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laser collimator confusion

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#1 donald41

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Posted 12 September 2019 - 09:11 PM

i have a orion 6" f/8 intelliscope and i collimate it with the cap that came with the scope. every thing is centered and round looking through the hole in the cap and im getting great veiws with the scope. got my new laser collimator the other day and used it on the dob, much to my amazement everything was way off. is there something i need to do to the collimator like align it or something. please help. don



#2 Jim Waters

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Posted 12 September 2019 - 09:25 PM

Rotate the laser collimator in the focuser.  The laser beam should stay on the same spot.  If not the laser collimator needs adjusting.


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#3 ngc7319_20

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Posted 12 September 2019 - 09:37 PM

If the focuser is tilted and not square to the tube and optical axis, it is possible for the pinhole cap and laser to disagree.  Also as Jim says, the laser could be out of adjustment.



#4 vtornado

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Posted 12 September 2019 - 09:43 PM

1) do a star test That is the ultimate test for collimation.

Find a bright star

Use around a  10mm eyepiece

Defocus until you see a set of rings.

Are the rings all nice and concentric or are they lopsided?

If lopsided you are not collimated.

 

2) regarding your laser.  I don't think it can be used like it is.  The focuser on the ST6 is hmmm not so good. (I have one)

The laser is guaranteed not to sit in the focuser squarely.  Do an experiment, put the laser in, collimate, take it

out put it back in, I bet it says you are off again.

 

Is you mirror center spotted?

Do you have a barlow?

 

Do this.

http://www.smartavtweaks.com/RVBL.html

 

This takes away the issue of the laser not sitting square in the focuser.

Works, takes 2 minutes, and can be done outside in the dark.



#5 sixela

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Posted 16 September 2019 - 03:29 AM

In addition to what was mentioned:

-the main function of the collimation cap is to set the tilt of the primary correctly, so that the optical axis and focused axis intersect close to the focal plane (I.e. where the cap is located)
-the main function of the laser collimation is to set the tilt of the secondary so that the reflected focused axis points to the middle of the primary.

So of course they can disagree: they're for different things!

Wif you don’t have a laser collimation or a sight tube you may just not touch the secondary at all and use the cap: you might end up with a tilted focal plane but since the eye can refocus when looking at different parts of the field it's often not much of a problem.
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#6 macdonjh

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Posted 16 September 2019 - 08:14 AM

+1 to what vtornado said about a star test.  If you're getting good views after collimating using the cap, then you must be close.

 

Everyone else is giving good advice, too, but here is one other variable specific to your scope.  I had that same scope and the stock focuser is very sloppy.  Sloppy enough to put your scope out of collimation while you're focusing.  So before you blame your laser collimator or get really frustrated constantly adjusting collimation searching for the perfect adjustment do this:

 

Insert your laser collimator in your focuser and look into the OTA.  Note where the laser spot is on your primary mirror.  Now rack your focuser in and out and watch what happens to the location of the laser spot.  My guess is the laser spot will move quite a bit.  If that happens, you really need to address your focuser before anything else. 

 

It's simple to shim the focuser's draw tube to remove most of the slop.  You can buy 0.010" and 0.020" thick plastic sheet on the internet (Amazon or Grainger among others).  You'll use a small portion of an 8-1/2" x 11" sheet.  Simply wrap the draw tube with the plastic (a piece cut to size) and glue it in place.  I have a pair of calipers, so I measured the OD of my draw tube and the ID of my focuser to calculate how much shim I needed, but you can also figure it out through trial and error.  While you have your focuser apart take the opportunity to clean the old grease off and put better grease in and get the tension knob adjusted just how you like it.

 

Once you've addressed the focuser, the advice above will be much more effective.  You can also replace for focuser...


Edited by macdonjh, 16 September 2019 - 08:16 AM.


#7 macdonjh

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Posted 16 September 2019 - 08:20 AM

Oh, another thing to keep in mind: the stock focuser has only one eye piece retainer screw, so placement and orientation of an eye piece or laser collimator in the focuser isn't really repeatable.  After modifying my XT6i I was able to get really nice views every time my son and I used it.  It just takes a little work to get the scope to the point where you can use "normal" set up and collimation procedures (e.g. set -up your scope, check collimation with your laser, tweak as necessary, start observing) to get those good views.  Without the mods, I think you'll be stuck collimating using a star test every time you set up, which is tedious.



#8 Starman1

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Posted 16 September 2019 - 01:24 PM

Star collimation is a process of refining PRIMARY mirror collimation, and does not tell you if the secondary is off.

So, for collimating the SECONDARY, which does, fortunately, have a looser tolerance than PRIMARY collimation, 

you need to collimate your laser well so it actually functions as a collimation device, and not just a cat toy.

http://www.stark-lab.../llcc/llcc.html
http://www.astromart...p?article_id=96
http://www.cloudynig...collimator-r509
http://www.visualast...collimator.html


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#9 Jon Isaacs

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Posted 16 September 2019 - 01:59 PM

Donald:

 

Something be aware of:

 

An 6 inch F/8 is very forgiving in terms of alignment of the primary. At F/5, the diffraction limited field is 2.75 mm in diameter. At F8. It's about 11.3 mm. That's 4 times as big and it means the slop allowed is 4 times as much.

 

Jon



#10 pgrunwald

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Posted 16 September 2019 - 03:16 PM

Attached File  collimation_with_a_Barlowed_Laser.pdf   237.66KB   164 downloads

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#11 Ken Watts

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Posted 18 September 2019 - 06:51 PM

Donald, I have an XT8 and it sounds like I have your problem.  I can align the secondary and make it look like all concentric rings, just like the book.  Using the collimation cap, I can get the primary also aligned.   BUT, when I drop the laser in, it reflects off the primary 2 inches from center.  Rotation of the laser and rotation of the 2-in to 1 1/4-in adapter show the laser not moving... Maybe tomorrow I will feel up to experimenting with the focus tube.  It is a crayford with 3 adjustment points.  When/if I figure this out, I'll pass along my findings.

 

As it is, the scope works great, but if it can be tweaked to another level, I am all in.

 

Don't let anyone badmouth any of your equipment, you paid good money for the best you can afford, I am sure!

 

Clear and steady skies,

 

Ken W


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#12 sixela

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Posted 19 September 2019 - 06:49 AM

It is, because of perspective effects, impossible to make "all the rings concentric" on scopes with a finite f/ratio ;-), at least if the outer "ring" is the opening of the focuser. And they aren't, from what you're telling us; if a laser collimator points off-course then by definition the reflection of the primary is not centred under the focuser opening (although it's possibly centred in the secondary's outline, but then they are both not centred under the focuser).

 

The situation you describe is one where the reflected focuser axis does not point to the primary, but with the collimation cap you have made the optical axis and focuser axis cross at the focal plane.

 

The result is good images in the middle of the field, but a tilted focal plane. At low power only a band across the middle will be in focus, but since your eye can accommodate for different focus even then you'll probably not notice it. At high power it will be irrelevant.

 

As for making the reflected focuser axis hit the middle of the primary (which also centres the reflection of the primary within the focuser opening), it's rather customary to use the tilt of the secondary to change that, but then you will shift the outline of the secondary with respect to the reflection of the primary that should sit within it (which may then force you to fiddle with the placement of the secondary again, and make you start from scratch). If you don't want to do that then yes, you can just collimate the focuser instead.


Edited by sixela, 19 September 2019 - 06:50 AM.


#13 Jon Isaacs

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Posted 19 September 2019 - 06:59 AM

Rotation of the laser and rotation of the 2-in to 1 1/4-in adapter show the laser not moving... Maybe tomorrow I will feel up to experimenting with the focus tube.  It is a crayford with 3 adjustment points.  When/if I figure this out, I'll pass along my findings.

 

 

Do not do this. Precise collimation does not require that the focuser be at 90 degrees to the OTA, there are Newtonians with the focuser mounted at 70 degrees. 

 

The alignment of the focuser axis with the primary mirror is adjusted by the secondary tilt. If that is aligned, the only remaining step is to align the primary tilt.

 

Sixela said it best:

 

"-the main function of the collimation cap is to set the tilt of the primary correctly, so that the optical axis and focused axis intersect close to the focal plane (I.e. where the cap is located)
-the main function of the laser collimation is to set the tilt of the secondary so that the reflected focused axis points to the middle of the primary."

 

If the laser and the collimation cap do not agree, trust the collimation cap.  Even with the best lasers, the return spot beam is not trustworthy for aligning the primary mirror because it is sensitive to errors in the tilt, in the collimation of the laser, to the fit of the laser in the focuser.  If you want to use your laser to adjust the primary tilt, use the Barlowed Laser Technique.  Nils Olof Carlin invented this method more than 15 years ago and it made accurate alignment of the primary with a laser possible.  This is a link to the original article in Sky and Telescope.

 

http://www.micosmos....lowed_Laser.pdf

 

The Barlowed laser is very simple.  It requires a paper target with a small hole in the center for the laser beam to pass through.  This is placed in the end of the Barlow.  The Barlow goes in the focuser, the laser goes in the Barlow. 

 

The Barlow expands the beam and illuminates a patch of the mirror that includes the center marker. The expanded beam is reflected back and viewed on the paper target.  What you see is the beam with ring shaped shadow, that is the shadow of the center marker.  Align the ring so it is centered on the out going beam.

 

To see just how effective this is, you can loosen the set screw holding the Barlow in the focuser and then, while looking that shadow on the paper target, rock the collimator/barlow. You will see the reflected beam move but the shadow does not move.  The beauty of the Barlowed Laser is that it does not require a precisely collimated laser, it does not require near perfect fit in the focuser and it is not dependent on the secondary tilt being properly set.

 

Jon


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#14 sixela

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Posted 19 September 2019 - 07:07 AM

Jon, if the primary's reflection already appears well centred under the outline of the secondary, it does make some sense to collimate the focuser rather than to tilt the secondary, when you are trying to change the reflected focuser axis.

 

Because the primary's reflection will then remain centred under  secondary if you tune at the focuser end. Whereas if you change the tilt of the secondary, the reflection of the primary will move with respect to the (decentred) secondary to centre itself under the focuser, revealing that you need to place your secondary differently if you want the focuser not to budge.

 

As you said yourself, the focuser axis need not be perpendicular to the tube, which will allow you to make small tweaks to its axis if this is easier than fussing with the secondary. Sometimes it's easier to tilt the focuser to where the secondary is located than to move the secondary to where the focuser is pointing...

 

But yes, it's no that common and not described in many collimation protocols because often the focuser is not collimatable, and because if it's square to the tube you can also leave it like that and place the secondary correctly without fussing with the focuser.

 

 

 

If the laser and the collimation cap do not agree, trust the collimation cap.

Seldom have wiser words been spoken ;-).

Little caveat, especially when you have focal plane tilt (see previous post)...

...it's important to place the collimation cap at the focal plane, i.e. at the level where the shoulder of a typical 25mm Plössl sits when focused (not a TV one, since that has the focal plane 9mm closer to the scope).

But the same holds for a barlowed laser, in which the perfect balance is for the focal plane to be halfway between the virtual point source created by the barlow (which sits at the eye side of the barlow) and the screen where you read the centre spot silhouette.

The better you made the reflected focuser axis point to the middle of the primary, the less it matters.


Edited by sixela, 19 September 2019 - 07:12 AM.


#15 Jon Isaacs

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Posted 19 September 2019 - 09:20 AM

 Jon, if the primary's reflection already appears well centred under the outline of the secondary, it does make some sense to collimate the focuser rather than to tilt the secondary, when you are trying to change the reflected focuser axis.

 

 

Alexis:

 

I can see that it might make "some" sense. But my concern is that people tweak the focuser tilt because they think it is required that the focuser be at 90 degrees to get the best possible planetary views. 

 

In the situation you describe, the motivation is perfectly centering the illuminated field of view which depends on the positioning of the secondary rather than the tilt of the secondary. This is not a critical in getting the optical axes aligned.

 

In this case, the laser and the collimation cap disagree, adjusting the tilt of the focuser won't change that.

 

Jon

 

Jon



#16 sixela

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Posted 19 September 2019 - 04:42 PM

You're right. It makes sense to tweak the focuser because it's easier than fiddling with the secondary, or because you want your spider vanes well behaved (which sometimes limits what you can do with respect to placement of the secondary), but not because you want the focuser axis exactly square to the tube.

 

And yes, in this case the cap and laser "disagree" because the focuser axis isn't pointing the right way (as detailed in earlier posts).



#17 Ken Watts

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Posted 20 September 2019 - 08:55 PM

With my XT8, I aligned the secondary per all the instructions.  Every reflection was concentric.  I taped graph paper inside the main tube and verified the secondary was exactly centered.  I checked out my laser collimator and it is right on.  Putting the laser in the focus tube and rotating the laser, the red spot on the main mirror did not move.  Next, I rotated the 2 to 1.25 adapter, again no change in where the spot was.  The spot where the laser hit the primary was about 2 inches off center.  I could see that the laser hit the secondary a long way away from center as well. It was offset toward the side of the secondary closest to the primary.

 

I did the unthinkable and actually started playing with the focusing tube.... By adjusting the 3 screws that hold the top of the focuser in place to the bottom plate, I brought the laser point on the main mirror inside the central "o" spot on the primary.  The spot where the laser reflected off the secondary was magically at the center of the secondary!  

 

Last night I took the scope out and had an excellent evening.  From outside focus through in focus to inside focus, images went from perfect doughnut to crisp to another perfect doughnut.

 

I am very happy with my work!

 

Clear and Steady skies!

 

Ken W



#18 Jon Isaacs

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Posted Yesterday, 12:50 AM

I did the unthinkable and actually started playing with the focusing tube.... By adjusting the 3 screws that hold the top of the focuser in place to the bottom plate, I brought the laser point on the main mirror inside the central "o" spot on the primary.  

 

 

That is one way to do it.  Normally this done by adjusting the tilt of the secondary so the laser is centered in the center marker. 

 

YMMV

 

Jon


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#19 Ken Watts

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Posted Yesterday, 01:19 PM

When I tried to adjust the secondary, and actually hit the spot with the laser, all the reflections of the secondary were way out of wack.  I admit it was weird, but now all seems to be OK.



#20 Jon Isaacs

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Posted Yesterday, 01:56 PM

When I tried to adjust the secondary, and actually hit the spot with the laser, all the reflections of the secondary were way out of wack.  I admit it was weird, but now all seems to be OK.

 

Which reflections were out of whack?  The reflections of the laser?

 

Jon




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