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Aligning a C8 corrector plate

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#1 1939Dodge

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Posted 22 December 2020 - 11:01 PM

I bought a basket case C8. Corrector had been removed, I don’t know if it was installed in the same position, or if the cork spacers were installed. Focus isn’t great, but collimation looks to be ok.

suggestions to position the corrector?



#2 Dynan

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Posted 22 December 2020 - 11:33 PM

There is (usually) a small engraving of numbers/letters on the outside of the corrector plate. They should be positioned at the three o'clock position (facing the corrector end of the scope) with the engraving on the outer side of the plate.



#3 macdonjh

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Posted 23 December 2020 - 09:15 AM

There is (usually) a small engraving of numbers/letters on the outside of the corrector plate. They should be positioned at the three o'clock position (facing the corrector end of the scope) with the engraving on the outer side of the plate.

+1

 

Sometimes the mark is a simple black line made with a Sharpie marker.

 

If there is no mark, you'll be left with trial and error.  Rotate your corrector plate 90o and recollimate your scope.  Make a note of how good the image is.  Rotate another 90o, recollimate and evaluate the image.  One more time.  Pick the best orientation.  If you think the view can be sharper still, rotate your corrector 45o from it's current "best orientation", recollimate and see if you made any improvement.  Continue to repeat that process until you get it perfect, or you've been driven insane.  smile.gif

 

Remember to mark your corrector once you have it correctly oriented. 


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#4 Dynan

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Posted 23 December 2020 - 10:38 AM

What macdonjh said is true, but hopefully as a last resort. It may be a sharpie mark. (The mark on the secondary mounting plate IS a sharpie mark that points to the 3 o'clock position also.)

 

LOOK CLOSELY for the engraving. I missed the one on my corrector TWICE before I hunkered down, got 3X magnifying glasses and took time to search. (Of course, your eyesight is probably better than mine.)



#5 1939Dodge

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Posted 23 December 2020 - 08:13 PM

Thanks. Assuming I can find the sweet spot, what do I do about the cork spacers? How do I position the corrector radially?



#6 macdonjh

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Posted 23 December 2020 - 10:22 PM

Thanks. Assuming I can find the sweet spot, what do I do about the cork spacers? How do I position the corrector radially?

So the center of the hole for the secondary mirror holder is centered over the primary baffle tube.  That's hard, so it may be good enough to just center the glass in the front casting.



#7 Dynan

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Posted 24 December 2020 - 06:58 AM

That 'centering' is a whole 'nother animal. When I had a C8 and a C11 I purchased a HOTECH Advanced CT Laser Collimator. It was expensive, but well worth it since I had two Casses to align. It was when I discovered 'cardboard spacers'  that I realized they had been unprofessionally adjusted. On the vintage C11 (de-forked) I added 4 adjustment set screws and aligned with the Hotech.

 

It aligns the main mirror to the tube, and the corrector and secondary to the main mirror.

 

A but touchy to learn, but it improved my imaging a good amount. I don't know of any cheap alternatives.

 

Did I mention 'pricy'?


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#8 KerryR

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Posted 24 December 2020 - 10:34 AM

Thanks. Assuming I can find the sweet spot, what do I do about the cork spacers? How do I position the corrector radially?

Here's how I centered the secondary over the primary baffle tube in my SCT's after I had them apart for venting: I start with the fairly bold assumption that the secondary is well centered in the secondary housing, and that the baffle is centered on the ota. I also assume that the center bore through the corrector may not be centered. Consequently, I assume the edges of the corrector probably won't be centered in the cell. I've read that this is a common occurrence. I point the scope straight up, and place the corrector in it's cell, with it's edge markings at 3 o'clock. I push the corrector all the way to one side, so that the edge hits the cell, it doesn't matter where. At this point, the secondary is off-center. I use Vernier calilpers (from the local hardware store) to measure the distance from the inner edge of the cell to the outer edge of the secondary housing. I do this in 3 places, separated by 120deg. Each of the three measurements will be different. I then average the measurements and set the calipers to that number, and lock the jaws. Using the calipers as a spacer, I move the corrector so that the calipers just fit at each of 3 120deg locations. This accurately centers the housing relative to the cell, and, we hope, to everything else if the ota was built to tight tolerences. I then placed shims consisting of custom-thickness paper and cardboard at 3 120deg locations, and installed the retainer. This worked extremely well on my 8 and 11" SCT's.

Here's what I saw when the secondary wasn't centered, and what drove me to pursue a solution: collimating on the diffraction rings on one side of focus yielded a collimation error on the other side of focus. Doing the above fixed the issue. The centering isn't terribly critical, I've read, as long as it's close, and you do your finest collimation on the in-focus Airy disk and first ring(s), seeing and tube thermals permitting.

