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What do people see in Schmidt-Cassegrains?

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#1 milleniumfalcon

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Posted 02 October 2014 - 07:40 AM

Something that I have wondered about for quite a while - why are Schmidt-Cassegrains so popular?

 

They are much more expensive than Newtonians of a given aperture - and never seem to perform quite as well.

Portability is commonly cited - but when factoring in a beefy mount and tripod, they will not be as portable as a dobsonian of equivalent aperture.

The goto and tracking popular on SCTs can be had on dobsonians - and often work extremely well with features such as dual encoders.

 

For a comparison, take the orion xt8g and the celestron cpc 800. The dobsonian performs slightly better, has an equally good goto system, costs half as much, and is more portable.

 

So what gives?

 



#2 Max Power

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Posted 02 October 2014 - 08:03 AM

Look at the planetary pictures on: ALPO Japan Latest

 

All the best ones are with Celestron 14"ers.

 

No refractors need apply and Newtonians need monster mounts.



#3 seawolfe

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Posted 02 October 2014 - 08:04 AM

Can you do long imaging exposures with a Dob?  As far as I know, most Dobs are on an Alt-Az mount which doesn't track the sky like a GEM mount which is for imaging.

 

My best friend has two Dobs, a home built 8 inch and an Orion XX12g.  While both give excellent views, he can't image outside of putting a smart phone or a little point and shoot to the eyepiece.



#4 jgraham

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Posted 02 October 2014 - 08:21 AM

Well let's see here...

 

"The dobsonian performs slightly better..." It does? Based on what?

 

"...has an equally good goto system..." Are you sure about that? I'm not too sure about that particleboard mount. But, to each their own.

 

"...costs half as much..." Now that's true, and easily measured.

 

"...and is more portable." No way. This may be a personal thing, but I find Dobs klunky and clumsy to transport. Tripods fold up, rocker boxes take up space and catch on things.

 

Cats in general pack a lot of scope into a small package. Dobs are economical. I use a great variety of scopes (Dobs, Cats, Refractors, commercial and homebuilt) and each has their niche. I just don't see that much overlap between Dobs and Cats when it comes to their respective spheres of performance.

 

Some like Chevy's, other Ford's, some even like Subaru's...



#5 MitchAlsup

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Posted 02 October 2014 - 08:21 AM

Why are they popular:

a) compact size--bot in use and in storage

b) equitorial mount with clock drive or more recently alt-az mount with goto

c) versitile--visual, photo, CCD, computerized

d) consistently tollerable optics

e) not "that" expensive



#6 Brian Carter

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Posted 02 October 2014 - 08:38 AM

Convenience. My 10" dob gives slightly better (and brighter) views than my C8, but I have to haul it in and out of the house, or the car, set it up, collimate it, and then star hop to see what I want.

My C8/C6 can be set up in five minutes, plus a little more for aligning the mount. It takes a bit to cool down, but I can still cool dinner while that is happening. I need to collimate them, but not often. It's an f/10, so I can save money on eyepieces.

Mostly though, I like the no-fuss factor. A Dob is always a project scope for me. I like to fiddle with it, improve it, add bits and pieces... I just don't have time for that part of the hobby. IT was great all through college and grad school, but now I'm employed, married, and a parent. There's bo time in my life for hours in the garage fiddling with a hobby. I just want to go out and look at something, with as few issues as possible.

The SCT just works. Not the best at anything, but it works just fine. All scopes are a compromise: a refractor is perfect, but limited aperture and expensive. A dob is a light bucket, but most don't have Goto or tracking (at least as well as a good dedicated mount), and fast mirrors need precise collimation, setup, and eyepieces. Plus, a big dob, despite what people like to say, is not a convenience scope. A Cat is not as great optically, but it works ok and I can comfortable sit and view through it without fuss. I like that more than I like optical perfection or a giant mirror.

Someday this compromise will change for me though, luckily there is a scope that will meet my next compromise. We are very fortunate to have options.

#7 milleniumfalcon

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Posted 02 October 2014 - 08:46 AM

jgraham - i see what you mean. The performance difference is slight - SCTs tend to have slightly lower contrast than equivalent newtonians (I've used star tests to check the optical quality of all of my scopes, and always fine tune collimation before tests).

 

By higher portability - i mean that the CPC 800 (I recently sold mine) has the tube and fork arms without the tripod that weigh as much as the dobsonian complete.

But yes, it can fold up into a small package.

 

I switched to the dobsonian as it did everything the CPC did, and weighed less. 

 

I've owned and used a lot of these goto dobs, and they all work very well, just as well as celestron and meade SCTs.



#8 MikeBOKC

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Posted 02 October 2014 - 08:47 AM

Taking everything into consideration, SCTs are the best all-purpose scopes period.



#9 evan9162

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Posted 02 October 2014 - 08:47 AM

Sounds like someone is trying to stir up an argument.

