Secondary screws

[Jamesacrux (James)]

What can I use to replace secondary mirror collimating screws on a 10 inch collapsible skywatcher dobsonian

James

[astro744]

Welcome!

Try Bobs Knobs. See http://www.bobsknobs.com/Newt/page66/SWdob.html

(You could ask Bintel but I’m not sure if they have them in stock or can get them for you).

[Jamesacrux (James)]

Thank you

James

[Eldest_Sibling (Alan)]

Bob's Knobs were developed for Tom Johnson's Schmidt-Cassegrains, and to keep them pretty, aesthetic. I get these from my local hardware, and for my Newtonians rather...



The socket-cap screws can be a mile long, as long as they don't flex and sway into the light-path. They can also be had in stainless-steel.

[5ash (Philip)]

Last year I purchased 5 stainless steel knurled thumb screws M4 or M3 (can’t remember thread size) for my 10”SW on eBay for about $5 posted, They work fine . If you remove one and check with your local hardware shop you can determine M4 or M3 , or someone here may know.
Regards philip

[gaseous (Patrick)]

As others have alluded to, M4 bolts from a hardware store, and any sort of knob which accepts an M4 thread will do the trick. I bought three small furniture knobs from Bunnings, (make sure they're small!) added the threads with a bit of Locktite glue, and they're perfect - you can easily adjust them by hand without the need for allen keys, etc.

[5ash (Philip)]

Originally Posted by 5ash

Last year I purchased 5 stainless steel knurled thumb screws M4 or M3 (can’t remember thread size) for my 10”SW on eBay for about $5 posted, They work fine . If you remove one and check with your local hardware shop you can determine M4 or M3 , or someone here may know.
Regards philip

Here’s a picture of the above thumb screws, total cost including postage was $5 for 5 screws.

[ausastronomer (John Bambury)]

Originally Posted by 5ash

Here’s a picture of the above thumb screws, total cost including postage was $5 for 5 screws.

That's the easiest and cheapest way to do it and you end up with the same result if not better. Bob's knobs are ridiculously expensive for what they are.

I replaced my secondary collimation screws on my 10" GSO dob with 304 grade stainless thumb screws about 15 years ago. I can't remember what length they were, but I remember they needed to be quite long to go right through the secondary holder and have enough length remaining to leave full adjustment of the secondary. They are definitely 4mm x .7mm pitch and I think they needed to be either 40mm or 50mm long. Measure the depth of the secondary holder and add about 15mm to give sufficient travel both sides of the holder. I also replaced the primary collimation screws on the GSO mirror cell with stronger stainless steel springs and 7mm metric thumb screws. I think they use a different primary collimation set up on the more recent manufacture scopes.

Cheers
John B