9 replies to this topic

[finiteguy]

Now that I have disassembled my C5 primary and set to have them recoated, when I re assemble, I’m wondering how I center the corrector with respect to the primary. I recall a procedure that I found here by putting thread cross hairs on the corrector mounting ring to center the secondary, but not sure about centering the corrector itself. I assume it’s important since the aspheric corrector needs to match the primary.

[markb]

It's important because the SCT design extremely sensitive to coincidence of the optical and mechanical axes. 

 

This is one of the reasons Meade epoxies the rear cell on (hopefully square- but not always), and Celestron has those dire warnings about ever loosening or removing the securing screws for the rear cell. And perhaps the only reason.

 

 

The primary is fixed, hopefully square to the sliding assembly, so the baffle tube sets both the mechanical axis and the optical axis to which the secondary must be aligned. 

 

The key here is the secondary, not the corrector. Secondaries are often glued a bit off center on the aluminum puck, and that eccentricity is sometimes used by the factory to aid the optical alignment. Even though it's spherical, or close to it, it can't just be placed anywhere, since the pivot point for the collimation is so far from the center of curvature. A misaligned secondary that is best collimated can either give soft images or, worst case, horribly comatic winged stars. I've had both, and resolved both.

 

People often can center the corrector and have everything work okay but it depends on whether you have a ideally positioned secondary on the puck, or yours is off center. I have a Meade where the rear cell was expoxied on crooked, and the secondary mirror was glued eccentrically and ended up ended up 8 or 9 mm from the center of the cell, and the corrector was flush to one side of the cell.

 

My used 11 was cleaned and badly reassembled  and was soft even when collimated (not disclosed by seller!). After secondary alignment it's very sharp. The corrector in the 11 is one or two millimeters off center, but the secondary mirror is spot on, as is performance.

 

 

Usually, on a cleaning, there should be 'correct reading' etched numbers -this can expose a corrector flip- at the right side of the corrector looking down the tube, and, hopefully, you have not disassembled the secondary. If you have there is a black sharpie stripe on the side of the glass secondary that should be aligned to that point as well. If it's a newer one with the corrector placed by the factory on a sticky rubber ring, the original eccentricity of the placement has to be duplicated on reassembly.

 

If you hadn't disassembled the secondary housing, and usually people don't, the exterior writing should read write side up when the tube is level with the exterior writing also reading right side up. That should work if folks have the cork spacers. I don't know about the C5 but later larger ones have sticky rubber that holds the corrector plate in the position that the factory placed it, so unless a record was made  that owner would need to perform a realignment or send it back.

 

 

 

Since you did disassemble everything to get the mirrors recoated, I would align the sharpie stripe and assemble the secondary housing so the writing is in that position. It makes it easier to fix if you accidentally rotate it later.

 

I've tried the optical crosshairs down the baffle tube from the back and found it very difficult (for me, completely useless). It might work if you use one of the camera alignment tools that I think they use one newtonians. 

 

 

 it can be done with a crosshair projection or concentric circle projection laser placed in the visual back, but it gets tricky to spot the laser image on the secondary. The crosshair or circles have to be completely centered on the secondary mirror. The properly collimated laser is of course essential.

 

 

Assuming the primary is square to its mount on the slider, the laser should strike the center of the secondary.

 

Ideally the corrector would be centered first and the secondary housing rotated to see if alignment could be reached. If it can't, the corrector may have to be moved a bit off center so the secondary is centered.

 

I have read from trusted sources that corrector centering is apparently not critical in the design, but I'm not the optical expert by any means. But my Meade the secondary off center from the corrector and, once the secondary was aligned to the centerline of the baffle, and primary, performance was excellent. I now assume my trusted sources were correct. 

 

I did a few rounds of this and got both the meade and the 11 properly aligned. 

 

My Meade underwent this process and is a very nice scope indeed even though the secondary is offset from the center of the corrector and the corrector is offset to the edge of the cell. 

 

 

Because it's so hard to spot the laser strike on the secondary I'm going to fine tune and finalize both by slipping a 3d printed white mask over the secondary, and check my original laser alignment of the secondary.

 

A hollow cylindrical (a round target with a short right angle rim) paper mask with the center exactly marked could be made and then carefully fitted over the secondary mirror to the same effect.

 

The corrector could be similarly centered with a simple paper circle in the perforation, but that would only work if the secondary mirror is perfectly centered in the housing, which requires that the screws be perfectly placed and the mirror be perfectly glued to be puck.  For a center of manufacturing tolerances secondary this might work, but I would still check the secondary mirror centering.

 

 

Going back a bit to collimation, not the totally separate instant task of optical and mechanical axes coincidence alignment,  ordinary laser collimators simply don't work. So this is completely unrelated to that issue (Kendrick had a really interesting 2-in thread on collimator with a grid for matching the original, collimated, off center dot return, which pretty much shows that a standard laser collimator won't work- after I started collimating using the slightly defocused diffraction ring concentricity method, fast and accurate before Airy disk refinement, I haven't bothered collimating any other way).

 

 

The secondary centering is not difficult, but it is fussy and will require a properly collimated laser, ideally with a holographic projection.

[davidc135]

Now that I have disassembled my C5 primary and set to have them recoated, when I re assemble, I’m wondering how I center the corrector with respect to the primary. I recall a procedure that I found here by putting thread cross hairs on the corrector mounting ring to center the secondary, but not sure about centering the corrector itself. I assume it’s important since the aspheric corrector needs to match the primary.

This question was extensively looked at in 'Schmidt corrector plates, to end one of those myths' OP; Shed9, started Jan 22, '25 in this forum.

 

The answer is that it doesn't matter whether the corrector plate or secondary is centered any more accurately than roughly with a ruler. i.e within 1 or 1.5mm as regards (an 8'') corrector plate. Tilting ie collimation compensates fully for any decentering of the secondary other than a little extra vignetting at one edge of the view.

 

Applies to classical scts, not ACF design.

 

It's possible that rotation of the corrector may affect performance.

 

David

Edited by davidc135, 16 March 2025 - 03:00 PM.

[KerryR]

Doesn't Celestron glue their SCT mirrors to the carrier with RTV compound? How did you release the mirror from the glue? Most coaters won't re-coat mounted mirrors. It used to be said that the Celestron primary mirrors were collimated on the mirror carrier in a jig and held in that state while the glue set, and that removing the mirror also removed collimation, and that some sort of collimation would have to be repeated when the mirror was returned and re-glued to the carrier.

 

My CPC11's primary is glued, I haven't had my C6 apart, so I don't know if that one is glued.

 

But, my Meade 8" isn't glued, nor are the mirrors in my 4 and 5" MCT's. I wonder what the difference is?

[davidmcgo]

When I redid my 1976 C5, I re attached the primary with a tight wrap of blue painters tape around the mirror carrier until I got a snug stable fit, then smeared a bit of silicone over that and the flat back part of the mirror, slid the mirror back on, and tightened the retaining ring.  Collimation and centering were fine.  It has been stable since, 5 or more years later.  It is a small light mirror, doesn’t need over thinking.

 

I did not redo the secondary, looked fine whereas my primary had deterioration at the edges.

 

Dave

Edited by davidmcgo, 18 March 2025 - 08:57 AM.

[sctbrd]

For centering the corrector itself, with the tube pointed up, run a straight solid object through the baffle tube and hold it to one side (if you can get something the same diameter, great). I used a wooden ruler for my C8.

Mount the corrector loosely and use the straight edge to center the inside hole all the way around.

(I tried a laser collimator, but mine was too loose in the EP holder and wasnt accurate enough).

Center the secondary on the corrector, by measuring at each 90 degree position to get the secondary aligned.

 

I did this for my C8 when I had very soft images and it sharpened my focus very noticeably.

 

I also used a level to ensure the corrector was parallel with the tube housing while pointed up by placing a couple rectangular blocks on the corrector on either side, and tightening the corrector screws in a torquing pattern to get it as close as I could. This was important for me as I use a Hyperstar, and tilt is a big deal. May or may not be as important in your case.

[KerryR]

When I redid my 1976 C5, I re attached the primary with a tight wrap of blue painters tape around the mirror carrier until I got a snug stable fit, then smeared a bit of silicone over that and the flat back part of the mirror, slid the mirror back on, and tightened the retaining ring.  Collimation and centering were fine.  It has been stable since, 5 or more years later.  It is a small light mirror, doesn’t need over thinking.

 

I did not redo the secondary, looked fine whereas my primary had deterioration at the edges.

 

Dave

Interestingly, My Meade 8" SCT, as well as the 4 and 5" MCTs use what appears to be masking tape on the carrier to soak up the slack between it and the hole in the primary. I think collimation of the mirrors must be done by precision machining of the back glass mounting surface and the front surface of the carrier. I've had the primaries off the carriers in all 3, on reassembly, I returned them to registration marks I'd placed on the parts, and all was well.

If memory serves, there 3 grub screws in the back of the carrier of my C(PC)11 that look like they'd bear on the back surface of the mirror base, probably for collimation before the RTV set. I didn't remove the primary from the carrier, so I don't know.

[finiteguy]

Doesn't Celestron glue their SCT mirrors to the carrier with RTV compound? How did you release the mirror from the glue? Most coaters won't re-coat mounted mirrors. It used to be said that the Celestron primary mirrors were collimated on the mirror carrier in a jig and held in that state while the glue set, and that removing the mirror also removed collimation, and that some sort of collimation would have to be repeated when the mirror was returned and re-glued to the carrier.

My CPC11's primary is glued, I haven't had my C6 apart, so I don't know if that one is glued.

But, my Meade 8" isn't glued, nor are the mirrors in my 4 and 5" MCT's. I wonder what the difference is?

[finiteguy]

The primary was glued with a white compound presumably RTV. I was able to remove the primary from the carrier. I’ll try to re align and replace the compound, but if it fails I suppose I will have to scrap the telescope.

[finiteguy]

Interestingly, My Meade 8" SCT, as well as the 4 and 5" MCTs use what appears to be masking tape on the carrier to soak up the slack between it and the hole in the primary. I think collimation of the mirrors must be done by precision machining of the back glass mounting surface and the front surface of the carrier. I've had the primaries off the carriers in all 3, on reassembly, I returned them to registration marks I'd placed on the parts, and all was well.

If memory serves, there 3 grub screws in the back of the carrier of my C(PC)11 that look like they'd bear on the back surface of the mirror base, probably for collimation before the RTV set. I didn't remove the primary from the carrier, so I don't know.