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collimation: reflection concentricity lost when aligning the secondary to the primary

collimation reflector beginner
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#26 Vic Menard

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Posted 25 February 2020 - 04:32 PM

Asbytec, on 25 Feb 2020 - 1:48 PM, said:
Finish collimation step one, secondary placement (and rotation), before starting step two, axial alignment. How simple is that?

 

I think I will remember that. A gem.

 

Thanks!

Except when you're close--then when you clean up your focuser axial alignment (tilt), you reassess your secondary mirror placement, and make any corrections (rotation or fore and aft), then clean up your focuser axis (tilt), then reassess...  Keep tilt for the focuser axis (cross hairs or outgoing laser dot) and rotation and/or fore and aft for secondary mirror placement (concentric circles) and each iteration will progressively reduce both errors, until both are correct. 

 

It's worth getting it right so you can evaluate your illumination profile with confidence.


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#27 pez_espada

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Posted 25 February 2020 - 05:49 PM

Asbytec, on 25 Feb 2020 - 1:48 PM, said:
Finish collimation step one, secondary placement (and rotation), before starting step two, axial alignment. How simple is that?

 

Except when you're close--then when you clean up your focuser axial alignment (tilt), you reassess your secondary mirror placement, and make any corrections (rotation or fore and aft), then clean up your focuser axis (tilt), then reassess...  Keep tilt for the focuser axis (cross hairs or outgoing laser dot) and rotation and/or fore and aft for secondary mirror placement (concentric circles) and each iteration will progressively reduce both errors, until both are correct. 

 

It's worth getting it right so you can evaluate your illumination profile with confidence.

I appreciate it Vic, I will try this as soon as I can tomorrow. You guys are great, wonderful community.



#28 Asbytec

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Posted 25 February 2020 - 09:10 PM

If I may, as Vic is explaining, when the cross hair is on the primary center the focuser axis is collimated. The primary reflection, then, must be centered under the focuser. And because it is, it can be used as a reference to center your secondary, too. So, when you focus out to the apex where the secondary and primary reflection appear the same size, you can easily see any residual tilt and rotation "error" with secondary placement.

As Vic goes onto explain (I repeat it only cuz it's beautiful thing) to use fore and aft movement and rotation (only) to center the secondary. It also sounds impossible because the secondary likely appears to need some tilt toward focuser center. But we can avoid the temptation to tilt by shimming the focuser (hopefully) slightly or minimal tension on one spider vane.

The idea is not to take the trouble to square the focuser to the tube's mechanical axis, rather to "square" the focuser to wherever the secondary is placed on or near the tube's mechanical longitudinal axis. In other words, the secondary is centered in the tube to some degree of accuracy, and it doesn't have to be perfect. The secondary is well centered in the tube in it's natural and neutral location, then we chase it's natural position by using a focuser shim if necessary.

When we do so, we reserve secondary tilt (only) for axial alignment. This also sounds impossible because when we tilt the secondary we induce some "unwanted" secondary tilt away from focuser center and induce some rotation error ending with collimation that looks like your pic above and my own pics. The cause is using the two secondary adjustment screws orthoganal to the focuser axis independently instead of in unison. They push and rotate the secondary away from where we placed it.

The key to avoiding those induced errors is to use only one secondary adjustment screw in line with the focuser axis to tilt the secondary so the cross hair will move onto the primary center in a straight line along the secondary major axis (parallel to the focuser axis). That sounds impossible, too, but it's not impossible when the primary center is also on the focuser axis (and secondary major axis) because we rotated the secondary to put it there.

Because we set up everything close to the focuser axis using the secondary major axis as a reference, we require only tilt along the focuser axis (and secondary major axis) to achieve focuser axial alignment. To allow that tilt, the other two screws orthoganal to the focuser axis are turned in unison in the same direction and by the same amount. They work together to allow the secondary to tilt instead of separately and opposing each other trying to tilt it.

This "teamwork" approach greatly reduces unwanted (and unneeded) tilt and rotation error of our carefully placed secondary induced during axial collimation using single direction tilt only along the focuser axis. If you miss the primary center when moving the cross hair along the secondary major axis (and focuser axis), back off and refine secondary rotation until you hit the primary center. Then evaluate your secondary placement at the apex relative to the *collimated* primary reflection. A few itterations will work out the residual secondary placement errors using the *collimated* primary reflection as a reference for secondary placement.

But this works only when the cross hair is on the primary center so the primary reflection is also centered under the focuser. This is one of the collimation signatures (focuser draw tube and primary edge) were trying to achieve. The other (secondary edge) is what we're trying to fix.

You can certainly achieve focuser axial collimation by using the three secondary adjustment screws independently (and intuitively like we do the primary axial tilt) by moving the cross hair along three random directions toward the primary center. But you have to compensate for induced rotation error and accept the secondary will likely be off center under the focuser. It's difficult, though, to hold those beautiful collimation signatures Vic describes. Edit: In a perfect world, that is, and truth be told, we can use some amount of tilt to help us position the secondary without much harm done.

