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9.25 SCT, it ain't no planet scope

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#201 Steve D.

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Posted 04 November 2018 - 04:41 AM

Where are these "stacks of interferometer tests" of 9.25" SCTs?  Please provide complete references or links.  

There are some test results on this page:   http://r2.astro-fore...astrofotografie


 

#202 Asbytec

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Posted 04 November 2018 - 08:23 AM

Where are these "stacks of interferometer tests" of 9.25" SCTs?  Please provide complete references or links.  

Ron, see here. Eddgie referenced this one before, it may be the one he's talking about now. One of many, but this one is good. 


 

#203 Ron359

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Posted 04 November 2018 - 09:51 AM

There are some test results on this page:   http://r2.astro-fore...astrofotografie

Yes, its says the 9.25 SCT is "SEHR GUT!" As do the other two he tested.   Even I can understand that much German.  So where are "the stacks" of data that say it ain't so?  


 

#204 Asbytec

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Posted 04 November 2018 - 09:57 AM

Yes, its says the 9.25 SCT is "SEHR GUT!" As do the other two he tested.   Even I can understand that much German.  So where are "the stacks" of data that say it ain't so?  

I think he was referring to the Ronchi lines and the stern tests. Compared to the refractors, anyway. Yea, on that site there is no quantitative data to go by, kind of have to wing it based on what we know. 


 

#205 grif 678

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Posted 04 November 2018 - 09:59 AM

Most Maks do not need collimation, they are preset and hardly never get out of collimation. Why can they not fix the SCT;s so they will be the same way. It seems that is the biggest problem with SCT's, and maybe a lot of us do not know how to collimate correctly. Back in the 60's, I read the ads about the celestron SCT's that have 1/20 wave, and now they advertise with 1/8 wave, why is that? If the celestron 9.25 had 1/20 wave, it seems that it would be a planet killer for sure.


 

#206 Billytk

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Posted 04 November 2018 - 10:17 AM

Wow my head is spinning... This thread has grown a lot since I first read it. After browsing through these pages, I have a few things I would like to bring to the table.

 

1: I had a 12" Dobsonian before I got my 9.25. The planetary views through the 12" were better than the 9.25 but I don't know if that is purely because of the size difference or due to scope design.

 

2: I think someone here mentioned wanting to see the "live view" from the camera that some of the pictures posted here have been taken from. Here is a link I posted on youtube of 1 of my raw AVIs taken through my 9.25. Not saying this proves anything, just for your viewing pleasure only.

 

https://www.youtube....h?v=eihw7Jge0tE

 

3: Someone also mentioned purposely de-collimating a scope to see what it does to the image. I have done this. These are 2 pictures taken with my 12" Dobsonian. one is collimated and one is way out of collimation. I don't think I need to point out which one is which.

Attached Thumbnails

  • Mars.jpg
  • before.jpg

 

#207 Asbytec

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Posted 04 November 2018 - 10:26 AM

Back in the 60's, I read the ads about the celestron SCT's that have 1/20 wave, and now they advertise with 1/8 wave, why is that? If the celestron 9.25 had 1/20 wave, it seems that it would be a planet killer for sure.

Lot of confusion about what 1/8 wave means. Surface, wavefront? Primary or entire system. Wavefront or RMS? Lower order spherical or total error? I doubt Celestron is what we'd like it to mean, say 1/8 total error on the wavefront. Almost definitely not 1/20. Not at those prices.

Not saying they are not "good enough." They can be diffraction limited in terms of Strehl, however, by some diffraction limited standard they use. This seems to be the cryptic advertised performance, if they advertise it at all, anymore. Even this standard can be debated as to how the achieve and determine it.

I think what most folks mean is SCTs are largely obstructed reducing small scale contrast below their liking. Some argue, and I agree, smoothness is important to planetary performance. Some SCTs tend to have some level of roughness. And maybe the reason is all the above, even if some IF tests show a mid 90's Strehl on many samples tested.

Collimation, thermal stability, and good seeing definitely matter.

Some folks are conisuers of fine optics and refractors and hate SCTs because they are cheaper made telescopes with focus shift and mirror flop. But they usually have goto.

Edited by Asbytec, 04 November 2018 - 10:32 AM.

 

#208 CHASLX200

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Posted 04 November 2018 - 11:40 AM

Most Maks do not need collimation, they are preset and hardly never get out of collimation. Why can they not fix the SCT;s so they will be the same way. It seems that is the biggest problem with SCT's, and maybe a lot of us do not know how to collimate correctly. Back in the 60's, I read the ads about the celestron SCT's that have 1/20 wave, and now they advertise with 1/8 wave, why is that? If the celestron 9.25 had 1/20 wave, it seems that it would be a planet killer for sure.

Most scope makers would really over state the WAVE figure to sell more scopes.


 

#209 Magnetic Field

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Posted 04 November 2018 - 01:19 PM

Wow my head is spinning... This thread has grown a lot since I first read it. After browsing through these pages, I have a few things I would like to bring to the table.

 

1: I had a 12" Dobsonian before I got my 9.25. The planetary views through the 12" were better than the 9.25 but I don't know if that is purely because of the size difference or due to scope design.

 

2: I think someone here mentioned wanting to see the "live view" from the camera that some of the pictures posted here have been taken from. Here is a link I posted on youtube of 1 of my raw AVIs taken through my 9.25. Not saying this proves anything, just for your viewing pleasure only.

 

https://www.youtube....h?v=eihw7Jge0tE

 

3: Someone also mentioned purposely de-collimating a scope to see what it does to the image. I have done this. These are 2 pictures taken with my 12" Dobsonian. one is collimated and one is way out of collimation. I don't think I need to point out which one is which.

 

Apart from collimation the first image is really amazing. It has got a very good 3 dimensional feel to it.


 

#210 Magnetic Field

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Posted 04 November 2018 - 01:26 PM

I think what most folks mean is SCTs are largely obstructed reducing small scale contrast below their liking.

But only for the same unobstructed aperture.

 

When people compare the MTF (Modulation Transfer Function) between say a 10" obstructed scope and a 7" refractor they always forget to also use a MTF with the correct scaling on the x-axis. They make the mistake to compare 2 MTFs where the x-axis is scaled from normalised 0 to 1 for both instruments. This is wrong. 

 

A 7" refractor will have run long out of resolution and steam for high frequency low contrast features. Those  features are still above the visibility threshold for the 10" obstructed scope.

 

If you use the proper x-axis scaled MTF for each instrument (e.g. 10" reflector vs 7") the reflector will always win.


Edited by Magnetic Field, 04 November 2018 - 01:30 PM.

 

#211 Erik Bakker

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Posted 04 November 2018 - 01:50 PM

Use scopes side by side and see which one shows you to most detail laugh.gif


 

#212 Magnetic Field

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Posted 04 November 2018 - 02:16 PM

But only for the same unobstructed aperture.

 

When people compare the MTF (Modulation Transfer Function) between say a 10" obstructed scope and a 7" refractor they always forget to also use a MTF with the correct scaling on the x-axis. They make the mistake to compare 2 MTFs where the x-axis is scaled from normalised 0 to 1 for both instruments. This is wrong. 

 

A 7" refractor will have run long out of resolution and steam for high frequency low contrast features. Those  features are still above the visibility threshold for the 10" obstructed scope.

 

If you use the proper x-axis scaled MTF for each instrument (e.g. 10" reflector vs 7") the reflector will always win.

A sketch (not to scale) for obstructed vs unobstructed.

 

Carefully compare the x-axis (on the top and bottom of each plot).

 

1. In the first instance they are scaled from 0 to 1 (left site panel). Wrong.

 

2. In the right site panel the x-axis on the bottom (red) denotes say a 10" reflector (Rayleigh criterion but the exact number does not matter here) and the x-axis on the top (green) shows it for say a 7" refractor. Beyond 0.8" there is nothing for the refractor (green in plots). By comparison the reflector (red) still has contrast (beyond the green vertical line on the right; again this is not to scale).

 

CloudyNightsMTF.jpeg


Edited by Magnetic Field, 04 November 2018 - 02:49 PM.

 

#213 Asbytec

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Posted 04 November 2018 - 05:54 PM

But only for the same unobstructed aperture.

 

When people compare the MTF (Modulation Transfer Function) between say a 10" obstructed scope and a 7" refractor they always forget to also use a MTF with the correct scaling on the x-axis. They make the mistake to compare 2 MTFs where the x-axis is scaled from normalised 0 to 1 for both instruments. This is wrong. 

 

A 7" refractor will have run long out of resolution and steam for high frequency low contrast features. Those  features are still above the visibility threshold for the 10" obstructed scope.

 

If you use the proper x-axis scaled MTF for each instrument (e.g. 10" reflector vs 7") the reflector will always win.

Yes, I agree comparing same aperture. Aperture itself increases contrast over small scales. It's called resolution on the right hand side of the MTF. Seeing and aberration can change the game a little in terms of contrast transfer at middling spatial frequencies where planetary detail slightly larger than the Airy disc is found. But that quickly normalizes at lower spatial frequencies. But you're right, we should compare same aperture when talking about obstruction effects. 


Edited by Asbytec, 04 November 2018 - 06:04 PM.

 

#214 Ptkacik

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Posted 04 November 2018 - 08:09 PM

Comparing a 8" Apo to an 8" Dob to a 8" SCT is silly. The price of each is dramatically different.

