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Equipment Discussions >> Cats & Casses

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MikeML
professor emeritus
*****

Reged: 10/09/04

Loc: NJ
Re: Why do people even buy SCTs??? new [Re: mmalik]
      #6227391 - 12/01/13 04:06 PM

Quote:

What gets lost in translation is visual vs. photographic performance. SCTs may be 'ok' for visual but it is a proven fact that they have lagged behind times in AP. An example here... (I hope OP wouldn't mind quoting him?) Regards




That may be true for the stock SCTs with the ubiquitous 6.3 reducer, but there are other reducers available and a new generation of SCTs.


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PowellAstro
scholastic sledgehammer
*****

Reged: 10/14/09

Loc: Tennessee
Re: Why do people even buy SCTs??? new [Re: MikeML]
      #6227406 - 12/01/13 04:17 PM

I am more than happy with the images from mine, they are sharp, detailed and all shot in Alt/Az in a small amount of time.

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Footbag
Post Laureate
*****

Reged: 04/13/09

Loc: Scranton, PA
Re: Why do people even buy SCTs??? new [Re: PowellAstro]
      #6227459 - 12/01/13 04:42 PM

I love imaging with an SCT. Sure, it's a bit slow, but there's nothing like having that reach for small galaxies or planetary nebula.

Yes. The long FL is hard to control and an SCT isn't suggested for beginners; but that doesn't mean it doesn't perform when operating on a sufficient mount.

The Edge, though, really is amazing. Perfect stars throughout the field. For long FL imaging, you cannot beat the price performance.


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rmollise
Postmaster
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Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: mmalik]
      #6227463 - 12/01/13 04:44 PM

Quote:

What gets lost in translation is visual vs. photographic performance. SCTs may be 'ok' for visual but it is a proven fact that they have lagged behind times in AP. An example here... (I hope OP wouldn't mind quoting him?) Regards




"Proven?" Maybe for you.


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kansas skies
sage


Reged: 12/02/12

Loc: Kansas, USA
Re: Why do people even buy SCTs??? new [Re: ATM57]
      #6227468 - 12/01/13 04:45 PM

Quote:

Quote:

Even for high resolution imaging the CO of an SCT does not reduce contrast to any ruinous degree or else images like these would not be possible.

CO's of 25-35% by diameter do have an effect on high magnification visual viewing of planets and the moon in scopes up to 8" in size when compared to same-sized (or slightly smaller) unobstructed scopes.

Once you get into 9" and larger scopes the central obstruction is a given in virtually every scope owned by amateurs - unless you know of some people with 9" or larger refractors in their backyards. From about 9" and up aperture and lens/mirror quality are pretty much the only variables worth considering.

I sometimes grow weary of the "CO is a disaster" way of thinking. As in so many debates here on CN, the truth is "it depends."

Dave




Excellent! Your statements are very true and I base this on almost 40 years of visual observation. Once you get past 8" the secondary obstruction becomes much less of an issue than it is exaggerated to be. Overall optical correction and temperature control are much more important. Even my 8" refractors had to temp equalize for their best performance.

Many are of the misunderstanding that "if it's obstructed then it is automatically inferior to my unobstructed instrument". As you have stated "it depends"

After 8" of aperture I find these points to be the most important regardless of design:

1. Optical design produced as close as possible to the design specification.
2. Excellent overall optical correction (final wavefront)
3. Smooth optical surfaces (very important)
4. Temperature control.
5. Good mechanical support.
6. Secondary obstruction. (Has an effect but seeing is normally the limiting factor as to what is seen through the eyepiece when you reach this size level. Reducing the obstruction reduces the instrument's sensitivity to seeing conditions)

I believe that a telescope that accomplishes these items will be an excellent performer.

I know some will disagree with this but it is based on decades of visual telescope use and experimentation and I'm sticking with it

Scopejunkie




Brings to mind that half-blind Cassegrain floating around out there in space.

