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Equipment Discussions >> Cats & Casses

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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: Vondragonnoggin]
      #6226751 - 12/01/13 11:04 AM

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


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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]
      #6226754 - 12/01/13 11:06 AM

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




Price!


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Vondragonnoggin
Post Laureate
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Reged: 02/21/10

Loc: Southern CA, USA
Re: Why do people even buy SCTs??? new [Re: drago]
      #6226756 - 12/01/13 11:08 AM

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




Size constraints. Mounting requirements, portability when using larger sizes. Cost prohibitive for a good majority. Doesn't have to be an optical compromise to be a compromise.


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rmollise
Postmaster
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Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: drago]
      #6226772 - 12/01/13 11:16 AM

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.


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: Vondragonnoggin]
      #6226774 - 12/01/13 11:17 AM

on that view all is compromise. i want an 2 metres of aperture with 1 kilogramm weight, and with compactness no larger than a 10x50 binoculars.
it is like to a demagogy. price also not a optical system compromise. i think, refractors is most perfect optical scheme. second will be newtonians, especially in designs without CO....
but ok, be a happy what your have. some folks were beware of looks in another optical systems, because all is relative, and can be perceived in comparison - if you never see and sit in ferrary, you can think, a vw passat VR6 is "very good,fast and sporty car"...:D


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Vondragonnoggin
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Reged: 02/21/10

Loc: Southern CA, USA
Re: Why do people even buy SCTs??? new [Re: drago]
      #6226778 - 12/01/13 11:20 AM

Quote:

on that view all is compromise. i want an 2 metres of aperture with 1 kilogramm weight, and with compactness no larger than a 10x50 binoculars.
it is like to a demagogy. price also not a optical system compromise. i think, refractors is most perfect optical scheme. second will be newtonians, especially in designs without CO....
but ok, be a happy what your have. some folks were beware of looks in another optical systems, because all is relative, and can be perceived in comparison - if you never see and sit in ferrary, you can think, a vw passat VR6 is "very good,fast and sporty car"...:D




I'm sure many are happy with their Ferrari's even if remote control size!


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: Vondragonnoggin]
      #6226794 - 12/01/13 11:27 AM


for small sizes rules refractors, for big - newtodobs
if the climate is stable, and temperature forever is about a 17 - 25 celsium, then i think, CATs may be more or less usable instrument. in our climate is only one way in this - cut a big holes in that OTA, and put ventilators there. as so,as in newtonians, but there sometimes is ok with ventilaors to rear side of mirror cell.


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rmollise
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Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: drago]
      #6226899 - 12/01/13 12:10 PM

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.


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EFT
Vendor - Deep Space Products
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Reged: 05/07/07

Loc: Phoenix, AZ
Re: Why do people even buy SCTs??? new [Re: drago]
      #6226913 - 12/01/13 12:15 PM

It takes surprisingly little ventilation to thermally stabilize an SCT. You really need to get out and try one so that you can see how well they do. Just make sure it is well collimated. Going off the theoretical analysis and the on-line arguments is never going to be the way to judge an optical design.

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mmalik
Postmaster
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Reged: 01/13/12

Loc: USA
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6226924 - 12/01/13 12:22 PM

Quote:

Solidity is a function of the mount, not whether you are using an SCT. In fact, mount for mount, an SCT with its short tube is usually MORE solid on any mount.




Missing the point Rod; I am referring to inherent fluidity of SCT design where all is shifting at times. SCTs just haven't reached that 'set-it and forget-it' solid-state quality over the years, and I understand cost is a factor but that's not to say such "shifty..." issues are not there. Regards


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6226928 - 12/01/13 12:23 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.




O'Meara look in 4" apo, btw
on beside this -i do not see a big difference in portability and weight compare 8" sct and 8" dob. dob is more compact ( if we account mount too), have less light loss, faster thermostab, and less price...:)


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Wmacky
Carpal Tunnel
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Reged: 11/24/07

Loc: Florida
Re: Why do people even buy SCTs??? new [Re: drago]
      #6226983 - 12/01/13 12:45 PM

Quote:

on that view all is compromise. i want an 2 metres of aperture with 1 kilogramm weight, and with compactness no larger than a 10x50 binoculars.
it is like to a demagogy. price also not a optical system compromise. i think, refractors is most perfect optical scheme. second will be newtonians, especially in designs without CO....
but ok, be a happy what your have. some folks were beware of looks in another optical systems, because all is relative, and can be perceived in comparison - if you never see and sit in ferrary, you can think, a vw passat VR6 is "very good,fast and sporty car"...:D




Again, unless use are very religious, I don't understand how a optical system that superimposes the sign of the cross on every bright star can be considered a perfect optical system. I think what you really state is just your opinion.


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: Wmacky]
      #6227031 - 12/01/13 01:04 PM

refractors not have crosses inducted by secondary support.
newtonians not perfect, but in that direction - newtonians without central obstruction too not have a crosses, and on traditional newtonians you have use a curved vanes, and also not have a cross.


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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: Vondragonnoggin]
      #6227110 - 12/01/13 01:44 PM

Vondragonnoggin said: "Was their a point to this whole thread?...Know your scopes," a lot of us sit here in the "Land of Plenty" and forget there are people who have only read about different designs and haven't had a chance to look through them. I'm sure Eddgie started this thread on a humbrum day just for fun, but a lot of good can come from a thread like this because of the info that is shared here on CN. And yes I'm still enjoying my Thanksgiving leftovers

Edited by A6Q6 (12/01/13 01:47 PM)


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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: drago]
      #6227194 - 12/01/13 02:11 PM

drago said: "O'Meara look in 4" apo, btw" I will also add Rodger W Gordon: 3.5" Questar. These people have eyes that most of us mortals don't have. Individual eyesight is a topic that really needs to be covered .

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rmollise
Postmaster
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Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: mmalik]
      #6227204 - 12/01/13 02:15 PM

Quote:



Missing the point Rod; I am referring to inherent fluidity of SCT design where all is shifting at times. SCTs just haven't reached that 'set-it and forget-it' solid-state quality over the years, and I understand cost is a factor but that's not to say such "shifty..." issues are not there. Regards




I get the point. I just disagree with you.


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Dwight J
scholastic sledgehammer
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Reged: 05/14/09

Loc: Lethbridge, Alberta, Canada
Re: Why do people even buy SCTs??? new [Re: mmalik]
      #6227303 - 12/01/13 03:13 PM

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.

Edited by Dwight J (12/01/13 03:46 PM)


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mmalik
Postmaster
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Reged: 01/13/12

Loc: USA
Re: Why do people even buy SCTs??? new [Re: Dwight J]
      #6227354 - 12/01/13 03:43 PM

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

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ATM57
sage


Reged: 01/01/10

Loc: Tehachapi, CA
Re: Why do people even buy SCTs??? new [Re: Cotts]
      #6227362 - 12/01/13 03:46 PM

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


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PowellAstro
scholastic sledgehammer
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Reged: 10/14/09

Loc: Tennessee
Re: Why do people even buy SCTs??? new [Re: mmalik]
      #6227365 - 12/01/13 03:49 PM

@ Dwight J

Very good post and acurate.


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