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Equipment Discussions >> Cats & Casses

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Roy McCoy
Pooh-Bah
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Reged: 10/13/05

Loc: Glendale, AZ
Re: Why do people even buy SCTs??? new [Re: Namlak]
      #6236319 - 12/05/13 10:05 PM

Quote:

Why would you want to use an SCT with a 38% central obstruction when you can use a refractor and look through the little hole in the center of the 62% outer obstruction?






Because just like a watermelon, the sweetest view is in the middle.


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: jgraham]
      #6236368 - 12/05/13 10:40 PM

Quote:

C14, 'cause I'd just love to have one. I also own a fair sized refractor (152mm f/8) but I'd have to go for that big beautiful cat.




They certainly are tempting.


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: Namlak]
      #6236371 - 12/05/13 10:42 PM

Quote:

Why would you want to use an SCT with a 38% central obstruction when you can use a refractor and look through the little hole in the center of the 62% outer obstruction?






Because wave mechanics differentiates between the inner and outer obstruction.


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TheObserver
sage
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Reged: 09/24/13

Loc: Light Polluted, NY
Re: Why do people even buy SCTs??? new [Re: Namlak]
      #6236509 - 12/06/13 01:44 AM

Quote:

Why would you want to use an SCT with a 38% central obstruction when you can use a refractor and look through the little hole in the center of the 62% outer obstruction?






they want the sct. so they can buy a focal reducer and have a bigger FOV thats 38% obstructed .


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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: TheObserver]
      #6236631 - 12/06/13 04:36 AM

I can have our club C14 at F2.8 using those reducers. Hyperstar gets a C14 to F2.1. Try that with your spyglass.

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Why do people even buy SCTs??? new [Re: drago]
      #6236679 - 12/06/13 06:20 AM

Drago, I understand what your driving at and you're right in principle. However, the CO has no affect on Strehl (outside maybe masking the paraxial rays), but it does affect peak intensity of the central disc through added diffraction (not aberration.) It adds to the reduction with the amount of aberration present. Same effect, really, it's just the CO effect not a function of Strehl. An optic with Strehl 1.00 with a 25% CO will still have a Strehl of 1.00. It's peak intensity will drop to 90%, though. That's correct.

So, any scope with a CO of any meaningful diameter needs to have a very good Strehl to hold the PSF peak intensity above 80%. Really, the limit for Strehl 1.00 is between 32% and 35% to keep peak intensity above 80% and very similar to the diffraction limit. It seems a lot of SCTs are in the mid 90's Strehl and with a CO can produce a PSF at something less than 80%. But, in fact, the SCT is really optimized for the entire range of the MTF, giving up a bit of lower contrast and adding a bit of high resolution near maximum spacial frequency.

They are a compromise design, like most any other designs, offering wide fully illuminated FOV and generous back focus. So, in addition to aperture in a short package, etc., if they suite your needs, the SCT is often a good scope for what you might want it to do. Some people don't care for the design because they apparently do not deliver what that person wants to the extent that person wants it. Its possible some folks have owned lemons or prefer refractors because they deliver views they expect.

In your link, "Well or 200 mm Newton shielded 20% is roughly equal to the resolution of 180 mm apo." That is true only for a relatively short span of spacial frequencies near the first ring, or slightly more (and not accounting for scatter, etc.) The obstructed scope actually excels at higher frequencies more so that a perfect unobstructed aperture...and there is no such thing...by a factor of 1/(1 - co^2). And at lower frequencies both designs begin to normalize.

Your link rightfully states there are two affects, one is "shading" estimated at S = 1 - co^2, which is the area of both apertures normalized to 1 - with 1 being the area of full aperture and co^2 being the area of the obstruction in terms of full aperture.) And the effect on peak intensity being (1 - co^2)^2. Multiply the result of the CO effect by your Strehl and you have an approximation of your final working intensity of the central disc.

Edited by Asbytec (12/06/13 09:05 AM)


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Why do people even buy SCTs??? new [Re: t.r.]
      #6236681 - 12/06/13 06:25 AM

Quote:

I did my best to stay out of this...but I finally caved . Here is the litmus test...

Two scopes side by side.

One a new C14.

The other a new AP 175 EDF refractor.

You can have one for FREE...BUT cannot sell it and must keep it forever as your only scope.

You have NO health or mounting/use issues.


Logic, MTF and Physics would dictate that you must pick the C14...right?

You MUST choose the C14.

How many of you can say that you would?

My




The C14 comes with a refractor.


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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: Asbytec]
      #6236714 - 12/06/13 07:11 AM

Ooooooooo, a C14 with a AP175 finder. Yeah, that's the ticket.

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: Dwight J]
      #6236737 - 12/06/13 07:36 AM

Quote:

I can have our club C14 at F2.8 using those reducers. Hyperstar gets a C14 to F2.1. Try that with your spyglass.




I saw the Hyperstar system for the first time at ASAE in Tucson a few weeks ago. It looked very interesting. I'll visit the guys at Starizona the next time I'm in Tucson (hopefully next month) and give that system a try.


