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Question about CDP Collimation (Catseye)...

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#1 Dave Bush

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Posted 23 September 2020 - 03:31 PM

UP to now I've been using the back and forth method of aligning both primary and secondary mirrors using the Catseye XLT and Infinity XLK.  I've always gotten good results.

 

The CDP procedure looked interesting so I though't I'd give it a try.  It does seem a bit quicker and seems to have less fiddling.

 

Questions:

 

  • Every time I would transport my scope to a dark site, I would have to tweak the collimation.  If using the CDP method, it seems to imply that the first part, where the secondary is aligned, might be a one time only affair and that any adjustments would be done only at the primary.  Is this the case or would I possibly have to tweak the secondary as well, stacking the P image to the #3 image?   Or, would I ever have to start the whole procedure over, moving the P image to the edge by de-collimating the primary, etc.?
  • Other than possibly simplicity and time is there anything about the CDP procedure that produces better collimation?

 

Thanks

 



#2 sixela

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Posted 23 September 2020 - 06:18 PM

To that last question: no -- CDP just avoids having to switch between tools (at the expense of having to rely on a reflection that is both faint and fairly hard to focus on with all the other stuff going on).

 

To that first question: I don't understand what you imply. If you have both your mirrors misaligned then you don't have to decollimate the primary -- it's already decollimated (not carefully). You then tweak the secondary to stack the correct images (P and 3) and then tweaking the primary wil bring all the remaining images into the fold. The fact that you stacked P and 3 means that you can't get into the "false stacking" scenarios where the images are not truly stacked but one of them has been eaten up, so the final Cheshire sanity check should reveal you're done. And yes, in the CDP protocol both the tweaks to the secondary and those to the primary are "one time affairs" and you don't need to iterate -- that's the whole point! You still need to verify, of course ;-).

 

The "D" in CDP is only necessary if you can't see four images when you start, which happens fairly often if the collimation is close; "3" can hide under something else and be really hard to see. Note: if the images that you do see are unstacked but you don't know where 3 is, then I always choose a decollimation of the primary that wil move the images so they're no longer on a line. It's easier to know where 3 should be if P, 1 and 2 are NOT in a line (since the inverted 3 is offset from inverted 2 as the non-inverted P is offset from non-inverted 1, you know where to look for it). But I suppose that is personal.


Edited by sixela, 23 September 2020 - 06:19 PM.


#3 Vic Menard

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Posted 23 September 2020 - 06:32 PM

The CDP procedure also shows the focuser axial error magnified 2X.

 

I'm not sure what a CatsEye XLT is, but if the OP is talking about the CatsEye BlackCat Cheshire, the two tools used in tandem should deliver excellent axial alignment.



#4 Dave Bush

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Posted 23 September 2020 - 09:58 PM

The CDP procedure also shows the focuser axial error magnified 2X.

 

I'm not sure what a CatsEye XLT is, but if the OP is talking about the CatsEye BlackCat Cheshire, the two tools used in tandem should deliver excellent axial alignment.

I meant "BlackCat XL"



#5 Dave Bush

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Posted 23 September 2020 - 10:05 PM

To that last question: no -- CDP just avoids having to switch between tools (at the expense of having to rely on a reflection that is both faint and fairly hard to focus on with all the other stuff going on).

 

To that first question: I don't understand what you imply. If you have both your mirrors misaligned then you don't have to decollimate the primary -- it's already decollimated (not carefully). You then tweak the secondary to stack the correct images (P and 3) and then tweaking the primary wil bring all the remaining images into the fold. The fact that you stacked P and 3 means that you can't get into the "false stacking" scenarios where the images are not truly stacked but one of them has been eaten up, so the final Cheshire sanity check should reveal you're done. And yes, in the CDP protocol both the tweaks to the secondary and those to the primary are "one time affairs" and you don't need to iterate -- that's the whole point! You still need to verify, of course ;-).

 

The "D" in CDP is only necessary if you can't see four images when you start, which happens fairly often if the collimation is close; "3" can hide under something else and be really hard to see. Note: if the images that you do see are unstacked but you don't know where 3 is, then I always choose a decollimation of the primary that wil move the images so they're no longer on a line. It's easier to know where 3 should be if P, 1 and 2 are NOT in a line (since the inverted 3 is offset from inverted 2 as the non-inverted P is offset from non-inverted 1, you know where to look for it). But I suppose that is personal.

OK.  So let me ask this, if I get it all lined up as it is currently, and assuming I am quite gentle with it (as I take pains to be), any likely misalignment is likely to show itself how?  In other words, if it gets only slightly misaligned, I won't be able to see four images using the Infinity XLK so would it simply be a matter of adjusting the primary to get them all to stack?  It would take a more severe misalignment for all four images to be seen again correct?d

 

And, finally, isn't the #3 image always the faintest one?  That's how to identify it correct? 



#6 cdndob

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Posted 24 September 2020 - 12:24 AM

And, finally, isn't the #3 image always the faintest one?  That's how to identify it correct? 

You can verify 3 but adjusting the primary mirror a bit, it won't move relative P.



#7 sixela

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Posted 24 September 2020 - 02:40 AM

OK.  So let me ask this, if I get it all lined up as it is currently, and assuming I am quite gentle with it (as I take pains to be), any likely misalignment is likely to show itself how?  

Since you have two tools: the Cheshire will confirm that if you see only one image in the autocollimator, it's a true stack. You can't get one of the "false stacking" scenarios.

 

If you use the Cheshire, then any remaining misalignment that leads to unstacked images is likely to be in the pointing of the focuser axis.

 

 

 

In other words, if it gets only slightly misaligned, I won't be able to see four images using the Infinity XLK so would it simply be a matter of adjusting the primary to get them all to stack?

No -- what mirror you shoud adjust obviously depends on what error there is. There is only one solution for the tilt of the secondary and the tilt of the primary that avoids both FAE and PAE errors. If you make adjustments to the wrong mirror, then you won't get a reading with no PAE and no FAE, no matter what.

 

There are tell-tale differences. If there is only one source of error, then the remaining images, if you have three, are always on a line. If you have only PAE (which means that you should only tilt the primary), not only is 3 stacked under P, but the P-1 and P-2 distances are the same. If 2 is further from P than 1, then you know that there is FAE.
 

 

And, finally, isn't the #3 image always the faintest one?  That's how to identify it correct?

 

#3 is faint and inverted (and focuses somewhere else than 2, the other inverted reflection -- some people find it hard to focus it). It also does not move relative to P when you tweak the primary. If you have trouble identifying it, don't bother with CDP.

 

If you know there is an error, you can do two things:

-switch to the Cheshire and remove PAE by making the Cheshire reading correct by tilting the primary; then stack the images as best you can by tilting the secondary (which will reduce FAE), evaluate PAE again with the Cheshire, etc. That's the iterative protocol. The trick is to use each tool for a different mirror, so that you have a reliable protocol that wil converge to a solution without running around in circles (where with one tool you undo what you fixed with the other). It's slow but simple, and using two different tools ensures that you cannot converge to a false solution (although the offset pupil in the newer autocollimator can also help avoid these "false solution" scenarios).

 

-Use CDP: you can always null FAE by stacking P and 3 (which is the same as making sure that 2 and 1 are at exactly equidistant from P at opposite sides of it, on a line, but that is an indirect reading that is less precise.) Once you've done that, then tilting the primary to stack the images will null PAE without affecting the (now zero) FAE. So that converges to a solution in one go (of course I always check with a Cheshire and make sure no little tweaks are necessary).

 

The good thing is that you can converge to the solution using only one tool. The danger of using only one tool is that if you think you're done with the secondary but you're not (i.e. 3 is not under P and P, 1 and 2 are either not exactly on a line or P-1 and P-2 distances are not equal), by tilting the primary you can end up in a false solution where some of the reflections disappear --because their light paths cross a pupil where there is no mirror-- before they are truly stacked. But the Cheshire will reveal the remaining PAE, which is why the Cheshire is always used in the final sanity check.


