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Celestron C11 has mild astigmatism

cassegrain Celestron collimation equipment optics SCT
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#26 Mitrovarr

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Posted 16 May 2019 - 10:31 PM

I think a situation where you have either astigmatism or collimation error almost makes it certain there is misalignment in the optical train.

#27 charlesyung

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Posted 16 May 2019 - 10:39 PM

I think a situation where you have either astigmatism or collimation error almost makes it certain there is misalignment in the optical train.

Agree!

 

In your opinion, if I remove the primary mirror from the OTA tube, will there be adjustment I can make to put the primary and secondary alignment back to the optical train?

 

Anyone??



#28 Mitrovarr

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Posted 16 May 2019 - 10:49 PM

If you take the corrector out, you can use a laser collimator to check the position of the primary optical axis (rotate the OTA in place - if the laser moves, it's off center). If it's off center you can decenter the secondary/corrector by an equal amount or try to center it.

I found this webpage and it looked helpful. I don't know its accurate but it looks like it should be.

http://www.wilmslowa..._alignment.html

#29 ngc7319_20

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Posted 16 May 2019 - 11:59 PM

Agree!

 

In your opinion, if I remove the primary mirror from the OTA tube, will there be adjustment I can make to put the primary and secondary alignment back to the optical train?

 

Anyone??

There is no adjustment to make.  The center baffle is permanently fixed to the rear casting.  The primary is glued to a sleeve that slides on the center baffle (to focus).  I guess you could un-glue the primary from the sleeve -- nasty job though.

 

http://www.darkatmos...ic/C11_Project/

 

http://astroshed.com...aga/c11saga.htm


Edited by ngc7319_20, 17 May 2019 - 12:08 AM.


#30 Mitrovarr

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Posted 17 May 2019 - 12:07 AM

Even if you can't adjust the position of the baffle tube you can decenter the corrector and secondary to align the optical axis.

#31 charlesyung

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Posted 17 May 2019 - 12:55 AM

There is no adjustment to make.  The center baffle is permanently fixed to the rear casting.  The primary is glued to a sleeve that slides on the center baffle (to focus).  I guess you could un-glue the primary from the sleeve -- nasty job though.

 

http://www.darkatmos...ic/C11_Project/

 

http://astroshed.com...aga/c11saga.htm

Thanks for the links! Those 2 articles give me some ideas of what to look for now.



#32 charlesyung

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Posted 17 May 2019 - 12:55 AM

Even if you can't adjust the position of the baffle tube you can decenter the corrector and secondary to align the optical axis.

 

Thanks, I think this will be a easy test to conduct!



#33 Vla

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Posted 17 May 2019 - 07:19 AM

attachicon.gif In-focus.jpgattachicon.gif Outside-focus-1.jpgattachicon.gif Outside-focus-2.jpgattachicon.gif Inside-focus-3.jpgattachicon.gif Inside-focus-4.jpgattachicon.gif 4-shadow-of-circles.jpg

I inspected the OTA tube and no sign of the scope being abused, or the rear/front casting was removed. Screws look untouched. 

 

I conducted the secondary mirror rotation test. With the mirrors rotated multiple times and tested covering a range of 270 degree, no improvement of the astigmatism error after collimation at each rotation test point. Rotating the secondary mirror has no effect on the orientation of the Astigmatism error ellipse pattern, i.e. the 90 degree of the ellipse shape state the same position from my camera preview screen.

 

One thing I noticed during the secondary mirror rotation test is that with the mirrors rotated 120 degree from the initial position, the airy disc change from ellipse shape to a perfect circle shape, and I think I have the Astigmatism error fixed. However, closer look at airy disc noticed that the collimation is way off. After collimating the secondary mirror, the Astigmatism error came back with the same orientation position.

 

My non-professional conclusion seems to tell me:

1) Introduce the collimation error helps to compensate and cancel the astigmatism error

2) Rotating secondary mirror will introduce collimation error, but does not fix the astigmatism error

3) Amount of collimation error requires to compensate the astigmatism error cause the image too burr, much worst than the image quality caused by the astigmatism error

4) The OTA scope does not seem to be disassembled by previous owners (I am at least the 3rd owner)

5) Took a picture of the astigmatism error with my DSLR, I can visually see 4 circle of 'green shadow' oriented at the 4 corner of the '+' sign. No sure if this fact means anything to anyone???

 

Is the above points enough to eliminate any cause of the astigmatism error, and narrow my tests?

Your descriptions are unclear and contradicting, starting with the thread title (cross-like pattern in focus is not "mild" astigmatism). "Perfect" diffraction pattern can't at the same time be "miscollimated" i.e. "blurred". If it is, it can only be from spherical aberration, and if it is not there when you use the telescope at infinity, it comes from despace error, i.e. having object (light source) too close. If astigmatism goes away when rotating secondary (alone or with the corrector) it can only be because it at the same time changes its position vs. primary's axis (whether because it is decentered on the decentered corrector, or because it is loose). Astigmatism in one or more optical surfaces would make it more complicated. In addition to what was suggested, you may want to check whether corrector's optical center (center of the ring formed by interferometric pattern against either a flat, or the primary) coincides with its physical center.



#34 ngc7319_20

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Posted 17 May 2019 - 01:43 PM

 

My non-professional conclusion seems to tell me:

1) Introduce the collimation error helps to compensate and cancel the astigmatism error

2) Rotating secondary mirror will introduce collimation error, but does not fix the astigmatism error

3) Amount of collimation error requires to compensate the astigmatism error cause the image too burr, much worst than the image quality caused by the astigmatism error

4) The OTA scope does not seem to be disassembled by previous owners (I am at least the 3rd owner)

5) Took a picture of the astigmatism error with my DSLR, I can visually see 4 circle of 'green shadow' oriented at the 4 corner of the '+' sign. No sure if this fact means anything to anyone???

 

Is the above points enough to eliminate any cause of the astigmatism error, and narrow my tests?

 

5) the "green shadow" with 4 circles is probably some reflection off the camera spatial filter or sensor.  Reflections off camera sensors often have these "4" patterns due to the pixel array.  I am not sure of the full reflection path here.  It may be from sensor out to the corrector plate and back to the sensor.

 

It sounds like the primary is mounted with some bad tilt or is loose.  This reference talks about a retaining ring on the front of the primary assembly being loose, and the primary flopping around on the center baffle (about half way down the scroll-down):

http://astroshed.com...aga/c11saga.htm

I'm am not sure of the details, but it sounds like that is something which could possibly cause the primary to tilt badly.  And yet pass Celestron quailty checks at the factory. I wonder if thats it.

 

Another random idea: Is it possible that stuff hung on the back of the scope is badly off center somehow?  For example, if the threads were crossed where some adapter threads to the back of the scope?


Edited by ngc7319_20, 17 May 2019 - 01:45 PM.


#35 charlesyung

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Posted 17 May 2019 - 09:35 PM

Your descriptions are unclear and contradicting, starting with the thread title (cross-like pattern in focus is not "mild" astigmatism). "Perfect" diffraction pattern can't at the same time be "miscollimated" i.e. "blurred". If it is, it can only be from spherical aberration, and if it is not there when you use the telescope at infinity, it comes from despace error, i.e. having object (light source) too close. If astigmatism goes away when rotating secondary (alone or with the corrector) it can only be because it at the same time changes its position vs. primary's axis (whether because it is decentered on the decentered corrector, or because it is loose). Astigmatism in one or more optical surfaces would make it more complicated. In addition to what was suggested, you may want to check whether corrector's optical center (center of the ring formed by interferometric pattern against either a flat, or the primary) coincides with its physical center.

Thanks for pointing out the confusion, let's see if this help.

 

There was no 'Perfect' diffraction pattern at the same time with 'miscollimation' in the scope. Rotating the secondary mirror change the shape of the image with Astigmatism error from ellipse shape to a circle shape. However, the circle shape was badly miscollimated.



#36 charlesyung

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Posted 17 May 2019 - 09:54 PM

5) the "green shadow" with 4 circles is probably some reflection off the camera spatial filter or sensor.  Reflections off camera sensors often have these "4" patterns due to the pixel array.  I am not sure of the full reflection path here.  It may be from sensor out to the corrector plate and back to the sensor.

 

It sounds like the primary is mounted with some bad tilt or is loose.  This reference talks about a retaining ring on the front of the primary assembly being loose, and the primary flopping around on the center baffle (about half way down the scroll-down):

http://astroshed.com...aga/c11saga.htm

I'm am not sure of the details, but it sounds like that is something which could possibly cause the primary to tilt badly.  And yet pass Celestron quailty checks at the factory. I wonder if thats it.

 

Another random idea: Is it possible that stuff hung on the back of the scope is badly off center somehow?  For example, if the threads were crossed where some adapter threads to the back of the scope?

Thanks for pointing out that the primary mirror may be badly tilted! I did more test, and I think I have a break-through now.

 

I decided to conduct a 'level test' using the cell phone level app: "Bubble Level app". I am trying to determine if the primary mirror and the secondary mirror/corrector plage are at the same level, i.e. no tilt for the entire OTA tube. Test result can be concluded as:

 

1) if Primary mirror and secondary mirror has the same level reading from the app, tilt happened inside the OTA, and the tilt is Primary mirror and Baffle related

 

2) if Primary mirror and secondary mirror do not have the same level readying from the app, tilt is caused by the OTA tube, i.e. the Rear cast, tube and the front cast are not aligned properly. 

 

3) tilt is a combination of the primary mirror, baffle, tube, rear cast, tube and front cast. There will be too many combination and too complex, and I'll ignore at this point until I exhausted all test and adjustment that I can do

 

I found that with a reference reading of the platform level first, then put the C11 tube on the platform vertically with the rear cast landed on the platform and the corrector plate facing up. Put the cell phone sitting on the secondary mirror holder with the phone having the same orientation while taking the reference reading, I found that the level of the secondary mirror is tilted in one direction by 0.4 ready on the. Adjust the phone to produce the same reference reading tells me the direction of the happened within the OTA tube.

 

I think this is a good news. Hypothesis is that by loosing the rear cast, tube and the front cast, then re-assemble to eliminate the tilt, that could fix the issue and have the Primary / Secondary mirror back on the same optical axis. 

 

Thanks for the people sharing those links of dis-assemble the tube and mirrors!

 

Question is how does the factory assemble team knows that the rear cast, tube, and front cast were put together with the right alignment, i.e. parallel to each other? I don't see any factory alignment mark as the corrector plate does.



#37 charlesyung

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Posted 17 May 2019 - 09:58 PM

Here are some pictures of the Bubble Level reading.

 

1st picture: reference reading

2nd: reading showed that primary and secondary were tilted

3rd: direction of tilt required in the tube to eliminate the primary/secondary tilt.

 

Reference-level.jpg

Primary-Secondary-tilted.jpg

Adjustment-required.jpg

 



#38 ngc7319_20

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Posted 17 May 2019 - 10:42 PM

 

Question is how does the factory assemble team knows that the rear cast, tube, and front cast were put together with the right alignment, i.e. parallel to each other? I don't see any factory alignment mark as the corrector plate does.

I have never heard of alignment marks in the castings or main tube.  I think the main tube just seats flush on ridges or a shelf inside the castings, and thats it.  There are screws and nuts that fasten the castings to the main tube.

 

If you take the corrector plate off, put on some nitrile gloves or something, aim the scope straight up, and grab the edges of the primary, and see if the primary mirror seems loose or wobbles on the baffle tube.  My feeling is that the scope probably left the factory OK, but something has happened since then -- something has come loose, etc.

 

I have never heard of this happening -- but I also wonder if the scope were dropped hard, if the baffle tube could become bent or tilted.  Maybe you could some how study this by looking in the rear of the scope with a pinhole device, and seeing whether the front edge of baffle tube made a concentric circle with the edges of the secondary mirror.   By pinhole device, I mean something that fits in the rear of the scope like an eyepiece, but that has only a pinhole at the center.

 

Along the lines of your level device... you could cut a flat board, or foam core, or similar that could be placed over the primary mirror (touching only at the edges), and then check if it is parallel to the rear casting, corrector plate, etc.


Edited by ngc7319_20, 17 May 2019 - 10:45 PM.


#39 Vla

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Posted 18 May 2019 - 07:19 AM

Thanks for pointing out the confusion, let's see if this help.

 

There was no 'Perfect' diffraction pattern at the same time with 'miscollimation' in the scope. Rotating the secondary mirror change the shape of the image with Astigmatism error from ellipse shape to a circle shape. However, the circle shape was badly miscollimated.

I don't think it's helpful to call spherical aberration arising from a small object distance "miscollimation". Even perfectly collimated SCT would be "miscollimated" under such condition, and that doesn't add up, does it? I'll just say once more: you can play with the elements' tilt forever, but if you want to take astigmatism out of the mid field - assuming it is not coming from one or more astigmatic surfaces - you need to make sure the elements - i.e. their optical centers - are on the same axis. Only then you'll have only coma to deal with, which you'll be able to take out tilting the secondary.



#40 ngc7319_20

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Posted 18 May 2019 - 02:45 PM

I don't think it's helpful to call spherical aberration arising from a small object distance "miscollimation". Even perfectly collimated SCT would be "miscollimated" under such condition, and that doesn't add up, does it? I'll just say once more: you can play with the elements' tilt forever, but if you want to take astigmatism out of the mid field - assuming it is not coming from one or more astigmatic surfaces - you need to make sure the elements - i.e. their optical centers - are on the same axis. Only then you'll have only coma to deal with, which you'll be able to take out tilting the secondary.

I think he is saying that he got rid of the astigmatism, but then there was a large coma -- what he is calling "miscollimation."  And when he got rid of the coma, the astigmatism came back.



#41 choward94002

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Posted 18 May 2019 - 05:45 PM

Thanks for pointing out that the primary mirror may be badly tilted! I did more test, and I think I have a break-through now.

 

I decided to conduct a 'level test' using the cell phone level app: "Bubble Level app". I am trying to determine if the primary mirror and the secondary mirror/corrector plage are at the same level, i.e. no tilt for the entire OTA tube. Test result can be concluded as:

 

1) if Primary mirror and secondary mirror has the same level reading from the app, tilt happened inside the OTA, and the tilt is Primary mirror and Baffle related

 

2) if Primary mirror and secondary mirror do not have the same level readying from the app, tilt is caused by the OTA tube, i.e. the Rear cast, tube and the front cast are not aligned properly. 

 

3) tilt is a combination of the primary mirror, baffle, tube, rear cast, tube and front cast. There will be too many combination and too complex, and I'll ignore at this point until I exhausted all test and adjustment that I can do

 

I found that with a reference reading of the platform level first, then put the C11 tube on the platform vertically with the rear cast landed on the platform and the corrector plate facing up. Put the cell phone sitting on the secondary mirror holder with the phone having the same orientation while taking the reference reading, I found that the level of the secondary mirror is tilted in one direction by 0.4 ready on the. Adjust the phone to produce the same reference reading tells me the direction of the happened within the OTA tube.

 

I think this is a good news. Hypothesis is that by loosing the rear cast, tube and the front cast, then re-assemble to eliminate the tilt, that could fix the issue and have the Primary / Secondary mirror back on the same optical axis. 

 

Thanks for the people sharing those links of dis-assemble the tube and mirrors!

 

Question is how does the factory assemble team knows that the rear cast, tube, and front cast were put together with the right alignment, i.e. parallel to each other? I don't see any factory alignment mark as the corrector plate does.

Hey Charles!  Good thread, and hopefully jhayes can chime in as he's 'da man' for optics, but here are some thoughts ...

 

- The primary mirror is resting on a spider of aluminum which has a baffle tube attached to it and the whole assembly rides on another tube that goes out the back of the OTA ... that said, the angle of the mirror/ spider will change relative to the central axis as the assembly rides along the central tube ... that's called "mirror flop" and it's a function of the SCT construction

- The secondary mirror is optically spherical, which means that it doesn't need to be coplanar with the mirror surface ... which is good, as that would mean the mirror flop would make the secondary useless as soon as that happened.  Light from the primary will still return on it's path even if the two mirror aren't coplanar ... so even though you have a bit of a tilt in the secondary mirror, it's not likely to be causing what you're seeing.  John did a study a few years back in relation to centering the secondary and found that it can be +/- 2mm out of optical center before it becomes a problem

- Astigmatism in SCT's is pretty well explained here [https://starizona.co...al/astigmatism/] but basically is a function of something getting "bent" somewhere and being "corrected" somewhere else.  To fix this at the factory (at least for Celestron) they take a primary which is "bent" and match it with a secondary which when rotated properly makes it "corrected".  This means that something has happened to interfere with this process, which isn't uncommon [sigh]

 

When I get in a new scope, they are always used and always have some kind of optical defect, as well as problems from when my scopes are exposed to the elements and elk want to use them as scratching posts [happened] ... in all of those cases the first thing I do is to place the scope in as close to a vertical position with the nose up and see if I can reproduce the issue that way.  In your case, I would collimate on Polaris and then do the in/out focus thing to get your ovals, take pix and then rotate the mount by 90deg and same thing.  Do this for 180deg and 270deg, and see if you can see changes in the size and shape of your ovals as the telescope orientation changes.  If there *are* changes then you've got something loose somewhere that needs to be tightened (I would look to the primary mirror locking hub, that loosens over time) as well as the collimation screws on your secondary (they should be "snug" torqued, not "chevy cylinder block" torqued).  If there aren't changes then it's possible that your corrector and secondary are out of alignment somehow ... you did good finding the serial number, everything should be lined up at the 3 o'clock position (primary, secondary, corrector plate) but sometimes things get jostled [tip: it's easier to see the primary alignment mark if you use a USB endoscope camera, about $45 on Amazon] ...

 

Good luck!



#42 Richard Whalen

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Posted 18 May 2019 - 07:01 PM

I have seen this before in a C11 that was dropped during shipment, box was beat up. Had a crack in edge of corrector and showed astigmatism. Lucky my buddy bought it second hand from a dealer, and he refunded money. I am always wary of 2nd and 3rd hand mass produced scopes, usually there is a reason they are not kept. And more likely than not due to either damage or optical quality. Hate to say it, but with a SCT usually better to buy new unless you can test it first or has a return policy. Good luck on getting it right, wishing you success.


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#43 charlesyung

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Posted 19 May 2019 - 01:45 AM

Hey Charles!  Good thread, and hopefully jhayes can chime in as he's 'da man' for optics, but here are some thoughts ...

 

- The primary mirror is resting on a spider of aluminum which has a baffle tube attached to it and the whole assembly rides on another tube that goes out the back of the OTA ... that said, the angle of the mirror/ spider will change relative to the central axis as the assembly rides along the central tube ... that's called "mirror flop" and it's a function of the SCT construction

- The secondary mirror is optically spherical, which means that it doesn't need to be coplanar with the mirror surface ... which is good, as that would mean the mirror flop would make the secondary useless as soon as that happened.  Light from the primary will still return on it's path even if the two mirror aren't coplanar ... so even though you have a bit of a tilt in the secondary mirror, it's not likely to be causing what you're seeing.  John did a study a few years back in relation to centering the secondary and found that it can be +/- 2mm out of optical center before it becomes a problem

- Astigmatism in SCT's is pretty well explained here [https://starizona.co...al/astigmatism/] but basically is a function of something getting "bent" somewhere and being "corrected" somewhere else.  To fix this at the factory (at least for Celestron) they take a primary which is "bent" and match it with a secondary which when rotated properly makes it "corrected".  This means that something has happened to interfere with this process, which isn't uncommon [sigh]

 

When I get in a new scope, they are always used and always have some kind of optical defect, as well as problems from when my scopes are exposed to the elements and elk want to use them as scratching posts [happened] ... in all of those cases the first thing I do is to place the scope in as close to a vertical position with the nose up and see if I can reproduce the issue that way.  In your case, I would collimate on Polaris and then do the in/out focus thing to get your ovals, take pix and then rotate the mount by 90deg and same thing.  Do this for 180deg and 270deg, and see if you can see changes in the size and shape of your ovals as the telescope orientation changes.  If there *are* changes then you've got something loose somewhere that needs to be tightened (I would look to the primary mirror locking hub, that loosens over time) as well as the collimation screws on your secondary (they should be "snug" torqued, not "chevy cylinder block" torqued).  If there aren't changes then it's possible that your corrector and secondary are out of alignment somehow ... you did good finding the serial number, everything should be lined up at the 3 o'clock position (primary, secondary, corrector plate) but sometimes things get jostled [tip: it's easier to see the primary alignment mark if you use a USB endoscope camera, about $45 on Amazon] ...

 

Good luck!

HI, thanks for the good explanation and the share of your thoughts, it's a very good learning!!

 

Did more 'level test',

 

1) I found that the central battle tube had the same level reading as the rear cast. I believe that means the battle is not tilted

 

2) Measured the level reading directly on the corrector plate, instead of the secondary mirror holder, and the corrector plate was not the same level as the rear cast. i.e. the corrector plate is definitely tilted against the rear cast

 

If the tilt of the corrector plate & secondary mirror will not cause the Astigmatism error since the secondary mirror is spherical, then, I think I should put my focus to the primary mirror. 

 

Since the central battle tube is not tilted, the primary battle tube travel along the central battle tube should not be tilted then. More inspection to the primary mirror

 

1) The focus knobs has some amount of backlash. Not sure if this is an indication of the focusing arm is not perpendicular with the mirror spider and causing the primary mirror tilted

 

2) Use finger to put little force to the edge of the primary mirror do feel that I can tilt the mirror, But I believed the primary mirror did come back to the position after the finger is removed. I assume this is normal

 

3) The secondary collimation screws are little bit hard to turn, I think that's what you mean 'snug' torqued, and assume that's normal

 

Do you have any reference URL for checking the primary alignment mark?

 

Anyway, this is my 3rd Celestron SCT scope, I have a C8 and C4 Mak. Both had problem when I received them used, and C8 is fixed by re-positioning the corrector plate and rotating the secondary mirror. The C4 Mak is totally out of alignment and disappointed for the first week since the image can't be focused. Then I found that there are alignment screws at the back, and need to remove the rear plastic cast to reach those screws. Thanks to the Internet and forum posting!!

 

I am just not aware that the C11 primary mirror has no collimation screws, may be that's why I am stuck now... 



#44 charlesyung

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Posted 19 May 2019 - 02:04 AM

I have seen this before in a C11 that was dropped during shipment, box was beat up. Had a crack in edge of corrector and showed astigmatism. Lucky my buddy bought it second hand from a dealer, and he refunded money. I am always wary of 2nd and 3rd hand mass produced scopes, usually there is a reason they are not kept. And more likely than not due to either damage or optical quality. Hate to say it, but with a SCT usually better to buy new unless you can test it first or has a return policy. Good luck on getting it right, wishing you success.

Hi Richard, thanks for sharing your experience! If I assume the previous owner dropped the scope which cause this astigmatism error, do you know what damage in the OTA I can find which will cause astigmatism? I don't see any visual damage of the OTA tube. Since the corrector plate does not have the same level as the rear cast, could that be an indication of the OTA being dropped?

 

Totally agree that buying a new SCT is the way to go. This C11 is my 3rd Celestron SCT scope, where I have a C8 and C4 Mak. I buy used because I want to experiment the size and the type that best suit my astro imaging needs before I commit for a high quality new scope, say a new C8/C11 EdgeHD. C4 is very portable and my recent post of the Jupiter is very reasonable with the price I am paying. I am waiting to get the C11 fixed so that I can compare the quality of the Jupiter image against my C4 and C8.



#45 choward94002

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Posted 19 May 2019 - 12:00 PM

Did more 'level test',

 

1) I found that the central battle tube had the same level reading as the rear cast. I believe that means the battle is not tilted

 

2) Measured the level reading directly on the corrector plate, instead of the secondary mirror holder, and the corrector plate was not the same level as the rear cast. i.e. the corrector plate is definitely tilted against the rear cast

 

... hmm ... that's hard to believe (not that it's hard to believe you, hard to believe what you're finding) ... the C11 corrector plate sits on a ring which goes around the inside of the tube, which is then sandwiched by a retaining collar [all of which you know] ... so if it's really tilted then something else is happening.  Let's try this: whenever you get a reading that's hard to believe it's best to verify it with an unrelated measurement ... make a simple "try stick" with a dowel or straw, put one end against the corrector plate and mark where the stick goes above the tube, then move around the tube and see if the mark "moves" ... this will verify that the plate ring is the proper distance from the end of the tube.  Take a tape measure and verify that the tube is the proper length from the tube end to the rear cell end (this isn't silly: the rear cell and the front cell are attached to the tube by screws and if someone took the tube apart and reassembled it they could have not been careful to put the cells back on square to the tube, and the cells will be misaligned)

 

Once we've done that we've verified that if the corrector plate is still tilted but the above dimensions show the corrector plate ring is square to the primary mirror cell, then there might be something between the corrector plate and the ring that shouldn't be there (piece of cork or paper)

 

If the tilt of the corrector plate & secondary mirror will not cause the Astigmatism error since the secondary mirror is spherical, then, I think I should put my focus to the primary mirror. 

 

Yes and no ... the secondary is spherical so a tilt of that won't cause a problem, but recall the corrector plate isn't simply a piece of glass, it's a weak lens as well (that's why Celestron rotates it to find a good match) ... if that lens really is tilted then it will be putting the light onto a different path than the intended one ... and at this point I'm completely out of my area of expertise, Frank or John Hayes would be the people to continue from here ...

 

Since the central battle tube is not tilted, the primary battle tube travel along the central battle tube should not be tilted then. More inspection to the primary mirror

 

1) The focus knobs has some amount of backlash. Not sure if this is an indication of the focusing arm is not perpendicular with the mirror spider and causing the primary mirror tilted

 

2) Use finger to put little force to the edge of the primary mirror do feel that I can tilt the mirror, But I believed the primary mirror did come back to the position after the finger is removed. I assume this is normal

 

Correct, all of that is normal ... your "wiggle" is the mirror flop, focus arm has backlash, that end of the assembly sounds good!

 

3) The secondary collimation screws are little bit hard to turn, I think that's what you mean 'snug' torqued, and assume that's normal

 

Yep, that's normal also and good ... although to be honest you'd have to be doing some serious deformation for that to be a problem as the collimation screws push against a metal plate which is where the secondary mirror is mounted but it's still a good thing to check ...

 

Do you have any reference URL for checking the primary alignment mark?

 

There are various CN discussions about it, look for "index mark" and "3'oclock" and "9'oclock" and you'll find them ... I don't have an easy ability to get them right now ...

 

Anyway, this is my 3rd Celestron SCT scope, I have a C8 and C4 Mak. Both had problem when I received them used, and C8 is fixed by re-positioning the corrector plate and rotating the secondary mirror. The C4 Mak is totally out of alignment and disappointed for the first week since the image can't be focused. Then I found that there are alignment screws at the back, and need to remove the rear plastic cast to reach those screws. Thanks to the Internet and forum posting!!

 

Sorry for your troubles, didn't even know about a MAK having alignment screws!

 

I am just not aware that the C11 primary mirror has no collimation screws, may be that's why I am stuck now... 

 

The C11 primary, like the C14, is simply an aluminum spider carrying a mirror sliding up and down a tube ... there are no rear adjustments other than the mirror locking hub ... at this point I'd first verify that there really is a tilt to the corrector plate, if there is then you can shim the corrector plate against the retaining ring to correct the tilt and see if that makes a difference ... be careful that you can still attach the retaining collar, and put your shims right where the screw holes are so you don't add any stress to the mirror from the shims ...

 

Interesting problem, to be sure!



#46 ngc7319_20

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Posted 19 May 2019 - 08:34 PM

 

2) Measured the level reading directly on the corrector plate, instead of the secondary mirror holder, and the corrector plate was not the same level as the rear cast. i.e. the corrector plate is definitely tilted against the rear cast

 

If the tilt of the corrector plate & secondary mirror will not cause the Astigmatism error since the secondary mirror is spherical, then, I think I should put my focus to the primary mirror. 

 

Do you have some quantitative measure of how much tilt there is between the corrector plate + secondary and the rear casting?  

 

If you are running out of things to try, you could try shimming the corrector plate to make it parallel to the rear casting.  I presume you could put 2 bits or cardboard, wood, etc. where the corrected plate seats, hence introducing some tilt.  Maybe a bit of double stick tape to hold the shims in place while you place the corrector plate.  If it is a large tilt (large shim thickness), you might need to get some extra-long screws for the corrector plate retaining ring.  I am thinking 2 shims to prevent rocking of the corrector plate.

 

While theory suggest this is not the problem making astigmatism, it is not a bad idea to test the theories sometimes.



#47 Selenite

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Posted 20 May 2019 - 01:49 PM

Hi,

I have an old orange C8 which gave poor high-power images due to astigmatism. When testing the scope with an artificial star I first tried different centerings and decenterings of the corrector, but this made no visible difference. When I rotated the corrector, however, the direction (but not magnitude) of the astigmatism changed. So I came to the conclusion that either there is significant astigmatism figured in the corrector or the secondary mirror, or the corrector and the secondary are incorrectly rotated to each other. Removing the secondary and trying to replace it at different angles seemed like too much work for me, so I tried a different approach.

 

A 0.25-diopter cylindrical lens cancels out the astigmatism of the scope when placed about 2.7 inches from the focus with a 2x Barlow and about 4 inches from focus with a 3x Barlow. Without a Barlow it would have to be impractically close to the focus, but the astigmatism in my scope is unnoticeable at low powers anyway.


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#48 ngc7319_20

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Posted 20 May 2019 - 02:47 PM

 

A 0.25-diopter cylindrical lens cancels out the astigmatism of the scope when placed about 2.7 inches from the focus with a 2x Barlow and about 4 inches from focus with a 3x Barlow. Without a Barlow it would have to be impractically close to the focus, but the astigmatism in my scope is unnoticeable at low powers anyway.

Great solution!  Where is the cylindrical lens relative to the Barlow lens?  Picture of setup might help people...  Thanks!



#49 Selenite

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Posted 20 May 2019 - 03:33 PM

It's between the Barlow and the eyepiece/camera. I'll try to take a picture tomorrow. My Barlow has a T-thread which makes things a little easier, otherwise the lens would have to be inside the Barlow tube.


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#50 charlesyung

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Posted 20 May 2019 - 11:25 PM

Hi,

I have an old orange C8 which gave poor high-power images due to astigmatism. When testing the scope with an artificial star I first tried different centerings and decenterings of the corrector, but this made no visible difference. When I rotated the corrector, however, the direction (but not magnitude) of the astigmatism changed. So I came to the conclusion that either there is significant astigmatism figured in the corrector or the secondary mirror, or the corrector and the secondary are incorrectly rotated to each other. Removing the secondary and trying to replace it at different angles seemed like too much work for me, so I tried a different approach.

 

A 0.25-diopter cylindrical lens cancels out the astigmatism of the scope when placed about 2.7 inches from the focus with a 2x Barlow and about 4 inches from focus with a 3x Barlow. Without a Barlow it would have to be impractically close to the focus, but the astigmatism in my scope is unnoticeable at low powers anyway.

Great to hear this solution!




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