The above method is less intuitive, but so easy to understand when it clicks. Even a noob like me can learn it (getting a piggy back ride on the backs of giants on CN) and it actually becomes intuitive. We often hear folks can collimate their scopes in little time, and that sounds impossible for a noob struggling for weeks to learn collimation. But it's not impossible to check and adjust collimation in 5 minutes when all we need to do is check secondary rotation, realign the cross hair to the primary center, then touch up the primary axis. That's it, done. It all starts with finishing the first step of collimation, secondary placement, refining secondary rotation before we begin tilt alignment.

Sorry for being verbose, but collimation can be fun. It really can...even if it sounds impossible. :lol:

Edited by Asbytec, 26 February 2020 - 02:00 AM.

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#29 Vic Menard

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Posted 26 February 2020 - 10:20 AM

...The primary reflection, then, must be (IS) centered under the focuser. And because it is, it can be used as a reference to center your secondary, too. So, when you focus out to the apex where the secondary and primary reflection appear the same size, you can easily see any residual tilt and rotation "error" with secondary placement.

Well, since you just corrected the tilt error, any residual secondary mirror placement error is assumed to be rotation and/or fore and aft (offset)--not tilt. 

 

As Vic goes onto explain (I repeat it only cuz it's beautiful thing) to use fore and aft movement and rotation (only) to center the secondary. ( waytogo.gif )  It also sounds impossible because the secondary likely appears to need some tilt toward focuser center. But we can avoid the temptation to tilt by shimming the focuser (hopefully) slightly or minimal tension on one spider vane.

I don't generally move on to shimming (or if the focuser has them, using the focuser "leveling" screws), until I've reached the point of diminishing returns where reiterating focuser axis (tilt) and secondary mirror placement (rotation and/or offset) will not reduce both errors. This usually happens when the focuser axis is pointing above or below the OTA axis, instead of somewhere along the OTA axis--which means either the secondary mirror needs to be moved or the focuser axis needs to be moved. And once the spider vanes have been correctly adjusted for centering and tension, I don't suggest adjusting the spider vanes to move the secondary mirror (unless there's a way to make the move without impacting the parallelism of the opposing vanes, or if the spider is not a 4-vane spider).

 

But you are correct, while it often seems like the secondary mirror just needs a little tilt adjustment to make the secondary mirror placement correction, because the tilt adjustment was just used to align the focuser axis (cross hairs aligned to the primary mirror center marker/donut), using tilt again to make the secondary mirror placement correction becomes a circular process with no resolution. Secondary mirror tilt for focuser axis, secondary mirror rotation and/or fore and aft for secondary mirror placement, and focuser leveling when reiteration will not resolve both errors.

 

...You can certainly achieve focuser axial collimation by using the three secondary adjustment screws independently (and intuitively like we do the primary axial tilt) by moving the cross hair along three random directions toward the primary center. But you have to compensate for induced rotation error and accept the secondary will likely be off center under the focuser. It's difficult, though, to hold those beautiful collimation signatures Vic describes. Edit: In a perfect world, that is, and truth be told, we can use some amount of tilt to help us position the secondary without much harm done.

I don't mean to rain on your parade, but you're not the first person to think that secondary mirror tilt would be better if the secondary mirror had only one adjustment axis. I've seen secondary mirrors that had two screws locked out with the third screw (on the long axis) spring loaded--one screw adjustment. Rick Singmaster used four screw secondary mirror holders and cautioned users that the two screws that weren't on the long axis should be considered "forbidden". And I'm OK with that, if the user has a good leveling base on his focuser (precise shimming is not and easy task). But without a leveling focuser, most users will need all three tilt adjustment screws, because the build precision in most commercially available "economy" Dobsonians is just not that precise. The front end geometry of a basic Newtonian (the focuser axis, spider centering and squaring, secondary mirror mount/stalk centering, tilt, and skew, and secondary mirror position/offset (or lack of offset), even the secondary mirror dimensions) is subject to a host of potential precision fails that make those other tilt adjustment screws useful.

 

IF all of the potential front end geometries are correct to significant precision, then the secondary mirror tilt adjustment will end up on the long axis and there's no need to allow for other tilt adjustments. BUT, since there are specific alignment signatures (cues) that clearly indicate whether the secondary mirror placement error is rotation and/or offset, the user need not be overly concerned with his scope's geometry precision when the observed errors can be corrected in spite of the geometry issues--although there are still other geometry errors that may ultimately require a part replacement to resolve a significant collimation error (a skewed eyepiece receptacle in the focuser drawtube come to mind)--but you cross that bridge after you've tried the other solutions. 

 

Thankfully, for most Newtonian users (those who aren't using their Newtonian for photometric studies or variable star work), secondary mirror placement is not a critical collimation concern. Close enough might even be considered a personal preference! The axial alignments have actual tolerances, but even those are pretty easy to achieve and maintain. Considering pez_espada's collimation requirements and his latest alignment image, I think he'll soon have the answers he needs.


Edited by Vic Menard, 26 February 2020 - 10:26 AM.


#30 Asbytec

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Posted 26 February 2020 - 06:22 PM

Well, since you just corrected the tilt error, any residual secondary mirror placement error is assumed to be rotation and/or fore and aft (offset)--not tilt.

 

That's a great point. Makes total sense. Tilt puts the cross hair on the center mark, it has to be (or assumed to be) right. That's what focuser axial alignment is. The rest is secondary placement. 

 

This usually happens when the focuser axis is pointing above or below the OTA axis, instead of somewhere along the OTA axis--which means either the secondary mirror needs to be moved or the focuser axis needs to be moved.

 

Yep. And you're correct about keeping the spider vanes straight as possible, which means not to really use tension. Maybe a little bit, but I am hesitant to recommend it, too, for that reason. 

 

It dawned on me one frustrating day as I was tilting the secondary laterally down toward the "bottom" of the OTA to bring the cross hairs downward onto the center mark, I was loosing the centered placement of the secondary. So, I heard you can use spider vane tension to pull it back to center. But, the tension required, after continuing to tilt the secondary, became so excessive I ran out of spider vane travel and the collimation signatures were just not there. At that point, I knew the spider vanes could not be kept parallel. And there should be no reason to use that much tension on the spider vanes. Something was obviously not right, so I decided to reset the spider and try again. And again. This is where better secondary rotation came into it's own. Not just relying on the face of the secondary looking circular under the focuser when using a colored mask to block reflections and show only the face of the secondary. Turns out, it may look good, but it's not necessarily that good. We can actually use the reflection of the primary instead of masking it. We 'need' to see the primary reflection. 

 

I don't mean to rain on your parade, but you're not the first person to think that secondary mirror tilt would be better if the secondary mirror had only one adjustment axis.

 

Ah, darn it. No, of course not, no rain on my parade. Just sharing the epiphany...the epiphany of a noob is my parade. lol.gif

 

With Don's help suggesting to toss the colored mask and use the primary reflection as a reference, it became immediately apparent why and how to ensure better secondary rotation. I absolutely know I am not the first to think of single axis alignment, but I am not sure how common that knowledge is among noobs and what significance it - single axis collimation - has on making life so much simpler. If my noob experience is any indicator, I think the tendency is to begin tilt collimation right away (after removing any colored mask) thinking the secondary placement step is done and assuming the secondary is well rotated because it looks like it. This is where many of us start to get into trouble almost immediately. The natural tendency is to look down the focuser and see the primary center at any random point and try to tilt the secondary to put the cross hairs on it. If we do not compensate for induced rotation, our signatures fall apart and we feel bad about it even though we are collimated. Hence the OP posting his picture above.  smile.gif

 

It became so easy to do the single directional focuser axial alignment along the secondary major axis, that I'd hope so many of us who post pictures like the OP (and me) could see the light (pun intended). Use a colored mask to make the secondary appear circular, then remove it and use the primary reflection as a reference to make sure rotation is, indeed, accurate. You almost cannot avoid being in a great starting position to begin single direction focuser axis collimation (with reiterations and refinements) until you hit the primary center within tolerance or better. And our collimation signatures hold nicely, too, so we feel good about it instead of frustrated. Dare I say different tools, then, tend to agree with each other, as well. And, as a bonus, normally we finish with the cross hairs moving onto the center mark adding some tension to the one inline secondary adjustment screw. This final bit of tension helps hold our secondary and collimation in place. 

 

Yea, I make no claim to inventing the idea (wish I could, it's a great idea), only sharing one of a few ways to skin the same cat. Only hope to make it more common knowledge, even a noob (like me) can understand it easily enough, so much so it quickly becomes intuitive, and we almost cannot avoid it if we actually finish the placement step by using the primary reflection as a reference for more precise secondary rotation. And of course, as you say, some iterations to nail the primary center mark and hold our good looking collimation signatures. With good initial secondary rotation, it's so much easier to approach the primary center from one planned direction rather than from three random directions. It's so easy, it's almost automatic. Automatic is a good thing for a noob. Or at least to minimize the amount of orthogonal tilt we need so we're not fighting with collimation. 

 

...(precise shimming is not and easy task). But without a leveling focuser, most users will need all three tilt adjustment screws, because the build precision in most commercially available "economy" Dobsonians is just not that precise.

 

Totally agree shimming to the OTA longitudinal axis is not easy, I spent wasted hours trying it years ago. However, shimming the focuser onto the secondary in it's natural location is not hard, and that's what is important, anyway. It's easier to square the moving parts to each other than to square everything to the tube's axis and hope they are squared to each other, too. Provided, that is as you say, the mass produced mechanics are good enough to make it easy to "chase" the secondary's location with a focuser shim (after secondary fore and aft placement). If the secondary is well placed fore and aft (toward or away from the primary along the X axis), really the focuser shim only has to be "up and down" across the tube along the Y axis. 

