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Is my C8 upside down?

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#1 AstroPotamus

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Posted 07 October 2021 - 11:54 AM

I'm attaching an image of the front and back of my classic C8 (ignore the ADM on a photo mount screwed to it).  I have problems getting crisp images from this scope on my AP work, but visually it's...."okay."  I'm wondering if my corrector plate might be upside down?  Can anyone tell me for sure?

 

For perspective, if you're looking at the back, the focus knob (pulled for a JMI electronic focuser) is on the right hand side if the scope is in what I consider to be the "upright" position.  But if I look at the front, the "Celestron" logo and serial number are upside down.  Have I been using this scope upside down all this time or are the front and back halves 180 degrees opposite each other?

 

I'm grasping at straws here because I'm out of other ideas.

Attached Thumbnails

  • c8.jpg

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#2 lee14

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Posted 07 October 2021 - 12:24 PM

You're correct, the rear cell shows the proper orientation with the focus knob on the right, since 90% of the population is right-handed. The corrector plate is apparently 'upside down', possibly it was reinstalled by a previous owner?

 

This should theoretically make no difference whatsoever since the optical path is symmetrical around the axis. It is unlikely, but possible, it was reinstalled this way in an attempt to better match the corrector plate and secondary mirror to the primary, compensating for some perceived error in one, or both. Equally unlikely, but conceivable, the system was originally optimized with both ends 'right-side-up', and is simply no longer performing as well as it did with the original configuration.

 

Try a star test. If a star under high power (in focus) remains centered in the diffraction rings, and those rings are all round, and appear the same on both sides of focus (when defocused), the system is good. If the star is not centered in either case, the collimation requires adjustment.

 

Lee


Edited by lee14, 07 October 2021 - 12:32 PM.

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#3 AstroPotamus

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Posted 07 October 2021 - 12:31 PM

You **** righties always making things for you.  Never thinking about us **** lefties.  :-)  It's fall in upstate New York which means crisp, beautiful days and cloudy, foggy nights.  If I ever get a chance to aim it at a star again, I'll try what you've suggested.  I've never actually collimated this scope, so duh, maybe that's the actual problem.  At any rate, good to know that I'm not crazy about it being half upside down.


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#4 lee14

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Posted 07 October 2021 - 12:41 PM

We share the same weather, I'm 90 miles east of you. When you can do a star test, pick one close to the zenith. There ought to be a couple of at least partially clear nights coming up in the next few days, all you need is a few minutes with a 'sucker hole' !

 

Lee


Edited by lee14, 07 October 2021 - 12:42 PM.


#5 AstroPotamus

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Posted 07 October 2021 - 12:51 PM

Thanks!  Now I just have to stay awake long enough....


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#6 Defenderslideguitar

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Posted 07 October 2021 - 12:52 PM

You **** righties always making things for you.  Never thinking about us **** lefties.

 

 

 

well   not always     my die cast orange c-8 was deforked and highly modified by a student astronomy club  it came with 2 losmandy bars   and I used it upside-down and right side up all the time

no matter.....I did prefer the focuser on my right hand side  but it depended more on the mount I was using

Attached Thumbnails

  • Celestron.diag2IMG_2937.jpg
  • c8losMansyIMG_9764.jpg
  • Ic8losmandyMG_9763.jpg

Edited by Defenderslideguitar, 07 October 2021 - 12:54 PM.


#7 jmillsbss

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Posted 07 October 2021 - 01:26 PM

Somehow, and likely through no fault of your own, you've ended up with the Australian version.  I wouldn't worry about it too much, unless you're unable to observe anything shown in a Northern Hemisphere star chart.  In that case, you need to work out a trade with an Aussie.  There's a few of them lingering here if you look for 'em!  lol.gif


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#8 AstroPotamus

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Posted 07 October 2021 - 01:30 PM

Somehow, and likely through no fault of your own, you've ended up with the Australian version.  I wouldn't worry about it too much, unless you're unable to observe anything shown in a Northern Hemisphere star chart.  In that case, you need to work out a trade with an Aussie.  There's a few of them lingering here if you look for 'em!  lol.gif

I would love to be able to observe the M. Clouds, so maybe this is a bonus?  :-)


Edited by AstroPotamus, 08 October 2021 - 06:55 AM.

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#9 markb

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Posted 07 October 2021 - 03:15 PM

Back to serious answers, I'll expand a bit on Lee's, based on my own similar but somewhat different experiences and research.

 

It is my understanding after restoring my own scopes and reading the available literature, that the factory 'spin' alignments are not done to tune the telescope for aberrations, but rather, to tune the telescope to make the optical and mechanical axes of the telescope coincident. Apparently Celestron did do some hand correction of secondaries and restoring the original spin alignment markings also places any adjustments back where they belong.

 

The primary optical access is presumed to be the same as the baffle mechanical axis. As long as the primary was properly glued squarely to the mount. I don't believe the factory compensates for a crooked primary but based that only on one online example of a c14 that had to be reglued.

 

I think there are two most likely explanations here.

 

One is that the corrector with secondary was removed and reinstalled rotated 180° out of its normal orientation, the second (and I think more likely) is that, secondary housings often being easily rotated by accident (so far every one I've had, and the reason for the starizona sorbothane gasket for its hyperstar), the secondary housing was simply rotated 180° out of normal position.

 

It has been my experience based on my massively misaligned Meade that a misaligned telescope can be collimated, but will deliver badly aberrated images. My Meade gave awfully comatic images even after apparent collimation.

 

While ideally, the rotation of the secondary and housing should make no difference, the 'spin' rotation final alignment at the factory is done to place the secondary mirror, not the secondary housing or the attachment puck, exactly centered over the baffle tube, which should be exactly over the optical center of the primary.

 

After the tuning, most Celestrons are marked with an etched number on the front edge of the corrector at the 3:00 position viewed from the big end, and opposite side from the focuser knob.

 

The secondaries are also marked, with a fat sharpie stripe on the ground edge at the same position, AFAIK only visible after the corrector has been removed from the tube. Meade used a different method.

 

Back in the orange tube days I believe all the correctors were simply centered within the tube with cork spacers, but at some later point before my 2006 GPS11 Celestron started using a sticky mat, and centered the corrector perforation and/or secondary over the baffle on assembly, often leaving, correctly, the corrector not concentric to the cell.

 

 

Since it's common for secondaries to be rotated by accident,  my FIRST step, no disassembly needed and easy to reverse, would be to simply rotate the secondary housing if it is free enough to rotate, to its normal name right side up position, and then see if the scope improves in sharpness.

 

If that doesn't do it, I suspect it will, I would then remove the retaining ring and look for the etched marking on the edge of the corrector and align the secondary housing with respect to the etching being in line with the writing, the corrector etched edge itself then being aligned to  the 3:00 position on the telescope.

 

In the worst case, like my once sad GPS 11 with inexcusably soft images not disclosed by the seller (still upset- most users would simply have condemned the scope or resold it with disclosures) I found that the corrector was in the wrong position, that the secondary sharpie marking was in the wrong position, and even the fastaer holder was in the wrong position relative to the secondary (usually the notch is also  at the 3:00 position as is the sharpie stripe on the secondary).

 

The corrector had also been improperly placed centrally in the cell, all due to a careless cleaning procedure, and the failure to follow any of the directions for installation of Bob's knobs. Those installation directions make sure that the alignment marks stay in the correct location.

 

On a scope that was built essentially centered, this wouldn't have made a difference. But my scope required offsets to the corrector and the secondary mirror to have them align properly with the baffle and primary.

 

After the alignment marks were properly placed, I had to also locate the correct position for the corrector, a couple of millimeters off center within the cell, and verify that the secondary mirror was absolutely centered over the baffle and, the primary optical axis once the factory alignments were restored to the secondary and the corrector edge.

 

This resulted in a very fine performing sharp GPS 11, a world away from what I was shipped.

 

I had also been working on a factory misassembled Meade 8 with a crooked baffle tube and a deteriorating secondary coating. Meade had actually glued the secondary off-center to the puck, intentionally, to allow the secondary to be rotated into place above the primary axis and the baffle axis. I can only assume they kept the stock of secondaries glued and varying distances from the center of the aluminum puck.

