Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Common star collimation mistake

This topic has been archived. This means that you cannot reply to this topic.
239 replies to this topic

#126 jtsenghas

jtsenghas

    Skylab

  • *****
  • Posts: 4397
  • Joined: 14 Sep 2014

Posted 08 March 2015 - 08:08 PM

 

 

The Question:

 

Is there any way, using just the cross haired sight-tube and just the Cheshire eyepiece, to demonstrate that the doughnut was deliberately misplaced by L?

 

It does matter what F is (for example, a 2-inch f/10 will have an L of 5mm).

If the error is easily resolved with the sight tube crosshairs, the answer to your question is "yes".

 

 

Gosh that was super quick Vic!

 

I suggested that F does not matter because you could imagine this scenario with any Newtonian for which the two tools were appropriate, right?  Otherwise no need to pose it as I did.

 

So here's why I considered the answer to be "no".

 

The secondary mirror is correctly placed with FAE = 0 relative to the misplaced doughnut as indicated by the sight-tube with cross hairs.

 

Even if the Cheshire eyepiece shows that the misplaced doughnut is middle-for-diddle inside the Cheshire ring showing that PAE = 0, relative to the misplaced doughnut, how is it possible to deduct that the doughnut has been intentionally misplaced?

 

Hint:  I desperately need the answer to be "yes".

 

I  am perplexed by Vic's answer and do not see how it can be determined with these tools that the spot was deliberately mislocated. If the focuser axis is aimed directly at the target that is slightly off-center, the crosshairs will show the focuser axis aimed as per a perfect collimation. The primary axis would subsequently have its axis aimed off by half the error of the spot misplacement. How would that deliberate error in the primary target location be detected with just those tools?



#127 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 08 March 2015 - 08:09 PM

Vic said: ". . you might also consider a new CatsEye triangular center spot. They're available in retro-reflective white (maximum visibility for autocollimators). . "

 

   I believe retro-reflective collimation marks do not work well for Barlowed collimation. The light they reflect tends to obscure the shadow on the Barlow screen.

 

Perhaps I used the wrong prefix. The CatsEye reflective triangles must be omni-reflective (they can be illuminated from any angle outside the focuser axis and appear illuminated from the focuser axis).

They do work well for Barlowed collimation (which illuminates the spot from the focuser axis).

 

Does that sound correct (omni instead of retro)?


Edited by Vic Menard, 08 March 2015 - 08:34 PM.


#128 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 08 March 2015 - 08:21 PM

I  am perplexed by Vic's answer and do not see how it can be determined with these tools that the spot was deliberately mislocated. If the focuser axis is aimed directly at the target that is slightly off-center, the crosshairs will show the focuser axis aimed as per a perfect collimation. The primary axis would subsequently have its axis aimed off by half the error of the spot misplacement. How would that deliberate error in the primary target location be detected with just those tools?

 

The sight tube has two alignment references--the sight tube cross hairs and the bottom edge of the sight tube.

If the bottom edge of the sight tube is carefully aligned with the edge of the primary mirror, the cross hairs will indicate the center of the primary mirror. If the primary mirror center spot is offset enough to be resolved by the sight tube cross hairs, then the error will be detected. (If the offset is too small, the error won't be resolved even though it's real.) If the focuser is aligned to the misplaced primary center spot, it will have a focuser axial error equal to the center spot displacement error. If the focuser is aligned using the bottom edge of the sight tube, the FAE will be zeroed. If the sight tube (aligned to the primary mirror using the bottom edge of the sight tube instead of the crosshairs) is used to align the primary mirror (by aligning the reflection of the sight tube cross hairs to the actual sight tube cross hairs), the PAE can be reduced to the resolution of the tool.

 

The Cheshire doesn't easily "see" focuser axial errors, so it will indeed align the primary mirror incorrectly, inducing a PAE of 1/2 the center spot placement error assuming the Cheshire alignment is "perfect".



#129 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 08 March 2015 - 08:30 PM

So here's why I considered the answer to be "no".

The secondary mirror is correctly placed with FAE = 0 relative to the misplaced doughnut as indicated by the sight-tube with cross hairs.

 

If the primary mirror center spot is incorrectly placed, aligning the (secondary mirror) focuser axis to the spot will introduce an FAE equal to the misplacement.

 

 

Even if the Cheshire eyepiece shows that the misplaced doughnut is middle-for-diddle inside the Cheshire ring showing that PAE = 0, relative to the misplaced doughnut, how is it possible to deduct that the doughnut has been intentionally misplaced?

 

 

And as I stated in my earlier post, the Cheshire uses the primary mirror center spot and the bright Cheshire ring as the two references for alignment. It does not "see" the focuser axis, whether it's correct or not. It also doesn't see the incorrect center spot placement. For the Cheshire to deliver precise results, the primary mirror center spot must be correctly centered (to a usable tolerance) and the Cheshire ring must be centered relative to the tool axis (which is a proxy for the eyepiece axis).



