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How accurately must I collimate?

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#1 cloud_cover

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Posted 03 March 2013 - 10:00 PM

I have an f/4.7 14" scope that I have yet to actually use, due to personal reasons. However, I've set it up a few times and run through collimation with a Cheshire and Barlowed laser (Howie Glatter)
I've noticed that although the laser spot appears to stay in the same place when moving it from horizontal to vertical, the center spot shadow (A simple paper doughnut sticker that you'll use with filing papers) moves about half its diameter across the face of my Barlow when going from horizontal to vertical.
The mirror support is a 4-point support and its firm although not completely tight (I'm not sure if tight will induce astigmatism in the mirror).
My question is: In light that the collimation shifts slightly in the barlowed Laser when changing altitudes, how accurate must I be in collimation since I clearly won't be re-collimating everytime I point the scope. Also, will there be any use for an autocollimator in my case since the collimation shift is already greater than the accuracy of my barlowed laser?
Or should I try to make the mirror even more snug (by shimming with rubber padding) so that it doesn't shift?
Thanks for your opinions! :)

#2 Deep13

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Posted 04 March 2013 - 01:01 AM

Short answer: it has to be quite accurate at f/4.7. But, the alignment changes when you change altitude? Is the barlowed laser tight in the focuser?

#3 cloud_cover

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Posted 04 March 2013 - 01:03 AM

Yup. Its tight to the point its almost difficult to fit in. The laser itself is perfectly collimated but that's besides the point, of course :)

#4 Deep13

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Posted 04 March 2013 - 01:13 AM

So, the secondary vanes are flexing from the weight of the secondary?

#5 Damo636

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Posted 04 March 2013 - 04:45 AM

My 12" f5 does exactly the same, from horizontal to vertical there is a slight shift in collimation. I stripped the mirror and cell out and replaced the adhesive pads securing the primary with cork pads and nylon screws for lateral support. My thinking was that the mirror was shifting slightly on the adhesive pads. Its made no difference though, I still see the slight shift as I raise or lower the ota through 90°. It is slight though so I just collimate at 45° & forget about it.

#6 nicknacknock

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Posted 04 March 2013 - 05:29 AM

My 12" f5 does exactly the same, from horizontal to vertical there is a slight shift in collimation. I stripped the mirror and cell out and replaced the adhesive pads securing the primary with cork pads and nylon screws for lateral support. My thinking was that the mirror was shifting slightly on the adhesive pads. Its made no difference though, I still see the slight shift as I raise or lower the ota through 90°. It is slight though so I just collimate at 45° & forget about it.


Couldn't agree more. Collimating while the scope is at 0° is not helping in anything. 45° gives you a "middle of the road" solution with little deviation if you shift towards zenith or horizon.

And yes, collimate. Otherwise good views will be limited to low power views only...

CS,

Nicos

#7 Jon Isaacs

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Posted 04 March 2013 - 05:32 AM

My question is: In light that the collimation shifts slightly in the barlowed Laser when changing altitudes, how accurate must I be in collimation since I clearly won't be re-collimating everytime I point the scope.


I am sure that Vic, Jason or Nils will respond but I believe that is too much shift. In general you want the primary mirror to be aligned within a small fraction of the coma free field, 2.3 mm for an F/4.7. I am sure if the is a factor of two in the Barlowed laser read out but it sounds to me like your shift is significantly larger than that.

There are possible sources of collimation shift... How much does the non-barlowed dot move on the face of the mirror when the scope is moved from horizontal to vertical?

Jon

#8 cloud_cover

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Posted 04 March 2013 - 09:34 AM

Thanks for the advice!
The laser dot either doesn't move or moves within 1-2mm when shifting the scope. I'll double check it tonight when I get home again.
Yeah, its definitely more than 2.3mm I'm using a 1.25 Barlow with a 1mm hole cut in the middle of the flat lenscap (Actually Jon, is your favourite GSO 2x Barlow :) ) and the circle moves probably at least half of the cap diameter, which makes it about 15mm.

#9 csrlice12

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Posted 04 March 2013 - 09:36 AM

Maybe consider replacing the collimation springs with better ones (or, how tight are the one's there)? Jon's right, it shouldn't shift a lot, if it is, something ain't quite right....

#10 cloud_cover

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Posted 04 March 2013 - 09:50 AM

Thanks for the advice! I'll have to check.
Its not quite the usual setup: Its an old Tscopes with a rigid square plate and 4 point supports. There's no collimation spring as such but 3 screws on the back and they feel plenty stiff. The 3 screws together push the base plate in their various directions while the base plus support columns move as one.
I suspect maybe I didn't secure the mirror enough. There's just a slight bit of play between the supports. I'll wedge in the mirror tighter and see if it helps. Then hopefully the weather will co-operate and I can do a basic star test :)

#11 catboat

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Posted 04 March 2013 - 10:15 AM

Flex or play somewhere.

Is your focuser board flexing? Grab hold of the focuser and see if you can induce some flex by force.

Another possibility might be play between draw tube and focuser body.

#12 Jarad

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Posted 04 March 2013 - 10:30 AM

If I understand correctly, the laser dot on the primary isn't moving. So that should rule out flex in the UTA (focuser, spider, secondary etc.) as the culprit. Sounds like it has to be something about the primary. I would check the following:

Collimation bolts
Attachment of the mirror cell to the rocker box
Each layer of supports in the cell (i.e. rocker bars, triangles, etc.).
Mirror edge supports (don't clamp down on the mirror from the top, but make sure that the bottom edge supports have no give to them so it can't slide down when you point to the horizon).

For a large movement like that, my money is on a loose connector somewhere that is allowing the cell to flop forward a bit as you tilt down. This will change the angle of the primary and cause the effect you are describing (primary still centered on the laser dot, but the return beam moving quite a bit).

Jarad

#13 cloud_cover

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Posted 04 March 2013 - 10:49 AM

Jarad: Thanks for that advice. It makes a lot of sense since I left the mirror slightly loose that it has a bit of wiggle space, thinking that to do so otherwise would cause astigmatism. I'll go rotate the supports so that all the layers of rubber padding are in contact and the mirror is then firmly wedged between the 5 point support.

#14 Starman1

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Posted 04 March 2013 - 03:57 PM

Yes, it's obvious the primary is moving, and this will have a bigger effect on the star images than slight movements in the secondary (though both are bad).
It's OK to leave your mirror loose so long as the points of support behind the mirror are in the right place.
The mirror should rest only on 2 points at the bottom edge of the mirror, 90 degrees apart.
Ideally, those two points of edge support should press against the mirror edge at the center of gravity (front to rear) of the mirror so the mirror neither tips forward nor leans back as the scope moves in altitude.
What you're looking for is to achieve ZERO movement of the shadow of the primary mirror's center marker on the bottom of the barlow through the entire range of motion of the scope.
Here is an edge calculator to figure out:
--methods of supporting the edge properly
--figuring out where the COG of your mirror is, front to back
--figuring how to improve your mirror cell to work with your mirror.

http://www.cruxis.co...ecalculator.htm

What you definitely don't want to do is to wedge pressure pieces in between the mirror and the cell--they will cause astigmatism to appear in the image.

Now it's possible your cell is moving rather than the mirror. If so, you need to seriously stiffen up the springs behind the mirror cell and/or use locking bolts to freeze the cell in place so it can't move. For springs, do a Google search for Belleville springs. High load, easily customizable, low height, no sideways snaking or flexure.

#15 cloud_cover

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Posted 06 March 2013 - 09:36 PM

Thanks Don! As you surmised, the Primary is moving somehow. It remains rock steady down to about 50deg then suddenly the shadow shifts about 3-4mm. Then it doesn't move again.
Looks like I've got to check my cell tightness...
Having said that, things don't look too bad and mild recollimation with a star isn't too demanding.

#16 FlorinAndrei

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Posted 07 March 2013 - 01:31 AM

I had some issues like that with a small mirror in a commercial cell. There were two problems with it:

1. The support pads were fluffy. Made of felt pads, piece of junk. Replaced them with 3 small nylon rings glued to the cell, then did some grinding with a flat plate to keep the upper faces of all rings in the same plane.

2. In terms of lateral support, the mirror was actually touching the lateral posts with the bottom part of its lateral side. So it suddenly tilted when the scope was crossing a certain angle. I made holes in the 3 lateral posts, corresponding to the middle of the thickness of the mirror, and put some nylon screws in those holes, which then became the lateral support points for the mirror. The cell is oriented so that the mirror has 2 lateral support points underneath it, at equal angles to the vertical. The 3rd point is centered on top.

The lateral nylon screws are not touching the mirror all 3 at the same time. When 2 of them touch the mirror (which is almost all the time), the 3rd is about 1mm from the mirror. This is to prevent pinching the optics. The nylon screws are locked in place with nylon hex nuts, so they don't creep out of perfect adjustment. The whole assembly feels surprisingly solid, despite all the plastic items.

The scheme is simplistic, but it should work for a mirror not too large. The concern with a larger mirror would be the mirror bumping into the lateral support points and breaking the nylon screws; in that case, steel screws with nylon tips might be better.

Now the primary is rock-solid, the laser (either the direct beam or the reflected one) does not move at all, at any angle, as if set in stone. Being an obsessive perfectionist, I am very pleased with that result.

#17 dave brock

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Posted 07 March 2013 - 05:20 AM

Its not quite the usual setup: Its an old Tscopes with a rigid square plate and 4 point supports.


If you mean 4 point support as opposed to the normal 3, 6, 9 etc that the back of the mirror rests on then the mirror is likely not resting on all four but resting on 2 and rocking back and forth to rest on only 3 at one time. This would cause the shift.

Dave

#18 csrlice12

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Posted 07 March 2013 - 09:21 AM

Post deleted by csrlice12

#19 nirvanix

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Posted 07 March 2013 - 09:52 AM

I don't believe that it's that difficult to collimate accurately, and it's certainly worth the effort. If you've never done it with your scope, you should do just once the following:

1) make sure the focuser is perpendicular. good focusers can be adjusted with screws to true them up, cheaper ones may need shimming.
2) the spider/holder and secondary mirror are dead centered in the scope.


You only have to do this once, but it's important so that your collimation efforts aren't wasted. After that it's always just collimating the optics. If you can't get bang-on collimation by adjusting the primary then it probably means your secondary needs adjusting to present a centered view of the primary to your eye. I really like this simple but thorough writeup on collimation by a british astrolady:

http://www.astro-bab...ation guide.htm

#20 acochran

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Posted 07 March 2013 - 04:50 PM

Consider replacing the primary mirror collimation springs with stronger ones.
Also, I have heard the more compressed the springs are, the stiffer the collimation.

#21 davidpitre

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Posted 10 March 2013 - 01:58 PM

Can you describe what you mean in terms of the mirror being "tight". If you mean the clips over the mirror that retain it from falling forward, there should definitely be some small space between them and the mirror, enough to slide a piece of paper.
What type of edge support is there?
As mentioned, a 4 point bottom support is more than suspect in that, if the support is hard, only 3 point will be in effect. If the point are cushioned ( such as soft felt), then the support will be unequal at different points. It is possible that the mirror shifts from one combination of 3 points to another combination as the scope moved in altitude.

#22 cloud_cover

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Posted 10 March 2013 - 07:49 PM

Thanks for all your replies!
My cell looks like this:
http://www.cloudynig...d=reflectors...

As you can see, the mirror is supported by essentially the 2 bottom posts, with the 2 top posts preventing it from sliding off the cell when horizontal. Its held down by screwable clips which are loose. There is a rubbery padding material which I suspect is added by the previous owner(s) although its absent in the stock photos. I'm now thinking its probably this rubbery material that is "sinking" as more mirror weight is placed upon it, which may explain why there is an abrupt 3-4mm shift in the barlowed laser shadow when pointing below (I think) 40deg or so.
I'm going to try removing them when I get home and see if the collimation shift is still there. I suspect the preivous guy placed them there to prevent the mirror from shaking about when being transported and banging against rigid support posts. A very good idea, in my opinion :)
Also want to mention: The mirror has Spectrum Max-R coatings and they have not been damaged by contact with the retaining surface of the clips! Truly a hardy coating.

#23 Starman1

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Posted 11 March 2013 - 02:29 PM

I suspect the pressure points between the bottom two posts and the mirror do not contact the mirror in the front-to-rear center of gravity. That means that when the scope points high the mirror is resting on the back support points and the two lower points, but when the telescope points low, the mirror tends to fall forward in the cell because of how it's supported on the bottom edge.

The bottom two contact points should subtend a 90 degree angle at the center of the mirror (1/4 pie), and they should contact the mirror with a relatively firm or hard surface at the COG of the mirror. When they do that, there is no tendency for the mirror to fall forward when the telescope points low.






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