Newbie with Skywatcher 200p - Tube rings affecting collimation?

[Mike_F]

Hi all,

Have just received (in incredibly fast time!) my new Skywatcher explorer 200p 8" Newt from FLO - which I'm putting on top of my EQ5 mount.

Of course, it's new kit - so it's cloudy outside So decided to spend a bit of time on setup instead.

It was quite a bit out of collimation when it arrived - mostly down to the locking screws on the primary being loose. But no probs, I have a cheshire and a Hotech SCA laser, so collimation should be the work of a few mins - and sure enough it was. I quickly got it into a nice collimation with the OTA off the mount.

Then, using the tube-rings and dovetail supplied with the OTA, I put it on the mount and re-checked collimation. To my surprise, it was out again by quite a lot. The secondary was no longer aligned with the centre of the primary.

At that point, with the OTA still mounted, I also noticed that one of the spider vanes had become a little loose. "Aha" thinks I, "root cause!".. but there was a nagging doubt in my mind - as I'd checked all the spiders when I unpacked it - and they were all tight enough to make a pleasant "ting" noise when flicked gently.

Thinking something else must have caused the spider to come loose, (and misalign the secondary) - I looked elsewhere and experimented with loosening the tube-ring clamp bolts. Lo and behold - the scope came back into collimation and the loose spider vane went tight again. What the devil?

I knew enough not to overtighten the tube rings, as they can pinch the OTA, and only mildly applied finger-tight pressure to the bolts - yet it distorted enough to cause the symptoms I'd observed.

Further experimentation leaving the HoTech laser in the focuser and tightening/loosening the rings reveals that even with the rings loose enough that I can slide the top-halves slightly up-and-down the OTA (using the play in the hinges) - it still affects collimation (principally secondary alignment), and I'd be worried about using the scope with the rings that loose. (Don't want the OTA sliding around in the rings when I'm slewing!)

My questions are these:

- I know large-ish F5 newts are sensitive to collimation - but blimey - should it be like that?

- Is it generally accepted practice then to tighten the rings enough so that you feel comfortable the OTA isn't going to slide out - and then collimate?

Thanks for listening to the ramblings of a newb.

Hope you guys can help.

Mike.

[dweller25]

I have the longer SW 8" F/6 newt mounted on an EQ mount. I can tell you that tightening the tube rings has no effect on collimation at all. Something is wrong - is the tube round or oval ????? and are the tube rings round ?????

[Merlin66]

Hmmm

Doesn't sound right. Usually the dovetail clamp rings which match the o/d of the tube won't distort the tube enough to cause mis collimation.....

In the meantime as you've probably figured out, you may have to clamp the tube and then collimate. Another option is to use a third ring above the dovetail rings; tighten it to the tube so it doesn't move, collimate and then leave the dovetail rings very slightly loose, the tube will be held by the fixed ring and you can then rotate the tube safely to bring the eyepiece to a comfortable position at any time. Hope this makes some sense...

[Mike_F]

Just got off the phone after seeking advice from FLO. The helpful chap at the other end suggested that collimation once setup and in the rings is the way to go. I wonder how much of a pain that will be - having to recollimate if I rotate the tube during the evening's viewing to make the eyepiece position more comfortable (and therefore loosening & re-tightening the tube-ring bolts).

I guess the only way to find out, is to give it a try. There's every chance I guess that the slight miscollimation caused by this might have no real impact on image quality. Time and a few cloudless evenings will tell..

[stuartsmith01]

its not right, send it back, its always going to be a concern.

[lukebl]

Hi,

Something to watch out for with a new Skywather 200, is that the morror itself sometimes slips around in its holder resulting in collimation changing from one minute to the next. It's something which Astro-baby pointed out with her identical scope, and I found out was a problem with mine. I first noticed it when there was a 'clunk' whenever I swivelled the tube round on the mount.

If I'd realised the problem sooner I would have sent it back to the supplier. Tightening the clamps didn't help, but I simply carefully wedged some card in between the mirror and each of the side clamps to stop the movement. Seems to have worked.

Regards,

Luke

[Doc]

I have never heard of this before...

Once collimated on or off the scope the collimation should be the same. Tightening tube clamps should not make the collimation go out.

Are you sure It's not something else that just happens to occur when you tighten the clamps. Have you checked all spider vanes, centre secondary screw. Is there slight movement in the primary when your scope is in the cradle.

[Bizibilder]

I had a problem when my scope was new in that I thought the spider had become loose - so I tightened the loose screw. I then saw that the screw was OBVIOUSLY a couple of turns loose again! The reason, it turned out, was that the tube is very thin and the spider arm threads were catching on the tube. The nuts on the spider were simply tightening onto the tube wall and were NOT tightening the spider. Once I realised this there have been no further troubles.

(I hope the above makes sense!) - It may help you as well.

[Astro_Baby]

Hmmmm - scope collimation on a reflector can slip from over tight tube rings. Most refelectors are quite thin walled but this does seem odd.

I'd check the following......

1/ Is the tube itself round with no obvious defects - the seam on the scope may be proud.

2/ Is the primary mirror slightly loose in its holder (this seems a very common problems with new scopes). The primary mirror clips should stop the primary moving but dont apply too much pressure. A good guide is you should be able to slip a sheet os standard paper between the mirror and its clips on the top of the mirror while the sideways movement shoud be restrained in a similar way.

I found the screws holidng the primary in were slightly too long on mine so I milled a them down by about 1mm and that did the trick.

Scopes today do seem to have very thin walls and of course the primary can be quite heavy. So tube flex can be a problem though I have never seen mine do what you describe.

Something else to think about - as the scope balanced ok ? Too much weight at one end may be causing excessive flex in the tube.

[FLO]

Just spotted this thread mike. It isn't unusual for a Newtonian's collimation to shift slightly if it is first collimated on a table then later fitted with rings and mounted outside. Collimation can also shift slightly as the night progresses and the tube and mirror cool. After saying that the laser spot shouldn't stray far from the primary mirror's central donut.

You have recieved some good advice in this thread, if you are still having problems or are concerned we'd be pleased to send you another OTA for comparison

HTH

[Deneb]

...we'd be pleased to send you another OTA for comparison

HTH

Could you please send me a SW 250PX for a comparison as well

PS. Im only joking...

[alfingido]

I just got my new Hotech laser collimator and as we have more cloud and fog here this evening, thought I would check collimation on my 200P.

It was out a little, now its spot on. But, I loosened the tube rings and turned the tube. Retightened the clamps and collimation had changed. After a few tightens and loosens of the clamps, it was obvious that each time I moved the screws, the collimation changed. I could actually recollimate it using the clamp screws!

The trick is to collimate and dont move the tube! I will be buying a step for viewing when the eyepiece is too high. The spider vanes didnt appear loose at any time.

After my experience, I would suggest mine is not the only 200p afected like this, and its a brand new black model.

Allan

[Mike_F]

All,

Thanks so much for all the valuable feedback. It's good to know that so many are willing to advise a self-confessed newb

Steve - great to see you're here to lend some balance.. and that's a very kind offer! Hopefully, the below will either allow you the luxury of saying "that's normal, don't worry" - or saying "ooeer! that's a bad'n - I'll replace it"

I thought the best way to proceed would be to show in a more precise fashion than words can manage - with a vid.

Hopefully, this will demonstrate my concern much more accurately, and we can reach some common ground. Is what I'm seeing normal, or abnormal?

YouTube - Collimation

[FLO]

I would say that is normal, the collimation does shift slightly when the tube-ring is tightened. The tube is made from rolled steel, it is never perfectly round so will move when clamped. Flexure in Newtonians is common which is why it is best to set them up before collimating. This is also why the Skywatcher 190MN (intended primarily for imaging) has a tube made from 1.5mm thick steel, to reduce flexure.

But, if you are still concerned, our offer to send another OTA for comparison still stands

HTH

[alfingido]

All,

Thanks so much for all the valuable feedback. It's good to know that so many are willing to advise a self-confessed newb

Steve - great to see you're here to lend some balance.. and that's a very kind offer! Hopefully, the below will either allow you the luxury of saying "that's normal, don't worry" - or saying "ooeer! that's a bad'n - I'll replace it"

I thought the best way to proceed would be to show in a more precise fashion than words can manage - with a vid.

Hopefully, this will demonstrate my concern much more accurately, and we can reach some common ground. Is what I'm seeing normal, or abnormal?

YouTube - Collimation

Mike

I get exactly the same results as you. So, I collimated with the tube rings tight. If I need to loosen them ( to rotate the OTA) collimation drifts then returns as I tighten up again.

Looks like a "feature" of the OTA !

Allan

[Rusty Strings]

Is the change in collimation noticable when viewing?

John

[Mike_F]

@Alan - many thanks for confirmation, that'll save Steve having to send out another example.

@John - That's probably the most pertinent question, and the one I arrived at for myself after speaking to Steve @ FLO. It'll be a right royal pain to have to recollimate after every tube rotation (especially if it means I have to remove the dewshield I'm about to purchase in order to do it) So, if it isn't really noticeable through the viewfinder - who cares? Unfortunately, it's been too cloudy so far since the 'scope arrived for me to test that.

I also can't quite believe given the lengths some are prepared to go for "perfect" collimation, that primary and secondary misalignment by 1/4" or so wouldn't be a visible difference. Time and a few cloudless nights will tell..

[SpookyKatt]

Hmmm I have a 200 (one of the blue ones) but I must admit

that I just keep the scope in the rings clamped all the times

and most of the time I never rotate the scope when observing

so I've never noticed this problem.

The tube walls certainly are thin and could flex while tightening the

clamps I guess.

I'm not sure there would be any way to fix this though.

Good video showing the problem though.

Kathleen

[FLO]

It isn't something unique to any particular Newtonian, it applies to Newtonians in general. Once the rings are fitted there really isn't any reason to remove them.

[Bizibilder]

Useful video! I now suspect that the ring you adjusted is slightly distorted - take the tube out and reclamp the ring as if the tube was in place then measure (carefully!) across the ring in the line of the hinges and clamps AND at 90 degrees to this ("top to bottom"). I bet the "hinge to clamp" dimension is smaller - the ring casting will have slightly distorted on cooling - the net result is that the ring is "pinching the tube" and is causing the slight movement of the secondary mirror.

[Astro_Baby]

I'd agree with Stsve. A certain amount of flex is always present in most newts especially when EQ mounted where the tube is under a bit of pressure.

[Mike_F]

Useful video! I now suspect that the ring you adjusted is slightly distorted - take the tube out and reclamp the ring as if the tube was in place then measure (carefully!) across the ring in the line of the hinges and clamps AND at 90 degrees to this ("top to bottom"). I bet the "hinge to clamp" dimension is smaller - the ring casting will have slightly distorted on cooling - the net result is that the ring is "pinching the tube" and is causing the slight movement of the secondary mirror.

Hi Bizbilder,

Slightly ahead of you on that one - I too originally suspected that the ring might be out-of-round a bit. Not having any accurate measuring equipment to hand, I decided to test the theory by swapping the rings around to opposite ends of the dovetail. No matter which way around they are, the same effect occurs. Lower tube-ring can be uber-tight with no effect on collimation - upper one causes enough tube deformation to misalign the secondary. So, if they are out - it's by an equal amount.

Thinking about it a bit from an engineering point of view, it makes sense. The lower (primary) end of the tube is effectively supported by a large cast-metal ring bolted to it, which holds the primary. The open end is already under tension from the spider vanes, and doesn't have the support of the cast-metal frame found at the primary end. Thus it should really be no surprise that the open end is a bit more sensitive to pressure from the tube rings.

I think the answer is to use as much pressure as is required on the lower ring to keep the tube secure on the mount, and go easy on the open-end ring to minimise the distortion and subsequent misalignment. I'd rather be observing than recollimating!

Let's hope we get some clear nights soon..

Steve - let's talk about some anti-dew equipment soon..