Let the story begin. I purchased a new Meade 12” F8 LX850 OTA and mounted it up on my EQ8, collimation was excellent. Rather than let sleeping dogs lay, I decided to check it out with my Hotech Advanced Laser Collimator, what the collimator showed was that the secondary holder was offset in the Corrector, "this worried me a tad". Having also an 8” LX200, I thought I would check that, guess what, its secondary holder is slightly offset in the corrector plate.
So I stared scouring the NET to see if this was normal, the first thing I came across was this:
The guy measured that his center hole in the corrector plate was 1.6mm offset in one direction. " I wouldn't recommend his solution"
I don’t believe that Meade are that sloppy given the technology today, it had to be done on purpose, and you don’t stay in business if you can’t produce a SCT that can compete with the main opposition ( Celestron).
I then found this an article in Cloudy night that suggests that the Secondary does not necessarily have to be centered, "hotly debated".
I'd like to respond to your query regarding the secondary mirror by sharing a bit of my experience with it. Like you, I was curious. So, after using my scope for a couple of years (and out of warranty), I fiddled with collimation. I think I got it fairly close, but the focus just didn't want to "snap" into place. I bought the Kendrick SCT collimation laser. What I determined by it was that the optical and mechanical alignment of the OTA wasn't exactly the same.
I also had heard that the secondary hole in the corrector was a bit bigger than the secondary holder. I reasoned that I could move it to try to get better optical/mechanical alignment. So, off came the corrector. To my surprise, I discovered that the secondary mirror was not centered in the holder. Apart it came. The secondary mirror is actually glued to an aluminum disk into which the collimation screws screw. At the center of the aluminum disk is a hole which the secondary pivots on. Still thinking I could improve my optics by getting everything centered, I pried the mirror from the aluminum disk, centered, and reglued it. I used a couple of layers of black electrician's tape around the secondary holder so that it fit snugly - and centered - into the corrector.
I put everything back together and....the astigmatism was so bad, my stars were now diamond-shaped. I could barely make out medium-sized craters on the moon. The next day, I called Meade and (sheepishly) told them what I did. Ollie explained to me that part of the process of setting the optics for each scope when it's manufactured is to align them such that they compensate for the inevitable mechanical misalignment of the OTA. In other words, the fact that my secondary was not centered - both in the corrector and in the holder - was on purpose. Ollie said I'd have to send my scope back and they'd "repair" it for the standard $500 fee - or install new optics for $550.
After many, many hours of trial and error, I was able to restore my optics back to where they were - and maybe even a bit better because I had so much practice at collimation. But, I think it was more a matter of luck than skill.
So, my suggestion regarding fiddling with the secondary is to be very careful what you do - and be very sure to mark it such that you can return it to its original position.”)
Which is a very interesting read indeed, this implies that you have to use a star or artificial star to collimate Meade’s correctly, you can’t use the Hotech, because it assumes mechanically and optically everything is centered, the above implies this is not the case, "intentionally". I might add that the corrector itself is accurately centered, well until the cork spacers finally compresses.
I would be interested hear from anyone who has some experience in this area, or thoughts on the subject.
Great story.
When I bought (new) my 8 inch meade, the secondary holder was loose in the corrector plate and could be easily rotated.
I just tightened it and hoped for the best. I did collimate it of course.
It seems to be ok . So either I'm lucky or it doesn't really matter.
Great story.
When I bought (new) my 8 inch meade, the secondary holder was loose in the corrector plate and could be easily rotated.
I just tightened it and hoped for the best. I did collimate it of course.
It seems to be ok . So either I'm lucky or it doesn't really matter.
Hi Jennifer,
I think that Meade purposely offset the secondary in the corrector plate. I had to do a lot of searching of the NET to find anything on the subject, and if the offset caused serious problems, then the NET would have lit up like a Christmas tree, when we have problems we are fast tell everyone about them. Meade has been around a long time and has a good reputation, if they where that sloppy they wouldn't be still in business .
Cheers
Peter
Last edited by PeterSEllis; 25-04-2017 at 04:47 PM.
Having overhauled many of these, I can tell you that centering the secondary likely won't center it in the corrector unless the secondary holder is, itself, dead center in the corrector.
It always seems to help to center the corrector plate in its cell, but note that there is a caveat to this: the primary itself may not be centered in the tube and the corrector needs to be centered on the primary, not the tube of the scope.
You only center the corrector if the primary is also centered. Otherwise, if the primary is off 1mm in a particular direction, offset the corrector in the same direction.
Then the alignment of the secondary: If the primary mirror's baffle does not have a front end (as seen through the back of the scope) that is concentric with the outline of the secondary (indicating the baffle, which is always perpendicular to the primary mirror) isn't pointed at the secondary mirror exactly), then the secondary must be off-center in the same direction the primary baffle indicates.
In the collimated scope, the corrector is off-center to match the primary, and the secondary is off-center to match the pointing of the primary baffle.
If you're extremely lucky, everything is centered in the tube as well.
When I worked on my old Meade 8", the only error was an off-center corrector, but everything else was perfectly aligned with the tube. And centering the corrector (and then re-centering the secondary) made a BIG difference (improvement) in the star images.
The final result was a tiny pinpoint with 1 or 2 diffraction rings around the stars.
But, from those I've seen, I was lucky. I guess they were making them a lot more slowly in 1993 than they are today.
Having overhauled many of these, I can tell you that centering the secondary likely won't center it in the corrector unless the secondary holder is, itself, dead center in the corrector.
It always seems to help to center the corrector plate in its cell, but note that there is a caveat to this: the primary itself may not be centered in the tube and the corrector needs to be centered on the primary, not the tube of the scope.
You only center the corrector if the primary is also centered. Otherwise, if the primary is off 1mm in a particular direction, offset the corrector in the same direction.
Then the alignment of the secondary: If the primary mirror's baffle does not have a front end (as seen through the back of the scope) that is concentric with the outline of the secondary (indicating the baffle, which is always perpendicular to the primary mirror) isn't pointed at the secondary mirror exactly), then the secondary must be off-center in the same direction the primary baffle indicates.
In the collimated scope, the corrector is off-center to match the primary, and the secondary is off-center to match the pointing of the primary baffle.
If you're extremely lucky, everything is centered in the tube as well.
When I worked on my old Meade 8", the only error was an off-center corrector, but everything else was perfectly aligned with the tube. And centering the corrector (and then re-centering the secondary) made a BIG difference (improvement) in the star images.
The final result was a tiny pinpoint with 1 or 2 diffraction rings around the stars.
But, from those I've seen, I was lucky. I guess they were making them a lot more slowly in 1993 than they are today.
Hi Don,
Many thanks for your reply. Hearing from somebody who has a lot of experience in overhauling Meade's, your knowledge is is invaluable. I'm running this thread in the cloudynights forum in parallel, and will copy over your comments. All the people following this discussion in both forums will certainly appreciate them.
As I have said to other people, my 12" F8 LX850 on first light produced circular stars edge to edge, until I started tinkering with it ( O why O why). I now have the collimation back to about 99.8% of its original collimation. I will do a lot of measurements of exactly where all the components lay with respect to each other, "before I fiddle with it again".
Cheers
Peter
Last edited by PeterSEllis; 05-05-2017 at 08:38 PM.
This amounts to a star based collimation, which requires good seeing at the time of collimation, and a scope that has settled down to ambient temperature. When done properly, the results are fairly acceptable.
I believe the collimation cannot be refined enough to compensate for incipient mirror shift. I have a crayford focuser as the main focusing device, and leave the mirror focus alone, but this must of necessity be a temporary solution. The LX200 OTA has a mirror lock which would at least allow for one less degree of freedom of the optical train, but the LX90 does not have this feature, and primary mirror shift is going to degrade collimation by significant amounts any time it occurs.
I bought a Glatter collimator with the concentric circle laser pattern, and was rather disturbed to see that it indicated the primary was **in no way** aligned with the secondary - eyepiece holder path, despite a reasonable star-tested collimation. When I moved the collimation to make the concentric circles properly aligned, the view through the scope was completely unacceptable.
Further reading convinced me that this state of affairs is endemic to modern SCTs, and true collimation is going to be a compendium of approximations rather than a path to a real goal. A bit like politics as practiced in Australia - many adjustments to get to the same rough end.
For my main purpose (occultation observing) this does not present a problem. A light curve is not significantly altered by slight mis-collimation, but the ease of setup and good pointing accuracy of the Meade SCTs is of real utility in finding dim, unremarkable stars that asteroids will pass in front of.
Views of planets and other small objects will however certainly be diminished by less than perfect collimation, and if an observer wishes to set sights on this very worthy goal, they could do much worse than take a page from Anthony Wesley's equipment setup (Mr. Wesley goes by the handle "bird" on these pages). He uses a very big (16") Newtonian and takes great care to remove air currents due to temperature fluctuations. The secondary obstruction is around 25% for a Newt, versus near 35 - 40% for a moderate SCT. This reduction in obstruction can only improve contrast. As well, collimation of a Newt is a much more confident path than the fudging of an SCT.
For myself, I cannot afford a Big Newt, and for my main purpose, the LX90-10" ACF does me well. I do feel the pain of those who would like to seem more clearly with their scope, but my suspicion is that the SCT as currently made is not able to be properly collimated to the degree that a purist would desire.
As always, your opinion may not coincide with mine, and you are welcome to hold differing views.
This amounts to a star based collimation, which requires good seeing at the time of collimation, and a scope that has settled down to ambient temperature. When done properly, the results are fairly acceptable.
I believe the collimation cannot be refined enough to compensate for incipient mirror shift. I have a crayford focuser as the main focusing device, and leave the mirror focus alone, but this must of necessity be a temporary solution. The LX200 OTA has a mirror lock which would at least allow for one less degree of freedom of the optical train, but the LX90 does not have this feature, and primary mirror shift is going to degrade collimation by significant amounts any time it occurs.
I bought a Glatter collimator with the concentric circle laser pattern, and was rather disturbed to see that it indicated the primary was **in no way** aligned with the secondary - eyepiece holder path, despite a reasonable star-tested collimation. When I moved the collimation to make the concentric circles properly aligned, the view through the scope was completely unacceptable.
Further reading convinced me that this state of affairs is endemic to modern SCTs, and true collimation is going to be a compendium of approximations rather than a path to a real goal. A bit like politics as practiced in Australia - many adjustments to get to the same rough end.
For my main purpose (occultation observing) this does not present a problem. A light curve is not significantly altered by slight mis-collimation, but the ease of setup and good pointing accuracy of the Meade SCTs is of real utility in finding dim, unremarkable stars that asteroids will pass in front of.
Views of planets and other small objects will however certainly be diminished by less than perfect collimation, and if an observer wishes to set sights on this very worthy goal, they could do much worse than take a page from Anthony Wesley's equipment setup (Mr. Wesley goes by the handle "bird" on these pages). He uses a very big (16") Newtonian and takes great care to remove air currents due to temperature fluctuations. The secondary obstruction is around 25% for a Newt, versus near 35 - 40% for a moderate SCT. This reduction in obstruction can only improve contrast. As well, collimation of a Newt is a much more confident path than the fudging of an SCT.
For myself, I cannot afford a Big Newt, and for my main purpose, the LX90-10" ACF does me well. I do feel the pain of those who would like to seem more clearly with their scope, but my suspicion is that the SCT as currently made is not able to be properly collimated to the degree that a purist would desire.
As always, your opinion may not coincide with mine, and you are welcome to hold differing views.
Regards,
Tony Barry
WSAAG
Hi Tony,
Thanks for your reply. I agree star collimation for SCT is probably the only way to go, it has certainly almost got mine back to where it should be, although not quite there yet as can be seen in the attached M83 picture taken last night (with the moon blazing like a beacon from hell). I'm 67 and my body doesn't take too kindly to massive large telescopes. The 12" Meade F8 SCT is at my limits. I guess you learn to live with imperfection of the SCT, or overhaul it yourself.