 

The corrector in that scope had to be placed flush to one edge of the cell. Obviously, this was an extreme case, but prior to the realignment it gave severely chromatic images even when collimated.

 

I can't wait to get it recoated, it gives every indication of being a excellent optic set. Once useless though, after a disassembly and careless reassembly.

 

I like the left-handed scope installation. As long as the optics are not touched it's a great solution.

 

Both were great learning experiences. But like many repairs it would have been nice not to learned them the hard way, reverse engineering why the factory made its alignments.


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#10 AstroPotamus

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Posted 07 October 2021 - 04:55 PM

This. Is. Priceless. Thanks. I think you've just described my scope and I can't wait to get my hands on it tomorrow too investigate further.
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#11 ccwemyss

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Posted 07 October 2021 - 09:08 PM

That may have been my C14, which needed the primary reglued. On the orange tube C8s, it's not that hard to remove the corrector, being sure to note the positions of the cork spacers. If it's stuck, a little isopropyl, lightly applied around the outer edge, to seep under, will usually unstick it. I've also seen reports of placing the scope face down over something soft, just held slightly off of the supporting surface, so gravity can gently pull the corrector out, with it falling just enough to be loose from the cell. Then lift the OTA off of it. 

 

The Fastar scopes have more issues with rotated secondaries, because people try to over tighten the ring. At one point, I was looking at a C14HD that was for sale, and noticed that the secondary was turned about 120 degrees. The seller said he had found the Fastar frustrating because he had to recollimate after every use. Once I pointed out the turned secondary, he realized he'd been shifting it each time he put it back. I decided not to buy it, and he decided to keep it and try to fix it. After pulling the corrector from my C14 a half dozen times, trying every possibility to figure out the massive mirror flop, I'm no longer as squeamish about it.

 

But getting the optical axes perfectly centered is still not something I'm equipped to do. 

 

Chip W. 


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#12 firemachine69

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Posted 07 October 2021 - 09:11 PM

You **** righties always making things for you. Never thinking about us **** lefties.



well not always my die cast orange c-8 was deforked and highly modified by a student astronomy club it came with 2 losmandy bars and I used it upside-down and right side up all the time
no matter.....I did prefer the focuser on my right hand side but it depended more on the mount I was using



What diagonal is that?

#13 AstroPotamus

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Posted 08 October 2021 - 07:20 AM

After the tuning, most Celestrons are marked with an etched number on the front edge of the corrector at the 3:00 position viewed from the big end, and opposite side from the focuser knob. The secondaries are also marked, with a fat sharpie stripe on the ground edge at the same position, AFAIK only visible after the corrector has been removed from the tube. Meade used a different method.

 

 

 

So I took off my corrector plate and found two things:

 

1)  An etching on the edge of the glass accompanied by a sharpie mark

2)  A bracketing set of lines on the edge of the retaining flange where the plate goes that are the same width apart as the width of the sharpie mark

 

I didn't take pictures, but one it was apart, it was obvious that the plate had been rotated approximately 180 degrees and that it "belonged" the other way round.  So I took a moment to clean out the OTA, clean off the primary and secondary, and get rid of a slight film on the inside of the corrector plate and put it all back together again.  There are three small cork pieces that hold the plate in place, and it was a snug enough fit that I didn't bother modifying anything while I was there.

 

So that all got done this morning (yes, I'm up early) and there's a possibility tonight of being able to have some clear skies, so I may try to see if it makes any difference.  Thanks for the help, @markb!

 



 


Edited by AstroPotamus, 08 October 2021 - 07:43 AM.

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#14 markb

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Posted 08 October 2021 - 11:19 AM

I'm keeping my fingers crossed, but it sounds like you will be on your way!

 

My GPS11 had an absolutely huge jump in performance after restoration, and it was only off a couple of mm. The only clue was the softness, but I wish I had done a careful Ronchi test. Suiter extrafocal patterns are tough to read without an expert, so I doubt they would have helped diagnosis. My Meade was out by 8mm or so at the corrector cell, even that resulted in huge amounts of coma. The misalignment performance change likely depends on how much the rear cell and baffle tube assembly was off from the tube centerline. 

 

As far as I been able to figure out, the factory must have kept groups of incrementally eccentrically glued secondaries to match to a particular tube and mirror assembly.

