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Secondary collimation question

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#1 Wallcreeper

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Posted 17 January 2020 - 07:55 PM

Hi,
I am using a astrosystems lightpipe to collimate my 8 inch dob. When I measure the length of the vanes that hold the secondary from above, my secondary is perfectly in the middle of the tube. It also looks round as seen through the focuser. However when seen through the focuser it is not perfectly centered. One side has a bit more space around it than the other. Does this matter? And if so, how do I fix it?
Thanks!

#2 Vic Menard

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Posted 17 January 2020 - 08:07 PM

One side has a bit more space around it than the other. Does this matter?

I don't know--how much is a bit? Your secondary mirror minor axis is about 23-percent of the primary mirror diameter, and at f/5.9, that should be at least a little oversized, which should, in turn, be able to accommodate a little bit of space differential surrounding the edge of the secondary mirror.

 

Can you provide a picture through your LightPipe?

 

Have you looked at stars through the scope? Did they look like tiny ball bearings or seagulls?



#3 TOMDEY

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Posted 17 January 2020 - 08:21 PM

Ideally, it should look centered in that tube. You can actualize that in numerous ways: adjusting the spider legs, moving the spider farther up/down the tube, moving the SM mirror mount along the stalk, tip-tilting and/or translating the focuser itself.

 

There is nothing special about getting an exact 90o deviation twixt focuser and tube axes. Some people sweat bullets over that, even though it doesn't matter at all.

 

By far, the most important requirement is to get the coma-free focal point of the Primary Mirror right on the center of the focuser draw-tube. All other adjustments matter only slightly, in comparison.    Tom



#4 Kipper-Feet

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Posted 17 January 2020 - 10:05 PM

Wallcreeper, kindly check your Personal Messenger, you have mail.



#5 Asbytec

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Posted 17 January 2020 - 10:50 PM

The secondary placement involves the center of the fully illuminated field and not axial collimation. So, it can be de-centered a little "bit" if, as Vic said, the secondary is a little oversize to capture the entire primary reflection. What's important for focuser axial collimation is the primary reflection be centered under the focuser draw tube, and it will be if the cross hairs are on the primary center. You can rack the focuser outward to the apex, the point where the primary mirror reflection and secondary are the same apparent size and their edges coincide as seen through the peep hole, and fine tune the secondary position and rotation. The primary mirror clips are not a collimation signature, but they can help to visualize the position and rotation of primary reflection within the secondary. 

 

As Tom said, if you want to center the secondary, you can move it laterally across the tube by pulling one spider vane in that direction or "translate" the focuser by adding a thin shim. I prefer the latter to keep the spider vanes as straight as possible thus avoiding the possibility of double diffraction spikes. I try to center the secondary using only adjustment toward or away from the primary and rotation (only) avoiding any urge to tilt the secondary at this point. Doing so helps maintain the three collimation signatures as Vic often describes: the focuser draw tube, the secondary edge, and the reflection of the primary. From there, it's just a matter of bringing the primary mirror coma free spot (and diffraction limited coma area) into the center of the eyepiece field of view using primary mirror tilt as seen through the Cheshire. 


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#6 Wallcreeper

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Posted 18 January 2020 - 09:35 AM

I was able to take a picture through the cheshire.

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#7 Wallcreeper

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Posted 18 January 2020 - 10:20 AM

Here is one with the primary blocked with a piece of paper.

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#8 Vic Menard

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Posted 18 January 2020 - 11:26 AM

I was able to take a picture through the cheshire.

Looks good! The focuser axis is slightly off (blue cross hairs), but still within the high magnification performance tolerance. It would require a small secondary mirror tilt adjustment (always followed by a final adjustment of the primary mirror tilt), but your alignment is certainly good enough to use "as is".

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#9 Wallcreeper

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Posted 18 January 2020 - 03:10 PM

That is good to hear! If I were to tilt the secondary a bit, any idea which way I should be tilting it?

#10 Vic Menard

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Posted 18 January 2020 - 03:40 PM

The focuser is currently pointing closer to the focuser side of the OTA than centered. Tilt the edge closest to the primary mirror end of the OTA away from the focuser to point the focuser axis toward the center of the OTA/primary mirror.



