Quote:
Originally Posted by bratislav
Disagree on all accounts. Visually SCTs suffer from huge central obstruction. There is no way around that - you can have a perfect SCT optically (and they are nowhere near that) but damage to contrast transfer has already been done. Not really that objectionable on Moon or Saturn (rings), but any ultra low contrast feature (all of Jupiter, detail in Saturn belts and fine features of Mars) will be hard hit.
Imaging wise, SCTs are hard work because they are dreadful in keeping collimation (I have to check and usually recollimate every time I change elevation more than about 20 degrees with my C11); but main issue is enclosed primary which lags in temperature and causes internal heat plumes. Short of actively tackling that via pelitiers and internal fans, you are at the mercy of local temperature gradient. Quite often SCT users will report "bad seeing" while Newtonian and Cassegrain counterparts will work close to their maximum resolution - same place, same time.
The only real advantage of SCT is comfortable observing. Using binoviewer (compulsory for people with floaters like yours truly) and having comfy chair is a real game changer. I know you can use bino in a Newtonian, but they often end up in all sorts of neckbreaking angles and it is impossible to seat back and relax. Planetary observing is a waiting game, being comfortable is a definite advantage.
Bratislav
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Bratislav,
I don't think you actually took the time to correctly read and comprehend what I was implying. At no time did I imply that a SCT was the best at anything, either visual or imaging. Truth is I wouldn't own a SCT if someone gave it to me.
Quote:
Originally Posted by bratislav
Disagree on all accounts. Visually SCTs suffer from huge central obstruction. There is no way around that - you can have a perfect SCT optically (and they are nowhere near that) but damage to contrast transfer has already been done.
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Isn't that exactly what I said in my post above, excepting that I expanded on it somewhat ?
Quote:
Originally Posted by ausastronomer
As I indicated in another recent thread physics supports my experience having looked through in excess of 100 different SCT's over the past 30 years from both Meade and Celestron ranging in aperture from 5" to 16". I am yet to look through one that can equal a top grade Newtonian of equal aperture as a visual planetary telescope. They do a very good job of planetary imaging because unlike for visual use the larger central obstruction and increased number of reflective and refractive surfaces does not affect their performance and their long focal length becomes an advantage due to the larger image scale.
Cheers,
John B
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If you read some of the posts
I made in this recent thread you will see exactly what I think of SCT's as visual instruments. Here are some of my posts replicated below
Quote:
Originally Posted by mental4astro View Post
Marc, you make SCT's sound like poison !
SCT's are fine visual instruments.
Quote:
Originally Posted by ausastronomer
Hi Alex,
I would say they are "reasonable" visual instruments. Pure physics determines that a SCT cannot equal the optical "visual" performance of an equivalent aperture newtonian, or refactor. Nor can a MCT for that matter, although a high quality MCT like a Questar, Quantum, Astrophysics or Intes and Intes Micro will put up a very good show.The 7" Meade MCT is an excellent scope, but it won't outdo a good 7" newt or refractor. Keep in mind also that these Maks are premium scopes and not typical of the mass produced SCT's. In 40 years of searching for the ultimate SCT that can prove the physics wrong, I am yet to find one, despite a heck of a lot of trying. That doesn't mean that some with good optics can't give nice views, they can't match the performance of an equal aperture newt or refractor, although the good ones with decent optics and properly cooled can put up a 1/2 decent fight. Unfortunately a lot of SCT's have marginal optics. Rod Berry (Rodstar) sold his rather expensive 10" Meade LX200 after 2 years of frustration. Every time we observed together, which was quite often back then, Rod would be continually dissappointed with the high power views in his scope compared to my 10" newt. I felt the performance of Rod's scope was typical of the performance of a lot of other SCT's I had looked through over the years from both Meade and Celestron, ranging in aperture from 5" to 16". Rod subsequently sold the SCT and bought the Mary Rose (20"/f5 SDM) and hasn't regretted his decision one bit. The performance of Rod's scope was partly due to mediocre optics and partly due to the cooling issues of a closed tube design. My 10"/f5 GSO newt, which has a great mirror in it for an $800 scope will pull 500x on night of good seeing. My 10"/f5.3 SDM with Suchting primary will pull 675x (5mm Pentax XW + 2.5x TV Powermate) on a night of good seeing. How many mass produced SCT's can do that?