Edited by sixela, 24 September 2020 - 02:45 AM.


#8 Dave Bush

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Posted 24 September 2020 - 04:51 PM

Since you have two tools: the Cheshire will confirm that if you see only one image in the autocollimator, it's a true stack. You can't get one of the "false stacking" scenarios.

 

If you use the Cheshire, then any remaining misalignment that leads to unstacked images is likely to be in the pointing of the focuser axis.

 

 

 

No -- what mirror you shoud adjust obviously depends on what error there is. There is only one solution for the tilt of the secondary and the tilt of the primary that avoids both FAE and PAE errors. If you make adjustments to the wrong mirror, then you won't get a reading with no PAE and no FAE, no matter what.

 

There are tell-tale differences. If there is only one source of error, then the remaining images, if you have three, are always on a line. If you have only PAE (which means that you should only tilt the primary), not only is 3 stacked under P, but the P-1 and P-2 distances are the same. If 2 is further from P than 1, then you know that there is FAE.
 

 

 

#3 is faint and inverted (and focuses somewhere else than 2, the other inverted reflection -- some people find it hard to focus it). It also does not move relative to P when you tweak the primary. If you have trouble identifying it, don't bother with CDP.

 

If you know there is an error, you can do two things:

-switch to the Cheshire and remove PAE by making the Cheshire reading correct by tilting the primary; then stack the images as best you can by tilting the secondary (which will reduce FAE), evaluate PAE again with the Cheshire, etc. That's the iterative protocol. The trick is to use each tool for a different mirror, so that you have a reliable protocol that wil converge to a solution without running around in circles (where with one tool you undo what you fixed with the other). It's slow but simple, and using two different tools ensures that you cannot converge to a false solution (although the offset pupil in the newer autocollimator can also help avoid these "false solution" scenarios).

 

-Use CDP: you can always null FAE by stacking P and 3 (which is the same as making sure that 2 and 1 are at exactly equidistant from P at opposite sides of it, on a line, but that is an indirect reading that is less precise.) Once you've done that, then tilting the primary to stack the images will null PAE without affecting the (now zero) FAE. So that converges to a solution in one go (of course I always check with a Cheshire and make sure no little tweaks are necessary).

 

The good thing is that you can converge to the solution using only one tool. The danger of using only one tool is that if you think you're done with the secondary but you're not (i.e. 3 is not under P and P, 1 and 2 are either not exactly on a line or P-1 and P-2 distances are not equal), by tilting the primary you can end up in a false solution where some of the reflections disappear --because their light paths cross a pupil where there is no mirror-- before they are truly stacked. But the Cheshire will reveal the remaining PAE, which is why the Cheshire is always used in the final sanity check.

First of all, I want to thank you for taking the time to explain all this.  Lots of good information for me to digest and clearly you know your stuff.

 

Before I re-read what you wrote and perhaps ask more detailed questions, is it essentially true that should the alignment be off after I transport the scope at worst I'd have to do the full CDP procedure over?  If so, that's still not a bad thing as it does seem to be quicker and perhaps more sure than the iterative process of adjusting both mirrors using both tools.  That process has in the past sucked up 10-15 minute of my time.



#9 sixela

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Posted 25 September 2020 - 05:09 AM

Yes, the CDP is all you want if you can see "3" for axial alignment. It's not particularly fast compared to other tools, though: with a Howie Glatter laser you can tweak the secondary from the front of the scope much faster, and then you'll see FAE is already nulled when you insert an autocollimator (which also has to be placed at the focal plane: if your reflections aren't all the same size then the tool needs to be racked in or out!)

 

That is, unless the focuser can't handle the weight of an HG laser collimator, in which case the tools disagree and thus reveal the mechanical weaknesses. The autocollimator is almost too sensitive in some settings: if it makes you point the focuser axis perfectly but then when you insert a TV Paracorr that focuser axis tilts, the extra precision has all been for nought.


Edited by sixela, 25 September 2020 - 05:09 AM.



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