 

The SCT design is wonderful in so many ways, but really is alignment critical. Many are happy with dead centered elements due to ideally located baffle tubes during assembly, but many needed that factory tuning tweak.

 

I would have bet on a simple secondary rotation.on yours, glad you checked the corrector edge. Odd about the sharpie marks, it sounds like someone marked it before disassembly, but somewhere along the line someone ignored the markings. I guess it could have been Celestron, too.

 

Did you check the secondary mirror edge for the sharpie marks also? Likely not essential, I think that to be the least likely misassembled item.

 

Mine was rotated during a bungled Bob's Knobs install, I believe. 


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#15 AstroPotamus

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Posted 08 October 2021 - 11:34 AM

Yes, I did check the secondary but didn't find any obvious markings.  I figure I'll take the chance that it's good where it is and that the front plate being off ~180 degrees is likely what's going on.  I can always take it apart again...


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#16 markb

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Posted 08 October 2021 - 11:38 AM

Chip, the one I read about was at Wilmslow Observatory, also a C14.

 

Most of my preliminary trusted information came out of the super informative Celestron edge HD white paper (it looks like they tried to fix as many early 70s design related alignment issues as they could to support both the baffle mounted lenses as well as the Fastar/Hyperstar extreme sensitivity to even minor misalignments, one example is the addition of corrector position adjusting set screws) and the Wilmslow pages.

 

Working on my crippled (no complaints, it was $75, sold as a project scope) Meade 8 provided the remainder of the experimental information I needed. $75, I wasn't particularly worried about accidentally messing something up.

 

Wilmslow has an absolutely wonderful JavaScript active calculation page for a really wide variety of astronomical formulas, too.

 

The analysis of their problem and how they fixed it with an amazingly simple and inexpensive test rig is at

https://www.wilmslow..._alignment.html

 

I did not use their procedure since I had a holographic laser collimator where the projected pattern let me do the centering directly, also, I did not have the time or space to try their procedure. Initially I used a concentric circle holographic projection, but later picked up a crosshair bullseye projection which was much easier to interpret.

 

Overall, I believe the Wilmslow optical solution is superior to my mechanical one.

 

Once I order a 3D printer, I will print a slip over mask for the secondary to allow me to easily print an accurate center point and match it to a laser, a clean secondary displays so little of the pattern it is difficult to fine tune.

 

Since part of what I need for the wilmslow setup is similar to what I'll need for a DPAC, I'll probably work on setting both up at the same time next year, now that I've accumulated all the bits and pieces I need. Will be interesting to test against a purely mechanical centering and alignment.


Edited by markb, 08 October 2021 - 12:06 PM.


#17 markb

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Posted 08 October 2021 - 11:55 AM

Potamus, I have read some secondaries are marked on the back of the support puck. But I doubt you'll end up looking.

 

Mine had, IIRC, had markings on the back but I'll have to see if I photographed them, as I recall them seeming a bit non-obvious with respect to the edge sharpie marking. Perhaps factory markings for curvature class or puck eccentricity, just guessing.

 

Anyway, I agree, your secondary probably was not removed from the holder.

 

The primaries are said to also be marked, but since they are fixed in place it doesn't come up for discussion.

 

The corrector rotation will hopefully be everything you need.

 

I do have a question for you, however.

 

I may have to clean one of my primaries, and I studiously avoid contact with SCT primary mirrors as a rule.

 

The tight confines of the tube are daunting, especially with my much larger than average hands.

 

What was the procedure you used to clean yours?

 

And I am greatly enjoying wandering your website!



#18 AstroPotamus

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Posted 08 October 2021 - 01:16 PM

I put three or four individual "peas" of styrofoam in middle of an empty spool of 3D printer filament.  If you're not familiar, it's like a 10" diameter spool of thread with a 2" central hole.  I then put the OTA on top of the spool such that the visual back hole was above the styrofoam.  Then, with the OTA upright, I took the corrector plate off.  I brought the crevice tool of a vacuum tube towards the top of the now open OTA.  Just enough to create a small updraft up and out of the tube, adjusting until the styrofoam was pulled up through the the focus tube.  I let that sit for a few minutes, assured I was pulling things up.  Then I took a new microfiber cloth, bunched it up around a crumpled sheet of newspaper to give it some depth, and then carefully dangled it into the OTA to barley touch the mirror.  Carefully went in circles until I was sure I'd gotten all the dusty bits.  There was one area that had a few non-dusty spots and I'll admit I cheated a bit and applied more than just newspaper+cloth+gravity force to it, but I cleared the spots.

