Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Can't remove corrector plate - Celestron Nexstar GPS 11"

Celestron catadioptric equipment optics reflector
  • Please log in to reply
19 replies to this topic

#1 Greg Stein

Greg Stein

    Lift Off

  • *****
  • topic starter
  • Posts: 16
  • Joined: 22 Sep 2012

Posted 06 November 2019 - 06:59 AM

Hello folks,

 

I wanted to clean up my SCT. I've removed 8 screws that hold the compression ring of the corrector plate, removed the ring and .... couldn't take off the corrector. It got to the point that when I tried to pull the collimation plastic part, the scope was almost in the air. What I'm doing wrong?

 

Thank you!

 

Kindest regards, Greg.



#2 macdonjh

macdonjh

    Aurora

  • *****
  • Posts: 4669
  • Joined: 06 Mar 2006

Posted 06 November 2019 - 07:35 AM

Nothing.  A few drops of denatured alcohol around the perimeter of your corrector will likely dissolve the sticky stuff holding the glass in.  Let it sit and work for a few minutes.  Be gentle with the glass.


  • Dynan likes this

#3 markb

markb

    Ranger 4

  • *****
  • Posts: 356
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 06 November 2019 - 09:24 AM

+1 on the drops of alcohol, and taking it very slow and easy. It worked well for me, too.

I would always prefer food grade Everclear from the liquor store over denatured though, if sold in your state. I can get in AZ but not in NY. That broadens what you can use in optics cleaning, as Everclear is pure enough to drink (AFTER you finish the job!). Denaturing agents seem to leave a slight residue?

The Everclear did a great cleaning job on my corrector, leaving no streaks.

If your SCT has shims, note their position and reuse them. From the question, I will guess you have a Celestron with the corrector 'stuck' to the rubber gasket, without edge shims, corrector seemingly randomly placed; it wasn't.

From my understanding, the corrector was placed in that position to compensate for mechanical and optical axis alignment concerns and to give them the same centerline. The EdgeHD scopes use set screws to maintain the alignment and corrector position I believe, from the whitepaper.

The corrector on scopes in the last decade are not necessarily centered in the cell, so do not assume they should be.

I would either make my own varying thickness shims to get the corrector in the same 'eccentric' to the cell perimeter, or measure the spacing, before loosening the corrector plate, to do so, or even find a way to mark the rubber gasket (my scope with this design is across the country so I cannot try this out).

Also, the rotational position of the corrector and secondary is key to avoiding turning a sharp SCT into a soft one. This rotational alignment is essential, I believe, confirmed from my experimenting with horribly performing SCTs. The spin alignment is not to offset zones on the optics (although Celestron did touch up figures by hand), but to maintain the same opto-mechanical axis coincidence mentioned above. Tilted backs (causing tilted baffle tubes) on assembly, secondaries not centered on support picks, plastic secondary housings etc all cause misalignment of the axes.

One way to tell is to see if a Robin Casady tabletop kitchen alignment (from the sky end, checking that all mirror and edge reflections of the elements and baffle output of an SCT are completely concentric) matches your star collimation. If not, something is likely miscentered or tilted. I believe this causes the great majority of soft SCTs, not poor optics. A ronchi or dpac will usually expose those with badly figured optics as opposed to those poorly assembled or rotationally misaligned. This test flagged both my problem scopes, and confirmed the C11 is not quite done yet.

Your corrector, secondary, and primary all have rotational alignment marks, all on the edge. They are to be at the 3 o'clock position viewed from the corrector/sky end of the tube. Celestron usually have tiny numbers inscribed on the edge face of the corrector, and Meade cleverly uses a smear of white out across the corrector and cell joint with a sharpie mark across the corrector-cell joint. My misassembled Meade 8 also had a Chinese character at the top of the corrector on the actual edge (box with a line through it) when the sharpie mark was aligned.

The secondary usually has a wide sharpie mark on the 3 o'clock edge to match.

My C11 had been disassembled and all parts rotated, EVEN the secondary puck to holder! Probably when the Bob's knobs were installed without following directions. Terribly soft, it is now sharp, and will improve more once I can finish the job when I move to join it again in AZ (someone had listed the wrong alignment point on CN but I ran out of time to resolve this, and from photos I discovered the secondary holder disassembly). The prior owner never disclosed the junk images. Maybe they thought it was the C11s supposed sensitivity to poor seeing. Or they or a prior owner cleaned and destroyed the alignment on reassembly and sold a bad scope, who knows.

Short answer is to follow the alcohol tip, and, if your scope was never disassembled, keep EVERY centering and alignment relative to the tube identical to what you started with.

The Meade white out and sharpie trick cam easily be applied to any part of the scope covered by a retainer, etc.

BTW, the Meade was unusable as delivered, used, to me, and is now sharp. It was super cheap due to the peeling silver secondary problem 20 years ago, and with be a very good performer once aluminized, and once I reglue the 3mm off center to the puck secondary it will still improve further. I think this one was (mis)built right after the company Christmas party or the day before a strike, with a badly tilted rear cell and baffle. Epoxied in place, yikes. The factory spin rotational alignment rendered it marginally acceptable (maximizing secondary offset to the tube helped this), and once that alignment was lost the scope was unusable.

Photograph, mark, and reassemble it identically to where it started.

Edited by markb, 06 November 2019 - 09:32 AM.

  • Greg Stein likes this

#4 macdonjh

macdonjh

    Aurora

  • *****
  • Posts: 4669
  • Joined: 06 Mar 2006

Posted 06 November 2019 - 11:12 AM

To markb's point: alcohol is a good choice because it should dissolve the goo, but still be kind to the plastic retaining ring.  As long as the alcohol is pure, it won't leave streaks.

 

Acetone would also work well, but you have to be careful, it can soften the plastic retainer.

 

Paint thinner would also work, but it streaks like crazy.

 

Isopropyl alcohol will also work, but most has quite a bit of water in it; some has lanolin, too.  So it will streak.

 

markb's points about maintaining the orientation of your corrector (and anything else you might disassemble) are also good information.  There isn't anything complicated about an SCT, but there are ways you can mess one up if you're not careful.  The good news is, as markb's stories show, there aren't many things you can do short of breaking glass to permanently mess up an SCT.  Most can be fixed.

 

Good luck with your cleaning project.  Glad to see you're trying it yourself.



#5 Greg Stein

Greg Stein

    Lift Off

  • *****
  • topic starter
  • Posts: 16
  • Joined: 22 Sep 2012

Posted 06 November 2019 - 12:06 PM

I gave up.

 

Tried now once more time. Pus some drops of alcohol and actually saw how it flows underneath the corrector plate around the edge. Tried slightly to move the corrector - nothing. Tried harder and harder... Put a plastic tool (I use for disassembling smartphones) between the edge and the corrector plate, tried to apply force. Nothing.

Tried even harder, and I have some strong hands (I'm a climber). NOthing. 

 

This thingy drives me crazy.

 

I think if I defork it and remove the outer ring from the carbon tube altogether with the corrector may be a way to go.

 

And no, unfortunately I live in Israel - so finding someone who can help is challenging.



#6 Don W

Don W

    demi-god and Chairman of the Finance Committee

  • *****
  • Administrators
  • Posts: 23501
  • Joined: 19 May 2003
  • Loc: Wisconsin, USA

Posted 06 November 2019 - 12:32 PM

Keep at it. Let the alcohol soak in. You are using the right tool. I was going to suggest that. Just pry a little harder and keep working around the plastic ring. I sincerely doubt that you will break it. Once off, you can use the same method on the corrector. Patience and gently prying is what will work.


  • markb likes this

#7 markb

markb

    Ranger 4

  • *****
  • Posts: 356
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 06 November 2019 - 09:24 PM

I agree with Don, give it time and go slow.

If you have drips on the far side you have too much alcohol, and position the tube so you don't get drips on the primary.

I don't recall how I popped the corrector loose (I think I also used cellphone screen tools), but I was never comfortable throughout the process. But I took it s l o w.