Bill


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PowellAstro
scholastic sledgehammer
*****

Reged: 10/14/09

Loc: Tennessee
Re: Why do people even buy SCTs??? new [Re: kansas skies]
      #6227511 - 12/01/13 05:06 PM

Yes and only the best images ever seen. Why doesn't someone contact them and let them know it should of been an APO or Newtonian.

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ATM57
super member


Reged: 01/01/10

Loc: Tehachapi, CA
Re: Why do people even buy SCTs??? new [Re: kansas skies]
      #6227568 - 12/01/13 05:40 PM

Quote:

Quote:

Quote:

Even for high resolution imaging the CO of an SCT does not reduce contrast to any ruinous degree or else images like these would not be possible.

CO's of 25-35% by diameter do have an effect on high magnification visual viewing of planets and the moon in scopes up to 8" in size when compared to same-sized (or slightly smaller) unobstructed scopes.

Once you get into 9" and larger scopes the central obstruction is a given in virtually every scope owned by amateurs - unless you know of some people with 9" or larger refractors in their backyards. From about 9" and up aperture and lens/mirror quality are pretty much the only variables worth considering.

I sometimes grow weary of the "CO is a disaster" way of thinking. As in so many debates here on CN, the truth is "it depends."

Dave




Excellent! Your statements are very true and I base this on almost 40 years of visual observation. Once you get past 8" the secondary obstruction becomes much less of an issue than it is exaggerated to be. Overall optical correction and temperature control are much more important. Even my 8" refractors had to temp equalize for their best performance.

Many are of the misunderstanding that "if it's obstructed then it is automatically inferior to my unobstructed instrument". As you have stated "it depends"

After 8" of aperture I find these points to be the most important regardless of design:

1. Optical design produced as close as possible to the design specification.
2. Excellent overall optical correction (final wavefront)
3. Smooth optical surfaces (very important)
4. Temperature control.
5. Good mechanical support.
6. Secondary obstruction. (Has an effect but seeing is normally the limiting factor as to what is seen through the eyepiece when you reach this size level. Reducing the obstruction reduces the instrument's sensitivity to seeing conditions)

I believe that a telescope that accomplishes these items will be an excellent performer.

I know some will disagree with this but it is based on decades of visual telescope use and experimentation and I'm sticking with it

Scopejunkie




Brings to mind that half-blind Cassegrain floating around out there in space.

Bill




I'd like to share a real experience that I have shared elsewhere. Small telescope users please don't be offended:

I will share my own obstructed aperture eyeopener many years ago. While at a star party, a friend and I were basking in the glow of how well our refractors were showing Saturn to the attendees. My Vixen 4" F/10 and his 4" F/15 Unitron were kicking butt. A lowly Celestron 8 was near by. We lowered ourselves for a look-see at Saturn and to our pleasure, the image couldn't match our views. Enjoying our obvious telescope superiority, we continued to show the planet to any and all who wanted a view. As the evening progressed, unknown to us, the C-8 (which happened to be a good one) was coming into it's own. The owner never said a word as to the stunning view the scope was beginning to produce. After a couple of hours, to give myself another pat on the back for bringing such a good scope to the star party, I went over to the C-8 for another view. To my amazement the view was stunning. I would guess the scope was running at about 350x. The view was so nice that I had a hard time walking away. My puny 4" couldn't come close to the view the C-8 was putting up. No matter how much magnification I tried to run, I couldn't touch it. Talk about a large slice of humble pie. The owner of the C-8 never said a word but he could tell that I had just been blown away by his scope.

On this date I came to the realization that all SCT's were not bad as I had thought they were.

Scopejunkie


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Rick Woods
Postmaster
*****

Reged: 01/27/05

Loc: Inner Solar System
Re: Why do people even buy SCTs??? new [Re: Vondragonnoggin]
      #6227616 - 12/01/13 06:00 PM

Quote:

Was their a point to this whole thread? Every design has a compromise built in. Know your scopes, buy what you deem to be the best compromise to you for the application you'll use it in.