#130 jtsenghas

jtsenghas

    Skylab

  • *****
  • Posts: 4397
  • Joined: 14 Sep 2014

Posted 08 March 2015 - 08:34 PM

Ahhh....I had taken it as a given that the tool didn't have the resolution to identify the small amount the spot was off center if the edge of the tool were used to center to the primary. I do understand the two references of the tool. I realize the FAE would be off by the displacement and the PAE by half of that if collimation were done that way.


Edited by jtsenghas, 08 March 2015 - 08:39 PM.


#131 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 08 March 2015 - 08:45 PM

Ahhh....I had taken it as a given that the tool didn't have the resolution to identify the small amount the spot was off center if the edge of the tool were used to center to the primary. I do understand the two references the tool.

 

That's why I stated focal ratio was important--longer focal ratio equals larger offset error (L).

Focal length will also be important for visual resolution (as well as the visual acuity for the person making the read).

 

Realistically, the placement error should not impact performance as long as any residual Cheshire alignment error is less than 1/2 of L.



#132 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7390
  • Joined: 21 Oct 2006

Posted 09 March 2015 - 02:59 AM

More results...

I have been working on improving my skills using Mike's star collimation technique. Below are the results of the last three experiments I completed tonight. Each row of the following corresponds to the "before" and "after" collimation photos. I did go all the way with the 3rd experiment in terms of initial miscollimation   :)

 

star_collimation_16.jpg

 

Here is what I did:

1- Collimated my scope to "perfection" using quality collimation tools.

2- Grossly miscollimated

3- Inserted my Paracorr and my 17mm EP (77X)

4- Defocused too much outward. Donut (secondary shadow) appeared mis-centered. Used typical star collimation technique to center the donut as much as I can. This was a "coarse" collimation. Note: If I defocused too much inward, the donut was somewhat centered as I have shown in the first post of this thread. Only outward defocus showed the off-center donut. Using a relatively low magnification EP made it easy to locate the star after every primary mirror iterative adjustment. 

5- At this point, I moved to fine tuning primary mirror collimation using Mike's technique. I inserted my 2-4mm TV EP. I started with the 4mm setting (330X). IMPORTANT: I found it extremely helpful to select a star that is bright enough to show the defocus bulge yet does not flare at focus (appears pin point). Avoid bright/flaring stars. At the end of this step, the bulging star is balanced/symmetrical/centered -- needs practice to judge. I used the fine focus knob of my Moonlite to test the "bulge" direction at both sides of focus. I defocused by a little while observing the outer outline of the bulge -- ignore the interior of the defocus star.

6- After completing step#5, I did the same but with the 2mm setting (660x)

 

Below are the final results but magnified (right column of that last attachment).

 

star_collimation_17.jpg

 

What does all this mean????

1- Mike's technique works and provides very good consistency

2- My quality collimation tools agree with star collimation

3- The placement of my Hotspot is accurate

4- Mechanical center of my primary mirror is the same as the optical center

 

Jason



#133 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7390
  • Joined: 21 Oct 2006

Posted 09 March 2015 - 03:11 AM

 

From what I've read so far, you still haven't actually tried Mike's technique (as noted by your step 2). Mike doesn't start by severely miscollimating the primary mirror. He suggests, "Go through your normal collimation procedure, without a coma corrector, paying particular attention to making the secondary mirror alignment as close to perfect as you can get it."

 

I believe Mike's normal collimation procedure would include a Cheshire-type alignment (Cheshire or Barlowed laser).

 

He then suggests adding the coma corrector to fine align the tool alignment using his technique.

 

 

 

My starting point has always been a well-collimated scope using my quality collimation tools. I grossly miscollimate the primary mirror then attempt to use star collimation to restore the primary mirror alignment,

 

My last post has more details and more results.

 

Jason



#134 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7390
  • Joined: 21 Oct 2006

Posted 09 March 2015 - 03:21 AM

To be clear, I am not advocating Mike's star collimation method as a replacement for quality collimation tools.

I am happy to have finally found a star collimation method (Mike's method) that works and provides a high level of consistency; however, it is time consuming. Back to the 3 experiments, each experiment took me 10 minutes to complete (from miscollimation to re-collimation). The 10 minutes average is accurate -- I timed myself. I am sure with more practice I should be able to reduce the time.

Star collimation validated my quality collimation tools. I will continue to use my quality collimation tools. Periodically, I will use Mike's method to validate/reaffirm the results  of my collimation tools.

Jason


Edited by Jason D, 09 March 2015 - 08:34 AM.


#135 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2227
  • Joined: 26 Jul 2004

Posted 09 March 2015 - 04:09 AM

The sight tube has two alignment references--the sight tube cross hairs and the bottom edge of the sight tube.


Here is a picture of my very first collimation tool ever - for a 6" f/5 reflector with no center marker - made from a vacuum cleaner tube (and padded with tape for a snug fit) with length/inner dia 5:1
To use it, I would pull it out enough to frame the secondary (piece of cardboard below) and center, then (remove the cardboard and) push in to frame the primary accurately.
The double "crosshairs" forms a well centered square about 1 mm. The peephole (not visible) was no more than 1 mm, to get a decent depth of field (I think commercial combo tools have too wide peepholes). I would illuminate the crosshairs, and then collimate the primary to center the small, distant reflection within the square itself.
For a first try, it worked well enough.

Nils Olof
PICT2313d.jpg

#136 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2227
  • Joined: 26 Jul 2004

Posted 09 March 2015 - 04:27 AM

How would that deliberate error in the primary target location be detected with just those tools?


Deliberately not answering the question as given, I recall a former thread about photographic checking the spot's placement.
My guess is you could take a picture of the primary (not too miscollimated) through the focuser (a centered peephole might help, but shouldn't be necessary if you center the camera reasonably). Counting pixels or measuring on the image should decide the spot's placement. If you find it well placed, no need to dismount the primary.

Nils Olof

#137 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22988
  • Joined: 22 Feb 2010

Posted 09 March 2015 - 05:09 AM

 

I am happy to have finally found a star collimation method (Mike's method) that works and provides high level of consistency

 

I haven't tried it but this sounds very cool for when collimating a scope with a uncoated primary.

 

I don't put donuts on them...

 

and I can sure use a technique that doesn't require any donuts, I'm on a diet. :grin:


Edited by Pinbout, 09 March 2015 - 09:54 AM.


#138 WyattDavis

WyattDavis

    Viking 1

  • *****
  • Posts: 970
  • Joined: 25 Dec 2013

Posted 09 March 2015 - 06:30 AM

 

If, as a beginner you were comfortable with the learning curve associated with an 8-inch f/6 (and maybe turned a screw or two), I don't think the alignment tolerances of a 10-inch f/4.7 should be a cause for anxiety. Dobsonians do require some mechanical, "hands on" adjustments, including balancing the OTA, adjusting the backlash, and cleaning the primary mirror. Each of us seems to have a comfort "limit" whether it's technical or physical (hauling that "cannon" around), although all we really want is to be able to see more cool stuff

 

 

Hi Vic. Thanks for your reply. At this point, I have selected the Catseye $300 anecdote to beginner collimation anxiety and am relatively worry free and focusing on seeing more cool stuff!   :waytogo:

 

It seems like the ongoing discussion is to whether or the price of admission could have been lower via a different approach on the tools. I do indeed have the $.10 collimation cap that came with the XT10, as well as the included instructions and a few other tools collected over time. On the next cloudy night, I will take some inspiration from you, Jason, et al, on this thread and try the cap along with a few other approaches mentioned and compare them with the results I get with the Catseye. (And, I will probably add a beer to the equation for good measure. I am confident in my tolerances regarding that variable. :) ) 



#139 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 09 March 2015 - 07:35 AM

The Notes:

 

1. We know that FAE is ordinarily measured relative to the true optical centre of the mirror but let's assume, for the purposes of this inquiry, that it is being measured relative to the misplaced doughnut.

2. It probably shouldn't matter, for this inquiry, what F is.

3. This inquiry does not allow for the position of the doughnut to be evaluated by any direct, photographic or star-test method.

 

The Question:

 

Is there any way, using just the cross haired sight-tube and just the Cheshire eyepiece, to demonstrate that the doughnut was deliberately misplaced by L?

 

OK, after a good night's sleep, I decided to revisit your experiment.

 

If you change condition 2 in your notes to F is 6 or less, I think I can comfortably answer your question with an unqualified "no". Or, if you change your question and omit the cross haired sight tube, I can also answer with an unqualified "no".

 

The problem scopes for your question as posted are the common 6-inch f/8 (L = 2.6mm, about 2-percent of the primary mirror diameter) and 4-1/4-inch f/10 Newtonians (L = 5mm, almost 5-percent of the primary mirror diameter). The 4-1/4-inch primary mirror is almost always left spherical--so there is no optical center other than whatever reference the user chooses. I also know of a 12.5-inch f/15 Newtonian at a college observatory in Fort Myers (L = 17mm!) that might be visually obvious (over 5-percent of the primary mirror diameter).

 

But let's say you allow star alignment to be used to detect the off-center primary mirror center spot (assuming L = the high magnification PAE tolerance). Will the error be visible with a "perfect" Cheshire alignment (actual PAE at the focal plane = 1/2 the PAE tolerance)? I feel pretty comfortable answering "no". How about with a Cheshire misalignment equal to 1/2 the PAE tolerance? Since the errors have direction, the possible outcomes are zero PAE to the full PAE tolerance. Again, I feel pretty comfortable answering "no". Even with the Cheshire misaligned to the full PAE tolerance, the outcomes vary between 1/2 the PAE tolerance to 1-1/2 times the PAE tolerance, with a good chance of achieving at least the high-magnification PAE tolerance ("lucky" collimation). 

 

So, even if you can "see" the primary mirror center spot misplacement with a sight tube, if its centering is kept within a reasonable tolerance, and the Cheshire alignment accuracy is similarly maintained, the primary mirror center spot placement error is unlikely to cause a significant (visible) impact on image performance. 

