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Secondary collimation

almcl

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almcl

almcl

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All the advice I've seen on collimating a Newtonian suggests that collimating the secondary shouldn't be needed very often, so I am wondering why this runs counter to my experience?

Every time the scope comes out of its box, the secondary is mis-aligned. Doesn't matter if I use the cheshire eyepiece, or the recently acquired Seben laser collimator, both indicate that we are out of whack. A moment or two with the thin allen key and then with the main mirror mount screws and all is well, but it makes me wonder if there's something I am doing wrong? The routine is to remove the scope (Skywatcher 200 P) from its home made wooden box in the unheated garage, carry it outside (carefully!), mount, collimate, observe, image or whatever, then carry it back to its box. Next time it comes out the laser won't be hitting the centre spot and it's necessary to do it again.

Anyone else have this problem? The vanes certainly seem very flexible and insubstantial, the tube is also less than rigid and the focuser is pretty flexible as a result and up till now I have assumed that it is a combination of some or all of these that creates the apparent secondary movement but would appreciate the community's thoughts.

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Tinker1947

Tinker1947

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Do you not use the 3 adjuster screws on the outside of the secondary holder to adjust the laser, the secondary mirror once set up correctly  rarely needs touching, only one of my newts had to have it tampered with after i flocked the tube, its been perfect over the last couple of years....

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Charic

Charic

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Something not right?

The central bolt with the Philips head moves the secondary mirror towards or away from the primary mirror. The secondary mirror is good when centred, as viewed though the focuser, too the focuser, and concentric to the wall/tube of the focuser.

Those adjuster/locks should be quite slack when you first adjust the secondary mirror, fully slack so that when you turn the Philips screw clockwise, it draws the secondary away from the primary, which also squares the secondary mirror to the spider mount that supports the mirror, then gently unscrew anti-clockwise to move the  secondary  towards the primary, until centred under the focuser.

When aligned, nip up those adjusters to just finger tight. Next is to tilt the secondary, which aligns with the primary. Were looking for those Three mirror clamps 120° apart. we see this by tilting the secondary mirror, enabled by adjusting those Alan bolt adjuster/locks? When you see all three mirror clips, and the secondary mirror is still concentric to the focuser tube, and circular ( not elliptical in appearance) then gently tighten the Alan bolts. observing you don't off-set the mirror. Once this is completed, you should not have to touch the secondary that often.

Thats the principle, and this was the guide that taught me. http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

HTH!

To be honest, If I didn't collimate just for the fun of it, my secondary never moves, and I have never had a slack  Philips Bolt/ Alan  Adjuster/lock or spider vane bolt ever come loose!  Note that I have a Skyliner Dob, not the Explorer EQ, but that shouldn't make any difference, and that my scope is fully assembled 24/7

I'm not sure why you need to adjust every time?

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almcl

almcl

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Thanks both.

Charic, I used Astro baby's guide, too. But unlike your experience, my secondary doesn't stay aligned (for whatever reason).

My experience is much more like Tinker1947's - that knob modification is a neat idea, might try that!

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Charic

Charic

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The knobs, akin to Bobs Knobs is a good idea, but if the Alan adjusters are coming loose, for whatever their reason, why should the thumb knobs be any different, and if you purchase the genuine Bobs Knobs, their not cheap?

The main advantage of the knobs is that its a tool-less, operation, and you can't loose the Alan key in the long grass at night! or drop a tool down the OTA, possibly striking/ hitting the Primary?

The only other conclusion I have for the adjusters to keep coming loose is that they have lost their threads, and therefore grip / tension is not being maintained.

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brantuk

brantuk

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"The vanes certainly seem very flexible and insubstantial"

The vanes should be very tight and not flex at all. Mine are rigid and don't budge. And they should position the secondary perfectly at the center of the tube - so ensure they are equidistant from tube edge to center. This also makes the tube more rigid which will lend support to the focuser tube. Sounds like everything has been slackened off at some point (possibly for flocking) and not put back properly. Hth :)

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geordie85

geordie85

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This may be a stupid question but how do you know it's your secondary that's out of collimation and not just the primary that needs tweaking?

Every time I set up my 200p I always check collimation and every time it's only the primary that needs re-alligning

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almcl

almcl

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Lots to answer here :-)

I can play tunes on the spider vanes, their mounting nuts are tight enough to be producing a slight dimple on the on the tube surface, yet the secondary can easily be twisted/moved by light finger pressure,so I stand by my comment that the vanes are not substantial enough .

The laser is collimated, although the spot is elliptical rather than oval.

The secondary adjusters have now been tightened a bit more, they weren't loose but I'll see if that improves matters.

The secondary needs adjusting if the laser spot isn't centred on the main mirror, this is quicker than using the Cheshire (and easier in the dark) but both give the same conclusion.

Waiting for a clear night to try again!

Thanks for all the comments.

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libraryman

libraryman

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My experience was the same with an Orion Optics 200mm f/4......it turned out to be flexure of the rings and tube on my losmandy plate, flexure of the focuser was amplified and the result was that the secondary appeared to move as I flipped the mount !!

Once I stiffened everything up the 4 vane spider held it in place.....

Ray

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geordie85

geordie85

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The secondary needs adjusting if the laser spot isn't centred on the main mirror, this is quicker than using the Cheshire (and easier in the dark) but both give the same conclusion.

Waiting for a clear night to try again!

Thanks for all the comments.

This sounds like it is your primary that needs adjusting, not the secondary.

As I said in my earlier post, every time I set up my scope I need to re-adjust the primary back to centre.

Next time instead of adjusting your secondary, just try adjusting your primary via the small screws at the bottom of your scope.

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almcl

almcl

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This sounds like it is your primary that needs adjusting, not the secondary.

Next time instead of adjusting your secondary, just try adjusting your primary via the small screws at the bottom of your scope.

Sorry, my description may not have been clear, but adjusting the primary does not centre the laser spot on it. It moves the returning beam into the centre of the eyepiece target, and this does indeed have to be done every time but only after the secondary has been correctly adjusted.

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Bart

Bart

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My 10" needs very little adjustment every time it comes out sitting on a sack truck. The secondary might need a little tweak with the laser (+barlow) and then the primary a little tweak also. Very small mind you.

The 16" needs the same, just a little more. The laser (unbarlowed) might be 1 cm off the centre ring on the primary. Bring that back in, put the laser through the barlow so that the reflection of the primary ring comes back into centre by adjusting the primary bolts and kerpow! off we go.

I consider it necessary to do or check this every time, even more than once if I'm out for hours, only takes 30-60 seconds really. Mind you, with the weather we've had in the last year, i haven't had cause to do it very often!

Barry

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Tinker1947

Tinker1947

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I recently joined the Dob Mob with a 300P Flextube and any time I get it out (which has only been 4 times in weeks---weather) I need to tweak the secondary. I did think this excessive.

How do you collimate a laser?

I have the same scope 300P and it does need a tweak every time i put it out, but its very small and probably wouldn't make and difference to the views through the EP's if the budgets will stretch to it a Hotech is well worth the outlay....I use a Baader 1.25/2" click lock to hold it in the focuser.....

http://www.firstlightoptics.com/hotech-collimation-tools/hotech-sca-laser-collimator.html

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Dave In Vermont

Dave In Vermont

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My 200mm F/4 Rich-Field often needs a tiny adjusting of the secondary and/or primary. I use both an Orion LaserMate II (which comes fully collimated and is guaranteed to remain so - and it has), and a Cheshire. They both always have agreed with each other. It's never bothered me that this happens. I attribute this to temperature variations, and the finicky nature of an F/4.

I also check the collimation with the draw-tube of the focuser racked all the way in, and then with it racked all the way out. It's never made a difference. I also rotate the laser with it lightly held in place by the retaining-screw while noting it's position centred on the mirror's centre spot. This, too, doesn't deviate.

All in all, I find the primary needs a tweak about twice as often as my secondary. One monkey-wrench about this scope: It has a three-way support vane, rather than the traditional four-way.

Sit, Spot! Sit!

Dave

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