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Equipment Discussions >> Cats & Casses

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: Ed Wiley]
      #6228129 - 12/01/13 10:51 PM

Quote:

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




Ed, are these doubles with roughly equally bright partners?

As you know, a central obstruction (CO) can make it easier to split close double stars of nearly equal intensity because it takes some energy out of the central peaks. However, a CO is presumably less helpful if the binary is composed of a relatively bright star and a relatively dim partner. I'd be interested in knowing at which point the advantage the CO brings to the table in this measurement is lost.


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kbastro
scholastic sledgehammer
*****

Reged: 04/20/08

Loc: Running from Clouds
Re: Why do people even buy SCTs??? new [Re: Dwight J]
      #6228286 - 12/02/13 01:08 AM

My on again and off again relationship with the SCT is that the bloody things are heat traps and dew magnets..

BUT on the good side they are not wind magnets and are easy to work with,,

the bad, they don't beat my refractors or reflectors for sharpness..

the good, a one man portable observatory type instrument (C14) with tons of accessories..

the bad, just calculate how many lens/mirror surfaces an edge hd scope has to go through with an attached focal reducer as compared to a Mak newt to form an image on your ccd

kbastro


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: Itz marcus]
      #6228319 - 12/02/13 01:57 AM

Quote:

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




Traitor!
But - you say, you want a 8" size. there are any objective limitation to this size, or you also can be get a 10 - 12" dob?
i think, if you compare 12" with your 8" sct, you fast "let 8" to go"...;)


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mmalik
Post Laureate
*****

Reged: 01/13/12

Loc: USA
Re: Why do people even buy SCTs??? new [Re: Dwight J]
      #6228364 - 12/02/13 03:29 AM

Quote:

The SCT design is capable in all areas but excels in almost none.




Well, isn't that the whole point? I think we are all going in circles trying to say the same thing but in different ways. Regards


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aa6ww
professor emeritus
*****

Reged: 10/23/11

Loc: Sacramento, Calif.
Re: Why do people even buy SCTs??? new [Re: jrbarnett]
      #6228377 - 12/02/13 04:03 AM

I keep buying and selling C8's. I don't get it. Ive had 6 or 8 of them now since the mid 80's. They have all been excellent scopes. I have tons of scopes, but the my Orange tube C8 gets the most use when I'm just by myself in my back yard. Especially now that the weather is colder, my vixen mount sets up fast and the my entire set up takes 5 minutes to get running. When I'm out with my friends under darker skies, my C11 gets the most uses.
I have tons of expensive refractors. I should just keep my TOA and sell the rest of them. I never use them.
SCT's are addicting. I have some weird love affair with C8's. It's very strange.

It's a fun addiction. I just sold an Edge 8 and want another one again! They shoulda made a C6 Edge. They should also make a carbon fiber C8 Edge!

...Ralph


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Baxstar
member


Reged: 09/15/12

Loc: The Netherland
Re: Why do people even buy SCTs??? new [Re: aa6ww]
      #6228399 - 12/02/13 05:07 AM

I really like my C9,25! I really like the viewing position, and with a newton i can be very awkward to look in the EP.

But I get your point. No it isn't the best scope right of the bet.. BUT! If you correct it you have a very very powerful scope! A dutch astronomer has corrected several scopes and the and now they are way and i mean WAY BETTER than any Newton or Dobson. No coma or any other errors, very stable image because they air inside the SCT is blown around. Just the air inside, not taking are from the outside. This way the air in de SCT get more homogeneous. And he re coated the corrector! AMAZING machine!

Casper


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: Baxstar]
      #6228408 - 12/02/13 05:25 AM

2Baxstar: air inside too can boil from thermal currents.
and try understand: light from observing objects cross tube inside currents ( if any):
in refractor: one time
in newtonian: two times
in SCT and MCT: three times.
as you would expect, the more crosses - the more is influence.
i not comment another your allegations...


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Joe Aguiar
Carpal Tunnel
*****

Reged: 02/10/07

Loc: none of your buss
Re: Why do people even buy SCTs??? new [Re: ATM57]
      #6228605 - 12/02/13 09:00 AM

OMG
i leave CN for one weekend and this thread goes from page 2 to 9 pages??


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why do people even buy SCTs??? new [Re: Joe Aguiar]
      #6228903 - 12/02/13 11:30 AM

I guess then the perfect optical system should be,
1.. Somewhat easy to make,

2..Not have a central obstruction,

3..No false color

4..Inexpensive to Produce
Ladies and Gentlemen you have just described a *********************Schiefspiegler!************************


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: Why do people even buy SCTs??? new [Re: JJK]
      #6229084 - 12/02/13 01:01 PM

Quote:

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).
<snip>
Now, if the new crop of SCTs have improved optical and thermal performance, I could be interested.




JJK, here's my take on Celestron SCTs, having owned 4 (3 Celestron, 1 Takahashi):

- Optical quality: mine ranged from run of the mill (a NexStar C11) to superb (my current C11HD). In the middle were a C9.25 (excellent) and a TSC-225, (slightly better than the C9.25 but not by a whole lot). See this recent thread about the C11HD's performance. My erstwhile C9.25 Roddier tested at 1/6 wave, 1/30 wave RMS (in green), an excellent optic by any measure. Taking it to 400x on Saturn was easy. The C11HD has such a good star test that I haven't bothered Roddier testing it, though that's on the todo list. To top it, it has a very flat field and is coma corrected. E.g., the components of the Double-Double look the same no matter where you place them in the field. But where it really shines is with lunar viewing because the entire field, with a decent eyepiece, is sharp. I sold the NS11 because I just couldn't stand its field curvature and coma. Otherwise, it had the terrific Gen 1 NS fork mount.

One thing that Celestron really paid attention to with the HD scopes is properly centering the corrector. I think a mis-centered corrector was the SCT bugaboo for the longest time resulting in poorer optical performance. They used to put in cork shims to center it and this probably wasn't very accurate to begin with and who knows what happened to the shims with time. My C9.25 showed a small amount of astigmatism in the Roddier test and I suspect that came from the corrector not being absolutely centered. Removing its effect made its RMS score distinctly better so there some performance lost here. With the HD scopes (and possibly some non-HD ones), they use centering screws on the corrector cell.

It's also recently come to light (with photographic proof) that Celestron assembles C11HD and C14HDs in California. So, I think they're paying more attention to at least this subclass.

- Focusing shift: It's gotten better over time. Celestron claims that they reject scopes showing more than 30 arcsec. My C11HD arrived with about 20 arcsec but it's gotten worse over time. I wonder if the primary mirror locknut has worked loose. The TSC225 was the best in this regard: almost no shift at all but the focuser was much harder to turn so I suspect it had some kind of spring loading. The HD scopes come with mirror locks, of course. It's a pity Celestron didn't re-introduce the 3-spindle belt-driven shift-free focuser which they probably invented and A-P borrowed for their Mak.

- Thermal management: The vents on the HD scopes help but are insufficient by themselves. Fortunately, there are the TEMPest fans from Deep Space Products which fit into the vents and both Starizona and DSP sell fans to put into the secondary. I am really uncomfortable with the secondary fan: taking the secondary out to cool the scope gives me heartburn but if you have a limited amount of time, it will cool your scope really fast. However, I find that in our climate, even the fans are unable to keep up with drops in temperature. I've constructed a Reflectix thermal blanket to keep the tube temperature stable once it cools to ambient and it really helps. This isn't scope specific, I think even the AP Mak would run into trouble with the top part of the tube cooling more than the bottom part.

- Misc. mechanicals: The Fastar secondary retaining ring still has the seizing problem. A simple solution, which Starizona sells, would be to install a better gasket. Fortunately, I don't do remove it often so I'm not particularly affected.

Digression: Many people worry unnecessarily about sub-mm play of the secondary in the corrector. It doesn't matter because the secondary is spherical. Any lateral shift can be corrected out by tilting it. All that'll happen is that the secondary shadow may be a tiny bit off-center, with uneven field illumination as well but this would take careful measurement with a CCD to figure out. But what you don't want is a loose secondary holder causing play. Every time it shifts it'll mess collimation up. Tighten it down but centering it is not required.

All this said, if you're looking for an A-P like guarantee, you won't find it, not at Celestron prices. Buying used may be a good strategy since you can recover all of the cost if you don't like the scope.

Tanveer.


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ATM57
super member


Reged: 01/01/10

Loc: Tehachapi, CA
Re: Why do people even buy SCTs??? new [Re: Baxstar]
      #6229198 - 12/02/13 01:50 PM

Quote:

I really like my C9,25! I really like the viewing position, and with a newton i can be very awkward to look in the EP.

But I get your point. No it isn't the best scope right of the bet.. BUT! If you correct it you have a very very powerful scope! A dutch astronomer has corrected several scopes and the and now they are way and i mean WAY BETTER than any Newton or Dobson. No coma or any other errors, very stable image because they air inside the SCT is blown around. Just the air inside, not taking are from the outside. This way the air in de SCT get more homogeneous. And he re coated the corrector! AMAZING machine!

Casper




I wonder where he got the idea to recirculate the air in the ota? I have been doing this for a long time and have published articles on the subject.

Scopejunkie


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WesC
Carpal Tunnel
*****

Reged: 02/06/13

Loc: La Crescenta, CA
Re: Why do people even buy SCTs??? new [Re: drago]
      #6229208 - 12/02/13 01:57 PM

Quote:

2Baxstar: air inside too can boil from thermal currents.
and try understand: light from observing objects cross tube inside currents ( if any):
in refractor: one time
in newtonian: two times
in SCT and MCT: three times.
as you would expect, the more crosses - the more is influence.
i not comment another your allegations...





At the speed of light? Really? That's just silly. False logic. It could cross 10,000 times and you would not see any detectable difference. Moving air in a tube is billions of times slower than the speed of light, it will not influence the light more just because its in the tube for a trillionth of a second longer.

Anyway, once you stabilize the tube, its a non-issue. My C11 Edge stabilizes in around 20-30 minutes with the TEMPest fans I have. That's about as long as it takes me to setup, balance and align the GEM.


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saemark30
Pooh-Bah


Reged: 02/21/12

Re: Why do people even buy SCTs??? new [Re: WesC]
      #6229234 - 12/02/13 02:06 PM

My old C11 has a problem with mirror tilting with changing azimuth. Has anyone experienced this, it would throw the collimation way off.

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csrlice12
Postmaster
*****

Reged: 05/22/12

Loc: Denver, CO
Re: Why do people even buy SCTs??? new [Re: jimbo728]
      #6229257 - 12/02/13 02:17 PM

Well, we needed some way to recycle all those toilet paper tubes........and they aren't long enough for dobs.

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ATM57
super member


Reged: 01/01/10

Loc: Tehachapi, CA
Re: Why do people even buy SCTs??? new [Re: saemark30]
      #6229259 - 12/02/13 02:20 PM

Quote:

My old C11 has a problem with mirror tilting with changing azimuth. Has anyone experienced this, it would throw the collimation way off.




I had an orange tube that did this. The primary retaining ring was loose. A loose secondary holder can mimic this as well.

Scopejunkie


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drago
sage


Reged: 01/11/08

Loc: Latvia
Re: Why do people even buy SCTs??? new [Re: WesC]
      #6229281 - 12/02/13 02:30 PM

Quote:



At the speed of light? Really? That's just silly. False logic. It could cross 10,000 times and you would not see any detectable difference. Moving air in a tube is billions of times slower than the speed of light, it will not influence the light more just because its in the tube for a trillionth of a second longer.




hot air speed over a campfire is also lot less speed than a speed of light, but you can see, it changes view throught it very dramatically. turbulence in atmosphere, as so as tube currents also is not faster speed than speed of light, but that also destroy a views. looks like you can have a very big problems with elementar physic understanding...


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ATM57
super member


Reged: 01/01/10

Loc: Tehachapi, CA
Re: Why do people even buy SCTs??? new [Re: orion61]
      #6229439 - 12/02/13 03:27 PM Attachment (12 downloads)

Quote:

I guess then the perfect optical system should be,
1.. Somewhat easy to make,

2..Not have a central obstruction,

3..No false color

4..Inexpensive to Produce
Ladies and Gentlemen you have just described a *********************Schiefspiegler!************************




Bulit one. True optical perfection in a minimalist design in both optics and mechanics. High res performance was off the chart for a 6". At F/29 is was a specialised instrument. I have a 10" two mirror on the drawing board but I can't find anyone who can make the optics. No, I'm not a glass pusher

Scopejunkie


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why do people even buy SCTs??? new [Re: ATM57]
      #6229674 - 12/02/13 05:01 PM

I agree 100% the standard SCT's can be made to perform much better, I have been using a Micrometer and lasers to align
mirrors for years now. It is a part of the service i go.
The problem is when you get into Commercial manufacturing
it begins to be good money thrown after bad time in the form of performance. The ave. SCT with 10 extra hours can be made to perform better, but if they took the extra time
during fabrication it would add hundreds of dollars to the price.
I have learned not to trust the alignment marks 100%.
I believe newer scopes have developed ways to get some of these things much closer.
I had a Meade 10" where the corrector factor markings were close to 1/3 of a rotation out. The difference in views was enough to change the system from fair to VG.
Factory installed active cooling will be the next big thing. The optics are pretty much there now.
BTW Nice Schief ATM57! Beautiful build quality.
Now a shorter FL with a field corrector and they will be an every day scope...


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maknewtnut
Member
*****

Reged: 10/08/06

Re: Why do people even buy SCTs??? new [Re: saemark30]
      #6229760 - 12/02/13 05:36 PM

Quote:

My old C11 has a problem with mirror tilting with changing azimuth. Has anyone experienced this, it would throw the collimation way off.




Ask Rod. He'll tell you you and everyone else that has ever experienced such a condition is wrong.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: maknewtnut]
      #6229767 - 12/02/13 05:40 PM

Quote:


Ask Rod. He'll tell you you and everyone else that has ever experienced such a condition is wrong.




No but I'll tell you you're wrong to start with.

OP: This condition as you were told can afflict some C11s, and is due to the primary being loose on the baffle due to a problem with the retainer.


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