Celestron C8 secondary not central to corrector plate

[Uplooker]

Hello,

As per the title my Celestron C8 secondary is not central to corrector plate. I discovered this a while ago after making a mask for it. I measured the diameter of the secondary holder, the diameter of the corrector plate and used a compass cutter to make the mask. When I came to fit it, it would not locate centrally. Not by much, maybe 5mm maximum.

This may explain why I may of had difficulties getting good collimation.

A little background. I bought the scope second hand. It is probably 20 yrs old but in good condition. I have previously removed the corrector plate for cleaning as on of the previous owners had left finger prints on the inside of the corrector and to remove some flocking / blackout paint that had settled on the primary. I did nothing to the secondary holder then.

As I am confident in removing and replacing the corrector plate, should I remove it and recenter the secondary?

On thinking about it, the corrector plate does not sit tightly within its housing, there is a small amount of movement prior to fitting the retaining ring. Is this why the secondary holder / corrector may not be central to the OTA.

I welcome your thoughts.

[nightfisher]

Never played with a C8 but if the corrector plate has lateral movement it might be worth using the compass cutter to make a jig that centres the secondary prior to doing up the retainer, then check collimation, my guess is it wants to be central

[Peter Drew]

Hello Ian.

 

Ideally the secondary/holder should be concentric to the primary. For some reason, economics?, early SCT correctors had rough cut edges and the corrector was shimmed with cork wedges to centralise them, current models have edges ground concentric to the holder aperture. I would recommend that you do your best to make your corrector concentric even though the primary is spherical. 

[Uplooker]

Hi Jules and Peter,

Thank you very much for your replies. I am sorry that I delayed replying for so long.

You have confirmed what I think I already knew. Looks like the corrector plate is coming off