#11 Asbytec

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Posted 18 January 2020 - 05:20 PM

That is good to hear! If I were to tilt the secondary a bit, any idea which way I should be tilting it?

You see that blue line running through the focuser and across the center mark? With three secondary collimation screws, one of them is very likely on that line and on the proper tilt axis. The other two adjustment screws are orthogonal to that axis and will work as a pair in unison with each other. Tilt only that one adjustment screw in line with the focuser "axis" (just as Vic said above) to move the cross hairs directly onto the center mark. The other two work as a pair in unison so they are adjusted (same direction by the same amount) to permit the single adjustment screw to actually tilt the secondary. Working those two in unison does not add any (or very little) unwanted tilt or inadvertent rotation orthogonal to the direction you want to move. 

 

I've learned never (sic) to use the secondary adjustment screws individually and intuitively, like we do the primary mirror adjustment screws, to approach the center mark from three directions. Rather approach the primary center mark from a single direction along that blue line. Use the one adjustment screw to tilt, then adjust the other two by the same amount in the same direction. This helps maintain your three collimation signatures by reducing unwanted rotation and secondary tilt away from center. 

 

In fact, once you centered the secondary, the proper rotation of your secondary caused the primary center mark to fall on that blue line running through the focuser (the blue line is kind of a projection of the focuser axis onto the primary mirror). That's the beauty of using rotation only when placing the secondary, we need rotation only to bring the primary center mark in line with the cross hairs and, by extension, in line with the focuser axis. Once in line, you only need tilt in a single direction. Not from three directions in a more or less random way. This use of secondary collimation screws might not be intuitive, but it is so much easier that trying to approach the center mark from all directions. Doing so reduces unwanted tilt of the secondary away from the focuser center and minimizes induced rotation helping to maintain your collimation signatures. When done, you collimation signatures fall into place nicely and your collimation just looks right, too. This sets up the conditions allowing you to tilt the secondary just as Vic said above. Every time. 

 

If you think about it, when the secondary is well placed (centered) under the focuser, the cross hairs will be centered on the center of the "apparently" circular secondary (but not at the geometric center of the actual elliptical secondary, which is correct). Using secondary rotation, then, the primary reflection can be rotated to be centered on the secondary major axis (again, the blue line running down the focuser in the image above). Doing so necessarily brings the primary center marker to the major axis of the secondary (again, the blue line as a projection of the focuser axis). This lines everything so that only tilt along that blue line (a projection of the focuser axis) is required to bring the cross hairs to the primary center. If you miss it by a tiny bit, you can avoid the temptation for orthogonal tilt by simply refining secondary rotation by that much. Then try tilting along that single axis, again, until you hit the center mark dead center.

 

This is exactly how I check and touch up my collimation prior to each use. I draw the focuser back to the apex (as show in the image above), loosen that single tilt screw, refine rotation, then tighten that single screw again until I hit the primary center marker dead center. Then, of course, check the primary tilt. It only takes a couple of minutes to collimate the scope instead of all weekend. lol.gif


Edited by Asbytec, 18 January 2020 - 06:08 PM.

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#12 dearchichi

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Posted 18 January 2020 - 07:43 PM

This is exactly how I check and touch up my collimation prior to each use. I draw the focuser back to the apex (as show in the image above), loosen that single tilt screw, refine rotation, then tighten that single screw again until I hit the primary center marker dead center. Then, of course, check the primary tilt. It only takes a couple of minutes to collimate the scope instead of all weekend. lol.gif

Neat idea, Asbytec. Thanks for drawing it out. Now, I only have to check on the positioning of the secondary collimation screws (the heads of the two of them on either side of the vane that lies along the focuser axis need to be at the same level) to gauge if an apparently collimated the telescope is obviously miscollimated. If their positions check out, the axial alignments and concentric placements could then be checked through tools.

 

None of the collimation procedures I have read (Don's, Vic's, Glatter's, Catseye's etc.) state the role of the secondary collimation screws while describing the first step of the procedure: centering of the secondary mirror below the focuser. How does these collimation steps sound in order to collimate a Newtonian with a sight-tube/Cheshire combination tool?

1. loosen all three of the secondary collimation screws so they don't affect the positioning of the secondary.