Move on and don't worry when someone spends time modding their scope. You can do it too if you want. If you never saw a difference after mods and someone else does, so what. No need to bake your noodle over it.

Enjoy your compromised scope whether its a big or small SCT, MCT, Apo, Dob, RC, Achro, or (name anything else because it all applies).






How dare you introduce common sense into a telescope debate!


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6227622 - 12/01/13 06:04 PM

Quote:

Quote:

Quote:

Was their a point to this whole thread? Every design has a compromise built in.




cannot imagine, what compromise is in an apo? :-O





Start with small aperture.




No need to go small with an apo.


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6227630 - 12/01/13 06:08 PM

Quote:

Quote:

on that view all is compromise.




No it's not. With 8-inches of aperture, you can see a lot of deep sky objects and details in may of them. In a 3 or 4-inch APO? Not so much. You can't defeat the laws of physics by throwing money at them.




You can see far more with a 20" Dob, and they aren't prohibitively expensive.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: JJK]
      #6227639 - 12/01/13 06:11 PM

Quote:

Quote:

Quote:

on that view all is compromise.




No it's not. With 8-inches of aperture, you can see a lot of deep sky objects and details in may of them. In a 3 or 4-inch APO? Not so much. You can't defeat the laws of physics by throwing money at them.




You can see far more with a 20" Dob, and they aren't prohibitively expensive.




But it's hard to take pictures with one. Or spectra...or... That's the thing...the SCT does many things well. And there's also no doubt that you'll see a hell of a lot more with an 8-inch as your primary scope than a freaking four.


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Itz marcus
Pooh-Bah
*****

Reged: 02/12/06

Loc: Brooklyn NY
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6227698 - 12/01/13 06:37 PM

Hi,
I didn't go for a sct because I was afraid of collimation and I also believed what was said about the co. I used my Eon 120 for he last 7 years? Man do I love he scope but I did want something bigger and I couldn't afford a six inch apo or a long focus achro (and I wanted an eight inch scope ). I finally took the plunge and bought. Used c8 at a fraction of the price of a used 120 Ed scope. Although I have to service it as the scope wouldn't hold collimation, when it was in collimation the views were amazing. Things were easier to see and I was able to see more in my light poisoned city than the 120. I hope to be able to keep the 120 but if I can't afford to keep both then it looks like the c8 will be the scope to stay and as much as I really love the Eon 120 I will reluctantly let it go.
Clear skies
Itz


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: Dwight J]
      #6227738 - 12/01/13 07:04 PM

Quote:

I collimated my old C8 twice in the 25 yrs. I owned it. And it was not pampered by never leaving home. It was packed up and moved three times a week and often on not nice roads. Pretty stable in my book. It was on a fork mount with a Byers drive and I took many astro photos with it not noticing how inherently unstable it was supposed to be. I wonder about all the hoopla about cool down time - it must be climate related b/c I have not had an issue with it. I know one club that sold their 7" AP apo refractor because all that glass would not cool down even giving it all night.
I don't relish laying on the ground to view the zenith and having to rais the tripod all the way up just for that privilege. I equally dislike climbing a ladder and having to bring one along. Moving my big dobs required a truck, dolly, and two guys.
Easy to critcize SCT's, everyone's whipping boy, by minimizing or glossing over the shortcomings of other designs. Sure, it would be nice to have a scope of every design to fulfill every need but a SCT offers the closest to meeting all those needs with one design. An overlooked quality that other designs often lack is backfocus, a serious issue with Newts.




Dwight, I don't see SCTs as "everyone's whipping boy". It's a legitimate and versatile design, and has brought real time astronomy to many folks at bargain-basement prices (I think they are still a great deal).

Oddly, some here use refractors and Newtonians as their whipping boys, which is silly. Those scopes, and others, are legitimate designs, and some manfacturers have implemented them exceptionally well.

For example, AP and Zeiss apos are indeed very expensive per inch or aperture. So what? They are a pleasure to use and the ones I've played with (and still do) produce stunning lunar, solar, and planetary images. The larger ones give some much larger scopes (e.g., Tak Mewlon 300) a run for their money on some DSOs (I have both, so I have no pony in that race).

