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Collimation is about to make me dump both my Dobs

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#1 titan7

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Posted 14 February 2012 - 05:25 PM

Okay I need some help from people who know how to do this.
Here is the issue.

I have a kendrick 2" laser with the 45 degree grid on one side. I have been putting it in the focuser, and moving the red dot to the center mark on the primary by adjusting the screws on the secondary. Then I move the primary adjustment screws to move the red dot to the center of the grid on the laser body. Done right?

Well then I put a Cheshire EP in and it's show collimation is off??? What's the deal?? I noticed that if I rotate the laser in the focuser the point of impact on the primary changes, that's should not be happening??

So I finally took the Laser out put the cheshire in and centered the with the primary mirror adjustments. I then put the auto collimator in and the primary donut and the (2)donut refections are all overlapping. checked back with the cheshire and it's still centered. Then I put the laser back in and everything is off??????????

should the POI of the laser move if rotating it??

#2 george golitzin

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Posted 14 February 2012 - 05:29 PM

your laser is miscollimated. you don't need it--sounds like you're doing fine with your other tools.
-geo

#3 james867

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Posted 14 February 2012 - 06:03 PM

"Collimation is about to make me dump both my Dobs"
Be patient,I almost made that mistake myself,it was very frustrating at first but I eventually got it down.My first views of Jupiter were mediocre,jump a year later after I significantly advanced my collimation skills and I saw a lot of detail in Jupiter that wasn't seen before.

#4 titan7

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Posted 14 February 2012 - 06:17 PM

What's strange is I had the mirror out of the cell and I put it back in it was rotated a bit from where it sat before. I had collimated with the laser and the cheshire and AC where pretty much right in-line. I am guessing that turning the mirror to a different position and caused this? Perhaps I was just getting lucky with the laser?

So this might be the dumb question but if I don't use the laser to adjust the secondary POI on the primary what do I use for that first step? Will the Cheshire and primary mirror adjustments handle that? I was told my Rick of Starmaster that if I use the auto collimator I don't have to have a perfect stack of the two primary donuts, it can be "close" at F4.55 and you will never see the difference. When I look through the AC I see the primary mirror donut and then the two reflections of the same donut are overlapping about 50% think of this view "OOO" with the center circle being the actual primary donut ane the two on the left and right are the refections of the primary donut. However, rather than being stacked on top of each other I have the circles on the left and right only stacked 50%. Rick said I won't see the difference between a perfect stack and 50% at F4.55. If I wanted to get it better then I need to adjust the secondary a bit. However, wont this mess up the Cheshire adjustments to the primary mirror???? Where's my refector, LOL.

#5 michael_m

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Posted 14 February 2012 - 06:48 PM

Where is Jason D? He is the ultimate collimation guru. Here is the link to Jason's thread, which is quite long. However, It pretty much says it all. I am not a master at this yet, but I will be with patience and practice. Every time I read through Jason's thread I understand better and see it "in my mind" better.

I think this thread is practically a must read for everyone. Here it is... Concise thread about autocollimators + improvements

Good luck..you can do it. Clear skies!

#6 Alan A.

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Posted 14 February 2012 - 08:00 PM

Hi Titan,

regarding your first post on this thread. Your laser is definitely off, it should not move from the center of your primary mirror spot if rotated.

On the return beam to align the primary mirror, the laser should be barlowed. Is that the way your laser is set up?

Alan

#7 titan7

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Posted 14 February 2012 - 08:08 PM

I have never barlowed the laser, in fact I don't own a barlow. Ok so the laser is going to in the trash. So how do I adjust the secondary so it's pointed at the center spot on the primary without the laser? I have the Techtron tools, I but the sighttool crosshairs blur out on me so I cant use the sighttool to adjust the secondary.

#8 Vic Menard

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Posted 14 February 2012 - 08:09 PM

...I had collimated with the laser and the cheshire and AC where pretty much right in-line. I am guessing that turning the mirror to a different position and caused this?

Unless the primary mirror center spot (donut) is offcenter, turning the primary mirror in its cell should have no significant effect on the axial alignment.

Perhaps I was just getting lucky with the laser?

I don't recommend using the return beam of a simple thin beam laser to achieve fine primary mirror alignment. If you can't Barlow the laser, then you should rely on the Cheshire eyepiece to fine align the primary. Both the Barlowed laser and the Cheshire eyepiece magnify the primary mirror axial error two times. After aligning the primary mirror with the return beam from a simple thin beam laser, you will almost always find a residual primary mirror axial alignment error with a Cheshire or Barlowed laser that can be easily corrected.

So this might be the dumb question but if I don't use the laser to adjust the secondary POI on the primary what do I use for that first step?

I'm not sure from your post what kind of autocollimator you're using. If you're using one of the CatsEye Infinity autocollimators, you can double check the focuser axial alignment achieved with the laser by carefully decollimating the primary mirror to spread the autocollimator reflections and then fine aligning the two center reflections by adjusting the secondary mirror. The two center reflections magnify the focuser axial error two times.

Then, assuming you're using the 2-inch Kendrick laser, you can match its alignment to the autocollimator's fine alignment by adjusting the laser's alignment screws, or finding a rotational position in the focuser where the laser's alignment agrees with the autocollimator alignment.

...if I use the auto collimator I don't have to have a perfect stack of the two primary donuts, it can be "close" at F4.55 and you will never see the difference.

The problem with this line of thought is, depending on the alignment procedure, "close" could mean your close enough, or you might have effectively hidden a residual error that could impact image performance. The axial alignment tolerances are not difficult to assess and maintain if you simply stick with the signatures and always end with the critical primary mirror alignment.

FWIW, after aligning the two center reflections with a carefully decollimated primary mirror, if you immediately follow with a Cheshire alignment, the resulting view in the autocollimator should be a "perfect" stack. Of course, if you end up with a "close jumble", and you can verify that the Cheshire alignment is good, it's quite likely that you're "close enough".

...at F4.55...If I wanted to get it better then I need to adjust the secondary a bit. However, wont this mess up the Cheshire adjustments to the primary mirror????

Yes. That's why you always finish with the Cheshire...

#9 Vic Menard

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Posted 14 February 2012 - 08:17 PM

I have never barlowed the laser, in fact I don't own a barlow. Ok so the laser is going to in the trash.

Not so fast! The laser has adjustment screws that you can use to improve the internal alignment.

So how do I adjust the secondary so it's pointed at the center spot on the primary without the laser? I have the Techtron tools, I but the sighttool crosshairs blur out on me so I cant use the sighttool to adjust the secondary.

That's a problem. The Tectron autocollimators were sensitive to temperature swings, and often ended up out of alignment after years of use. And the Tectron tools are 1.25-inch tools, so their accuracy is dependent on their registration with the 2- to 1.25-inch adapter and the adapter's registration with the focuser's 2-inch drawtube.

I'm not saying you can't achieve good axial alignment with the tools you have, but it's not going to be as easy as it could be with modern 2-inch tools.

#10 michael_m

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Posted 14 February 2012 - 08:21 PM

The thread I linked to has a deceptive name. It really is basically all about collimation while using autocollimators as just one of the tools. All of your answers are here if you take the time to read it and study. ANYTHING you want to know about collimation Jason can teach you in this thread as well as other threads that he has participated in (many).

You have an autocollimator, site tube and a cheshire. You have the very items that Jason can teach you how to use in the thread.

The search engine allows for using a poster's name as well. Type collimation in the key search terms fields, and Jason D in the username field. Search in the reflectors forums.

Sorry Vic. You posted while I was typing and I did not see your post.

#11 GeneT

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Posted 14 February 2012 - 08:41 PM

If you like a laser collimator, I recommend the Howie Glatter laser collimator with TuBlug.

#12 mogur

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Posted 14 February 2012 - 08:51 PM

I have never barlowed the laser, in fact I don't own a barlow. Ok so the laser is going to in the trash. So how do I adjust the secondary so it's pointed at the center spot on the primary without the laser? I have the Techtron tools, I but the sighttool crosshairs blur out on me so I cant use the sighttool to adjust the secondary.


When aiming the crosshairs at the primary center dot(adjusting the secondary) back your eye away from the cheshire about a foot or so until you only see the very center + of the crosshairs through the peephole. That way you should be able to keep both in focus. Works for me.

#13 David Pavlich

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Posted 14 February 2012 - 10:30 PM

It could be that your laser is not fitting into the focuser properly. Try this: First, make sure that your sets screws on the focuser are backed out far enough that they have no contact with the laser or the brass compression ring. Now, get some electrical tape and make one complete wrap around the laser base making sure that you don't overlap the tape and then stick it in the focuser and wiggle it. If it's loose, add another wrap of tape. Do this until you get a snug fit. Don't use the set screws to hold the laser in place. There should be enough tape on the laser to create a friction fit. Now do your collimation and see if there's any improvement.

David

#14 spaceoddity

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Posted 15 February 2012 - 11:53 AM

The kendrick laser has 2 tiny nylon set screws on the body that can be backed out just enough to get a snug fit in the focuser. This could throw off the calibration of the laser a little bit. Depending on your focuser they may not need adjusted at all. I would check these screws and see if they are the problem. Maybe they need backed out a little to get a snug fit.

Then you will want to collimate the laser itself by making a V-block and a target a few feet away on the wall. Adjust the allen screws(my kendrick laser has 4) until the laser hits the center of the target while rotating it a few times in the V-block.

Even after you get the laser collimated, I would still just use it for secondary tilt and stick with the cheshire for primary adjustment. A cheshire is more accurate than an unbarlowed laser, no matter how good the laser. You can also try the barlowed laser method for the primary which is as accurate as a cheshire.

#15 titan7

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Posted 15 February 2012 - 01:10 PM

Thanks for all the tips. I will make another run at collimation this evening. I will collimate the laser and try again. I only plan on using the laser for the secondary tilt and then go to the cheshire and AC. I read through Vic's book on collimation and got a bad headache, sorry Vic, there just has to be a easier way to do this that the book. I think this is one of those things that is simple once you are shown how it looks in person by somebody rather than trying to figure this out from a book. This is one of the reasons I have not used dobs, it's beyond frustrating for something that should be pretty easy. At least I know the focuser is squared up and the secondary is positioned correctly (Rick does this on all his scopes) I have Feathertouch focusers on both the Obsession and Starmaster so I know thoses are GTG.

What gets confusing is the natural offset of the secondary when using the Cheshire when your brain wants everything to be concentric. Fighting mechanical alignment vs optical alignment is frustrating.

When the Cheshire/primary adjustment is completed you move to the auto collimator and adjust at the secondary to close the system. Isn't this going impact the adjustment I just did with the laser on the secondary tilt and the Cheshire on the primary tilt?

#16 Vic Menard

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Posted 15 February 2012 - 07:18 PM

[quote name="titan7"]...I only plan on using the laser for the secondary tilt and then go to the cheshire and AC.[/quote]
Once you've sorted out the laser's internal alignment and registration, you should have no problem assessing and correcting the axial alignments.

[quote]I read through Vic's book on collimation and got a bad headache, sorry Vic, there just has to be a easier way to do this that the book.[/quote]
Ahh, but it is easier than reading the book (I hope you're reading the fifth edition). FWIW, about 60 of the 80 pages in New Perspectives... actually deal with Newtonian alignment, and most of the illustrations are in those pages. When you consider I sifted through hundreds, perhaps thousands of pages in the collimation discussions in this forum alone, to finally condense the essence of those discussions in 60 pages...well, I gave it my best shot.

As I stated earlier, if you confine your alignment to specific signatures, and your alignment procedure is approached systematically, you shouldn't have any problem achieving excellent collimation with the axial alignments well within tolerance.

[quote]I think this is one of those things that is simple once you are shown how it looks in person by somebody rather than trying to figure this out from a book.[/quote]
I agree! It's even easier when someone shows you how using your tools on your scope! It's why I always do "in the field" workshops at every star party I attend, and why I encourage those who know to share their knowledge.

[quote]This is one of the reasons I have not used dobs, it's beyond frustrating for something that should be pretty easy.[/quote]
I'm sure that I could have your scope axially aligned in a matter of minutes. Part of the frustration you're experiencing is not knowing what's important. The other part is you have tools that do not deliver consistent results, and worse, they're confusing you when they seem to indicate different axial alignments.

[quote]At least I know the focuser is squared up and the secondary is positioned correctly (Rick does this on all his scopes)...[/quote]
I owned and used a 22-inch StarMaster for almost 8 years. Rick leaves the secondary centered although the alignment is offset, which means the focuser is almost certainly not square to the optical axis. But that doesn't matter! As long as the focuser axis is pointed to the center of the primary mirror (what you accomplish when you use a laser and adjust the secondary mirror tilt) and the primary mirror (optical) axis is pointed toward the center of the eyepiece fieldstop (what you accomplish when you align the primary mirror center spot to the bright Cheshire ring), the axial alignment will deliver good image performance.

[quote]I have Feathertouch focusers on both the Obsession and Starmaster so I know thoses are GTG.[/quote]
I personally find the Feathertouch 2- to 1.25-inch adapters to be to shallow. If you want better consistency with your 1.25-inch collimating tools, you might want to look into the Glatter parallizer.

[quote]What gets confusing is the natural offset of the secondary when using the Cheshire when your brain wants everything to be concentric.[/quote]
When you use the Cheshire to align the primary mirror, don't worry about the secondary mirror alignment (concentric circles). Concentrate on the bright Cheshire ring and the primary mirror center spot.

[quote]Fighting mechanical alignment vs optical alignment is frustrating.[/quote]
FTR, we haven't entered that discussion yet. Optical alignment with or without offset can be accomplished without any consideration for mechanical alignment. (Mechanical alignment achieves orthogonality between the optical axis and the mechanical axes.)

[quote]When the Cheshire/primary adjustment is completed you move to the auto collimator and adjust at the secondary to close the system.[/quote]
Actually, that's not what I suggested. Use the autocollimator to assess the focuser axial alignment with a carefully decollimated primary mirror axis (using the signature alignment of the two center reflections to align the focuser axis), and then use the Cheshire to align the primary mirror axis (which "closes" the system). Using a "darkened" autocollimator by itself is not a conclusive alignment--the carefully decollimated primary mirror signature is a conclusive focuser axial alignment (with 2X magnification), as is the Cheshire signature (which magnifies the primary mirror axial error 2X).

[quote]Isn't this going impact the adjustment I just did with the laser on the secondary tilt...[/quote]
Carefully decollimating the primary mirror has no impact on the focuser axial alignment achieved with the laser, but the autocollimator does multiply any residual error 2X. But you're probably going to need a better autocollimator than the Tectron.

#17 uniondrone

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Posted 15 February 2012 - 07:36 PM

It could be that your laser is not fitting into the focuser properly.


This is the problem with a large percentage of miscollimations coming from laser collimators. There are two things to look for: 1) poor fit of the laser into the focuser (does it wobble? If so, does the wobble cause a significant shift in where the beam strikes the primary mirror?), and 2) shift in the focuser drawtube itself (if you grab the focuser tube and try to shift it around, does the beam shift noticeably?)

P.S. I get better results from a simple collimation cap, than from my laser. The collimation from my laser used to be terrible, but since I upgraded my focuser to a premium model it is merely sub-par due to poor fit. Your milage may vary, and premium lasers will likely do much better than my low-end Orion model.

#18 Vic Menard

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Posted 15 February 2012 - 08:20 PM

This is the problem with a large percentage of miscollimations coming from laser collimators. There are two things to look for: 1) poor fit of the laser into the focuser (does it wobble? If so, does the wobble cause a significant shift in where the beam strikes the primary mirror?), and 2) shift in the focuser drawtube itself (if you grab the focuser tube and try to shift it around, does the beam shift noticeably?)

I'm not sure what you mean. These both seem to be registration issues. If the laser OD is significantly smaller (1 or 2 hundredths) than the focuser drawtube ID, then there are two kinds of registration issues. The first is lateral offset (1 or 2 hundredths) and the second is angular tilt, which may translate to an inch or more at the reference primary mirror center spot (measured as the distance between the laser dot and the primary mirror center spot).

P.S. I get better results from a simple collimation cap, than from my laser.

The collimation cap is a primary mirror axial alignment tool. The simple thin beam laser is a focuser axial alignment tool. Neither one is very good at what the other tool is designed to do.

The collimation from my laser used to be terrible, but since I upgraded my focuser to a premium model it is merely sub-par due to poor fit. Your mileage may vary, and premium lasers will likely do much better than my low-end Orion model.

Premium lasers do indeed do "much better".

#19 Alan A.

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Posted 15 February 2012 - 08:29 PM

I had a different experience using Vic's book. I thought it was fantastically clear and it helped me understand the whole process of collimation at a much higher level - a great read. I still however, run into the occasional collimation conundrum.

#20 Vic Menard

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Posted 15 February 2012 - 08:35 PM

...I still however, run into the occasional collimation conundrum.

Me too! :foreheadslap:

#21 uniondrone

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Posted 15 February 2012 - 08:55 PM

I'm not sure what you mean. These both seem to be registration issues. If the laser OD is significantly smaller (1 or 2 hundredths) than the focuser drawtube ID, then there are two kinds of registration issues. The first is lateral offset (1 or 2 hundredths) and the second is angular tilt, which may translate to an inch or more at the reference primary mirror center spot (measured as the distance between the laser dot and the primary mirror center spot).


You're right; they are both registration issues, but from different sources. One is registration of the laser. The other is registration of the drawtube. If you have them both, it can be an especially frustrating experience--hence why I stopped using my laser.


The collimation cap is a primary mirror axial alignment tool. The simple thin beam laser is a focuser axial alignment tool. Neither one is very good at what the other tool is designed to do.


Well, I failed to mention that I use a sight tube for the focuser axis and the collimation cap for the primary axis. :grin:

Regarding laser collimators, are you saying that they are only intended for adjustment of the secondary tilt, i.e. the focuser axis? I always thought (not that I am any kind of a collimation expert) that people generally first align the laser with the primary center spot (focuser axis alignment), followed by adjusting the primary so that the return beam hits the target on the laser (primary alignment). Wouldn't this make for the laser to be intended as an all-in-one tool?

#22 Vic Menard

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Posted 15 February 2012 - 09:18 PM

...You're right; they are both registration issues, but from different sources. One is registration of the laser. The other is registration of the drawtube. If you have them both, it can be an especially frustrating experience--hence why I stopped using my laser.

OK, I'm still not sure I understand what you're saying. When you say "registration of the drawtube" do you mean the registration of the drawtube relative to the focuser body? (Modern Crayford focusers have all but eliminated that problem.)

Regarding laser collimators, are you saying that they are only intended for adjustment of the secondary tilt, i.e. the focuser axis? I always thought (not that I am any kind of a collimation expert) that people generally first align the laser with the primary center spot (focuser axis alignment), followed by adjusting the primary so that the return beam hits the target on the laser (primary alignment). Wouldn't this make for the laser to be intended as an all-in-one tool?

That's certainly the way they're sold, but simple thin beam lasers are not as accurate as Cheshire eyepieces and their derivatives when assessing and correcting primary mirror axial errors. If you think about it, the laser's return beam is simply the reflection of the focuser axis. It doesn't represent the primary mirror axis unless the focuser axis is perfectly corrected. If there's a residual focuser axis, one half of the residual error will be propagated back to the laser emitter via the return beam, which means a perfect return beam alignment is actually imperfect! And there's no magnification factor either.

So, while you can expect excellent alignment accuracy with a Cheshire eyepiece or collimation cap, it's quite possible to end up with a significant error (the kind that actually impacts image performance) when you use the unBarlowed return beam from a simple thin beam laser to align the primary mirror.

That doesn't mean there aren't some simple workarounds to get the job done with a good simple thin beam laser. The 2-inch Glatter is machined to sufficient precision to all but eliminate pesky registration errors, and with the standard 1mm aperture stop, the diffraction pattern around the laser dot can be used to bring the silhouette reflection of the primary mirror center spot back to the emitter/target. With this tool, I can get a precise read of the focuser axial error and the primary mirror axial error.

#23 titan7

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Posted 15 February 2012 - 09:50 PM

No offense Vic, I did learn from your book and I am glad it's out there. I am going to try things again after I collimate the laser. So its back to the refractor until I can figure this out. Perhaps I can run into somebody who can show me the right way to do it, I am more of a learn by observing and doing guy.

#24 cheapersleeper

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Posted 15 February 2012 - 10:00 PM

Titan7, being a bit off in collimation does not mean that your scope won't work. It will still work and if it is close, it will work fairly well. Don't give up the enjoyment to be had by being overly concerned with perfection.

Collimation is a lot easier if you think of it as several separate operations that when completed add up to being collimated.

Sight tube: get the secondary centered under the focuser
Laser: Get the cetnered secondary aimed at the center spot of the primary
Cheshire: get the circle, or triangle, or radioative sign into the glowing circle

There it is. And I did not say "axial" one time. :lol:

Regards,
Brad

#25 michael_m

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Posted 15 February 2012 - 11:10 PM

:waytogo:

And usually this sequence is repeated at least once because by the time you complete the last step, the first step got slightly affected. Each repetition produces less error. The remaining error from the first iteration is reduced in the second iteration and so on. A couple-three times through the sequence and you have a collimation within the "tolerance envelope" with good performance of the system.

The focuser-secondary combo generally are a "once in a while process" leaving more often than not just the primary. The hardest part is the initial setup of the first collimation of the telescope. Thereafter it is much easier.


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