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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: WesC]
      #6230722 - 12/03/13 02:49 AM

Quote:

Quote:


Wes, are you suggesting that the path of a light beam passing through turbulent air (i.e., spatially varying indices of refraction) isn't perturbed?





Absolutely not!

What I am, and have been saying is that it doesn't become visibly MORE perturbed by the light passing through the disturbed air more than once. Its does it all so fast and to such a small degree that the cumulative effect isn't much different that it would be if it passed though that same air once.

Could it be mathematically measured? Maybe, but I'm doubtful.

Could the human eye detect it? No way.





but that light beam no cross the air in same manner.
until it drops to primary - it carrier a wide beam. when it was in way from primary to secondary - it is a conical beam from prinmary to secondary diameter, and so on. as so, turbulent layer each time the light cross it, disturb the image in various way.
even it is not that, if cross always is the same light, the errors from turbulence summs. 1+1+1 =! 1
deformed wavefront becomes more deformed, when you get it throught the same deforming conditions again.

P.S. do you also think, your understanding cannot be see from your posts? Do you also think, "education" is equal to "understanding", and become a anyone "smart" ?


Edited by drago (12/03/13 02:50 AM)


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: WesC]
      #6230762 - 12/03/13 04:30 AM

Quote:

Quote:


Wes, are you suggesting that the path of a light beam passing through turbulent air (i.e., spatially varying indices of refraction) isn't perturbed?





Absolutely not!

What I am, and have been saying is that it doesn't become visibly MORE perturbed by the light passing through the disturbed air more than once. Its does it all so fast and to such a small degree that the cumulative effect isn't much different that it would be if it passed though that same air once.

Could it be mathematically measured? Maybe, but I'm doubtful.

Could the human eye detect it? No way.




Perhaps I don't understand your point, but in a compound telescope like a SCT, a given light ray doesn't trace the same path after hitting either the f/2 primary or the secondary. In addition, it's extremely likely that at a fixed distance from the corrector, the index of refraction of the air in the tube varies along the radius. The same is true of all rays entering the scope. Thus, at a given distance from the corrector, a given ray will likely pass through three different index of refraction "boundaries".

Now, because the sum of all light rays become focused as they are reflected into the baffle tube, it's conceivable that some of the "tube currents" are caused by the single pass through a relatively warm narrow baffle. It's also possible that the turbulent boundary layer above a relatively warm primary contributes to the patterns visible at the focal plane.


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: azure1961p]
      #6230763 - 12/03/13 04:31 AM

Quote:

Quote:

Quote:

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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...




Nice cheap shot there. You know nothing about me or my level of education. But thanks for taking potshots anyway.

FWIW, you're still incorrect.




I've got to agree with Wesc here it is apparent for the reasons mentioned. It seems infinitesimally minuscule and how after all could such a fine discrepancy be witnessed by and eye/brain but alas light spells it out unmistakably - even exceedingly sensitive at that. I had questions of this sort - and not THAT long ago. At the point of focus (and on either side) its plain as day. I am sympathetic to your questioning it - but once you see how light fleshes out these discrepancies it all makes sense. Even within the extremely short distance of an optical assembly. Doesn't matter. It reveals all.

Pete




???


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saemark30
Pooh-Bah


Reged: 02/21/12

Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6231602 - 12/03/13 02:42 PM

Rod so the mirror flop only occurs with some C11s? How about C14s with heavier mirrors?
I know of one Meade 16 SCT LX200 had issues.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: saemark30]
      #6231638 - 12/03/13 03:05 PM

Mirror flop can occur with ANY SCT in that there is always a little slop between carrier and baffle tube. If your focusing leaves it in an offbalance condition, it can move slightly when the scope changes in attitude, like from one side of the Meridian to the other. What was mentioned here is entirely different and much worse. I suppose it could afflict other scopes, but I have only seen it in older C11s.

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6231648 - 12/03/13 03:12 PM

Quote:

Mirror flop can occur with ANY SCT in that there is always a little slop between carrier and baffle tube. If your focusing leaves it in an offbalance condition, it can move slightly when the scope changes in attitude, like from one side of the Meridian to the other. What was mentioned here is entirely different and much worse. I suppose it could afflict other scopes, but I have only seen it in older C11s.




Rod, many years ago, the issue with SCT mirror slop (non-uniform mirror movement when the focuser was turned) was caused by a poor design (two points do not define a plane), no doubt used to keep costs down (its hard to sell scopes at great prices by putting in Ferrari-level components). Do you know if Celestron & Meade have improved this issue, and if so, do you know how they did it? I'm a bit reluctant to ask someone at a star party if I can take the back of their scope off!


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Bruce FitzGerald
sage


Reged: 09/17/10

Loc: Arizona
Re: Why do people even buy SCTs??? new [Re: Eddgie]
      #6231652 - 12/03/13 03:14 PM

Sayonara, Eddgie! Guess we won't be seeing you around here anymore!