 

I realize that is an assumption I make, that we can place a thin piece of plastic to shim the focuser high or low of the tube longitudinal axis to catch and center the secondary pretty well. I believe you about the other screws should not be totally "forbidden", we can use some tilt to help center the secondary, too, if needed. But, with a shim, the amount of tilt needed to center the secondary is much less. And if the single axis collimation brings us close to the primary center mark, we can "cheat" a little with minimal secondary tilt to hit the primary center without much damage to the concentric signatures. All is well that ends well. smile.gif

 

IF all of the potential front end geometries are correct to significant precision, then the secondary mirror tilt adjustment will end up on the long axis and there's no need to allow for other tilt adjustments.

 

Interesting. So, in a sense, to use a 'true' single directional collimation, it's more critical to be mechanically centered and squared? It kind of makes sense, though I don't bother will all that mechanical squaring (dare I say mumbo jumbo?). Just square the moving parts (focuser and secondary) to each other and simply iterate through collimation, rotation, and placement until axial alignment is within tolerance and it looks good. I guess I can see where precise mechanical alignment is needed for precise single axis focuser alignment. But, in practice, I find such mechanical precision is not necessary. Just shim the focuser to chase the secondary location (high or low of the tube axis), rotate the secondary so the primary reflection and center mark are on the "long" axis, and go from there. 

 

I mentioned above getting a piggy back ride on the back of giants, rather than standing on your shoulders. The latter implies I can see further than giants, which is not the case. I might be able to only see some of what you see, the view from a piggy back ride. Just thought that was cute.  lol.gif

 

My coffee is cold. I think it's a great discussion, though, and I hope the OP is following along without pulling his hair out. Collimation can be difficult to explain, more difficult than actually doing it. smile.gif


Edited by Asbytec, 26 February 2020 - 06:36 PM.


#31 Asbytec

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Posted 26 February 2020 - 06:23 PM

Yikes, I knew that post was gonna be long, just not that long. Sheesh...sorry. 

 

As a noob, I found coming at collimation almost automatically from one direction was easier to understand and to actually do it. Collimation made sense, finally, and kept me out of the proverbial and frustrating rabbit hole. My hope is this will be true for other noobs, too. To avoid the rabbit hole of excessive tilt and rotation and fighting with collimation. Because as a noob, I think we all want to see nice collimation signatures as well as being collimated. It makes us feel better about the work we put into learning collimation when the result is as beautiful as it is functional. There is little doubt we finally did it right. I remember being frustrated a decade ago with my 12" Dob using a laser, because even though it was collimated it just looked wrong through the focuser. The secondary was rotated and tilted not unlike the OP's pic above. It did not inspire confidence.


Edited by Asbytec, 26 February 2020 - 06:58 PM.


#32 Vic Menard

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Posted 26 February 2020 - 07:40 PM

In the good old days when I was building telescopes, we used Novak spiders and secondary holders. The cool thing about the holders was that they had four little screws that attached the shell (that held the secondary mirror) to the metal plate (that the four "tilt" adjustment screws pressed against). Those four little screws that held the shell were spaced in thirds, two close together at the seam (which was usually at the narrow side closer to the focuser) and the other two that were spaced away from the seam. The screws had round heads that you could see surrounding the silhouette reflection of the secondary mirror IF the secondary mirror placement was good (the 45-degree cylindrical shell surrounding the secondary mirror was coaxial with the center of the primary mirror). If one of those screw heads (not the paired set close to the seam) was showing proud and the other one wasn't visible--you knew you had a tilt/rotation error.

 

Of course, the paired screws should also be equally visible, but as it turns out, that wasn't the case. The reason (of course) is that the shell surrounding the secondary mirror was not coaxial with the primary mirror axis, because it was mechanically centered relative to the tube assembly, and the alignment procedure was offset.

 

Although glued secondary mirrors don't have mounting screws for reference, it's often possible to see the side of the secondary mirror with a significant tilt/rotation error. Secondary mirrors are cut from glass cylinders, and when they're correctly aligned, the cylinder edges should be aligned to the optical axis (except for that residual error that's leftover when the secondary mirror is centered in the OTA but offset in the alignment procedure).

 

 

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#33 Vic Menard

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Posted 26 February 2020 - 08:12 PM

...This is where better secondary rotation came into it's own. Not just relying on the face of the secondary looking circular under the focuser when using a colored mask to block reflections and show only the face of the secondary. Turns out, it may look good, but it's not necessarily that good. We can actually use the reflection of the primary instead of masking it. We 'need' to see the primary reflection.

Yep.

 

...However, shimming the focuser onto the secondary in it's natural location is not hard, and that's what is important, anyway. It's easier to square the moving parts to each other than to square everything to the tube's axis and hope they are squared to each other, too.

