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SCT Bob's Knob's users?

Rob

By Rob
in Discussions - Scopes / Whole setups

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Rob

Rob

Posted
Posted

What are your experiences?.. can you hold collimation well?. I've just fitted mine and I must say I'm on the fence after last nights 3 collimation's experience. The screws were just fine before!!. I may put them back.

I have the correct ones and left the spacers in... just cant work out why its not really holding good collimation.

Rob

:?

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John

John

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I used to swear by Bobs Knobs with my C5 but ended up going back to the original screws when I fitted a set to my C8. I was having exactly the same issue that you are Rob :D - never did really get to the bottom of it.

John

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JOHN MILLER

JOHN MILLER

Posted
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I use Bob's Knobs on my C8 SE and have had no problems - unusually for me as I'm anything but technically minded! I was just having a look at the fitting instructions that come with them, which are quire detailed - it might be worth revisiting them to check you've followed them exactly. For Celestron users you have to " Loosen each collimation screw in sequence by a fraction of a turn until tension is just removed from the screws. Do not remove any of the screws during this process" - and then remove the screws one at a time and replace with a knob, before going on to the next one, etc.

So presumably following the instructions is critical - possibly otherwise you might end up with a big variation in tension between different knobs. I had a similar problem with my OMC Mak-Cass, having got the collimation screws all out of kilter and it took a trip back to Orion Optics to identify the problem . Could that be something to do with it? Just a thought.

Regards

John

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Whippy

Whippy

Posted
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My Intes MK66 came with a set already installed and they seem to be holding the collimation nicely. Not sure they'd be high on my list of upgrades for my other scopes but they do make life a little easier I suppose.

Tony..

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Rob

Rob

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I have indeed follow the instructions by the letter and fitted them as specified. I can achive collimation, it does not seem to hold it that well... at one stage I collimated perfect (and I'm a perfectionist!!). moved the scope toward Mars for a detail check with out the dewshield on... Perfect!. Moved bck to horizontal, pushed the dewshield on which slipped and dropped back on the Baader dovetail bar with a bump... it was out of collimation again?.. that seems a little too sensitive to me :?

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markcol

markcol

Posted
Posted

This appears to be a Celestron problem. I've got a set of the equivalent Astro-Engineering knobs on my Meade 8 in LX10 ota and I hardly ever have to touch them. It was a bit scary originally when I fitted them before I'd learnt to collimate, but since then even when I deforked the tube to put it on a dovetail it was still in collimation!

(Nothing against Celestrons - in fact I fancied getting a C9.25 or C11 myself...)

Hope you get it sorted- are there extra spacer washers needed?

Mark

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John

John

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I have indeed follow the instructions by the letter and fitted them as specified. I can achive collimation, it does not seem to hold it that well... at one stage I collimated perfect (and I'm a perfectionist!!). moved the scope toward Mars for a detail check with out the dewshield on... Perfect!. Moved bck to horizontal, pushed the dewshield on which slipped and dropped back on the Baader dovetail bar with a bump... it was out of collimation again?.. that seems a little too sensitive to me :?

I suppose rather than collimation knob problems, this might be a thing called mirror flop which is something that afflicts SCT's - especially older ones. Basically the primary mirror moves slightly when the scope moves - typically when crossing the zenith - and collimation is affected. I think you can get mirror locking devices to sort this out if that's what it is :D

One cure for this is to run the focussing knob through it's full range in both directions which spreads the lubricant out evenly along the screw thread that's insude the back of the scope.

John

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strutsinaction

strutsinaction

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I added Bob's Knobs to my 8SE soon after getting my scope and since the initial fitting I haven't had to collimate.

I suggest you revert back to your original screws and see if that helps. If you're still losing collimation after that, at least you'll know it wasn't Bob's Knobs causing the problem.

Regards

John

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MikeP

MikeP

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Mine arrived today. I installed them and did a rough collimation by looking into the tube as Bob described in the instructions.

Its hard to put into words but ... I got the impression that all three knobs could be quite slack or quite tight yet in both cases put the secondary into the correct position. If they were quite slack, I'd expect the secondary to move when the scope moved.

Is it possible you could have been a bit too gentle and all three knobs need tightening? Obviously you need to be careful to avoid damage to the secondary or the knobs themselves.

I could be talking rubbish, because until I get my scope outside, collimate properly and do a test, I can't be sure I won't have the same problem.

Good luck.

Mike

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Astroman

Astroman

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Posted

I added Bob's Knobs to my C8 a few years ago. I've never been really happy with them, as it seems I need to recollimate more often than previously, but since I've gotten over my fear of collimation, I just do it and shut up. I figure, I should check it any time I move the scope anyway, so it's no big deal just to check/align. It's part of the routine, and I'd rather do it with the knobs than hold that screw driver over the corrector in the dark.

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Rob

Rob

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Not sure to be honest. I took the BK out. re sat the philips screws, collimated the nre moved and replaced by the instructions once again. I also swapped the spacers round on the knobs. The hold for collimation seems a lot tighter + the overall collimation less hassel.

I'm not going to question it! :D

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MikeP

MikeP

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I'm pretty sure it will have been reversing the washers on the screws. They have a sticky out bit that goes into the hole in the plastic cover and stops things wobbling about. Fortunately, I noticed this in the instructions and put mine in that way round from the outset.

The C11 is outside now, so I'll know in a few hours whether I can collimate with Bob's Knobs and whether it will keep the collimation.

Mike

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MikeP

MikeP

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Well that was a piece of cake.

My attempt at a rough daytime collimation was a fair bit off, so I did quite a lot of twiddling knobs and re-centring the unfocussed star. Apart from the normal difficulty of not knowing which knob to turn in which direction, the process was so much easier to carry out. There was no danger of scratching expensive glass, which was my main motivation for the change.

Only time will tell if it keeps the collimation - a move or two in and out of the house should be a pretty good test.

Mike

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