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Equipment Discussions >> Cats & Casses

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JohnH
Carpal Tunnel
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Reged: 10/04/05

Loc: Squamish BC Moved!!!!!
Re: Why do people even buy SCTs??? new [Re: MikeML]
      #6223578 - 11/29/13 02:51 PM

SCT's are for the most part, the equivalent of a Ford Focus or some other car costing around $10K-15K.

They fill the need of people needing to get around to work, shopping kids from school and the occasional road trip. Good all around. Inexpensive, able to do most things well but not outstandingly.

SCTs are the telescopic version. A good scope to learn on and is all most people will ever need. Does imaging and visual; is easy to maintain and keep clean; and is useable most of the time.

Newtonians have the edge for imaging faint objects visually but are heavier and need more adjusting; Maksutovs also stay cleaner and well adjusted and are heavier and more expensive; corrected D/K's and R/C's are suited for demanding imaging with higher costs and less utility for visual use


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kkokkolisModerator
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Reged: 09/23/09

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Re: Why do people even buy SCTs??? new [Re: jimbo728]
      #6223597 - 11/29/13 02:59 PM

SCTs (and all Cats) are in fact heavily moded Newtonians. Take a fast Newtonian, replace the secondary with a f/5 mirror, add a Schmidt lens to seal the tube, voilà. Then open the tube because sealing brings problems, add fans, then dew heaters, then dew shields (that make them long again) and ofcourse focal reducers to make fast again what you made slow. Fast again but with added glass! Then add a refractor/ finder for widened views and a diagonal to replace the removed diagonal/ secondary. You still see a reversed image! Have an Amici somewhere close for terrestrial. Find a capable mount. Now add Ethos corrected to f/4 although your SCT isn't f/4 at all. Or remove the secondary to make it fast again for astrophotography. Oh! I forgot the coatings! Add coatings. And the glass should be good. Beware not to brake it.
Now you have your SCT.
And you start moding it. Bob's knobs is just for start.
Somewhere in the moding process, one might make it a Newtonian again!


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: Footbag]
      #6223600 - 11/29/13 03:03 PM

built quality?
be more real. look at interferometer test on mr. Rohr or on VAGO - these objectively show us most SCT "quality" - strehl ratio even without central obstruction be about 0.5 !
http://fidgor.narod.ru/Observers/Test/test_104.html
http://fidgor.narod.ru/Observers/Test/test_22.html
http://fidgor.narod.ru/Observers/Test/test_30.html

yes, there are some exemplars who show strehl about 0.85 and up, even 0.96 or so - but when we count central obstruction, who in general is larger than on newtonians, big problem with thermal equilibrium, a rough corrector plate plishing and so on - SCT have no chances to good newtonian, imho.


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WesC
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Reged: 02/06/13

Loc: La Crescenta, CA
Re: Why do people even buy SCTs??? new [Re: kkokkolis]
      #6223608 - 11/29/13 03:08 PM

I don't agree with that. SCTs and Newtonians are very different designs. They work differently, run differently and have different pros and cons. The only similarities are that they are folder optics and need to be collimated. But even those similarities are different.

Nope. Apples to pomegranates.


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Footbag
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Reged: 04/13/09

Loc: Scranton, PA
Re: Why do people even buy SCTs??? new [Re: drago]
      #6223609 - 11/29/13 03:08 PM

Quote:

built quality?

SCT have no chances to good newtonian, imho.




So... What is a good Newtonian?

Strehl doesn't matter if your focuser cannot hold your CCD camera.


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WesC
Carpal Tunnel
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Reged: 02/06/13

Loc: La Crescenta, CA
Re: Why do people even buy SCTs??? new [Re: Footbag]
      #6223611 - 11/29/13 03:11 PM

And for every link you post showing a bad SCT. I can post an excellent one.

This proves NOTHING!!!!!


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Why do people even buy SCTs??? new [Re: jimbo728]
      #6223629 - 11/29/13 03:26 PM

Even without a flocking mod - man I'm bonkers over my sct. I don't care about apo arguments and the like. For my dollar and effort - my c6 is a rock star.

You are right though Ed - the sct seems to get kicked like a whipping boy at times - particularly the C11 .

Pete


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rmollise
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Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: kkokkolis]
      #6223633 - 11/29/13 03:29 PM

Quote:

SCTs (and all Cats) are in fact heavily moded Newtonians. Take a fast Newtonian, replace the secondary with a f/5 mirror,




Well...kinda...sorta...not really. The primary mirrors are different...the Newt's primay, unless it is of a very high focal ratio, must be a parabola. The SCT's is a sphere. And the secondary is not an "f/5 mirror," it is a convex mirror that magnifies 5x. It would be just as correct to say the Newtonian is a simplification of the earlier Cassegrain design.


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kansas skies
sage


Reged: 12/02/12

Loc: Kansas, USA
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6223637 - 11/29/13 03:32 PM

As for me, I really don't care for the Dobsonian design. I like the motorized equatorial tracking (although a mechanical clock drive would be cool). For this reason, I find the 8" SCT to be a perfect setup for adequate aperture and portability. The key here, as someone already mentioned, is proper collimation. I might add that I have nothing against Newtonians, and I really would like one day to build something in the 16" to 20" range. If and when I do, it will have either a driven fork or a driven yoke equatorial mount.

Bill


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crow
sage
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Reged: 07/09/12

Loc: BC, Canada
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6223662 - 11/29/13 03:46 PM

I honestly think a lot of refractor heads worry about the performance of a good sct as great optics in combo with a large amount of aperture you'd have an ideal scope. Just as portable in many respects, I think my 1100 Edge weighs as much as a TOA130, pretty easy to handle.

You pay a hell of a lot for a good refractor so SCT's have to be bad lets face it.

If you find a good one, which is far easier than its made out, stick some fans in it or buy a cooler and enjoy. Some simple cooling is about all you need. learn how to Collimate it, so as not to be intimidated and thats it.

I enjoy a good 4' refractor too, to me its the ideal combo.


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orion61

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Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6223663 - 11/29/13 03:46 PM

Quote:

Quote:



they cannot hold even against cheaper newtonians. newtonians have smaller central obstruction, less light loss and scatter taht catadioptrics. imho only cat's pluss in general is their compact size. all another compared to newtonians and refractors, is minuses...




Newtonians are fine if you just want to look. If you decide you want to do more than that, the SCT is far more versatile and adaptable.



Possibly if you are talking about the Average SCT's of years back. But the Quality of todays Schmidts are MUCH more consistent, and I firmly believe better.
My C6 SCT regularly performs up to my Criterion RV6.
There was a Time when I'd never have believed it.
I have never owned a 9.25, but My opinion of the Celestron C6 is that it is the perfect scope. Portable, and enough Horses under the hood to perform with the Big boys!
I also agree with Rod, they are far more flexible than a Newtonian. Binoviewing, Photography, Ease of transport,and other tangible features. Including the best deal for the money.
I used to love the C5, but it cannot compete with the C6, just like the C6 cant compete with an Edge 8.
Great thread guys!


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A6Q6
professor emeritus


Reged: 05/31/11

Loc: Stroudsburg,Pa,U.S.A
Re: Why do people even buy SCTs??? new [Re: azure1961p]
      #6223668 - 11/29/13 03:51 PM

Ok Pete and Eddgie, Whats up with the "flocking mod", what will it do for my old classic orange tube C5 that I'm having so much fun with? What do I have to do and is it really worth It?

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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Why do people even buy SCTs??? new [Re: Eddgie]
      #6223677 - 11/29/13 03:54 PM

I used a C8 only for about 10 years. It served me well. For the past 18 years, I have viewed only with Dobs. For viewing only, I agree, at least for me, that the Dob is the better choice. For AP? Probably refractors or SCTs or MAKs.

I see one important question in all this: over the years, why haven't manufacturers made adjustments and upgrades to the SCT? The Edge and AC optics are an improvement; but, I believe many more could have been done. Mirror cool down issues alone would keep me from buying another SCT. True, you can make the modifications yourself. But, why not build them in? Doing so would increase the cost? Yes, but modifying an SCT also costs, and many people would find making their own modifications to be a daunting task.


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kkokkolisModerator
Carpal Tunnel
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Reged: 09/23/09

Loc: Piraeus, Greece
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6223700 - 11/29/13 04:11 PM

Quote:

Quote:

SCTs (and all Cats) are in fact heavily moded Newtonians. Take a fast Newtonian, replace the secondary with a f/5 mirror,




Well...kinda...sorta...not really. The primary mirrors are different...the Newt's primay, unless it is of a very high focal ratio, must be a parabola. The SCT's is a sphere. And the secondary is not an "f/5 mirror," it is a convex mirror that magnifies 5x. It would be just as correct to say the Newtonian is a simplification of the earlier Cassegrain design.




I was joking ofcourse. Lost, perhaps, in translation.


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Kon Dealer
professor emeritus


Reged: 01/05/11

Loc: Cambridge UK
Re: Why do people even buy SCTs??? new [Re: GeneT]
      #6223728 - 11/29/13 04:31 PM

I love my 6SE. My 8SE gives even better views.
That said I use my 4" refractor more than the 6 and 8SE put together.
The basic problem is at 52 North the seeing raely supports over 150x. That along with cooling..
Still for the occassional times the SCTs are well worth it.


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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Why do people even buy SCTs??? new [Re: drago]
      #6223731 - 11/29/13 04:32 PM

Quote:

built quality?
be more real. look at interferometer test on mr. Rohr or on VAGO - these objectively show us most SCT "quality" - strehl ratio even without central obstruction be about 0.5 !
http://fidgor.narod.ru/Observers/Test/test_104.html
http://fidgor.narod.ru/Observers/Test/test_22.html
http://fidgor.narod.ru/Observers/Test/test_30.html

yes, there are some exemplars who show strehl about 0.85 and up, even 0.96 or so - but when we count central obstruction, who in general is larger than on newtonians, big problem with thermal equilibrium, a rough corrector plate plishing and so on - SCT have no chances to good newtonian, imho.




Ill say one thing about the two Parks interfero's I've seen of scopes Ive owned - the d---ned lines are STRAIGHT. None of this swirly swaying grass in the wind garbage. I feel bad for anyone owning those mirrors - not that I haven't had equal or worse in the past.

Ill tell you though - the optician who opens shop with the offer of straightening out those poor sct optics will have his work cut out.
At anyrate I can't believe the C6 I have would put out the whimsical interferograms linked to.

Pete


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Why do people even buy SCTs??? new [Re: orion61]
      #6223758 - 11/29/13 04:52 PM

Quote:

I also agree with Rod, they are far more flexible than a Newtonian. Binoviewing, Photography, Ease of transport,and other tangible features. Including the best deal for the money.




My first serious scope was an orange tube C-8. Since then I have owned a Meade 8 inch and two 5 inch Celestron as well as setup alongside many SCTs. Just a few thoughts:

In the smaller sizes, SCT's are easier to transport but this is not true once the scopes are large enough that a Truss Dobsonian makes sense. Setup, that depends on the mount.. But typically a Dobsonian is more easily setup but part of that is simplicity of the mount. A 16 inch Dob can be an easy one person setup, a 25 inch is still a one person setup. A 16 inch SCT is not really a portable, one person setup... 25 inch SCTs, they just don't exist as portable scopes.

Versatility, flexibility: At the eyepiece, Newtonians are more versatile, more flexible, they cover a wider range of magnifications, they can compete with refractors in terms of low power, wide field views. My 130mm F/5 will do 3.7 degree TFoV. That is not happening with an SCT.

Binoviewing, as Eddgie is doing, is possible with a Newtonian as is photography though probably not as easily done as with an SCT.

Jon


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jgraham
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Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Why do people even buy SCTs??? new [Re: Jon Isaacs]
      #6223990 - 11/29/13 06:37 PM

Hmmm, I love my SCTs. They are the best general purpose scopes that I own. None of mine have been modified in any way.

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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why do people even buy SCTs??? new [Re: azure1961p]
      #6223999 - 11/29/13 06:42 PM

Quote:

Quote:

built quality?
be more real. look at interferometer test on mr. Rohr or on VAGO - these objectively show us most SCT "quality" - strehl ratio even without central obstruction be about 0.5 !
http://fidgor.narod.ru/Observers/Test/test_104.html
http://fidgor.narod.ru/Observers/Test/test_22.html
http://fidgor.narod.ru/Observers/Test/test_30.html

yes, there are some exemplars who show strehl about 0.85 and up, even 0.96 or so - but when we count central obstruction, who in general is larger than on newtonians, big problem with thermal equilibrium, a rough corrector plate plishing and so on - SCT have no chances to good newtonian, imho.




Ill say one thing about the two Parks interfero's I've seen of scopes Ive owned - the d---ned lines are STRAIGHT. None of this swirly swaying grass in the wind garbage. I feel bad for anyone owning those mirrors - not that I haven't had equal or worse in the past.

Ill tell you though - the optician who opens shop with the offer of straightening out those poor sct optics will have his work cut out.
At anyrate I can't believe the C6 I have would put out the whimsical interferograms linked to.

Pete



About every 6-8 months someone dredges up those toilet test results.
The worst of the worst. It's a wonder they don't dig up every bad Dynamax that has been sold and resold on EBAY!
Why don't you show some of the bad mirrors from Cave that went out in the last year or so. Sad fact. I love those old Caves, and Criterion, Parks, Edmund so on, and on.
Anyone is more than welcome to stop by and compare the views from my RV6 and C6.. at least there are no annoying diffraction spikes, or in the case of the RV6, (big clips) 6 of them. I will never sell my old criterion, I love it, but it is much more limited and frankly a pain to move and
use, Especially in the slightest of breezes.
Set minds are just that. A Donkey sitting in the middle of the road..
I used to be one of them...


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Why do people even buy SCTs??? [Re: Eddgie]
      #6224004 - 11/29/13 06:45 PM

I can think of just one reason, really, but it's a pretty good one. SCTs offer an attractive tube-length-per-unit-of-aperture (TLPUA) ratio. How else are you going to fit an 8" mirror into a 16" long package?

- Jim


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