Instead of getting a C-11, I decided to buy an AP 10" f/14.6 Mak-Cass. It is a well designed instrument (great optics and thermal management), and it too provides stunning views of the Moon, planets, and brighter DSOs.

Rod, with all due respect, this scope is much better than an equivalent aperture SCT (of course, this is meaningless, because one can't get this AP scope anymore), but when I want to look at DSOs in detail, I use a much larger Dob-Newt. Of course, if one can only have one scope, then you learn to get the most out of it. For the longest time, I observed solely with an AP Traveler (105 mm f/5.9). I pushed that thing to its limit and even star hopped to find DSOs (I used to enjoy that process over the use of go-to mounts). However, if one can afford more than one scope, or have friends with different scopes, that opens up new possibilities.

To others here, ignoring the limitations of any scope design or implementation, doesn't help anyone. When I was looking into getting an 11"-class scope over decade ago, I was put off by the poor image quality and the poor focusing mechanics of Celestron's offerings at the time (I never had and still don't have a preferred one scope design, so don't over interpret my observations). I looked into the latter and realized why it worked so poorly and how that issue could be easily mitigated. When I mentioned this on a SCT blog, I was soundly chastised for my blasphemy! Keeping one's head in the sand is absurd. Now, if the new crop of SCTs have improved optical and thermal performance, I could be interested.


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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Why do people even buy SCTs??? new [Re: JJK]
      #6227811 - 12/01/13 07:49 PM

Quote:

Quote:

Quote:

on that view all is compromise.




No it's not. With 8-inches of aperture, you can see a lot of deep sky objects and details in may of them. In a 3 or 4-inch APO? Not so much. You can't defeat the laws of physics by throwing money at them.




You can see far more with a 20" Dob, and they aren't prohibitively expensive.




The 20" Newt (Dob is a mount) is great, but I prefer not to have to climb up a 16 foot ladder to start at f/10.


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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Why do people even buy SCTs??? new [Re: EFT]
      #6227890 - 12/01/13 08:24 PM

Quote:

The 20" Newt (Dob is a mount) is great, but I prefer not to have to climb up a 16 foot ladder to start at f/10.




Why would you do that? A 20" F/3.8 is about the same focal length as an 8" F/10 SCT and doesn't require a ladder. For big light grasp at an achievable price, it's hard to beat a newt on a dobsonian mount.

Of course, everything is a tradeoff. That fast newt is going to be extremely sensitive to collimation. Its critical focus zone is going to be razor thin. It may have issues with narrow band filters. It's going to have limited back focus.

I've not had time to go through this thread, but I see two advantages to SCTs: Versatility and packaging. They may not be the best option at any given use, but they are going to be "good enough" at just about every use. The second advantage is that they pack a lot of aperture into their size and weight.

If I were forced to live with only one telescope, it would be an 8" SCT.


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jgraham
Postmaster
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Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Why do people even buy SCTs??? new [Re: WadeH237]
      #6227928 - 12/01/13 08:45 PM

+1 on the 8" SCT.

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: EFT]
      #6227980 - 12/01/13 09:17 PM

Quote:

Quote:

Quote:

Quote:

on that view all is compromise.




No it's not. With 8-inches of aperture, you can see a lot of deep sky objects and details in may of them. In a 3 or 4-inch APO? Not so much. You can't defeat the laws of physics by throwing money at them.




You can see far more with a 20" Dob, and they aren't prohibitively expensive.




The 20" Newt (Dob is a mount) is great, but I prefer not to have to climb up a 16 foot ladder to start at f/10.




Almost everyone here knows that the term "Dob" is colloquially applied to large Newtonian telescopes. I'm sure Newton wouldn't care about such usage.

