Schmidt Cassegrains and DSOs

[Newfie]

I have a cpc 1100 and find it performs well visually on DSO'S allthogh i have not had a chance to compare it against much else.

[HD-AP]

Optical quality is most critical in planetary work, as you are looking at very fine detail. A scope which excels at planetary and lunar can hardly be poor quality. One more point to realize is that a scope below about 6" in diameter (and few of us have APO scopes beyond that) "looks" through a single seeing cells (roughly), which means seeing tends to make stars wobble about a bit, but not to blur so much. In bigger scopes, light from the same point source passing through different parts of the aperture travel through different seeing cells at each point along the path, which results in more blurring and less wobble.

I agree optical quality is critical when looking at very fine detail ,my points were regarding reasons schmidt cassegrians not appearing to perform as well as apo refractors and newtonians of high quality . I wouldn't say the sct's excel at planetary and lunar they are ok , but it depends what you are comparing them to.

All of my scopes are slow by modern standards > f6 Apo 7" f9 to name one.

I was not saying that a 4 inch scope would out perform an 11 inch scope ! And i do know how seeing cells affect different apertures. All reasons why no one scope fits all.

[michael.h.f.wilkinson]

I agree optical quality is critical when looking at very fine detail ,my points were regarding reasons schmidt cassegrians not appearing to perform as well as apo refractors and newtonians of high quality . I wouldn't say the sct's excel at planetary and lunar they are ok , but it depends what you are comparing them to.

All of my scopes are slow by modern standards > f6 Apo 7" f9 to name one.

I was not saying that a 4 inch scope would out perform an 11 inch scope ! And i do know how seeing cells affect different apertures. All reasons why no one scope fits all.

The OP mentioned his scope was fine on planets and the moon, and not everyone knows the extent of seeing effects, hence my comment. The OP also argues that his (modest-sized) fracs perform better on DSOs than the C11, which is odd, as the C11 should be great on faint fuzzies (just not on the extensive ones). I do find SCTs vary quite a bit in performance (collimation and figure of the optics can vary a lot, but certainly for planetary imaging, they seem to be amongst the best, especially in the hands of guys like Damian Peach. The views of planets through my C8 can be absolutely stunning (and the C9.25 in particular has earned a reputation for outstanding planetary performance, regardless of its 34% secondary obstruction.

Having said that, SCTs will be outperformed by Newtonians or apochromatic refractors of equal aperture and optical figure. An 8" APO of good quality would certainly outperform my C8 in all aspects, except price and portability. However, even the difference in optical performance may be smaller than some people think. For me, chromatic errors are far worse than the slight increase in diffraction rings caused by a large SA.

[HD-AP]

                             

[allen g]

Permit me to add to the mix. I have a TV NP-101 (4")refractor and a Meade LX 90 8" ACF SCT. Comparing them I find, hands down, that the refractor is superior on star clusters and nebulae, especially open and wide, respectively. For example I can see wide star clusters in their entirety that I could see in my sct. I saw the veil like it was in a book with my refractor but could not pull that off with my sct. On faint DSO like globulars the 8" sct wins. So, they make a great companion to each other. Now on planets I find something of interest. At 180X on my refractor I get wonderful views of planets. It takes approximately 300x in my sct (of course on good nights) to get a view of maybe equivalent quality. The resolution in the refractor appears better on planets at less magnification. Do others have similar experiences.

[ollypenrice]

'Resolution' is a can of worms as a term in astronomy.  You can define it as the Dawes limit if you like, in which case it is entirely determined by aperture in diffraction limited optics.

However, the word greatly predates the calculations of Mr Dawes and means something different in different contexts. To my mind the only useful defintion for a visual observer is 'the fine detail which can be detected.' Sometimes the small scale contrasts of refractors, and their smaller beam width through 'cells' of bad seeing, may make a difference.

However, my affection for refractors is not based on that in any shape or form. It's simply based on the sense of engagement I find that I get when refracting! I'm certainly not alone in this since the term 'refractor-like images' is a commonplace. I don't see as deep or, necessarily, resolve as much detail in a refractor but I just enjoy myself more. I also think that peoples' eyes may well vary in terms of the optics which best suit them since this old chestnut will never go away.

Olly

[Moonshane]

excellent post Olly. I agree with the 'vive la difference' approach too. just wanted to make sure I understood what resolution was in this case. it seems more like your description.

[Naemeth]

Imad, what is your longest FL eyepiece? As others' have said, you may simply be using too much power on DSOs, and you would probably be better with long FL eyepieces, like the 55mm TV plossl . You don't need to worry about a large exit pupil in an F/10.

[Pig]

Ive never tried a refractor unless my finder scope counts, the views of DSO through my SCT are better than my brothers 200p

2 totally different scopes.

Im not comparing scopes here I am just commenting on what we have both observed when using the same eyepieces at the same location

[Moonshane]

at the same sort of magnification and all other factors being equal, that's what I'd expect. that said, 'better' is probably in the eye of the beholder I suspect.

[Mark in Macc]

Sorry to say it but a load of people are still banging on about slow scopes not being suitable for DSO observing. That is one whole load of foetid dingo's kidneys. An F/10 scope with a 20mm EP gives the same visual appearance as an F/5 with a 10mm (assuming similar optical quality). The amount of light per unit of solid angle reaching the eye is exactly the same. For visual, aperture is king. The only drawback of a slow scope is that the maximum FOV is a bit more limited (1.34 deg in my 8" F/10) for visual work.I have both an 80mm F/6 triplet APO (from APM), and an 8" SCT. Guess what? The SCT is the better DSO hunter by far on about 95% of the 793 DSOs (including 395 galaxies) I have bagged to date. This is simply a matter of aperture being king in visual. My C8 bagged me a mag 13.2 galaxy this summer; try that with an 80mm. Only for a handful of wide-field objects (M31, M33, the Veil, the North America Nebula and Pelican, the M45, the Magellanic Clouds, and perhaps half a dozen more) does the 80mm come into its own with its 5.3 deg FOV with the 31T5. The view of pinpoint stars is stunning, but remember that this only in part due to the lack of central obstruction. The remainder is due to the simple fact that you do not use as much magnification in the smaller scopes (65.5 times is the minimum mag I use in my C8, vs 15.5x for the 80mm, the C8 regularly gets good views at 203x, which I never use with the 80mm). This means I will see much more of the contribution of the atmosphere in my C8. With a C11, these problems are worse.So, for me, which is the better DSO hunter, for the vast majority of faint fuzzies, given the choice of an 11" SCT or a 6" APM-class APO? The 11" SCT, any time. Will a 12" Dob outperform it visually on DSOs (and at a fraction of the cost)? Sure it will, assuming the optical figures are similar.The APM does show more contrast at a given exit pupil size. At the same magnification the situation can be different, as the resolving power, and light gathering power of the C8 are so much larger.

I couldn't agree more on this point. My C9.25 is excellent for DSOs, albeit with a narrow FOV.

[Pig]

The OP mentioned his scope was fine on planets and the moon, and not everyone knows the extent of seeing effects, hence my comment. The OP also argues that his (modest-sized) fracs perform better on DSOs than the C11, which is odd, as the C11 should be great on faint fuzzies (just not on the extensive ones). I do find SCTs vary quite a bit in performance (collimation and figure of the optics can vary a lot, but certainly for planetary imaging, they seem to be amongst the best, especially in the hands of guys like Damian Peach. The views of planets through my C8 can be absolutely stunning (and the C9.25 in particular has earned a reputation for outstanding planetary performance, regardless of its 34% secondary obstruction.

Having said that, SCTs will be outperformed by Newtonians or apochromatic refractors of equal aperture and optical figure. An 8" APO of good quality would certainly outperform my C8 in all aspects, except price and portability. However, even the difference in optical performance may be smaller than some people think. For me, chromatic errors are far worse than the slight increase in diffraction rings caused by a large SA.

Go get em Floyd

[hobsey]

As we are on SCTs and DSO.

What sort of DSO would you get with a 127? I wouldn't expect mind blowing views but just enough to give a newbie the wow factor.

I have been waiting for someone to mention the 127s and what they can get but no one has chipped in as I guess this discussion is all about aperture and high end scopes.

[RikM]

The 127 Maks are nice compact scopes that have a very loyal and passionate following. 5" is a bit on the small side for DSO observing but you can get respectable views of the brighter objects. They don't take up that much room in the car for going on holiday with either and pack a serious punch under dark skies.

The thing to think about is, if it's DSO's you are interested in, you can get an 8" Dob for almost £100 less than a GoTo Mak, or a 6" for £200 less. Either Dob will give you better views of...well, everything, by a considerable margin. The downside is bulk and finding things yourself (which may not be a downside, depending on your point of view).

[hobsey]

At the moment I'm not too bothered about DSO, I was just curious to know what sort of images you may get in case I get board of looking at the same things every time I go out.

Many thanks for the reply Rik. 

[Moonshane]

as Michael said above, a 5" scope at 100x will provide the same kind of views as any other 5" scope at 100x within reason, especially of fainter objects. the field will be the same and the brightness of objects will be the same. there will be subtle differences due to design but they will be approximately similar. so a 130mm newtonian will be roughly the same as your 127 mak. the best way to find out what sort of views your scope will provide with DSOs (or any other target for that matter) is to try it and look

[hobsey]

try it and look

I'm trying, believe me but its quite hard living in a flat with no garden. Fingers crossed I will get a decent go with it soon.

[Moonshane]

I see (and feel) you problem. hope you get a chance soon.

[michael.h.f.wilkinson]

I'm trying, believe me but its quite hard living in a flat with no garden. Fingers crossed I will get a decent go with it soon.

In that case a scope that you can easily transport somewhere else is ideal. The compact size of the Cassegrain derivatives is useful in that case

[Luke]

Well I have had a few different types of scope so far and it would be boring if we all liked the same scopes, but I have to say, the scope that surprised me the most was the C6. That was one very impressive scope in a highly portable package.

We got it with a CG5 mount and were mainly interested in the mount (Celestron's pricing meant that the scope plus mount cost almost the same as just the mount).

We were in no hurry to try the C6, as we had an 8 inch scope as far as I recall.

When i finally tried it, I got hooked on it. What a magnificent scope for its size. Sarah, my other half, wondered why I was starting to hog it. A few views later, and she was highly impressed too.

I do love fracs, they do give such a fantastic view, and I wonder if Olly's awesome Tec140 under his dark skies would be a bit like an 8 inch refractor under my okay UK skies, in which case I can imagine it would be wonderful for viewing DSO's.

I prefer 8 inch aperture upwards for general DSO observing under my home skies. At the SGL star party's darker sky, my C6 felt more like an 8 inch scope at home. I saw some amazing sights through bigger scopes, but I returned back to have a look at the same objects in my C6 and the view, while not as bright, was still a very nice sight indeed, so I was very chuffed at the punch my C6 packed, especially under those darker skies. It was handy that the tube was so compact, with tent etc. stuffed in the car on the long journey.

My C6 has since gone but it took an age to part with it because we were so impressed, and it's only gone because we have a very nice Edge 8 that's portable enough for our needs.