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Secondary Mirror Not Centered in Sight Tube When Spider Vanes are Equal

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#1 ShooieLewis

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Posted 18 January 2021 - 02:06 PM

Hello,

 

I'm going through the process of collimating my telescope for the first time and am finding that the position of my secondary is not close to centered in the sight tube when I "center" the secondary holder in the OTA by ensuring that all spider vane lengths are equal.  The photo below illustrates what I mean.  Note that in this photo all of the spider vane lengths have been adjusted to be equal in order to perfectly center the secondary holder in the OTA, yet the secondary appears off center as seen through the sight tube.  I have also centered the secondary in the up/down tube axis (as seen through the sight tube) using the center screw on the secondary holder. 

 

1...AcrossTube.jpg

 

I've done quite a bit of research on this, searched all forums, reddit, and have read through all of the recommended guides and it seems that everyone more or less assumes that the secondary will be centered "across" the tube and only advises to center in the up/down tube direction using the center screw on the secondary holder.  Some guides mention to "center" the secondary using the three tilt screws, however my understanding is that these are used to "point" the mirror and not position it so that it is well-centered under the eyepiece (maybe I'm wrong here but I don't see how it would be possible to correct the offset I'm seeing by adjusting the tilt screws).  Some guides have mentioned to adjust the spider vanes in order to center under the eyepiece but when I do this the secondary holder is clearly offset in the OTA as can be seen in the photo below.  Note that the thread engagement is very different on the cross tube vanes and the secondary holder is 1/8" - 1/4" off center in the OTA - this seems to contradict almost every guide I've seen which always starts by instructing to perfectly center the secondary holder in the OTA.

 

2.SpiderVanesNotEqual_MRKP.jpg

 

I've also read a lot about how secondary mirrors are supposed to be offset in fast scopes (this scope is f/5) - however I believe that is referring to something else entirely - correct me if I'm wrong but the outline of the secondary SHOULD appear concentric with the sight tube, correct?

 

I've tried shimming the focuser but even this is not enough to center the secondary in the sight hole - even when I shim the upper side of the focuser to the extent of what the threads will allow.  To shim any more I would actually need to get longer screws - and it seems like I'm shimming beyond where the focuser would be perpendicular to the tube. 

 

3...FocuserTilt.jpg

 

So in short, my questions are: 

 

1: Does it even matter if the secondary is centered as seen through the Cheshire sight tube in the "cross" tube direction? 

2: If #1 is critical then what is the recommended method to center the secondary in this axis?  Is it acceptable for the spider vanes to be unequal in length thus placing the secondary holder in an off-center position in the OTA? 

3: Is there something else at play here that I'm missing?  Possible poor placement of the secondary mirror onto the holder or something else? 

4: Is shimming the focuser until everything is physically centered the answer - even if it is no longer perpendicular to the OTA? 

 

Thanks for bearing with me - I know this is a long post and hopefully I've explained well.  I've been driving myself crazy trying to achieve proper collimation and nothing seems to work or make sense at this point.  Feel like I've scoured the internet looking for an answer so if I've missed some other guide or post that explains this well then please point me in the right direction. 

 

Any advice is very much appreciated. 

 



#2 Vic Menard

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Posted 18 January 2021 - 02:52 PM

...Some guides mention to "center" the secondary using the three tilt screws, however my understanding is that these are used to "point" the mirror and not position it so that it is well-centered under the eyepiece (maybe I'm wrong here but I don't see how it would be possible to correct the offset I'm seeing by adjusting the tilt screws).

You're half right in your thinking here, as I always suggest using the tilt adjustment to align the outgoing laser beam (or to align the sight tube cross hairs) with the primary mirror center marker. But in your case, I believe you have a secondary mirror rotation error, and to correct this error, you'll need to also adjust the secondary mirror tilt. From your picture, the secondary mirror appears to be pointed too high (toward 12 o'clock in your first image). To "center" the secondary mirror, you'll need to adjust the tilt screws to lower it. This will of course change the alignment of the sight tube cross hairs relative to the primary mirror center marker, so, to finish the secondary mirror placement error, you'll need to rotate the secondary to bring the sight tube cross hairs back into alignment with the primary mirror center marker. You may not be able to get the cross hairs accurately aligned with the center marker, so get the center marker aligned on the long axis ("up" tube, "down" tube). Then finish the cross hair alignment using the tilt adjustment screws. Your secondary mirror placement is correct when these three circles are aligned:

the bottom edge of the focuser drawtube (or the bottom edge of the sight tube)

the actual edge of the secondary mirror, and

the reflected edge of the primary mirror (if you can't see the edge of the primary mirror, the alignment of the primary mirror center marker relative to the sight tube cross hairs is the same alignment).

