DIY Bob's Knobs

[Hypernova]

Looking for some thumbscrews to make collimation easier on my Skywatcher 200p dobsonian, I balked at the idea of spending £30+ on a total of 9 screws shipped in from the USA. I decided it would be easier to get some from another source in the UK, it wasn't.

Most suppliers won't sell at a reasonable cost to the public and may necessitate ordering large quantities of one item, many don't include VAT until the checkout and even the delivery charges can extortionate for such a lightweight order and many times I could not find the correct length (this was especially true when it came to the secondary mirror screws). I spent ages pouring through the websites of various fastener suppliers such as WDS and RS-Online and eBay but came across the issues above. I tried looking for push on caps for socket cap screws but the only suppliers would've made me buy 20 or more of each size (m4 and m5) + VAT + postage.

In one last ditch attempt I came across these guys:

https://www.cromwell.co.uk/index.php?q=0&p=browse&c=220104&ss=thumb+screws

From them I managed to order exactly what I required which ended up being:

• 3x M4 X 30mm - For the secondary
• 3x M5 X 15mm - For the primary adjustment
• 3x M5 X 25mm - For the primary locking screws

This set me back a total of £10.61 with postage, which was £4.99 standard flat rate via Parcel Force, there was no other option.

The heads of these bolts are larger than others that I have been looking at which makes things quite tight but still workable on my scope, although this may be an issue on smaller scopes where space is tighter. The secondary screws are very close together but there is still ample room for my fingers to grip them. Around the primary though I hit a small issue, the head of the shorter adjustment bolts is very close to the thread of the locking bolts, dunno why Skywatcher put them so close together. On 2 out of three adjusters there is small but distinct gap between the 2 but on the remaining pair the the head ever so slightly touches the thread and causes a noticeable notchy feeling when turning the mirror adjusting screw. I put this down to slightly sloppy machining of the holes for the locking screws as the screws themselves are true. The screws are still usable and I could actually file the offending thread off where it rubs as it is only the lower 8mm or so of the thread that is engaged in the screw hole, the rest is there to give some height so the pair of screw heads are staggered in height.

I hope this information is of use to someone on here as it was to me.

[xtreemchaos]

thats a great tip if only a week earler it would of save me buying some bobs knobs but i will keep it in mind for the next scope,nice bit of imfo..thanks for sharing..charl..

[Demon Barber]

Yeah Cromwell are great ,Astroboot uk is a good source. ,also try Essentra, website here:www.essentracomponents.co. uk   free samples available.

Regards

Mike

[Spikey]

Looks good Tom, well done

[knobby]

I admire your persistence but isn't the head of the secondary screws in the light path ...

[Hypernova]

I admire your persistence but isn't the head of the secondary screws in the light path ...

You are right in your assumption, whilst 2 of the screw heads are well hidden by the offset of secondary mirror there is one that protrudes slightly into the light path as seen from the focuser end with the focuser tube fully drawn out to maximise the effect. The offending screw can be seen at the 9 o'clock position, the other two are at 2 o'clock and 5 o'clock or thereabouts but can't be seen.

Whether or not this will impact on the views is yet to be seen but the intrusion is minimal. It shouldn't be a problem with a faster scope which may have a larger secondary that will hide over sized screws better but I can definitely see it being an issue with a slow or small aperture scope.

Maybe a better source or affordable screws will pop up with suitably-sized heads that are better for this application.

[happy-kat]

I have found a similar place also online in Brighton who do metric and imperial stuff and can buy just 1 of items.

I used them for the bits for my barn door, your post reminded me of them, so thank you.

[knobby]

You are right in your assumption, whilst 2 of the screw heads are well hidden by the offset of secondary mirror there is one that protrudes slightly into the light path as seen from the focuser end with the focuser tube fully drawn out to maximise the effect. The offending screw can be seen at the 9 o'clock position, the other two are at 2 o'clock and 5 o'clock or thereabouts but can't be seen.

20150712_182133.jpg

Whether or not this will impact on the views is yet to be seen but the intrusion is minimal. It shouldn't be a problem with a faster scope which may have a larger secondary that will hide over sized screws better but I can definitely see it being an issue with a slow or small aperture scope.

Maybe a better source or affordable screws will pop up with suitably-sized heads that are better for this application.

I'm sure it won't make any difference to viewing ... I think I may have been having a bit of a bad day when I posted [emoji3]

[Tim99]

Just what I was looking for.  Thanks!