 

I then took some 75% isopropyl alcohol, put it on the clean side of the microfiber cloth, and did one gravity-only circular swipe around the mirror, and then let that dry out.  Did a visual inspection with overhead LED worklights to check the reflection, found nothing else, and closed it up.

 

I knew I had crap on my mirror from previous imaging, and I'm confident I got all the crap without doing anything to any coatings.  Given that this is a 40+ year old scope with no XLT type coatings, I'm confident I have done no harm.



#19 ccwemyss

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Posted 08 October 2021 - 02:04 PM

Chip, the one I read about was at Wilmslow Observatory, also a C14.

 

 

The analysis of their problem and how they fixed it with an amazingly simple and inexpensive test rig is at

https://www.wilmslow..._alignment.html

 

 

That's a useful resource. I should really check mine. After re-gluing the primary, they shipped it back to me without locking the mirror, and with the screws that hold the tube to the rear cell loose. Their advice was to put the OTA face down, and just be sure that the cell was seated evenly before tightening the screws. But ever since I got it back, it has not seemed to deliver the sharpness that I remember from before, despite being collimated. 

 

Chip W. 


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#20 markb

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Posted 08 October 2021 - 02:57 PM

Thanks Potamus!

 

I avoid all the drugstore alcohols, I've had contamination issues resulting in streaking, etc. I use either 190 proof everclear, and only that brand for its purity, but have also just found on CN a link to an Amazon seller that sells lab grade isopropyl alcohol, which should have zero issues. I understand it to be a better solvent on lenses than ethanol but ethanol is pretty good. You can PM me for a link. It sounds like your particular batch of isopropyl did not cause any issues though.

 

I use 3M microfiber loop towelettes all the time for cleaning lenses, but had no idea if they were mirror coating safe. I'll have to try them on an expendable first surface mirror, but it sounds like they will be safe.

 

I had assumed I would use the old standby of pure cotton wadding.

 

Dust I usually ignore, but your method sounds very practical and safe.

 

If you didn't see any sleeking with the overhead LEDs the isopropyl plus a gravity only microfiber wipe certainly sounds safe and doable.

 

Direct lighting like flashlights or bright overheads usually highlight even inconsequential defects, so it sounds like you created no damage.

 

How did you dangle the microfiber coated newspaper, or do the single microfiber wipe? Did you hold it by hand or use something else?


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#21 AstroPotamus

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Posted 08 October 2021 - 03:11 PM

Just my hand.  And I have a supply of safe alcohol for my 3D printing projects, but always interested in other sources.


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#22 markb

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Posted 08 October 2021 - 03:41 PM

Chip, I'd definitely try an alignment, especially if the primary mounting structure was shipped loose and tightened by you, their instructions were probably to minimize any shifting of the screws within the clearance of the screw holes.

 

Your before and after experiences would seem to indicate a misalignment, and I have certainly become personally aware how much even slight misalignments can damage SCT images.

 

SCT collimation is separate, but has a similar reputation for sensitivity.

 

The wilmslow page does have his experiences on centering the battle tube and tightening the retaining collar for the primary. Both can only be done with the tube in certain positions.

 

I've seen references to image shifts due to a loose collar retainer but have no personal knowledge.

 

My image shift on my scopes is relatively minimal and I don't do AP, but I do plan to try one of the baffle tube modifications that are said to decrease image shift, for high powers. I've seen the Dow silicone vacuum product suggested, as well as repeatedly running the mirror from stop to stop to redistribute the factory installed grease, but I may one day try using 1 mil Kapton tabe or 3 mil UHMW 3m tape on the outside of a cleaned baffle tube, both having friction levels in the range of Teflon. The UHMW tape has already worked wonderfully in slicking and smoothing a Vixen R&P as well as Synta clones. This one is very long-term for me, since it requires far more disassembly than I think I'd be willing to undertake.

 

On the alignment issue I assume there are similar alignment markings for your c14 as for smaller scts and you have maintained them.

 

It is possible you might be able to reduce the measured run out of the baffle as suggested on the Wilmslow page, but doing a centering alignment should have close to the same end result.

 

It's possible to do the alignments I've done with paper targets and a plain but known well aligned laser.

 

The only holographic lasers I've seen, remarkably, are the pricey Glatter lasers, and the holographic inserts are not easy to find although a source was posted recently on CN. Cannot recall at the moment though.

 

It's kind of ridiculous because it's my understanding that a holographic film insert is all you need, and they sell them by the ton in Christmas decoration lasers from China.

 

As to aligning with paper targets, I have avoided them because I'm incapable of drawing straight lines, curves are finding a correct center. Odd, because I can do pretty much anything mechanical at any level. I recognize my limitations.

 

If the threads on the cast back are independent of the baffle tube, I certainly would align that first.

 

For all of the following the procedure would be essentially to make a paper target for the object being aligned with an accurate center for for aiming.

 

Blue painters tape can be put over the object and a true center be located with a compass. This can then be used as a target for alignment with the laser.

 

A paper or blue tape Target at the end of the baffle tube should allow you to align that to the visual back threaded onto the cast back. This would be similar to the run out adjustment that Wilmslow did.

 

If the c14s are assembled is carefully as I would expect that the factory, since the reputation is that they are hand assembled, I would assume that might solve your issues, alone.

 

On the corrector, blue painters tape or an ordinary paper target can be put over the perforation and a true center be located with a compass.

 

On C8s or other older SCTs with cork centered corrector plates within the cell, it's possible that the secondary centering adjustments lie only with rotation of a slightly eccentrically glued secondary mirror assembly within a centered corrector.

 

On mine, I started with the corrector perforation centered over the baffle tube, keeping in mind that I might have to shift the corrector plate slightly if it was required to get the secondary mirror centered over the baffle, which was not necessary on the 11, but was necessary on the extreme Meade 8 with a badly tilted baffle due to a misglued crooked rear cell.

 

The last step I made was to get the secondary mirror itself centered over the baffle tube, I was barely able to make out the pattern of the holographic crosshairs but was able to verify centering to the best of my current ability.

 

One option I considered but haven't done so far was to make an open cylinder out of paper that could slip over the secondary and provide an easy to see centered target dot.

 

Since I plan to buy a 3D printer I will simply 3D print a snug fitting slip on target to finish mine, or just verify that it's done.

 

Any of the paper options would easily duplicate what I have done with the holographic laser.

 

But, when I get around to the making the Wilmslow tester and the needed layout, it will be very interesting to see if any additional tweaks will be necessary to my alignment adjustments.

 

I think the wilmslow tester is functionally equivalent to the very expensive three laser holographic commercial alignment tool.



#23 markb

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Posted 08 October 2021 - 03:53 PM

Thanks, I may brave cleaning the 2 spots on one of the SCTs.

 

The product I bought, but haven't used, is 

 

https://smile.amazon...duct/B005DNQX3C

 

Oddly, the high purity 95% distilled water, 5% isopropyl Zeiss fluid is now easily found in large bottles at your local Walmart, of all places.



#24 markb

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Posted 08 October 2021 - 04:00 PM

Since Chip and the OP both image, I should mention that I suspect the excess clearance in between the secondary housing and the corrector preparation maybe the culprit in some collimation shifts.

 

I used thin strips of the UHMW tape to take up the clearance, but I think the Sorbothane gasket sold by Starizona, or a homebrew equivalent, is the best way to make the clearance, as well as housing rotation, a non-issue.



#25 Defenderslideguitar

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Posted 08 October 2021 - 06:33 PM

What diagonal is that?

Hello there  thanks for asking    not to go too far off topic but

 Actually I believe it is  an older  celestron 2 inch diagonal  with the 1.25 adapter

that scope came with two finders   two diagonals  two losmandy bars  three anti mirror flop knobs and some student notes written on the tube in pencil

all for 150 dollars


Edited by Defenderslideguitar, 08 October 2021 - 06:34 PM.

  • markb likes this


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