#8 GoFish

GoFish

    Surveyor 1

  • *****
  • Posts: 1645
  • Joined: 30 Nov 2016
  • Loc: Kentucky / Colorado

Posted 06 November 2019 - 10:12 PM

There are 4 plastic-tipped grub screws around the edge of the corrector. They center the plate. Loosen 2 of these using a small Allen key to free up the corrector. Leave the other 2 alone if you wish to preserve the original collimation of the corrector. 


  • Dynan likes this

#9 Greg Stein

Greg Stein

    Lift Off

  • *****
  • topic starter
  • Posts: 16
  • Joined: 22 Sep 2012

Posted 10 November 2019 - 03:41 AM

Hi,

 

Finally I succeed with getting the corrector out. But only by completely removing the front casting completely. So I unscrewed everything on the front casting and the nuts fall into the tube. It is crucial to do this while the OTA is in horizontal position so the nuts and much bigger and heavier brackets do not fall onto primary. Even after removing the front casting with the corrector it wasn't easy to remove the corrector plate itself. 

 

 

According to what I've read online, it appears to me that the whole triplet of primary, secondary and the corrector was misaligned. While I did marked the original orientation with masking tape, I think I need to redo the alignment of all pieces. The article on nexstarsite.com (https://www.nexstars...ctorRemoval.htm) says that the marking on secondary's edge should be aligned with the tiny serial number on the corrector and this wasn't the case with my scope.

 

 

So what I'll try is to align the secondary with the corrector, then put the corrector back on to OTA WITHOUT securing it with the screws. Try to collimate and check several orientations of it in relation to the primary. Maybe collimation for each of the orientation positios of the correct is necessary and the process could be not as easy. Probably far light source would be better than a star because I'm going to do this without tracking.

 

Would really appreciate if someone can suggest how to proceed with this alignment procedure. Maybe taking pictures of defocused light source and then compare on a computer?

 

Thank you!

Attached Thumbnails

  • IMG_1925.jpg
  • IMG_1933.jpg


#10 markb

markb

    Ranger 4

  • *****
  • Posts: 356
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 10 November 2019 - 08:33 AM

Simple mechanical alignments should cure your issues.

Since the best guess is that the corrector was removed from the cell previously, I doubt its position within the cell is the factory one. On my C11 even the secondary puck was removed and rotated on reassembly, which is a slight additional complication.

I got stuck working my way through this with two totally misaligned SCTs, successful 'fixing' both.

The Celestron corrector and secondary markings are a bit easier than Meade, since they should be aligned so the primary marking (I don't think this one was changed unless the primary was reglued), the sharpie stripe on the edge of the secondary, and the tiny numbers on the corrector all have to be at the 3 o'clock position, which is the side opposite the focuser knob, viewed from the corrector end. Meades risk having the whiteout and sharpie marks removed by overzealous cleaners.

Celestron does or did hand correct one of the optics, but it is my understanding the reason for the rotational alignment is to align the optical center axis of the scope to the mechanical center axis, set by the baffle tube and how accurately the rear cell is mounted to the tube, not to offset optical errors.

I used a holographic projection laser collimator to do my hopeless Meade 8 (bought super cheap with known massive issues making it useless) and the corrector perforation centering on my very soft C11, where the corrector, the secondary, and even the secondary housing were randomly misaligned on reassembly by a prior owner. If they didn't get it that way (blaming soft images on seeing sensitivity of big scopes??) I assume they just dumped it on me after they messed it up. Very soft and marginally useful. Collimation was achievable but only made the most of a bad mechanical alignment.

Reread my earlier post first, but here is my method for getting the opto-mechanical axes coincident.

Reassemble the front cell as close to 90 degrees to the tube as possible. Perhaps you can try to get it parallel to the rear cell with a digital inclinometer, cheap at Harbor Freight IIRC, I use it for scope and pier leveling too. Money well spent. Most folks can skip this step.

Holographic concentric circle collimators are now hard to get, but you can use an Agena Astro refractor Cheshire collimator, orther Cheshire, or a visual back plug with an accurated centered small hole to do the rest.

