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SCT's, the perennial bridesmaid??!

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#1 rdl800

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Posted 23 January 2011 - 07:39 AM

I often read that SCT's are great all-purpose telescopes, but, there is nothing that they do that can be considered THE best. In other words, SCT's hold a lot of kings in the deck but no aces..... What does this forum think about that?
 

#2 skybsd

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Posted 23 January 2011 - 07:56 AM

Hello,
I've also often read the "Jack of all trades, master of none." comments..,

However, consider this:
SCT = Aperture/Price x Portability

As a visual-only DSO observer that wants GoTo, the SCT's numbers from the above formula makes it a winner in my book - every time.

When looking at telescope designs, to me, its not about what's the theoretical best - its about what actually works for me, at my location, for my requirements.

For some..., for what they deem important to them, its Dobs, or Newtonians, or Maks, or RCs, or refractors...

Others will no doubt have their own impressions and opinions..,

Regards,

skybsd
 

#3 teskridg

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Posted 23 January 2011 - 08:15 AM

Add in comfort/ergonomics and the SCT excels even more. Among amateur enthusiasts, I suspect the SCT is likely at least as popular as refractors and Newtonian reflectors sales wise. Tim
 

#4 t.r.

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Posted 23 January 2011 - 08:21 AM

If your going to have one scope to do it all, taking into account portability/price/aperture, anyone that would choose to argue against the selection of an SCT, has already lost the debate. To select another scope design requires a compromise in one of these criteria. The SCT is the compromised design. There is a reason it is the most sold telescope design in the world. It is the "Jack of all trades-Master of none" by its very nature. This should be viewed as its hallmark, not as its detriment!
 

#5 SandyHouTex

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Posted 23 January 2011 - 08:24 AM

In my opinion they excel at portability by aperature and equatorial driveability by aperature. Meaning I can't beat the amount of driven aperature my C14 gives me. A 14 in. refractor is a beast which must be permanently mounted and a DOB doesn't have the long term drive capability. The closest to meeting these two advantages would be a 14 in. Newtonian, but I still think my C14 on a Losmandy G11 can't be beat. Just my $0.02.
 

#6 Eddgie

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Posted 23 January 2011 - 09:19 AM

In terms of absolute optical performance, the SCT is not even a "Mid Pack" player. It is near the bottom of the stack.

The desing is highly compromised towards packaging and ease of manufacture.

It has the biggest obstruction of any telescope in common use visually (some people use astrographs visually, but the contrast is very poor on scopes with a 40% obstruction, marginal on a telescope with 34%, but you can fix that with aperture).

The obstruction robs contrast.

It has multiple optical elements. This impacts light throughput. While you can improve it considerably with advanced coatings, in the end, when you also factor in the shading of the primary from the secondary and baffling, you loose a lot of light. It is the least efficient of all telescope designs in common use.

The standard SCT has coma and field curvature. The coma by itself is not really any worse than maybe an f/6 reflector, but the f/6 Newt will have an almost flat field, while the SCT will have a field that is curved to about 1/4 of the radius of the focal length. This field curvature means that the comatic blur itself is not sharply focused at the edge of the field, so that you need a corrector to manage this. The EdgeHDs incorporate this corrector, but at the expense of another couple of percent of light loss. The ACFs fix the coma, but the blur remains unfocused.

The baffles in the SCT design restrict the scope to a relativly narrow field. When pushed, it vignettes.

In fact, the SCT design is not even a King. It does nothing as well as any other design except for the fact that you can get a big one and mount it on a GEM or Alt-Az fork, and have a lot of telescope. In the bigger sizes, the transmission and contrast become less of an issue compared to much smaller scopes. I big SCT still makes a better planetary telescope for exampe, than a small reflector or refractor.

So, the performance isn't even "Good all around" in an ABSOLUTE sense. Reflectors and refractors made to the same quality standards and same aperture will outpreform an SCT in every way.

This is why I vasty prefer refractors as small scopes. Up until the aperture gets to be about 8", I think the SCT compromises too much. 8" and above though, the PACKAGING becomes so important to me that it becomes the "preferred" telescope for my own personaly needs. Also, at this size, the contrast can still be relativly high as compared to much smaller scopes, and the light transmission losses are relativly less important. And rarely do you need much more than a half degree of FOV for the most challanging deep sky objects. I can fit M42 in the field of a low power eyepeice even in my C14.

Jack of all trades to me overstates the absolute performace of the SCT. It is simply a brilliant compromise in terms of getting the very edge of acceptable performance in a decent size, easily manageable telescope package that is easy to manufacture.

Sorry.. Don't intend to burst any bubbles, and I LOVE SCTs, but to call it a King overplays it's rather poor hand of optical perfomance. Only when they are big do they make much sense to me.
 

#7 rdl800

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Posted 23 January 2011 - 10:05 AM

great replies thus far... keep em coming!
 

#8 alrosm

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Posted 23 January 2011 - 10:41 AM

The other day I saw a short documentary from ESOcast, guys looking for a new site, they want to build another big one in Chile, the portable scope they had with them on top the hill to test the site was a SCT, ok not even middle of the pack optically but kinda of good enough for them...
 

#9 woodsman

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Posted 23 January 2011 - 10:45 AM

Ok, here's my take. I liked especially that "the SCT is at the bottom of the pack" comment. I'll just say this, I'm a visual observer. I don't get into astrophotography. I could go out and get something like a 12 inch refractor for $20,000. I could go out and get a 16" refractor I suppose, and spend more. Instead I have as one of my "pack" scopes an old C14. I paid a little less than $3K with the fork mount, wedge, and tripod. I'll put that up against any Takahshi for the same price any day of the week. Will it have the same contrast as the Tak? No. Will the stars be as pinpoint sharp as the Tak? No. So what? I can see much more nebulosity, fainter galaxies than the Tak as well. I can take my C14 anywhere. I can set it up faster than a 14 inch refractor, and I would assert that I can set it up faster than a Dob as well. What time does it take to set up a 14 inch truss tub Dobsonian? It takes me 5 minutes to set up my C14. I don't have to collimate each time I use it. I can set up my C5 in faster time than that. My C8 is about the same.

I was out last night with my C5. The stars were nearly pinpoint sharp, and I didn't notice ALL this Coma that is talked about on a regular basis here. For those who have heard all the horror stories about comatic problems with SCTs, let me say this, don't believe all the talk. Its no big deal. I don't even notice it, when I'm viewing. Its so miniscule that for years and years I simply didn't believe that my C8 or my C14 had it. I thought that my mirrors were corrected so well, that it was gone. Then I began to really try to see it, and yes, at the very edge of the field, you can just notice a little bit of elongation of the stars. It is subtle, but its there. Again, so what??

I'd disagree with some of the posters here, the performance all-around in my estimation is not good, its GREAT.

My feeling is that hey, you could compare a $25K vehicle with a $75K vehicle. I would hope that you get a bit more for the money. One may not have heated seats, and GPS, but they both get you where you want to go. To be honest, all this talk about comparisons is a lot of hogwash. For the money, the SCT for me is king, both in what is offered for price, the quality, and the ease of use. Sorry, I'd say you get a lot of bang for the buck!

For me, there is no comparison. :grin:
 

#10 Arizona-Ken

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Posted 23 January 2011 - 10:46 AM

Ya know, I've got a really good scope. I really like it.

I love to go out to dark sites with the club and work on lists of deep sky objects. (I love to look at deep sky objects.) I can put the scope in my little SUV with all the accessories I need, and easily put it up all by myself at my dark sky sites. It is in such a great, small package for its aperture size that the stock alt-az mount on the tripod is absolutely rock-steady for my visual work. I don't have to worry about long moment arms causing vibration during slewing or having to consider dampening times when I touch the telescope.

It's aperture is large and collects a lot of light and provides a lot of detail to me visually on deep sky objects and even on the planets.

I don't have to get on a ladder or step-stool to see through it. I also don't have to get into any contorted positions to use the scope. I use a pneumatic stool, a drummer's throne, that I easily move up and down to settle in and get those great views.

It has a GOTO computer on it, and even one of those GPS things that tell the scope where it is. Really easy to set up. Out here in Arizona, dew is a consideration only a few months a year during the winter, and then only occasionally. I get a lot of great views all the time, and the climate and typical seeing conditions out here are certainly a large part of that.

It wasn't exactly cheap, but I know guys who have spent far more money for far less aperture, and also guys who have spent less money for a bulkier, harder to handle scope.

I've read all the prior posts and largely agree with them. I understand all the subtle, theoretical things about light transmission and throughput, baffle tubes, contrast, obstruction percentages and the like. I also think that these items have been presented clearly and very well. But ya know, sometimes my head hurts to think about these things when I can instead plan for my next dark sky outing.

The scope does everything I want it to do. It's a SCT, a CPC1100, and it really works for me.

Ya know, I've got a really good scope. I really like it.

Arizona Ken
 

#11 ahlberto

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Posted 23 January 2011 - 10:49 AM

HI.From mi short experience on astronomy since my first scope was bought about 4 years ago and having tried many scopes,the C8 is my weapon of choice.For planetary on those few and rare nights of good seeing the views are outstanding on jupo per ex. with the 7mm Pentax XW giving razor sharp views.I envy no scope on planetary.For deep sky,well,bigger is better but this trusty old friend keep me well satisfied.For ex. at a star party at a good dark sky i was comparing a few deep sky objects between a Orion XT10" and the C8 at targets like NGC4565,the Needle Galaxy,the difference was almost none.
Anyway,all designs have their limitations.None is perfect and maybe the SCT design is taking the heat because having manage the shortcomings inherent to newts and fracs designs...expectations...I ,like a true cat lover,expect he performs like a really good and well collimated cat would do,-i do not want frac or newt views ,and for that, i have a newton wen i want newt views and so on...
Bottom line is..Do i am happy?!?! You bet,very happy.All my scopes receive equal attention and care and love them all. ;)
 

#12 darkman

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Posted 23 January 2011 - 11:02 AM

Well, let’s see: my current stable is a Cat, an RC, and a refractor. Just recently parted with an 8” Dob, 5” Mak, and a 5” Newt.

I used to take the Dob out to hunt DSO’s, or if it was Planets on the menu the Mak came out. The newt spent it’s time on the Moon or clusters. So, three different scopes for three different applications.

Now days (nights actually), the Cat goes on the mount and I can chase Nebula, Galaxies, Planets, and if I’m feeling frisky I can look at satellites and asteroids, all in the same night with the same scope, an SCT. I can use bino’s if I want, or slap in a Radian, or if I want to take photos, just mount the cameras and refocus. The moving mirror design means (to date) I’ve never had a combination that wouldn’t foucs.

That’s why the other scopes moved on to new homes, just don’t need them anymore. The SCT does the same work, it’s easier to set up, much easier to use.
 

#13 Eddgie

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Posted 23 January 2011 - 12:16 PM

It seems that we are in violet agreement.

The things you said are true, but all of them basically boil down to what I said earlier...The packaging is great. This is why I own two.

The optical performance though, inch for inch, simply doesn't stack up very well.

It is supposed to be the "Jack of all trades and master of none" but this implies that it is perhaps better than some things than some scopes and not as good at some things as other scopes.

Per inch of aperture, except for a pure astrograph with large central obstruction, the SCT finishes last to all other designs (except cheap MCTs with similar sized central obstructions).

I LOVE SCTs. My FAVORITE scope is an SCT.

But just like you, it is the packaging that draws me to it. I like sitting to observe, and I demand a tracking mount. Nothing provides this like an SCT does.


No, the SCT is not a particularly competent performer when compared to almost any other design on an inch per inch basis, both on and off axis (and off axis is where refractors REALLY shine), but you can get a big one and put it on a GEM or Alt-az fork, and view for hours without fatigue. Maybe it isn't as good as a similer size f/8 reflector, but the difference in performance is small enough that the compromise to me is worth it and I don't know anyone with an equitorilly mounted 14" reflector that can sit in a chair with a blanket over their lap in the winter while observing Jupiter for 30 minutes.

The SCT is ALL about a compromise to packaging and it does this by pushing the envelope as far as possible to get a telescope with "Acceptable" optical performance. The on axis performace losses just about the maximum contrast that is acceptable, the off axis field curvature combined with the coma push it to the level that the scope can barely pass muster at the edge of modern wide field eyepieces, and the off axis field illumination is at the very edge of what it could be without being obvious to the observer.

In other words, it is a BRILLIANT design in that it puts a LOT of aperture in the smallest possible package, but it does this by pushing acceptable limits.

Not great. Not even better at this or that then the other choices... Just passable.

I love them and am not bashing them, but at the same time, the physics of the desing simply mean that compared to a similar sized f/6 reflector or similar sized f/8 refractor, they just don't deliever the same level of performance.

Supersize it though (as compared to the other scopes) and it can hold its own.. A little extra aperture fixes a lot of sins.

And that again is the beauty of the SCT. You CAN get extra aperture and still wind up with a small, compact scope that is comfortable to use.

We both like them for the same reason.
 

#14 SandyHouTex

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Posted 23 January 2011 - 12:36 PM

I have to disagree with you Eddgie. I purchased an 8se about 9 months ago and the optics are really outstanding. Properly cooled, I don't really feel like I'm compromising at all when it comes to optical performance.
 

#15 woodsman

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Posted 23 January 2011 - 12:39 PM

Maybe it isn't as good as a similer size f/8 reflector


So stick a focal reducer in the light path, and now you have a f/6.3 or f/7 scope..

How about the idea that the mirrors are somewhat protected against salt air in areas of the country where that is a problem. How many times have I read about scopes that are for sale on one coast or the other, whereby they need complete recoating of their optical surfaces? I have a 1972 C14 with the silver coated mirrors on 3 surfaces (diagonal). That's 39 years old. My C8 is 33 years old. My C5 is 37 years old. For each one, I'd say the mirrors look showroom new, aside of a few specks of dust. VERY rarely do I hear about SCT mirrors needing recoating.

Just curious as to what your take is on that?
 

#16 woodsman

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Posted 23 January 2011 - 12:50 PM

By the way, I would certainly disagree with your assertion regarding the optical performance.

Not great. Not even better at this or that then the other choices... Just passable

Yes, I certainly do enjoy going out on a regular basis and observing through a scope that I can barely stand to look through because of ALL those optical abberations.. :roflmao:

I'd respecfully have to disagree with your assertions so far. I DO believe that you are going just a TAD bit overboard in your assertion that SCTs are "just passable". Tell me what kind of car do you drive? Is it a Bentley because ALL the other ones have such flaws? :grin:
 

#17 woodsman

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Posted 23 January 2011 - 01:02 PM

One last thing, something that I believe can be considered THE best, since that was what the original poster was asking. A fork mounted SCT is in MY opinion THE MOST comfortable way to visually observe the night sky. There you go ONE ACE in the deck. I've used refractors on GEM mounts and I personally don't like GEM mounts. For the same aperture and focal length refractors LOSE BIG time in the comfort department. I've spent many a night ON THE GROUND trying to use a refractor to observe stars near the zenith. I've never observed in a Newt, but if I was to look through an f/8 Newt say on the order of 12 inches in diameter mirror, I'd need a ladder, I would bet. The Same with a Dob. I can comfortably look through my C14 at stars near the Zenith and I'm not on the ground, or in the sky. There you Go, ONE ACE in the hole!
 

#18 Arizona-Ken

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Posted 23 January 2011 - 01:16 PM

Edggie:

We are in agreement. I also find your posts are well done, thoughtful and accurate. I think your experience with all scope types lends credence to your opinions and posts and they subsequently are valuable to us all.

I certainly don't think you're bashing SCTs, but I think you might be selling their optical quality a little short.

The total package really is the beauty of the design. I guess that's a main reason why they're so popular.

Arizona Ken
 

#19 jimb1001

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Posted 23 January 2011 - 01:43 PM

It seems that we are in violet agreement.

The things you said are true, but all of them basically boil down to what I said earlier...The packaging is great. This is why I own two.

The optical performance though, inch for inch, simply doesn't stack up very well.

It is supposed to be the "Jack of all trades and master of none" but this implies that it is perhaps better than some things than some scopes and not as good at some things as other scopes.

Per inch of aperture, except for a pure astrograph with large central obstruction, the SCT finishes last to all other designs (except cheap MCTs with similar sized central obstructions).

I LOVE SCTs. My FAVORITE scope is an SCT.

But just like you, it is the packaging that draws me to it. I like sitting to observe, and I demand a tracking mount. Nothing provides this like an SCT does.


No, the SCT is not a particularly competent performer when compared to almost any other design on an inch per inch basis, both on and off axis (and off axis is where refractors REALLY shine), but you can get a big one and put it on a GEM or Alt-az fork, and view for hours without fatigue. Maybe it isn't as good as a similer size f/8 reflector, but the difference in performance is small enough that the compromise to me is worth it and I don't know anyone with an equitorilly mounted 14" reflector that can sit in a chair with a blanket over their lap in the winter while observing Jupiter for 30 minutes.

The SCT is ALL about a compromise to packaging and it does this by pushing the envelope as far as possible to get a telescope with "Acceptable" optical performance. The on axis performace losses just about the maximum contrast that is acceptable, the off axis field curvature combined with the coma push it to the level that the scope can barely pass muster at the edge of modern wide field eyepieces, and the off axis field illumination is at the very edge of what it could be without being obvious to the observer.

