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RC secondary collimation with a newtonian laser & artificial star collimation

Collimation Reflector
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#1 WillYang

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Posted 20 September 2021 - 02:51 AM

Some thoughts of RC telescope collimation.

 

If my knowledge is correct, the secondary mirror must be squared to the optical axis (or sensor).

 

Assume there is no tilt in the focuser and the tilt plate is set to "0", and the secondary has no offset with the optical axis (centered).

Can I use a newtonian laser for aligning the secondary mirror by adjusting the secondary and make the laser beam reflected back to the center of the laser collimator (it means the secondary is squared to the optical axis)?

 

And later I will use an artificial star to adjust the primary mirror.

 

In the theory of the DSI collimation method, adjusting the primary mirror will eliminate on axis coma, and adjusting the secondary will eliminate off axis astigmatism. So if I aligned the secondary with the laser mentioned above, there won't be any off axis astigmatism. The only thing left is on axis coma, which I need to adjust the primary mirror (with a artificial star).

 

So here's the full collimation steps.

1. Use the newtonian laser collimator to adjust the secondary mirror, the laser beam should be reflected back to the center of the collimator.

2. Now change the laser with a sensor or camera.

3. Set the artificial star in the center of FOV, defocus the image in order to see the donut.

4. Adjust the primary mirror until there is no on axis coma.

 

Am I correct about the collimation workflow or theory?

 

I will be using a GSO RC10, so there won't be any problem about focuser tilt affecting primary mirror. 

 

Thanks in advance!

 

GSO-1-25-newtonian-Laser-collimator-with-Calibration-plate-and-fine-tuning-screw-Telescope-lator.jpg


Edited by WillYang, 20 September 2021 - 06:50 AM.

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#2 macdonjh

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Posted 20 September 2021 - 05:07 PM

You can, sort of.  The convex secondary mirror will cause your laser beam to diverge, becoming a large blob of red light by the time it returns to your collimation tool.  I've tried it with SCTs and classical Cassegrains, it's of limited benefit.

 

A laser collimator can help you ensure your secondary mirror is centered "over" your focuser if your mirror is center spotted...



#3 Terry White

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Posted 20 September 2021 - 06:02 PM

Some thoughts of RC telescope collimation.

 

If my knowledge is correct, the secondary mirror must be squared to the optical axis (or sensor).

 

Assume there is no tilt in the focuser and the tilt plate is set to "0", and the secondary has no offset with the optical axis (centered).

Can I use a newtonian laser for aligning the secondary mirror by adjusting the secondary and make the laser beam reflected back to the center of the laser collimator (it means the secondary is squared to the optical axis)?

 

And later I will use an artificial star to adjust the primary mirror.

 

In the theory of the DSI collimation method, adjusting the primary mirror will eliminate on axis coma, and adjusting the secondary will eliminate off axis astigmatism. So if I aligned the secondary with the laser mentioned above, there won't be any off axis astigmatism. The only thing left is on axis coma, which I need to adjust the primary mirror (with a artificial star).

 

So here's the full collimation steps.

1. Use the newtonian laser collimator to adjust the secondary mirror, the laser beam should be reflected back to the center of the collimator.

2. Now change the laser with a sensor or camera.

3. Set the artificial star in the center of FOV, defocus the image in order to see the donut.

4. Adjust the primary mirror until there is no on axis coma.

 

Am I correct about the collimation workflow or theory?

 

I will be using a GSO RC10, so there won't be any problem about focuser tilt affecting primary mirror. 

 

Thanks in advance!

 

attachicon.gifGSO-1-25-newtonian-Laser-collimator-with-Calibration-plate-and-fine-tuning-screw-Telescope-lator.jpg

I think you may be misunderstanding what the DSI guide says. Nowhere in the guide does it say to use traditional tools to collimate your RC. In fact, it makes it quite clear that traditional tools (lasers and collimation telescopes) will not archive a good collimation, for several obvious reasons. To quote:

 

"The traditional method of collimating references several physical points within the system and relies on certain assumptions that may or may not be true. First, the method requires a center spot (or circle) on the secondary mirror. This spot is usually located in the physical center of the secondary mirror. It assumes the mechanical center of the secondary mirror is also the optical center of the secondary mirror. Further, the method references the edges of both the primary and secondary baffles. It assumes both baffles are coaxial with the OTA and that the system optical axis will eventually be coaxial with these as well. Neither of these will be the case to at least some degree. That is why it is often reported that when a scope is collimated with the traditional method that it still does not perform well. Conversely, a well collimated scope may not look collimated when viewed with a collimation telescope."

 

and,

 

"The mechanical centers of both the primary and secondary mirror are almost never the optical center. They can vary by a few thousands of an inch to as much as a tenth or more. This is often not taken into account when the instrument is assembled. The procedure presented here is very tolerant of this. Optical analysis as well as practical experience shows that this procedure produces good results in the presence of these types of issues."


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#4 WillYang

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Posted 21 September 2021 - 03:35 AM

Thanks both for the replies.

 

But for the laser collimation of secondary, I don't use the center mark or the edge of baffles for alignment. Just simply use the reflected beam, so I think I won't be affected by the not coaxial mechanical axis and optical axis since I'm using something optical (laser and reflection) to align the secondary?

The only "physical point" I'm using is the focuser and the laser collimator, I believe the focuser will be precisely centered. So the secondary should be coaxial to the focuser axis after this process, and so as the sensor (assuming there is no tilt in the imaging train)?

 

I don't trust my eyes so I won't consider the takahashi collimation scope (and it's super expensive for that, I know takahashi has excellent mechanical accuracy).

 

I understand that the laser beam will be enlarged after reflecting from the secondary. But I think it will be easy to be solved if my laser beam is small enough?


Edited by WillYang, 21 September 2021 - 03:42 AM.


#5 Rasfahan

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Posted 21 September 2021 - 08:02 AM

Thanks both for the replies.

 

But for the laser collimation of secondary, I don't use the center mark or the edge of baffles for alignment. Just simply use the reflected beam, so I think I won't be affected by the not coaxial mechanical axis and optical axis since I'm using something optical (laser and reflection) to align the secondary?

The only "physical point" I'm using is the focuser and the laser collimator, I believe the focuser will be precisely centered. So the secondary should be coaxial to the focuser axis after this process, and so as the sensor (assuming there is no tilt in the imaging train)?

 

I don't trust my eyes so I won't consider the takahashi collimation scope (and it's super expensive for that, I know takahashi has excellent mechanical accuracy).

 

I understand that the laser beam will be enlarged after reflecting from the secondary. But I think it will be easy to be solved if my laser beam is small enough?

Due to the hyperbolical shape of the secondary, the laser reflecting back on itself does not guarantee proper alignment. You can use the laser to align your focuser on the center dot of the secondary.

 

Also, the assumptions you make above (especially about focuser alignment) can, in general, not be made - that‘s what makes it so difficult. 

I found there is no shortcut to RC alignment. It is taxing, tedious and requires diligence and patience. The Tak scope works to get close. For better eyes, the REEGO does, too. Both will not get you to „good“. The DSI-method or something similar is always needed. 


Edited by Rasfahan, 21 September 2021 - 08:15 AM.


#6 WillYang

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Posted 21 September 2021 - 11:34 AM

Due to the hyperbolical shape of the secondary, the laser reflecting back on itself does not guarantee proper alignment. You can use the laser to align your focuser on the center dot of the secondary.

 

Also, the assumptions you make above (especially about focuser alignment) can, in general, not be made - that‘s what makes it so difficult. 

I found there is no shortcut to RC alignment. It is taxing, tedious and requires diligence and patience. The Tak scope works to get close. For better eyes, the REEGO does, too. Both will not get you to „good“. The DSI-method or something similar is always needed. 

Big thanks for your reply.

Okay...so can I achieve a very good collimation only with a 9um artificial star (of course, placed far enough)?

 

Last time I was collimating my RC8 using a real star, it took me 2 hours and it's still not perfect, not even close. I know that GSO has changed the mechanical design on 10" truss or above models so the focuser is decoupled with the primary mirror.

 

Maybe I am willing to spend several days using only a artificial star for perfect collimation......

 

And I think it's no harm trying to align the secondary with a laser.

I always want to make RC collmation simpler but it seems impossible...



#7 Rasfahan

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Posted 21 September 2021 - 11:44 AM

Yeah, if you find a method to make it quicker or easier than the DSI-method, I'll award you a medal. If it's the first time, I'ld say 2 hours is getting to know your telescope a bit. Took me at least 12 to get my CFF to acceptable - and that was with my previous experience of the RC8 and the fact that the CFF is much better adjustable.

 

I haven't tried an artificial star yet, as I'ld have to move to some kind of sports field to do that. I got an (allegedly) 5 um star and I think I recall that for collimation the distance should be something of about 30m for my 8 inch. Also, you'ld have to move it around the field: The astigmatism will not be visible in the center, only the coma will. I guess that would be even more tedious than on a real star field.

 

On a bright note, the collimation holds quite good on my GSO RC, so it's not something that needs to be done too frequently. 



#8 fuadramsey

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Posted 21 September 2021 - 12:29 PM

I went through all this too. No tools will really work that great because everything is not aligned perfectly as noted above. I was used to using lasers to collimate reflectors in about a minute so  wanted the same for my 6" RC. But an RC is not as forgiving as a regular reflector.

 

Try the defocus star method first. It's really not that hard. The DSI method works great and is not that hard. I read the instructions about 6 times before I did it. To start I would only adjust the secondary. And in doing so, you will want to adjust the stars on the outer side of the frame (or make the outer star field flat). Bob's knobs also help out a lot. But warning, you will lose collimation if you install these, so just be ready for that.

 

You can also use a tri-bahtinov mask. I just made one for my RC6 since no one sells one, and there were not any designed. Works great! 

 

https://www.thingive...m/thing:4970713


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#9 Terry White

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Posted 21 September 2021 - 03:02 PM

You can also use a tri-bahtinov mask. I just made one for my RC6 since no one sells one, and there were not any designed. Works great! 

 

https://www.thingive...m/thing:4970713

You can get 3-D printed Tri-Bahtinov masks here, and Duncan masks here.


Edited by Terry White, 21 September 2021 - 03:05 PM.


#10 WillYang

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Posted 22 September 2021 - 10:41 AM

Thank you all.

 

I think the best way, perhaps, is to adjust first with an artificial star and final adjust using a true star field.



#11 akulapanam

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Posted 22 September 2021 - 10:01 PM

Some thoughts of RC telescope collimation.

If my knowledge is correct, the secondary mirror must be squared to the optical axis (or sensor).

Assume there is no tilt in the focuser and the tilt plate is set to "0", and the secondary has no offset with the optical axis (centered).
Can I use a newtonian laser for aligning the secondary mirror by adjusting the secondary and make the laser beam reflected back to the center of the laser collimator (it means the secondary is squared to the optical axis)?

And later I will use an artificial star to adjust the primary mirror.

In the theory of the DSI collimation method, adjusting the primary mirror will eliminate on axis coma, and adjusting the secondary will eliminate off axis astigmatism. So if I aligned the secondary with the laser mentioned above, there won't be any off axis astigmatism. The only thing left is on axis coma, which I need to adjust the primary mirror (with a artificial star).

So here's the full collimation steps.
1. Use the newtonian laser collimator to adjust the secondary mirror, the laser beam should be reflected back to the center of the collimator.
2. Now change the laser with a sensor or camera.
3. Set the artificial star in the center of FOV, defocus the image in order to see the donut.
4. Adjust the primary mirror until there is no on axis coma.

Am I correct about the collimation workflow or theory?

I will be using a GSO RC10, so there won't be any problem about focuser tilt affecting primary mirror.

Thanks in advance!

GSO-1-25-newtonian-Laser-collimator-with-Calibration-plate-and-fine-tuning-screw-Telescope-lator.jpg


Yep this is effectively what Alluna, ASA, RCOS, and Officina recommend with the exception that they sometimes substitute the laser with a Tak collimation scope. Both work.

Worth noting you need a nice laser like a Howie Glatter to ensure the laser is collimated and you get a nice small dot.

DSI sure. I see that thrown out and I wonder if people have actually used that method. It works if you have great seeing and you can make really small adjustments.

http://www.alluna-op...ion-English.pdf

http://www.rcoptical...collimation.pdf

 

https://drive.google...vsl-bSQIUD3eyFg


Edited by akulapanam, 22 September 2021 - 10:55 PM.


#12 Rasfahan

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Posted 23 September 2021 - 12:12 AM

Yep this is effectively what Alluna, ASA, RCOS, and Officina recommend with the exception that they sometimes substitute the laser with a Tak collimation scope. Both work.

Worth noting you need a nice laser like a Howie Glatter to ensure the laser is collimated and you get a nice small dot.

DSI sure. I see that thrown out and I wonder if people have actually used that method. It works if you have great seeing and you can make really small adjustments.

http://www.alluna-op...ion-English.pdf

http://www.rcoptical...collimation.pdf

 

https://drive.google...vsl-bSQIUD3eyFg

Yes, and a similar procedure works for my CFF RC. It does not work for my GSO RC. The DSI method does. To get to perfection, it needs good seeing - but not to get it good enough for the seeing you actually have. Before going into further details: How many GSO RCs have you successfully collimated by one of the methods you recommend?



#13 akulapanam

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Posted 23 September 2021 - 12:22 AM

Yes, and a similar procedure works for my CFF RC. It does not work for my GSO RC. The DSI method does. To get to perfection, it needs good seeing - but not to get it good enough for the seeing you actually have. Before going into further details: How many GSO RCs have you successfully collimated by one of the methods you recommend?

A couple.  I don't have all my historical scopes listed but I owned a GSO 10" V2 Truss RC from Altair Astro amongst others.  I also recently (like ~9 months ago) helped out on a TPO branded GSO 10" truss tube.  A few others as well.  Go to a star party and you run into a few.  I have not touched any of the solid tube GSO but those have a different issue stemming from the focuser attachment. Both GSO scopes had very rough adjustments which make the DSI method more challenging.  Also I see people try and use the DSI method thinking they have astigmatism but the reality is that they have tilt (particularly when a reducer is introduced) and thus get into an endless cycle to adjusting secondary and primary without actually solving the problem they have.



#14 WillYang

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Posted 23 September 2021 - 04:43 AM

A couple.  I don't have all my historical scopes listed but I owned a GSO 10" V2 Truss RC from Altair Astro amongst others.  I also recently (like ~9 months ago) helped out on a TPO branded GSO 10" truss tube.  A few others as well.  Go to a star party and you run into a few.  I have not touched any of the solid tube GSO but those have a different issue stemming from the focuser attachment. Both GSO scopes had very rough adjustments which make the DSI method more challenging.  Also I see people try and use the DSI method thinking they have astigmatism but the reality is that they have tilt (particularly when a reducer is introduced) and thus get into an endless cycle to adjusting secondary and primary without actually solving the problem they have.

Do you mean the tilt of the sensor (ie sensor not squared to optical axis)?

 

I'll replace the stock GSO focuser with a TS optics 3.7" R&P focuser which APM also uses on some refractor models. And the reducer I'm using will be the APM riccardi 0.75x reducer M63. The full imaging train will be thread connected so I think I can avoid any possible tilt?



#15 akulapanam

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Posted 23 September 2021 - 08:59 AM

Do you mean the tilt of the sensor (ie sensor not squared to optical axis)?

I'll replace the stock GSO focuser with a TS optics 3.7" R&P focuser which APM also uses on some refractor models. And the reducer I'm using will be the APM riccardi 0.75x reducer M63. The full imaging train will be thread connected so I think I can avoid any possible tilt?


Yeah tilt of the sensor and or rarely adapters that you attach to add sensor that are not used when you collimate.

#16 George N

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Posted 23 September 2021 - 05:18 PM

.....

 

"The mechanical centers of both the primary and secondary mirror are almost never the optical center. They can vary by a few thousands of an inch to as much as a tenth or more. This is often not taken into account when the instrument is assembled. The procedure presented here is very tolerant of this. Optical analysis as well as practical experience shows that this procedure produces good results in the presence of these types of issues."

Exactly what John Stiles of Optical Guidance Systems  told me one time at NEAF and again over the phone. ( I own an OGS 10" RC and regularly image with an OGS RC20 ) He and his guys have found that not only is the physical center of the secondary often not the optical center - there is at times a slight flat spot there making any laser beam reflect off at an odd angle. He recommends not using a laser to collimate an RC secondary.

 

OGS assembles their RCs on a double reflection rig with 40-inch flat. John has gotten quite popular with the members of his local astronomy club - by volunteering to put their SCTs on his collimation rig. Quote "I can sometimes improve the performance of an SCT by conpensating for slight errors in the primary by adjusting the secondary to compensate."


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#17 akulapanam

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Posted 23 September 2021 - 07:02 PM

Exactly what John Stiles of Optical Guidance Systems told me one time at NEAF and again over the phone. ( I own an OGS 10" RC and regularly image with an OGS RC20 ) He and his guys have found that not only is the physical center of the secondary often not the optical center - there is at times a slight flat spot there making any laser beam reflect off at an odd angle. He recommends not using a laser to collimate an RC secondary.

OGS assembles their RCs on a double reflection rig with 40-inch flat. John has gotten quite popular with the members of his local astronomy club - by volunteering to put their SCTs on his collimation rig. Quote "I can sometimes improve the performance of an SCT by conpensating for slight errors in the primary by adjusting the secondary to compensate."

That is certainly an issue but you also need to remember, within reason, that for a given position of a secondary there is a corresponding position for the primary and focuser to match and vice versa. The within reason part comes from the question of whether or not you can move the other pieces to align with that position. If you can get stuff mechanically aligned you are going to either be on, or at least very close which makes something like DSI easier.

I assume that OGS is using a DPAC with Zygo or phasecam + 1/20 wave flat. That is a whole $150k+ of equipment that will absolutely improve performance.

Also looking at just how much collimation error you can have before really impacting diffraction limited much less seeing limited performance. See below page 12 on.

https://www.astrosys...against-cdk.pdf

Edited by akulapanam, 23 September 2021 - 07:07 PM.


#18 rockenrock

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Posted 27 September 2021 - 12:49 PM

Will, 

Roger: The others are correct, the best method is the DSI method. Forget the first pages of the DSI instructions as it is just complaining the other methods. Good stuff at end. For your list:

Will wrote:

So here's the full collimation steps.
1. Use the newtonian laser collimator to adjust the secondary mirror, the laser beam should be reflected back to the center of the collimator.

Roger: Partially correct...

-- You cannot assume the focuser is pointing down (up?) the optical axis. So you must adjust the focuser alignment to make the beam hit the center of the donut on the secondary. But FIRST:

-- Check your laser pointer is not by itself misaligned by rotating it while it is (very slightly loosened) in the focuser and observing the beam hitting the a piece of paper temporarily stuck over the secondary mirror. Put a black dot on the paper, or print concentric circles on it. Don't worry the paper mark is aligned with the secondary donut at this stage.

-- Once your collimator is proven good, then rotate your focuser to assure the beam also does not draw a circle as you rotate it.

-- Push side ways and up/down on the focuser tube (use the marked paper) with everything tight to simulate the weight of your camera, FW, etc. A lot of motion is going to affect your optics. Now you are ready to collimate!...

-- Yes, Now adjust the focuser to point at the center of the donut. It kind of "disappears" when it in the center of the donut and not reflecting off the black donut. The laser beam should be reflected back to the center of the collimator. Yes, adjust secondary to get it to reflect back to the collimator.
2. Now change the laser with a sensor or camera.

Roger: Yes, and include the reducer you plan to use.
3. Set the artificial star in the center of FOV, defocus the image in order to see the donut.

Roger: Yes
4. Adjust the primary mirror until there is no on axis coma.

Roger: 5. Then either move your artificial star to each of the 4 corners, star defocused, and take images. Fine tune primary mirror to balance any coma. All 4 corners equally good! This is following the DSI method as I recall. Write down every adjustment and the resulting effect on coma. You only need to adjust any 2 of the 3 locking positions. If you adjust all 3 you will eventually be changing the distance between primary and secondary. Just put a piece of tape over the 3rd position so you never want to use it. And don't overtighten the screws!

 

Remember your reducer may make coma show up all by itself. You cannot collimate to overcome the reducer effects. Different reducers are matched to different types of optics. 

 

For step 5 I prefer to image an open cluster with lots of stars. Then with one image you can instantly see coma balance in the 4 corners, and in the center of FOV. Also, your scope will be pointing up (same as imaging) and not horizontal like the artificial star. 

 

Threaded connections, not compression connections are better for your optical components, especially if your camera stuff is a bit heavy.

 

You should budget at least 4 hours doing steps 1 to 4, then 1 hour for step 5 if pointed up at the stars.

 

Every now an then, every few months, (or if you take out your reducer or other major adjustment), you can just do a quick check by imaging a cluster with out of focus and see all star images are balanced.

 

Welcome to the world of (relatively) long focal length RC imaging! Forget Andromeda unless you want to make a several images mosaic!

 

Roger


Edited by rockenrock, 27 September 2021 - 12:51 PM.

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