Shimming is always a pain. I used to carry tiny thin stainless steel washers that would fit around the focuser mounting screws between the mounting plate and the OTA. I would add one at a time, to each of the screws on the side I needed to lift, and sometimes I would end up with two on one screw and three on the other to try to split the difference to get the right tilt (leveling screws are so much easier and much more precise). But I do agree that "squaring" the focuser to the secondary mirror is easier than trying to make everything orthogonal to the OTA, as long as you realize that "squaring" will probably not be "square" (90-degrees)!

 

Interesting. So, in a sense, to use a 'true' single directional collimation, it's more critical to be mechanically centered and squared?

Yes, except I would suggest effective instead of "critical". It might be easier to understand by randomly picking one of the front end elements and making it incorrectly fixed, (for example, a spider that isn't squared to the OTA, or a secondary mirror that is mounted skewed (twisted) relative to the stalk) and then visualizing how that might impact finding a usable secondary mirror placement correction.

 

...Collimation can be difficult to explain, more difficult than actually doing it. smile.gif

Post #122   https://www.cloudyni...ment/?p=3052981

 

waytogo.gif


Edited by Vic Menard, 26 February 2020 - 08:28 PM.


#34 Asbytec

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Posted 27 February 2020 - 12:22 AM

I gave up on washers, just a thin piece of plastic under one side was plenty. :)

Godspeed, Vic.

#35 pez_espada

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Posted 27 February 2020 - 08:54 AM

Dear all,

 

Wow lots of more discussion added since last time I dropped by. I tried to do what Vic suggested, i.e iterating the procedure hoping to get closer to nearly perfect collimation with each time.  However I thought I ended up messing up the whole setup and started from scratch again following Asbytec first directions. And I think I came back to be in ballpark similar to my first attempt but I don't think I have improved any further my collimation beyond of what Vic points out in post #25.

 

Asbytec what do you mean by "focuser shimming"? is it simple to focus in and focus out until you frame the secondary slightly smaller in size than the field stop of the focuser/sight tube? How focuser shimming is carried out? I am reading and re-reading all your posts -with the scope in front of me and the tools in hand -and some time I think 'voila! it has finally clicked!' and other times I find myself confused and re-reading the whole thing again.

 

Just for me to understand, when you say fore-aft secondary movement means the movement induced by only playing with the center bolt of the secondary holder (and not using the three secondary's collimation screws )? Secondary rotation does mean the movement of the secondary on its own axis determined by this center bolt of the holder? I am still struggling with the jargon.

 

Vic I'm now aware you wrote a well regarded book on the topic but I don't know if it is too advanced for me now.

 

Best and many thanks.


Edited by pez_espada, 27 February 2020 - 08:56 AM.


#36 Vic Menard

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Posted 27 February 2020 - 10:22 AM

...Vic I'm now aware you wrote a well regarded book on the topic but I don't know if it is too advanced for me now.

While I do dedicate a couple of pages to tilt/rotation errors in New Perspectives..., I've tried to keep the most recent edition of the book oriented more toward the beginner/intermediate level. That said, the discussion we've been having in this thread is only 36 posts long, and because of the specific requirements for your application, we've spent more time than usual in the "deep end" of secondary mirror placement and optimization. (You might want to revisit the final paragraph in post #29.)

 

The link I provided in post #33 to Jay Scheuerle's quote, is in fact one post of 429 posts in a discussion titled "Useful info about secondary mirror alignment" (the first six posts in the discussion include some excellent animations and a quick summation in post #6). Considering the many tangents of the 429 post discussion, I'm sure there's more than enough material for an entire book on the subject, but it would likely be too advanced for most readers, and especially difficult to write for a less advanced audience.


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#37 Starman1

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Posted 27 February 2020 - 03:12 PM

For a basic tutorial:

https://www.cloudyni...escope-v4-r2599

 

For a more in-depth analysis of secondary placement issues (with great illustrations and gifs):

https://www.cloudyni...rror-alignment/

A very long thread, worth reading.


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#38 pez_espada

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Posted 27 February 2020 - 03:18 PM

For a basic tutorial:

https://www.cloudyni...escope-v4-r2599

 

For a more in-depth analysis of secondary placement issues (with great illustrations and gifs):

https://www.cloudyni...rror-alignment/

A very long thread, worth reading.

 

Don

Many thanks Don



#39 Asbytec

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Posted 27 February 2020 - 05:54 PM

Dear all,

 

Asbytec what do you mean by "focuser shimming"? is it simple to focus in and focus out until you frame the secondary slightly smaller in size than the field stop of the focuser/sight tube? How focuser shimming is carried out? I am reading and re-reading all your posts -with the scope in front of me and the tools in hand -and some time I think 'voila! it has finally clicked!' and other times I find myself confused and re-reading the whole thing again.

 

Focuser shim. It's simply tilting the focuser by placing some washers under the screws holding it to the tube. It's adjusting the focuser angle a little so it can find the secondary centered under it. Instead of tilting the secondary to find the focuser center, we're tilting the focuser to "chase" the secondary. I find cutting a small strip of thin plastic and slipping it under one edge of the focuser to be a good way to do it. Just loosen the screws and slip the thin plastic into the gap to hold it there.

 

But only if you need or desire a shim to center the secondary. It may be okay as is, check it. I shimmed mine a tiny bit so I did not have to tilt the secondary toward focuser center. 

 

The advantage is not having to make significant tilt of the secondary from it's natural position being approximately centered in the tube (as seen and measured from the front) so the spider is in a more relaxed and neutral position to begin collimation. The secondary adjustment screws are roughly the same height so the secondary tilt is more neutral, too, and tilted more or less toward the primary mirror. It's just a clean way to begin collimation. 

 

If the spider is in it's neutral position centered in the tube (as measured along the spider vanes from the spider center bolt to the tube inside diameter) with no excessive tilt to start with, look down the focuser to see where the secondary is positioned. Yes, use a white paper background opposite the focuser (as you did earlier), and draw the focuser down until the site tube field stop is just larger than the secondary with a thin gap (give or take) of white paper around it. If the secondary is already close to center, then some tilt can bring it to center. No worries, adjust rotation a bit, too. If you need or prefer a little more movement, you can tilt (shim) the focuser a little to capture the secondary directly under the focuser. 

 

To center the secondary, we need only move it along two axes X and Y. To my perspective, the X axis runs the length of the telescope tube (longitudinal axis) from the primary to the secondary. If the secondary is not centered under the focuser on the X axis, then use the spider center bolt to adjust this "fore and aft" adjustment toward or away from the primary mirror. The Y axis is lateral across the telescope tube. If the secondary in it's neutral starting position appears "high or low" as seen through the focuser along the Y axis across the telescope tube, then you either tilt the secondary a little to bring it toward the focuser center or, alternatively, you can tilt (shim) the focuser a little to "chase" the secondary. You really only have to shim along the Y axis, if needed, because the center bolt will take care of the X axis. 

 

I am still struggling with the jargon.

In terms of jargon, this secondary mechanical centering is what we (or I) mean by "squaring" the focuser and the secondary to each other. To align the "moving parts" mechanically to each other using fore and aft secondary movement (spider center bolt) along the X axis and either a little secondary tilt or a thin focuser shim along the Y axis. When the secondary is mechanically centered under the focuser (and rotated), the white paper background will show a thin white gap between the spider and the site tube field stop and the secondary will appear circular and concentric. 

 

Now, this is the whole point I make about rotation at this point (I find useful and easy, and I hope you do, too). Just because the secondary looks circular under the focuser does not mean rotation is good enough, not yet. Don't begin tilt collimation right away, resist the urge to put the cross hair on the center mark until you refine secondary rotation. This was the epiphany moment when it all came together for me. This is the small part I share, learned from Don, that got me into trouble as a noob. It clicked so hard, my wife asked me what that noise was. lol.gif

 

It's pretty hard to tell very small differences between circular and slightly elliptical even with the site tube field stop as a reference. Small fractions of a degree. If the secondary appears circular, it probably means the entire primary reflection will be seen in the secondary, though, and that's great. You can evaluate how well the secondary is rotated by looking at the primary reflection. The primary reflection might be seen anywhere in the secondary. If so, then refine your secondary rotation so the primary reflection *and it's center mark* are as close to the secondary major axis as you can get it.

 

Now, IMO, you're (really) finished with the secondary position step and ready to collimate the focuser axis using secondary tilt only (if possible, and it is mostly possible). When the primary reflection is centered on the secondary major axis (parallel to the focuser axis), best you can, it will not take much orthogonal (off axis) tilt to put the cross hair on the center mark thus reducing the amount of "unwanted" tilt away from center and induced secondary rotation caused by the secondary adjustment screws during tilt. Most of the tilt will be along the secondary major axis because the primary center is already on (or very near) that same major axis (again, parallel to the focuser axis). This in-line tilt along the focuser axis also reduces adverse tilt away from focuser center and induced or skewed rotation we worked so hard to achieve when we centered and rotated the secondary. Reducing these induced "errors" helps hold your collimation signatures. That's the key!

 

Check and refine your collimation signatures at the apex, as needed, a couple of times until you're happy with the secondary placement then align the primary axis. Done. It's that easy, really. (LOL) 

 

Just for me to understand, when you say fore-aft secondary movement means the movement induced by only playing with the center bolt of the secondary holder (and not using the three secondary's collimation screws )?

 

Yes. 

 

Secondary rotation does mean the movement of the secondary on its own axis determined by this center bolt of the holder?

 

Yes. 

 

As you know, Vic "wrote the book" on collimation (I believe collaborating with Jason and a few others). It's a good read. And I believe those same folks also developed some nice collimation tools, too, like the auto collimator. They've been around the block a few times, so you're in great hands. I only share what helped me understand what's going on and what they are telling us, it's not easy to explain.  And sometimes hard to understand until it clicks. Even then there's more to learn. Like earlier, Vic kind of got me thinking in more detail about mechanical squaring and how that might affect "single axis" collimation I find so easy to use. The bottom line is, your collimation in tolerance and your scope will perform well. Instead of being frustrated, we actually have the luxury of good collimation to improve performance. That's nice. When you see two nice Airy discs drifting across the FOV, you'll know it...

 

waytogo.gif


Edited by Asbytec, 27 February 2020 - 07:20 PM.

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#40 pez_espada

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Posted 29 February 2020 - 08:11 AM

Asbytec,

Once again I greatly appreciate your explanations and clarifications. They do help me (and I hope others too) to understand more every time. I am just organizing and printing out some exerts for myself to have them close by my scope!

 

Regarding focuser shimming, thanks for the clarification I probably want to try this. I think my secondary was not centered in the 'Y-axis'  under the focuser originally so I had to tweak the secondary tilt to reach a 'neutral' position to begin with when following your directions (namely, firstly centering the secondary on the focuser/Cheshire fieldstop by only rotational and fore-aft shifting, and secondly centering the primary reflection and matching cross-hair and primary's donut by mostly tilting the secondary along its major axis to avoid unwanted extra rotation errors).

 

I think that starting the procedure with a secondary that, to begin with, was tilted away from its 'natural' position to "chase the focuser" center (as I have it now) instead of doing it the other way around (i.e. to shim the focuser to chase the secondary, as you have very well explained) may complicate the possibility of using the method of centering the primary reflection and hair-cross with this method. I guess this since the major axis of the previously tilted secondary may not be inline any longer with the screw corresponding to its major axis in the secondary holder. Maybe the mapping between the secondary's major axis and the its collimation screws in the holder loses the symmetry we take advantage of with this method. But I am only guessing. Anyway I want to try again by "chasing the secondary" instead by shimming the focuser and see. After all I am learning and that's the main purpose of this my first Newtonian.

 

Regarding the shimming by sliding some plastic shim between the focuser and the tube (e.g. some old piece of a credit card?) I think I can try that but, pardon my ignorance,  I have spotted two small Allen screws in 'X-axis' and 'Y-axis' positions in the focuser axis (see pic). Aren't they to be used to 'shim' or 'square' the focuser, or shouldn't I touch those?

 


When you see two nice Airy discs drifting across the FOV, you'll know it...

 

waytogo.gif

 

 

 

That's what I want to be with my well-collimated Newton + coma corrector, this and a symmetric and as well illuminated FOV as possible!

 

Clear skies,

 

 

 

Attached Thumbnails

  • focuser_screws_3.jpg


#41 Asbytec

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Posted 29 February 2020 - 10:18 AM

Pez, I really appreciate your kind words. I am just happy you are able to understand my hen scratching. It's not easy to explain. 

 

I kind of lost track of your primary purpose and touched on collimation a bit too much. I do want to help you place your secondary for field illumination. You were so close in your post #20. I'll have to leave the conversation of paracorr to Vic and others, and the actual field illumination you achieve to the math of your secondary size, etc. One thing Vic does well is get to know your scope so he can help with these things. I describe only the method I used to hold my signatures and avoid your first image above. 

 

I am shy to say I am not familiar with your focuser, but those do look like "leveling" screws that can be used instead of a focuser shim. If so, it's the same idea, though, to chase the secondary position in the tube. Mine does not have that luxury. 

 

Okay, so to answer your questions. 

 

...may complicate the possibility of using the method of centering the primary reflection and hair-cross with this method.

 

Unless I am missing something, I believe it should not complicate things. If you can chase the secondary position, then rotate the secondary so the primary reflection and center mark are on the secondary major axis, the secondary should appear very much circular under the focuser. It'll be centered "up and down" along the Y axis in the secondary, tilt will bring the primary center to the secondary center on the *minor* axis, as well. Of course, the primary reflection will be circular, and the secondary should appear circular, too, with the same amount of secondary surface surrounding the primary. This, by the way and as I bet you know, is an indication the off axis illumination you are seeking.

 

The beauty is, you can focus outward to the apex where the primary clips appear at the edge of the secondary, the primary reflection and the secondary mirror will appear about the same size. (The primary clips, as Vic explains, are not a collimation signature, they are a proxy for the primary edge). You should be able to see whether or not the two mirrors are concentric with each other, as well. I bet they will be at least close. Remember, this only works when the cross hair is on the primary center, in other words the focuser axis is well collimated which means the primary reflection is also dead center under the focuser. You can use the dead centered primary reflection as a reference to refine your secondary position and rotation. (Affectionately referred to as Jason's Method). You will see residual tilt, position, and rotation errors of the secondary. The idea is to refine these secondary placement errors until the secondary edge coincides with the collimated and circular primary reflection as a reference. 

 

If your cross hairs do not align with the primary center after initial attempt to tilt the secondary along the secondary major axis, back off and and rotate the secondary a tiny bit, then re-tighten the adjustment screw until you hit the primary center. You might induce secondary positional error, but that's okay because the focuser axis is collimated and the primary reflection IS centered when the cross hair is on the center mark. The primary reflection must be centered (to within the accuracy of your center mark). But, if you induced some errors in secondary "placement", that's what you want to see. Those are the placement errors you will fix as you iterate through this step at the apex a few times until the primary reflection and the secondary are concentric with each other and BOTH are centered under the focuser - when the focuser axis is collimated to the primary center. Those concentric circles are, of course, the signatures we're trying to achieve. This is the precision you are trying to achieve and your secondary should also appear very much circular (relative to the primary reflection and the site tube) as seen from the focuser.  

 

I guess this since the major axis of the previously tilted secondary may not be inline any longer with the screw corresponding to its major axis in the secondary holder.

 

Yea, maybe not. It should be very close, however. I am not entirely familiar with this quirk, but if the slightly off axis adjustment screw does not collimate *exactly* along the secondary major axis you still may be okay. You may have to use some orthogonal tilt from the other two screws. But the amount you need is likely very minor compared to your initial attempt. So, you will induce only a tiny amount of error and you can still see and fix any residual rotation error at the apex. You should still be able to be as precise as you like adjusting the secondary position to the collimated primary reflection at the apex. If it requires too much secondary tilt for your liking, you can adjust the focuser tilt, again, to lessen that secondary tilt required. Repeat until you get it where you want it. (I am playing this out in my mind's eye, so I hope that's right). 

 

I believe this is one of the quirks Vic explained to me (us) earlier. I get my signatures very well concentric to the point I cannot see any error visually, but I think you are looking for higher precision. I mean, we're talking the slightest drop off right at the edge of the fully illuminated field (of whatever dimension your scope provides) which is important to your precise work. I am not aware of or use any tools for secondary placement to that level of accuracy. Maybe just be assured of and settle for a few arc seconds smaller fully illuminated field of view? 

 

I have spotted two small Allen screws in 'X-axis' and 'Y-axis' positions in the focuser axis (see pic). Aren't they to be used to 'shim' or 'square' the focuser, or shouldn't I touch those?

 

As above, I think those are "leveling" screws for that very purpose. I'm sure they allow you to tilt the focuser on both axes. I used a thin piece of plastic, not even as thick as a credit card. I recall it was a small snipped piece form a ice cream box. But, it looks as though all the discussion of "shimming" is built into your focuser.

 

Okay, it's dark outside and the skies are clear. Hope I said things correctly and was correct in saying them. Gotta run. smile.gif


Edited by Asbytec, 29 February 2020 - 10:27 AM.

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#42 Kipper-Feet

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Posted 29 February 2020 - 11:01 AM

To pez_espada, please check your Personal messenger, you've got mail.


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#43 Asbytec

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Posted 29 February 2020 - 02:06 PM

Of course, the paired screws should also be equally visible, but as it turns out, that wasn't the case. The reason (of course) is that the shell surrounding the secondary mirror was not coaxial with the primary mirror axis, because it was mechanically centered relative to the tube assembly, and the alignment procedure was offset.

 

Although glued secondary mirrors don't have mounting screws for reference, it's often possible to see the side of the secondary mirror with a significant tilt/rotation error. Secondary mirrors are cut from glass cylinders, and when they're correctly aligned, the cylinder edges should be aligned to the optical axis (except for that residual error that's leftover when the secondary mirror is centered in the OTA but offset in the alignment procedure).

hmm.gif

 

Interesting, need to think that through. One immediate question comes to mind, if the secondary is perfectly centered and the focuser axis is perfectly aligned then the edges of the secondary won't be visible? Or will they because the secondary is *mechanically* centered relative to the tube and not necessarily on the primary mirror. In any case, the secondary surface should be perfectly(sic) circular, yes? Great, it's always nice to have something to think about. :)  



#44 Starman1

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Posted 29 February 2020 - 03:08 PM

If the secondary is centered in the tube and centered under the focuser and the scope is collimated in an f/5 or shorter scope:

--the reflected angle at the secondary will be >90°

--the optical axis will be tipped toward the focuser

--the appearance of the secondary will not be PERFECTLY round but appear to be slightly smaller in the long axis than the short axis of the ellipse, from the focuser

--mounting screws on a secondary holder will not be uniformly visible.



#45 Asbytec

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Posted 29 February 2020 - 03:16 PM

Thanks, Don, great points. That's true with the new model of singe directional offset, yes? I recall Jason's illustrations showing the secondary slightly tilted upward for focuser alignment. 



#46 Starman1

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Posted 29 February 2020 - 03:18 PM

Yes.  Bi-directional offset results in a 90° reflection angle at the secondary.


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#47 Asbytec

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Posted 29 February 2020 - 09:24 PM

Yes.  Bi-directional offset results in a 90° reflection angle at the secondary.

Like many mass market scopes, mine has bi directional offset built in. I'd imagine the OP's scope does as well. 




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