Truss Dob Collimation Shift

[niallk]

Hi - Weather has been terrible so... I'm looking at collimation shift vs altitude in my recently acquired 15". I'm using a barlowed laser, and looking at the shifting of the shadow of the centre mark donut over say ~10° to 90° (zenith). I see a shift of ~2mm - I will check the tightness of everything. The absolute distance shift is somewhat ambiguous for comparison I guess - it is divergence angled that counts...?

It's not too big a deal to tweak if viewing low to horizon, but just wondering do others see such non-zero shift - is it just par-for-the-course?

Would certainly be grateful of any comments/suggestions from others - I'm new to truss dobs + using a laser to aid collimation

Thanks,

-Niall

[mister_wavey]

it's not unusual, see the middle section of this page: http://www.dobsonians.co.uk/Mirror%20Care.html

[Moonshane]

Usually tightening the spider vanes helps

[niallk]

Thanks yes the vanes seem pretty taut, but will check over whole scope including mirror cell with tools.

I also expect that I might need to adjust the sling a little bit possibly.

I guess what I'm wondering is if after fiddling and checking things, does a mm or two sound reasonable as residual shift? Seems plausible from Lurehurst link (+thanks).

Or do people typically find their systems approach zero shift...?

Before collimating, I push the scope horizontal, give a gentle push on the back of the mirror, swing gently up to zenith and back down a couple of times, then start collimating at say 45° or so.

[Beulah]

Before collimating, I push the scope horizontal, give a gentle push on the back of the mirror, swing gently up to zenith and back down a couple of times, then start collimating at say 45° or so.

That's what I do, it seems to speed up the collimation process a bit. Get fed up with 'chasing the polo' around...need to save the body warmth for actual observing 

[ollypenrice]

I can't say that my fairly rustic and primitive 20 inch truss Dob suffers too much from this. I have only ever collimated it once a night. I'm not obsessive about it, though.

Olly

[russ.will]

Why would you view at 10deg above the horizon? Stop it! 

My truss shifted with altitude and as I normally stuck to 30deg+ I collimated it at 60deg and just got on with it. They all do and there's no way of avoiding it, if you still want to be able to lift the thing. David Lukehurst probably summarizes it better in the middle section of this page.

Russell

[niallk]

Thanks for the helpful feedback. Agreed about observing too low to the horizon! What I'm looking at is just testing the mechanics of the scope ... indoors due to poor weather

I'm pretty happy that its behaving well over normal use range. The deviation is more pronounced over the last 20° as weight shifts more onto sling I imagine. I've to check tightness of everything (minding small but destructive kids over the weekend and can't draw attention to the scope have to sneak off to check it ..but that's another story ).

I had an issue with crisp focus at first light - conditions were also woeful, and found the centre donut a bit off centre (!). Hence I'm trying to check everything...

Thanks again - Niall.

[swamp thing]

Check your sling is wound tightly, or is it the new wire type one?

As Olly not had this problem myself.

[Dannae]

Hi Niall,

It is likely to be a small combination of things already mentioned but the largest factor may be the sling and the orientation of the spider vanes with respect to 'vertical'. I changed my obsession sling to the glatter cable sling and it reduced the collimating error due to stretch, movement and any chance of torsion ( which would show up as a movement in the laser also). I found that on nights of heavy dew and high delta T's the Kevlar sling would affect the collimating despite never being personally able to stretch or deform the damn thing once I'd replaced it, go figure!

I never changed the orientation of the spider vanes but I know that teeter now fits them parallel to the altitude motion ( in line with the Uta clamps/truss pole apices) and there has been a very favourable response with almost zero shift.

http://chesmontastro.org/node/8545

This page has some insights as well as teeters yahoo group on the subject of drift in alt.

Regards

Dannae

[niallk]

Thanks Dannae, Swamp Thing; I definitely have more checking over to do, and suspect a little re-seating of the sling/Velcro (its not a glatter cable sling type). I also suspect that any small stretch that needs to happen has not bedded in yet due to lack of load time so far - stored upright not under load.

Yes the vanes are more 'x' than '+', and I see Teeter additionally favouring the focusser horizontal more recently.

Living in hope that after checking things are all tight and adjusting the sling that it will be fine at the eyepiece which is all that counts... Hope not to have to do any surgery

Again, I appreciate the helpful comments. I guess by posting I was trying to get a sense from people for either "yeah a little shift is to be expected & it doesn't sound excessive" or "no: any shift should be really negligible if everything is right".

One thing: it is a great learning experience to assemble the scope: certainly fast lose any fears about having to collimate from scratch. I always felt I got off lightly with the 250px - holds almost too well. I must check if the mirror clips are borderline tight - but I get good views. My dirty little secret is that I've never taken the mirror out to wash it. With the new barlowed laser, I actually find collimation kinda fun! Novelty will wear off no doubt...

Just need some good seeing/non-sick soundly sleeping kids/wife contentedly watching tv/tinker time & to get out under the stars!

[russ.will]

I did forget to add - although I'd hope an Obsession gets this right - check the center spot is actually the center. Admittedly, all of my mirrors have been Chinese, but on the two 12" examples, collimating with a slightly defocused star resulted in the laser (Farpoint, so no slouch) being significantly off center at any elevation. Checking the centering of the spot on both, revealed why.

Russell

[niallk]

I did forget to add - although I'd hope an Obsession gets this right - check the center spot is actually the center. Admittedly, all of my mirrors have been Chinese, but on the two 12" examples, collimating with a slightly defocused star resulted in the laser (Farpoint, so no slouch) being significantly off center at any elevation. Checking the centering of the spot on both, revealed why.

Russell

Thanks - Yip I found after installing the mirror (sheesh... should have checked first) that the centre donut is not correctly placed. There is an etched centre mark - just on the inner edge of the donut. Quick measurements show this looks pretty accurate as geometric centre. A bit disappointing to not have the donut placed correctly with care - but easily remedied I guess. I take it into account when collimating. I had 1 brief session after I caught this and Jupiter looked pretty good, despite the mirror not being cooled fully. Still awaiting a good night to really test drive... (Typed to the sound of rain beating on the window... After I thought "maybe..." earlier on!

[Dannae]

Hi Niall,

My mirror was a Pegasus one and it had a crosshair also (noit sure if Torus/OMI use a crosshair) but I remember having to nervously centre spot the mirror myself - are you the second owner? Dave K doesn't spot the mirror as it arrives directly from the mirror maker.

If you are going to replace, check out the catseye hotspot which comes with the necessary template to find the centre:

http://www.firstlightoptics.com/catseye-collimation-tools.html

Regards

Dannae