 

This consideration for center spot placement is equally important for primary mirrors that have an optical axis that is not coincident with the measured center of the mirror blank. Professional mirror makers like Mike Lockwood and Carl Zambuto have stated their finished parabolic mirrors are optically centered relative to the glass mirror blank diameter to within 0.01- or 0.02-inch. I assume most economy mirrors won't be quite that precise, although I've center spotted and collimated dozens (perhaps hundreds?) of economy mirrors with good (high magnification performance) results...



#140 howard929

howard929

    Member

  • *****
  • Posts: 4603
  • Joined: 02 Jan 2011

Posted 09 March 2015 - 08:03 AM

 

In keeping with the tone of this thread, i question that accuracy claim. That mask can be nothing but hit or miss as with any physical process involving hand and eye and parallax views. It may be quick and easy but for accuracy it would require repeatable results to a high tolerance and there, it's lacking. If "good enough" allowed a full 1mm error, it may not even be good enough. 

 

Oh, come on Howard! I've used CatsEye's templates to center spot dozens of mirrors and I've never left a 1mm error in the placement (although many of the spots I've replaced had 1mm plus errors). I routinely get 1/2mm accuracy without even trying--and here's my secret. I don't press hard on the center spot when getting it to adhere to the mirror surface until I've ensured the placement is good (pretty easy with a 1mm precision rule). If I messed up and left an error more than a 1/2mm (a 1mm difference in the read with the rule), I just remove the spot and give it another shot (with big mirrors I'll use a Sharpy marker to indicate the actual center in the center spot perforation). I have to do this adjustment about 1 out of every 2 or 3 times, and I'm sure it's because I just don't worry about it that much when I'm using the template. FTR, I never have to do a third run, and I always get the spot placed to an accuracy of 1/2mm or better.

 

FWIW, a 1mm placement error induces a 1/2mm error on an otherwise perfect Cheshire alignment. Since it's pretty easy to get the (calibrated) Cheshire alignment almost perfect (1/4mm isn't that hard), the final error could be as much as .75mm or as little as .25mm (because the errors are likely to be in different vectors). The high magnification tolerance (up to 50X per inch of aperture) at f/4.7 is about 1/2mm. The "moderate" magnification tolerance (up to about 20X per inch of aperture) in less than perfect seeing may be as much as twice that amount. Still, I recommend 1/2mm or less for the placement tolerance below f/5--you never know when the seeing will suddenly improve and you'll want to "crank it up"!

 

 

Lets talk about this Vic.

 

You go on and on about how exacting the collimation is with the tools you use to the point where perfection, if there is such a thing, is the ultimate goal. At the same time, you advocate the use of a mask that you admit is inaccurate and then accept that your perfect collimation is based on a know misplaced center spot and shrug at the mention of a photographic method which is clearly more accurate. For someone seeking accuracy as THE  goal, this makes no sense to me.

 

BTW - the spot on the f/5 reflector I have was placed at exactly .3mm from the center of the mirror with a photographic method on the first try. I was going for less then .5mm and just left it as more then good enough.

 

Was it fast? Not really. Was it easy? It was for me, I do this sort of thing all the time. Was it accurate?  As accurate as my shaky hand can be but at least I KNOW where the center of the mirror is becuase the photo shows me that.


Edited by howard929, 09 March 2015 - 08:25 AM.


#141 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7390
  • Joined: 21 Oct 2006

Posted 09 March 2015 - 08:43 AM

. I would illuminate the crosshairs,

 

How do you do that with the sight-tube inserted?

 

Jason

 

Edit: To clarify, I was referring to evenings? Do you just shine a red light down the OTA? If yes, wouldn't it be somewhat blinding?


Edited by Jason D, 09 March 2015 - 10:15 AM.


#142 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 09 March 2015 - 10:50 AM

Lets talk about this Vic.

 

You go on and on about how exacting the collimation is with the tools you use to the point where perfection, if there is such a thing, is the ultimate goal. At the same time, you advocate the use of a mask that you admit is inaccurate and then accept that your perfect collimation is based on a know misplaced center spot and shrug at the mention of a photographic method which is clearly more accurate. For someone seeking accuracy as THE  goal, this makes no sense to me.

 

BTW - the spot on the f/5 reflector I have was placed at exactly .3mm from the center of the mirror with a photographic method on the first try. I was going for less then .5mm and just left it as more then good enough.

 

Was it fast? Not really. Was it easy? It was for me, I do this sort of thing all the time. Was it accurate?  As accurate as my shaky hand can be but at least I KNOW where the center of the mirror is becuase the photo shows me that.

 

OK Howard, let's talk.

 

First, I do "go on and on" about the read accuracy of the various collimation tools. That's important to know when trying to achieve a defined tolerance (not "perfection"). I also advocate going beyond the defined tolerance when it's easy, because these critical alignments are subject to change when the scope is moved (gravitational flexure) and as the temperature changes (thermal coefficient variances). I don't tell the user he must achieve perfection, but if he expects a certain level of performance through a night of multi-target observing, he will need to maintain a prescribed tolerance for the type of observing he has planned.

 

Second, by mask I assume you're referring to the CatsEye acetate overlay. I'm not sure where you read that I admitted it's inaccurate--I'm sure I never said that. I've been pretty clear that with my "quick and easy" procedure, I've been able to achieve centering to about 0.5mm in a few minutes with one or two tries. I'm sure I could make it even better, but I usually stop there because the precision rule I use (as my second reference standard) is graduated in 1mm increments (the 0.5mm increments are too hard to read). I can easily read a 1mm differential (perhaps less) from one side to the other--a 0.5mm error. Since that measurement accuracy has worked well for all of the scopes I've worked on (down to about f/3.6), I think 0.5mm accuracy is fine given the collimation procedures I advocate. At 0.33mm (assuming the actual paraboloid is centered to the same threshold), I believe your center spot should be good enough to achieve excellent primary mirror alignment down to about f/2.75. I believe Jason was able to achieve 0.25mm accuracy with his center spot placement (also using digital imagery), and his star tests seem to confirm his centering is "good enough" (using Jason's meaning of "good enough"!)

 

FTR, I have no problem advocating the photographic method for those who are properly equipped and have a primary mirror focal length that doesn't require any special gymnastics or tall unsteady ladders. But for most of the scopes out there today, especially the economy Dobs, the CatsEye overlay is much more than "adequate" and consistently delivers the necessary accuracy with minimal fuss. If you prefer the photographic method--I would suggest that you use it--hey, it works.



#143 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2227
  • Joined: 26 Jul 2004

Posted 09 March 2015 - 10:56 AM

. I would illuminate the crosshairs,

 
How do you do that with the sight-tube inserted?
 
Jason
 
Edit: To clarify, I was referring to evenings? Do you just shine a red light down the OTA? If yes, wouldn't it be somewhat blinding?


This was at least 23 years ago <grin> but I recall using a flashlight obliquely over the edge of the telescope tube, directly illuminating the crosshairs (protruding just inside the focuser) but not visible direct or reflected - easier on the eyes than having to illuminate a spot with an AC.

Wow - what a thread you started! thanks

Nils Olof

#144 howard929

howard929

    Member

  • *****
  • Posts: 4603
  • Joined: 02 Jan 2011

Posted 09 March 2015 - 01:34 PM

 

Lets talk about this Vic.

 

You go on and on about how exacting the collimation is with the tools you use to the point where perfection, if there is such a thing, is the ultimate goal. At the same time, you advocate the use of a mask that you admit is inaccurate and then accept that your perfect collimation is based on a know misplaced center spot and shrug at the mention of a photographic method which is clearly more accurate. For someone seeking accuracy as THE  goal, this makes no sense to me.

 

BTW - the spot on the f/5 reflector I have was placed at exactly .3mm from the center of the mirror with a photographic method on the first try. I was going for less then .5mm and just left it as more then good enough.

 

Was it fast? Not really. Was it easy? It was for me, I do this sort of thing all the time. Was it accurate?  As accurate as my shaky hand can be but at least I KNOW where the center of the mirror is becuase the photo shows me that.

 

OK Howard, let's talk.

 

First, I do "go on and on" about the read accuracy of the various collimation tools. That's important to know when trying to achieve a defined tolerance (not "perfection"). I also advocate going beyond the defined tolerance when it's easy, because these critical alignments are subject to change when the scope is moved (gravitational flexure) and as the temperature changes (thermal coefficient variances). I don't tell the user he must achieve perfection, but if he expects a certain level of performance through a night of multi-target observing, he will need to maintain a prescribed tolerance for the type of observing he has planned.

 

Second, by mask I assume you're referring to the CatsEye acetate overlay. I'm not sure where you read that I admitted it's inaccurate--I'm sure I never said that. I've been pretty clear that with my "quick and easy" procedure, I've been able to achieve centering to about 0.5mm in a few minutes with one or two tries. I'm sure I could make it even better, but I usually stop there because the precision rule I use (as my second reference standard) is graduated in 1mm increments (the 0.5mm increments are too hard to read). I can easily read a 1mm differential (perhaps less) from one side to the other--a 0.5mm error. Since that measurement accuracy has worked well for all of the scopes I've worked on (down to about f/3.6), I think 0.5mm accuracy is fine given the collimation procedures I advocate. At 0.33mm (assuming the actual paraboloid is centered to the same threshold), I believe your center spot should be good enough to achieve excellent primary mirror alignment down to about f/2.75. I believe Jason was able to achieve 0.25mm accuracy with his center spot placement (also using digital imagery), and his star tests seem to confirm his centering is "good enough" (using Jason's meaning of "good enough"!)

 

FTR, I have no problem advocating the photographic method for those who are properly equipped and have a primary mirror focal length that doesn't require any special gymnastics or tall unsteady ladders. But for most of the scopes out there today, especially the economy Dobs, the CatsEye overlay is much more than "adequate" and consistently delivers the necessary accuracy with minimal fuss. If you prefer the photographic method--I would suggest that you use it--hey, it works.

 

 

Lets be clear. After using the Catseye mask you mentioned that you use a rule to center the spot on the mirror. If that isn't an admission that the mask is inaccurate, I don't know what is. On top of that, you still advocate the use of that mask to people who will be first time users, unlike yourself who has "a trick" and has done so as many times as you have. 

 

I bought one of those masks you advocate so hardily. After checking it with a caliper I tossed it where it belonged. In the garbage. That's an easy admission for me to make. Jim Fly, nice guy that I know he is isn't and will never be, a pal of mine so I have no need to defend his products. 


Edited by howard929, 09 March 2015 - 01:56 PM.


#145 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 09 March 2015 - 03:59 PM

Lets be clear. After using the Catseye mask you mentioned that you use a rule to center the spot on the mirror. If that isn't an admission that the mask is inaccurate, I don't know what is. On top of that, you still advocate the use of that mask to people who will be first time users, unlike yourself who has "a trick" and has done so as many times as you have. 

 

OK--let's be really clear. 

 

I get it that you don't like the CatsEye template, and Jim is not your "pal". Your experience with the template is different than mine (and many other users). Before I switched to the CatsEye template, I used to make my own. I've been center-spotting mirrors for about 30 years--but I've only been using the CatsEye templates for about 7 or 8 years. So before you condemn all CatsEye templates because you bought one that didn't pass your caliper test, I hope you'll understand that my "trick" is also a method to quantify the precision I routinely achieve numerically--something I expected you would ask.

 

It wasn't a trick to achieve higher precision--it is a quick and easy trick to let the template do the heavy lifting and then, if necessary, finish with a tweak. Including the mm rule clearly shows the precision (in mm) you can easily achieve with the CatsEye templates. I almost always use this procedure for apertures up to 12-inches, which covers the majority of the "economy" scopes I'm routinely asked to center spot for friends and acquaintances I meet at star parties. Over 12-inches of aperture, the mm rule is no longer helpful (it's only 6-inches long). I've considered a 12-inch precision rule, but instead I just use the CatsEye template "carefully". 

 

I can still accomplish the same precision by simply rotating my position around larger apertures and repeating the "gentle-stick" procedure, if necessary. With larger apertures, the "common" read problem is caused by failing to get directly above the edge alignments. I also rotate the template to get separate reads at 120-degree intervals (this pretty much eliminates any alignment errors). It takes more time, but it gets the job done. And when I can, I also find it's "easier" when you're center spotting an aperture over 20-inches to have two people watching the alignment from opposite sides--"easier"--not "necessary". I proved to myself some years ago that the template was capable of delivering center spot alignment precision less than 0.5mm using this rotational process by verifying the alignment afterwards with the precision rule on a few "smaller" apertures.

 

FTR, Jim's website used to show a 13-year old successfully center spotting a primary mirror (it may still be there but I couldn't find it). I endorse Jim's and Howie's products because they're both driven to deliver excellent collimation results for beginners and experts alike. If I didn't know the CatsEye templates work--I wouldn't be "heartily" advocating their use    :waytogo: .


Edited by Vic Menard, 09 March 2015 - 04:10 PM.


#146 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42903
  • Joined: 23 Jun 2003

Posted 09 March 2015 - 04:42 PM

Seeing that the template could possibly move to the side, and lacking evidence I was putting the center marker dead center as I was doing it, I decided to combine two methods and the results are that my center marker is centered to better than 0.25mm, using the Catseye acetate template.

Here's how:

1. I place the mirror on a piece of stiff paper (we used to call it construction paper) and draw a circle around the edge with a pencil.

2. I cut out the circle with scissors, fold it in half twice (creating a pie slice) and nip off the corner.  I open it and smooth it down.  There is now a tiny square hole dead center.

3. I place the paper on the mirror on the mirror and tape the edge to the side of the mirror at 4 places.

4. I use a new, very pointed, Sharpie pen through the center of the paper to place a dot on the mirror.  That dot is maybe 0.5 wide.

5. I remove the paper and use a compass with two points to measure the distance from the dot to the edge.  If I'm satisfied it's centered, I then proceed to the clear template.

6. I place the center marker on the template as per the instructions, and invert the template and lay it down on the mirror.  Since the template inevitably sags into the center of the mirror, as it "rests down", I look through the template to be certain the center marker is surrounding the dot, putting the dot in the very center (as well as I can see) of the marker.

At this point, only a tiny bit of the marker 'may' be contacting the mirror and i can usually scoot it sideways if needed by pulling the template. 

7. Once I am certain the dot is as centered as I can identify by eye, I press the template down with a pencil eraser, making certain the template does not slide sideways.  Since the pencil eraser has some friction with the template, you can even control the "landing" as the center marker touches down.

8. After one last check of proper placement, I use the same pencil to press the entire center marker down into contact with the mirror and slowly peal the acetate template off the mirror. This took 5 minutes to write and it takes 15 seconds to actually do.

 

When I checked my mirror after receiving it, the Catseye Hotspot center marker did not look perfectly aligned with the template when I just held it on the mirror.  It's somewhat difficult to guarantee (without taping it down) that you have the edge marks perfectly lined up with the mirror, so I did my paper/template/Sharpie mark to see how far off it was.

It was about 1mm off from dead center--its center was off center by that much.  Normally, I would have just let it go, but instead I peeled up the old marker and laid down a fresh one.  I cannot measure any off center at all right now: the acetate template shows the marker centered to its resolution, and the compass confirms it.  Whatever error exists is below the level of resolution of the template, the compass, or the center mark.  Several washings of the mirror have faded the dot to invisibility, but the mark hasn't budged.

 

Judging from my Sharpie dotting technique, I'd say that it is a pretty accurate way to find dead center.  I have done about 10 mirrors this way, and the compass has yet to find any resolved error in placement greater than, perhaps, 0.25mm [1/100 inch] (I use it from the edge of the glass blank to the center of the dot--I ignore the bevel, which could vary in width).  The compass leaves a tiny shiny point in the center of the black dot and it is to that point I refer when I check from multiple points around the edge..

But one thing seems clear to me, anyway, and that is that there doesn't seem to be any easier method to place a center marker on a mirror than the acetate template.

 

I only use the dot technique as a cross check when laying the spot on the mirror with the template.  If I did mirrors often or in large numbers, I might dispense with the dot and just use the template.


Edited by Starman1, 09 March 2015 - 04:46 PM.


#147 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7390
  • Joined: 21 Oct 2006

Posted 09 March 2015 - 05:55 PM

We have beaten the subject of Catseye template accuracy to death in multiple threads where Howard made the same claims. I am not sure derailing this thread to discuss the same subject will be of good value. I suggest to Howard to revive one of the old threads if he has interest in resurrecting the same discussion.

From my own perspective, I have been very happen with Catseye template. I have used it multiple times without issues. Below is my method of using it which I shared multiple times in older threads.

 

I place spacers with the proper height around the mirror -- covering top/down/right/left areas. I place the template on the top along with weights to hold it in place and keep it stretched. I make the necessary adjustment. I even used a handheld magnifier to fine tune placement. That is it!!!

 

4335320-center2.png

 

Unlike Howard's experience, I have nothing but praise for Jim Fly's customer service and products. I have the same praise for Howie Glatter's customer service and products. I love my Glatter's paralllizer.

 

Jason


Edited by Jason D, 09 March 2015 - 05:56 PM.


#148 Joe G

Joe G

    Surveyor 1

  • *****
  • Posts: 1608
  • Joined: 10 Jan 2007

Posted 09 March 2015 - 06:24 PM

I guess I have a question on the relevance of the autocollimator given the discussion in this thread.  Unfortunately this will be somewhat long.

 

I have "quality" tools including the Glatter laser with the 1mm aperture stop and barlow attachments.  I have a Catseye sight tube, the Blackcat cheshire, and the old, one pupil Catseye autocollimator.

 

Skipping the sight tube adjustments, my understanding for rough collimation, is that you align the laser to the center spot on the primary by using the knobs on the secondary mirror.  Then you adjust the primary mirror knobs until the return hotspot image is centered in the barlowed laser aperture white face, or the white cheshire ring.  For finer collimation you could then iterate the cheshire and autocollimator to stack the hotspot or triangle reflections keeping the hotspot centered in the cheshire.

 

I think if I understood Vic correctly, he often tweaks the secondary so that the return beam of the laser with the 1mm aperture stop coincides such that you see the laser line up and also see the diffraction rings on the face of the laser's white face.  I took this as an alternative to using the autocollimator in the middle of the night when one is too tired to get the red flashlight to tweak the secondary.

 

There has been skepticism expressed that star collimation is largely useless given good tools or is beyond the skill set that most of us have.  Jason gave some examples of trying to utilize star collimation and then check the results using the Blackcat cheshire.  His first night out indicated that star collimation was hard and his images illustrated that he got close to decent collimation, but certainly not perfect collimation.

 

Mike Lockwood then chimed in for Jason to follow his method as outlined on his website.  Mike argues that star collimation is not as difficult as it seems once learned and that it leads to better results.  He states that his method should be used after rough collimation with decent tools, and that he mostly uses a laser (assuming Glatter) and does not own a "quality" cheshire.  He also said that he tweaks the collimation with a coma corrector in place under a high power eyepiece by adjusting the primary mirror knobs.  Because of the various registration errors (paracorr tightened in the focuser, the eyepiece in the paracorr, the quality of the focuser and structure, etc.), I believe he is saying that final star collimation will give the best results essentially incorporating all the "issues" of the optical train into the final collimation.

 

In Mike's words, "Lasers have improved greatly (thank you Howie Glatter and others), but still there are errors introduced by imperfect tools, the fit of the tool in the focuser, and other small errors.  No tool is perfect, but some are better than others.  So, the only method that avoids all of the errors is final collimation using a star."

 

Jason then re-read Mike's treatise and tested his skills yet again.  I think his take was that he understood star collimation better and his results after comparing to the Blackcat cheshire were indeed better.  Further he indicated that his test verified that his hotspot was placed correctly and the optical center of his premium mirror was consistent with the mechanical center . He indicated that the tools (Blackcat cheshire) and Mike's method were consistent with each other.  But the tools are easier to use.  I don't believe Jason used a laser or autocollimator to check for end collimation in these tests, rather just the Blackcat cheshire.

 

Nils says, "I have said it before - the only meaningful purpose of star collimation that I can think of is to verify the placement of the primary's spot.  You might be the first (bold mine) to find that a spot carefully centered on the blank is not at the optical center."

 

Nils also asked a star tester whether he checks his star collimation afterwards with a quality cheshire for consistency. 

 

Don P says. "Jason, I could be wrong, but I don't think he (Mike Lockwood) uses Catseye tools, especially the autocollimator.  He did say he didn't have a good cheshire.  When I follow up the Glatter laser tools with a Catseye AC, I always find residual errors.  It seems that Mike may be using a star to do the same thing."

 

Nil's responds, "Don.  If instead you follow up just with the Blackcat, what do you find?  The limitation of the AC is that it is pretty useless for collimating the primary! It may be a bit better in showing the focuser axis collimation, but not really that great)."

 

So here we have very experienced folks not necessarily in agreement on some of the finer points.  Nils seems skeptical of using star collimation to collimate and seems to indicate that the Blackcat cheshire and the hotspot would do the job for the most part.

 

Don P is skeptical that most of us can accurately collimate using a star and that the tools are better/easier.  Further he fine tunes his collimation to eliminate residual errors with the autocollimator.

 

Mike Lockwood indicates that only collimation on a star will give perfect collimation because the tools/scope mechanics are not perfect which is not to say that quality tools are not desirable or that you don't want a Feathertouch focuser.  Collimating on a star with the end optical train as one would use it would at least adjust for the errors in the train (at least as I interpret it).

 

That would also imply to me that tweaking collimation with an autocollimator might be somewhat futile in the sense that once you take the AC out and put a Paracorr and eyepiece in the focuser, you have re-introduced errors that mess up your tweaking.  I know from personal experience using an AC that you just need to put mild pressure on any one component in the system and your stacks will become un-stacked.  I further assume this is why Nils is a little dubious of the value of the autocollimator versus just using the Blackcat cheshire.

 

Of course, Jason perfected the AC to its current state.  He was dubious that you could get accurate results by star collimation and is likely a little less dubious now having tried and tested Mike's methods.  But in practice Jason says quality tool based collimation is consistent with and easier than star based collimation.  Not sure at this point whether Jason thinks that the AC would add anything over star collimation although I would assume that he would say to use an AC if the seeing supports it.

 

Of course, the title of this thread is, "Common star collimation mistakes" for beginners who are attempting star collimation.

 

Personally I am not sure I want to go through the process of using and implementing star collimation.  I find the process of aligning the secondary with a laser simple to implement.  I prefer the cheshire to the barlowed laser as I can read and interpret it better (as I said I own the Glatter barlow attachment and the Tublug).  And I then tweak it with the laser and 1mm aperture stop.  But sometimes I am a little over obsessed with collimation and after reading this thread am wondering if the autocollimator offers much value, especially given what I have pointed out above.  And especially that the folks I have quoted are all gurus in the art of collimation.  (I apologize if I misrepresented someone's point of view.)

 

So is the AC necessary from a practical perspective?

 

Thanks.

 

Joe

 

 

 

 

 

 

 



#149 Steve Harris

Steve Harris

    Sputnik

  • -----
  • Posts: 39
  • Joined: 21 May 2010

Posted 09 March 2015 - 07:29 PM

One thing I recommend about the Catseye template, is if you have a mid-size scope in the "teens" range, to get the one for *large* mirrors (it comes in two sizes), then trim it to just a little over the size of your actual mirror.  That way, you can check the registration of the mirror vs. the template circle all the way around the mirror, and not just in two arcs on two sides of the mirror.  With the template used that way, I've found the template can lay spots with high accuracy.



#150 Vic Menard

Vic Menard

    Fly Me to the Moon

  • *****
  • Posts: 6895
  • Joined: 21 Jul 2004

Posted 09 March 2015 - 07:54 PM

In response to Joe G.:

 

"...is the AC necessary from a practical perspective?"

 

 

I think maybe you're seeing subtle disagreement where there is actually veiled admiration. Nils Olof Carlin--the brilliant theoretician, Mike Lockwood--the master mirror maker, Jason Khadder--the technology wizard, and Don Pensack, who just wants to find a way to distill all of the complexity of this hobby (including collimation)--that he has lived--into a palatable recipe that can be enjoyed by everyone else. Each is a giant in his area of expertise--and smart people know to listen when they have something to say.

 

Regarding the usefulness of the autocollimator, I guess it depends on how you use it. For me, the autocollimator is the ultimate quality control inspector. IME, I've found that when I'm sufficiently attentive to my axial alignment reads using my Glatter laser (and making precise corrections), I can achieve axial alignment that is ostensibly perfect in my autocollimator better than 50-percent of the time. This is important to me because my Glatter is my "after dark" alignment tool. To keep my skills at this level, I routinely use my autocollimator with my Glatter laser when there's daylight. So for me, my autocollimator is indispensable for my collimation needs.


Edited by Vic Menard, 09 March 2015 - 07:59 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics