Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Reflectors

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | >> (show all)
turtle86
Pooh-Bah Everywhere Else
*****

Reged: 10/09/06

Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3533387 - 01/01/10 11:57 AM

Great work! I've saved this as a favorite thread. Would love to see this in article/pdf format.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Concise thread about autocollimators+improvements new [Re: turtle86]
      #3533575 - 01/01/10 01:56 PM

Thank you everyone for the kind words.

This thread is a labor of

I do realize that there is so much material in this thread and it is inevitable that some of the presented information is unclear and would need more clarification. Please feel free to ask questions and to challenge some of the concepts presented. If you have questions but you are hesitant to post them because you think they are too basic to ask, be assured that many others will have the same questions in mind but are also hesitant to post them for the same reason. Just post your questions no matter how basic they are.

Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
Tim L
Carpal Tunnel
*****

Reged: 12/17/08

Loc: Missouri
Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3533965 - 01/01/10 05:56 PM

Stickey! Stickey!

Post Extras: Print Post   Remind Me!   Notify Moderator  
Don W
demi-god
*****

Reged: 05/19/03

Loc: Wisconsin, USA
Re: Concise thread about autocollimators+improvements new [Re: Tim L]
      #3533998 - 01/01/10 06:18 PM

I will put this in with the Best of Reflectors list.

Post Extras: Print Post   Remind Me!   Notify Moderator  
backwoody
Pooh-Bah


Reged: 01/08/07

Loc: Idaho USA
Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3534327 - 01/01/10 10:45 PM

Jason, I've got some work ahead of me to understand all the detail of your explanation. Thanks for the lessons from a guru...

c/s,


Post Extras: Print Post   Remind Me!   Notify Moderator  
Nils Olof Carlin
Carpal Tunnel
*****

Reged: 07/26/04

Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3534575 - 01/02/10 06:38 AM

Kudos to Jason for not only identifying the potential, previously unknown (or at least ignored) problems with the AC:

* the vanishing reflections (that can be seen in the offset pupil),
* the defocus problems (solved by the CAM),
* the consequences of having the AC mirror away from the focal plane

but also solving them!

Nils Olof


Post Extras: Print Post   Remind Me!   Notify Moderator  
hudson_yak
Carpal Tunnel


Reged: 11/15/07

Loc: Hyde Park, NY, USA
Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3534714 - 01/02/10 09:33 AM

The pictures and explanations as this topic progresses are truly excellent, Jason. Nice work.

Then we get to the table (what many readers are going to take away from this), and I have to wonder about a few things there:

1) Minor quibble, the cheshire in a combo tool can also be calibrated to a primary mirror mark, so could also earn an Excellent.

2) Was interested to see that there are no Excellent FAE tools(?)

3) Was interested to see that the CAM AC did not get promoted into the PAE row at all, since I thought its intent, one of them, anyway, was to make a cheshire completely redundant.

4) There ought to be some reemphasis that PAE and FAE are really the only two collimation axes. CAE and LAE are interesting to help talk about AC theory only.

Mike


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3535104 - 01/02/10 01:15 PM

Hi Nils Olof,
My work is based on the strong foundation laid out by your mathematical analysis of the autocollimator
Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Concise thread about autocollimators+improvements new [Re: hudson_yak]
      #3535203 - 01/02/10 02:12 PM

Quote:

The pictures and explanations as this topic progresses are truly excellent, Jason. Nice work.




Thank you, Mike

Quote:

Then we get to the table (what many readers are going to take away from this), and I have to wonder about a few things there:




For the record, I did include a disclaimer explaining that the table is subjective and based on my own opinion.

Quote:


1) Minor quibble, the cheshire in a combo tool can also be calibrated to a primary mirror mark, so could also earn an Excellent.



My grading is relative. A “Good” rating should not be construed as a bad rating just because there is an “Excellent” rating. Using a cheshire like Catseye’s with a triangle and a ring does add more reading accuracy than the sight-tube/combo tool with the distracting cross-hairs. I am speaking from experience since I have used both tools extensively.

Quote:

2) Was interested to see that there are no Excellent FAE tools(?)



Correct. Getting the “Excellent” rating has high standards. It should involve visual cues that are parallax-free, sharp, and highly-readable. Stacking P+1 does not meet this criteria. A laser collimator does not either.
Let me put it this way, a “Good” rating provides accuracy to within ~0.5mm. An “Excellent” rating provides accuracy to within ~0.2mm.

Quote:

3) Was interested to see that the CAM AC did not get promoted into the PAE row at all, since I thought its intent, one of them, anyway, was to make a cheshire completely redundant.



The table is meant to rate “direct” and “explicit” alignments – not “indirect” alignments. The CAM AC does not get a “rating” for PAE because it is impractical to “directly” align the pupil reflection against reflection “P”. HOWEVER, as long as a tool gets “direct” excellent rating for two error elimination, it implies “indirectly” the remaining two error elimination get the same rating.
OK, Mike I'll grant you that table could be confusing. I need to think about how to update that post later today.