In other words, it is a BRILLIANT design in that it puts a LOT of aperture in the smallest possible package, but it does this by pushing acceptable limits.

Not great. Not even better at this or that then the other choices... Just passable.

I love them and am not bashing them, but at the same time, the physics of the desing simply mean that compared to a similar sized f/6 reflector or similar sized f/8 refractor, they just don't deliever the same level of performance.

Supersize it though (as compared to the other scopes) and it can hold its own.. A little extra aperture fixes a lot of sins.

And that again is the beauty of the SCT. You CAN get extra aperture and still wind up with a small, compact scope that is comfortable to use.

We both like them for the same reason.


I've had many different scopes, starting with 3" Unitron 40 years ago.

Now, its a Celestron 9.25 and a 90mm refractor.

Is coma an issure with the Celestron? I don't know, I put objects for view in the middle of the eyepiece, seldom at the edge.
My 9.25 has no false color. It gathers so much more light than the refractor that I only use the 90mm for photography, never visual. And that's only because of the wider field.
So I think you need to be more specific. Is a 9.25 better than a 90mm refractor? Imho, in every way.
Is a 4" sct better than a 4" refractor? Never.
Generalities don't shed much light on the subject when talking telescopes.
 

#20 Chassetter

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Posted 23 January 2011 - 01:45 PM

:gotpopcorn:
 

#21 rdl800

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Posted 23 January 2011 - 02:27 PM

a wealth of great posts.. exactly what I was hoping for..

Topics like this I never get tired of. I thought I was starting to become an expert on SCT's, but judging from the quality of these posts I got a long way to go.

Thanks to all who have made this debate such a good one!
 

#22 stevew

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Posted 23 January 2011 - 02:55 PM

SCT's hold a lot of kings in the deck but no aces..... What does this forum think about that?

:fishing: :fishing: :fishing: :smarty:
 

#23 davidpitre

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Posted 23 January 2011 - 02:56 PM

If your going to have one scope to do it all, taking into account portability/price/aperture, anyone that would choose to argue against the selection of an SCT, has already lost the debate.

My 10" Chinese dob is considerable lighter than my 11" SCT, considerably easier to move, I can sit in a chair to use it, can resolve double stars considerably better, has much better contrast (much smaller central obstruction), has almost 2x the FOV, can cool down in half the time despite the fact that the SCT has active cooling, and cost 1/3 the price.
Portability/price/aperture the newtonian wins ...and it has better optics than any SCT I've ever used, and I've owned at least one sample of every SCT size available less than 16".

I'd say Eddgie's comments are spot on. There are good reasons to own them, and they are fun scopes to use. But be realistic.
 

#24 bobhen

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Posted 23 January 2011 - 03:26 PM

The SCT advantage is that it puts the largest aperture in the most compact tube.

Today, when most speak of an SCT they speak of mass-produced SCTs and the mass-produced SCT also adds reasonable cost to the advantage side of the ledger. For most, compact aperture and reasonable cost are good reasons to purchase.

In general, the mass-produced SCT is less well figured than other custom-crafted scopes of various designs and has some inherent design and manufacturing compromises that also negatively impact absolute performance to one degree or another.

Bob
 

#25 MikeBOKC

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Posted 23 January 2011 - 03:43 PM

Most people who enter this hobby (or who have been in it for a long time) are not in the income/wealth category to buy a TEC or AP refractor or a giant Newtonian. They do learn that aperture matters a lot, especially if they are like the majority of posters I see here on CN who must contend with appreciable light pollution. Hence an SCT between 8 and 12 inches is very often their best possible choice to get maximum aperture, utility and portability.

Adam Smith was right . . . the market determines everything.I don't know what the annual sales figures are for each type of scope, but I would bet that the general umbrella covering all composite scopes (SCTs, Maks, Mak-Newts, etc.) comes in right near the top. The simple truth is that even more avid amatuer astronomers are very happy with something short of perfection optically that they can afford and use, and that is very often an SCT.

I have owned all three general types of scopes, and currently have an SCT and a large achro refractor. Even if I had a medium or large Newtonian, most nights my scope of choice would be the CPC1100. It's the best performer I've ever had, with all the bells and whistles I care about, and really no operational drawbacks like awkward eyepiece positions. I also suspect that with the optical advances like the Celestron Edge SCTs will gobble up an even larger share of the imaging community.

There really is no king, but there are sure three equivalent princes -- SCTs, refractors and Newts. To me it comes down to persnal preference, with the SCT haveing a slightly broader audience because it just does more across the board.
 


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