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Spacetravelerx
Carpal Tunnel


Reged: 12/23/12

Loc: New Mexico
Re: Why do people even buy SCTs??? new [Re: JJK]
      #6231668 - 12/03/13 03:23 PM

Quote:

Quote:

Mirror flop can occur with ANY SCT in that there is always a little slop between carrier and baffle tube. If your focusing leaves it in an offbalance condition, it can move slightly when the scope changes in attitude, like from one side of the Meridian to the other. What was mentioned here is entirely different and much worse. I suppose it could afflict other scopes, but I have only seen it in older C11s.




Rod, many years ago, the issue with SCT mirror slop (non-uniform mirror movement when the focuser was turned) was caused by a poor design (two points do not define a plane), no doubt used to keep costs down (its hard to sell scopes at great prices by putting in Ferrari-level components). Do you know if Celestron & Meade have improved this issue, and if so, do you know how they did it? I'm a bit reluctant to ask someone at a star party if I can take the back of their scope off!




JJK,

My Meade 14" f/8 ACF (new) does not experience focus shift, at least none I can detect. With my very old 10" LX200 it would drive me crazy! Mirror flop is also no existent with my 14", however this because I can lock the mirror.

Oh, I purchased my 14" SCT for the aperture and very good optics. Though I have also purchased 2 APOs. And I have a 125ETXand That is how I dealt with the issue - buy all types!


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PowellAstro
scholastic sledgehammer
*****

Reged: 10/14/09

Loc: Tennessee
Re: Why do people even buy SCTs??? new [Re: Spacetravelerx]
      #6231689 - 12/03/13 03:37 PM

Meade has not only improved it, but eliminated it! Their new baffle slider, is no longer a slider. It now rolls on six high precision bearings, like a Crayford focuser tube. All mirror flop is gone!

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: Spacetravelerx]
      #6231696 - 12/03/13 03:42 PM

Quote:

Quote:

Quote:

Mirror flop can occur with ANY SCT in that there is always a little slop between carrier and baffle tube. If your focusing leaves it in an offbalance condition, it can move slightly when the scope changes in attitude, like from one side of the Meridian to the other. What was mentioned here is entirely different and much worse. I suppose it could afflict other scopes, but I have only seen it in older C11s.




Rod, many years ago, the issue with SCT mirror slop (non-uniform mirror movement when the focuser was turned) was caused by a poor design (two points do not define a plane), no doubt used to keep costs down (its hard to sell scopes at great prices by putting in Ferrari-level components). Do you know if Celestron & Meade have improved this issue, and if so, do you know how they did it? I'm a bit reluctant to ask someone at a star party if I can take the back of their scope off!




JJK,

My Meade 14" f/8 ACF (new) does not experience focus shift, at least none I can detect. With my very old 10" LX200 it would drive me crazy! Mirror flop is also no existent with my 14", however this because I can lock the mirror.

Oh, I purchased my 14" SCT for the aperture and very good optics. Though I have also purchased 2 APOs. And I have a 125ETXand That is how I dealt with the issue - buy all types!





That is very good news.

I agree that it doesn't pay to favor one kind of instrument. So far, mine run the gamut from apos, apos w/fluorite, Mak-Cass, Newt-Dob, CDKs. A good 14" SCT would be fun to play with for visual and imaging work.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Why do people even buy SCTs??? new [Re: JJK]
      #6231954 - 12/03/13 06:49 PM

Quote:

Quote:

Mirror flop can occur with ANY SCT in that there is always a little slop between carrier and baffle tube. If your focusing leaves it in an offbalance condition, it can move slightly when the scope changes in attitude, like from one side of the Meridian to the other. What was mentioned here is entirely different and much worse. I suppose it could afflict other scopes, but I have only seen it in older C11s.




Rod, many years ago, the issue with SCT mirror slop (non-uniform mirror movement when the focuser was turned) was caused by a poor design (two points do not define a plane), no doubt used to keep costs down (its hard to sell scopes at great prices by putting in Ferrari-level components). Do you know if Celestron & Meade have improved this issue, and if so, do you know how they did it? I'm a bit reluctant to ask someone at a star party if I can take the back of their scope off!




In my experience, flop and focus shift are better now. With the mirror locks on the Edge SCTs flop is, naturally, not a problem.


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The Ardent
Pooh-Bah
*****

Reged: 10/24/08

Loc: Virginia
Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6232027 - 12/03/13 07:31 PM

Any chance there will be SCT's with fixed mirrors?

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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: The Ardent]
      #6232124 - 12/03/13 08:24 PM

Quote:

Any chance there will be SCT's with fixed mirrors?




You can already have this by either using a rear cell focuser or using one of Optec's SCT secondary focusers (I can't find them on the website, but they had them out at ASAE). Using either of those methods, the primary can be locked in place and left. Of course the secondary focuser still result in a moving mirror similar to that in a Mewlon 300.


