Installing Bob's Knobs on a SCT

[want a be]

Can anyone offer any advise on installing a set of Bob's Knobs on my 8" Celestron CPC. I read the instructions on Bob's site and I'am concerned about not being able to get the first knob in after taking the first factory screw out. Something about the secondary mirror plate shifting so the Bob's Knob won't reach the threads to screw in right. If anyone has done this can you please offer me some do's and don'ts on the install...Thankyou

[Omaroo]

Just follow the instructions. I did it on my 12" LX200GPS and found no problem. You need to remove and then replace one screw at a time - as this way the other two are holding the secondary in place while you work on the one you're replacing - so (at least in my case) the secondary didn't budge at all - allowing me to screw the one I was working on in without a hitch. If you have any doubts, tilt the OTA down so that even if the secondary does dislodge for any reason - it will slide down the corrector plate and stop before it hits your primary.

A very worthwhile effort.......

Cheers
Chris

[Dennis]

Here’s the procedure I used to fit my Bob’s Knobs, as I was concerned about losing the original collimation, so decided to do this in the day time using a domed cap on a metal pole as a reference.

In the day time, I set my C9.25 up on a workbench, horizontal, pointing through an open doorway so I could focus on a metal pole some 100m away.

I shimmed the OTA until the cap on the pole was more or less centred in a reticule eyepiece that has an adjustable position reticule. I then tweaked the X-Y position of the reticule to centre the cap.

Next, I removed one secondary screw and replaced it by one Bob’s Knob and then returned to the eyepiece to see the effects. I then tweaked that Bob’s Knob to bring the cap of the pole back to the centre.

I repeated this for Bob’s Knobs 2 & 3, one at a time, thus retaining the original collimation. On the next clear night, I touched up the collimation using the star test.

Cheers

Dennis

[snowyskiesau]

As Dennis and Chris have described.
I recently did this on a C8 with no dramas.

[sheeny]

Yes, I concur...

The only variation in my procedure was I was able to aim the OTA at a feature that was about 100m away but downhill... that meant there should not be any chance of the mirror falling up away from the screw...

It worked easily anyway!

Al.

[acropolite]

I did the same with my LX90, however due to a change in secondary holder design the threads were a little short on the knobs. I managed by first tightening all the screws then removing one and replacing with the knob.

[want a be]

I did it...I replaced my collimation screws with the Bob's Knobs,I loosened each screw a 1/4 of a turn to take the stress off the secondary,then removed 1 screw replaced it with the bob's knobs,screwed it in till I felt there was no play then backed it off a 1/4 turn.Repeated for the other 2 knobs...Piece of cake...Now I have to do my star alienment for the 1st time to collimate the scope,hope that goes as well....

[Omaroo]

Piece of cake huh! :thumbsup:

[want a be]

It was easy,I hope the star collimation goes as well as its my 1st time .I know the scope is off now since I replaced the screws,but I did a rough alignment with a laser collimator in the eye piece. I've read that some say to star collimate with the diagonal in and some say to do it with out going right into the rear cell or visual back....Anyone have advise for a first timer?

[Omaroo]

Laser-collimate an SCT? Don't think that's possible....

They way I do a "rough" collimation is this: In daylight I stand back about 8-10 mirror diameters away from the scope and look straight into the primary through the corrector plate from the front. Move back and forth a little until you see what should be a series of separate concentric dark circles. If the circles are grouped closer together on one side or another, start adjusting via your Bobs Knobs - as you screw one out - screw another in. After a little while you'll start to see the relationship of what you're doing to the way the concentric circles move. The plan is to get the circles as concentric as you can (with the gaps between the circles the same right the way around the circle), and when you have, you'll be surprised how good your collimation already is. Follow that up at night with a star test and fine-tune from there. The secret is to screw the Bobs Knobs in and out NO MORE than a 1/8th of a turn at a time - it's that fine an adjustment. Do the star-test collimation with the eyepiece mounted straight in the back - without a diagonal.

Cheers
Chris

[want a be]

I also did the rough collimation as you stated and it was hard to tell if the circles where concentric, but they did look the same on all sides the best as I can tell. I did this inside the house,maybe I need to take the scope outside in the briter lite as you stated. As far as the laser I just looked at the the red dot though the corrector plate and as I replaced each screw with the bob's knobs I adjusted each till the dot was where It started at. When I bought the scope the man at the store did it that way and adjusted the screws to move the dot a little. I think he through the collimation off because I could not get saturn to focus clearly at higher mag. I never knew about a star test so I didn't check it when it was bought home from the store...Thanks to people like you us newbie's are learning alot,Thanks again for taking the time to help...Roger