Bob's Knob's Experience

[rwilkey]

Hi, I recently upgraded my secondary mirror grub screws with Bob's Knobs as I had read so many good things about them and I thought they sounded great. Unfortunately, they put the secondary mirror way out and I couldn't get it back to the right position. The long and the short of it was I had to get a stargazing friend to help correct it again and collimate, which meant putting the original grub screws back etc. So this turned out to be a very bad experience.

What I want to ask, has anyone else had a bad experience using these? We think one of the Bob's Knobs screws could have had a faulty thread....

[spaceboy]

Never had problems with them and I've use them on all my Newts.

[AndyUK]

Funny you should mention it - I've also just gone back to the original grub screws on the secondary as well... They seem to give a much better / finer and more secure adjustment than the Bob's knobs. (However, the primary knobs are excellent...)

[rwilkey]

Hi Andy, I'm glad I'm not the only one, I have the newer 200P so primary mirror screws were OK. I have to admit, there's nothing wrong with the grub screws, they do seem to hold the secondary more securely. Thanks for your post, thanks also to Spaceboy.

[John]

I've put Bob's Knobs on the secondary cell of my Orion Optics 10" F/4.8. I've also put a large hard steel washer behind the secondary support hub as the old bolts had dug pits in the alloy surface. So far, so, good. Collimation is a little easier and is being held. I did wonder whether to either round off or file to a point the end of each "knob" where it bears on the washer / secondary hub but I've not done that as yet.

They are expensive for what they are though and I'm sure there are lower cost options that work just as well.

[spaceboy]

Ahh Sorry Robin never gave it a thought. John just reminded me, I also use a washer. If you have to remove the the grubs anyway I figured why not put a washer behind the secondary. The aluminium holder and stainless steel Bob knobs can be a recipe for disaster if slightly over zealous tightening during collimation. The washer is a worth while mod anyway even if using the standard grub screws. Dion on AS recommends a bottle washer also so that the secondary rotates easier.

[Matthew]

I bought replacement secondary thumb screws from a company online . WDS. . They cost about £5 and they have plenty of Clearance between the heads for collimating.

[Gregger]

My order took 6 days to get here

$19, seems very good service

[Macavity]

Unsure of the exact length, I ordered over-length screws, with knurled heads, for my Newt secondary. I cut them to length, smoothed the ends etc. Not a lot can go wrong, frankly? Sources of "knurled things", things with "knobs on the end" (of requisite longer length) are not THAT common in the UK. My time versus a "few dollars more"? Lesser of evils, I reckoned. Quite fun to get a package (quickly) from the States too?