2. make the OTA axis vertical so the secondary does not carry a tilt - that is, it's geometric center is along the OTA axis

3. center the secondary along its major axis under the focuser only by turning the central (forward-aft) screw of the secondary holder

4. center the secondary along its minor axis under the focuser by rotating the secondary so the center marker of the primary intersects major axis. The secondary is now centered under the focuser.

5. gently tighten the secondary collimation screws enough so they contact with the secondary while ensuring the alignment achieved until #4 above remains. Move the OTA axis in altitude to ensure such alignment holds.

6. loosen/tighten the secondary collimation screw on the vane that lies along the focuser axis, and tighten/loosen the other two the same amount until the center of the primary aligns with the intersection of the crosshairs. Thus, FAE = 0.

7. Turn the primary collimation knobs so the Cheshire ring is centered around the intersection of the crosshairs. Thus, PAE =0.

 

Collimation is now complete.



#13 Wallcreeper

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Posted 18 January 2020 - 07:58 PM

There is indeed a screw in line with the focuser. It is the closest one if the 3 to the focuser. So to tilt the edge closest to the primary away from the focuser, I would tighten that one a bit and perhaps loosen the other 2 a bit by equal amounts?
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#14 Vic Menard

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Posted 18 January 2020 - 09:53 PM

I would start by simply tightening the one screw--depending on how tight the tilt alignment screws are, that may be all you need. A little bit of tilt goes a long way when you're already close...



#15 Vic Menard

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Posted 18 January 2020 - 10:15 PM

...None of the collimation procedures I have read (Don's, Vic's, Glatter's, Catseye's etc.) state the role of the secondary collimation screws while describing the first step of the procedure: centering of the secondary mirror below the focuser...

I spend quite a bit of time describing tilt/rotation errors in New Perspectives... and a lot more time in these forums. Resolving secondary mirror placement errors is only one part of a series of complications associated with the focuser/secondary mirror/spider/optical axis geometry (which can become further complicated with a truss tube OTA). This makes it pretty much impossible to define a sequence of corrections that leads to perfect secondary mirror placement every time.

 

That said, tilt/rotation is the number one error I see in forums and on the observing fields (offset runs an easy second). Invariably, it enters the discussion when a user tries to make the three circles (the bottom edge of the focuser/sight tube, the actual edge of the secondary mirror, and the reflected edge of the primary mirror) concentric. 

 

This is why I always suggest limiting the secondary mirror tilt adjustment to focuser axis alignment (making the sight tube cross hairs and/or simple thin beam laser dot aligned with the primary mirror center marker) and secondary mirror rotation and/or position fore and aft (closer to or farther from the primary mirror end of the OTA) for centering the secondary mirror under the focuser (via the three circles). Of course, the two alignments are fully involved in each other's correction, which is why the procedure--correct the axial alignment (tilt), then correct the rotation, then correct the axial alignment again (tilt), then correct the rotation again... is iterative with both errors diminishing after each correction.

 

Finally, you will reach a point where the secondary mirror placement and focuser axial alignment are both correct, or you will reach a point where neither can be correct at the same time! This is where the other geometry complications I mentioned above come into play.


Edited by Vic Menard, 19 January 2020 - 12:20 PM.

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#16 SteveG

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Posted 18 January 2020 - 11:16 PM

Neat idea, Asbytec. Thanks for drawing it out. Now, I only have to check on the positioning of the secondary collimation screws (the heads of the two of them on either side of the vane that lies along the focuser axis need to be at the same level) to gauge if an apparently collimated the telescope is obviously miscollimated. If their positions check out, the axial alignments and concentric placements could then be checked through tools.

 

None of the collimation procedures I have read (Don's, Vic's, Glatter's, Catseye's etc.) state the role of the secondary collimation screws while describing the first step of the procedure: centering of the secondary mirror below the focuser. How does these collimation steps sound in order to collimate a Newtonian with a sight-tube/Cheshire combination tool?

1. loosen all three of the secondary collimation screws so they don't affect the positioning of the secondary.

2. make the OTA axis vertical so the secondary does not carry a tilt - that is, it's geometric center is along the OTA axis

3. center the secondary along its major axis under the focuser only by turning the central (forward-aft) screw of the secondary holder

4. center the secondary along its minor axis under the focuser by rotating the secondary so the center marker of the primary intersects major axis. The secondary is now centered under the focuser.