 

-Compact and lightweight for its aperture

-Simple collimation

-Longer focal length means easier to reach higher powers w/o barlows or extremely short focal length eyepieces

-Easier on eyepieces than faster scopes

-Versatile focusing.  Eyepieces, cameras, binoviewers all come to focus w/o additional optics or extension tubes

-Reasonable eyepiece position, even on an EQ mount

-Non-moving accessory holder means heavy eyepieces/cameras/binoviewer don't affect focuser operation

-No diffraction spikes around bright objects



#10 rmollise

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Posted 02 October 2014 - 08:53 AM

Something that I have wondered about for quite a while - why are Schmidt-Cassegrains so popular?
 
They are much more expensive than Newtonians of a given aperture - and never seem to perform quite as well.
Portability is commonly cited - but when factoring in a beefy mount and tripod, they will not be as portable as a dobsonian of equivalent aperture.
The goto and tracking popular on SCTs can be had on dobsonians - and often work extremely well with features such as dual encoders.
 
For a comparison, take the orion xt8g and the celestron cpc 800. The dobsonian performs slightly better, has an equally good goto system, costs half as much, and is more portable.
 
So what gives?


OK, I'll assume you are not engaged in pot-stirring for the sake of watching the fur fly (yeah, mixed metaphor :lol:)...

The versatility.

Leaving aside your comment that the little Dob "performs better" (not in my experience, especially compared to an Edge 800), an SCT can do many things since almost all are sold on tracking mounts; usually even the fork mounts can be used in equatorial mode. That opens up imaging, spectroscopy, photometry, any number of things beyond the "just looking" of a little Dob. A C8 on an inexpensive VX mount will be capable of far better tracking than one of the goto dobs. Even tracking dobs much more expensive than the Syntas Orion is selling.

SCTs are, especially when mounted on GEMs, much more portable than a Newtonian.

The SCT is the PC of telescopes. It's been around forever and just about every accessory that can be imagined is available and will work even with old SCTs.

I could go on. :lol:

Edited by rmollise, 02 October 2014 - 08:55 AM.


#11 t.r.

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Posted 02 October 2014 - 08:57 AM

Well let's see here...

 

"The dobsonian performs slightly better..." It does? Based on what?

 

"...has an equally good goto system..." Are you sure about that? I'm not too sure about that particleboard mount. But, to each their own.

 

"...costs half as much..." Now that's true, and easily measured.

 

"...and is more portable." No way. This may be a personal thing, but I find Dobs klunky and clumsy to transport. Tripods fold up, rocker boxes take up space and catch on things.

 

Cats in general pack a lot of scope into a small package. Dobs are economical. I use a great variety of scopes (Dobs, Cats, Refractors, commercial and homebuilt) and each has their niche. I just don't see that much overlap between Dobs and Cats when it comes to their respective spheres of performance.

 

Some like Chevy's, other Ford's, some even like Subaru's...

 

Ditto :waytogo:



#12 dotnet

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Posted 02 October 2014 - 09:18 AM

Schmidt-Cassegrains are very suitable, convenient, comfortable to use, and portable narrow-field visual instruments, but most of all they are a great gateway drug into the world of Maksutov-Cassegrains  :p

 

I love Dobs, own and use one, but I recognise that they are compromised by their very concept. Out of necessity (keeping the physical size and weight manageable) their are much faster than one would like for visual use, and with that comes a grab bag of issues – the sky looks washed out, all aberrations are magnified, there's heaps of coma, the light cone is steep requiring extra-special eyepieces, etc. They are also less suited for narrow-field observing, if that's a concern. Nevertheless, they are the cheapest ticket into the large aperture show, unless one demands premium build and mirrors.

 

In the end, unless you specialise in one particular type of observing, you'd want one of each. Just my humble opinion.

 

Cheers

Steffen.



#13 eklf

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Posted 02 October 2014 - 09:19 AM

Speaking for myself, after owning an Xt8 on tracking platform and comparing to a C8 on an alt az over several months, here is why I favored the C8.

 

 

1) Ease of transport - both the dobson base and newtonian OTA are too large to store long term in the trunk of my car - I needed to move them each time I observed.  Its not just the weight, but the bulky unwieldy nature that was bothersome to move to and from my 3rd story apartment.  It can certainly be done, and I did it for years.  The SCT on other hand is just easier.  My alt az mount - Vixen custom D - folds up compactly and is stored long term in the trunk - very easily.  Form factor of C8 is such that I can easily carry it one one hand (with an attached handle) and don't have to undergo multiple maneuvers while going up and down the four flights of stairs.  I grab the C8 in one hand, eyepiece case in the other and walk down the stairs.  Dob required three trips (four if using the tracking mount).  I feel old and lazy enough where this is a big deal to me.

 

2) Easy viewing position.  I use a vestile adujustable star chair, but have to move the height less frequently.  Also, the dob required me to bend forward at an awkward angle that put too much pressure on a busted lumbar disc.  The SCT feels like I am bending ergonomically and does not irritate my lower back.  I can view in more comfort.  

 

3) I recently acquired a nexstar mount and I can still carry the mount + eyepeice + battery in one go (the nexstar tripod rests besides the vixen in the trunk.  

 

I retained the C8 and sold the Xt8.


Edited by eklf, 02 October 2014 - 09:24 AM.


#14 jhayes_tucson

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Posted 02 October 2014 - 11:26 AM

Upper:  DOB

Lower:  SCT

 

Contest over.

:lol:

 

 

...John

 

 

 

Attached Thumbnails

  • Star Trails sm.jpg
  • M27 24 subs 96m cropped neutralized stretched PS V2 sm.jpg


#15 Misha

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Posted 02 October 2014 - 11:39 AM

I have a click-lock on my EdgeHD 8". I am strictly visual. I love the fact that I can almost always rotate the eyepiece to a position where I can see comfortably while sitting. Not possible with a dob.



#16 WesC

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Posted 02 October 2014 - 12:18 PM

LOL@John!

 

 

Yeah this is nothing but a thread to stir the pot.



#17 ewave

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Posted 02 October 2014 - 01:31 PM

ummmm..the Moon, the stars and some planets.



#18 Jon Isaacs

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Posted 02 October 2014 - 02:11 PM

 

SCTs are, especially when mounted on GEMs, much more portable than a Newtonian.

 

 A few thoughts:

 

My first serious scope was an Orange Tube C-8..  First:

 

What do people see in SCTs:  Pretty much the same things they see in other scopes. The longer focal lengths mean for a given aperture, the maximum possible field of view will be considerably less, for a given aperture, the contrast will be less than a refractor and somewhat less than a Newtonian. But in general, people see the same things in an SCT that they seen in other scopes.

 

To address Rod.. 

 

In the smaller sizes, compared to a Dobsonian, SCTs are portable and are reasonably easy to setup but somewhere between 12 and 14 inches, the tide turns.   Setting up a 16 inch SCT is definitely not the easy 10 minute, one person setup of a 16 inch Strut or Truss Dob.  

 

As far as Dobsonians being inexpensive, certainly many are but just as certainly, one can invest in a premium Dob with essentially perfect optics, drives and GOTO as desired but it won't be cheap..  

 

SCTs are black boxes, you get what you get. Dobs, they are open architecture, as was said, they require more fiddling and tuning. For some, that's a virtue, for some, that's a hindrance.

 

Jon Isaacs

 

P.S. For J. Hayes: 

 

6287012-10 inch Dob on EQ platform.jpg
 
No star trails there.. Yes, the mount costs more than $2500 new but it will handle a 18 inch scope, has dual axis drives and autoguiding.. 
 
Jon


#19 jhayes_tucson

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Posted 02 October 2014 - 02:22 PM

Cool!

 

I wanna see a photo through it!   ;)


Edited by jhayes_tucson, 02 October 2014 - 02:24 PM.


#20 Eddgie

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Posted 02 October 2014 - 02:48 PM

Something that I have wondered about for quite a while - why are Schmidt-Cassegrains so popular?

 

They are much more expensive than Newtonians of a given aperture - and never seem to perform quite as well.

Portability is commonly cited - but when factoring in a beefy mount and tripod, they will not be as portable as a dobsonian of equivalent aperture.

The goto and tracking popular on SCTs can be had on dobsonians - and often work extremely well with features such as dual encoders.

 

For a comparison, take the orion xt8g and the celestron cpc 800. The dobsonian performs slightly better, has an equally good goto system, costs half as much, and is more portable.

 

So what gives?

 

 

Compact size.   My EdgeHD 8" and 8SE mount can be packed into a space smaller than the base assembly of my 12" dob.  If I go on vacation in my Prius, it is feasible to put the 8SE in the back and not loose all of my luggage room, and I have a lot more aperture and better performance on all targets than my 110mm ED refractor.

 

Comfortable seating position.   The SCT places the eyepiece very close to the mount, and this means that from horizon to zenith, it is possible to easily reach the eyepiece without having to stand or kneel on the ground.   My 8" EdgeHD 8" on my 8SE mount is the most comfortable scope to use that I own.

 

Better planetary performance than small APO refractors costing three times as much..  Why carry out a 4" APO for planets when it weighs as much as the EdgeHD 8", has dim images at 200x, and can't show as much detail.

 

Cost is reasonable.   Not as cheap per inch as a Dob, but far less than a 120mm ED refractor, again, with much better performance on almost all object types.

 

Per inch of aperture, SCTs tend to give up to almost all other designs in terms of absolute performance, but getting a slightly bigger SCT will offset those dificienceies, and you still have all of the advantages I mentioned earlier.

