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Help with collimation

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#1 stargeeser

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Posted 18 February 2018 - 01:02 AM

I have a 12 inch dob. When using an Orion LaserMate 2 which is collimated ( I checked by rotating it on v blocks) it still is making a circle when rotated in my scopes focuser. I am using a Baader helical focuser insert to do the rotation and it makes the same circle as when I rotate it in the focuser without the baader. Please help. 


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#2 Gipht

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Posted 18 February 2018 - 01:18 AM

The Orion Laser  I have  is somewhat loose in my dob.  In my astograph, the laser fits nice and tight.   In my case, with the Dob, I can do a star test if I am concerned between two possible settings.  I prefer using a Cheshire  with site tube and a crosshair on the Dob  for that reason.  So far the star tests have favored the Cheshire,  at least on the Dob.



#3 Tdesert63

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Posted 18 February 2018 - 02:15 AM

I agree with Viking, Cheshire all the way with my 12" dob, I heard a little masking tape like the blue painters kind will  work on the laser and make it seat better and will come off ez



#4 aeajr

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Posted 18 February 2018 - 05:37 AM

i had an Orion Laser mate.   It was so loose in my Orion XT8 that I sent it back.



#5 Mike W.

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Posted 18 February 2018 - 08:42 AM

Morning, why not collimate your laser in the helical focuser, 

Checking a laser in a "V" block is ok, 

I have a different laser but the process is still the same, with the laser secured in the turntable, loosen the laser's alignment screws just a bit, and like adjusting the secondary start adjusting the laser as you rotate it in the turntable?

When your're done then mark the laser housing to the turntable, tape and a marker will work, mark the location of the laser to the turntable, and the barrel of the helical focuser. 

Now remove the laser or loosen the laser and rotate the turntable under it and check it the turntable is a bit off axis, it may move the spot across the primary, 

Now rotate the turntable back to the marks so they line up, is the laser centered on the primary?


Edited by Mike W., 18 February 2018 - 08:50 AM.

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#6 Itz marcus

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Posted 18 February 2018 - 10:59 AM

Hi,

I think that the op is wondering more as to why it is moving when rotated in the focuser and not when on the v block.

 

Does it make a difference how far the laser is placed (when using a v block? Will placing it to close give a false read of good collimation? 

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#7 zawijava

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Posted 18 February 2018 - 11:03 PM

Could be your focuser is not squared to the OTA.



#8 Mike W.

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Posted 19 February 2018 - 08:52 AM

I had the same issue not with the op's laser but two higher end lasers, both from the same manufacturer.

The Self Centering system was the cause of the mis-alignment, every time the laser was installed the dot hit a different spot on the primary, even when it had just been used to colllimate and with out touching a thing on the scope.

So I came up with solution because for what's paid for them they should work, and work easily.

My solution is a dedicated 2" to 1.25" adapter, my focuser is 2" so for all but three manufacturers you're going to be using an adapter.

Instead of using the self centering system to secure it to the adapter I cut feeler gauge strips the length of the lower section of the laser, the total clearance between the laser base and the inside of the adapter was about .006 of an inch, so I cut three pieces of the feeler gauge strip .003 inch thick and pushed them in between the lower section of the laser and the inside of the adapter, I placed the strips evenly around the laser's base and since I decided on a smooth sided adapter the least expensive was the GSO zero profile, smooth sided to keep the eyepiece holding bands from grabbing just the edge of the adapter when installed in the focuser eyepiece holder.

I removed the thumb screw and replaced it with a set screw that will recess into the side of the adapter, and with the screen facing back towards the bottom of the scope I centered the laser to the center of my primary.

I checked alignment of the laser with a known good high end 2" laser so that I adjusted to a known good collimation.

With the laser mounted and secured in the adapter with the set screw I can just spin the adapter in the eyepiece holder and watch the dot on the primary as described in post #5, you won't need the other laser to confirm with, I did it because I needed to check my results and feel confident about recommending this up-grade.

 

Now collimation is as easy as advertised for the newer generation less expensive lasers with 45° rear facing target screens for adjusting the primary mirror.

All you need to do this up-grade for your laser is an automotive type feeler gauge strip and an adapter, measure the total clearance and cut the needed strip.

About $25 total.not including the price for the laser.


Edited by Mike W., 19 February 2018 - 08:58 AM.


#9 frito

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Posted 19 February 2018 - 11:42 AM

a common issue with mass produced dobs, the easiest solution is get a glatter parallizer thats what i used to use on my XT10, it also makes for a much better 2" to 1-1/4" adapter to use on the daily as well. another solution is get a HOTECH SCA laser but even then if its really loose in the focuser before tightening down it could end up in slightly different positions sometimes. 


Edited by frito, 19 February 2018 - 11:42 AM.


#10 Mike W.

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Posted 20 February 2018 - 10:46 AM

 but even then if its really loose in the focuser before tightening down it could end up in slightly different positions sometimes. 

Sometimes?

 

How or when do yo know it isn't?



#11 aeajr

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Posted 20 February 2018 - 11:14 AM

I have a 12 inch dob. When using an Orion LaserMate 2 which is collimated ( I checked by rotating it on v blocks) it still is making a circle when rotated in my scopes focuser. I am using a Baader helical focuser insert to do the rotation and it makes the same circle as when I rotate it in the focuser without the baader. Please help. 

 

 

I had previously purchased an Orion Lasermate but the fit in the focuser was so sloppy that I could not get a consistent dot position.  The laser itself was aligned but the fit was poor.  I sent it back.  I don't know if this was just the one I got or if this is a problem with this product.   The barrel seemed undersized leading to a lot of slope in the fit in the 1.25" adapter on the scope.  I don't notice any such slop with my eyepieces.

https://www.telescop...yword=lasermate

 

 

I recently added an SVBONY Red Laser Collimator - 1.25'' with 2" adapter.  It has 7 Bright Levels. 

I checked the Laser when I received it and it came in perfectly collimated itself, based on a test on a V block to insure it was collimated.  The included 2" adapter fits snugly in the focuser and if I turn it the dot remains steady. The laser dot is not pinpoint but spreads out a bit.  I only used it once on the XT8i just to test the fit and to be sure the dot did not move around.   It has been loaned to a friend for his new 12 Meade truss Dob.  Easy to use and seems to work well.  It appears to be yielding good results in his scope.   We have only used it with the included 2" color so I don't know if it would perform differently if I put it in the adapter that came with my scope.

https://www.amazon.c...L70_&dpSrc=srch


Edited by aeajr, 20 February 2018 - 11:15 AM.


#12 Starman1

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Posted 20 February 2018 - 12:24 PM

I have a 12 inch dob. When using an Orion LaserMate 2 which is collimated ( I checked by rotating it on v blocks) it still is making a circle when rotated in my scopes focuser. I am using a Baader helical focuser insert to do the rotation and it makes the same circle as when I rotate it in the focuser without the baader. Please help. 

One of two possible errors:

--the bore in the focuser adapter is not parallel to the outside of the focuser and when it is rotated causes the beam circle.

This would cause a small circle as the focuser is rotated. (It could be the outside of the adapter if the adapter is being turned)

--the bore in the helical focuser adapter is parallel to the focuser line but offset from center, which would cause the beam to rotate around in a circle as the helical focuser is rotated.

This would cause a small circle in rotation.

 

However, neither of these would be the cause if the laser makes a circle when rotated in the focuser adapter without focuser adapter movement.

That has other causes:

1) despite your check, the laser isn't collimated  Collimated means the laser beam has NO circle of movement visible at 20', i.e. the beam never moves as the laser is rotated.

2) the registration of the laser in the focuser isn't being checked correctly.  You rotate the laser 90° a few times and lightly tighten the thumbscrew at each position.  Wobble in the hole of the focuser adapter would result in visible errors if the laser is not at least lightly tightened in place.

3) gravity is moving the laser in the focuser adapter, i.e. causing it to be irregularly lined up with the focuser adapter when inserted.

Gravity could be causing movement in the helical focuser adapter as well, so it could be a combination of registration and focuser adapter movement.

 

Since the laser is 1.25" and is being used in an adapter in the focuser, switch to another adapter.  It's just that simple.  Not all adapters are accurately made.

But since you have the issue with 2 adapters, one regular, one helical, I suspect either #1 or #2 above.


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#13 Vic Menard

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Posted 20 February 2018 - 12:47 PM

I have a 12 inch dob. When using an Orion LaserMate 2 which is collimated ( I checked by rotating it on v blocks) it still is making a circle when rotated in my scopes focuser. I am using a Baader helical focuser insert to do the rotation and it makes the same circle as when I rotate it in the focuser without the baader. Please help. 

Any cylinder (laser barrel) that can be "freely" rotated inside another cylinder (focuser drawtube/adapter) will suffer from runout (https://en.wikipedia.org/wiki/Run-out). To reduce runout, you should tighten the setscrew in the focuser drawtube/adapter) as Don pointed out in post #12.

 

Additionally, I would also expect your helical focuser to have some runout. This type of runout can sometimes be reduced with heavy grease, but then you have a greasy mess to deal with. 

 

Your real concern should be whether or not your scope's mechanics can maintain the alignment tolerances necessary to ensure that the image in the eyepiece is not degraded. The outgoing laser beam tolerance is about 3-percent of the primary mirror diameter, or 0.36-inch for a 12-inch Dob (laser dot relative to the center spot). The primary mirror tilt alignment is much more critical, about 0.005mm times the focal ratio cubed, or 0.6mm (0.024-inch) for an f/5 Newtonian mirror. I doubt your laser will be up to the task, but if you have access to a simple collimation cap or a Cheshire eyepiece (either one magnifies the primary mirror tilt error 2X), the actual read will be 0.05-inch, which is easy to see and easy to correct (collimation cap pupil/bright Cheshire ring relative to the center spot).



#14 MrJones

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Posted 20 February 2018 - 12:49 PM

I have a 12 inch dob. When using an Orion LaserMate 2 which is collimated ( I checked by rotating it on v blocks) it still is making a circle when rotated in my scopes focuser. I am using a Baader helical focuser insert to do the rotation and it makes the same circle as when I rotate it in the focuser without the baader. Please help. 

For exactly this reason I collimate my GSO laser in my scope (Z12). It's probably never going to be perfect with all the imperfect Chinese machining involved but this has proven to be very reliable for me.

 

If you have multiple 1.25" to 2" adapters it's also worth picking the one that minimizes the problem.


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