That's not to say SCT's don't have their place. They make an excellent all rounder if you plan to do both visual and imaging with the one telescope and they offer a portable, transportable package. The quality of lunar planetary images Damian Peach takes with 11" and 14" SCT's is testament to that. However, if you want ultimate visual performance from a telescope then a SCT is not the best choice IMO.
Richard has indicated he is only interested in visual astronomy. If storage and transport are not a concern then a 10" GOTO dob ($1,499) is a way better choice than an 8" SCT IMO. He can buy one of these new for what an 8" SCT will cost him 2nd hand if he's lucky; and will see a whole lot more tnan the 8" SCT can show him.
http://www.bintel.com.au/Telescopes/...oductview.aspx
If transport and storage are issues then a 10" collapsible tube dob may be a better option.
http://www.ozscopes.com.au/skywatche...telescope.html
If Richard wants to try imaging at a later time he could start with a small refractor for imaging and keep the newtonian for visual.
Cheers
John B |
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Quote:
Originally Posted by SkyWatch View Post
I assume that when John talks "physics", he is referring to the large secondary of the SCT's that causes a loss of contrast and therefore slightly poorer view to an equivalent newt or refractor. This however would only be if the newt had an optimised (minimum size) secondary
Quote:
Hi Dean,
There's a bit more to it than just the size of the secondary central obstruction (CO), although that in itself is important enough. In addition the SCT, ACF or MCT have an additional 2 refractive surfaces, as well as the 2 similar reflective surfaces of a Newtonian.
I have attached a table which shows the light loss comparing an unobstructed 10" aperture, to an optimised Newtonian with an 18% CO, to a mass produced Newtonian with a 25% CO and a SCT design with a 34% CO. Some SCT models have the CO down as low as 32%, some are as high as 38%. Some Newtonians have a lower CO as well. As you can see from the table the Optimised Newtonian has an 11.5% light gathering advantage over the SCT design and the mass produced Newtonian has an 8.2% light gathering advantage over the SCT design. The experts who have conducted laboratory tests on human visual perception will tell you that it takes a 5% change in light intensity to be perceptible to the human eye. In both cases the numbers are well in excess of this. Notwithstanding any perceptible difference in the view on bright targets it can make a significant difference in the observers ability to detect targets on the verge of visibility for a given aperture. In essence a 10" Newtonian will see "deeper" on threshold targets than a 10" SCT.
In addition the larger central obstruction and additional refractive and reflective surfaces of the SCT design cause further degradation of the image quality and reduce contrast. Each optical surface in a system introduces a small degree of light scatter and diffraction. The less of these surfaces the better.
If you have a look at Damien Peach's Optical Simulation you will see the effect of a change in Central Obstruction from 20% to 30% in part 2 of the simulation examples. It's not huge but noticeable.
http://www.damianpeach.com/simulation.htm
This website explains the effects of central obstruction on the MTF curves
http://www.telescope-optics.net/obstruction.htm
In addition to the central obstruction effect the additional refractive surfaces cause further image degradation.
Cheers,
John B
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Here is the table I posted in the other thread.
Now, if I don't really like SCT's, particularly as a visual instrument, why would I make the comment "If you want one scope for both visual and imaging they are a great choice and probably the best choice."
They are "reasonably" portable, transportable and storable. I live on an acre of land with a 3 car garage as a storage shed. I have no problems storing 18", 14" and 10" dobs. If I lived in a 2 bedroom unit in Sydney I doubt that would be the case. SCT's do a "reasonable" job of both planetary and deep sky visual astronomy and planetary and deep sky imaging. While not the best at anything they can do a reasonable job on all fronts. Because of their short tube length and thus moment arm they can be used for imaging on a medium sized EQ mount. Dobsonians make easily the best visual instruments IMO, for both planetary and deep sky, but they are a poor choice for imaging. IMO an EQ table doesn't help. Just ask Paul Haese who spent about 20K on an 18" SDM dob for planetary imaging and sold it 6 months later and bought a SCT. Refractors give aesthetically nice images but because of their limited aperture they cannot match a medium / large aperture high quality Newtonian as a visual instrument. Ant that is how I came to make the comment I did, nothwithstanding that I would never own a SCT.
Cheers,
John B