 

Now, that being said, my primary observing scope is a 12" Go-To dob.  I also own a 6" APO (Talk about expensive and requiring a big mount!!!) and a 110mm ED refractor.

 

My EdgeHD 8 is my least used scope, but when I need something to take on a trip, or to use for neighbors or guests, the scope I almost always turn to is the EdgeHD 8".  

 

Heck, even Roland Christen used to have a C8...  Probably still does...

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Edited by Eddgie, 02 October 2014 - 02:53 PM.


#21 rmollise

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Posted 02 October 2014 - 03:06 PM

As far as Dobsonians being inexpensive, certainly many are but just as certainly, one can invest in a premium Dob with essentially perfect optics, drives and GOTO as desired but it won't be cheap..


True, but the pilgrim asking for eternal wisdom specifically mentioned the Orion Dob. :lol:

#22 SteveGR

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Posted 02 October 2014 - 03:52 PM

For some of us, diffraction spikes are a deal killer.  While I don't HATE them, and accept them as unavoidable in Hubble images or large research scopes, I definitely don't like them and don't want them in the images I make.  So, say I want to image, and I don't like spikes, Newts are out, RCs and DKs are out, and Maks have cool down issues, limited aperture and long focal length, essentially leaving me with (barring some of the really complicated designs) refractors and SCTs.  If I want aperture, on my budget, that means SCTs.  That's my reasoning, which is why I will be buying a SCT and a little refractor for wide field stuff.  :)



#23 jzeiders

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Posted 02 October 2014 - 04:02 PM

Perhaps one should consider that when SCTs became popular most amateur telescopes were 6-8" Newtonians on 100 pound equatorial mounts, often undriven and frequently ATM products. I had a mount made of 2" cast iron pipe that was turn on threads, no drive for my ATM 8" f/8. John Dobson was hauling long focal length low tech, low cost large telescopes around the west coast and beyond,  setting them up right in the middle of big cities and many national parks bringing astronomy to the general public. A few had refractors, mostly 2.4" (60mm) or maybe a 3" Unitron or Tasco. Very rare was a big long focus Jeagers you might see in someones back yard observatory. Most refractors were f/15 doublets.

 

The Celestron SCT offered a compromise product that could be made to do just about anything ok for a high but reasonable price. Back then $1k was a lot of money. You got a f/10 8" scope complete with a drive, tripod and an eyepiece. You could transport it in a regular car and it fit in a small corner of a closet. Bang for the buck was really good and still is. In todays dollars they are very cheap. 

 

Technology is relatively recent and not always easy to use. Goto Dobs are ok as are goto Alt-Az mounts. If it makes you happy and meets your needs fine. The humble SCT isn't the best at much of anything, but it does just about everything ok and at a reasonable price.

 

I prefer a collection of scopes of different types for different reasons and purposes. Yes, there is an old C-8 from either the late '60s or very early '70s, I'm not sure. It has a concentric focuser.  It gets used now and then and still works fine. Yes, it has the old sand cast fork mount and AC spur gear drive.

 

Jack



#24 Stephen Kennedy

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Posted 02 October 2014 - 06:00 PM

I have never owned, or even used, a dobsonian and the only SCT I have is a Japan Special Optics (JSO) Space 10.  The Space 10 has an aperture of 100 mm and is F/10.  It has a helical focuser instead of a moving primary mirror so I am not 100% sure it qualifies as an SCT but is free of the mirror shift that many people do not like about conventional SCTs.  During its brief existence in the 1980s and 1990s JSO made some really great telescopes and this is one of them.  Optically, it is as good as a 3.5" Quester but with (100/88.9) = 1.125 the resolving power and (100/88.9)^2 = 1.265 times the light gathering capacity.  The build quality is exquisite and while mounted on A Vixen Super Polaris mount is not as portable as a Quester, you do not have to find a table to set it up on when out observing.

 

I like the JSO SCT and at times wish it had more aperture but I am reluctant to purchase a large Meade or Celestron SCT because I fear they simply will not match the JSO in optical and mechanical quality.  If someone like JSO came along and built a larger SCT as a hand made premium product rather than a mass market consumer item, I would take a close look at it. 

 

I do most of my observing with a 210 mm F/7 Mikage Newtonian reflector on a Pentax MS-5 GEM with a PE of +/- 3.5 arcseconds.  It is optically and mechanically as close to perfection as you can get but is 1980s technology and at 175 kg is not very portable.  Still, I would never trade my steel GEM and OTA for a telescope made out of particle board that you have to keep pushing if you want objects to stay centered.     



#25 Jon Isaacs

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Posted 02 October 2014 - 07:34 PM

Cool!

 

I wanna see a photo through it!   ;)

 

I would too but honestly, I am not into A-P.. 

 

Jon




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