SCT corrector plate replacement

[Westmoorland]

I posted not long ago about a crack which appeared in my C9.25 corrector lens/plate. I made inquiries and I can purchase a replacement from Celestron UK for around £170.  However, I can't find anyone who'll fit it: Rother Valley optics say I need the original box for shipping (which I haven't got) and the widescreen Centre say they don't perform repairs of this kind. In light of this I wondered if this is something a novice can do by themselves. I mean, I can screw it in okay but my understanding from recent research is that it has to be rotated in place at the factory in order to provide optimal performance with the mirrors. Yesterdays email from Celestron UK tech supports 'engineering team' said this:

''When new telescopes are assembled or any repair scopes with new correctors installed go through the matching process. The hand figuring is very minor but does optimize the optics for the best correction of spherical aberration.''

I obviously want optimal performance but if I can avoid extra cost and the hassle of postage then I would try it myself. Does anyone have any experience with this they can help me with?

 

[Steve Ward]

Try Steve Collingwood who's now working with Gary Walker at Pulsar Observatories in Downham Market , there's not many things he cannot sort out telescope-wise and he's a thoroughly pleasant chap as well ... 😉

https://www.pulsarastro.com/about-us-1-w.asp

[RobertI]

If you need convincing about Steve’s credentials:

http://sctelescopes.com/sc-telescopes-broadhurst-clarkson-and-fuller-fullerscopes/

[newbie alert]

54 minutes ago, RobertI said:

If you need convincing about Steve’s credentials:

http://sctelescopes.com/sc-telescopes-broadhurst-clarkson-and-fuller-fullerscopes/

I'm not so sure that Steve is still running SCTelescopes, hope that he is

[Peter Drew]

This procedure is something I've done many times in the past when I had Celestron and Meade agencies for SCT's.  It's fairly straightforward but needs patience and benefits greatly if an optical bench is available.  If not, an artificial star is the next best option.  I have considerable doubt that Celestron hand figure the corrector plates, they are too weak in optical power to make it suitable for hand figuring.  My understanding is that if there is any hand work applied it would be on the small secondary, as a result it would be this that was matched to the primary, the corrector acting as a carrier and turned radially to optimise the two mirrors.  Once the position is found it would be marked relative to the corrector and the corrector relative to the tube.  Replacing a corrector with fitted secondary after cleaning, the importance of the original position of the corrector ensures the correct position of the secondary relative to the primary.🙂

[davidc135]

I suggest that you could try to install the corrector yourself, going slowly and carefully and following advise from Peter Drew and others on this forum. You will need an artificial star or a ball bearing and torch that acts as such and a reasonable distance between 'star' and scope. Doesn't have to be huge for the purpose of collimation.

It may be that rotation to find the optimal orientation for corrector and 2ndry won't be needed and even if it is it's not a big problem. Just takes time with recollimation after each clocking.

If it doesn't work out you still have the option of posting it off to the professionals.

If you do decide to go ahead I'm sure there will be many SGL members who will be interested in following progress.

David

[Westmoorland]

Thanks all. Rother Valley Optics seem like the only people enthusiastic about helping me. There's too much scope for further cock-ups if I attempt it myself, but I am intrigued by artificial star tests etc.

[Westmoorland]

23 hours ago, Steve Ward said:

Try Steve Collingwood who's now working with Gary Walker at Pulsar Observatories in Downham Market , there's not many things he cannot sort out telescope-wise and he's a thoroughly pleasant chap as well ... 😉

https://www.pulsarastro.com/about-us-1-w.asp

He looks familiar. I wonder if he had anything to do with the scope shop in Southery (just south of Downham Market) where I used to live. I bought a SW Mak 127 there in 2013.

[Steve Ward]

4 hours ago, Westmoorland said:

He looks familiar. I wonder if he had anything to do with the scope shop in Southery (just south of Downham Market) where I used to live. I bought a SW Mak 127 there in 2013.

No , that was David Tatton , who used to work at the Astronomy and Nature Centre in Witchford just outside Ely , with Robert Dalby and the sadly no longer with us Ralph Bell .

Gary Walker , the other director of Pulsar was also based there at one time.

Edited October 21, 2022 by Steve Ward

[Westmoorland]

8 hours ago, Steve Ward said:

No , that was David Tatton , who used to work at the Astronomy and Nature Centre in Witchford just outside Ely , with Robert Dalby and the sadly no longer with us Ralph Bell .

Gary Walker , the other director of Pulsar was also based there at one time.

Thanks for clearing that up. I'll message pulsar optical today.

[Gabby76]

If I remember correctly, Celestron marks the corrector and secondary at the 3 o'clock position to help keep things aligned during replacement.

It is not a hard job just requires a bit of patience and there are many good tutorials online walk you through it if you want to give it a try.  

[Westmoorland]

Are there? I must have overlooked them. I'll wait to see if Pulsar Optical get back today.

[Carl Au]

You could try Orion Optics UK. They say they will service any scope, any brand 

[Gabby76]

This writeup is from Michael Swanson who was a beta tester for Celestron for many years

https://www.nexstarsite.com/OddsNEnds/SCTCorrectorRemoval.htm

Another quick one from Don Pensack of Eyepieceetc

https://www.cloudynights.com/topic/81066-how-to-remove-and-clean-a-corrector-plate/

[Shimrod]

2 hours ago, Gabby76 said:

This writeup is from Michael Swanson who was a beta tester for Celestron for many years

https://www.nexstarsite.com/OddsNEnds/SCTCorrectorRemoval.htm

Another quick one from Don Pensack of Eyepieceetc

https://www.cloudynights.com/topic/81066-how-to-remove-and-clean-a-corrector-plate/

These are guides for removing and putting back a corrector place. @Westmoorland needs a guide for replacing a broken corrector plate with a new one. 

[Westmoorland]

1 hour ago, Shimrod said:

These are guides for removing and putting back a corrector place. @Westmoorland needs a guide for replacing a broken corrector plate with a new one. 

Thanks. Trouble is when I get a new 9.25 plate it won't have marking so I will have no idea which way to orient it. 

[Westmoorland]

On 20/10/2022 at 20:38, RobertI said:

If you need convincing about Steve’s credentials:

http://sctelescopes.com/sc-telescopes-broadhurst-clarkson-and-fuller-fullerscopes/

He tersely stated he no longer works on scopes. 

[RobertI]

11 minutes ago, Westmoorland said:

He tersely stated he no longer works on scopes. 

Oh dear, sorry about that. 😟Mind you, I didn’t promise he would be polite!

[Westmoorland]

2 minutes ago, RobertI said:

Oh dear, sorry about that. 😟Mind you, I didn’t promise he would be polite!

Well, not rude, but a little abrupt. Mind you he is a/an FRAS, though.

[Peter Drew]

I think the key is to mark the current position of the secondary in relation to the primary and also its centricity to the tube.  If these dimensions are retained following replacement of the new corrector all should be well.    🙂

[davidc135]

7 minutes ago, Westmoorland said:

Thanks. Trouble is when I get a new 9.25 plate it won't have marking so I will have no idea which way to orient it. 

It's not a big problem. Install the corrector randomly and collimate with the artificial star eg Hubble £19.00 from FLO. at a suitable distance. If the at/near focus image at high power shows some astigmatism or irregularity at best collimation the (now marked) plate can be rotated in 20 degree steps and the process repeated.

If there are errors on the corrector/2ndry combo and also on the primary there will be two positions on the clock face at 180 degrees where the errors most add and another two 90 degrees to the above where they best subtract. If there are no errors on the primary then the astig just rotates.

It's potentially possible that the corrector and 2ndry need to be rotated relative to each other but it's also very possible that the scope will be fine straight away.

I imagine that it will cost around £200 inc postage(?) to have someone further afield do this plus the uncertainty of carriage.

Removing the secondary assembly from your plate will show you how it works.

It can be a fiddle so best done indoors or at least not under the stars.

David

[Westmoorland]

2 hours ago, davidc135 said:

It's not a big problem. Install the corrector randomly and collimate with the artificial star eg Hubble £19.00 from FLO. at a suitable distance. If the at/near focus image at high power shows some astigmatism or irregularity at best collimation the (now marked) plate can be rotated in 20 degree steps and the process repeated.

If there are errors on the corrector/2ndry combo and also on the primary there will be two positions on the clock face at 180 degrees where the errors most add and another two 90 degrees to the above where they best subtract. If there are no errors on the primary then the astig just rotates.

It's potentially possible that the corrector and 2ndry need to be rotated relative to each other but it's also very possible that the scope will be fine straight away.

I imagine that it will cost around £200 inc postage(?) to have someone further afield do this plus the uncertainty of carriage.

Removing the secondary assembly from your plate will show you how it works.

It can be a fiddle so best done indoors or at least not under the stars.

David

Great info. I'd rather keep the cost to about £170ish for just the price of the new plate and not have to post it away. I'm not sure I know how to identify astigmatism but any more advice will be appreciated as I'm resolved to sort it myself.

[davidc135]

8 hours ago, Westmoorland said:

Great info. I'd rather keep the cost to about £170ish for just the price of the new plate and not have to post it away. I'm not sure I know how to identify astigmatism but any more advice will be appreciated as I'm resolved to sort it myself.

At high power you'll be looking for an Airy disc surrounded by circular diffraction rings at focus when collimated. Mild astigmatism shows as thickening of the diffraction rings at the four points of a cross at focus and slightly oval out of focus star images. Severe astig destroys the Airy disc which is replaced by a cross at focus. The defocused ovals whose major axes are at right angles to each other inside and outside of focus are very obvious.

Do you have some room in a garden to set up the artificial star? Twenty yards will do or less at a pinch. This short distance will cause some over-correction but that shouldn't affect collimation. It's a good idea to get the Hubble artificial star or similar and practise with the existing scope and cracked corrector.  By tilting the 2ndry see what gross mis-collimation looks like and how it is corrected.

The procedures can be set out step by step.

David

Edited October 25, 2022 by davidc135

[Westmoorland]

3 hours ago, davidc135 said:

At high power you'll be looking for an Airy disc surrounded by circular diffraction rings at focus when collimated. Mild astigmatism shows as thickening of the diffraction rings at the four points of a cross at focus and slightly oval out of focus star images. Severe astig destroys the Airy disc which is replaced by a cross at focus. The defocused ovals whose major axes are at right angles to each other inside and outside of focus are very obvious.

Do you have some room in a garden to set up the artificial star? Twenty yards will do or less at a pinch. This short distance will cause some over-correction but that shouldn't affect collimation. It's a good idea to get the Hubble artificial star or similar and practise with the existing scope and cracked corrector.  By tilting the 2ndry see what gross mis-collimation looks like and how it is corrected.

The procedures can be set out step by step.

David

I'll order the artificial star today, If you're quite sure it's what I need. I thought spherical aberration was what I'd be trying to correct in this case?

[Westmoorland]

On 25/10/2022 at 07:43, davidc135 said:

At high power you'll be looking for an Airy disc surrounded by circular diffraction rings at focus when collimated. Mild astigmatism shows as thickening of the diffraction rings at the four points of a cross at focus and slightly oval out of focus star images. Severe astig destroys the Airy disc which is replaced by a cross at focus. The defocused ovals whose major axes are at right angles to each other inside and outside of focus are very obvious.

Do you have some room in a garden to set up the artificial star? Twenty yards will do or less at a pinch. This short distance will cause some over-correction but that shouldn't affect collimation. It's a good idea to get the Hubble artificial star or similar and practise with the existing scope and cracked corrector.  By tilting the 2ndry see what gross mis-collimation looks like and how it is corrected.

The procedures can be set out step by step.

David

I'll order the artificial star today, If you're quite sure it's what I need. I thought spherical aberration was what I'd be trying to correct in this case?