[Sunfish (Ray)]

Opinion seems to be divided on the accuracy of centering of the secondary required for imaging.

Some opinion is that spherical elements do not require perfect centering and that collimation corrects for this.

The secondary on my classic C8 varies in offset to the scope wall by 0.5mm. I have measured the centering of secondary in the cover plate while cleaning and this seems good so the external measurements hold.. The images are OK when very carefully collimated but not exceptional, particularly for planets.

Due to the rough cut corrector centering ( I presume), to centre the secondary I would need to push the corrector to one side and take out the already thin black paper spacer and use a slightly larger cork spacer opposite. Brrr.

Is there an easy way to establish the centre of the primary coincides with the secondary without removal and will it make a difference?

Ah I can see now when ask the question whether, logically , the corrector lense also needs centering that Multiweb has already posted the answer to this question. More reading required.

So if the hole in the corrector is not central but the corrector was figured to suit the final position then it is going to be very hard to know exactly if the corrector is centred on the primary without some sophisticated equipment. And if rotation is exactly critical also then perhaps marking should be exact to replace in exactly the same rotation.