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Collimation

E_ri_k

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E_ri_k

E_ri_k

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Hey guys. Again I have another question about collimating my 10" Newtonian....

I am using a Glatter laser and TuBlug which are working great.

I notice that when the scope moves, the collimation shifts, usually quite a bit. You can hear the primary mirror shift around sometimes and "clunk", when moving from on part of the sky to another.

Is this normal? Should I tighten things up a little. Obviously there has to be a small clearance between the mirror and the clips, but I'm not really confident to remove the mirror from the OTA to check, in case I stuff something up...

Thanks.

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E_ri_k

E_ri_k

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Yeah the locking screws are done up tight. I cant tell if the clips are too loose either without removing the mirror. What should the gap be between the mirror and the clips?

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E_ri_k

E_ri_k

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I might check the clips for clearance. I usually have the scope less than 35 deg as its a bit awkward getting up high enough to see down the tube, even with my little ladder....

I'll let you know what clearance I have when I check it tomorrow.

Thanks.

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AlBoning

AlBoning

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Primary mirrors are mounted so that no force is imparted from the mirror cell. The mirror clips should not be in direct contact with the mirror. If in fact the problem is the mirror shifting in the cell you might consider a few thin pieces of felt around the perimeter of the primary mirror to inhibit such movement.

However, I'm guessing here now, if your OTA is rolled steel you might be having the problems I discovered. The tube wasn't round but a slight oval. Measured across the vanes the internal diameter differed by 4 mm. Tightening and loosening the tube ring bolts, and/or rotating the tube in the rings changes the forces on the spider which in turns throws the secondary mirror out of alignment. Second the tube isn't quite as stiff as one would like it to be and sags just a little bit. Of course the direction in which it sags changes in an EQ mounted scope each time you change targets that throws off the primary's alignment.

My solution was another pair of tube rings to stiffen the OTA and hold it more consistently round from one tube rotation to another. I also went to a longer dovetail mounting plate.

These two articles were my source material ...

Andy's Shot Glass - Wilson Rotating Rings - Astronomy & Astrophotography for Non-Gazillionaires

Rotating Rings for Reflector Telescopes

Now my scope rings can remain loose all the time while the Wilcox rings stiffen the tube and hold it round from one rotation to the next, the collimation doesn't change with tube rotation or change with altitude, and I can freely rotate the tube so that the eyepiece is where I want it to be and that is never again over the counter-weights preventing me from placing my observing chair so that I have a comfortable seated viewing position.

Adding another dovetail shoe on the other side of the focuser means I can rotate the tube so the eyepiece is on the other side and still have the finder on top. Mounting the Telrad on a dovetail allows me to switch from from finder to Telrad as needed.

Once I installed 1/32" bondable teflon to lower the frictional forces the tube started rotating so freely (since it was really no longer necessary for the tube rings to be tight, and I most certainly didn't want to bothered any more with tightening/loosening them for each rotation) that I resorted to a doorstop between the OTA and dovetail mounting plate to hold it in place.

It occurs to me that the SkyWatcher OTA have primary mirrors measured in mm, but the Orion (US) branded versions are in inches so there is a slight difference in the primary mirror size, for instance an 8" mirror is 203 mm not 200 mm. It could be though that they share the same mirror cell. Having checked I know there is no room for lateral movement of the primary mirror in the mirror cell of my 8" Orion, but I can see how were it a 200 mm mirror that there would be.

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Tinker1947

Tinker1947

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My Newt doesn't suffer from any clunking ect, i would run some check on the screws ect that attach it to the tube and also the mirror clips, my clamping screws are just finger tight and star checks show no movement since it was last collimated several months back....i use a Cheshire to collimate with...

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E_ri_k

E_ri_k

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Thanks for the info guys. I like the idea of felt around the mirror. I will pull it out tomorrow and let you know how I go. The screws holding the cell in the back of the tube are tight. I'm starting to think the mirror may just be sliding around a little, but I have never had the mirror out, so I cant say.

Ill pull it out and take some photos tomorrow.

Erik

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E_ri_k

E_ri_k

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Just to add. When I star test, everything looks great. I just worry that as the scope tracks an object the collimation slowly gets worse and in turn stuffs the subs?

Erik

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umadog

umadog

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The moving mirror does suggest that something is loose in the cell or in the way the primary is held into the cell. Move the scope around in the light and see if you can figure out what is moving and when. I doubt your problems is the locking screws: if the springs are sufficiently strong then you don't need the locking screws at all. In fact, they'll cause more problems than they solve. Leave them loose and if you find you need them just upgrade the springs instead.

Also, check the tension on the spider. If you pluck the vanes, they should all be of similar tension. If they make a dull "thwonk" sound then you need to tighten them a little. If the vanes are loose, the secondary will move when shift to different elevations. Make sure you check this because it could be that the sound being made by the primary is a different issue.

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E_ri_k

E_ri_k

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Ok, so today I pulled the mirror out to see what was going on. Found that the mirror was free to move around the cell in a circular pattern, with about 1mm gap between the mirror and the cell. The rubber clips were also quite loose.

I put a bead of silicone around the cell and replaced the mirror, also tightened up the rubbers, but they are only resting on the mirror, not pushing down, they can still move freely when wiggled.

Did a rough collimation in the daylight, and tweaked the secondary mirror a touch.

So went out tonight and collimated properly using the Glatter Laser collimator.

I'm pretty happy with the silicone, now when I can move the scope in almost any direction and the collimation stays spot on, according to the laser, nothing moves. Before when I used to move the scope, you could see the laser spot shift.

Tried to get some photos of the star test, but they are not fantastic. Visually everything looked really good except for a bit of "wobblyness" around the edge of the star.

Attached are some photos:

Focused out on star

Loose rubber clip

Loose rubber clip

Tightened Rubber clip

Focused in

Focused in

What do you guys think?

Erik.

post-27504-133877761868_thumb.jpg

post-27504-133877761876_thumb.jpg

post-27504-133877761885_thumb.jpg

post-27504-133877761894_thumb.jpg

post-27504-133877761901_thumb.jpg

post-27504-133877761907_thumb.jpg

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umadog

umadog

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Looks good. Note, however, that those aren't star tests you have photographed. Those images are taken very far out of focus and the only thing they will help you evaluate is whether your mirror has cooled and the position of the secondary.

A true star test looks like this: http://www.willbell.com/tm/IMAGES/StarTest2.jpg The scale is deceptive: the images look larger than the the star test will look through the scope. You need to work with a high power eyepiece to see those patterns. Note that in those images you don't see the secondary shadow and the spider. The defocus is fairly mild; you should be seeing only the first few rings (perhaps 8 or 10 or so). The star test is used to evaluate axial alignment and imperfections in the optical train: Star Testing Astronomical Telescopes, A Manual for Optical Evaluation and Adjustment, Second Edition by Harold Richard Suiter

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jimmyjamjoejoe

jimmyjamjoejoe

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Thanks from me too, I was making the same mistake on the star test and will use high power next time we get a clear night :D

It's recommended to go crazy high power (like 400x or so) for a star test, but barlows will mess with the results... Not many people have an eyepiece that gives 400x with no barlow though, so just go as high as you can with no barlow, and very, VERY slowly take it out of focus, you'l see the above image, but it'l be small and like everything astro, will take a serious looking at to see it properly.

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pajr777

pajr777

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It's recommended to go crazy high power (like 400x or so) for a star test, but barlows will mess with the results... Not many people have an eyepiece that gives 400x with no barlow though, so just go as high as you can with no barlow, and very, VERY slowly take it out of focus, you'l see the above image, but it'l be small and like everything astro, will take a serious looking at to see it properly.

Hmm I can only muster x100 un-barlowed, possibly not worth the time?

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