But this is part of the problem. Lack of understanding and too much marketing blurb. If the normal person off the street took greater interest in optical statistics/measurements, all marketing buzz words are removed from the equation. One has to simply browse through the astro mags to be dazzled by crap. Statements such as “Company X is proud to introduce a revolutionary coating system that outperforms any other coating in the commercial telescope market. Our most popular Schmidt-Cassegrain telescopes are now available with this high quality optical coating at an incredible value.” – What the? Am I supposed to believe this? Marketing blurbs mean nothing without sufficient optical data to back such statements up. Unfortunately, obtaining this information is difficult, certainly from the commercial manufacturers.
Quote:
Originally Posted by Gama
This whole point about the RCX was not that the RCX is a better scope, just another optical system that performs well, not that its better than an RC..
Understood. There are many optical designs out there that perform well. Designs are shifting. As I mentioned in another thread, we are beginning to see optical designs starting from the focal plane forward (instead of the conventional telescope back). With the rise and increasing availability of large format cameras, it makes perfect sense that this shift is occurring. Understandably astrophotographers get annoyed when they’ve payed large dollars for a 35mm+ CCD chip, only to find that their telescope optical design restricts the light path or aberrations are present.
Quote:
Originally Posted by Gama
It also doesnt matter were any data was obtained to descibe the pros and cons to the different optical methods, the point is its true. Which do you believe to be incorrect ?. To say that the RC's are the best optical scope is easy but i havent found any data that says its better than a CDK, if there is please let me know, as i havent been able to find much to say this..
There are pros and cons with every optical design and again the selection criteria are based on application. I don’t doubt that the CDK is a solid optical design and it would appear there are optical statistics available partly confirm performance; however there is simply not a large market presence to validate these claims. As you mention in your previous post – how a telescope performs in a lab environment can be different to the real world. There are so many variables to consider. The RC install base is large with a real world clear and proven performance record. This of course doesn’t mean the design is the best.
Quote:
Originally Posted by Gama
The cheaper RC's which are being released, do not perform as good as the current RC's, and if you know RC's, then collimation will be a bigger problem with the cheaper versions.
Of course the cheaper RC’s don’t perform as well as the professional RC series. You pay what you get. The ion-milling process of the professional series isn’t cheap - http://www.rcopticalsystems.com/ionmill.html
Collimation will affect the performance of any Cassegrain. I do acknowledge that many underestimate the importance of collimation especially with these scopes. The collimation characteristics between the RC Astrographs and RC professional series is identical. Seriously, the difficulty in collimation of the RCs is overrated. Its not hard. If you’re purpose is solely imaging, you will not find a better way of squaring your optical axis to your CCD camera by using CCDWare’s CCDInspector.
Quote:
Originally Posted by Gama
Not much point in talking about the designs if you dont believe there is a better optical system. You didnt comment on the Quoted comment post, as that was also posted at another person who also believed that RC's were the the greates on the cloudynights forum, id like to know your views on it.
You can’t admit defeat. This is a healthy and informative debate. I don’t believe RC’s are the greatest, however I’ve yet to see another design with a proven optical record. As I stated in my previous post, if there is greater optical design, why aren’t we seeing more installations of it? Perhaps over the next decade we will see the emergence of more CDK’s or other RC designs such as BRC.
Quote:
Originally Posted by Gama
Something also you may not know, you dont need the corrector at the front of the SCT type scopes, it can actually be placed further down the optical train.
Indeed, you are correct. However this is not practical with the SCT design. When you look at ray-traced light paths in such a design, the corrector lens close to the focal plane needs to be large for the correction to occur. Depending where the lens is located, it may require an oversized baffling tube which inturn would affect primary mirror diameter/light gathering power.
Quote:
Originally Posted by Gama
I agree with you the RC's are a magnificent scope, and it does have its pros and cons, as do all things on earth. But i looked form many months for my next scope and the RC was bumbed off by the CDK. Up until then it was on the top of my list. I am open to any data you have to show me a better optical scope, and possible save me (Or cost me) money.
One other optical system was also the Newt with a corrector, this is also high on my list of scopes.
I’m far from a “telescope consultant”, but you have not alluded to your application. What do you want to do – “pretty pictures” (wide, narrow, deep, hi-res etc) or make the next scientific discovery. You appear to have done all your research and if you’re true to your word, then I look forward to seeing the results of your CDK. After all, it takes people like yourself to make that step, similar to what Steven did with the Allira 14”. I’ve seen about three sites that provide astro images taken through the CDK optical design. It does appear a strong performer. Still no definite Strehl ratios forthcoming so I remain reserved. I'm happy for you to surprise me.
Im glad to read your analysis of the my posts, i was thinkg it was going to be a Ford vs Holden issue again, but after reading you last post, it shows your fundemental considerations governing specs and the final product.
I totally agree that printed material is bogus until its built and proven.
But its the start of the buying process.
My requirments are set at imaging with ability of viewing. Even though the RC is a Imaging system, it has been and is used visually by many users. But like anyone, i need a reliable instrument (Well, that leaves Meade Out.. JUST KIDDING) and superb optics. I dont like the cheaper RC's because of the lower quality in performance. Still, i wouldnt say no to one, and was on a list for a short time.
As i said, i have bounced between a CDK and the corrected Newtonian, and i have racked my brains trying to finalise which one. Then in the back of my mind pounding away trying to get in the decision is the 20" RCX400 optical tube. As said, money is the final judge and jury of the purchase.
There are some observatories using the CDK, but i have only seen them in the 1 meter size and smaller. They are found in some Cloud Absorption Radiometer's from NASA and other manufacturers, but small in size. Even Optical Guidance Systems make them for specialised purposes.
I think one drawback is a 20" and bigger corrective lenses for big observatory scopes may tend to diminish the idea of building one.
Im glad to read your analysis of the my posts, i was thinkg it was going to be a Ford vs Holden issue again, but after reading you last post, it shows your fundemental considerations governing specs and the final product.
I totally agree that printed material is bogus until its built and proven.
But its the start of the buying process.
No, there is no point being so fanatical when in comes to telescope designs. Every design has its merits and place. I know I’ve harped on the point of application, but this is really a critical one. What your intentions are will be the driving force in choosing a suitable design to meet your requirements/expectations. I do get frustrated by marketing blurbs, in particular with commercial manufactures. Bold statements, with weak/non-existent optical measurements as they spar for market share. I’ve yet to hear of high-end equipment manufacturers bickering over each others designs. Tak make the FRC-300 and BRC-250 (the latter uses a corrector), but have never claimed their design is superior to an RC. Too much ego-tripping in the commercial market for my liking.
We’ve already touched on the external influences such as tracking, guiding and seeing. Have good optics is a great start, but there are so many other factors. The astrophotographer plays a major role in processing the image. If you can’t process you’re wasting your time. I found a good snipet of info on processing and astro software that puts a few things into prospective;
“The more time I spend in Astrophotography, the more I realize that imaging raw data is the easy part. Its what you do with it once you have it.
This is where software comes in. People have a hard time justifying expensive software purchases because it does not weigh 47 lbs and breaks your back putting it in your trunk...but if you think about it, software for image processing has a far greater impact on the final product of all your labors than the equipment you use.
Give me an 8" LX200 and a ST7XME and put me up against a 14" RC with a ST10XME. If the owner of the 14" RC cant image process, then I produce the finer image.
My point, think of software as being as valuable as your imaging equipment and then you start getting the proper perspective on value.
My requirments are set at imaging with ability of viewing. Even though the RC is a Imaging system, it has been and is used visually by many users. But like anyone, i need a reliable instrument (Well, that leaves Meade Out.. JUST KIDDING) and superb optics. I dont like the cheaper RC's because of the lower quality in performance. Still, i wouldnt say no to one, and was on a list for a short time.
As i said, i have bounced between a CDK and the corrected Newtonian, and i have racked my brains trying to finalise which one. Then in the back of my mind pounding away trying to get in the decision is the 20" RCX400 optical tube. As said, money is the final judge and jury of the purchase.
Your decision will be about compromise in optical design. If you had of said imaging or visual only, then the direction is clearer. Not all imaging scopes perform well visually. A classic example is the Pentax SDP and SDHF modified petzval APO refractors. They follow a similar design to the Takshashi FSQ. Visually, I found the Pentax SDP100 rather disappointing. It didn’t deliver the contrast I was expecting, but photographically it’s a real strong performer. They certainly give the FSQ a run for the money with the same image circle size of 88mm and are cheaper. Of course a dedicated planetary imaging scope is considerable different to a dedicated DSO imaging scope.
Quote:
Originally Posted by Gama
There are some observatories using the CDK, but i have only seen them in the 1 meter size and smaller. They are found in some Cloud Absorption Radiometer's from NASA and other manufacturers, but small in size. Even Optical Guidance Systems make them for specialised purposes.
I think one drawback is a 20" and bigger corrective lenses for big observatory scopes may tend to diminish the idea of building one.
Will certainly keep my eyes open for more amateurs using the CDK design. Would really like to see more optical statistics on the design before I make further comment.
The Klevzov-Cassegrain aspherical system presents a rather radical form of catadioptric telescope that combines the compactness of a Schmidt-Cassegrain with similar enviable high resolution image quality and contrast of modern Maksutov designs. Klevsov's revolutionary catadioptric design employs a sub-aperture meniscus correcting lens, combined with a Mangin mirror-lens secondary. Their fast focal ratios, flat field and excellent correction or coma, spherical aberration and astigmatism (14 microns star images at a distance of 10mm from the optical axis) makes these great instrument for astrophotography. I have not seen sufficient optical tests to make further assessment.