5. gently tighten the secondary collimation screws enough so they contact with the secondary while ensuring the alignment achieved until #4 above remains. Move the OTA axis in altitude to ensure such alignment holds.

6. loosen/tighten the secondary collimation screw on the vane that lies along the focuser axis, and tighten/loosen the other two the same amount until the center of the primary aligns with the intersection of the crosshairs. Thus, FAE = 0.

7. Turn the primary collimation knobs so the Cheshire ring is centered around the intersection of the crosshairs. Thus, PAE =0.

 

Collimation is now complete.

This is interesting, and I'm not sure what you mean about #2.

Mine would read as follows if it is a standard Newtonian:

 

1. Looking into the top of your tube assembly, center the secondary within the tube using the spider vanes, making sure they are straight and inline/perpendicular to each other. Once you have that, do not use the spider vanes for collimation, as the rest can be achieved with the secondary adjustments. Make sure they are very tight.

2. Looking into the focuser (preferably with a site tube or Cheshire), center the secondary under the focuser by turning the central (forward-aft) screw of the secondary holder.

3. Again looking into the focuser, center the secondary along its minor axis  by rotating and/or tilting it, so the center marker of the primary intersects major axis. Make sure the center screw is nice and tight. The secondary is now centered under the focuser.

4. Looking into a Cheshire or site tube, make minor adjustments to the 3 small secondary screws until the center of the primary aligns with the intersection of the crosshairs.

5. Turn the primary collimation knobs so the Cheshire ring is centered around the intersection of the crosshairs.


Edited by SteveG, 18 January 2020 - 11:16 PM.

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#17 Asbytec

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Posted 19 January 2020 - 02:26 AM

 

None of the collimation procedures I have read (Don's, Vic's, Glatter's, Catseye's etc.) state the role of the secondary collimation screws while describing the first step of the procedure: centering of the secondary mirror below the focuser.

 

How does these collimation steps sound in order to collimate a Newtonian with a sight-tube/Cheshire combination tool?

1. loosen all three of the secondary collimation screws so they don't affect the positioning of the secondary.  Yes, Don recommended I do this...

2. make the OTA axis vertical so the secondary does not carry a tilt - that is, it's geometric center is along the OTA axis  I don't think it's really necessary, just let the secondary rest on the adjustment screws, just enough tension to prevent it from slopping around. It needs to be tight enough to hold a neutral tilt (for now) and loose enough to rotate. 

3. center the secondary along its   major   axis under the focuser only by turning the central (forward-aft) screw of the secondary holder  Yes, the aft edge and the fore edge of the secondary are equidistant from and concentric with the focuser draw tube as seen against the white paper background. The latter is harder to see because it's blackened and dark. You can illuminate the fore edge of the secondary and its holder if needed. Shim the focuser or pull one spider vane slightly to center the secondary laterally across the tube (your step 4). 

4. center the secondary along its minor axis under the focuser by shimming the focuser then rotating the secondary so the center marker of the primary intersects major axis. The secondary is now centered under the focuser. Don recommended I do this, too. Also, the primary center mark is on the secondary major axis, as well, and things are coming together nicely to begin axial collimation using a single tilt direction along the focuser axis/secondary major axis. smile.gif

5. gently tighten the secondary collimation screws enough so they contact with the secondary while ensuring the alignment achieved until #4 above remains. Move the OTA axis in altitude to ensure such alignment holds. Sure...they should hold, but able to rotate. 

6. loosen/tighten the secondary collimation screw on the vane that lies along the focuser axis, and tighten/loosen the other two the same amount until the center of the primary aligns with the intersection of the crosshairs. Thus, FAE = 0.  waytogo.gifIf you do this right, you will hit the center mark by tightening one screw and thus ending collimation by adding some tension to hold it. Another nice feature. And your collimation signatures will come together as you finish focuser axial alignment. 

7. Focus out to the apex and verify the primary reflection and secondary edge coincide with each other and both are well centered under the focuser draw tube. Refine secondary position and rotation as needed. 

8. Turn the primary collimation knobs so the Cheshire ring is centered around the intersection of the crosshairs. Thus, PAE =0. Done! 

 

Collimation is now complete.