FWIW, the 20" Obsession Dob-Newt I had didn't require a 16-foot ladder. These things used to come in relatively short focal ratios like 4.5 and 5 (even faster now in others' models). If you require a relatively long focal length for image scale, you don't need f/10 when you have a 20" primary mirror (the math is easy). I used a 3 step ladder (Little Giant Type 1A Safety Step Step-stool) with 7" deep non-slip rubber-coated steps, with a safety bar at the top one can hold onto). I usually used 0, 1, or 2 steps and it was a really comfortable viewing experience.

If you're really afraid of heights (I would certainly understand not wanting to go higher than 5' off the ground in the dark), the 18" Obsession or its equivalent requires at most a 6-pack of beer as a step stool to observe at Dobson's Hole.

Why would you want a 20" f/10 scope?

I regrettably sold that 20" Obsession (that had a pick of the litter Galaxy Optics primary). It provided phenomenal tack-sharp images. I used to store the scope in my Living Room and when I wheeled it outside, the thermals were terrible (no surprise; I should have stored it in my garage to shorten the equilibration time). Over time, what others call "tube currents" improved and a few hours after bringing it outside, the mirrors figure snapped into place. After that, M42 was a sight to behold (tack sharp E & F Trap stars, a sprinkling of reddish-appearing jewel-like stars in the heart of the nebula just S of the Trap, very deep and smokey nebulosity, the obvious blue/green nebulosity, thundercloud purple in the gossamer wings, obvious pastel rose, orange, yellows). The North American Nebular looked better than any B & W film image I've seen, the even the Trifid showed pastel colors (beyond the normal green and blue one perceives with any decent size scope). I resolved Omega Centauri to the core from a mountain ridge in WV (the cluster was only 3* off the horizon, we had great seeing that night).


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: WadeH237]
      #6227991 - 12/01/13 09:23 PM

Quote:

Quote:

The 20" Newt (Dob is a mount) is great, but I prefer not to have to climb up a 16 foot ladder to start at f/10.




Why would you do that? A 20" F/3.8 is about the same focal length as an 8" F/10 SCT and doesn't require a ladder. For big light grasp at an achievable price, it's hard to beat a newt on a dobsonian mount.

Of course, everything is a tradeoff. That fast newt is going to be extremely sensitive to collimation. Its critical focus zone is going to be razor thin. It may have issues with narrow band filters. It's going to have limited back focus.

I've not had time to go through this thread, but I see two advantages to SCTs: Versatility and packaging. They may not be the best option at any given use, but they are going to be "good enough" at just about every use. The second advantage is that they pack a lot of aperture into their size and weight.

If I were forced to live with only one telescope, it would be an 8" SCT.




Mine would be the AP 175 f/8. It's a great astrograph (the optional focal reducer/flattener gets you to f/6.1 or it can be used near prime focus at f/8.3 with the standard field flattener), and a superb visual instrument.


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Ed Wiley
scholastic sledgehammer


Reged: 05/18/05

Loc: Kansas, USA
Re: Why do people even buy SCTs??? new [Re: Cotts]
      #6228050 - 12/01/13 09:58 PM

I'm with Cotts on this one, otherwise it would not be possible for me to image doubles at F50 with my C11 and get within 0.2" of the Dawes Limit.

Ed


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Dwight J
scholastic sledgehammer
*****

Reged: 05/14/09

Loc: Lethbridge, Alberta, Canada
Re: Why do people even buy SCTs??? new [Re: JJK]
      #6228115 - 12/01/13 10:46 PM

SCT's may not be everyone's whipping boy, certainly not mine nor Uncle Rod's and many satisfied users. As far as bargain prices, they may be more economical to buy now but in 1983 my Super C8 cost $2000.
I do know what it is like to look through top quality optics. When I obtained my MN and compared it to my C8, the difference in contrast was obvious and my C8 was no slouch. It tested out as a very good example by Peter Cerevolo. The MN is optimized for visual and many cameras do not reach focus and the fan needs to run for optimal views. So even though it is a premium scope, it isn't completely capable in all areas. It also weighs more than my C11. The SCT design is capable in all areas but excels in almost none.


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