TheSkySearchers

Can anyone bend the barlowed laser collimation method to me of iron wire?
I don't really seem to know where to put the paper target and where to look at it.
I'd probably go wrong, if I started guessing, because so many others, with far better knowledge of optics, have got it wrong (I've read).

I'd like to do that, because every piece in the image train can be off for all I know.
I understand that with this method I could at least get the main mirror right regardless of how off everything else is (as long as I can see through the system). Then I could collimate the rest based on the idea that the main mirror is pretty close to right.

I have a laser collimator, but I don't think it is well collimated. I think it has 3 LEDs, because when I point it to a wall, I see a round dot with 3 "ears" (LEDs?). Then again, what do I know with my very short and limited experience...

Could you please post a link to the method in question?
Nils Olof Carlin had extensive knowledge of optics and collimation techniques so need to read the process first.

Unfortunate that he passed a while ago, he was a bright light in our hobby.

Lady Fraktor wrote: ↑Sun Aug 09, 2020 10:26 pm Could you please post a link to the method in question?
Nils Olof Carlin had extensive knowledge of optics and collimation techniques so need to read the process first.

Unfortunate that he passed a while ago, he was a bright light in our hobby.

Here: http://w1.411.telia.com/~u41105032/koll ... arlowlaser

And here are his pages: http://w1.411.telia.com/~u41105032/

The end of the laser collimator is a hollow tube and you can either place the target inside the tube or make it the same outside diameter and use thin tape to hold it in place on the end.
There is usually enough tolerance for this to fit inside the focuser tube.
The hardest part is getting a clear hole in the paper, you do not want a oval as it will change the shape of the returning beam making it harder to center.

Thank you for posting this link, I used it years ago to make different focal length combination tubes and I could not remember after all these years where I had found the information

I have a 2" Kendrick laser collimator that was supplied with a Barlow attachment, but I never got this to work and could not see the advantage of the Barlow. I removed the Barlow lens, placed a white sticker around the laser hole, on the inside of the laser hole I blackened it with mat-black paint. I have a 15" Obsession that has to be collimated after every assembly by myself, with this setup it takes only a fey seconds: I adjust the secondary to place the beam on the centre of the primary, then I lie on my back and adjust the primary's knobs till the laser beam is reflected back into the hole of the collimator, while looking up at the face of the collimator, this is now very easy because the reflected light is bright outside the hole and very dim when it shines inside the hole. So its a case of adjusing all three knobs for minimum refection. The centre of my primary has a 'X" engraved on it and when my collimation is perfect the "x" is superimposed over the hole of the collimator.

PS I collimated my laser by placing it on a "V" shape piece of metal, clamped in a vise, and rotating it while shining the beam on a paper about 5 meters away and adjusting its four adjustment screws till the beam stays on a dot while I rotate the collimator.

Nice. I've messed it again... I have this... https://www.meade.com/1-25-laser-collim ... apter.html

What makes the method insensitive to other collimation errors except primary mirror?

turboscrew wrote: ↑Mon Aug 10, 2020 9:37 am What makes the method insensitive to other collimation errors except primary mirror?

You've been talking to computers for too long! Your questions are often not easy to understand. Can you explain what your question means?

I presume you mean, why must this be done to collimate the primary. Well if you can center the laser beam on the doughnut before collimating the primary, there is no reason. Assuming that is the case, ignore the Barlowed laser idea.

Piet has it exactly right, see if you can do the same and you're good.

SkyHiker wrote: ↑Mon Aug 10, 2020 2:57 pm

turboscrew wrote: ↑Mon Aug 10, 2020 9:37 am What makes the method insensitive to other collimation errors except primary mirror?

You've been talking to computers for too long! Your questions are often not easy to understand. Can you explain what your question means?

I presume you mean, why must this be done to collimate the primary. Well if you can center the laser beam on the doughnut before collimating the primary, there is no reason. Assuming that is the case, ignore the Barlowed laser idea.

Piet has it exactly right, see if you can do the same and you're good.

This method is supposed to be insensitive to bad collimation of other components except the main mirror. It shouldn't make much difference if the secondary mirror is somewhat off. To my understanding, that's what makes this method special.
"The new method is preferable in several ways to the standard one in which the laser collimator is used alone. The most striking improvement is that the new method is insensitive to collimation errors of the other elements in the optical path, namely the secondary and the focuser. These should have been collimated before the tilt of the primary is adjusted, of course, but decoupling the primary from the other elements vastly simplifies the final part of the work, namely, adjusting the primary mirror." https://flylib.com/books/en/3.311.1.57/1/

I'm confused by sentences like this: "You need to put a target over the lens end of the Barlow". Which end is "lens end"?
Or "make it a tight fit to the lens cell". What is "lens cell"?
And do you need to look from in the telescope into the focuser tube? I kind of guess you do. I don't believe that the doughnut image is visible through the cardboard target.

turboscrew wrote: ↑Mon Aug 10, 2020 3:16 pm

SkyHiker wrote: ↑Mon Aug 10, 2020 2:57 pm

turboscrew wrote: ↑Mon Aug 10, 2020 9:37 am What makes the method insensitive to other collimation errors except primary mirror?

You've been talking to computers for too long! Your questions are often not easy to understand. Can you explain what your question means?

I presume you mean, why must this be done to collimate the primary. Well if you can center the laser beam on the doughnut before collimating the primary, there is no reason. Assuming that is the case, ignore the Barlowed laser idea.

Piet has it exactly right, see if you can do the same and you're good.

This method is supposed to be insensitive to bad collimation of other components except the main mirror. It shouldn't make much difference if the secondary mirror is somewhat off. To my understanding, that's what makes this method special.
"The new method is preferable in several ways to the standard one in which the laser collimator is used alone. The most striking improvement is that the new method is insensitive to collimation errors of the other elements in the optical path, namely the secondary and the focuser. These should have been collimated before the tilt of the primary is adjusted, of course, but decoupling the primary from the other elements vastly simplifies the final part of the work, namely, adjusting the primary mirror." https://flylib.com/books/en/3.311.1.57/1/

The "other components" in your case are irrelevant except the secondary. It's all about the optical path hitting the center of the primary exactly perpendicularly. So if you get the secondary to reflect the beam right into the donut there is no error to be doubled by the reflection, which is what the Barlowed laser idea aims to accomplish.

If you are so worried about these details just get a Howie Glatter with tublug, it is the best system that I know to prevent collimation errors. Using a cheap laser collimator will work if you do it diligently (collimate the laser, rotate the laser in the focuser to check the effect of slop, inaccurate dot shape). It has the Barlowed laser built in and is a joy to use.

The point is: I bought the scope second hand. I have no idea whether anything is in the right place. Something might be just compensated: two wrongs make one right. The barlow (2x) actually spreads the laser dot into a disc about 2 inch in diameter on the primary mirror.

turboscrew wrote: ↑Mon Aug 10, 2020 6:26 pm The point is: I bought the scope second hand. I have no idea whether anything is in the right place. Something might be just compensated: two wrongs make one right. The barlow (2x) actually spreads the laser dot into a disc about 2 inch in diameter on the primary mirror.

Is this a question? I am trying to help, so does anyone else, and your incoherent sentences are not helpful.

If the Barlow causes a 2" disc on the primary you won't have enough reflection to be of any use.

If you want the primary to be accurate just get a collimation cap! They are cheap. You can use your laser for the secondary.

Again if you can get the secondary to reflect the laser beam in the donut you will need no Barlowed laser.

Have you actually tried the collimation or is this just about theory?

Well, first about the laser and the beam (Nils Olof Carlin's own words): "Put your laser collimator into a Barlow lens, and aim it at a wall, at a distance about the focal length of your mirror. You will see a much larger spot, perhaps not quite round, but if it is large enough to cover the main mirror spot, you are ready to go on ( if it is too small, try a shorter Barlow!)."

Also, when I was, obviously, trying it wrong, I saw that the reflection from the disc back to the focuser was well visible through a paper. I think the reflection is strong enough.

And like I said, I'd like to make sure that the thing is properly collimated. The primary, the secondary and even the focuser may be wrong - just such that the error of the secondary mirror compensates the errors of the focuser and primary mirror. (That was what I meant with "two wrongs make one right".) And even if the laser beam gets back from the primary mirror into the hole in the collimator, the errors may pop out in some other respect, like non-symmetrical field curvature or coma. The previous owner had an electronic focuser, so the current focuser has been at least removed and put back. It may not be properly aligned.

Still, back to the track: I wasn't asking about collimation in general, but specifically about the barlowed laser method, to get at least one of the components right. The others can then be collimated based on the knowledge that possible collimation errors are not due to the primary mirror.

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm Well, first about the laser and the beam (Nils Olof Carlin's own words): "Put your laser collimator into a Barlow lens, and aim it at a wall, at a distance about the focal length of your mirror. You will see a much larger spot, perhaps not quite round, but if it is large enough to cover the main mirror spot, you are ready to go on ( if it is too small, try a shorter Barlow!)."

Also, when I was, obviously, trying it wrong, I saw that the reflection from the disc back to the focuser was well visible through a paper. I think the reflection is strong enough.

Good, nice that you got it working.

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm And even if the laser beam gets back from the primary mirror into the hole in the collimator, the errors may pop out in some other respect, like non-symmetrical field curvature or coma.

Coma is irrelevant for collimation because collimation plays only on the optical axis where there is no coma. If you collimate using a laser in the standard way your scope is collimated, period, it will not somehow introduce coma elsewhere. What you suggest just does not happen.

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm Still, back to the track: I wasn't asking about collimation in general, but specifically about the barlowed laser method, to get at least one of the components right. The others can then be collimated based on the knowledge that possible collimation errors are not due to the primary mirror.

Do not make the mistake of collimating the primary first then the secondary. The collimation of the primary depends on that of the secondary. So if you collimate the primary first, then the secondary, you must redo the primary. There is no point in collimating the primary first. The collimation of the secondary does not depend on that of the primary.

SkyHiker wrote: ↑Tue Aug 11, 2020 4:57 am

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm Well, first about the laser and the beam (Nils Olof Carlin's own words): "Put your laser collimator into a Barlow lens, and aim it at a wall, at a distance about the focal length of your mirror. You will see a much larger spot, perhaps not quite round, but if it is large enough to cover the main mirror spot, you are ready to go on ( if it is too small, try a shorter Barlow!)."

Also, when I was, obviously, trying it wrong, I saw that the reflection from the disc back to the focuser was well visible through a paper. I think the reflection is strong enough.

Good, nice that you got it working.

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm And even if the laser beam gets back from the primary mirror into the hole in the collimator, the errors may pop out in some other respect, like non-symmetrical field curvature or coma.

Coma is irrelevant for collimation because collimation plays only on the optical axis where there is no coma. If you collimate using a laser in the standard way your scope is collimated, period, it will not somehow introduce coma elsewhere. What you suggest just does not happen.

What I meant was that if the collimation is off, the coma may be asymmetrical. I think coma corrector can't handle that.

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm Still, back to the track: I wasn't asking about collimation in general, but specifically about the barlowed laser method, to get at least one of the components right. The others can then be collimated based on the knowledge that possible collimation errors are not due to the primary mirror.

Do not make the mistake of collimating the primary first then the secondary. The collimation of the primary depends on that of the secondary. So if you collimate the primary first, then the secondary, you must redo the primary. There is no point in collimating the primary first. The collimation of the secondary does not depend on that of the primary.

I understand that that's the beauty of the barlowed laser. It's not dependent on the collimation of secondary mirror or focuser.
And I think it's worth the while to be a bit more picky now. I checked the spider, and it's obviously turned 90 degrees. I also think that in my scope the offset is 2 mm, because the left side vanes were 2 mm shorter than the right side vanes.
I think the shorter vanes belong to back and the longer in front. It looks like the vanes were not meant to be adjustable, so I need to rotate the spider. But the laser in focuser hits the primary within the doughnut...
I think it's hard to collimate the traditional way when you can't trust _anything_.
Next I probably need to figure out how to measure the height of the secondary. Then how to figure out how to check the focuser direction.

turboscrew wrote: ↑Tue Aug 11, 2020 1:21 pm

SkyHiker wrote: ↑Tue Aug 11, 2020 4:57 am

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm Well, first about the laser and the beam (Nils Olof Carlin's own words): "Put your laser collimator into a Barlow lens, and aim it at a wall, at a distance about the focal length of your mirror. You will see a much larger spot, perhaps not quite round, but if it is large enough to cover the main mirror spot, you are ready to go on ( if it is too small, try a shorter Barlow!)."

Also, when I was, obviously, trying it wrong, I saw that the reflection from the disc back to the focuser was well visible through a paper. I think the reflection is strong enough.

Good, nice that you got it working.

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm And even if the laser beam gets back from the primary mirror into the hole in the collimator, the errors may pop out in some other respect, like non-symmetrical field curvature or coma.

Coma is irrelevant for collimation because collimation plays only on the optical axis where there is no coma. If you collimate using a laser in the standard way your scope is collimated, period, it will not somehow introduce coma elsewhere. What you suggest just does not happen.

What I meant was that if the collimation is off, the coma may be asymmetrical. I think coma corrector can't handle that.

turboscrew wrote: ↑Mon Aug 10, 2020 8:37 pm Still, back to the track: I wasn't asking about collimation in general, but specifically about the barlowed laser method, to get at least one of the components right. The others can then be collimated based on the knowledge that possible collimation errors are not due to the primary mirror.

Do not make the mistake of collimating the primary first then the secondary. The collimation of the primary depends on that of the secondary. So if you collimate the primary first, then the secondary, you must redo the primary. There is no point in collimating the primary first. The collimation of the secondary does not depend on that of the primary.

I understand that that's the beauty of the barlowed laser. It's not dependent on the collimation of secondary mirror or focuser.

Yes it is. If you change the secondary you change the optical axis to the primary. This is not vice versa.

turboscrew wrote: ↑Tue Aug 11, 2020 1:21 pm And I think it's worth the while to be a bit more picky now. I checked the spider, and it's obviously turned 90 degrees. I also think that in my scope the offset is 2 mm, because the left side vanes were 2 mm shorter than the right side vanes.
I think the shorter vanes belong to back and the longer in front. It looks like the vanes were not meant to be adjustable, so I need to rotate the spider. But the laser in focuser hits the primary within the doughnut...
I think it's hard to collimate the traditional way when you can't trust _anything_.
Next I probably need to figure out how to measure the height of the secondary. Then how to figure out how to check the focuser direction.

Read Don Pensack's article on collimation. I am done.

So Nils Olof Carlin lied?

turboscrew wrote: ↑Tue Aug 11, 2020 3:52 pm So Nils Olof Carlin lied?

No. His Barlowed method would also need a correction when collimating the primary before the secondary. Changing the secondary causes a change, albeit a small one, in the Barlowed reflection.

Here's a link to Gary Seronik's collimation method. He is a bit of an odd duck in that he starts with the primary, then the secondary, then corrects the primary for changes in the secondary. The latter are small changes and may be negligible in many cases. He just uses a collimation cap.

https://garyseronik.com/a-beginners-gui ... llimation/

Perhaps starting out you may want to learn the collimation cap and cheshire methods before trying using a barlowed collimator.

Squaring the secondary under the focuser first is essential to aligning the primary, the method linked by Henk is a good one as well but adds a step that may not be required.

What makes the method insensitive to other collimation errors except primary mirror?
When Nils Olof Carlin mentions this he means using a Cheshire not the laser.
Reading some of your questions/ comments it seems you are possibly reading to far down the page and combining Schmidt and spherical mirrors.