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Star-testing MTS-SN6

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#1 AlMuz

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Posted 22 July 2020 - 11:50 AM

Hello everyone,

 

I need some advice on MTS-SN6 which I've recently got from Craigslist in terrible dusty/rusty condition.

It took me 3..4 days to clean it out and collimate, but star-testing results are not looking satisfactory.

 

My MTS-SN6 came from factory with "Multi-Coated Optics Group". After 35 years of previous ownership Schmidt-corrector AR coating was in terrible condition, so I had to remove it before using the scope. Luckily this old tech AR-coating was not bonded that hard and it came off easily after soaking for few hours in bathroom cleaner and whipping it off with a cotton ball.

 

MTS-SN6 Schmidt-corrector with removed AR-coating

 

After telescope reassembly I did a star-test by Polaris, and could not see Airy disc at perfect focus with super high magnification (400x ..600x). I can see it in Orion XT8 flickering and distorting from atmospheric turbulence. But MTS-SN6 is like incapable to produce a crisp focus of a star, similar to being washed-out by spherical aberration - just a blob of blured light.

 

I have concluded that this "washout" comes from Schmidt-corrector (hex pattern from over-tightened mounting bolts):

 

MTS-SN6 star-test by Polaris

 

At tiny intra-focus I see that there is bright ring of a wave-front forming and about to collide in perfect Airy image, but it is being overlayed with dim disc of light of much larger diameter. From hexagon pattern of the disk I can conclude that it is produced by corrector plate. Could it be some reflection because of stripped-off AR coating?. Weather did not allow me to check it again with mounting bolts released yet.

 

Schmidt-corrector artifact on MTS-SN6

 

Moon images with prime focus DSLR and with 2.5x Barlow are good enough though, so I am wondering - maybe I am demanding too much from this old boy? Or should I research a possibility to re-coat Schmidt-corrector?

Moon APS-C MTS-SN6 w. 2.5x Barlow
Album: MTS-SN6
12 images
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Edited by AlMuz, 22 July 2020 - 11:51 AM.

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#2 mashirts

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Posted 22 July 2020 - 12:04 PM

I never go further than 80x on my mts-sn8.  If I was interested in double stars or other high mag use I found a sub-aperture mask did great to do splitting.

 

I just enjoy what seems to me to be bright images while looking at nebula at f4.



#3 davidc135

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Posted 22 July 2020 - 12:55 PM

I suppose the obvious question is can the corrector mounting bolts be loosened? I doubt if another AR coating would help much other than increasing brightness slightly. It may well be spherical aberration caused by corrector errors that is responsible for the wash. Could you check with a Ronchi eyepiece or one of a number of other tests?

 

But it'll still be a great low power scope.

 

David



#4 AlMuz

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Posted 22 July 2020 - 01:11 PM

I suppose the obvious question is can the corrector mounting bolts be loosened? ...

Yes its possible, but as I mentioned, did not have a chance to check it out yet because  of the weather.

My expectation is even if hex-corners will go away - the dim disk will still be there.

Will post new results here as soon as I will have clean skies in my area.



#5 CHASLX200

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Posted 23 July 2020 - 05:32 AM

None of them scopes seems to have super optics. Reason i never bought one.



#6 starman876

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Posted 23 July 2020 - 07:43 AM

I never go further than 80x on my mts-sn8.  If I was interested in double stars or other high mag use I found a sub-aperture mask did great to do splitting.

 

I just enjoy what seems to me to be bright images while looking at nebula at f4.

this scope is meant for deep sky and not high power planet viewing.  Therefore, expecting high power views is not going to be rewarding. 



#7 DAVIDG

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Posted 23 July 2020 - 09:34 AM

this scope is meant for deep sky and not high power planet viewing.  Therefore, expecting high power views is not going to be rewarding. 

 There is nothing inherent in the design of Schmidt Newtonian that would stop if from giving excellent high power images. From the star test images the optics are just poorly made and unfortunately an example of the optical quality of many commercial telescopes.  

  The weird star test pattern has nothing to do with the lack of anti reflective coating . It looks to be that either the corrector is figured poorly and/or it is being distorted from to much pressure from the retainer ring.   

If it was my scope I would test each optical element and see which one(s) has the problem and if it could be corrected easily, as in one being pinched.  So the first thing I would do is remove the corrector and Foucault test the primary. It can be tested  while still in the tube and it an easy test.  It should be a perfect sphere. Next test the secondary against a known flats. If both of the those are fine, then it has to be the corrector so either  test  the surface against my 8" flat and/or reassemble the scope and test the complete unit by double pass autocollimation. 

 

                     - Dave 


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#8 starman876

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Posted 23 July 2020 - 10:18 AM

 There is nothing inherent in the design of Schmidt Newtonian that would stop if from giving excellent high power images. From the star test images the optics are just poorly made and unfortunately an example of the optical quality of many commercial telescopes.  

  The weird star test pattern has nothing to do with the lack of anti reflective coating . It looks to be that either the corrector is figured poorly and/or it is being distorted from to much pressure from the retainer ring.   

If it was my scope I would test each optical element and see which one(s) has the problem and if it could be corrected easily, as in one being pinched.  So the first thing I would do is remove the corrector and Foucault test the primary. It can be tested  while still in the tube and it an easy test.  It should be a perfect sphere. Next test the secondary against a known flats. If both of the those are fine, then it has to be the corrector so either  test  the surface against my 8" flat and/or reassemble the scope and test the complete unit by double pass autocollimation. 

 

                     - Dave 

I thought because of the low F ratio and the central obstruction that high power views would not be rewarding.



#9 DAVIDG

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Posted 23 July 2020 - 01:44 PM

I thought because of the low F ratio and the central obstruction that high power views would not be rewarding.

  A large central obstruction reduces contrast but this an  F/5 system so the obstruction is not huge and listed at 1.83"  so some  contrast loss for planetary viewing but not enough not to get very good views. Most of the commercial Schmidt cassegrains have central obstructions in the same range  as this 6" f/5 Schmidt Newtonian.  You also view the planets in the center of field  were the optical correction is best so off axis performance is not an issue for high power viewing. It all comes down to optical quality and you can have the best design in the world on paper but if it made poorly it doesn't matter.  In the case of  a Schmidt Newtonian, on paper it has less coma than a standard f/5 parabola and the spherical aberration is just as good but again you have to make it  right to enjoy what theory shows it can provide.  

 

 

                    - Dave 


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#10 starman876

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Posted 24 July 2020 - 07:08 AM

  A large central obstruction reduces contrast but this an  F/5 system so the obstruction is not huge and listed at 1.83"  so some  contrast loss for planetary viewing but not enough not to get very good views. Most of the commercial Schmidt cassegrains have central obstructions in the same range  as this 6" f/5 Schmidt Newtonian.  You also view the planets in the center of field  were the optical correction is best so off axis performance is not an issue for high power viewing. It all comes down to optical quality and you can have the best design in the world on paper but if it made poorly it doesn't matter.  In the case of  a Schmidt Newtonian, on paper it has less coma than a standard f/5 parabola and the spherical aberration is just as good but again you have to make it  right to enjoy what theory shows it can provide.  

 

 

                    - Dave 

Thanks Dave for clarifying that.   Makes sense waytogo.gif



#11 RichA

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Posted 25 July 2020 - 02:03 AM

Hello everyone,

 

I need some advice on MTS-SN6 which I've recently got from Craigslist in terrible dusty/rusty condition.

It took me 3..4 days to clean it out and collimate, but star-testing results are not looking satisfactory.

 

My MTS-SN6 came from factory with "Multi-Coated Optics Group". After 35 years of previous ownership Schmidt-corrector AR coating was in terrible condition, so I had to remove it before using the scope. Luckily this old tech AR-coating was not bonded that hard and it came off easily after soaking for few hours in bathroom cleaner and whipping it off with a cotton ball.

 

 

 

After telescope reassembly I did a star-test by Polaris, and could not see Airy disc at perfect focus with super high magnification (400x ..600x). I can see it in Orion XT8 flickering and distorting from atmospheric turbulence. But MTS-SN6 is like incapable to produce a crisp focus of a star, similar to being washed-out by spherical aberration - just a blob of blured light.

 

I have concluded that this "washout" comes from Schmidt-corrector (hex pattern from over-tightened mounting bolts):

 

 

 

At tiny intra-focus I see that there is bright ring of a wave-front forming and about to collide in perfect Airy image, but it is being overlayed with dim disc of light of much larger diameter. From hexagon pattern of the disk I can conclude that it is produced by corrector plate. Could it be some reflection because of stripped-off AR coating?. Weather did not allow me to check it again with mounting bolts released yet.

 

 

 

Moon images with prime focus DSLR and with 2.5x Barlow are good enough though, so I am wondering - maybe I am demanding too much from this old boy? Or should I research a possibility to re-coat Schmidt-corrector?

Good of you to post this.  There is scant info out there on these scopes. 



#12 CHASLX200

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Posted 25 July 2020 - 06:09 AM

 There is nothing inherent in the design of Schmidt Newtonian that would stop if from giving excellent high power images. From the star test images the optics are just poorly made and unfortunately an example of the optical quality of many commercial telescopes.  

  The weird star test pattern has nothing to do with the lack of anti reflective coating . It looks to be that either the corrector is figured poorly and/or it is being distorted from to much pressure from the retainer ring.   

If it was my scope I would test each optical element and see which one(s) has the problem and if it could be corrected easily, as in one being pinched.  So the first thing I would do is remove the corrector and Foucault test the primary. It can be tested  while still in the tube and it an easy test.  It should be a perfect sphere. Next test the secondary against a known flats. If both of the those are fine, then it has to be the corrector so either  test  the surface against my 8" flat and/or reassemble the scope and test the complete unit by double pass autocollimation. 

 

                     - Dave 

If it is anything like my Comet catcher was then it is just flat out bad.  Meade was not making good optics in them days other than the Newts were very good. Meade stepped up the plate with their SCT's optics in the mid 90's and were beating Celestron by a mile.
 


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#13 AlMuz

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Posted 29 July 2020 - 03:28 PM

UPDATE:

 

I had corrector mounting bolts loosened to the level of "barely tight" (just enough not to have any gap between the bolt head and retention ring).

Then was waiting a good skies for a week to take the scope out of my apartment.

 

Hex pattern did not go away, but it was more "crisp-er" this time :

 

MTS-SN6 Star Test #2

 

And I could see clear airy rings at best focus flickering  through atmospheric turbulence.

Now I suspect it was just some hazy high-altitude cloud on original day of testing playing bad joke with me.

 

Also got more/less decent picture of Jupiter:

 

Jupiter in MTS-SN6
 

Pentax K5 got too large pixel pitch to deal with this relatively short scope for planetary imaging. This was taken in with 2.5x Barlow-ed prime focus.

 

I did not grab my screwdriver to re-tighten the Schmidt-corrector back, to make sure the tightening level was not a problem. So have to wait for another night now. 

The plan is to make two shots with the same camera settings and focus setup, with loose and tightened corrector.


Edited by AlMuz, 29 July 2020 - 03:40 PM.

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#14 DAVIDG

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Posted 29 July 2020 - 04:45 PM

When you examine the images of the defocused star you can see that the size of the shadow of the secondary is always smaller on all the Intrafocal ones vs the Extra focal ones. Note the two image  on the left and the right  of middle image of the focused star. On the right side image ( Intrafocal) it  is a bright circle  with no shadow of the secondary visible but on  the left image ( Extrafocal) you easily see the shadow of secondary as a  black circle in the middle. The image are nowhere similar. That indicate a very large amount of over corrected spherical aberration. Well over 1/4 wave. 

   The mirror is suppose to be a sphere and the corrector plate corrects the spherical aberration of the primary. What I think has happen is the mirror is poorly figured and is not a sphere, but is aspheric so it has less spherical aberration then it should and the corrector is now adding more then it should hence the results in your star test.

   If you were to do a side by side test of this scope vs one that had true 1/8 wave optics, the image difference would be  like the  picture quality of the old analog TV vs a modern digital HD 8K image. Jupiter would show amazing detail when the seeing snapped that you'll never see in this scope. 

   

 

                         - Dave 


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#15 tim53

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Posted 29 July 2020 - 05:30 PM

It would be interesting to test the primary by itself, to see if it's spherical.  If it's not, it might be easy to fix the scope.



#16 AlMuz

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Posted 29 July 2020 - 06:10 PM

When you examine the images of the defocused star you can see that the size of the shadow of the secondary is always smaller on all the Intrafocal ones vs the Extra focal ones. Note the two image  on the left and the right  of middle image of the focused star. On the right side image ( Intrafocal) it  is a bright circle  with no shadow of the secondary visible but on  the left image ( Extrafocal) you easily see the shadow of secondary as a  black circle in the middle. The image are nowhere similar. That indicate a very large amount of over corrected spherical aberration. Well over 1/4 wave. 

 

It would be interesting to test the primary by itself, to see if it's spherical.  If it's not, it might be easy to fix the scope.

Thanks for interesting points!
From the differences in/out of focus I've understood that there is too much spherical aberration, but I did not have enough experience to judge into which side (over- or under- corrected).
As a follow-up experiment I will try to 3D print a spider of exact diameter and thickness to shoot the same series of pictures again (meaning - without corrector plate in-place).

If you are saying it could be an aspherical mirror - we will see how much it contributes, will be much of surprise if the scope will perform better as regular Newt. 


Edited by AlMuz, 29 July 2020 - 06:13 PM.


#17 RichA

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Posted 29 July 2020 - 07:25 PM

If it is anything like my Comet catcher was then it is just flat out bad.  Meade was not making good optics in them days other than the Newts were very good. Meade stepped up the plate with their SCT's optics in the mid 90's and were beating Celestron by a mile.
 

Comet Catcher wasn't a SC.



#18 CHASLX200

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Posted 29 July 2020 - 07:40 PM

Comet Catcher wasn't a SC.

I know but mine was just mushy. This was before i had Nags and SWA's.



#19 DAVIDG

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Posted 29 July 2020 - 08:50 PM

Thanks for interesting points!
From the differences in/out of focus I've understood that there is too much spherical aberration, but I did not have enough experience to judge into which side (over- or under- corrected).
As a follow-up experiment I will try to 3D print a spider of exact diameter and thickness to shoot the same series of pictures again (meaning - without corrector plate in-place).

If you are saying it could be an aspherical mirror - we will see how much it contributes, will be much of surprise if the scope will perform better as regular Newt. 

It would be far easier to make a simple Foucault tester from a razor blade, LED and simple wooden stand and Foucault test the mirror.   You can test the mirror in the scope and just remove the corrector and secondary which comes out as a unit. Then there will be no question to what the figure is. Since the mirror should be a sphere and the Foucault test is a null test for a sphere, there is nothing to measure. If it nulls it is sphere if you see any error it is not.   

 

               - Dave 


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#20 davidc135

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Posted 30 July 2020 - 02:05 AM

Interesting to see where the problem(s) lie. My money would be on the corrector as far as the over correction goes. The c.p would only have to correct the 1 wave S.A of a spherical mirror. Hmm, (half) wonder if they put in a paraboloid by mistake.

 

David



#21 DAVIDG

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Posted 30 July 2020 - 07:28 AM

 What most likely what happened was that what ever the figure was on either the corrector or the primary mirror when they came off the polishing machine was what when into the telescopes. If the buyer was happy great, if not send it back and give them their money back or get another one to try.   

    Amateurs that purchase mass produced telescope need to understand that the manufacture can't make a profit at the price they sell these telescope for if they were testing and correcting optics. So what ever the quality is when they come off the polishing machine is what you get. The problem is consumers believe they are getting precision optics that are  super great when a majority of the time  you are  lucky if  they at 1/2 wave if not worse. This is just another example of this. The test images shows the real story and you'll  see the truth when you start to test optics.    

   It is only when you get into the lower volume higher end scopes that you see consistently better optical quality and much better chance of getting a true 1/8 wave system. 

    It always comes down to the fact that quality optics takes time to make and time equals money. 

 

                  - Dave 



#22 AlMuz

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Posted 30 July 2020 - 02:16 PM

I forgot to mention that I was using 2.5x Tele Vue Barlow to make all these star-test pictures with Pentax K5 DSLR.
And wonder how much of spherical aberration it can potentially introduce to affect start-test images?

 

The Barlow itself probably made at the same time as a telescope (the Craigslist seller just gave away the scope to me with a bunch accessories he had).
I can not find the same model on sale now, so I assume it is at least 20 y.o.



#23 AlMuz

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Posted 08 August 2020 - 04:25 AM

Hello Gentelmen, Sorry for delay with my response.

 

I wanted to make Foucault Test setup earlier and desired it to be somewhat decent.

With proper features - like precise adjusters mounted with DSLR/CCTV to do some image analysis with dedicated software.

 

So ...as a main mechanical component I've ordered an 80mm X-Y adjustment stage form Amazon and waited for a week for it to arrive.

After few hours of design and 3D-printing here is my first prototype:

 

Foucault Tester

 

And here is the first measurement with 0.5 mm steps of MTS-SN6 mirror:

 

Foucault test of MTS-SN6 Spherical Mirror

 

Higer res pic at the center of curvature:

 

Foucault Test of MTS-SN6 Spherical Mirror

 

Loos like spherical to me (with somewhat questionable surface quality).

So the root problem of my scope is over-correcting Schmidt Corrector undecided.gif . 


Edited by AlMuz, 08 August 2020 - 04:28 AM.

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#24 tim53

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Posted 08 August 2020 - 08:26 AM

 What most likely what happened was that what ever the figure was on either the corrector or the primary mirror when they came off the polishing machine was what when into the telescopes. If the buyer was happy great, if not send it back and give them their money back or get another one to try.   

    Amateurs that purchase mass produced telescope need to understand that the manufacture can't make a profit at the price they sell these telescope for if they were testing and correcting optics. So what ever the quality is when they come off the polishing machine is what you get. The problem is consumers believe they are getting precision optics that are  super great when a majority of the time  you are  lucky if  they at 1/2 wave if not worse. This is just another example of this. The test images shows the real story and you'll  see the truth when you start to test optics.    

   It is only when you get into the lower volume higher end scopes that you see consistently better optical quality and much better chance of getting a true 1/8 wave system. 

    It always comes down to the fact that quality optics takes time to make and time equals money. 

 

                  - Dave 

"Oh, there's an old man on the floor, so I summon my charm

I said "Hey Stormband, has there been an alarm?"
He said "Here, they're selling off eternal youth
They all got afraid 'cause I'm the living proof
My name is Einstein. Do you know time is a curve?"
I said "Stop old man! you got a nerve
'Cause there's only one rule that I observe
Time is money and money I serve!"

 

-Peter Gabriel "One Way World"



#25 tim53

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Posted 08 August 2020 - 08:35 AM

Hello Gentelmen, Sorry for delay with my response.

 

I wanted to make Foucault Test setup earlier and desired it to be somewhat decent.

With proper features - like precise adjusters mounted with DSLR/CCTV to do some image analysis with dedicated software.

 

So ...as a main mechanical component I've ordered an 80mm X-Y adjustment stage form Amazon and waited for a week for it to arrive.

After few hours of design and 3D-printing here is my first prototype:

 

 

 

And here is the first measurement with 0.5 mm steps of MTS-SN6 mirror:

 

 

 

Higer res pic at the center of curvature:

 

 

 

Loos like spherical to me (with somewhat questionable surface quality).

So the root problem of my scope is over-correcting Schmidt Corrector undecided.gif . 

The next thing to try would be double pass autocollimation (DPAC).  If you don't have an optical flat, you can "make" one using a pan of oil and a stand made of plywood and bolts to set the OTA on, facing down into the flat.  There are a number of threads here and on the ATM forum as to how to go about doing this.  It's surprisingly simple to make, and can give the best results, since the light is passing through the complete optical train twice.

 

-Tim.




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