SCT corrector plates

[Bugdozer]

I saw a couple of videos which showed removing the SCT corrector plate to clean the primary mirror. In these, the person made a reference mark so they could put the corrector plate back exactly at the same rotation it was before, apparently because the plate is "matched" to the mirror in some way. 

I was curious about this so looked up about SCT manufacturing but couldn't find anything regarding any sort of rotational matching between the corrector plate and the primary mirror. Indeed, if the corrector plate and mirror have been made properly, they should have complete rotational symmetry, so however you put the plate back in shouldn't make a difference, should it? 

Can anyone explain this? 

[GrumpiusMaximus]

47 minutes ago, Bugdozer said:

I saw a couple of videos which showed removing the SCT corrector plate to clean the primary mirror. In these, the person made a reference mark so they could put the corrector plate back exactly at the same rotation it was before, apparently because the plate is "matched" to the mirror in some way. 

I was curious about this so looked up about SCT manufacturing but couldn't find anything regarding any sort of rotational matching between the corrector plate and the primary mirror. Indeed, if the corrector plate and mirror have been made properly, they should have complete rotational symmetry, so however you put the plate back in shouldn't make a difference, should it? 

Can anyone explain this? 

I've no idea what the manufacturing tolerances are but I can't see a reason not to re-align the plate to its initial position.  I recently took the corrector plate off my C5 to clean it and tighten up the secondary mirror housing (a story for another day) and was sure to put it back in the same place.  No harm done at all, just took a while to collimate it because I'm a ham-fisted twit.

Edited November 5, 2023 by GrumpiusMaximus

[Cosmic Geoff]

It is said that (depending on when and where they were manufactured) some SCT optics were individually figured so that it mattered how the corrector was put back when re-fitted.  If you don't know for sure whether your SCT is affected or not, it make sense to mark the corrector and put it back how it was...

On the other hand, it is apparently possible to purchase replacement corrector plates from a 3rd party manufacturer, which is clearly a case of crossing fingers and hoping it'll be OK.

Edited November 5, 2023 by Cosmic Geoff

[newbie alert]

There has been several posts,u tube videos etc to do with matching the orientation of the corrector, but I've also read of people that have had replacement correctors that obviously won't match having no noticeable issues

If taking yours out for cleaning purposes then there's no harm marking and replacing in the same position

It maybe a expensive bit of optical equipment but doesn't mean it's precision made, or not as precisely made as you would hope

[Bugdozer]

1 hour ago, Cosmic Geoff said:

It is said that (depending on when and where they were manufactured) some SCT optics were individually figured so that it mattered how the corrector was put back when re-fitted.  If you don't know for sure whether your SCT is affected or not, it make sense to mark the corrector and put it back how it was...

On the other hand, it is apparently possible to purchase replacement corrector plates from a 3rd party manufacturer, which is clearly a case of crossing fingers and hoping it'll be OK.

Exactly. If the corrector plate can be replaced, then there clearly cannot be any real value in matching the orientation. I have not seen any guidance from manufacturers on this either. I do wonder if this is just "telescope folklore".

[newbie alert]

39 minutes ago, Bugdozer said:

Exactly. If the corrector plate can be replaced, then there clearly cannot be any real value in matching the orientation. I have not seen any guidance from manufacturers on this either. I do wonder if this is just "telescope folklore".

Folklore or not if you have the option to mark it's orientation before removal then I'd do it as a failsafe option

If you have to replace the corrector then you no longer have that option

Chance it or not?

[Bugdozer]

Just now, newbie alert said:

Folklore or not if you have the option to mark it's orientation before removal then I'd do it as a failsafe option

If you have to replace the corrector then you no longer have that option

Chance it or not?

I agree that's a sensible option if it's possible to do it, my point is it may be a precaution against nothing. The aim of this thread was to try and establish if it is. So far, nobody has any information either way.

[Albir phil]

1 minute ago, newbie alert said:

Folklore or not if you have the option to mark it's orientation before removal then I'd do it as a failsafe option

If you have to replace the corrector then you no longer have that option

Chance it or not?

I agree mark it , refit it then it's as before, simple.😃

[Albir phil]

2 minutes ago, Bugdozer said:

I agree that's a sensible option if it's possible to do it, my point is it may be a precaution against nothing. The aim of this thread was to try and establish if it is. So far, nobody has any information either way.

I think the lack of information is probably because no one is willing to find out if it dose matter 

[newbie alert]

10 minutes ago, Bugdozer said:

I agree that's a sensible option if it's possible to do it, my point is it may be a precaution against nothing. The aim of this thread was to try and establish if it is. So far, nobody has any information either way.

There's plenty of information on this subject on here and cloudy nights

Peter Drew on here made a  decent statement that he doesn't think that Celestron hand figure and match correctors but goes on to say that a optical bench makes easier work to align the corrector, or failing that as it be for the most of us a artificial star

A quick Google brings these up

Edited November 5, 2023 by newbie alert
Spelling

[Peter Drew]

Has anyone asked Celestron directly ?     🙂

[Bugdozer]

30 minutes ago, newbie alert said:

There's plenty of information on this subject on here and cloudy nights

Peter Drew on here made a  decent statement that he doesn't think that Celestron hand figure and match correctors but goes on to say that a optical bench makes easier work to align the corrector, or failing that as it be for the most of us a artificial star

A quick Google brings these up

A quick Google didn't bring it up for me. And I got banned from Cloudy Nights before even making a post there. 

Since you seem to have found the information I was asking about and I can't, could I trouble you for a link? 

[MartianHill]

My Fastar C8 was a bit average optically and had no manufacturers markings on the corrector, so after reading a thread on CN I marked the corrector myself then rotated it 90’ and tested it and it improved.

I then rotated it another 90’ and the contrast and sharpness was excellent, so that’s how I left it.

A result, but don’t ask me why 🤷🏻

 

Edited November 5, 2023 by MartianHill

[davidc135]

In theory, no optical surface can be a perfect figure of revolution, all must be astigmatic even if of microscopic degree. Therefore, if two or more surfaces work together in an optical train there will always be a best and a worst orientation where astigmatic errors either subtract or add.

In practice, I doubt if you'd often see the difference. Especially so as manufacturing standards have improved. It might apply more to older and larger scts where the secodary is spherical. If it's the case that the Meade ACF series uses apheric 2ndries then there may be issues of decentration so I'd definitely mark those.

I can't give any links but I've come across posts where the poster claimed that rotation of the corrector plate solved a problem. One authoritative example comes to mind featuring a C14.  Edit see post above!

Most mentions however are hearsay and speculation.

When time allows I'll experiment with my decent 2003 example of a Meade 8''. I bet it will make no difference but I'll see.

David

PS Just read MartianHill's post

Edited November 5, 2023 by davidc135

[newbie alert]

https://stargazerslounge.com/topic/401578-sct-corrector-plate-replacement/#elControls_4304353_menu

[newbie alert]

Think I've managed the right link

[Bugdozer]

OK, that thread contains the answer. Celestron specifically state they adjust it until it's optimal. 

Since my scope is a Celestron, I will assume that keeping the same rotation will be optimal if I ever have to take it off.