Of course we understand all who give input on collimation, and Vic who literally wrote the book, do a great job and put in tireless hours explaining collimation, what needs to be done, and the steps to do it. It's not easy to explain. But, yea, few of us talk about "how" to manipulate the secondary screws to achieve those tried and true collimation steps. Truth is, you can do it using them independently, but you have to reiterate and correct for induced rotation and unwanted tilt. But, there is a better way than is intuitive. 

 

I kept using the secondary collimation screws independently to center the secondary under the focuser, despite Vic saying not to do so. I do recall Vic saying use for and aft placement and "rotation only". Rotation only stuck in my mind because it seemed *impossible* to rotate the secondary into the center of the focuser draw tube without tilting it into position. And when I did tilt it, I found myself tightening the spider vane on the opposite side to bring it back to center and correcting some rotation, too. I was chasing the rabbit down the hole in an endless cycle of weekend after weekend of frustration. When I finally got it, finally, Vic's advice suddenly made perfect sense, and I realized how folks can collimate their scopes in just a few minutes. Kipper Feet and I also exchanged PMs, he has an unequaled knack for explaining things.

 

The epiphany came when I was having trouble and Don mentioned to neutralize the secondary collimation screws back to a scratch starting position (as you describe above) and to center the reflection of the primary on the elliptical secondary major axis (Later, focuser axial tilt will bring the primary reflection to center on the elliptical secondary *minor* axis, thus centered in the secondary. An if all goes well, the cross hairs will also hit the center mark at the same time. Which is so nice to see come together. smile.gif ). Then take what Vic said about fore and aft placement and rotation "only" for step 1 placement of the secondary...meaning it's not time for tilt yet. We're still in step 1. Tilt is reserved (primarily) for step 2 focuser axial alignment, but we need the secondary centered first without using tilt to the extent possible. A tiny bit of focuser shim put the (my) "neutralized" secondary dead center of the focuser draw tube. Only then was it possible to use "rotation only" to center the reflection of the primary not only in the secondary major axis, but also under the focuser draw tube at the same time because the secondary is centered under the focuser already. Now, it becomes a matter of "tilt only", again as I recall Vic saying in a thread long ago, to bring the focuser axis into alignment.   

 

So, I think folks are saying it, but maybe not directly. I stumbled onto the single axis movement because I realized after following Don and Vics guidance, the cross hairs were in line with the focuser and set to move directly to the primary center mark using only one adjustment screw. It just kind of happens automatically when we properly place the "neutralized" secondary (by shimming the focuser or slight pull of one spider vane, if necessary), then rotate only the secondary to set up conditions for collimation with the cross hairs and primary center on the same line. You almost cannot avoid the single directional movement of one of the secondary adjustment screws to bring the cross hairs onto the center mark. The trick is to use only that screw and resist the tilt movement of the other two. Refine missing the center marker with secondary rotation instead of orthogonal secondary tilt. Very little or no secondary tilt is required by the other two screws except to allow the one screw to tilt the secondary. 

 

It just clicked one day and collimation instantly made sense and became unbelievably easy after weekends of fighting unwanted tilt, induced rotation, and frustration. So, yes, I share the experience because the use of secondary adjustment screws turns out to be important to obtaining and maintaining our collimation signatures we worked so hard to achieve. The steps are just fine, it's the "doing" that can be frustrating. Imagine if you had one of the high end spiders with 4 adjustment screws! Now you know how it's done and you'll be collimating your scope in 10 minutes rather than all weekend.  lol.gif


Edited by Asbytec, 19 January 2020 - 03:06 AM.

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#18 Vic Menard

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Posted 19 January 2020 - 09:07 AM

...Imagine if you had one of the high end spiders with 4 adjustment screws! ...  

My 22-inch f/4 StarStructure secondary mirror has 4 tilt adjustment screws. It's actually much easier to work with since two of the screws lie on the same line as the secondary mirror major axis, so all you have to do (after rotation has been corrected) is loosen one screw and tighten the other. Also, rotation and fore and aft adjustments have been decoupled from the tilt adjustment screws on the secondary mirror holder and moved to the threaded mounting stalk that attaches the holder to the spider. Again, easier to adjust and simpler to understand. This type of secondary holder (with 4-screw tilt) was introduced by AstroSystems about 30 years ago and has found its way into many Newtonian and Cassegrain designs.

 

(Edit: I believe it was Kenneth Novak who first decoupled the tilt adjustment screws from the mounting stalk rotation/offset adjustment--but he still had 3 tilt adjustment screws. I think Novak preceded AstroSystems by 10 or 15 years...  )


Edited by Vic Menard, 19 January 2020 - 09:23 AM.

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#19 Vic Menard

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Posted 19 January 2020 - 09:19 AM

Looking at step 2--neutralizing secondary mirror tilt--by either letting it hang free from a vertical position or trying to loosen the three tilt screws until they are loose but equal may be hit or miss for some mounted secondary mirror. Similarly, loosening the secondary mirror and holding it with your hand so you can freely move it around until you can see the primary mirror reflection is no guarantee that you've neutralized tilt or rotation (or offset)! Until you know that the primary mirror is, in fact, centered under the focuser (FAE is zeroed), centering the primary mirror reflection is at best, a starting position from which you can begin to assess secondary mirror placement. 

 

Jason's procedure (laser and collimation cap) uses this paradigm for resolving these first two complications  https://www.cloudyni...ment/?p=5260727


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#20 Asbytec

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Posted 19 January 2020 - 10:20 AM

"Until you know that the primary mirror is, in fact, centered under the focuser (FAE is zeroed), centering the primary mirror reflection is at best, a starting position from which you can begin to assess secondary mirror placement."

Great point. As I gather it, the primary reflection is centered under the focuser when the cross hair or collimated thin beam laser is on the primary center mark. I.e., FAE = 0. No guarantee the secondary is similarly centered or rotated correctly (only tilted correctly). But, (I believe
Jason said in the link step 1) when FAE = 0, then checking at the apex can refine secondary placement (including rotation) relative to the primary reflection which is already centered on the focuser axis. Yes?

When I check collimation, I pull back to the apex to see how it looks. If rotation or tilt is off, I refine secondary rotation on the primary reflection (by loosening the one adjustment screw so the secondary can rotate freely), then tilt the secondary back so the cross hair hits the primary center mark. If I miss the center marker, a slight refinement of secondary rotation then tilt, again, at the apex until the cross hair does land on the primary center mark. From the apex, you can even assess and refine secondary fore and aft placement.

Usually this, in my experience, means all three signatures look good and FAE is back to zero. I find this to be more accurate at hitting the center mark, too, and very easy to do.

Edited by Asbytec, 19 January 2020 - 10:36 AM.

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#21 Vic Menard

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Posted 19 January 2020 - 11:23 AM

...But, (I believe Jason said in the link step 1) when FAE = 0, then checking at the apex can refine secondary placement (including rotation) relative to the primary reflection which is already centered on the focuser axis. Yes?

Yes. But Jason's reason for checking close to the apex was to make the apparent size of the secondary mirror similar to the apparent size of the primary mirror reflection--because the bottom edge of the focuser (remember, he's using a collimation cap for secondary mirror placement) is too large in apparent size for a viable reference. 

 

...When I check collimation, I pull back to the apex to see how it looks. If rotation or tilt is off, I refine secondary rotation on the primary reflection (by loosening the one adjustment screw so the secondary can rotate freely), then tilt the secondary back so the cross hair hits the primary center mark. If I miss the center marker, a slight refinement of secondary rotation then tilt, again, at the apex until the cross hair does land on the primary center mark.

 

If you're using a Cheshire/sight tube combination tool, you have the advantage of seeing both the secondary mirror placement (the three circles) and the focuser axial alignment (alignment of the cross hairs and the primary mirror center marker) simultaneously. This is why I always suggest a good sight tube for assessing and correcting secondary mirror placement. Unfortunately, I still see far too many people abandon the focuser axial alignment in order to center the primary mirror reflection (make all of the primary mirror clips visible) in the secondary mirror. Remember, a small, residual tilt/rotation error or tilt/offset error will have no significant impact on image performance, while a residual focuser axial error that exceeds about 3- to 4-percent of the primary mirror diameter will have a visible impact on image focus (much more so if a coma corrector is in use).

 

...From the apex, you can even assess and refine secondary fore and aft placement.

Usually this, in my experience, means all three signatures look good and FAE is back to zero. I find this to be more accurate at hitting the center mark, too, and very easy to do.

 

With a good sight tube, there's nothing magical about the apex. If the cross hairs are aligned to the primary mirror center marker, the reflection of the primary mirror is centered (difficult to assess with a collimation cap). The more critical alignment reference for secondary mirror placement then becomes the other two circles--the bottom edge of the sight tube and the actual edge of the secondary mirror.

 

Without question, when all three circles are reasonably visually concentric, any residual focuser axial error is usually quite small (a minor tweak adjustment of the secondary mirror tilt). Since potential residual errors in the concentricity of each element is additive, you would want to limit residual errors to 1- or 2-percent of the primary mirror diameter if you didn't have another method for assessing the actual focuser axial error.

 

Finally, I feel like I'm playing the role of Devil's advocate here, but when it comes to secondary mirror placement, the associated geometries, and the available adjustments, as they say, "The Devil is in the details." I'll give you an example, and leave it at that. Years ago, I owned a StarMaster Dobsonian with a 4-screw secondary mirror tilt setup. If there are any StarMaster owners reading this, you know that Rick Singmaster (the owner/designer/builder of StarMaster Telescopes) called the two screws that were not in line with the secondary mirror major axis the forbidden screws! This is because Rick set up the rotation adjustment with plastic washers to facilitate easy rotation--which in turn made the secondary mirror prone to rotation error. Of course, with two "no-tool" knurled knobs available to "fix" the error, almost every StarMaster had tilt/rotation errors! So, the "forbidden" screws were summarily removed from the StarMaster alignment procedure. (Another complication soon followed when Rick realized that the focuser board on the larger aperture Dobsonians was prone to warping from torque forces associated with  the truss attachments. But that's another story.)


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#22 dearchichi

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Posted 19 January 2020 - 03:29 PM

Many thanks to Vic, Asbytec and SteveG for their responses. I do have some questions regarding the same.

However, before I hijack the thread a little more, I'd like to know if the OP has fixed the issue based on the advice he has received since that's the primary reason for the thread.

Thanks.
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#23 Starman1

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Posted 19 January 2020 - 07:25 PM

No one has brought up the elephant in the room, and I'm mystified why.

The OP is asking why the black shadow of the secondary mirror is not centered in the reflection.

And so far, no one has said, "It shouldn't be!"

When the scope is collimated, the offset is built in as if the secondary were offset away from the focuser and down the tube toward the primary.

This results in the optical axis being tilted slightly toward the focuser, so the image from the focuser shows the secondary mirror, focuser, sight tube, and tool reflection

all concentric with the silhouette of the secondary offset to one side.

It should look a bit like this when collimated:

 

Attached Thumbnails

  • Secondary & Primary Aligned - full view.jpg

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#24 Vic Menard

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Posted 19 January 2020 - 09:02 PM

roflmao.gif

 

Well, the OP never mentioned the reflection of the secondary mirror! He even included an image with no reflections at all. But I suppose you could be right--although the actual secondary mirror placement did have "...a bit more space around (one side) than the other".  ubetcha.gif

 

So, what do you say, Wallcreeper?  



#25 Wallcreeper

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Posted 20 January 2020 - 08:53 AM

roflmao.gif

 

Well, the OP never mentioned the reflection of the secondary mirror! He even included an image with no reflections at all. But I suppose you could be right--although the actual secondary mirror placement did have "...a bit more space around (one side) than the other".  ubetcha.gif

 

So, what do you say, Wallcreeper?  

My original thought had to do with the placement of the secondary, with the primary blocked so that no reflections are visible. However, I had been wondering about the black shadow of the of the secondary too! I had assumed that at F5.9 there was not going to be any off set.

 

I have been reading and re-reading all the very thoughtful posts here. Thank you to all who contributed!

 

Here is my understanding so far:

1) Make sure that a line from the center of the focuser runs through the center of the primary. If not then tilt the secondary sideways until it does. (using the 2 screws that are not in line with the focuser)

2) Use the screw on the secondary that is in line with the focuser to tilt the secondary away/toward the focuser until the line perpendicular to the focuser also runs through the center of the primary.

3) adjust the primary until the center mark is directly behind the crosshairs.

 

I tilted the secondary using the 1 screw that is in line with the focuser and here is what my scope looks like right now:

Attached Thumbnails

  • 20200120_083545.jpg



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