Let's compare views by price. Now we are comparing a 3" Apo to a 10" Dob to an 8" SCT. The Dob should win. However, when we bring in the hassle factor of manual tracking and ground level eyepieces, the SCT starts to lead.

SCT's aren't the answer to all problems. They are just a great compromise that we can all live with. They require more cooling and collimation love and attention. In contrast, the eyepiece doesn't climb to the roof and fall to the ground; they typically track without hassle, they are relatively easy to transport.

I prefer Mak's as I'm a planetary guy but I love the hassle factor of folded optics.

Clear skies,
Peter
 

#215 CHASLX200

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Posted 04 November 2018 - 08:18 PM

I would rather have a old Cave 8" F/8 vs a 8" APO.  Is the view in the APO worth $59k more than the 8" Newt?


 

#216 earlyriser

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Posted 05 November 2018 - 03:05 AM

I would rather have a old Cave 8" F/8 vs a 8" APO.  Is the view in the APO worth $59k more than the 8" Newt?

Not to mention the cost of the observatory you'd need to permanently house it. A telescope for the wealthy, that's for sure. 


 

#217 BarrySimon615

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Posted 05 November 2018 - 05:36 AM

I suppose anyone who purchases a schmidt- cassegrain (new or used) or even a newtonian or a refractor and feels it should be in perfect collimation from the factory and stay that way, feels the same way about any musical instrument be it guitar, piano or anything else.  Just saying............

 

 

Barry Simon


 

#218 CHASLX200

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Posted 05 November 2018 - 07:07 AM

I suppose anyone who purchases a schmidt- cassegrain (new or used) or even a newtonian or a refractor and feels it should be in perfect collimation from the factory and stay that way, feels the same way about any musical instrument be it guitar, piano or anything else.  Just saying............

 

 

Barry Simon

Just shipping alone will do a number on Collimation with SCT's or Newts.  I have never needed to collimate a refractor.  That Meade 7" ED i paid 6k for went back to Meade at least 6 times that i know of and would always come back out of center. No way i was gonna collimate it after paying that amount of money. I made the seller take it back. Total hunk of junk.


 

#219 Redbetter

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Posted 05 November 2018 - 01:26 PM

Just shipping alone will do a number on Collimation with SCT's or Newts.  I have never needed to collimate a refractor.  That Meade 7" ED i paid 6k for went back to Meade at least 6 times that i know of and would always come back out of center. No way i was gonna collimate it after paying that amount of money. I made the seller take it back. Total hunk of junk.

That highlighted part above is self-contradictory.  The same Meade refractor needed collimation six times.  You needed to collimate it, but you chose to return it to the seller.  I am not saying that it was an acceptable cell design, but it was one that needed attention.

 

I take claims that refractors never needed collimation with a grain of salt, same as when Mak owners claim it.  (That and advertising as "Never cleaned!" as if it was a good thing.)  It sometimes says more to me about the visual acuity/care in star testing of the observer using them, than the scope.  My experience with refractors is that some need collimation or fiddling with them. 

 

For example the previous owner of my 110ED never noticed the small amount of coma visible in the first diffraction ring when star testing the scope.  It annoyed me, but was not hurting the images that much, so I didn't try to correct it for a year.  I finally researched enough to try my hand at collimating the undocumented cell.  This eliminated the coma and allowed the scope to provide incrementally more planetary detail and magnification. 

 

Relying on factory collimation is folly for many commercial optics.


 

#220 Deep13

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Posted 05 November 2018 - 05:16 PM

That highlighted part above is self-contradictory. The same Meade refractor needed collimation six times. You needed to collimate it, but you chose to return it to the seller. I am not saying that it was an acceptable cell design, but it was one that needed attention.

I take claims that refractors never needed collimation with a grain of salt, same as when Mak owners claim it. (That and advertising as "Never cleaned!" as if it was a good thing.) It sometimes says more to me about the visual acuity/care in star testing of the observer using them, than the scope. My experience with refractors is that some need collimation or fiddling with them.

For example the previous owner of my 110ED never noticed the small amount of coma visible in the first diffraction ring when star testing the scope. It annoyed me, but was not hurting the images that much, so I didn't try to correct it for a year. I finally researched enough to try my hand at collimating the undocumented cell. This eliminated the coma and allowed the scope to provide incrementally more planetary detail and magnification.

Relying on factory collimation is folly for many commercial optics.


Maybe, but there seems to be no way to collimate a TV101 eithout returning it for a $700 servicing.
 

#221 Spikey131

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Posted 05 November 2018 - 05:48 PM

Maybe, but there seems to be no way to collimate a TV101 eithout returning it for a $700 servicing.


It’s only $395 plus shipping. Just sayin’.
 

