OMG! What have I done? Have I done anything?

[Stargazer33]

I am currently thinking about getting some thumb screws to replace the secondary collimation screws on my Celestron C8 SGT (XLT). I have also noticed a large amount of dust/swarf on my secondary (reflected in the primary), when I look down the OTA from the corrector plate. So I thought I would remove my secondary - I have the Fastar system with the removable secondary - check what size collimation screws I have and clean the secondary with my bulb blower.

All seemed to go okay until I came to screw the secondary back in and I realised that I would have to remove the corrector plate in order to hold the inner collar/shield for the secondary.

This in itself didn't go badly until everything was back together and then I thought: 'did I flip the corrector plate when I re-secured the secondary?'

So my question is - does it matter if the corrector plate is flipped outside for inside? As it is called a corrector plate, I would have thought it was either convex or concave in shape which would mean it is vital that it is replaced in the same way.

I am pretty sure that I did put it back in the same way, but you know what happens when doubt creeps in!

[Ags]

You also need to be sure you put it back in with the same rotational orientation as before - so the bit that was at 12 o'clock when you took it out is still at 12 o'clock when you put it back in. I recall that the SCT elements are oriented to minimize aberrations, so you have to be careful to preserve the orientation.

[Cath]

I would imagine it makes a great deal of difference which way up it goes - if the corrector plate is not equally curved on both sides.

I heard it also needs to be fitted back with the same rotation angle due to it being matched with the primary mirror at manufacture. I've also heard that that's not right as manufactures don't bother matching up optics (as previously thought).

[Stargazer33]

Hi Ags,

I have been on the Celestron website and read a reply to someones question that the pin on their Fastar system wasn't in the 3 O'clock position as it should be but was in the 11 O'clock position and was this a problem/fault. Celestron replied that it was the case with the original Fastar system but not with the latest. Is this what you mean or is this another thing I have to worry about?

[Stargazer33]

Hi Cath,

I have absolutely no idea what the orientation of the corrector was! I was looking for ages before I removed the Fastar system for information on how/what to do both on the Celestron site and generally on the internet. I found no information at all on Celestrons site and very little information on the internet generally, so (foolishly in hindsight) I decided to just go ahead!

Oh hell!

[Peter Drew]

I'd try it first off, I'm pretty sure there will be problems if it is the wrong way round. If you remove it again, check the corrector edge for any indication of correct assembly, the difference between the shape of the front and rear surfaces is too small to judge by eye. Winess marks on the coating like dewspots or cleaning sleeks are often a clue to which surface is the outer.

[Pig]

When I read "thumb screws" I thought you were getting them to torture some one --- pheew

[Stargazer33]

Thanks Peter, I will try that. No chance of getting out at the moment - complete cloud cover. I may try setting the scope up indoors later and see if I can focus on something at the other end of the hall. I have managed this before so that will be a good indicator.

I hope there is a witness mark on the corrector for me to realign to.

The stupid thing is I work in a repair/service workshop and I often put witness marks on things before I disassemble them! Why didn't I do it with this? Unfamiliarity I guess!

[Stargazer33]

When I read "thumb screws" I thought you were getting them to torture some one --- pheew

Only myself if I have messed up my OTA!

[Stargazer33]

I've been thinking over dinner about this problem.

Surely if the secondary holder is designed to be taken off to make the scope 'fast', then if you want to put it back to an f10 scope, for observing say, you would have to remove the corrector plate in order to screw the secondary back in. Therefore, there should be a witness/index mark placed by the OEM in order for the user to realign their corrector plate properly. If there isn't then it is a serious oversight by the manufacturer as the only way you can screw the secondary holder back in is to remove the corrector plate.

I am now going to go away and see if I can find anything on the corrector plate and its housing.

[Stargazer33]

Well, it's half good news anyway! I set my scope up indoors and focused on our front door blind from about 32 feet away and I could focus perfectly seeing every strand of the weave in the material.

Bad news is that apart from four instances of what looks like 'A2' or possibly 'AV' scratched into the edge of the corrector plate glass there are no alignment marks at all. I did wonder if the 'A2' or 'AV' marks lined up with the securing screw holes, but they don't. As there are only four and there are six screws I wouldn't know which ones they lined up with anyway.

So it looks like I am going to have to write to Celestron and see if I need to return this for realignment!

[si@nite]

DONT PANIC! Ive removed my corrector plate a few times on my c9.25 to remove debris & smears from inside the ota. i marked mine with masking tape before removing, but dont think its critical, i also have faststar & my location peg was at 6 o clock. I think its more importent not to over tighten the correcror plate retaining ring screws. maybe im lucky but collimation was still good, i too was worried but i dont think they seem to be that precise at manufacture.

[Stargazer33]

Hi Simon,

Thanks for the reassurance!

I have pretty much come to the conclusion that either the primary and corrector plate are so well made now that it doesn't matter what the orientation of the corrector plate is. Or probably a more realistic conclusion is that the OEM can live with the aberrations caused by the miss matched elements in the optical train.

I must say that I haven't seen a location pin on my secondary holder, not that I looked for one! As I mentioned above, I read a question on the Celestron site that inquired about the position of the 'pin'. I'm not going to take mine apart again to look for it, unless I absolutely have to.

I do think it is really bad that there is nothing in the latest online manual for the C8 regarding the removable secondary/Fastar assy.

I am currently waiting for a reply from Celestron as to whether I have messed up my optics or not. Fingers crossed.

[spaceboy]

I would try not to worry yourself too much otherwise the problem you might have now is you will not relax and enjoy the views but more so try to pick out any little fault in them putting it down the the corrector being misaligned. These faults may or may not have been present originally but now you are looking for them you will notice any little thing. I would say unless the views are obviously worse, to just leave things alone otherwise you could risk causing further problems.

[Stargazer33]

Thanks spaceboy, I can be very critical at times!

My wife and I were out last night and when we got home i noticed what a lovely clear sky it was - very cold with snow still on the ground - but clear. As it was 22:00 it was a bit late to start setting up the mount & scope for a test of the optics. Hopefully I will be able to get out Wednesday night - I believe it is supposed to be getting slightly warmer then also.

Thanks to everyone for their advice and support. I am finally starting to calm down and think that as there is no witness/alignment mark then the plate doesn't have an exact position that it needs to be put back to every time it is removed. I am still waiting to hear back from Celestron but hopefully they will confirm my suspicion.

[Stargazer33]

I finally got a reply from Celestron today as follows:

Hello Bryan,

Thank you for contacting Celestron Technical Services.

If it's a new black or orange tube, it does not need to be aligned. The optics do not have marks.

Thank you

Wil

Celestron Technical Services

So that is a result. Obviously the new scopes don't have matched optics.