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Laser on primary moves as focuser racked in/out

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#1 mtb54703

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Posted 04 March 2013 - 10:20 PM

I'm sure this qualifies as another dumb and obvious collimation question, but here goes... :)

I use a Glatter laser/tublug to collimate (I'm still learning the auto collimator) and I thought I had my focuser squared. I've got a spot marked on the inside of the UTA opposite the focuser that I placed by measuring equal distances from the center of the focuser around the inside of the UTA and equal distance up from the bottom edge of the UTA.

Using the laser I adjust the tilt of the focuser (FeatherTouch on an AstroSystems filter slide/focuser board) so the laser hits the spot, the re-install the secondary holder and align it with a sight tube.

Back with the laser in the focuser, I tweak the collimation screws until the laser hits the middle of the center marker on the primary (with the focuser racked all the way in).

Then if I rack the focuser all the way out - the dot on the primary moves. The best way I can describe which direction it moves is that if you imagine the line from the laser to secondary - project that line down to the primary - it moved along that line. Not much, just from center of the CatsEye hotspot hole to the edge.

This movement indicates that my mark on the inside of the UTA opposite the focuser needs to be either closer or further away from the bottom edge of the UTA.

As I've typed this all out I think I've explained this to myself, but I'll toss this out just to get a confirmation.

#2 DavidC

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Posted 04 March 2013 - 11:50 PM

Is it a rack and pinion focuser? It kind of sounds like you have excessive play when racked all the way out. Once the mounting bolts/screws are tight on the focuser, the red dot should not move on the opposite side of the tube. Look at the focuser barrel to make sure there's no looseness when its racked out. I'm not sure how you would go about tightening the focuser barrel though, hopefully someone will chime in with more answers.
David

#3 ThreeD

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Posted 05 March 2013 - 01:17 AM

Technically it is not necessary to actually square the focuser as adjusting the tilt of the secondary should fix everything. The only consequence of not having the focuser square with the OTA is that the optical axis for the primary will not end up being coincident and parallel with the axis of the OTA. In theory this could result in clipping of the light path by the OTA but I think things would need to be very far out of square for this to happen.

What you are seeing is the result of the focuser drawtube not being parallel to the axis of its travel motion. That is, the laser is held at an angle rather than along the focuser axis. Thus when you move the focuser in and out, laser beam itself doesn't hit the same point on the secondary. (If it did then it would also hit the same point on the primary.)

Is the drawtube sagging under the weight of the laser when it is extended? Does the drawtube have any wobble? I believe the feathertouch has screws for adjusting the tension against the drawtube -- perhaps it too loose.

I know my glatter laser is nice solid fit in my focuser. Does if feel like it just fits in the focuser as you insert it or does it seem to have any play? If there is any play then that too could be a source of the problem as the laser may get cocked as the tension ring (or set screw) tightens against it.

#4 Jason D

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Posted 05 March 2013 - 01:30 AM

Technically it is not necessary to actually square the focuser as adjusting the tilt of the secondary should fix everything. The only consequence of not having the focuser square with the OTA is that the optical axis for the primary will not end up being coincident and parallel with the axis of the OTA. In theory this could result in clipping of the light path by the OTA but I think things would need to be very far out of square for this to happen.

What you are seeing is the result of the focuser drawtube not being parallel to the axis of its travel motion. That is, the laser is held at an angle rather than along the focuser axis. Thus when you move the focuser in and out, laser beam itself doesn't hit the same point on the secondary. (If it did then it would also hit the same point on the primary.)

Is the drawtube sagging under the weight of the laser when it is extended? Does the drawtube have any wobble? I believe the feathertouch has screws for adjusting the tension against the drawtube -- perhaps it too loose.

I know my glatter laser is nice solid fit in my focuser. Does if feel like it just fits in the focuser as you insert it or does it seem to have any play? If there is any play then that too could be a source of the problem as the laser may get cocked as the tension ring (or set screw) tightens against it.


:waytogo:

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#5 mtb54703

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Posted 05 March 2013 - 09:44 AM

What you are seeing is the result of the focuser drawtube not being parallel to the axis of its travel motion. That is, the laser is held at an angle rather than along the focuser axis. Thus when you move the focuser in and out, laser beam itself doesn't hit the same point on the secondary. (If it did then it would also hit the same point on the primary.)

Is the drawtube sagging under the weight of the laser when it is extended? Does the drawtube have any wobble? I believe the feathertouch has screws for adjusting the tension against the drawtube -- perhaps it too loose.


Sagging - none that I can detect.
Wobble - same.

I don't see anything that would make me suspect that the focuser itself is at fault. Even with the drawtube racked out it's still tight with no detectable wobble or play.

I know my glatter laser is nice solid fit in my focuser. Does if feel like it just fits in the focuser as you insert it or does it seem to have any play? If there is any play then that too could be a source of the problem as the laser may get cocked as the tension ring (or set screw) tightens against it.


My Glatter is a tight fit as well. If the laser is inserted up to it's collar, it can be wiggled just a tad before the thumb screws are tightened.

And the laser collimation appears dead on - it passes the rotate, clamp, rotate, clamp, rotate, clamp test - hitting the same spot on the primary each time.

This all came to light when I set up the other night and had just finished adjusting the secondary to align the laser in the center of the primary when I noticed the focuser was racked out, so I racked it in and then dot had moved. Then I re-centered it and rack the focuser out, and the dot moved the other direction. I had never thought about racking the focuser in/out with the laser in - I just typically do the collimation with the focuser racked in.

Jason posted some great pictures. But I don't think the collimator is tweaked in the drawtube as illustrated. If the laser is parallel with the sides of the collimator (which it appears to be), and the collimator is held flat against the inside of the drawtube, then I would think it's safe to assume the laser is parallel to the drawtube as well.

For grins and giggles, I tried clamping the laser partially inserted, halfway inserted, full inserted - and the dot on the primary hit the same spot.

So is the drawtube not traveling parallel to the focuser body? If it was some non-crayford, non high-end focuser I would say that's a possibility, but not in this case.

Too much cold weather, snow, and clouds in WI leads one out into the weeds when you can't look at the stars.

#6 Jason D

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Posted 05 March 2013 - 09:59 AM

Jason posted some great pictures. But I don't think the collimator is tweaked in the drawtube as illustrated. If the laser is parallel with the sides of the collimator (which it appears to be), and the collimator is held flat against the inside of the drawtube, then I would think it's safe to assume the laser is parallel to the drawtube as well.


Since you have a Glatter, I do not doubt its good fit in the drawtube -- assuming the drawtube meets the high standard tolerences of the Glatter. My illustration is a representative of the drawtube being un-parallel to the focuser wall.

For grins and giggles, I tried clamping the laser partially inserted, halfway inserted, full inserted - and the dot on the primary hit the same spot.


Just to make sure I understood: If you pull out the laser then tighten it the problem goes away but if you inserted all the way in and tighen it then you notice the laser movement. Did I get that right?

Jason

#7 mrowlands

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Posted 05 March 2013 - 10:11 AM

Just to clarify, does the spot move while you're racking the focuser, or does it only move when you hit the end stop(s)? I've noticed something similar, but only when you bump into the stops in either in-focus or out-focus (I think).

Mike R.

#8 mtb54703

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Posted 05 March 2013 - 10:27 AM

For grins and giggles, I tried clamping the laser partially inserted, halfway inserted, full inserted - and the dot on the primary hit the same spot.


Just to make sure I understood: If you pull out the laser then tighten it the problem goes away but if you inserted all the way in and tighen it then you notice the laser movement. Did I get that right?

Jason


No - what I was attempting to say was that regardless of how far the collimator is inserted into the focuser, once the thumbscrew on the drawtube is tightened the laser hits the primary in the same location. I was not racking the focuser in/out when doing this. I was just trying to determine if collimator was being clamped down at an angle, which I would say is not the case.

I had inserted the collimator about 3/4", then about halfway, then full inserted.

My gut tells me that the entire focuser needs to be be tweaked forward/backward just a tad.

#9 mtb54703

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Posted 05 March 2013 - 10:35 AM

Just to clarify, does the spot move while you're racking the focuser, or does it only move when you hit the end stop(s)? I've noticed something similar, but only when you bump into the stops in either in-focus or out-focus (I think).

Mike R.


Nope - you can actually watch the laser dot on the primary move as you rack the focuser in/out.

The primary is center spotted with a HotSpot. If the focuser is racked about 1/2 way, and the laser is centered in the HotSpot, racking the focuser in the laser moves towards one edge of the HotSpot hole, racking the focuser out the laser moves back towards the other edge.

And it moves exactly in the same path, whether the scope is pointed straight up, or at 30 degrees altitude, so that should eliminate any slop in the focuser.

You getting snowed in down your way too Mike?

#10 dan_h

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Posted 05 March 2013 - 10:52 AM

My gut tells me that the entire focuser needs to be be tweaked forward/backward just a tad.


I agree. You need to remove the secondary and go back to square one. Ensure that as the focuser is racked in and out, the beam does not move off the spot on the far side of the tube. Until this is set up, you can't expect anythng else to fall in line. The trouble is that the distance across the tube is short compared to the distance to the primary so the movement of the beam at the primary is much easier to see.

dan

#11 nevy

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Posted 05 March 2013 - 10:58 AM

My dot use to move a bit when the focuser was racked up & down, I took the draw tube out and flattened the flat bit on the shaft with a diamond wetstone , it only moves a tiny bit now normally when the draw tube is at its highest point.

#12 mrowlands

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Posted 05 March 2013 - 12:25 PM

Hi Mike,

I think I botched my last reply, so...

It's snowing pretty hard - they predict up to 8 inches here. I hope I get to see (through) your scope this summer. I bought a SIPS and am really liking it.

I'm at a loss as to what your problem could be! The Glatter is heavy enough to cause very slight flexure somewhere but that doesn't explain why the spot always follows the same line. Let us know what you find.

Mike R.

#13 Jason D

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Posted 05 March 2013 - 01:04 PM

My gut tells me that the entire focuser needs to be be tweaked forward/backward just a tad.

I am not sure I follow.

Did you try to exert downward pressure on the laser -- along the gravity direction? Does the laser beam on the primary mirror move along the same path? If yes then you problem correlates with gravity and the most likely scenario is your drawtube sagging a little under the weight of the laser collimator.
Jason

#14 ThreeD

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Posted 05 March 2013 - 01:21 PM

For grins I googled and I see a few different references on other websites where people have asked about their feathertouch focuser and the exact collimation problem you are describing. Unfortunately there were no solutions in any of those threads.

The bottom line is that the direction (location on the other side of the OTA) the focuser is pointed should not change when the focuser is moved in/out. The only way for that point to move with drawtube movement is if the laser is not aligned with the axis of movement. This could be caused by laser miscollimation, how the laser is sitting in the focuser (is it cocked?), and/or by the drawtube itself effectively being cocked.

From what you've said I think the only answer left is the the drawtube itself is cocked and thus the axis of the movement is different than the centerline of the tube. This is disappointing to hear given the reputation and cost of a feathertouch. I'd hope that a reexamination of the focuser shows that the tube was sagging under the weight and it was fixed by increasing the tension.

#15 nevy

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Posted 05 March 2013 - 01:31 PM

My dot use to move a bit when the focuser was racked up & down, I took the draw tube out and flattened the flat bit on the shaft with a diamond wetstone , it only moves a tiny bit now normally when the draw tube is at its highest point.

I just checked and the dot stays put , so when I flattened the draw tube shaft it cured the problem.

#16 mtb54703

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Posted 05 March 2013 - 02:19 PM

My gut tells me that the entire focuser needs to be be tweaked forward/backward just a tad.

I am not sure I follow.

Did you try to exert downward pressure on the laser -- along the gravity direction? Does the laser beam on the primary mirror move along the same path? If yes then you problem correlates with gravity and the most likely scenario is your drawtube sagging a little under the weight of the laser collimator.
Jason


I'll attribute my 'gut feeling' to gas and based on everyone's input I will start from scratch again tonight.

After looking back at your illustrations it's now apparent that if the collimator, drawtube, and focuser body are all parallel, and move in parallel, then I should not be seeing what I'm seeing. I agree that either the collimator, or drawtube must be tweaked, or get tweaked as I rack in/out and I'm not seeing it... yet.

I actually have another feather touch mounted on another not yet finished scope that I will swap in and see if it behaves the same.

#17 Starman1

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Posted 05 March 2013 - 06:27 PM

Look at the focuser drawtube top and bottom. Is the hole centered at both ends? Forgive me for asking, but I have seen many that weren't, albeit not on Feather Touch focusers. Measure with calipers.
If the drawtube center hole is not completely parallel with the outer diameter of the drawtube, then it would move as you describe as you move it in and out.

Sag is another possibility. You can test that with a significantly lighter tool, like a sight tube.

I had a little wobble visible in the focuser axis with in/out movement that was, I discovered, due to eccentric bearings in the focuser and a worn/dimpled drawtube. Changing bearings and drawtube solved the issue. It was only visible in the lateral pupil of a Catseye XLK autocollimator, though, so of a several powers of ten difference from your issue. And it wobbled back and forth, while yours moves in one direction.
So the "loose focuser drawtube" possibility could be real. but that's essentially the same as sag.

As could the tipped laser in the focuser scenario. Hence, my first paragraph.
Pulling the laser in and out and tightening it at different points wouldn't correct that problem, and nor would precision in focuser movement.
I'm going to bet a change of focuser drawtube is called for.

#18 mtb54703

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Posted 05 March 2013 - 10:09 PM

Well......

First, since I figured I'd be taking pictures eventually I made a template and attached it on the mirror cover which sits firmly just above the mirror. After swapping between two focusers multiple times to make sure the I could repeat the results I took some pictures. All I swapped was the focuser, not the mounting plate. The old focuser is about 3 years old, the new focuser has never been used (it's waiting for optics in the new scope).

The short story - there was virtually no movement detected with the new focuser.

This animated gif is the the old focuser. I took three shots - racked in, 1/2 way, racked out:

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#19 mtb54703

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Posted 05 March 2013 - 10:11 PM

Then I racked the focuser back in, rotated the collimator 90 degrees and took three pictures:

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#20 mtb54703

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Posted 05 March 2013 - 10:12 PM

Here's the new focuser, collimator rotated back to the original position:

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#21 mtb54703

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Posted 05 March 2013 - 10:13 PM

And lastly, the new focuser, collimator rotated 90 degrees again:

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#22 mtb54703

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Posted 05 March 2013 - 10:25 PM

Nothing appears loose on the old focuser, but something is certainly amiss.

I'll add that it doesn't appear to be gravity related. All these pictures were taken with the scope at 45 degrees. If I pointed the scope straight up I see the same movement along the same line on the template.

Also, when I racked the focuser in/out I did so slowly, as I did notice that if I hit the stop at either end of travel quickly the laser dot would jump (as Mike R noted), so when I took the picture I was careful to not introduce that effect.

#23 Starman1

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Posted 05 March 2013 - 10:37 PM

This is fair proof the drawtube either:
a) has a bore not colinear with the exterior surface, or
b) has a travel not in-line with the focuser body.
I suspect A.

#24 nevy

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Posted 05 March 2013 - 11:06 PM

Could it be that the flat on the draw tube isn't quite flat & uniform ? Mine did that and after I used a wetstone on it it cured the moving dot.

#25 mtb54703

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Posted 06 March 2013 - 12:24 PM

After posting the pictures last night I also fired off an email with the same problem description/pictures to Starlight Instruments and asked what I needed to do to get the focuser back in shape.

They replied to send the focuser to them and they will adjust it and/or replace the drawtube if necessary under warranty.

More places should have super customer support/service like that!






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