Quote:

4) There ought to be some reemphasis that PAE and FAE are really the only two collimation axes. CAE and LAE are interesting to help talk about AC theory only.



I disagree with this line of thinking. PAE, FAE, LAE, and CAE are all means to an end which is “Axial Alignment”. I will follow whichever path that gets me to my axial alignment destination with the greatest accuracy whether it is PAE, FAE, LAE, CAE or even XAE -- whatever that is

Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
CatseyeMan
Vendor (Cats Eye Collimation)
*****

Reged: 12/16/04

Loc: Madison, AL USA
Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535247 - 01/02/10 02:43 PM

Quote:

Quote:

2) Was interested to see that there are no Excellent FAE tools(?)



Correct. Getting the “Excellent” rating has high standards. It should involve visual cues that are parallax-free, sharp, and highly-readable. Stacking P+1 does not meet this criteria....

Jason




What about stacking P & 3 in the CDP?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Shawn H
Postmaster
*****

Reged: 05/16/07

Loc: Equatorial Guinea, West Africa
Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535266 - 01/02/10 02:55 PM

Jason
Incredible drawings again! You really know how to make a blind man see!


Post Extras: Print Post   Remind Me!   Notify Moderator  
Nils Olof Carlin
Carpal Tunnel
*****

Reged: 07/26/04

Re: Concise thread about autocollimators+improvements new [Re: hudson_yak]
      #3535285 - 01/02/10 03:08 PM

Since you invite comments, Jason, in #3533575, here are some on account of Mike's comments:
Quote:

There ought to be some reemphasis that PAE and FAE are really the only two collimation axes. CAE and LAE are interesting to help talk about AC theory only.




Indeed true - IIRC CAE=2*PAE+FAE and LAE=2*PAE+2*FAE (vector sums), and thus, the CAE and LAE should perhaps not be seen as "fundamental" in the sense of being independent of the others (you could, in principle, derive PAE and FAE from CAE and LAE, but this would be pointless).

It could also be made clearer that the PAE and FAE are indeed independent of each other - you may have accurate collimation of one axis and still have some miscollimation of the other (even if that's what you try to avoid). It is only the PAE that causes coma in the center of the FOV, the FAE will cause a tilt of the focal planes but no error in the center of the FOV.

Nils Olof


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Concise thread about autocollimators+improvements new [Re: CatseyeMan]
      #3535335 - 01/02/10 03:38 PM

Quote:

What about stacking P & 3 in the CDP?




It depends on the focal length of the scope. For my 1200mm scope, it is hard to see reflection 3 primarily because of its fuzziness.

The left photos are what I see and the right ones is with the camera focused at infinity.



Those with longer focal lengths scopes, reflection 3 will be clearer.

For a reference to others, the following posts explain the relationship between focal length and reflections 1&3 fuzziness:
post1
post2
post3

But the trend seems to be moving towards scopes with shorter focal lengths (< F3.0 scope). Not to mention that imaging scopes tend to have a short focal length – the very scopes that require more accurate focuser axial alignment.

Jason

Edited by Jason D (01/03/10 04:58 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Nils Olof Carlin
Carpal Tunnel
*****

Reged: 07/26/04

Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #3535338 - 01/02/10 03:39 PM

One interesting thing, very well illustrated in Jason's post #3532773 (and perhaps easier seen in photos than directly by eye), see the lower set of images:

The reflections occur at different apparent distances from the pupil.
P and 2 are seen one (mirror) focal length away, the AC and CAM reflections at infinity, and reflections 1 and 3 at minus one focal length (that is, behind your head).

When the camera is focused on P and 2, 1 and 3 are obviously fuzzy. On the CAM images, the camera is focused at infinity and both pairs are equally fuzzy (while the CAM rings are sharp), but only half as much as in the previous case. If the camera lens could be moved inwards from the infinity setting, it would be possible to get 1 and 3 sharp - but "normal" cameras do not allow it (a weak negative add-on lens might do the job).

(The thought of seeing images occuring behind your head may be strange - but those images are virtual, and it wasn't before I realized that they are indeed behind the observer's head that I could even begin to make sense of all the reflections.)

Nils Olof


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3535379 - 01/02/10 04:06 PM

Just check the post submitted 1 minute before your post. The photo and included links explain the unintuitive concept of seeing an image behind our heads. Of course, you were the first to have explained it to me, Nils Olof. When the camera focuses at infinity then P&2 images are formed behind the retina and 1&3 images are formed in front of the retina by about the same distance – that is why both sets of images would look similar in fuzziness. For scopes with longer lengths, the distance between the four images and the retina is reduced which brings them all closer to focus (retina).
But the eye has the tendency to focus on P&2 on the expense of 1&3 since the eye is designed to see images in front of the eye – not behind.


Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #3535404 - 01/02/10 04:20 PM

Quote:

Indeed true - IIRC CAE=2*PAE+FAE and LAE=2*PAE+2*FAE (vector sums), and thus, the CAE and LAE should perhaps not be seen as "fundamental" in the sense of being independent of the others (you could, in principle, derive PAE and FAE from CAE and LAE, but this would be pointless).

It could also be made clearer that the PAE and FAE are indeed independent of each other - you may have accurate collimation of one axis and still have some miscollimation of the other (even if that's what you try to avoid). It is only the PAE that causes coma in the center of the FOV, the FAE will cause a tilt of the focal planes but no error in the center of the FOV.




I covered the PAE/FAE relevant importance in the 3rd post of this thread.

What helped me to think out-of-the-box is thinking of axial alignment as what it takes to coincide two lines – the focuser axial line with the primary axial line. I was not thinking in terms of PAE and FAE – that is how CAE and LAE got more attention from me. Since the objective of this thread is to attain the most possible accurate axial alignment, there is no distinction between the importance of eliminating one residual error over another. The objective is to eliminate all errors: PAE, FAE, LAE, and CAE. Of course, eliminating two is enough to eliminate all four.

Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
CatseyeMan
Vendor (Cats Eye Collimation)
*****

Reged: 12/16/04

Loc: Madison, AL USA
Re: Concise thread about autocollimators+improvements [Re: Jason D]
      #3535412 - 01/02/10 04:23 PM

Quote:

Quote:

What about stacking P & 3 in the CDP?




It depends on the focal length of the scope. For my 1200mm scope, it is hard to see reflection 3 primarily because of its fuzziness.

The left photo is what I see and the right one is with the camera focused at infinity.


Those with longer focal lengths scopes, reflection 3 will be clearer.

For a reference to others, the following posts explain the relationship between focal length and reflections 1&3 fuzziness:
post1
post2
post3

But the trend seems to be moving towards scopes with shorter focal lengths (< F3.0 scope). Not to mention that imaging scopes tend to have a short focal length – the very scopes that require more accurate focuser axial alignment.

Jason




... so the question is: What other tool/methodology is out there that is "better" than stacking P & 3 in the AC for insuring no significant FAE?

I'm curious as to how the "diffraction ring" effect (using the Glatter 1 mm aperture option) of the return beam around the Barlow Screen exit pupil stacks up on accuracy? (Vic has mentioned this phenomenon)?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: Concise thread about autocollimators+improvements [Re: CatseyeMan]
      #3535431 - 01/02/10 04:35 PM

Jim,
FAE, PAE, LAE, and CAE are not independent. Eliminating two out of the four with great accuracy also implies that remaining two are eliminated with the same level of high accuracy. This is a mathematical fact.
Therefore, eliminating LAE with P+2 stacking and CAE with the CAM with great accuracy implies both PAE and FAE are also eliminated with the same great accuracy.
Having said the above, it is highly recommended to start off the steps with CDP. If reflection 3 is clear enough to eliminate all or most of FAE then the rest of the collimation steps will coverage quicker. If reflection 3 is not clear enough to accurately execute CDP then the same accurate axial alignment can still be achieved but it might require and additional iteration or two.
I can't comment about the diffraction ring affect of the laser collimator because I have never tried it.
Jason


Post Extras: Print Post   Remind Me!   Notify Moderator  
CatseyeMan
Vendor (Cats Eye Collimation)
*****

Reged: 12/16/04

Loc: Madison, AL USA
Re: Concise thread about autocollimators+improvements [Re: Jason D]
      #3535469 - 01/02/10 05:01 PM

Quote:

.... Therefore, eliminating LAE with P+2 stacking and CAE with the CAM with great accuracy implies both PAE and FAE are also eliminated with the same great accuracy.




Sounds like the practical CAM option needs to happen sooner than later stay tuned....


Post Extras: Print Post   Remind Me!   Notify Moderator  
hudson_yak
Carpal Tunnel


Reged: 11/15/07

Loc: Hyde Park, NY, USA
Re: Concise thread about autocollimators+improvements [Re: Jason D]
      #3535550 - 01/02/10 05:40 PM

Quote:

Using a cheshire like Catseye’s with a triangle and a ring does add more reading accuracy than the sight-tube/combo tool with the distracting cross-hairs. I am speaking from experience since I have used both tools extensively.




I agree the xhairs do get in the way, in fact I took mine out for that reason, so mine isn't quite as combo as it used to be. Regardless, it is possible to make a mirror mark that matches up well with the cheshire in a combo tool. For example I made a 8mm diameter circle mark which made an easy eccentricity read within the 9.5mm cheshire dark circle of the combo tool.

Just a general comment on the table, it's a good summary of the foregoing posts in the thread, in the context of your intent to discuss AC theory. I was mostly thinking about a relative novice dropping in here, skipping over all that stuff, seeing the table and thinking ah, here's what I need to buy, and perhaps getting a bit mislead as to how best to start. The disclaimer that the table is just your opinion doesn't really help in that respect.

One way to avoid this would have been to not include the table at all.

Mike


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | >> (show all)


Extra information
20 registered and 18 anonymous users are browsing this forum.

Moderator:  ausastronomer, Phillip Creed, JayinUT, okieav8r 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 45628

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics