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Catseye vs. Howie Glatter and Blug??

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#26 Starman1

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Posted 23 November 2008 - 04:59 PM

I ordered the Howie Glatter and the Blug. I thought this problem was solved. Why exactly do you need to collimate [better] at f /4.5 anyway? What is the process of collimating at f/4.5 [that's different from other f/ratios]. Aren't we splitting hairs here?

In my opinion, all the Catseye stuff is too expensive for my blood. Since there are no other 2" autocollimators on the Market? - What about this: Would this work with a 1.25 to 2" adaptor?
http://www.agenaastr...alc-js-etii.htm

Seems to me I have purchased a telescope that has numerous design flaws and won't hold collimation. Maybe I'll just cancel the Order, and all the accessories, and buy an SCT.


There is no difference in the process of collimation at f/4.5 than at f/8. The difference is that the maximum allowable tolerances for miscollimation shrink substantially. Therefore, a user of a shorter f/ratio scope needs to hone his skills at collimation.

The likelihood is that the Glatter laser and BLUG, if repeated (recheck after using each tool once), will get you close enough to perfect collimation to give you star images as close to perfect as the scope and seeing conditions allow.

The autocollimator is a far more sensitive tool, and collimation using it is closer to perfect than not using it. Will you see it? Perhaps not. But if the scope's collimation drifts slightly during the night, if you were AC perfect at the start, the miscollimation that occurs is less likely to drift to a point where miscollimation is visible.

And, if you use a Paracorr (you may, you may not), tolerances tighten by a factor of 6, making use of the autocollimator nearly essential.

A good 1.25" autocollimator will work fine. I would suggest the one from Astrosystems or the one from Tectron (discontinued, but still out there) If you can afford a dedicated 2" adapter for it, you essentially have a 2" tool. The 1.25" AC won't have as wide a field of view as a 2", but I've seen them in use, and they work well. A used Catseye might be found. And there are a couple more brands coming out soon. It seems more and more people are discovering the advantages of using one.

You haven't bought a scope that is inherently flawed, where collimation is concerned, just a scope that needs collimation. Once collimated, there doesn't appear to be any reason why the scope shouldn't retain collimation. Even very expensive scopes need to have tweaks made, though. I've helped some owners of very large scopes more expensive than my car go through the "shakedown" problems one experiences with nearly every new scope. I had one friend with a $60K+ scope have collimation problems for almost a year before they were satisfactorily eliminated.

And spending a few hours on the mechanicals of a scope seem negigible when amortized over the life of a scope. I don't even want to mention how many hours I spent rebuilding and refinishing parts of my scope. :foreheadslap: :bawling: :lol:
But it was worth it, I think.

#27 Shawn H

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Posted 23 November 2008 - 05:02 PM

Matt
I went with CatsEye passive tools because lasers can be knocked out of collimation! But Howie Glatter is high end stuff, Soooo there you go, decisions, decisions :grin: Shawn

#28 Starman1

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Posted 23 November 2008 - 05:13 PM

I also plan for a permanent position. Just how do you collimate at f/4.5? It seems both the Meade & the AT go out of collimation when moved up and down. I do believe there are some design issues that Meade or GSO should correct.

Every scope seems to have a few issues that the manufacturer has ignored. Jayscheuerle (user name) here on CN has rebuilt a Meade from scratch. If you read some of his past posts, you will get the lowdown on everything he encountered on his Meade along the way.
It does seem, from other posts here on CN, that a few star washers or screw changes would go a long way on the 16" Meade.
Meade has made running changes to these scopes as time has passed. I wouldn't be surprised to see some changes come along as the years go by. The first LightBridges didn't have an altitude brake, or a steel washer under the secondary collimation screws, and had innumerable other problems now fixed.
And if you read Rod Mollise's treatise on the SCTs in the market over the years, you'll discover there have been a few, uh, problems with that type of scope through the years.

The key is to not expect perfection when you are paying an affordable price. Perfection requires a "military-sized" budget. :lol:

#29 JR1560

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Posted 23 November 2008 - 06:10 PM

i heard that GRS did replace the springs on the AT16 i reviewed with heavy duty ones. the scope still did not hold collimation.

--------------------
dave bonandrini

what we are talking about here is that the scope does not stay in collimation WHILE assembled.

you are right that every dob should be collimated EVERY time you take it out and use it.

(just thought i would clear that up before i get any more emails!)

--------------------
dave bonandrini



It's also interesting to observe how much flexure are in some of these dobs. You can see the stacks moving all over the place when moving the scopes up and down.

D.Mounsey


In my opinion, regardless of price or quality, the collimation should hold moving the scope up or down. I see there are collimation issues with both the AT and the Meade. Or if you want to nail it down - GSO. I suspect there are design issues here.

So Don, we don't agree, but for now maybe you can explain
how an f/4.5 is collimated, and does Howie Glatter have a device that would autocollimate the LB 16.

#30 matthew2000tx

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Posted 23 November 2008 - 06:18 PM

... CatsEye has too much ...


No doubt there is a "lot" of information I've put out on the site over the years, but I have something there for everyone's level of comprehension and interest; however, I'm always open to suggestions for improvement. If there's some non-useful or too technical information on the site that needs to be eliminated, let me know via email flyj@catseyecollimation.com or PM :grin:


Jim, I'm a visual person. I find it very hard to figure out the steps of using the catseye tools. There is no doubt that your tools are great, but I'm getting having trouble visualizing it all.

You have a lot of great info on your site, but what about making a "How to video with narration" that takes us through the whole process with Catseye from marking the Primary all the way to the last collimation check. Something like what Andy's Shot glass has on there website.

As Example!!

I think If I could see the whole process done before my eyes that would help.

#31 Starman1

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Posted 23 November 2008 - 06:46 PM

In my opinion, regardless of price or quality, the collimation should hold moving the scope up or down. I see there are collimation issues with both the AT and the Meade. Or if you want to nail it down - GSO. I suspect there are design issues here.

So Don, we don't agree, but for now maybe you can explain
how an f/4.5 is collimated, and does Howie Glatter have a device that would autocollimate the LB 16.


John,
We've communicated before about a few possible mods that could eliminate any movement that results from design-related issues, so I will just make some notes about the tools used and collimation. Glatter does not make an autocollimator.

Step 1: Center the secondary under the focuser and make it round by rotating the oval until it appears round. This is typically done once, and not again until the mirror is removed for cleaning. The tool to use is a sight tube, or, if you can hold your head steady enough, the focuser by backing your head away a few feet until you see the inside diameter of the focuser *just* larger than the outline of the reflective surface of the secondary. When these are concentric, you're done here.
Step 2: using a single beam laser, or a sight tube, angle the secondary to put the beam in the center of the primary (laser) or the centermark behind the crosshairs (sight tube). Go back to step one to see if the secondary moved. If so, repeat step one and step two as many times as necessary until both agree at the same time.
Step 3: using a barlowed laser (or a Blug), or a cheshire, or a Krupa collimator, align the primary to the optical axis. If the movement was more than a tiny bit, repeat step two to see if there is any movement. If so, repeat step two and three again until both agree at the same time.

To all intents and purposes, you can stop here. When all steps agree, then the return beam of the single beam laser should return to its source. You don't use this to collimate originally because there are many tilt solutions for the secondary and primary that can return the beam to its source that are not correct collimation, but once the scope is collimated, the return beam should return to its source.

At this point, if the laser wanders on the primary mirror when the scope moves up and down, something somewhere is moving. It could be flex in the truss poles, but more likely it is elsewhere--the screws, the spider tightness, the focuser, etc.
But, no matter where it is, it can be tracked down and, if not eliminated, tamed to insignificance.

The autocollimator, at this point, can be used to collimate even more exactly. Some would argue that it's not necessary. Probably true, since, if the previous steps all show good agreement to your ability to discern the details, you are probably within tolerances. But if you want to collimate more exactly, the autocollimator can do so. If it shows no errors at all, it was the final arbiter of collimation. But, more likely, it will show that a further accuracy is possible.

But say you see no collimation movement with the earlier steps. Does that mean you will not see collimation movement with the AC? No. It is incredibly sensitive, displaying changes with movement in the low thousandths of an inch. You can be assured that if you see no movement in the AC with altitude change, there is none to speak of.

If, at a future date, you decide to get a Paracorr to eliminate coma, then you should definitely invest in an autocollimator. Until then, just be careful with steps 1-3 and I think you'll be fine.

If there are some design issues in GSO scopes that result in collimation changes with altitude, but are easily fixed by $0.50 of parts from Home Depot, I don't regard that as a design flaw. It's probably due to an economy in manufacturing. No big deal.

#32 JR1560

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Posted 23 November 2008 - 07:36 PM

If there are some design issues in GSO scopes that result in collimation changes with altitude, but are easily fixed by $0.50 of parts from Home Depot, I don't regard that as a design flaw. It's probably due to an economy in manufacturing. No big deal.


I don't think I would buy a car from you Don. I think the issues I raised are valid. I'll send you the bills if it's more.

#33 CatseyeMan

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Posted 23 November 2008 - 08:39 PM

... but what about making a "How to video with narration" that takes us through the whole process with Catseye from marking the Primary all the way to the last collimation check.


Thanks for the feedback; coincidentally, such a video is already in the making. In the meantime, see this written step-by-step with graphics:

7 Steps for Perfect Collimation

and see this "visual" Pictorial Sequence by my friend Mike Sedonio:

Mike Sedonio CATSEYE Pictorial Sequence

#34 Wave Vector

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Posted 23 November 2008 - 09:40 PM

Mr. Starman what metrics does one use in evaluating an auto-collimator?

#35 Starman1

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Posted 24 November 2008 - 01:10 AM

Here, Nils Olof Carlin argues the autocollimator does not have any inherent advantage in accuracy over a cheshire or barlowed laser in achieving accurate collimation of the primary mirror:
http://web.telia.com...Acoll/Acoll.htm

Carlin implies that secondary alignment (actually focuser alignment) can be improved by use of the autocollimator but only over the use of an imprecise sight tube, not necessarily a laser.

But let's look at the necessary pointing accuracy of the sight tube or laser at f/4.5:
.005x(4.5)^3=0.46mm maximum allowable tolerance of focuser axis alignment. At the distance to the mirror, can you estimate the position of the laser to that accuracy? That's less than 1/50 inch (.018, or 18 thousandths). And that's the maximum allowable error. Say you have a laser with a beam point small enough at the mirror's distance that you might estimate its position to within an error circle of that diameter.
Then you are within tolerances.

But if you use a Paracorr, the tolerances tighten by a divisor of 6 to .003", and I don't care what laser you use--you simply cannot place the beam on the primary to that degree of accuracy. A laser's beam is not fine enough to do it. [Someone with more knowledge of the math involved can point out my error if I make one here.]

This refers to focuser axis tilt error, not primary mirror tilt error.

Yet, an autocollimator can easily see that amount of error, and, if read correctly, can correct the focuser-secondary axis to that level of accuracy. I've used a well-collimated laser with a small aperture stop in my own scope to collimate the focuser-secondary alignment, and, after collimation, still saw residual errors with the autocollimator that could be reduced.

I say reduced and not eliminated because my eyes are simply not capable of focusing the 4 stacked images in the autocollimator at the same time. They do not appear, to my eye, *exactly* the same size and the edges of the centermark stack appear soft because different focus is needed to see each image. I can only stack them until they appear stacked to my eye, and the reflection of the AC pupil appears as a black dot in the center of the stacked images.

Now, if I was able, with the laser, of getting the focuser axis within tolerances, would I see a visual improvement at the eyepiece with the AC? No.
Would I be able to improve on the laser's collimation with an autocollimator?
Yes.
But is it likely I can achieve the same level of collimation accuracy with a laser that I can achieve with an autocollimator?
My answer to that explains why I do not collimate my scope with a laser.

I'll go on to elaborate that I do not collimate my scope in the dark. If I did, and I did not want to set up some imaginative way to illuminate the centermark on the primary so I could see the 4 reflections, then I probably would use a laser and use a barlow or Blug.

#36 Relativist

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Posted 24 November 2008 - 01:24 AM

I have both a laser and catseye system. I use the laser only when I'm in a hurry, and maybe one step with the catseye if I can. Otherwise I do two steps with the laser and the catseye.



#37 DeepSpaceTour

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Posted 24 November 2008 - 07:24 AM

.....[snip]Yet, an autocollimator can easily see that amount of error


....But you can't see any visual improvement at the eyepiece.


Now, if I was able, with the laser, of getting the focuser axis within tolerances, would I see a visual improvement at the eyepiece with the AC? No.
Would I be able to improve on the laser's collimation with an autocollimator?
Yes.


I hesitate to go here and say this,for fear of being accused of heresy,but the difference between accurate lazer collimation,and a chance that the images are not perfectly stacked,but almost,and having the images absolutely perfectly stacked with the autocollimator makes absolutely NO,NONE,NOTA,difference at the eyepiece,these are my findings through trial and error...there is a point of collimation where "it's not going to get any better visually" your just stacking the images perfectly to achieve,as perfect collimation as your tools allow,but makes no difference visually at the eyepiece,basically to satisfy yourself,but that's it.......soooo I don't use the autocollimator anymore,why bother,when it makes no difference at the eyepiece,a well collimated single beam and barlowed collimator gets you accurately collimated enough.

As someone already stated when you collimate a scope,moving the scope around will make the collimation move around a little bit,there is flex going on in the scope at different points,no matter what....put in an autocollimator and move the scope around.....you'll see,but that little bit of movement,makes no difference at the eyepiece,at least not that the average(none super human) eye can detect.

These are my findings on this collimation issue.....now... :getem:.....here it comes... :tonofbricks:....:lol:

Clear skies.

#38 Vic Menard

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Posted 24 November 2008 - 08:39 AM

...let's look at the necessary pointing accuracy of the sight tube or laser at f/4.5:
.005x(4.5)^3=0.46mm maximum allowable tolerance of focuser axis alignment.

I think you meant .005xD for the allowable focuser axial tolerance (with Paracorr) which works out to about 2mm. If the scope is used without a Paracorr, the tolerance is a whopping 12mm!

The tolerance you were evaluating above is the primary mirror axial tolerance for high performance applications. At f/4.5, the 0.46mm tolerance might seem almost impossible to maintain. But given the fact that both the Cheshire eyepiece and Barlowed laser magnify the error 2X, the magnified tolerance only needs to be maintained to +/- ~1mm, which is a very easy read with a properly matched center spot. For example, if one uses a Cheshire with a 0.5-inch perforation and a center spot with a 0.4-inch outside diameter, the annulus between the two with perfect alignment will be 0.05-inch wide. A 1mm error is about 0.04-inch, which means the annulus will appear to be scrunched on one side to 0.01-inch and open on the other to 0.09-inch--in other words, it will stick out like a sore thumb! Yet even this obvious error is within tolerance...

Now, if I was able, with the laser, of getting the focuser axis within tolerances, would I see a visual improvement at the eyepiece with the AC? No.

I personally find that if I take the time to carefully align the focuser and primary mirror axes with my Glatter laser, I can skip autocollimator verification--and I'm working at f/4! And that's pretty much what I do after dark. But if I have the ambient light to work with, the autocollimator lets me see any residual errors as well as any potentially troublesome mechanicals so I can resolve these problems before I start observing.

#39 Vic Menard

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Posted 24 November 2008 - 09:45 AM

...the difference between accurate laser collimation,and a chance that the images are not perfectly stacked,but almost,and having the images absolutely perfectly stacked with the autocollimator makes absolutely NO,NONE,NOTA,difference at the eyepiece...

Assuming you're within tolerance for the performance level you require, the two tools become redundant. I personally don't see a problem with the concept of redundant verification.

...there is a point of collimation where "it's not going to get any better visually" your just stacking the images perfectly to achieve,as perfect collimation as your tools allow,but makes no difference visually at the eyepiece,basically to satisfy yourself,but that's it...

While it is satisfying to be able to achieve a perfect stack with the autocollimator, I think it's much more important that the perfect stack should fall between the close jumbles at the zenith and working horizon to maintain axial tolerances (+/-) within the various mechanicals.

...soooo I don't use the autocollimator anymore,why bother,when it makes no difference at the eyepiece,a well collimated single beam and barlowed collimator gets you accurately collimated enough.

Like I said above, that's pretty much what I do once it gets dark. But along with redundant verification, the autocollimator also is a valuable tool for assessing where mechanical flexure is originating (specifically, for evaluating the impact on the focuser axis). And at f/4, with a Paracorr, even though I can get the axial alignment nearly perfect with my Glatter, I still consider the autocollimator to be an essential tool.

At least now, we seem to have conquered most (if not all) of the collimation gremlins with modern tools and procedures. There are, of course, more image killers on the other side of accurate collimation--inadequate mirror support, temperature differentials, limited seeing, and if you're using your eyes instead of a camera... Given the typical signal to noise ratio most of us deal with on a nightly basis, it's nice to know we can control some of the problems!

...there is flex going on in the scope at different points,no matter what....put in an autocollimator and move the scope around.....you'll see,but that little bit of movement,makes no difference at the eyepiece,at least not that the average(none super human) eye can detect.

I agree with your statement regarding flexure, but I'm not sure that your conclusion is always correct. As long as the axial error attributed to mechanical flexure falls within the prescribed tolerance for your performance expectation, then the alignment is "good enough". A good example would be using a Dob to view Eta Carina at the Winter Star Party. Although the target object is only a few degrees above the horizon where the axial alignment is surely outside of the high performance tolerance, I usually use lower magnifications (rarely more than 10X per inch of aperture) to get as much of the nebula as possible in the field of view. Lowering the magnification relaxes the axial tolerances... There are many good reasons we wait for planets to rise well above the horizon before we start powering up.

#40 Starman1

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Posted 24 November 2008 - 10:45 AM

For example, if one uses a Cheshire with a 0.5-inch perforation

I presume you are talking about the diameter of the central black area in the cheshire.

and a center spot with a 0.4-inch outside diameter, the annulus between the two with perfect alignment will be 0.05-inch wide. A 1mm error is about 0.4-inch

I think you mean 0.04 inch

which means the annulus will appear to be scrunched on one side to 0.1-inch and open on the other to 0.9-inch

Don't you mean squeezed to 0.01 inch on one side (.05-.04) and expanded to 0.09 (.05+.04)on the other?

in other words, it will stick out like a sore thumb! Yet even this obvious error is within tolerance...

Agreed. A difference of 9 times the gap is going to be visible.

Well, the implications of your post, along with Bill's, is that the autocollimator's improvement in collimation produces empty collimation, since it can't be seen.

But I'll make a few contentions that seem to be well supported by what I see in the field:
--having collimation that is more accurate can help compensate for collimation shifts that occur in many scopes' mechanicals during the night as the scopes are shifted in altitude and mechanical flexures and movements occur. Such small errors will stay within tolerances if the original collimation was closer to perfect.
--the ease of use of an autocollimator to achieve this higher level of collimation accuracy exceeds the ease of use of a barlowed laser. I admit this is a personal preference.
--the exact collimation of the primary using a BLUG depends on the accuracy of the focuser (not all have perfectly bored drawtubes, or perfectly round ones for that matter) and the accuracy of placement of the Blug into the focuser.
--At least one laser manufacturer saw the difficulties of using a laser for primary collimation and decided to produce a cheshire to sell with its laser.
--the use of a barlowed laser attachment that is merely a white circle on the bottom of the laser is difficult unless the focuser drawtube is short or the scope is a larger aperture.
--When you add a barlow to a laser you add an additional source of registration error to the system. The exception is the white faced barlow thread-in adapter on the bottom of some lasers.
--most users of lasers in the field have neither an accurately collimated laser, nor use a barlowed laser technique to achieve primary collimation (they rely on the return beam).
--the use of a laser/barlowed laser combination doesn't result in the same degree of accuracy as the autocollimator, even if the laser is accurate enough. If two tools quickly result in the same effect: collimation good enough to show good star images, but one is more accurate/sensitive than the other, then the latter is the superior tool. That doesn't mean that both techniques don't work, merely that one technique is inherently more accurate than the other.

One tool, which I wish was in production, eliminates many of my above objections to the use of a laser for primary collimation: the focused Krupa collimator ( Krupa ). This tool could easily replace the barlowed laser for primary collimation. It should be relatively cheap, too, since an LED is going to be inexpensive to obtain and manufacture.

Now Vic knows I am, to a certain degree ;), playing devil's advocate here. Because I know that good collimation can be obtained with lasers and barlows.
But not entirely. Passive tools can be safely used to collimate and they are not prone to miscollimation, are easier to use (personal judgment there), and in the case of the autocollimator possessed of a higher degree of accuracy with greater mechanical simplicity than the equivalent barlowed laser technique.

Use what you like and patience will pay off with the goal of collimation that produces your scope's best images.
But put me down as a passionate advocate for passive tools rather than lasers.

#41 Island Mike

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Posted 24 November 2008 - 11:52 AM

I'll drop this back to the mundane. I have a Catseye system and have used it for 3 yrs or so.

I've liked it very much, but will be replacing it with a laser; specficially the Hotech. Or, I should say, that I will replace it once I've done a test to make sure the Hotech does a good job.

I've enjoyed the Catseye, but here are my issues:

1. I find it difficult to use at night; specifically I had trouble seeing the required triangle / donut in the dark.

Since I'm collimating in the field after driving or at least moving the scope after nightfall, it limited its usefullness. Shining a light into the OTA helps, but requires an extra person, and in my case wasn't the best for my night vision (even with a red led flashlight)

2. Making adjustments is possible with only one person, but frustrating. The idea of being able to collimate alone, without having to run from the back of my dob to the focuser is a dream.

3. This isn't an issue with the Autocollimator, but with my scope, but it impacts none the less: I found that no matter how tight my spider is, AND even with my Bobs Knobs replacement secondary screws, that adjusting them during autocollimation was a frustraging backlash goose chase. I would twist a screw, it would throw the triangles way off, I would let go, they would bounce back, still a bit off, never perfect. In the end, I was always satisfied at the primary alignment alone in my F5.

Down the road if I get a F4 and need a parracor, I will likely be going back to the Catseye and maybe I will have better luck with the XL version for seeing in the dark. Again, a good product, but I'm looking to try something new.

Why the Hotech over the Howie or the Kendrick? Price. The alternatives are not cheap! I know you get what you pay for, but I'm not coninced the 'premium' price of the other two lasers are justified in my case.

Cheers,

Mike

#42 DeepSpaceTour

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Posted 24 November 2008 - 12:05 PM

Vic;

I agree with all that you have said,just let me clear a few things up;

First I agree with redundant verification,and I still use the autocollimator once in awhile now just to make sure my earlier findings are correct.

While I was figuring this all out I used to use the autocollimator all the time to back up my results,what I used to find was that the stacked images in the AC were just that, perfectly stacked, after using the 2" Kendrick in single beam and barlowed collimation mode,but some times the images were a "BIT" and I mean ever so slightly not stacked,in fact you could still see the black pupil or hole through the center of the stacked images,just not so big and round as when "perfectly" stacked...so that's how close things are when I am collimated...this is where the experimentation comes in,some nights I would observe with the scope collimated as described above,other nights I would get the collimation absolutely dead on perfect with the AC...and you know what???I found no difference at the eyepiece between the two above scenarios.

So I think what Don eluded to "empty" or "redundant" collimation is true after a certain point,but we are talking about very,very close collimation tolerances,nothing we are talking about here is a "mile out" or anything,I am not sure how far out of dead perfect stacks in the AC makes a difference at the eyepiece,because I never had my collimation out that far,I am a stickler for having the laser register dead on in both single beam and barlowed mode,and I go back and forth between SB and BLC mode,until it's right,and the AC has always shown that this is right on,or very close enough for visual and average human eyes.

Now this all goes without saying,that,the initial secondary alignment was done with a sight tube to get the secondary axial alignment right and the secondary centered under the focuser.....to begin with(during initial alignment which usually does not have to be touched for a long time,if it gets bumped or jolted hard or something)I am just taking "this" important step,as a given,we "are" just talking about the routine collimation we all go through every night,before we commence observing........of course!!!!

Clear skies.

#43 Vic Menard

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Posted 24 November 2008 - 12:06 PM

[quote name="Starman1"]...I think you mean 0.04 inch...[/quote]
Yep--I fixed the post.

[quote]Well, the implications of your post, along with Bill's, is that the autocollimator's improvement in collimation produces empty collimation, since it can't be seen.[/quote]
Not empty--redundant. And only redundant if the laser is capable of delivering a consistent, precise read (internal alignment, registration, and beam size are all optimal).

[quote]...having collimation that is more accurate can help compensate for collimation shifts that occur in many scopes' mechanicals during the night as the scopes are shifted in altitude and mechanical flexures and movements occur. Such small errors will stay within tolerances if the original collimation was closer to perfect.[/quote]
Agreed and included in my post above.

[quote]...the ease of use of an autocollimator to achieve this higher level of collimation accuracy exceeds the ease of use of a barlowed laser. I admit this is a personal preference.[/quote]
Well, with a carefully decollimated primary, the autocollimator magnifies the focuser axial error 2X--a simple thin beam laser is 1X. Ease of use has many variables, including personal preference...

[quote]...the exact collimation of the primary using a BLUG depends on the accuracy of the focuser (not all have perfectly bored drawtubes, or perfectly round ones for that matter) and the accuracy of placement of the Blug into the focuser.[/quote]
I'll probably get flamed here but I personally prefer the 1mm aperture stop over the Blug for precision. While the Blug facilitates primary mirror alignment from the rear of the OTA, I still have to get up and get close to the Blug for a more precise read.

[quote]...At least one laser manufacturer saw the difficulties of using a laser for primary collimation and decided to produce a cheshire to sell with its laser.[/quote]
No argument there, especially if the laser isn't Barlowed.

[quote]...the use of a barlowed laser attachment that is merely a white circle on the bottom of the laser is difficult unless the focuser drawtube is short or the scope is a larger aperture.[/quote]
Agreed. That's why Howie introduced the Blug and the tuBlug.

[quote]...When you add a barlow to a laser you add an additional source of registration error to the system. The exception is the white faced barlow thread-in adapter on the bottom of some lasers.[/quote]
I'm not sure the registration issue would be problematic, especially if a 2-inch Barlow is used. If an adapter is used, I agree with your point.

[quote]...most users of lasers in the field have neither an accurately collimated laser, nor use a barlowed laser technique to achieve primary collimation (they rely on the return beam).[/quote]
No argument there. I would like to call this "improper" usage, but too many manufacturers encourage this practice.

[quote]...the use of a laser/barlowed laser combination doesn't result in the same degree of accuracy as the autocollimator, even if the laser is accurate enough. If two tools quickly result in the same effect: collimation good enough to show good star images, but one is more accurate/sensitive than the other, then the latter is the superior tool. That doesn't mean that both techniques don't work, merely that one technique is inherently more accurate than the other.[/quote]
Again, I use the tools redundantly, and when I take the time to get the Glatter (with 1mm aperture stop) precisely aligned with the CatsEye triangle, I find that the Infinity XL autocollimator also shows perfect alignment about 60- or 75-percent of the time. I'm not suggesting that everyone's results will be the same as mine, only that it can be done. For a novice user, the autocollimator may seem more like an exercise in frustration. But if they're not achieving the necessary level of alignment precision to realize their scope's performance ("...my 4-inch refractor outperforms my 8-inch Dob"), the autocollimator is just showing them that they need to do a better job.

[quote]One tool, which I wish was in production, eliminates many of my above objections to the use of a laser for primary collimation: the focused Krupa collimator ( Krupa ). This tool could easily replace the barlowed laser for primary collimation. It should be relatively cheap, too, since an LED is going to be inexpensive to obtain and manufacture.[/quote]
I personally feel that we can already achieve the necessary level of precision for axial alignment with a variety of tools that are available today.

[quote]...Passive tools can be safely used to collimate and they are not prone to miscollimation, are easier to use (personal judgment there), and in the case of the autocollimator possessed of a higher degree of accuracy with greater mechanical simplicity than the equivalent barlowed laser technique.[/quote]
Playing the devil's advocate from this side of the fence--I rarely use a Barlow attachment on my Glatter laser anymore--the 1mm aperture stop shows both axes simultaneously. I also rarely use my Cheshire anymore. I use my TeleCat XL combo tool and Infinity XL autocollimator about equally (people should really use a good sight tube to evaluate the secondary mirror alignment more often!) and the Glatter with 1mm aperture stop almost exclusively after dark for axial spot checks.

[quote]Use what you like and patience will pay off with the goal of collimation that produces your scope's best images.
But put me down as a passionate advocate for passive tools rather than lasers. [/quote]
And I truly hope your passion never wanes! :waytogo:

#44 Vic Menard

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Posted 24 November 2008 - 01:11 PM

...2. Making adjustments is possible with only one person, but frustrating. The idea of being able to collimate alone, without having to run from the back of my dob to the focuser is a dream.

I'm not a big advocate of collimating the primary mirror with the reflected focuser axial beam, but if it meets your needs, that's all that counts.

3. This isn't an issue with the Autocollimator, but with my scope, but it impacts none the less: I found that no matter how tight my spider is, AND even with my Bobs Knobs replacement secondary screws, that adjusting them during autocollimation was a frustraging backlash goose chase. I would twist a screw, it would throw the triangles way off, I would let go, they would bounce back, still a bit off, never perfect. In the end, I was always satisfied at the primary alignment alone in my F5.

Without a Paracorr, you were likely already in the tolerance window. I'm somewhat surprised you had so much difficulty (even with Bob's Knobs), but there is often a certain amount of finesse required to tweak the focuser axial alignment with some secondary mirror holders.

...Why the Hotech over the Howie or the Kendrick? Price.

The 2-inch HoTech is $120 and the 2-/1.25-inch Glatter is $150 (I think that's correct). The Glatter isn't a windowed laser, but with the 1mm aperture stop (standard on all Glatter lasers) I consider it a top contender. (You can add other Glatter accessories later if you choose to.)

#45 Vic Menard

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Posted 24 November 2008 - 01:36 PM

...what I used to find was that the stacked images in the AC were just that, perfectly stacked, after using the 2" Kendrick in single beam and barlowed collimation mode,but some times the images were a "BIT" and I mean ever so slightly not stacked,in fact you could still see the black pupil or hole through the center of the stacked images,just not so big and round as when "perfectly" stacked...so that's how close things are when I am collimated...this is where the experimentation comes in,some nights I would observe with the scope collimated as described above,other nights I would get the collimation absolutely dead on perfect with the AC...and you know what???I found no difference at the eyepiece between the two above scenarios.

I don't doubt that you were operating within tolerance (you didn't say whether you were using a Paracorr or not). Considering that the offset reflections in a close jumble magnify the focuser axial error 2X and 4X, and assuming your Barlowed laser alignment was precise, your focuser axis could have been as close as two or three hundredths from perfection (without a Paracorr the tolerance for a 17-inch is about a half inch--even three hundredths is 10X better than the allowable tolerance).

...I am not sure how far out of dead perfect stacks in the AC makes a difference at the eyepiece,because I never had my collimation out that far...

Again, assuming the primary mirror axis is properly aligned, the flanking reflections magnify the focuser axial error 2X and 4X.

I am a stickler for having the laser register dead on in both single beam and barlowed mode,and I go back and forth between SB and BLC mode,until it's right,and the AC has always shown that this is right on,or very close enough for visual and average human eyes.

That's my experience too, and why I use the Glatter with the 1mm aperture stop after dark--plus it's easier on your dark adaptation than the brighter Barlowed laser!

...the initial secondary alignment was done with a sight tube to get the secondary axial alignment right and the secondary centered under the focuser.....to begin with(during initial alignment which usually does not have to be touched for a long time,if it gets bumped or jolted hard or something)

Or if you're using no-tool knobs and the secondary mirror adjustments are hand tightened instead of wrench tightened. It only takes a few moments to evaluate the secondary mirror alignment with a good sight tube--a few moments that I find to be worth the effort.

I am just taking "this" important step,as a given,we "are" just talking about the routine collimation we all go through every night,before we commence observing........of course!!!!

At f/4, even with my robust StarStructure mechanicals, I find that I can get away with simple axial alignment a couple of times before the secondary mirror alignment needs to be reassessed. But I also tweak the axial alignment every hour or two of active, multi-target observing to keep the high magnification performance optimized when the seeing permits. That's where I find the Glatter to be particularly useful.

#46 hudson_yak

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Posted 24 November 2008 - 01:36 PM

I alternate between the cheshire and autocollimator for final collimation. You can stack the images and have the primary slightly off, so I bring the stack together, check the primary alignment with the cheshire (often, it needs a tiny tweak), then check the stack in the autocollimator again.


I've read this sort of comment a number of times in forums. I don't understand it, I guess. Under what circumstances can the AC images be stacked but the primary still be off as shown by a cheshire? The only thing I could think to attribute it to would be differences in how the respective tools sit in the focuser (iow, where the respective axes lie).

I do understand the converse situation, that it's possible for the cheshire to show alignment but the AC not (due to focuser tilt error).

Mike

#47 Vic Menard

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Posted 24 November 2008 - 01:51 PM

2. Making adjustments is possible with only one person, but frustrating. The idea of being able to collimate alone, without having to run from the back of my dob to the focuser is a dream.

3. This isn't an issue with the Autocollimator, but with my scope, but it impacts none the less: I found that no matter how tight my spider is, AND even with my Bobs Knobs replacement secondary screws, that adjusting them during autocollimation was a frustraging backlash goose chase. I would twist a screw, it would throw the triangles way off, I would let go, they would bounce back, still a bit off, never perfect. In the end, I was always satisfied at the primary alignment alone in my F5.

Just an afterthought--at f/5 the primary mirror axial tolerance for high performance is 0.6mm (about 2-hundredths of an inch). To utilize the return beam, you'll have to add one half of any residual focuser axial error to the remaining error at the laser target. This means that if you can align the focuser axis (by adjusting the secondary mirror tilt) to a precision of 0.02-inch, you'll need to keep the primary mirror axial alignment almost perfect (+/- 0.01-inch). To sum up, if 3. is real issue, then 2. is probably a pipe dream. :shrug:

Of course, if you follow up with a Barlowed laser or Cheshire procedure, you'll only need to keep the primary mirror alignment accurate to +/-0.05-inch--an easy read.

#48 Vic Menard

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Posted 24 November 2008 - 02:00 PM

...Under what circumstances can the AC images be stacked but the primary still be off as shown by a cheshire?

Actually, it's possible to stack the wrong reflections and end up with the two axes intersecting at the radius of curvature and only the primary mirror center spot visible in a darkened autocollimator. More likely, by "stacking" I think Don means tightening the jumble. If you do this by adjusting the secondary mirror, the primary mirror axis will be affected. The better solution is to decollimate the primary mirror to separate the reflections, and then to stack the right reflections to zero the focuser axis. Then you can finish by fine aligning the primary mirror with the autocollimator, Cheshire, or Barlowed laser...

#49 GardnerPacificCA

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Posted 24 November 2008 - 02:05 PM

I just ordered Vic's new book: http://www.catseyeco...rspectives.html
I haven't decided on which dob yet...with so much info and discussion on this thread though...I think I better get a head start on understanding collimation.

#50 hudson_yak

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Posted 24 November 2008 - 02:25 PM

More likely, by "stacking" I think Don means tightening the jumble. If you do this by adjusting the secondary mirror, the primary mirror axis will be affected.


To me this means you never really got it stacked, just close. Or else it means that there's some AC tool-reading limitation where they look stacked but really are not.

Mike


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