#222 Redbetter

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Posted 05 November 2018 - 06:37 PM

Triplet and quadruplet designs rely on having a cell that holds its factory collimation, because they are not as practical to collimate as an doublet, SCT, Mak, or Newt.  Still, some of the more complex apos have problems at times, including one of the Tak models where it has been chronic.  The difficulty of collimation adjustment by the user is one of the reasons I have not been as interested in triplets.

 

But the issue that is apparent in this thread (and many others on CN) is that new owners, even experienced ones, often don't even do a basic star test to see if the optics are functioning as they should.  Why not?  I don't get it.  It is basic due diligence.  Is the collimation poor, good, spot on?  If you don't check it then you don't know. 

 

During first light of a new or new-to-me scope I turn to a star and do a quick check.  Any obvious pinch?  Astigmatism?  Coma?  I am not adept in judging levels of spherical aberration, but I can check to see if the focused star image looks right.  The main limitations are seeing and thermals...it can be awhile before thermals settle or it might take another night to get adequate seeing depending on the aperture, but at least this gives some idea of what needs to be further examined.


 

#223 CHASLX200

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Posted 05 November 2018 - 07:18 PM

That highlighted part above is self-contradictory.  The same Meade refractor needed collimation six times.  You needed to collimate it, but you chose to return it to the seller.  I am not saying that it was an acceptable cell design, but it was one that needed attention.

 

I take claims that refractors never needed collimation with a grain of salt, same as when Mak owners claim it.  (That and advertising as "Never cleaned!" as if it was a good thing.)  It sometimes says more to me about the visual acuity/care in star testing of the observer using them, than the scope.  My experience with refractors is that some need collimation or fiddling with them. 

 

For example the previous owner of my 110ED never noticed the small amount of coma visible in the first diffraction ring when star testing the scope.  It annoyed me, but was not hurting the images that much, so I didn't try to correct it for a year.  I finally researched enough to try my hand at collimating the undocumented cell.  This eliminated the coma and allowed the scope to provide incrementally more planetary detail and magnification. 

 

Relying on factory collimation is folly for many commercial optics.

You need to read up a little my man. Me collimating it was not the problem. It was the cell that was the problem and this was before Meade had a fix and redid the sell mel.  You could collimate it till the cows came home, but once you moved it around it came out of center.  It was not so much the collimation, but the cell would not hold the lens in center.  Once Meade had a fix, and that was near the end , a few lucky owners had a pretty good scope. But many before the cell fix had a bad scope.

 

I don't expect a end user to have to collimate a $6K scope.  It goes back and i get my money back, simple as that.


 

#224 Redbetter

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Posted 05 November 2018 - 11:01 PM

You need to read up a little my man. Me collimating it was not the problem. It was the cell that was the problem and this was before Meade had a fix and redid the sell mel.  You could collimate it till the cows came home, but once you moved it around it came out of center.  It was not so much the collimation, but the cell would not hold the lens in center.  Once Meade had a fix, and that was near the end , a few lucky owners had a pretty good scope. But many before the cell fix had a bad scope.

 

I don't expect a end user to have to collimate a $6K scope.  It goes back and i get my money back, simple as that.

I already understood the problem was the cell, and someone buying a Meade ED refractor used should have been aware that the type had problems already.  scratchhead2.gif   If you buy something with a known problem, then you better have a fix in hand or accept it for what it is.  Regardless, the point stands that you had a refractor that needed collimation/centering tweaking rather than what you stated: "I have never needed to collimate a refractor." 

 

Perhaps the cell would have worked once adjusted if in a permanent mounting location.  I don't know, but that seems to be about the only thing it was fit for.  Spending $6K on a Meade refractor...how could anything go wrong...


 

#225 CHASLX200

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Posted 06 November 2018 - 06:16 AM

I already understood the problem was the cell, and someone buying a Meade ED refractor used should have been aware that the type had problems already.  scratchhead2.gif   If you buy something with a known problem, then you better have a fix in hand or accept it for what it is.  Regardless, the point stands that you had a refractor that needed collimation/centering tweaking rather than what you stated: "I have never needed to collimate a refractor." 

 

Perhaps the cell would have worked once adjusted if in a permanent mounting location.  I don't know, but that seems to be about the only thing it was fit for.  Spending $6K on a Meade refractor...how could anything go wrong...

Keep dreaming. No one at the time i bought that scope used knew of the problem. I bought it right when it came out from a seller that knew it was bad and yet sold it to me.  He told me it went it to Meade to be collimated and like a dumb SOB i took his word for it.  I took a 2 sec look at a star and that was it.  So it was not known when i bought it. You jump the gun too fast.  I am done typing about it so need to reply and lets get back to SCT's.


 


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