 

1: Does it even matter if the secondary is centered as seen through the Cheshire sight tube in the "cross" tube direction? 

2: If #1 is critical then what is the recommended method to center the secondary in this axis?  Is it acceptable for the spider vanes to be unequal in length thus placing the secondary holder in an off-center position in the OTA? 

3: Is there something else at play here that I'm missing?  Possible poor placement of the secondary mirror onto the holder or something else? 

4: Is shimming the focuser until everything is physically centered the answer - even if it is no longer perpendicular to the OTA? 

1.) Reasonably centered is usually good enough (you could almost certainly use your current secondary mirror placement "as is". This assumes your secondary mirror is large enough to accommodate the small placement error. I'm still scratching my head over the (factory installed) 2-inch focuser!

2.) See above (do NOT make the spider vanes unequal).

3.) It could be some other geometry issue, but combined tilt/rotation errors are the common cold of Newtonian collimation.

4.) Generally speaking, if shimming is required there is some other geometry error that can't be solved with tilt/rotation. 

 

And finally, welcome to Cloudy Nights!


Edited by Vic Menard, 18 January 2021 - 02:54 PM.

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#3 wrvond

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Posted 18 January 2021 - 02:53 PM

You probably shouldn't oughta done that. Centering the vanes in the tube is going to result in a mirror that is not centered on the light cone being reflect by the primary mirror. Your secondary mirror surface should be centered which requires a certain amount of offset of the mirror holder/vanes.

The surest thing to do is call the dealer and ask what the offset is supposed to be for your particular model of telescope, set it and then leave that alone. It is not normally a part of the collimation routine.


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#4 ngc7319_20

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Posted 18 January 2021 - 02:57 PM

 

1: Does it even matter if the secondary is centered as seen through the Cheshire sight tube in the "cross" tube direction? 

2: If #1 is critical then what is the recommended method to center the secondary in this axis?  Is it acceptable for the spider vanes to be unequal in length thus placing the secondary holder in an off-center position in the OTA? 

3: Is there something else at play here that I'm missing?  Possible poor placement of the secondary mirror onto the holder or something else? 

4: Is shimming the focuser until everything is physically centered the answer - even if it is no longer perpendicular to the OTA? 

1. It is not critical that the secondary is centered.  But if it is not, the focal plane might end up tilted, and images might be a little soft at the top and bottom of the eyepiece. (Probably the images at the field edge will be a little soft anyway due to coma in the primary, or eyepiece aberrations.)

 

2. The spider vanes should be equal length, but it is not critical (see 1.)

 

3. There could be many things going on. Are the holes for the spider in the correct places? Does the secondary mirror need to be rotated on the spider hub?  Does the secondary need to be moved towards / away from the primary mirror using the 4 screws on the spider hub? Is the focuser square on the tube in both directions?  Is the sight tube parallel to the focuser (does it wobble?  What if you rotated it?).    Have you checked the aiming of the primary mirror with a laser?

 

4. The focuser should be square to the tube in both directions.  Otherwise the focal plane may be tilted (see 1.).  But it is not critical.

 

The most critical adjustment is the aiming of the primary mirror.  But we have not discussed that.  Usually it is done with a center mark on the primary, and a laser collimator.


Edited by ngc7319_20, 18 January 2021 - 04:14 PM.


#5 Vic Menard

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Posted 18 January 2021 - 03:00 PM

Green circle is optimal (you can see that you're actually pretty close already)...

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Edited by Vic Menard, 18 January 2021 - 03:14 PM.

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#6 Vic Menard

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Posted 18 January 2021 - 03:11 PM

OK, we've had a few more comments. So I'll reiterate--do NOT make the spider vanes unequal. Each vane should be inline with the opposing vane (easy to check with a straight edge). If the manufacturer built in the offset, it will most likely have been done by offsetting the placement of the secondary mirror on the stalk. Also, as long as the sight tube cross hairs (or if you have a laser, the outgoing laser beam) are aligned with the primary mirror center marker, your focuser axial alignment will be correct. This axial alignment is responsible for keeping the focal plane perpendicular to the eyepiece axis. The secondary mirror does not have to be precisely centered or offset, and the focuser doesn't have to be perpendicular to the tube assembly to achieve good focuser axial alignment. Just align the three circles, then fine adjust the secondary mirror tilt (focuser axis), and always last, fine align the primary mirror tilt--and you should be good to go.


