Secondary centering expert help needed
Posted 12 November 2012 - 05:59 PM
Posted 12 November 2012 - 08:02 PM
Move a bright star to the very exact center of your field.
Collimate as best as possible.
Defocus on a very bright star and note that there will be a little tiny point of light at the center of the secondary shadow.
Defocus just enough so that you see the inner bright ring come out from under the secondary shadow, but again you still should see this tiny dot near the center of the shadow.
Does the dot look perfectly centered in the inner bright ring?
If not, this may indicate that the secondary is not directly over the primary. To center it, just move loosen the screws holding the corrector plate and push it in the direction required to center the point in the secondary shadow.
It does not have to be perfect perfect perfect, but you should be able to improve it using this method.
Plese let me know how this works for you (if it works).
I know it worked for me on two scopes, but one was an EdgeHD and one was a C5 that had some other modifications.
But I think it will work.
Again, would appreciate your feedback to know if this helped.
Posted 12 November 2012 - 08:19 PM
I will let you know the results once we get some good seeing here. I do have one question though, should it be that obvious looking through the eyepiece hole?
Posted 12 November 2012 - 08:26 PM
Visual performance of the instrument is now very good. I also had to secure and adjust the rear cell mounting to ensure proper alignment of the primary mirror assembly, but that is another subject entirely.
Posted 12 November 2012 - 08:55 PM
Posted 12 November 2012 - 09:18 PM
Are you sure that the front corrector cell and the mirror cell are both flush to the edge of the tube ends?
You need to eliminate any chance that the baffle is tilted with respect to the optical axis. If this condition occurs, you can have the secondary perfectly centered in the corrector cell, but when you sight though the baffle, it will appear off center.
If you removed the baffle, then you are going to have to check it to ensure that you got it back perfectly centered and perfectly parallel to the optical axis.
If you pulled off the mirror box from the tube, you need to ensure that that it is fully seated back against the end of the mirror tube. No gap at all anywhere around the outside. The mirror cell has to be exactly square to the tube itself.
Posted 12 November 2012 - 11:11 PM
2) The secondary was perfectly centered and even measured.
3)I saw that the secondary was off center when i sighted through the baffel.
4) Could this be the true reason for astigmatism?
**Please give me some steps of what you would do to correct this please i need to get some nice Jupiter shots soon.
4)How do you square the tub to the mirror housing?
Posted 13 November 2012 - 09:17 AM
Posted 13 November 2012 - 09:23 AM
Posted 13 November 2012 - 10:19 AM
It could be the cause of astigmatism, other's have complained of the same astig and centering issues. But, if your secondary was off at some point, then there might be two problems to deal with.
The first one would be to accurately place the secondary back in the corrector according to spec. It may have some sort of hash mark showing proper rotational orientation (in reference to what I don't know. Another hash mark?) Surely Celestron will give up the info.
Once rotational orientated, then it really needs to be placed radially so that it sits above the primary baffle. You could square the primary mirror and baffle assembly, but that may not be needed. It was just fine before. Unless you wanna knock your scope out of commission for a few days and pull our hair out.
There should be some play in the secondary hole in the corrector to allow for some radial adjustment (maintaining proper orientation of both the corrector and the secondary.) Then maybe some sufficient play in the corrector shims to allow more, if needed. You should be able to work out that 1/2 inch as seen from the visual back and get it back to where it was.
Alright, lot of words but how to do it? Eddgie and Ed Holland offered some methods. There was a guy who used rolled up paper sticking from his primary baffle (just long enough to reach the secondary baffle) and set the corrector that way. His process was a bit more involved than that, but that's the idea. It was on the internet somewhere, sorry I don't have a bookmark. Google, "how the *BLEEP* do I center my *BLEEP* *BLEEP* secondary mirror and corrector for *BLEEP* sakes?"
I used Eddgie's method on my scope with the same problem. Focusing in and outside, my donut was not concentric on both sides. It was skewed one way inside and the opposite way outside. I could tell my focal plane wasn't moving orthogonal to the optical axis - same effect as you might have. Rotating my optical components (with a fixed, spot secondary) 90 degrees eventually got everything spot on. As Edggie says, I made sure all components were as flush with the tube as they could be.
So, first thing to do is to align your secondary's rotation back to spec. I believe there might be some surface correction applied to it, so it has to go in one way. And that one way probably means the corrector is the same. It goes in one way, too. (They add some 5th order aspheric terms and that may mean some hand figuring with optics in some alignment. Someone can correct me if that's not true.)
Once rotation alignment is restored, then radial centering can be done. By then, you should be back to normal without ever touching the primary mirror assembly...unless you have way too much hair and need to pull some of it out.
Good luck, Duke. My hair is growing back. Yours will, too.
Posted 13 November 2012 - 10:30 AM
I don't even think there is enough play in the corrector to correct for that much decentering. Ususally there is only about 1/4 inch of total play between the edge of the corrector and the cell.
Dude, I don't know how you could be off so far unless the baffle was tilted with respect to the optical plane.
Did you mark the primary cell to ensure that it was put back in the same orientation on the tube? It sounds like your baffle is not square.
There are other tests you can do.
One is to use a low power eyepeice and defocus a star. Now, move the star to the north, south, east, and west.
What you want to see is that the fresnel pattern remains whole as it approaches the edge of the field of the eyepecie.
If you see the pattern have a circular intrusion at any point on one side and not the other, this indicates that the baffle is tilted with respect to the focal plane.
The Fresnel pattern should remain whole in every direction.
The SCT design is not free from astigmatism, but the dominant abberation is coma. If you your baffle is tilted, or you secondary is not centered, you will see coma as the primary abberation, but there is indeed an astimatic component in the SCT. Same with a Newtonian mirror. Both have astigmatimsm, but the coma is much stronger so it does more damage. But if you pass through focus on star at the edge of the field even using excellent eyepeices, there could be an astigmatic component visible.
I just don't know how you could be so far off with the secondary mirror.
What was the result of the point test? Did you observe the little point I mentioned? Was it centered in the inner ring?
Posted 13 November 2012 - 11:24 AM
If it's a half inch, that may be a bit much to account for with corrector centering itself. Maybe the mirror assembly was re-installed 90 degrees out. Re-installing both incorrectly could give that much displacement, ya think?
This seemed to be true in my case, the baffle pointed at the secondary when I rotated the mirror assembly 90 degrees (to match the mounting holes.) Not sure how the CGE is constructed, but if the primary mirror assembly is rotated, then the baffle could tilt away from the installed secondary position. Rotating the mirror assembly might re-point the baffle at the secondary, or get closer than 1/2 inch. From there, some corrector adjustment would finish the alignment.
Chances are, it's not perfectly square as a result of mass production. This is still okay as long as the secondary can be positioned directly above (centered) it and shining directly down it (verified by your Poisson spot method, or checking the secondary shadow is concentric both sides of focus.)
Posted 13 November 2012 - 11:40 AM
Posted 13 November 2012 - 12:51 PM
I had to remove the corrector so that the rear cell fasteners could be accessed, then adjust & re tighten and replace the corrector. At that point I used the (well known) method of sighting the scope from the front to check for a concentric pattern of circles & reflections. This has the potential to be a time consuming and frustrating process... care is needed each time the tube is reassembled if the effect of resetting the rear cell is not to be confounded by some other variation.
However, once I was happy with alignment as gauged by the "circles" test, the telescope required only a minor adjustment of the secondary mirror to attain excellent collimation. The difference between the scope performance before and after realignment of the rear cell was very significant. Pin sharp stars, surface features on Mars etc. were all now within easy reach.
Posted 13 November 2012 - 01:20 PM
Posted 13 November 2012 - 02:51 PM
In the case of my scope, a previous owner had obviously removed the rear cell It was loose on the main tube, leading to a tilted, and tilting primary, which made collimation impossible and images astigmatic.
I had a phone call this morning with the OP and suggested this exact same thing... That the rear cell itself may be tilted very slightly.
He is going to center the corrector the secondary hole in the corrector cell, remove the secondary, put a piece of paper over the hole, then put the alignment laser into visual back to see if it strickes the center of the hole. or if it misses.
It may be possible to loosen the screws on the rear cell and tilt the entire rear cell in much the same way that refractor focuser tubes can be sometimes tilted slightly by loosening the screws holding the focuser housing. In this case, the entire rear mirror cell might have to be very slightly tilted.
Waiting to hear the result.
We discussed several other things to do as well to diagonose the problem, but I remain confident that this is an axial alignment problem of some sort...
Posted 13 November 2012 - 06:08 PM
The secondary should be positioned properly in the corrector if still in the original factory configuration. If that has been altered it needs to be fixed. A secondary decentered from the plate will result in on-axis coma. To 'collimate' a scope in that condition will result in other aberrations. The centering of the secondary in the corrector plate is of paramount importance, even more so than centering the plate over the primary.
If the primary is not tilted then you may find the best placement for the corrector plate is not necessarilly concentric to the scope tube. It could be off as much as 1-2 mm. A decentered plate will introduce astigmatism as well as a mixture of some other components.
Let me know (you have my email) if you need any notes or procedures or additional information concerning the above.
Posted 14 November 2012 - 09:12 AM
If you do have any run out (mine was about 2mm), then loosen the nuts/screws that hold the rear casting to the OTA and use a piece of wood and mallet to gently tap the tube in the direction required. You will probably find the tube is seated hard back against the rear casting, so the initial adjustments are going to be tapping the front casting forwards away from the rear casting.
My guess is that Celestron has a fixture that holds the tube and baffle alignment when the retaining screws are tightened so that the baffle is parallel to the tube wall.
And the 2mm figure is interesting because this is what I mentioned on our phone call... A tiny amount of tilt of the baffle could put you outside of the diffraction limited field of the mirrors.
Looking forward to hearing how it goes. Please keep us updated.
Posted 14 November 2012 - 04:17 PM
Posted 14 November 2012 - 06:03 PM
Posted 14 November 2012 - 06:05 PM
Posted 18 November 2012 - 08:57 AM
If you get the time, perhaps you could augment your basic collimation writeup with these other, troubleshooting/mechanical adjusting, steps so that we have a comprehensive reference.
The above was very interesting. I don't think I have a problem with my scope, but I will run through the tests and procedures to verify secondary and baffle tube alignment.
Posted 18 November 2012 - 10:55 AM
Posted 20 November 2012 - 11:09 AM
"Commercially made SCTs all have slight optical and mechanical alignment errors. The error typically is in the centering of the secondary mirror. As the result, when a laser is installed and emitted from the focuser, the beam will seldom return dead center, even though star testing would indicate perfect collimation. By default, the laser will have to be position in the optical center of the SCT. Also, the convex secondary mirror expands the fine laser dot to a larger spot, and magnifies any axial error by about 5 times. The error of deviation for a typical well collimated SCT will return laser beam at around 0.125" to 0.250" off center. In order to compensate the optical and mechanical alignment errors of SCT, a different collimation approach is required in compare with Newtonain telescope. If the procedure is followed through correctly, users can still accurately collimate their SCT."
If you apply this process and your secondary returns beam at greater than 0.125" to 0.250" off center it needs to be centered. The SCA collimator can assist you in making those correctors.
You should get a return that looks something like this: