C9.25 and fitting Bob's knobs in daylight

[Dennis]

To fit my set of Bob’s knobs to the C9.25, (a first ever collimation exercise for me), I though I would try it in daytime on a fixed terrestrial object. So, I placed the optical tube on my Black & Decker workmate (portable bench) and clamped the dovetail rail in the bench jaws. I then inserted a star diagonal and a 9mm reticle eyepiece and focused on a bolt head on steel pole some 70m away.

I centred the bolt head in the innermost circle of the reticle, clamped the mirror lock bolt on my focuser (from Hutech) and removed the first Celestron star head screw and inserted Bob’s knob #1.

To my consternation, after carefully tightening Bob’s knob #1, I could not re-acquire the bolt head, nor see anything other than a blank FOV. Oh well, maybe I should remove the 2nd Celestron screw, which I did. After inserting Bob’s knob #2, the FOV was still blank. Hmm, gone too far now to turn back.

After inserting Bob’s knob #3, the FOV thankfully returned and I tweaked all three Bob’s knobs until the bolt head on the steel post was re-centred in the innermost circle of the reticle.

A thunderstorm and hail just came through my suburb, so it’s unlikely I’ll get out tonight to check collimation. At least the OTA is in the same state as it was before I removed the three Celestron screws.

Cheers

Dennis

[atalas]

Good move buying Knobs Nobs Dennis ! they make life a lot easier with a SCT thats for sure.

[Robert_T]

Hi Dennis, I just fitted Bob'sKnobs to my C9.25 yesterday. Nice idea about centering on a terrestrial object whilechanging screws. I've fitted these to other scopes and as long as it's done one screw at a time it seems to come back pretty close on the final screw like you found. A good collimation check is to sight down the tube from a distance of between 2-3m. If you get the secondary so it lies just within it's own shadow on the primary mirror you should see alternating concentric rings of dark (shadow) and light surrounding this out to the mirrors edge - if they're not concentric they can be collimated using the knobs in a similar way to defocussed star collimation.


cheers,

[Hitchhiker]

Thanks for posting this Dennis.

I was going to install my Bob's knobs in my 10" SCT today but it hasn't stopped raining for long enough! I was going to set up an artificial star and change one at a time while trying to keep the collimation as close as possible. I may not bother now and just use a bolt or other feature like you did.

Adam.