Ideally you want to remove the secondary housing to start with, but IIRC the two parts in Celestrons are glued. In that case you can try to center the housing or Fastar ring first instead of the actual perforation (hopefully centering the corrector plate), then the secondary mirror edges.

First and foremost, use the factory marks! Place the corrector etched edge at the 3 o'clock position viewed from the corrector end, should be opposite side from the focuser), and align the sharpie stripe on the edge of the secondary mirror glass body to the etched corrector marking and thereby also to the 3 o'clock position.

Having these marks will greatly simplify things or resolve them completely, but the cell removal may require more mechanical axis alignment efforts.

Using the peep or the projection, shift the corrector until the perforation (or secondary housing baffle edge) is centered in the baffle tube view through the peephole eyepiece. No, the corrector is not necessarily centered in the cell! That's why the EdgeHD has the locating set screws and the older ones have no shims, they centered and placed it on the sticky rubber at the factory. Older scopes may have been shimmed into the center; it is my understanding this is less critical than other centerings.

Next, the important one is to get the secondary mirror edges centered with the baffle tube using the peep on the Cheshire. If you already put the corrector edge etching and the secondary sharpie mark at 3 o'clock, you should already be in the right spot, just confirming the centering. Unless the cell removal shifted things, or it was off from the factory etc. If the marks are aligned but the secondary itself, not the housing, is not concentric to the baffle, and I would shift the corrector plate to achieve a concentric, mechanically aligned, view of the secondary mirror edge.

The goal is to match optical and mechanical centerlines. Mercifully, the spherical secondary has no single optical centerline, so we can focus on getting it mechanically centered.

At that point, the primary (hopefully) is centered to the baffle tube (sometimes they are glued a bit crooked), you have centered the perforation to the baffle, and then centered the secondary itself to the baffle, then you should the have the mechanical axes aligned, and regular collimation will finish alignment of the optical axis.

I suspect the reason the factory does the rotational alignment, marking same, at the factory is, in large part, to simply shift the secondary to a spot directly in line with the primary optical center, which SHOULD match the baffle mechanical center. While Celestrons does or did some hand figuring, the rotation process is not done to offset zone or astigmatism (except resulting from misalignment I assume), but to locate the secondary in the mechanical centerline of the scope.

My Meade 8 has the rear cell glued on at least 5_-7mm out of alignment with the corrector plate. By putting the corrector totally off center and one edge flush with the cell it is still a couple of mm off. The secondary is actually glued 2-3mm off center to the puck, so by maximizing the secondary offset to the same direction as the corrector offset, mechanical alignment was almost perfect. And now the scope is reasonably sharp pending final tuning, a big change from uncollimatable and beyond useless.

I suspect that they use slightly off-center glued secondary mirrors (my Meade is 2-3mm off-center!) In the corrector/secondary rotation process to align optical and mechanical centers to coincide.

Edited by markb, 10 November 2019 - 08:42 AM.

  • Greg Stein and Dynan like this

#11 Greg Stein

Greg Stein

    Lift Off

  • *****
  • topic starter
  • Posts: 16
  • Joined: 22 Sep 2012

Posted 10 November 2019 - 04:29 PM

Mark, thank you so much for this informative answer. 

 

Since I live in Israel, we don't have everclear here. Denaturated alcohol does leave some residue (you can see the photo of the corrector - see those spills of alcohol I've done when I tried to remove the corrector?), so I thought if I get some 70% alcohol solution from a drugstore, will it have some solvents in it?

 

My scope does have cork shims (4 of them) spread very asymmetrically: 12, 3, 5 and 10 o-clock. After removing the corrector plate I looked at the what I thought is a rubber ring. But it wasn't rubber. Instead it looked to me very solid layer of something glued to the inner ring attached to the front cell.

IMG_1927.jpg

 

I will buy the Cheshir collimator you mentioned.

 

Removing the secondary from the corrector plate is very easy. It was so loose I thought it could fall inside the scope smile.gif

 

I forgot to mention that the corrector plate has some strange markings at the perforation I attach the images here. Don't know if this even important.

IMG_1931.jpg

IMG_1932.jpg

And also, my scope is an older model it was completely produced in US including the motors. At least I was told so.

