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C9.25 corrector & secondary centering & looseness problem

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#1 Axunator

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Posted 04 December 2017 - 02:55 PM

I just received a new-to-me second-hand and probably rather old C9.25 XLT (standard, non-Edge) I bought from a UK dealer with a good reputation for selling preowned scopes. The price was very reasonable and on the surface the scope is in a good, clean, but obviously used condition. No problem with that.

 

I noticed from the reflection that there was quite a bit of big dust particles on the secondary, and since it has Fastar secondary assembly, I figured it would  be easy to remove the secondary and give it a good blow with a blow bulb. The "Fastar" text was obliquely upside down, but I didn't pay much attention to it first. Removed the secondary, blew the biggest dust bits away, and re-installed the secondary and tightened the locking ring. Dang! Only then I realized the whole secondary assembly was rotating freely. Not super-loosely, but without excessive force anyway. Quick internet search revealed that I'm not alone with this problem, and that it should be possible to tighten the assembly by pulling the corrector plate and tightening the assembly rings from both sides.

 

However, the assembly was not only rotating, but also wiggling sideways in any direction as well. Maybe appr. 1 mm or so. Another quick search, and I came across John Hayes' instructions on how to check the corrector plate centering by looking at the concentricity of reflections from a distance with the secondary removed. Dang again! The corrector plate itself seems to be decentered, because wiggling the secondary holder as much as it could move in its slightly larger hole was not sufficient to make the reflections concentric.

 

The third problem related to all this was that - probably due to lateral movement - there's a ring of glue (or something) residue visible around the secondary holding ring on the corrector glass. Not much, maybe extending appr. 2 mm from the side of the ring here and there, and perhaps invisible to the primary because of secondary baffle, but still a bit nasty.

 

Now, if this was a brand new scope, I would obviously return it. I may consider returning it even it's a second hand scope (I contacted the dealer and told about my findings - still waiting for a reply), but then again, I'm pretty sure that these are issues that come with aged SCTs, and I might as well bite the bullet and try and learn to deal with them. Especially since I immediately fell in love with the size and form factor of C9.25 - easily handled, reasonably light, yet not a small scope anymore smile.gif

 

If I'll take that last route, I probably need some advice from you guys.

 

- How am I going to re-centralize both the corrector plate and the secondary holder ring based on concentricity of reflections, since I now have two degrees of freedom (decentered secondary ring & decentered corrector plate)

 

- Am I able to detach the whole secondary holder from the corrector plate to clean the glue (or whatever that gooey stuff is) - or should I even care, since it's such a narrow ring?

 

Thanks in advance!


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#2 Axunator

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Posted 04 December 2017 - 11:44 PM

Did some actual measurements to help you put the problem into perspective:

- the freedom of lateral movement of the secondary within its hole in the plate is less than I thought: 0.4-0.5 mm. Perhaps this is so little, that I don't really have to care about its exact position within the hole, as long as I get it tight enough so that it doesn't move by itself (free, uncontrolled movement of 0.5 mm would obviously make preservation of accurate collimation impossible, I presume). E.g. John Hayes in his corrector centering instructions tells that the accuracy of "concentricity of reflections" -method is appr. 0.5 mm. Still, let me tell ya, when something's moving that's not supposed to be moving, it feels SCARY, and 0.5 mm feels like 5 mm bigshock.gif

- as I wrote above, based on concentricity of reflections, the corrector plate is decentered. Just based on simple ruler measurements, it seems to be off center by about 2 mm (i.e. much more than explained by the loose secondary).

Now, my thinking of the solution to the problem would be:

1) pull out the corrector (making notes of its orientation to ensure proper re-installation), tighten the secondary holder, without stressing too much about its lateral position within the hole of the corrector.

2) now, with the appropriately immobile secondary holder, center the corrector plate using the "concentricity of reflections" -method

3) collimate secondary using a star

4) observe and be happy

Comments?

P.S. Note that I still don't know what to do with the gooey residue around the secondary holder. Maybe cleaning that should happen somewhere during step #1.

Edited by Axunator, 05 December 2017 - 06:30 AM.


#3 TG

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Posted 05 December 2017 - 02:40 PM

Loose secondaries seem to be endemic to C9.25s for some obscure reason. I had one and it had a loose secondary too.

 

Now, a secondary doesn't have to be centered absolutely. This is because it is spherical and a lateral motion of a sphere off the optical axis can be compensated by tilting it, i.e., collimating it to bring it back onto the optical axis. You'll just end up with slightly uneven field illumination at the edges but that is all. What is more important is to have it in the same rotational position as the marks on the secondary indicate. There is usually a mark/serial number visible at the edge of the corrector when you remove the corrector retaining ring. There is also usually a mark on the secondary edge. Both should align for best performance. In my case, I verified this with Roddier testing.

 

Regarding the secondary centering, this is important but I would assume it was centered correctly at the factory and if you remove it to tighten the secondary assembly, either measure its position beforehand, using the corrector cell rim as a reference, or mark the positions of any shims and make sure they are replaced exactly as before. It also goes without saying that the corrector should be reinstalled in the same rotational position as before. If you have centering screws for the corrector, people have recommended loosening only two a measured number of turns and then tightening them the same amount.

 

Starizona sells replacement gaskets that keep the secondary securely in place.

 

Good luck!

 

Tanveer.


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#4 Axunator

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Posted 05 December 2017 - 03:55 PM

Thanks Tanveer! I got excellent instructions also from John Hayes on how to address this problem - they were very much in line with your experience. I also ordered a new gasket from Starizona. 

 

My secondary mirror edge itself does not have any markings or serial numbers. If I rotate it so that the "fastar" text is horizontal and upright, this will place the alignment notch of the Fastar assembly at 6 o'clock position, but I don't know whether the orientation of the text has any real meaning for this purpose. I'll have to check if there's an alignment mark on the corrector plate to align the notch with, as per John's suggestion.

 

It's obvious that the corrector plate of my scope is clearly off center. While it remains a possibility that it was adjusted this way at the factory on purpose, I have a suspicion that the previous owner has removed the plate at some point (hopefully reinstalling it at proper rotational orientation!), and may have messed up the centering while doing this. I may try to re-center it using John's method.



#5 TG

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Posted 05 December 2017 - 10:13 PM

 

My secondary mirror edge itself does not have any markings or serial numbers.

If you can't find any markings on the glass edge of the secondary mirror, that would be strange...


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#6 jhayes_tucson

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Posted 05 December 2017 - 11:13 PM

Thanks Tanveer! I got excellent instructions also from John Hayes on how to address this problem - they were very much in line with your experience. I also ordered a new gasket from Starizona. 

 

My secondary mirror edge itself does not have any markings or serial numbers. If I rotate it so that the "fastar" text is horizontal and upright, this will place the alignment notch of the Fastar assembly at 6 o'clock position, but I don't know whether the orientation of the text has any real meaning for this purpose. I'll have to check if there's an alignment mark on the corrector plate to align the notch with, as per John's suggestion.

 

It's obvious that the corrector plate of my scope is clearly off center. While it remains a possibility that it was adjusted this way at the factory on purpose, I have a suspicion that the previous owner has removed the plate at some point (hopefully reinstalling it at proper rotational orientation!), and may have messed up the centering while doing this. I may try to re-center it using John's method.

Forget about the text orientation.  As I mentioned in my instructions, you should orient the notch in the secondary holder to line up with the mark on the corrector plate.  Sometimes the mark on the glass is on the bottom where it is bonded to the tilt plate so that you can't see it.

 

John



#7 Axunator

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Posted 06 December 2017 - 05:01 AM

 

 

My secondary mirror edge itself does not have any markings or serial numbers.

If you can't find any markings on the glass edge of the secondary mirror, that would be strange...

 

No markings whatsoever. There's a picture of it below, looks like that all around the edge of the glass. However, it's not a problem, because the secondary holder can go into the metal ring in the corrector plate in only one orientation, because of the notch in the holder ring, also pictured below. This notch dictates the rotational orientation of the secondary, just like John mentioned.

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  • IMG_2556.jpg

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#8 Axunator

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Posted 06 December 2017 - 05:13 AM

Forget about the text orientation.  As I mentioned in my instructions, you should orient the notch in the secondary holder to line up with the mark on the corrector plate.  Sometimes the mark on the glass is on the bottom where it is bonded to the tilt plate so that you can't see it.

 

 

John

 

waytogo.gif Gotcha! My next problem will be these pegs pictured below. They prevent me from detaching the outer corrector plate retaining ring (there are two of them on the opposite sides). Their function is to hold the front dust cap in place. They are not screws, and they seem to be in place very tightly. There is not enough space to slide the retaining ring and the corrector plate under the pegs. I'm afraid to use a lot of force and metal tools so close to corrector plate.  I don't want to cut them, because then the dust cap would not stay in place anymore. And I need to be able to remove the corrector plate to 1) check the markings to orient the notch in the secondary holder, and 2) install the new secondary holder ring gasket once it arrives.

 

How should I remove these pegs?

 

EDIT: These pegs are basically rivets. It seems impossible to remove them without destroying them (half destroyed the other one already by trying - luckily no functional damage..). How can I pull out the corrector from the front then? confused1.gif

 

Right now I'm thinking of cutting the other one, drilling and tapping a hole through it, and replacing it with a small screw.

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Edited by Axunator, 06 December 2017 - 07:23 AM.

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#9 ad701xx

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Posted 06 December 2017 - 01:56 PM

The ring is flexible plastic. You should be able to bend it a little to get it out even with the pegs in place.


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#10 GolfSierra

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Posted 06 December 2017 - 04:00 PM

The ring is flexible plastic. You should be able to bend it a little to get it out even with the pegs in place.

I second that, you have to be careful when bending the ring but it will come off over the pins and on again.I actually thought of splitting the the ring to ease a future removal.It never ceases to amaze me how they always make the easiest solution the most difficult. I can hear it now "why would you want to remove the retaining ring" so says tech support. It makes you wonder if anyone there ever puts to use what they manufacture.


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#11 Cpk133

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Posted 06 December 2017 - 05:06 PM

If the pins weren't right next to the retaining ring fasteners, a couple of small notches would be a better solution.  


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#12 jhayes_tucson

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Posted 06 December 2017 - 11:43 PM

Sheeze.  Celestron probably saves 1.0356 cents per scope by using those things instead of a threaded screw that would serve the same purpose.  That just seems stupid.

 

John


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#13 TG

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Posted 07 December 2017 - 01:56 AM

 

Thanks Tanveer! I got excellent instructions also from John Hayes on how to address this problem - they were very much in line with your experience. I also ordered a new gasket from Starizona. 

 

My secondary mirror edge itself does not have any markings or serial numbers. If I rotate it so that the "fastar" text is horizontal and upright, this will place the alignment notch of the Fastar assembly at 6 o'clock position, but I don't know whether the orientation of the text has any real meaning for this purpose. I'll have to check if there's an alignment mark on the corrector plate to align the notch with, as per John's suggestion.

 

It's obvious that the corrector plate of my scope is clearly off center. While it remains a possibility that it was adjusted this way at the factory on purpose, I have a suspicion that the previous owner has removed the plate at some point (hopefully reinstalling it at proper rotational orientation!), and may have messed up the centering while doing this. I may try to re-center it using John's method.

Forget about the text orientation.  As I mentioned in my instructions, you should orient the notch in the secondary holder to line up with the mark on the corrector plate.  Sometimes the mark on the glass is on the bottom where it is bonded to the tilt plate so that you can't see it.

 

John

 

Yes, I remember the marking on my C11HD secondary lined up with the notch.



#14 Axunator

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Posted 07 December 2017 - 03:55 AM

Thanks y'all, I'll give it another try.

 

 I can hear it now "why would you want to remove the retaining ring" so says tech support. It makes you wonder if anyone there ever puts to use what they manufacture.

 

funnypost.gif

 

 

If the pins weren't right next to the retaining ring fasteners, a couple of small notches would be a better solution.  

Notches would be a good idea. And they wouldn't have to be right next to screw holes, because you could always rotate the ring after removing the screws to align the notches with the pins.

 

 

Sheeze.  Celestron probably saves 1.0356 cents per scope by using those things instead of a threaded screw that would serve the same purpose.  That just seems stupid.

 

John

My thoughts exactly.

 

Come to think of it, it might be a good idea to replace both of them with detachable screws anyway. Even if it is possible to remove the retaining ring and the corrector plate with the pins in place, it 1) makes it unnecessarily difficult, and 2) poses an unnecessary risk of damaging the corrector plate, as I can already see with my mind's eye how I will hit the edge of the corrector plate to that tiny, sharp pin, chipping the plate bawling.gif


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#15 Axunator

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Posted 07 December 2017 - 02:52 PM

Mission almost accomplished! grin.gif  I was able to remove the retainer ring and corrector plate, now that I had the courage to bend the ring enough (still, I think that those unremovable pegs are just inexcusably bad engineering). Lo and behold, there was a black line drawn at the edge of the corrector plate (and no other markings), which I believe indicates the direction of the notch in the secondary holder ring, just like John said there would be! Moreover, there was an old tape remnant at the edge of the corrector cell, which gives me hope that the previous owner had been smart enough to mark the rotational orientation of the corrector plate before removing it. So now I hopefully have the rotational alignment of both front elements in order (or at least as they were set at the factory). Huge thanks to all of you for your help and encouragement! bow.gif flowerred.gif

 

I don't have the new gasket from Starizona yet, so I will have to do the whole exercise once more, but at least now I know how to do it laugh.gif I did tighten the secondary holder ring from both sides a bit while I had the corrector off, so now it's good enough to at least have a test look through the scope and practice collimating the secondary, while waiting for the gasket.

 

Today I also received Baader ClickLock 2" visual back and ADM Vixen-Losmandy Dual Saddle for my HEQ5 (really nice piece of work, albeit quite a bit heavier than the original saddle - the C9.25 came with Losmandy dovetail so ADM was a no-brainer), so I'm starting to get ready to rock'n'roll with this baby! Now I just have to get rid of the clouds somehow...tongue2.gif


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#16 outpostbob

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Posted 01 October 2021 - 05:43 PM

I have a further complication. The corrector was loose in the cell, so when I went to unscrew the secondary retainer the whole corrector assembly rotated. And I think this was a problem for the prior owner.... one of the screws is missing on the corrector retainer. So who knows how much the corrector has turned.

 

I have read enough of the above that I don't THINK I missed this... but what should the corrector plate mark be aligned to in order to maintain its orientation with the primary?



#17 Axunator

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Posted 02 October 2021 - 12:37 AM

I have a further complication. The corrector was loose in the cell, so when I went to unscrew the secondary retainer the whole corrector assembly rotated. And I think this was a problem for the prior owner.... one of the screws is missing on the corrector retainer. So who knows how much the corrector has turned.

I have read enough of the above that I don't THINK I missed this... but what should the corrector plate mark be aligned to in order to maintain its orientation with the primary?


The lore says it should be at 3 o’clock position; that’s how I aligned mine and have been quite happy with the scope’s optical performance ever since. At least some SCT primary mirrors are known to have alignment markings on the side or back of the mirror as well, but mine doesn't - I have since disassembled and reassembled the whole thing and there were lots of cryptic writings on the back, but nothing that could be interpreted as rotational alignment marking. So I've settled on using the 3 o'clock tradition (as seen from the front, with dovetail at 6 o'clock).

 

Presumably, if rotational alignment has an effect, misalignment should show up as astigmatism. My C9, with secondary holder pin and corrector plate marking at 3 o'clock, has very little.

 

Here is a WinRoddier analysis of my SCT at red light (to reduce seeing effects, as this was done on a real star under the jet stream at 60 deg N):

 

Dia1.jpeg

 

Strehl of 0.936.

 

With Zernike polynomial terms transferred to OpenFringe, I could play with including or excluding different aberration terms. This is with all aberrations still included:

 

Dia2.jpeg

 

As you can see, my collimation while capturing the star test was not perfect, as coma terms affect the Strehl (here 0.938, i.e. practically identical to WinRoddier as it should, since the Zernike terms are from WinRoddier).


Edited by Axunator, 02 October 2021 - 07:34 AM.


#18 Axunator

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Posted 02 October 2021 - 04:59 AM

If I eliminate the coma terms (i.e. simulating perfect collimation, which of course should be the goal in high magnification use anyway), Strehl increases to 0.962 despite the residual astigmatism (which is visualized quite nicely at 4-10 o'clock angle in the coloured wavefront error visualization graph) and other aberrations:

 

Dia4.jpeg

 

This residual astigmatism is so insignificant for the image quality, that I'm not even thinking of trying to eliminate that by rotating the corrector away from 3 o'clock position, since I don't have an optical bench to try that effectively (WinRoddier is waaayyy too cumbersome, labour-intensive and circumstance-sensitive tool for that). Especially without any a priori evidence that changing the rotational alignment would further improve the wavefront of this particular specimen - it may as well be at or near its optimum as it is, based on the already excellent Strehl value for a mass-produced scope of this size.

 

Of course, the actual Strehl value numbers (with their unrealistic 3-digit accuracies!) must be taken with a pinch of salt, as WinRoddier analysis has its sources of error, including atmospheric seeing during star test capturing.


Edited by Axunator, 02 October 2021 - 10:05 AM.


#19 tturtle

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Posted 02 October 2021 - 07:00 AM

I’ve read through the posts on your issues getting this scope up and running and first off I sympathize as I’m sure you weren’t expecting to dig this deeply into it when you bought it. I have this scope (w/out fastar) and love it so I would encourage you to persevere. I have removed the front corrector numerous times now as part of a fairly intense project to rebuild the OTA to reduce the weight of the scope in half. Removing the big retaining ring with the Celestron lettering is easy as noted it is flexible so once you remove the screws you can just slide/wiggle it out. Also the corrector diameter is smaller than the pegs for the dust cap so they do not interfere with the removal. There is a mark on the edge of the corrector with a line and 2 dots that apparently shows how the corrector should be oriented rotation wise so when you see it note how it is oriented relative to the primary focuser and return it likewise when you reassemble. There has been heated debate lately on whether the orientation really matters so if you feel like the original factory rotation of the corrector was lost by the previous owner you may be fine.  Post any questions You have as you get it back together and we will help you through the process.



#20 Axunator

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Posted 02 October 2021 - 07:26 AM

I’ve read through the posts on your issues getting this scope up and running and first off I sympathize as I’m sure you weren’t expecting to dig this deeply into it when you bought it. I have this scope (w/out fastar) and love it so I would encourage you to persevere. I have removed the front corrector numerous times now as part of a fairly intense project to rebuild the OTA to reduce the weight of the scope in half. Removing the big retaining ring with the Celestron lettering is easy as noted it is flexible so once you remove the screws you can just slide/wiggle it out. Also the corrector diameter is smaller than the pegs for the dust cap so they do not interfere with the removal. There is a mark on the edge of the corrector with a line and 2 dots that apparently shows how the corrector should be oriented rotation wise so when you see it note how it is oriented relative to the primary focuser and return it likewise when you reassemble. There has been heated debate lately on whether the orientation really matters so if you feel like the original factory rotation of the corrector was lost by the previous owner you may be fine.  Post any questions You have as you get it back together and we will help you through the process.

Hi Tom,

 

I am not sure whether you were replying to me (the OP - note that I started this thread 4 years ago!) or to outpostbob (post #16) who resurrected this old thread of mine, after having experienced similar problems, and to whom I was replying today.

 

Anyway, I was able to fix my C9 already years ago, and it turned out to be an excellent performer, as demonstrated above. Perhaps my reply to outpostbob was lengthier than necessary, but I wanted show some proof that, at least in my case, the optical result was superb by just aligning the corrector marking and secondary holder notch at 3 o'clock, without knowing with absolute certainty how they left the factory.


Edited by Axunator, 02 October 2021 - 07:31 AM.

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#21 Old Speckled Hen

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Posted 02 October 2021 - 08:09 AM

Hi Tom,

 

I am not sure whether you were replying to me (the OP - note that I started this thread 4 years ago!) or to outpostbob (post #16) who resurrected this old thread of mine, after having experienced similar problems, and to whom I was replying today.

 

Anyway, I was able to fix my C9 already years ago, and it turned out to be an excellent performer, as demonstrated above. Perhaps my reply to outpostbob was lengthier than necessary, but I wanted show some proof that, at least in my case, the optical result was superb by just aligning the corrector marking and secondary holder notch at 3 o'clock, without knowing with absolute certainty how they left the factory.

One of the worse things about the internet is trying to decide the validity of information you receive

 

I have a further complication. The corrector was loose in the cell, so when I went to unscrew the secondary retainer the whole corrector assembly rotated. And I think this was a problem for the prior owner.... one of the screws is missing on the corrector retainer. So who knows how much the corrector has turned.

 

I have read enough of the above that I don't THINK I missed this... but what should the corrector plate mark be aligned to in order to maintain its orientation with the primary?

It should match the line on the back of the primary which in turn should be directly opposite the focus knob [on the rear casing]

So from FRONT [and depending if no one has moved dovetail from factory position ] with scope sitting on dovetail the 3 o'clock position. Also note that the non farstar secondaries have two small square 'ish holes in which a line can sometimes be seen, this IS NOT the alignment mark for the secondary but the final digit of the mirror set number in my case the lower bar of the number 2 on the backing plate of the secondary.

 

post-331710-0-54301300-1590540769_thumb.


Edited by Old Speckled Hen, 02 October 2021 - 08:44 AM.

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#22 Axunator

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Posted 02 October 2021 - 08:24 AM

One of the worse things about the internet is trying to decide the validity of information you receive

 

It should match the line on the back of the primary which in turn should be directly opposite the focus knob [on the rear casing]

So from FRONT [and depending if no one has moved dovetail from factory position ] with scope sitting on dovetail the 3 o'clock position. Also note that the non farstar secondaries sometimes have two small square 'ish holes in which a line can sometimes be seen, this IS NOT the alignment mark for the  secondary.

 

 

There seems to have been a lot of variation on how Celestron marked the mirrors and corrector plates over the years. As said above, my corrector had a marking on the side, but the secondary mirror did not (but being a Fastar scope, the holder had the notch & pin), and the primary mirror did not either:

 

IMG_1115.jpeg



#23 Old Speckled Hen

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Posted 02 October 2021 - 08:52 AM

There seems to have been a lot of variation on how Celestron marked the mirrors and corrector plates over the years. As said above, my corrector had a marking on the side, but the secondary mirror did not (but being a Fastar scope, the holder had the notch & pin), and the primary mirror did not either:

 

attachicon.gifIMG_1115.jpeg

Unfortunately the images of my C8, which were in a topic on both my c8 and Joe's C11 stripdowns, on the Chicago Astronomer site, from about 2011'ish, have been culled by proboards mad.gif



#24 tturtle

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Posted 02 October 2021 - 08:54 AM

Hi Tom,

 

I am not sure whether you were replying to me (the OP - note that I started this thread 4 years ago!) or to outpostbob (post #16) who resurrected this old thread of mine, after having experienced similar problems, and to whom I was replying today.

 

Anyway, I was able to fix my C9 already years ago, and it turned out to be an excellent performer, as demonstrated above. Perhaps my reply to outpostbob was lengthier than necessary, but I wanted show some proof that, at least in my case, the optical result was superb by just aligning the corrector marking and secondary holder notch at 3 o'clock, without knowing with absolute certainty how they left the factory.

Oh that’s funny - I should have read the date on your original post, but glad you have it working now. I am reading all the posts that come up these days about disassembly of SCTs for my build project in the ATM forum. Going back to the discussion on corrector rotation I think the 3 o’clock position also puts the edge mark directly opposite the focuser bolt attached to the primary. This might explain why they don’t always have alignment marks on the back of the primary?


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