Jump to content

 

1825338873_SNRPN2021banner.jpg.68bf12c7791f26559c66cf7bce79fe3d.jpg

 

Bobs Knobs for secondary on 12" Flextube Dob


Recommended Posts

Hi Guys

I am considering buying a set of Bobs Knobs for the secondary mirror on my Skywatcher 12" Flextube dob.

They appear to come in either 30mm or 35mm screw length.

Anyone know whether to go for the 30mm or 35mm on this dob?

thanks

Graham

Link to comment
Share on other sites

Can't you get these knobs from any decent hardware shop? Buying a set of bolts from America seems like a waste of money to me.

The scopes come with a perfectly good set of hex bolts in already, but rather than messing around, in the dark, with a fine hex bolt, these replacements have excellent moulded ends allowing you to do the adjustment by hand, makes life much easier and safer, worth the little bit extra in my mind.

Steve..

Link to comment
Share on other sites

The scopes come with a perfectly good set of hex bolts in already, but rather than messing around, in the dark, with a fine hex bolt, these replacements have excellent moulded ends allowing you to do the adjustment by hand, makes life much easier and safer, worth the little bit extra in my mind.

Steve..

I know what they're for. I just don't see the point in buying them in America when thumbscrews are available here for a fraction of the price...

naoutlet items - Get great deals on items on eBay.co.uk Shops!

Link to comment
Share on other sites

I believe the major advantage of the Bobs Knobs is that the centre cap can go back on after adjustment. That won't happen with a regular wingnut.

Also I'm not happy fumbling with an Allen key around an expensive corrector plate in the dark! :)

Link to comment
Share on other sites

That won't happen with a regular wingnut.

Might that be because a wingnut is the wrong tool for the job?:)

I actually bought some m4x25 thumbscrews from the site I posted a while ago, after somebody recommended them as a cheap alternative to Bob's knobs, but I can't find the post and I've not fitted them yet. I'm waiting until I finally get round to flocking my scope, so I can't really say how well they fit, but I don't see why they won't. I can't believe Bob, or skywatcher, get their thumbscrews/bolts specially made.

TBH I'm not 100% certain that they are m4x25, they look it but looks can be deceiving as far as threads are concerned, so I'd be tempted to remove one of the original bolts and take it down to a hardware shop to see if they have anything I could use before forking out +£15 on 3 thumbscrews, but if you've all got money to burn...

Edited by scogyrd
Link to comment
Share on other sites

It's not that I have money to burn but they are designed for the job, work perfectly, fit exactly and save me time and effort. After the amount spent on the equipment I (personal opinion only) feel they are a small price to pay. It's not about bragging rights because everyone in the dark looks down the opposite end of the scope:). If there are other things out there that do the job just as well then I say great but by problem is I am maybe a bit lazy and more importantly would not know where to start looking. I have a shed full of bolts but none of them fit any of my astro related gear properly. It is a bit like the Astrodevelopment bolts that I also have fitted that could probably have been bought cheaper elsewhere.

Link to comment
Share on other sites

I just bought some " 8" SCT" collimating screws from the Astro-boot linked to in the Astrolonge on sgl.

They're for a Meade but might fit the C8. If not, for £4 they're worth a shot and I might rethread them to the right size ( I'm an Engineer/ Toolmaker).

They had some left last time I looked, for various size SCTs.

Link to comment
Share on other sites

I bought the M4X25mm from that eBay seller last year as I too wasn't ready to fork out £15 on 3 bolts from the US. They fitted perfectly into the threaded holes left when I took out the hex screws on my 8" dob and yes I can fit the lid back into place so there is no issue there.

Link to comment
Share on other sites

  • 1 year later...

Useful thread this one and glad I found it.

I have noted that even with Bob's knobs that the end of the thumbscrew that is in contact with the back of the secondary mirror is flat and ends with the last bit of thread edges in contact with the mirror back - so a not entirely regular contacting surface. As the movement of the secondary is determined by this contact would a domed end not offer more predictable adjustments? (particularly when fine adjustments are critical). Whether or not any one agrees with what I have suggested, I wonder if anyone knows where I could source thumbscrews with a neatly machined dome shaped contour at the end.

Cheers

Anthony

Link to comment
Share on other sites

I bought some 30mm thumbscrew 0.7mm pitch from amazon for £3 each. turns out the entry thread had just enough leading edge to score the back of the secondary mirror holder and start to cut a thread when under pressure, which these always are. This was resulting in some very odd movements when collimating as the screws were randomly holding on or letting go of the mirror. Obvioulsy as soon as i noticed the root cause for the odd behaviour I extracted the thumbscrews and replaced the original ones which worked fine. I have 6x of these screws sitting here doing nothing now.

It pays to get good quality is the point. I'll get some bobs knobs now and not look to cut a corner. There is a good reason why bobs knobs is still selling alot when it seems so easy to source these sorts of things for cheaper, his obviously are good quality and fit for purpose.

Link to comment
Share on other sites

That is a much better description than I was able to put forward Graham!!! Many thanks.

I have a (little used) GSO newt f4 that I bought 2nd hand. It had Bob's knobs fitted and when I removed the secondary holder I saw that the Bobs Knobs thumbscrews had a leading edge and thus scored the secondary holder creating the effect you have described. That's why I wondering about the domed end being better if done well.

The main reason I have posted here is that a very nice lady on this forum recently sold me her 12" Flextube and I'd like to place thumbscrews on the secondary of that.

Link to comment
Share on other sites

they're the ones I got that have the leading edge on them :/

thanks for the heads up on bobs knobs anthony! I wont bother with them then. I'll work on flattening the edge of the ones I have. My dads an engineer so probably has a tool to correct the issues im facing.

Link to comment
Share on other sites

thanks for the heads up on bobs knobs anthony! I wont bother with them then. I'll work on flattening the edge of the ones I have. My dads an engineer so probably has a tool to correct the issues im facing.

I must say though - it could well be that 99% of Bob's Knobs thumbscrews are finished well and mine are in the 1% that aren't. I don't know, I only have experience of 1 set after all.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.