 



#12 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 43238
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 10 November 2019 - 04:55 PM

It has been my experience, after overhauling more than a hundred of these, that the off-center correctors were done solely to center the secondary on the baffle tube,

assuming the baffle tube was straight and perpendicular to the rear cell of the scope.

However, for the best optical correction, the corrector, primary, and secondary must all have their "centers" align.

That is simple to achieve if you have an accurate laser and a tube on the back of the scope with a window in its side.

[I had such a tube from Howie Glatter back in the day].

After collimating on a star at night at high power, you insert a laser in the visual back + tube.  If the optics all align in the tube, the laser will reflect

from the secondary and return to the source.

If the secondary, corrector, or primary do not line up, and a misalignment was necessary to collimate the scope, the laser beam will not return to its source.

 

We discovered the biggest culprit was having the secondary off center relative to the corrector.  When the secondary was centered in the corrector, and the corrector

was centered in the corrector's cell, we usually got a scope collimated at the same time the laser returned to its source.  In other words, everything was centered.

We did note that several scopes done this way did not appear to have the secondary centered in the baffle when looking through the back of the scope.

This was determined (by simple measurement) to be a primary baffle tube that was not in line with the outer tube.  We resisted the urge to start out with the secondary 

off-center to correct that, as that would result in the same skew that we corrected.  Our aim was to align the optical center line with the corrector, the secondary,

and the tube.

 

It's interesting to note that this resulted in easier collimation, with more equal tension on the 3 secondary screws, and generally better star images.

 

What we did not do was to spend a lot of extra time rotating the corrector relative to the scope, leaving the secondary and primary fixed.

There is ample literature on-line to show that simple experimentation with 30-45° rotations of the corrector will likely end up with better correction than the original setup,

and put the corrector in a slightly different orientation than the original.

 

If I ever get another SCT, the first thing I will do is to overhaul it, flocking the entire interior, and perform the same centering corrections we performed when overhauling

a ton of them.  The results spoke for themselves.  Why didn't the factory do that?  Time = money, and the margin is not high.  That level of care is more of a Questar thing,

and they were nowhere near the cheap prices of SCTs.


Edited by Starman1, 10 November 2019 - 04:56 PM.

  • markb likes this

#13 markb

markb

    Ranger 4

  • *****
  • Posts: 356
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 10 November 2019 - 04:58 PM

Good luck!

There are threads on cleaning solvents, but I personally would avoid drugstore denatured alcohol.

Everclear is just liquor store 150 or 190 proof pure alcohol for making your own fancy drinks or cordials using fruit, etc. They might sell something similar there, a high proof unflavored alcohol made for human consumption.

But there are alternatives.

You likely have access to Amazon, so the Zeiss lens cleaning fluid sold there is well regarded and affordable. 5% lab grade isopropyl, the rest is distilled water.

The Baader cleaning fluid is great but not imported to the USA as far as I can tell. You may be able to get it there or buy it out of Europe.

I have no clue on the perforation markings, Mike Swanson might know. The etched number on the edge should be all you need, though.

Avoid flipping the corrector by accident! I think the etching is on the front surface, readable when installed. You can tell before cleaning by looking for your alcohol streaks. If it does happen, the best info seems to indicate it is noncritical.

I might keep the shims, but since the secondary holder was removable I would almost certainly center the perforation, modifying shims if needed (thinner, thicker, etc) to center the perforation, and use thin cork to center the secondary holder (and keep it in place) within the perforation.

My C11 holder cannot be unscrewed, and has at least .5mm of slop in the fit.

Good luck, I hope you end up with a very sharp scope!

#14 markb

markb

    Ranger 4

  • *****
  • Posts: 356
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 10 November 2019 - 05:17 PM

I skipped your post, Starman1, just reading it now.

The laser info confirms what I was thinking, thanks for adding that experience info. I have an old Kendrick SCT collimator. I think it is essentially identical to what you describe, a 45 degree face visible through a cut in the tube-holder.

Lasers got a mostly deserved bad rep with SCTs, I assume because they only worked on scopes hand tuned for coincident optical and mechanical axes as described. My experience in the alignment process was also colored by a Meade with a baffle tube way out of alignment with the tube.

Thank you for providing your very extensive experience and observations!

Edited by markb, 10 November 2019 - 05:36 PM.


#15 mclewis1

mclewis1

    Thread Killer

  • *****
  • Posts: 19228
  • Joined: 25 Feb 2006
  • Loc: New Brunswick, Canada

Posted 11 November 2019 - 09:31 PM

Greg,

 

A few thoughts from when I worked on my 2001 era C11 (cf tube, fastar compatible secondary) ...

 

- the edge spacers around the corrector were also evening distributed and seemed to be thinner than cork. They appeared to be just paper. I replaced them with similar paper (heavy card stock).

- in my C11 the ring of material the corrector is sitting on is cork. It was very compressed and a bit sticky but a close look at it showed it was some form of cork.

- I didn't bother with or worry about using anything special liquid wise. I used dilute Windex and it released my corrector easily and also removed any "sticky bits" from that cork ring on the backside of the corrector.

- I added a sorbothane gasket (homemade, not the Starizona product) to keep my fastar compatible secondary mirror holder from rotating.

- spending extra time to center the secondary mirror holder seems to have paid dividends in returning everything to where it was easy to collimate again.

- My C11 is now quite a bit sharper optically. I don't think there was anything wrong with it originally (it hadn't been opened or altered at all, but the secondary holder had rotated slightly), it was however a bit soft optically. The reason for pulling the corrector was to add the gasket. It seems that the re centering of the secondary and then a fine collimation (it was easier to collimate than I remember the untouched scope being) has yielded a sharper scope.



#16 Greg Stein

Greg Stein

    Lift Off

  • *****
  • topic starter
  • Posts: 16
  • Joined: 22 Sep 2012

Posted 13 November 2019 - 08:51 AM

Thank you all for your time spent answering me. I really appreciate that!

 

Oddly enough, I work for Carl Zeiss. So I went to the lab and asked our optics guy what do they use for cleaning optics. Found this:

 

 

 

Attached Thumbnails

  • IMG_1944.jpg
  • IMG_1946.jpg

  • gfstallin likes this

#17 Cfreerksen

Cfreerksen

    Viking 1

  • *****
  • Posts: 653
  • Joined: 18 Aug 2018
  • Loc: Tooele, Ut

Posted 16 November 2019 - 12:59 AM

Thank you all for your time spent answering me. I really appreciate that!

 

Oddly enough, I work for Carl Zeiss. So I went to the lab and asked our optics guy what do they use for cleaning optics. Found this:

Enough cleaning fluid to last a lifetime!

 

Chris


  • Traveler and gfstallin like this

#18 Greg Stein

Greg Stein

    Lift Off

  • *****
  • topic starter
  • Posts: 16
  • Joined: 22 Sep 2012

Posted 26 November 2019 - 04:49 AM

Saga continues... unfortunately.

 

After cleaning all of the optics and assembling back, aligning everything to 3-o'clock. I found the marking on secondary edge (barely seen if shining with a flashlight when secondary is in its housing). Aligned it with the serial number of the corrector plate. The housing of secondary mirror still rotates freely despite I tried to tighten it.

 

The picture of the scope isn't blury it's a total mess frown.gif Even when looking through the visual back without eyepiece, it is obvious that something isn't right. Take a look, it's not centered at all. What could be a problem in your opinion?

 

Attached Thumbnails

  • not centered.png


#19 markb

markb

    Ranger 4

  • *****
  • Posts: 356
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 26 November 2019 - 09:26 AM

Don't panic, you have a relatively straightforward mechanical axis misalignment that should be easily fixable.

Short answer, ditch the corrector shims, align the corrector with secondary MIRROR, not the secondary housing, to the baffle tube with the Cheshire, and see if that gets you aligned.

Do all preliminary collimation in daytime using Casady, link below. This preliminary collimation can be very tough at night IMHO, while final tuning is not too bad at night. So I ALWAYS start with a Casady daytime (pre)collimation.

That said, I use artificial stars in daytime, too, for near-final collimation, finishing at night. A small ball bearing at 50+ meters works, as does the Hubble 5 star light http://hubbleoptics....cial-stars.html .

Your issue should be curable. My misassembled M8 was far far worse, and now is only .5mm out, with the secondary 6-7mm off-center in the cell but almost in line with the primary and baffle tube. And that is an extreme example. It WORKS now where it was trash before.

Your scope sounds as if it has been apart in the past, but the possibility of factory misassembly exists, as mentioned by Starman. Worst case,on a Celestron you may be able to loosen the rear cell and shift it to center the baffle tube at it's end, but that is only a last resort. I would avoid it!

My procedure:

1)

Although it sounds silly, which 3 o'clock?

All the markings go to the 3 o'clock viewed from the corrector end, that is, the e o'clock on the side opposite the focuser knob.

If it were a Meade with no hand figuring, where spin alignment appears to only use eccentric positioning of the secondary to get the secondary mechanically centered with the baffle tube axis, I would devote all efforts to the secondary-baffle tube mechanical alignment. With yours, start with checking that you used the correct reference point

If you used the wrong reference point, fix it and you very possibly will be done, check with Cheshire.

2)

Prep your knowledge base and reread my long posts.

Also, read
http://www.robincasa...ro/collimation/
to see how to do a daytime setup. It works amazingly well.

Read the Wilmslow C14 pages http://www.wilmslowa..._alignment.html
for a inexpensive final mechanical-optical test setup. But you are not ready to implement it until you use the Cheshire to get most of it done. It also lays out the possible issues.

DON'T mess with the primary! Do everything else, but if you damage the primary it's game over.

3)

Reread the Casady tabletop method, it DOES work at getting a near perfect initial collimation. AFTER you get your axis alignments fixed.

Keep using it along with the Cheshire to gauge progress.

If your sky or artificial star in focus, collimation still has the Casady reflected 'circles' nonconcentric in a post-collimation check, some minor mechanical alignment issues might remain. If slight, don't go nuts trying to cure them.

4)

Time to adjust!

Remove the factory corrector shims as you are obviously out of the baffle tube defined mechanical (and hopefully primary-optical center). Celestron, with Fastar on, went to a true perforation/corrector centering, unshimmed on rubber, or, now, grub screw located at the edges. Cork spacers worked well only on baffle tubes correctly centered. That's one reason for soft images, as well as lost spin alignments, and simple misassembly.

You may need to glue or otherwise secure the semicircular paper corrector cushions to the front cell lip, only, if the factory glue failed and they keep falling into the tube. You will be shifting the c. plate a few times.

Ideally I center the corrector perforation separately from the secondary, but you may find this hard to do, and it may require a means of visualizing the geometric center of the secondary. Therefore it is likely most practical to do the centering with only the sec. housing assembled to the corrector.

Use the Cheshire or (much more accurate, but none currently in production, why?) a concontric circle holographjc projection laser collimator to make your centering adjustments.

You have two freedoms of movement in the corrector secondary assembly; the corrector within the cell (5+mm), and the secondary housing within the corrector (about 1mm). Use them to get the secondary mirror centered, the goal. With the Cheshire method, I'd adjust them while the corrector and secondary are assembled. That's what I cover below.

If working alone, you may be forced to use a diagonal to work with the tube near vertical let gravity hold the corrector in place as you work,not ideal but...

Or adjust the retainer screws to let a slight amount of friction let you shift the plate but still hold it in place. Tricky though.

The cheshire method kind of forces these choices on you.

I compared my laser method to the Cheshire, and the laser was easier to get perfect, but the Chesire did work. Absolute concentricity is hard to judge though.

Shift the corrector-secondary together until you have the two thin bright outer circular lines in your photo concentric to the bright secondary circle. And the baffle tube.

As to the pattern in the very centermost reflection in your photo, I could not figure how to use it (doing so messed things up) so ignore it. Reexamining your photo it appears to simply be an image of the camera used, of no use to us in alignment.

You will have to remove the corrector shims and shift the plate. You may also have to shift the secondary housing within the corrector perforation; optical pros here seem to indicate this will not create a new problem. Don't worry about shim removal! You can add custom thickness ones if desired at the end.

My C11 is mechanically assembled fine so I shimmed the secondary holder concentric to the plate with UHMW tape, Teflon plumbing tape should also work, but in my M8, the corrector and the secondary housing, both, are shimmed totally to one edge of the hole. I shifted the corrector plate to center the perforation and secondary, and confirmed the alignments with projection and Casady reflections.

Yours does not look as mechanically close to alignment, but start with a centered in perforation housing, and shift it if needed as you work.

The secondary housing can shift in the perforation, as you have found, so either shim it within the perforation once done, or use a thin rubber/sorbothane gasket to help keep it in place. They are sold pre-cut for C11 and C8s, see https://starizona.co...n-kit-c8-gasket

I am sure you can make one if thin material is available, e.g. Amazon. It also reduces the possibility of secondary tightening induced pinching.

If the movement freedoms of the corrector and secondary housing are not enough, you can see if rotation of the corrector and/or secondary can finish the job. The secondaries are apparently often slightly eccentric to the housing and pick.

--

I assume the misalignment will be accomplished simply by shifting, and then 'locking down' the corrector and secondary.

The method mentioned by Starman can be utilized with a regular laser collimator with a 45 degree target and a side hole to view the return hit, such as made popular with barlowed laser Newtonian collimation, and are available on EB and elsewhere, BUT the geometry of the secondary collimation adjusters, pivot point, aluminum puck and the scope might require this be left as a final, iterative tuning. Any turning of collimation screws affects the return point even with a centered secondary because the pivot is not at the center of curvature and tilts the return, I believe.

If you can safely (without scratching the secondary) find a way to get the secondary level and parallel to the housing and corrector, it might allow the laser to be used as a primary centering method.

I think it is safest to leave the laser return method to the very end, and I personally found it was not useful in final tuning as the holograph method was reliable. I did get a return within 1-2mm of the laser return target center when done. This was, I thought, an impressive confirmation of the centering method used above, as was attaining a textbook star collimation pattern.

--

As I have mentioned, I have used the C Edge white paper and the Wilmslow C14 sites, and CN posts, as other resources and hands-on experimentation in figuring out the seemingly simple (but actually complex) issues in SCT alignment, pre-collimation.

I applied this to my 'crooked' M8, and my totally 'spun' out of factory alignment C11. Over my AZ trip last week the C11 is now spot-on, with star and Casady alignments matching, and my M8 is as close as possible to being opto-mechanically aligned without bending the baffle tube a bit (apparently possible) or regluging the secondary an extra 1mm further off center, obviously safer. Both now give great images, although the Meade needs a secondary recoat.

The Meade was the best $75 learning tool I've bought in a long time, and my kids got a great scope out of it.

Good luck, keep calm, and be nondestructive in all attempts.

Edited by markb, 26 November 2019 - 12:35 PM.


#20 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 43238
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 26 November 2019 - 02:08 PM

Basic mechanical collimation can be done indoors in the daytime.

If collimation is way off, then the images you see can be way off.

You can't really evaluate the mechanical misalignments easily using reflections in a miscollimated scope.

Here is the indoor collimation technique:

http://www.mira.org/...ures/collim.htm

So, first get the scope at least into a fair collimation using the indoor technique, then evaluate the mechanical line up.

Mark's post has a lot of information in it for a step-by-step analysis.

I can't tell from your image if the corrector is off center, the secondary is off center, or what.

But if you cannot get the system moderately collimated with the daylight technique, then the mechanicals are way off.

And careful measuring might get you better aligned.

Corrector should be centered on the baffle (trusting the baffle is perpendicular to the primary).

Secondary should be centered on the baffle and centered in the corrector (not the housing, per se, but the secondary mirror).

I'm a little concerned the secondary housing is still loose in the corrector.  Did you remove the shims between the housing and the corrector?


Edited by Starman1, 26 November 2019 - 02:10 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Celestron, catadioptric, equipment, optics, reflector



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics