C11 mechanical alignment

[multiweb (Marc)]

Just received the flocking material to do my C11. I'm going to open it again. I did a test shot here last week-end that showed the alignment was much better. I think I did the right thing but I'm just checking with the gurus out there.

When the corrector plate and the secondary are centered in the tube, the view from the baffle tube with a cross tube shows the secondary holder shifted to the left and down. The cross tube was held flash with the threaded piece on the mirror cell (focuser removed) so it was square.

So here's what I did. I shimmed the secondary within the corrector plate to shift it right and shimmed the corrector plate right as well and up until I got the edge of the baffle tube and the secondary holder perfectly concentric.

From there I placed the secondary mirror back in its cell and shimmed the focuser this time until the cross hair/cheshire aligned with the dark spot.

From there I did a star test at Crago (...sort of , windy, seeing not too good) but did shoot the keyhole and the field was uniform and the stars were round on both sides of the field, not round on one half and elongated on the other half like my very first shot pre-mods.

Does this sound all good to you guys? Or did I make a mistake by shifting the secondary and the corrector plate? Just checking the procedure and steps are correct and there's nothing wrong with what I did. Thanks for any feedback.

[Paul Haese]

I would have thought that your OTA would have been near perfectly aligned if you got it from new. Sticking to the initial alignment would have been best, however given the field looks even now, well mabe not. It might be instructive though to take the field with a DSLR, this image seems a little smaller than I would have expected.

Just looking at it the field seems good, some out of focus stars would be good too.

[multiweb (Marc)]

Originally Posted by Paul Haese

I would have thought that your OTA would have been near perfectly aligned if you got it from new. Sticking to the initial alignment would have been best, however given the field looks even now, well mabe not. It might be instructive though to take the field with a DSLR, this image seems a little smaller than I would have expected.

Just looking at it the field seems good, some out of focus stars would be good too.

I got it second hand. Originally the corrector plate was shimmed already to the right so I guess it was for compensating for the same problem I saw in the baffle tube. But I don't know for sure if the tube had been opened before. My concern was not knowing if the corrector plate and secondary should be exactly centered in the tube regardless of any offset seen within the baffle tube.?

[AlexN]

if the images are better i'd say the realignment was a success.. Overall i've heard many things about centralizing the corrector and secondary, alot of what i read was contradictory. Some say keep everything centered, even keeping the corrector in the same orientation, others say this is not a big concern. As i said, i go by the images. If they look good, or better than original, then all is sweet.

[multiweb (Marc)]

Sure is a lot of litterature and different opinions on the corrector plate placement. Some people go as far as saying the corrector plate and mirrors are mass produced and not necessarily matched, just generic. My gut feeling is that the corrector plate and the primary and the secondary have to be parrallel and also not rotated vis a vis one another. Having said that I'm am not convinced (dont' really know but) that a slight offset in the corrector plate axis/secondary and primary is a "picture killer". Having the corrector plate not parallel to the primary would be very bad I would have thought though. It's like your camera not being squared. Your field looks like

I'm also hearing that not having concentric shadows when looking at the front of the OTA or the back with a draw tube/cheshire is not a concern because of the SCT design and doesn't mean it's not aligned. That I'm not buying though It doesn't make sense to me. Anyone disagree or care to explain why it wouldn't matter? I don't get it.