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kbastro
scholastic sledgehammer
*****

Reged: 04/20/08

Loc: Running from Clouds
Re: Why do people even buy SCTs??? new [Re: EFT]
      #6232142 - 12/03/13 08:35 PM

there was one SCT that I would have liked to buy

takahashi tsc 225


kb


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jgraham
Postmaster
*****

Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Why do people even buy SCTs??? new [Re: kbastro]
      #6232182 - 12/03/13 08:53 PM

I made a simple tensioner for my 10" LX6 that greatly reduced my focus-shift to he point that it has become a non-issue.

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Why do people even buy SCTs??? new [Re: rmollise]
      #6232260 - 12/03/13 09:33 PM

Quote:

Quote:

Quote:

Mirror flop can occur with ANY SCT in that there is always a little slop between carrier and baffle tube. If your focusing leaves it in an offbalance condition, it can move slightly when the scope changes in attitude, like from one side of the Meridian to the other. What was mentioned here is entirely different and much worse. I suppose it could afflict other scopes, but I have only seen it in older C11s.




Rod, many years ago, the issue with SCT mirror slop (non-uniform mirror movement when the focuser was turned) was caused by a poor design (two points do not define a plane), no doubt used to keep costs down (its hard to sell scopes at great prices by putting in Ferrari-level components). Do you know if Celestron & Meade have improved this issue, and if so, do you know how they did it? I'm a bit reluctant to ask someone at a star party if I can take the back of their scope off!




In my experience, flop and focus shift are better now. With the mirror locks on the Edge SCTs flop is, naturally, not a problem.




Thanks. I'll check out the new offerings.


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PowellAstro
scholastic sledgehammer
*****

Reged: 10/14/09

Loc: Tennessee
Re: Why do people even buy SCTs??? new [Re: JJK]
      #6232271 - 12/03/13 09:36 PM

Any of the meades with mirror lock are nice. Add a Crayford focuser, then adjust the main mirror where you need it, then lock it down.

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fjs
Pooh-Bah
*****

Reged: 03/25/13

Loc: Olympic Peninsula, USA
Re: Why do people even buy SCTs??? new [Re: Eddgie]
      #6232290 - 12/03/13 09:47 PM

To the original post: The answer is obvious. It's all about convenience. Unit of aperture per unit of bulk, and at a reasonable price. The O.P. knows this. He wanted to start a discussion. I await the reason for it. What answer was he looking for?

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Spacetravelerx
Carpal Tunnel


Reged: 12/23/12

Loc: New Mexico
Re: Why do people even buy SCTs??? new [Re: PowellAstro]
      #6232374 - 12/03/13 10:38 PM

Quote:

Any of the meades with mirror lock are nice. Add a Crayford focuser, then adjust the main mirror where you need it, then lock it down.




The new Meade f/8 ACFs all have Internal Crayford focusers, and the LX850 and LX200 also come with Zero Image-Shift Microfocuser.

It was an epic leap from my old SCT. I won't lose any hair focussing, lol.


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maknewtnut
Member
*****

Reged: 10/08/06

Re: Why do people even buy SCTs??? new [Re: fjs]
      #6232473 - 12/03/13 11:39 PM

Quote:

To the original post: The answer is obvious. It's all about convenience. Unit of aperture per unit of bulk, and at a reasonable price. The O.P. knows this. He wanted to start a discussion. I await the reason for it. What answer was he looking for?




Frank nailed it.

Some continue their love affair with the design after decades. Others might seek attributes they feel the SCT can't provide, or don't provide on a consistent basis. Whatever attribute one might desire, it's impossible to deny those listed by Frank.

Damn near every telescope has what one person might consider pros and cons. The next person might recognize the same list of attributes, yet not consider a con to be of any consequence to them.

IMHO, issues such as the occasional poor fit between primary mirror and baffle or ever-thickening grease are minor (regardless of the design type or who produces it). They are easy and inexpensive to eliminate or minimize. The list goes on in that regard. Other issues that are not easy or cheap to 'fix' are a different story, and yet still subject to individual preference or perception.

Regardless of minor or major, common or uncommon, it's in the best interest of every participant of this forum to learn about how to maximize the performance of their scope, if they so desire. A majority of telescope mechanics really aren't that complex. Those with some mechanical ability and a few simple tools can handle much. A few extra tools, a bit of analysis and patience, attention to detail, and a never ending focus on keeping optical surfaces clean will allow you to accomplish most any mechanical issue a telescope could throw at you.

On a personal note...
Rod, I got a little feisty earlier in this thread. My apologies. You are considered by many to be an authority on the SCT. Tips about the cause of primary mirror flop is a fine example. It helps some folks make the very best of theirs. At the very least, it helps all to be informed and aware.

Is the mass produced the cat's meow for everyone? No. Nothing is.


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