Edited by Vic Menard, 18 January 2021 - 03:11 PM.


#7 ShooieLewis

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Posted 18 January 2021 - 03:27 PM

Thanks all for the responses.  

 

Apologies if I'm being a little slow here, but I feel like I'm still getting some conflicting information (or I'm just not understanding correctly).

 

You probably shouldn't oughta done that. Centering the vanes in the tube is going to result in a mirror that is not centered on the light cone being reflect by the primary mirror. Your secondary mirror surface should be centered which requires a certain amount of offset of the mirror holder/vanes.

The surest thing to do is call the dealer and ask what the offset is supposed to be for your particular model of telescope, set it and then leave that alone. It is not normally a part of the collimation routine.

 

This is where I start to get confused about what we're referring to when we talk about "offset" secondary.. I've read various guides that discuss offset ("new" vs "old" etc. - for example http://www.mikehotka...anTelescope.htm ) and even still, they show that concentric circles are the goal.  Even in the guides I've read that DO discuss setting proper offset it sounds like this is done by moving the secondary in/out towards the focuser as well as up/down the tube... NOT in the direction that I centered my secondary (across the tube - please correct my terminology if there is a standard term for this axis).  I did not touch the spider vanes that move the secondary in/out with respect to the focuser.  

 

 

@Vic Menard :

 

Looks like I'm cross posting you at this point -- can't type fast enough!  Commenting on all your posts collectively:  I think I'm following as far as optically aligning everything in terms of crosshairs, centers of circles etc.... what is bugging me is that I can't achieve the concentric green circle you drew without making significant adjustments to the "cross axis?" spider vanes.  If I'm following you though it sounds like that is not necessarily critical, so long as everything else is optically aligned i.e. crosshairs intersecting the mid-point of the various circles?  I'm following you as far as tilting the secondary etc... I hadn't actually tried to collimate anything in photo 1 EXCEPT for trying to center the secondary outline in the focuser.  Would it be acceptable to leave the secondary in that position, and then follow the remaining steps (rotate until round, tilt secondary until primary well centered, then adjust primary until secondary reflection is centered)? 

 

Also - Thanks for the warm welcome.  Seems like a very helpful and welcoming community around here. 

 

@ngc7319_20

 

I think I've got it down as far as collimating the primary mirror using the adjustment screws.. what's concerning me is the fact that secondary is so far off center in the "cross" axis (i.e. up/down when looking through the focuser).  In response to #3 the secondary is secure and I rotated and tilted until the mirror appears round and the primary reflection was centered - I am not positive that the focuser is square on the tube, but as I've played with tilting the focuser I'm concerned with just how much it appears I need to tilt the focuser in order to center the outline of the secondary.
 


Edited by ShooieLewis, 18 January 2021 - 03:34 PM.


#8 SteveG

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Posted 18 January 2021 - 03:41 PM

Thanks all for the responses.  

 

Apologies if I'm being a little slow here, but I feel like I'm still getting some conflicting information (or I'm just not understanding correctly).

 

 

This is where I start to get confused about what we're referring to when we talk about "offset" secondary.. I've read various guides that discuss offset ("new" vs "old" etc. - for example http://www.mikehotka...anTelescope.htm ) and even still, they show that concentric circles are the goal.  Even in the guides I've read that DO discuss setting proper offset it sounds like this is done by moving the secondary in/out towards the focuser as well as up/down the tube... NOT in the direction that I centered my secondary (across the tube - please correct my terminology if there is a standard term for this axis).  I did not touch the spider vanes that move the secondary in/out with respect to the focuser.  

 

 

@Vic Menard :

 

Looks like I'm cross posting you at this point -- can't type fast enough!  Commenting on all your posts collectively:  I think I'm following as far as optically aligning everything in terms of crosshairs, centers of circles etc.... what is bugging me is that I can't achieve the concentric green circle you drew without making significant adjustments to the "cross axis?" spider vanes.  If I'm following you though it sounds like that is not necessarily critical, so long as everything else is optically aligned i.e. crosshairs intersecting the mid-point of the various circles?  I'm following you as far as tilting the secondary etc... I hadn't actually tried to collimate anything in photo 1 EXCEPT for trying to center the secondary outline in the focuser.  Would it be acceptable to leave the secondary in that position, and then follow the remaining steps (rotate until round, tilt secondary until primary well centered, then adjust primary until secondary reflection is centered)? 

 

Also - Thanks for the warm welcome.  Seems like a very helpful and welcoming community around here. 

 

@ngc7319_20

 

I think I've got it down as far as collimating the primary mirror using the adjustment screws.. what's concerning me is the fact that secondary is so far off center in the "cross" axis (i.e. up/down when looking through the focuser).  In response to #3 the secondary is secure and I rotated and tilted until the mirror appears round and the primary reflection was centered - I am not positive that the focuser is square on the tube, but as I've played with tilting the focuser I'm concerned with just how much it appears I need to tilt the focuser in order to center the outline of the secondary.
 

Follow what Vic is telling you. Forget offset altogether - as it will happen naturally when you center the secondary under the focuser (you are already there). In your first picture in post 1, everything is off, but in the second picture labeled “centered”, it is close enough. Leave it there and focus on your secondary tilt, followed by your primary tilt. You should not have to tilt the focuser at all.



#9 ShooieLewis

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Posted 18 January 2021 - 03:54 PM

Follow what Vic is telling you. Forget offset altogether - as it will happen naturally when you center the secondary under the focuser (you are already there). In your first picture in post 1, everything is off, but in the second picture labeled “centered”, it is close enough. Leave it there and focus on your secondary tilt, followed by your primary tilt. You should not have to tilt the focuser at all.

 

Thanks Steve.  So I should not be concerned that in the second photo (labeled centered) - my spider lengths are unequal by over 1/8" and therefore the secondary holder is not actually centered looking down the OTA?  The first photo was only to demonstrate where the outline of the secondary sits when I adjust the spider vanes so that they are equal... the second photo was my attempt at full collimation BUT the spider lengths are drastically different lengths in order to achieve the centering of the outline of the secondary. 



#10 Vic Menard

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Posted 18 January 2021 - 04:11 PM

...in the guides I've read that DO discuss setting proper offset it sounds like this is done by moving the secondary in/out towards the focuser as well as up/down the tube... NOT in the direction that I centered my secondary (across the tube - please correct my terminology if there is a standard term for this axis).

Generally speaking, movement of the secondary mirror closer to or farther from the primary mirror end of the tube assembly are considered to be offset adjustments. If the offset is incorrect, the three circles will not be concentric. (Interesting tidbit--you only really need to know the actual offset distance if you want a "centered" (no offset) secondary mirror placement! This secondary mirror placement requires the actual edge of the secondary mirror to appear offset away from the primary mirror instead of concentric with the other two circles.) 

 

When you see the secondary mirror placement above or below (decentered "across" the tube), you'll also see the silhouette reflection of the secondary mirror (violet circle in attached illustration) bulging above or below (toward 1:30 or 4:30 on a clock face, instead of 3:00--toward the primary mirror end of the tube assembly). Tilt/rotation errors "skew" the secondary mirror presentation above or below the long axis (up or down the tube). 

 

...what is bugging me is that I can't achieve the concentric green circle you drew without making significant adjustments to the "cross axis?" spider vanes.  If I'm following you though it sounds like that is not necessarily critical, so long as everything else is optically aligned i.e. crosshairs intersecting the mid-point of the various circles?  I'm following you as far as tilting the secondary etc... I hadn't actually tried to collimate anything in photo 1 EXCEPT for trying to center the secondary outline in the focuser.  Would it be acceptable to leave the secondary in that position, and then follow the remaining steps (rotate until round, tilt secondary until primary well centered, then adjust primary until secondary reflection is centered)?

As I noted above, your tilt/rotation error is pretty small. You can probably move directly to fine tuning the focuser axis (secondary mirror tilt adjustment) to get the sight tube cross hairs aligned to the primary mirror center marker. This may make your primary mirror reflection seem "off center" relative to the secondary mirror. What's important is that the primary mirror reflection is centered in the focuser (which means your focal plane will NOT be tilted). Many beginners figure it's better if they can see all of the primary mirror clips, so they keep adjusting the secondary mirror tilt to make all of the clips visible--which ruins the focuser axial alignment and guarantees a tilted focal plane! The takeaway here is that the visibility of the primary mirror clips is NOT a valid alignment goal unless the focuser axial alignment is also correct!

 

Your secondary mirror should be centered in your tube assembly, which usually means the spider vanes are all equal (assuming the tube assembly is round).

 

...I think I've got it down as far as collimating the primary mirror using the adjustment screws.. what's concerning me is the fact that secondary is so far off center in the "cross" axis (i.e. up/down when looking through the focuser).  In response to #3 the secondary is secure and I rotated and tilted until the mirror appears round and the primary reflection was centered - I am not positive that the focuser is square on the tube, but as I've played with tilting the focuser I'm concerned with just how much it appears I need to tilt the focuser in order to center the outline of the secondary.

It's actually not "so far off center". It's just that it's painfully obvious--which sends most beginners down the rabbit hole trying to make the primary mirror clips visible, or in your case, trying to make the actual edge of the secondary mirror precisely centered. Fine tune your secondary mirror tilt first (cross hairs aligned to the primary mirror center marker). If I'm seeing the primary mirror center marker in your first image, the tilt adjustment screw closest to the focuser should get the job done. Leave the focuser alone.

 

Forgot the attachment.

Bottom edge of the focuser -- light blue circle

Actual edge of the secondary mirror -- green circle

Reflected edge of the primary mirror -- red circle

Silhouette reflection of the secondary mirror -- violet circle

Reflection of the underside of the focuser -- yellow circle

 

Note that all circles are concentric except the violet circle -- which appears offset toward the primary mirror end of the tube assembly. (Original illustration by Jason Khadder. Color annotations by Vic Menard.)

 

Attached Thumbnails

  • offset2.jpg

Edited by Vic Menard, 18 January 2021 - 04:19 PM.


#11 ShooieLewis

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Posted 18 January 2021 - 05:47 PM

Thanks Vic - That clears a lot up.  I played around with it a little more and somehow I was able to get the secondary fairly centered while simultaneously keeping the spider vane lengths equal.. Not sure if the secondary had bound up in an awkward position or what.  I also removed the shim underneath the focuser per your recommendation so now all adjustments are being done purely with the four secondary screws and the three primary screws.   

 

Here's where I'm at after a little more adjusting.  My cell phone camera doesn't want to keep all planes in focus at once but I think this gets the idea across...thicker crosshairs are the cheshire and the thinner lines are the spider vanes. Thinking if I can get the center mark on the primary slightly down and to the left using the three tilt screws on the secondary (and then adjust the primary as needed) I should be good... am I on the right track? 

 

 

Collimated_SML.jpg



#12 Vic Menard

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Posted 18 January 2021 - 08:29 PM

...am I on the right track? 

 

Yes!  waytogo.gif

 

First, adjust the secondary mirror tilt to center the primary mirror center marker (red circle) relative to the sight tube cross hairs, and then adjust the primary mirror tilt to center the yellow circle (the reflection of the underside of the focuser/bright Cheshire ring) relative the primary mirror center marker (red circle). And that's pretty much all that's left...

 

Take your time, make small adjustments, and your persistence will pay off. 

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Edited by Vic Menard, 18 January 2021 - 08:31 PM.


#13 SteveG

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Posted 18 January 2021 - 11:15 PM

Thanks Steve.  So I should not be concerned that in the second photo (labeled centered) - my spider lengths are unequal by over 1/8" and therefore the secondary holder is not actually centered looking down the OTA?  The first photo was only to demonstrate where the outline of the secondary sits when I adjust the spider vanes so that they are equal... the second photo was my attempt at full collimation BUT the spider lengths are drastically different lengths in order to achieve the centering of the outline of the secondary. 

You certainly want the vanes equal length, and very tight. As Vic noted, they should be straight in line with each other. The spider vanes are not a means of adjusting secondary or primary collimation. They are there to hold the secondary mirror hub firmly in place, that’s all. Make sure the vanes aren’t twisted - they want to be perpendicular to the open tube. This will minimize diffraction spikes.

 

Another hint; In post #11 you show both the spider vanes lined up with the crosshairs in your site tube. I recommend twisting the site tube about 45 degrees, so that you are not confusing or referencing the vanes, as they won’t line up with the crosshairs (on one axis). The picture in post #10 shows what I’m talking about.



#14 ShooieLewis

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Posted 20 January 2021 - 12:12 AM

Thanks all - I really appreciate the advice.  Collimation is looking much better now and I'm seeing a noticeable improvement through the eyepiece.  




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