49 replies to this topic

[stargeeser]

Is there a way to tell if a corrector plate was installed backwards (by the company) on a sct scope and if yes is there a way to take off the secondary mount and install it the other way? If yes, is it easy to realign the secondary to center.

 

[BWAZ]

Check if there is a serial number carved which should be facing the primary mirror.

 

As a side note IIRC it makes little difference even if the corrector is backwards.

[yukosteel]

I was centering secondary today on 8SE It's not easy at all. The corrector moves to about 1-2mm, and secondary frame as well.

I've spent a lot of time moving them and looking at reflections from different distances until all rings appear concentric.

Edited by yukosteel, 02 June 2017 - 12:27 AM.

[gnowellsct]

My scope optics don't have a serial number (newer chinese).  My much older C14 does. My old c8 did.  GN 

[Max Lattanzi]

Check if there is a serial number carved which should be facing the primary mirror.

 

Are you sure about it?

 

I have evidences of the opposite -- serial on the plate at 3 o'clock (you facing the plate itself) and serial facing outside (i.e. me, not the main mirror).

 

Others?

[DAVIDG]

 It does not make any optical difference what side of the plate faces the sky. If you want, take a small optical flat and place around the 70 to 86% of the plate and the side that has a Schmidt curve will show strong fringes at the position while the "flat side will show basically straight ones. 

 

                 - Dave 

[SandyHouTex]

 It does not make any optical difference what side of the plate faces the sky. If you want, take a small optical flat and place around the 70 to 86% of the plate and the side that has a Schmidt curve will show strong fringes at the position while the "flat side will show basically straight ones. 

 

                 - Dave 

I'm pretty sure DavidG is right.  At one time I was thinking of making a Wright telescope, and when it came to the corrector, you could put all the figuring on the front, or the back, or split it between the two, with no change to the ray trace results.

[Steve OK]

 

Check if there is a serial number carved which should be facing the primary mirror.

 

Are you sure about it?

 

I have evidences of the opposite -- serial on the plate at 3 o'clock (you facing the plate itself) and serial facing outside (i.e. me, not the main mirror).

 

Others?

 

On my C11, the serial number (at the 3 o'clock position) faces outward.

 

Steve

[NMBob]

Are you sure you can't see traces of any kind of mark left by the gasket material, or the glass rubbing directly on the front ring (had one of those once), on one side (inside), and no marks on the other side (outside)? There's the ring of gasket that it sits on, and then there are two breaks in that ring about 2" long that may be visible on the inside surface.

 

I've never removed the secondary. Is one side of the hole more likely to be scratched from the secondary holder being screwed together -- if that's how it works -- than the other side?

[MKV]

 It does not make any optical difference what side of the plate faces the sky. 

Spot on, Dave. 

[GlennLeDrew]

To add to the fact of the unimportance of the plate's facing direction, it can even be tilted a couple of degrees to little detriment to the image. (The secondary mirror, naturally, must not be tilted so. )

[yukosteel]

I've moved out 8SE corrector yesterday, and didn't locate any serial number on it.

I also think it doesn't make any difference because the glass is flat and coating looks same on both sides.

 

The position of the corrector and the secondary making difference though. When they are both shifted to the side, the secondary collimation is more tough.

But that's just my personal experience.

Edited by yukosteel, 02 June 2017 - 01:30 PM.

[NMBob]

The corrector is not flat. Someone had a broken corrector from a C14 and it went from .220 to .222 and back to .220" over the diameter (or something like that). They are not flat. I don't know if the curves are on both sides or just one side.

 

Yes, if the secondary does not get back into the center with respect to the primary mirror it can cause trouble. There are sometimes shims around the edge to get the corrector back in the middle, and I think some scopes now also have screws around the edges to get and keep the corrector (and secondary mirror) in the middle.

[MKV]

To add to the fact of the unimportance of the plate's facing direction, it can even be tilted a couple of degrees to little detriment to the image. (The secondary mirror, naturally, must not be tilted so. )

In fact, it should be tilted a little to avoid ghosting.

 

Mladen

[yukosteel]

The corrector is not flat. Someone had a broken corrector from a C14 and it went from .220 to .222 and back to .220" over the diameter (or something like that). They are not flat. I don't know if the curves are on both sides or just one side.

 

Yes, if the secondary does not get back into the center with respect to the primary mirror it can cause trouble. There are sometimes shims around the edge to get the corrector back in the middle, and I think some scopes now also have screws around the edges to get and keep the corrector (and secondary mirror) in the middle.

Thanks for this important point! I didn't know the corrector surface is aspheric, the coating reflections look quite straight on C8, but it makes sense with such minimal thickness difference so hard to notice.

[yukosteel]

Just found some details on Celestron site. They tell a lot about expected marks and positions. But they don't say anything about few mm play of corrector and secondary frame.

 

I didn't know the radial position of corrector is important, will probably need to make some tests and re-alignment now : )

 

http://www.celestron...econdary-mirror

[MKV]

The corrector is not flat. Someone had a broken corrector from a C14 and it went from .220 to .222 and back to .220" over the diameter (or something like that). They are not flat. I don't know if the curves are on both sides or just one side.

 

Yes, if the secondary does not get back into the center with respect to the primary mirror it can cause trouble. There are sometimes shims around the edge to get the corrector back in the middle, and I think some scopes now also have screws around the edges to get and keep the corrector (and secondary mirror) in the middle.

Actually, for an 8-inch f/2 mirror the corrector is about 20-23 μm deep at its deepest. That's about 0.0008". 

 

Mladen

[smeyer8015]

Flat side facing out. Curved side in.

 

https://www.google.c...496522375256049

 

 

Scott

[yukosteel]

Found good article with picture samples and factory marks exposed on C8

http://www.astronomy...aryremoval.html

 

also great article about optics with a lot of details about corrector plates at the end

http://www.faculty.v...s2/optics2.html

Edited by yukosteel, 02 June 2017 - 04:05 PM.

[NMBob]

 

The corrector is not flat. Someone had a broken corrector from a C14 and it went from .220 to .222 and back to .220" over the diameter (or something like that). They are not flat. I don't know if the curves are on both sides or just one side.

 

Yes, if the secondary does not get back into the center with respect to the primary mirror it can cause trouble. There are sometimes shims around the edge to get the corrector back in the middle, and I think some scopes now also have screws around the edges to get and keep the corrector (and secondary mirror) in the middle.

Thanks for this important point! I didn't know the corrector surface is aspheric, the coating reflections look quite straight on C8, but it makes sense with such minimal thickness difference so hard to notice.

 

It's such a small amount. I don't know how you can tell. I guess with an optical flat? I don't think they have those at Walmart. I've looked at both sides of several of mine when I had them out and I guess the only way to tell is to just not take the secondary off.

 

Nice find in your post #19.

 

Bob

[yukosteel]

Just went through awesome 2014 thread about correctors on CN :
https://www.cloudyni...ackwards/page-2
 
Just to summarize, I'll quote what TS Michael Miles wrote in #39 there:

Hey guys:

Great discussion - theoretical and otherwise.  Here are my takeaways:

1) Turning the corrector around did seem to have a noticeable effect.
2) I'll check to see which side is the flat side of the corrector
3) I'll mount it as originally mounted in the original tube and make sure it's centered

If I still have the problems,  I'll try turning it around.

More as the experiment develops,

Michael

It looks most important to set corrector in proper radial position, and properly center the secondary, while corrector aspheric side direction is the least important.

[PETER DREW]

During the 1980's I had an agency for both Celestron and Meade 8" SCT's. The early Celestron corrector plates were relatively rough cut discs which were centred by wedges of cork, Meade correctors were better edge ground and Celestron soon followed suit. Each telescope was optical bench tested at 500x prior to delivery and those that fell short of acceptance were usually brought up to muster by carefully adjusting the radial orientation of the corrector, + or - 5 degrees often made a significant improvement. Current models seem to be uniformly of high quality, I have a 8se which I can't fault optically in any respect, the telescope is limited only by design, aperture and seeing. There is still an ongoing debate as to whether SCT optics are individual sets or can be mixed successfully. 

[freestar8n]

If you assume everything is made perfectly and symmetric then reversing makes no difference. But if you look at the actual wavefront of the complete system, or even the Airy patterns of actual stars, you can see things like trefoil astigmatism. This is because each surface in the system isn't perfectly rotationally symmetric - and it's also why there are alignment marks.

The Schmidt corrector won't be rotationally symmetric and it also won't be perfectly symmetric in each half of the axis specified by the alignment mark. So when you flip it the wavefront will either be slightly worse or slightly better.

As usual, the answer based on ray traces and the assumption of perfect optics is very different from what would happen with a real system. Ray traces in Oslo don't have alignment marks.

My guess is that if you flipped it, the optimal orientation would be different and you would need to try different angles to see which is best.

Frank

[yukosteel]

Found 2 marks in different places of 8SE corrector border

[freestar8n]

My guess is that flipping it would not make much difference. But if there is a preferred angle for the corrector, then it is probably not the same when flipped - because the asymmetry along the axis of that angle would reverse. If the angle isn't critical in the first place, or if the two halves are very symmetrical - it wouldn't matter much.

But it's hard to say because when you see the final wavefront - it contains beneficial cancellation of errors in all the surfaces. You don't get a direct view of how big those errors are.

Frank