If the mechanical centering doesn't work, due to construction errors, then you can use the collimation error issue mentioned above to center the housing, through trial and error.
 


Edited by KerryR, 24 December 2020 - 10:35 AM.

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#9 Old Speckled Hen

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Posted 24 December 2020 - 11:13 AM

Here's how I centered the secondary over the primary baffle tube in my SCT's after I had them apart for venting: I start with the fairly bold assumption that the secondary is well centered in the secondary housing, and that the baffle is centered on the ota. I also assume that the center bore through the corrector may not be centered. Consequently, I assume the edges of the corrector probably won't be centered in the cell. I've read that this is a common occurrence. I point the scope straight up, and place the corrector in it's cell, with it's edge markings at 3 o'clock. I push the corrector all the way to one side, so that the edge hits the cell, it doesn't matter where. At this point, the secondary is off-center. I use Vernier calilpers (from the local hardware store) to measure the distance from the inner edge of the cell to the outer edge of the secondary housing. I do this in 3 places, separated by 120deg. Each of the three measurements will be different. I then average the measurements and set the calipers to that number, and lock the jaws. Using the calipers as a spacer, I move the corrector so that the calipers just fit at each of 3 120deg locations. This accurately centers the housing relative to the cell, and, we hope, to everything else if the ota was built to tight tolerences. I then placed shims consisting of custom-thickness paper and cardboard at 3 120deg locations, and installed the retainer. This worked extremely well on my 8 and 11" SCT's.

Here's what I saw when the secondary wasn't centered, and what drove me to pursue a solution: collimating on the diffraction rings on one side of focus yielded a collimation error on the other side of focus. Doing the above fixed the issue. The centering isn't terribly critical, I've read, as long as it's close, and you do your finest collimation on the in-focus Airy disk and first ring(s), seeing and tube thermals permitting.

If the mechanical centering doesn't work, due to construction errors, then you can use the collimation error issue mentioned above to center the housing, through trial and error.
 

Good effort!!!

 

I measured the distance from the end of the baffle tube to side wall 90 degree in turn  x 4 measurements averaged, it was centered. Carried out the same for the secondary holder  [mine was v loose due to dodgy washer, I made another out of gasket paper] to edge of corrector once again x4 averaged. Got it down to the low hundredths of a mm. Had a issue with the marking on the back of secondary and with etchings on corrector edge, had  x2 180 degrees apart!!! think that was my first post here!!


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#10 KerryR

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Posted 24 December 2020 - 11:49 AM

Good effort!!!

 

I measured the distance from the end of the baffle tube to side wall 90 degree in turn  x 4 measurements averaged, it was centered. Carried out the same for the secondary holder  [mine was v loose due to dodgy washer, I made another out of gasket paper] to edge of corrector once again x4 averaged. Got it down to the low hundredths of a mm. Had a issue with the marking on the back of secondary and with etchings on corrector edge, had  x2 180 degrees apart!!! think that was my first post here!!

I didn't measure baffle runout, but that would be a good thing to check; if it's not centered, then my method wouldn't work very well, at which point I might try center spotting the secondary and using a cross-hair site tube to center the secondary.

 

Quite some time ago, there was a cool website that discussed adjusting the baffle runout and primary collimation of a C14.


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#11 Old Speckled Hen

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Posted 26 December 2020 - 10:25 PM

I didn't measure baffle runout, but that would be a good thing to check; if it's not centered, then my method wouldn't work very well, at which point I might try center spotting the secondary and using a cross-hair site tube to center the secondary.

 

Quite some time ago, there was a cool website that discussed adjusting the baffle runout and primary collimation of a C14.

Yes indeed, re the C14 reference, required reading IMO!

 

Be difficult to accurately X reference the back of rear cell [optical axis] to the secondary without a center point marking or ring, IMO. It is doable but your eye has to be having a good day.


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#12 davidc135

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Posted 27 December 2020 - 05:22 AM

A little while ago instrument designer John Hayes tucson claimed that a c.p lateral shift of +/- 1.5mm would make no noticeable difference to performance. (Have I got his name right? Nothing came up in a search.)

 

But that runs opposite to the experience of some here.

 

For my part both the c.p and primary in a DX-8 have astigmatism which can be reduced by rotation of the c.p. The secondary is clear. In trying to remove the residual stig I moved the c.p from being centered to as far as it would go in different directions within it's cell, each time recollimating by tilting the secondary to remove axial coma. I also tried tilting the c.p. Nothing made a scrap of difference to the residual stig.

 

As the secondary was a snug fit within the c.p I didn't alter it's centering.

 

As the basic all spherical sct design has very little off axis astigmatism and with axial coma being cured by secondary tilt, and spherical aberration not being affected I'm not sure how improved centering would show itself? But Celestrons may have their secondaries retouched enough to be aspherical and that might explain it??

 

I can see that it would or might be different in the ACF and Edge designs. 

 

David



#13 1939Dodge

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Posted 27 December 2020 - 11:27 AM

Thank you. 
most of this is beyond me, but here’s what I plan to do:

 

-try to locate the marks and place them at 3 o’clock

 

-center the corrector plate

 

-collimate

 

-then sell it if I can’t make it work.....



#14 KerryR

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Posted 27 December 2020 - 12:53 PM

A little while ago instrument designer John Hayes tucson claimed that a c.p lateral shift of +/- 1.5mm would make no noticeable difference to performance. (Have I got his name right? Nothing came up in a search.)

 

But that runs opposite to the experience of some here.

 

*snip!*

 

David

This might be referring to centering the curves of the corrector relative to the center of the primary, and not secondary centering, but I certainly don't know.

Based on my observations, secondary centering does seem to affect the efficacy of the common practice of collimating outside of focus. This can be dealt with simply by finishing collimation at focus, which we should be doing anyway. This should render secondary centering moot, unless, perhaps, the secondary is aspherized. (I know that Celestron aspherized, if necessary, in the past, and Meade didn't, relying, instead, on the 'mix and match' approach, but I'm not sure what the state of things are currently. Most of the articles that I've read that discuss this aspect hale from the days when the ota's were made in California. It wouldn't surprise me if modern manufacturing methods dispense with aspherizing the secondary...)

In practice, though, I find I often don't have the seeing and thermal management requirements met for collimating at focus, and so I end up collimating on either side of focus using the shadow, diffraction rings and Poison spot. Having the secondary centered helps with this because it appears to elliminate the intra-extra focal differentiation issue I was observing and helps produce good collimation without that final tweak on the Airy disk. If I can, I still like to verify and/or tweak at focus at highest powers... but that's doesn't happen as often as I'd like.



#15 choward94002

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Posted 27 December 2020 - 01:01 PM

A little while ago instrument designer John Hayes tucson claimed that a c.p lateral shift of +/- 1.5mm would make no noticeable difference to performance. (Have I got his name right? Nothing came up in a search.)

 

But that runs opposite to the experience of some here.

 

For my part both the c.p and primary in a DX-8 have astigmatism which can be reduced by rotation of the c.p. The secondary is clear. In trying to remove the residual stig I moved the c.p from being centered to as far as it would go in different directions within it's cell, each time recollimating by tilting the secondary to remove axial coma. I also tried tilting the c.p. Nothing made a scrap of difference to the residual stig.

 

As the secondary was a snug fit within the c.p I didn't alter it's centering.

 

As the basic all spherical sct design has very little off axis astigmatism and with axial coma being cured by secondary tilt, and spherical aberration not being affected I'm not sure how improved centering would show itself? But Celestrons may have their secondaries retouched enough to be aspherical and that might explain it??

 

I can see that it would or might be different in the ACF and Edge designs. 

 

David

John Hayes used to be a pretty prolific poster and he's (among other things) an optical teacher who really knows the C14's front and back ... he did indeed do some measurements with a C14 in relation to positioning with a Hyperstar and indeed did find that the secondary can be mis-aligned by up to 1.5mm off the center axis without problems ...

 

The trick is that the secondary is a spherical mirror, which means that at any point it has the same curvature presented to any rays.  It's like a christmas tree ball ... no matter how you move it, you'll always get a focus to the same location

 

Optical problems (and here I speak of the C14 and C11's, I only use my C8's for terrestrial viewing where I wouldn't notice an optical problem) can come down to three problems: primary, secondary and corrector

 

Primary problems come from misaligned mirrors on the base (very uncommon but still possible) and mirror shift from when the mirror changes it's vertical orientation.  Many solutions for mirror shift, it's a common cause of collimation loss 

 

Secondary problems come from overly aggressive tightening of adjustment screws (will cause astigmatism) and misalignment with the corrector and primary.  All three components have flaws and when the scopes were assembled those parts were rotated until they "worked", marked and boxed up.  It's pretty common for people accidentally rotate the secondary either because it's gotten loose or they took it out/ replaced it for cleaning, curiosity, etc.  There should be a mark on the secondary and it should be on the side opposite the focuser

 

Correctory problems come from alignment problems, again from folks accidentally rotating the corrector somehow ... this too has a mark (usually an etched serial number) which needs to be opposite the focuser.  You will also sometimes run into a problem where the plate is no longer properly seated in the front cell (usually when someone is fiddling with the spacers) and ends up slightly tilted.  That won't bother the secondary (spherical mirror) but will definitely throw off the optics as it needs to be square to the primary mirror.

 

Getting back to the OP's question and some of the replies, first check to see if the corrector serial number is opposite the focuser, then that the secondary mark is opposite the focuser.  Once that's done make sure there's nothing in the corrector plate seat on the front cell to throw things off and then start measuring/ moving the plate until it's centered in the OTA (the poster who mentioned using a digital caliper to measure things was pretty good, I personally have a plywood disk that fits over the secondary and is about 1/8" smaller than the plate.  I put the plate over the secondary and shim that up with strips of paper so it's snug, then move the plywood disk and measure distances with my caliper until the disk is centered).  

 

That will get things about 90% of the way done .. verify your corrector and secondary markings, collimate vertically, in the same orientation as you will be viewing at and you're done ...

 

To get another 8% you need to realize that the Celestron folks didn't spend hours tuning each scope ... they got it "close enough" after five minutes and moved on.  To improve on the optical orientation you're going to need to invest in a Hotech laser collimator (or a Hubble artificial star for the 8"), CCD Inspector and about 4hr of night time.  Use the Hotech for an initial collimation, then find a bright globular cluster somewhere high in the sky, and start taking pictures.  Take five in the original position (corrector and secondary opposite focuser) and start doing plate rotations:  with the secondary fixed, rotate the corrector plate 45deg and take five more pix, rotate another 45deg and five more pix, etc.  

 

Once you've got your stack of pix, run them through CCD Inspector looking at collimation: what you will see is a "shift" in collimation as the corrector is rotated.  Choose the orientation that gives the best CCDI results, and it's time for more rotation pix but this time of the secondary.  Take five in the new "original" position, rotate the secondary by 45deg, another five pix, etc. and then it's back to CCDI.

 

You should see a position for the secondary that gives the best collimation results, and that's your "new" orientation.  More than likely you'll find you haven't moved from the original markings (which is good and expected, the Celestron folks were pretty good in those five minutes on the bench), if they have then I'll make notes of the "new" position relative to the factory markings (making new markings is just going to confuse you/ the next owner) and put those notes someplace safe.  Recollimate with the Hotech/ Hubble star and you're good ...

 

Otherwise, good C8's on CN get about $300 these days ...


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#16 davidc135

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Posted 27 December 2020 - 05:33 PM

KerryR and choward94002, Thanks for the replies. 

 

I agree that rotation of the corrector/ secondary can improve astigmatism. Which suggests that not only has the c.p got a manufacturing fault but it depends on a fault in the primary mirror (or 2ndry) with which it can be cancelled! The astig in the corrector that I made isn't strong enough to cancel that in the primary completely. I didn't make it badly enough! Unfortunately, the 2ndry is free of stig and it's independent rotation can't contribute to a solution.

 

I can also see KerryR's point that where poor seeing prevents high power in focus collimation, a mechanically centered system is needed when using inside and outside of focus collimation.

 

If you are able to collimate at high power using an actual star or artificial one on the ground then the centre of field Airy disc and diffraction rings should show any faults or improvements. My results are based on viewing an artificial star in the garden, ignoring the little spherical aberration that the too-short distance produces. So, assuming that the scope is collimated this way, then it doesn't seem to matter where the corrector is in it's cell, centre or off-centre so long as it isn't free to move around.

When making the corrector I had it and the secondary mounted separately in a jig and was able to tilt the c.p at an angle as well as as offset it and look at any effects on the 'star'. With c.p offsets up to 2mm and a tilt of up to a huge 4mm I didn't see much change. I was looking for the extra tool to tackle the astigmatism but I don't think there is one.

 

Eventually there can be too much off-set or tilt but otherwise secondary tilt adjustments choose the symmetric optical axis. An sct can be drawn and a line through the primary's CofC and the corrector's optical centre will always be the axis. There is a distortion due to the ep being on a slightly separate axis but I think that secondary tilt can cancel that. Or so it seems to me.

 

Still, it's good to centre everything mechanically. You might as well. Again, these comments relate to the visual use of basic Scts such as the OP's and not to more advanced photographic systems.

 

David


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#17 moonrider

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Posted 31 December 2020 - 01:28 AM

One simple way of improving a SCT's mirror flop (with out disassembly) is to rack the focuser totally from stop to stop to somewhat redistribute the baffle tube grease. If the corrector is off, better to carefully reapply Dow non gassing silicone grease in a thin layer and rack the focuser, then remove any excess.  It of course won't eliminate but will significantly reduce mirror flop. 




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