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David PavlichAdministrator
Transmographied
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Reged: 05/18/05

Loc: Mandeville, LA USA
Re: Why do people even buy SCTs??? new [Re: JJK]
      #6236832 - 12/06/13 08:49 AM

We must also remember that this is the Cats section. I would think that the poll may come out a little differently in the Refractor section. Still, it shows that most here desire aperture, me included.

David


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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: David Pavlich]
      #6237012 - 12/06/13 10:29 AM

Absolutely, in the refractor forum I'd go foran APM175 with a C14 finder.

Even the 48" Schmidt camera at Palomar was bought as 'auxilliary equipment' for the 200".


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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: jgraham]
      #6237040 - 12/06/13 10:40 AM

Why would I buy an SCT? Because I don't have one and want to see what an Edge or ACF view is like at 8" to 10" aperture and not going for dob and have two 6" scopes already. The SCT design is tops for compact scope.

Makes a lot of sense to me in terms of how much I'd use a larger aperture instrument.

I'm not afraid of naysayer comments. My 6" scopes are an iOptron MCT and an F/5 refractor. I have naysayers to spare....

Bring on the SCT criticism!

Edited by Vondragonnoggin (12/06/13 10:42 AM)


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NHRob
Post Laureate
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Reged: 08/27/04

Loc: New Hampshire
Re: Why do people even buy SCTs??? new [Re: Dwight J]
      #6237107 - 12/06/13 11:05 AM

Why would I want to? And how flat is the fiel and how well do your eyepieces do with f/2.1?
I wouldn't want one f/2.1 even with a refractor.

Anyway, if I want aperture I prefer to go with a premium dob rather than an SCT.

Quote:

I can have our club C14 at F2.8 using those reducers. Hyperstar gets a C14 to F2.1. Try that with your spyglass.




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Bill Barlow
Carpal Tunnel
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Reged: 12/03/07

Loc: Overland Park KS
Re: Why do people even buy SCTs??? new [Re: Namlak]
      #6237157 - 12/06/13 11:27 AM

Quote:

Why would you want to use an SCT with a 38% central obstruction when you can use a refractor and look through the little hole in the center of the 62% outer obstruction?

The CO of the C14 is only 31-32%, the smallest of all Celestron and Meade SCT's. I own one and it puts up some amazing views through the eyepiece. I would take it over the AP 7" just for much better deep sky performance.

Bill

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Chaz659
sage


Reged: 12/08/12

Loc: Islip, NY
Re: Why do people even buy SCTs??? new [Re: NHRob]
      #6237163 - 12/06/13 11:30 AM

I'm telling ya, it moved, it flinched. There! See it!



15 pages?!?! You guys are scaring me...


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nicklane1
sage
*****

Reged: 03/04/08

Re: Why do people even buy SCTs??? new [Re: NHRob]
      #6237167 - 12/06/13 11:32 AM

Would you rather watch the big game on a 100" standard definition TV or 60" high definition TV?

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bob midiri
Carpal Tunnel
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Reged: 06/17/04

Loc: pa 19320
Re: Why do people even buy SCTs??? new [Re: Bill Barlow]
      #6237213 - 12/06/13 11:56 AM Attachment (6 downloads)

Quote:

Quote:

Why would you want to use an SCT with a 38% central obstruction when you can use a refractor and look through the little hole in the center of the 62% outer obstruction?

The CO of the C14 is only 31-32%, the smallest of all Celestron and Meade SCT's. I own one and it puts up some amazing views through the eyepiece. I would take it over the AP 7" just for much better deep sky performance.

Bill



Not sure why anyone would buy one, personally I can't stand the SCT's


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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: NHRob]
      #6237299 - 12/06/13 12:39 PM

Well Rob, of course it is not for visual. I rarely do visual. It certainly works well with the small chip of a Mallincam. Hyperstar is used with DSLR's so the optics involved flatten the field. SCT's don't require an additional infrared blocking filter to prevent star bloat when using cameras sensitive in that area while most refractors happily pass out of focus infrared bloating star images. Refractors are not immune to curvature of field either given the need to add a field flattener for imaging. AFAIK, only Petzvall refractors have native flat fields. NO telescope design is perfect. The SCT's just happen to be good to very good at most things while a refractor excels in a few areas but fails miserably in others (weight, long moment arm on larger, price per inch, CA, field curvature, tall mounts, laying on ground to view zenith).

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Martin Lyons
sage
*****

Reged: 10/06/08

Loc: Cape Town, South Africa
Re: Why do people even buy SCTs??? new [Re: Dwight J]
      #6237333 - 12/06/13 12:58 PM

I would take the refractor........and use it as a finder on the 16" SCT....

Edited by Martin Lyons (12/06/13 12:59 PM)


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Why do people even buy SCTs??? new [Re: nicklane1]
      #6237387 - 12/06/13 01:25 PM

Quote:

Would you rather watch the big game on a 100" standard definition TV or 60" high definition TV?




Since resolution is directly proportional to aperture and Newtonian optics can more carefully figured, I would choose the 100" Retina display..



Jon


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