Bint dob knurled head secondary screws?

[Kokatha man]

Hi people - decided to roll my sleeves up, get out of the insanity of my RA/DEC thread for a mo and collimate my recently arrived Bint 10" dob. (Well heck, the viewing outside has been atrocious these last few nights and tho' it has maintained pretty good collimation since arriving I figured I had to get back into it sooner rather than later - haven't collimated anything for over 30 years!)

Went the whole hog and started fiddling with the secondary (which was really just about spot-on already) and when I finally had everything back at least as ok as when I started (for me that's pretty damn amazing!) I thought, why don't I get knurled heads for the 3 "screws" and centre "bolt" to facilitate matters and alleviate my almost manic phobia about dropping phillips' head drivers down the chute.

Anyone done this - I suppose I should just go to Sweetmans/Unimeyer or whatever they call themselves nowadays but thought I'd ask IIS forum members.

Regards, Kokatha man.

[erick (Eric)]

Yes, I made my own. It's an M4 thread. Same as the bolts used to fit handles on cupboards. You can buy packets of them from hardware stores. They have the reasonable length that you need. I had some wooden plugs that fill holes over screw heads on wooden furniture (yes, hardware store), drilled holes through them, fitted against the screws with washer and nut, flutted the edges with a round file to give me grip (and to get the edge out of the light path around the secondary holder), and fitted them. No more screw driver phobia!

Eric

Originally Posted by Kokatha man

........Anyone done this - I suppose I should just go to Sweetmans/Unimeyer or whatever they call themselves nowadays but thought I'd ask IIS forum members.

Regards, Kokatha man.

[Tannehill]

there have been some posts here on home-made versions of this, using hex-heads or some other. Perhaps a search will reveal them. If I recall the thread spec on those bolts is odd enough that it isn't stocked in local hardware stores. 4mm is it? I don't know the specialty stores here as well, maybe the store you mention would carry those...

I have a 12" Bintel (GSO) and just ordered the BOb's Knobs for all the collimation bolts. I don't think they sell one for the center bolt, and in fact you don't want one for there. The bolt head of the center bolt on the secondary is designed to fit just so into the hub of the spider, allowing the bolt to "pivot" (wiggle) while still connected to the secondary. This permits the variable positioning of the secondary by the three coll bolts. It's the compression spring over the bolt, between the secondary and spider hub, that "locks" the secondary mirror in place while adjusting the collimation bolts. Dynamic tension between the spring and the bolt(s). If you swapped out that head, you'd lose that anchoring-but-still-wiggly feature of the secondary center bolt. Plus, that center bolt should never really need adjusting once the secondary is centered under the focuser, so it doesn't routinely get adjusted as do the coll bolts....

But, yes, I share your trepidation regarding dangling a screwdriver over one's primary mirror. Such a thing...it's just not...natural. I always lay my tube horizontal on bed while doing it, thereby practically making it impossible for a screwdriver to damage the mirror. Short of a sudden change in the direction of gravity, or a poorly timed earthquake, of course..

Regards

Scott

[erick (Eric)]

Alternative, which I did first after seeing the suggestion. on someone's website. Connect some form of cord - leather? - I used some electrical wire - to your screwdriver - I drilled into the plastic. Slip it over your wrist so if you drop the screwdriver, it just dangles!

[astronut (John)]

KM,
I changed over my primary and secondary collimation knobs on my 12"lightbridge, with ones from www.Astronomy-Shoppe.com
Since the LB's are made by G.S.O they share the same spider assembly.
Del is about 1 week.
They have turned a nightmare of a job into something I actually look forward to.
It's now a NO TOOL collimation process.

[OneOfOne (Trevor)]

I would check that they are in fact 4mm you may find that some "metric" bolts are not actually metric. I bought a packet of "6mm metric" bolts once from a hardware store to find that in fact they were Whitworth, 1/4" to be precise. The easiest way to make sure would be to take one with you and check that the pitch of the screws line up. If you place the two threads together they should mate perfectly, non metric ones will slowly "walk" past the threads on the bolt. Unfortunately the pitch on some imperial threads are very close to metric and you can actually screw the screw several turns into the thread before it jams. A vernier to check the diameter, if you have one would also be handy.

One of the first things I changed on my C11 was the silly little Philips head screws to a set of Bob's Knobs...with and SCT you don't even have to drop it to do some serious damage...just slip...

[Kokatha man]

Thanks for all the responses fellas - oneforone is quite correct re "alledged" thread types and how to check them: I thought my first stop would be the Sweetmans/Unifasteners/unimeyers or whatever their current corporate identity is (if you haven't been to them for 6 months or more they've changed their name - but I believe they have incarnations in all major cities.)

Will try for those allen key head types with an extended knurled barrel head around the allen socket first: if unavailable most probably go for home-modified hex-head or somesuch, I'm that broke (or conscious of just about being there) that I'll steer away from purpose-made stuff, what with buying the scope at Xmas and having just ordered a Brian Read Round Table Platform.(Thankfully the primary cell has all knurled heads as standard.)

My new question is this: after my first collimation attempt in 30+ years and thinkink it looked pretty reasonable, I'm wondering if I overtightened the locking screws a bit for the primary mirror cell.

Everything looked ok (all concentric, centred) but in operation I found that it was extremely hard to focus on anything below the zenith areas: more particularly on higher power. I also began to think the Crayford type 10:1 focusser was "playing up" but I may have been exacerbating my perceptions. Viewing conditions were fine and when I de-focussed (which seemed far easier than focussing?!?) the expanded star disks displated a pretty-well centred black spot for the secondary.

Another query that the notes didn't (for me!) articulate fully in describing collimation - once the cental bolt in the secondary assembly is adjusted to centre it in the drawtube is that the end of any adjusting of that bolt: in other words once its position in the tube is set are the only adjustments to be effected periodically the 3 aforementioned screws I wish to convert to knurled heads: ie will they give me the alignment (at the secondary) by concerted impinging/relaxing their thrust onto the sprung bolt assembly OR DOES THE CENTRAL BOLT AND ITS SPRING ALLWAYS HAVE TO BE RELAXED/LOOSENED BEFORE ANY ADJUSTMENTS CAN BE MADE TO THE 3 AFOREMENTIONED SCREWS?

Hoping for some erudite responses - Kokatha man. ps Happy New Year to all.

[Tannehill]

It's very easy to overtighten those locking bolts; any degree of tightening, actually, will make your collimation drift a bit. If you tighten each one ever so lightly, the exact same amount, it should be okay. But check your primary collimation (Chesire or barlowed-laser) after to confirm. If you swap out the old primary springs for heavier ones, the locking bolts are less necessary. In fact I don't use my locking bolts at all; with the heavier springs there is no drift in collimation once I dial it in, AND I recollimate every night - takes 5 minutes. Someone who wanted to avoid recollimation (again, not a big deal once you learn) might be more keen to use the locking bolts. [all the above is based on only my experience....as they say, your mileage may vary]

There are few causes for an apparently touchy focus or inability to achieve pinpoint focus in newts. Seeing, poor collimation to begin with, and, yes, a malfunctioning focuser. I'd say a hot mirror and bad seeing are #2, and poor collimation is #1 on that list, respectively. That is, those are the stats I'd expect from a large sample of users. Was the focus sharp at any time? If so, that eliminates poor collimation, leaving the seeing and mirror thermals and focuser issues as possible causes.

You say you noted that when you defocused the spider was nicely centered. That shows one aspect of collimation, but you could still be grossly decollimated and that picture could still show the spider "centered" in the large bright disc when you defocus on a bright star.

What collimation tools do you have?

Lastly, its true that once you center the secondary mirror under the focuser drawtube, you should not need to readjust that centre bolt. You do NOT have to loosen or tighten that each time you want to collimate the secondary with those three bolts. However, if your spring happens to be floppier than desired, you may find you have too little dynamic tension to prevent the secondary from flopping about as you tighten those three knobs. (see my Review on the 302 mm Dob here on IIS - I talk about my struggles with this very feature). And as you turn those three collimation bolts, you are actually pivoting the mirror on the centre bolt, not moving its center down or up. The only caveat is, if each night out your recollimate, and you find yourself "always" tightening one or more of these three bolts, never loosening, after a few sessions you will soon find it hard to go much farther: you are now fighting that centre bolt...and it'll win. Then you should loosen instead of tighten. It'll be fairly easy to figure out as your experiment. In truth, on my GSO, there isn't a heck of a lot of "room" to move those three collimation bolts before it gets too hard, or the bolt comes off the fender washer fully. You have to find that sweet spot and use that for collimation. If you find yourself turning a bolt, and the laser spot either jumps or doesn't move, it may be because you are at the 'end' of one or more bolts' effective travel.

It's easy and safe to remove your secondary...good to see the anatomy of the assembly so you can troubleshoot in the future. Just, do it horizontal, because as you loosen the centre bolt, the secondary will fall off, AND immediately thereafter that spring will drop off. If you are ready for the former and not the latter, that spring will make a very musical 'dong' on your primary mirror. Probably followed hard upon by expletives from the person holding the secondary mirror in his or her hand....

Ding "Crap!"
Dong "S#%#!"

hope this helps.

Scott

P.S. Not sure what you mean by erudite..is that a mineral or gemstone? Mayhap my info is more coprolite....



Another query that the notes didn't (for me!) articulate fully in describing collimation - once the cental bolt in the secondary assembly is adjusted to centre it in the drawtube is that the end of any adjusting of that bolt: in other words once its position in the tube is set are the only adjustments to be effected periodically the 3 aforementioned screws I wish to convert to knurled heads: ie will they give me the alignment (at the secondary) by concerted impinging/relaxing their thrust onto the sprung bolt assembly OR DOES THE CENTRAL BOLT AND ITS SPRING ALLWAYS HAVE TO BE RELAXED/LOOSENED BEFORE ANY ADJUSTMENTS CAN BE MADE TO THE 3 AFOREMENTIONED SCREWS?

Hoping for some erudite responses - Kokatha man. ps Happy New Year to all.[/quote]