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Collimation Autopsy

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#1 havieair

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Posted 23 May 2020 - 03:14 PM

Lots of great advice given in the forum recently but didn't want to hijack anyone else's thread.

 

I'm hoping I can get a little help collimating my 16" f4 NMT (full upper cage). I've been referencing Vic Menard's book (I definitely need to give it another thorough reading) as well as the extremely helpful diagrams Asbytec posted in the First Time Collimation Trouble thread https://www.cloudyni...mation-trouble/

 

Here's are the tools I'm using: Catseye Telecat sight tube, HG 1.25" laser & TuBlug (2" to 1.25" HG Parallizer), and the Infinity Autocollimator.

 

I know my collimation is off so hopefully you all can point out where my mistakes are. It's probably a combination of misinterpreting the concepts and inexperience. I'll admit on previous scopes I used to just use a simple laser and randomly start turning adjustment screws but it's time to step up. This has been my approach:

 

Step 1. Verified secondary holder is parallel to the front aperture and centered in the upper cage (slight tightening of spider vanes to get this exact), removed secondary and inserted long bolt into holder hole and, using the laser in the focuser, collimated the focuser base until the laser was hitting the center of the long bolt, and then reinstalling the secondary.

 

Step 2. Using the the sight tube loosen the secondary bolt holder until the secondary is centered longitudinally in the sight tube. After this step I'll slowly tighten the 3 adjustment screws just to to the point where they ever so slightly engage with the secondary clutch disk. After this point I'll grab the secondary and rotate it until it's as concentric as possible with the bottom edge of the sight tube. At this point I haven't further tightened any of the adjustment screws.

 

Step 3. With the sight tube still in the focuser, uncover primary mirror and grab the secondary and slightly rotate it so that the primary hotspot is centered with the secondary major axis. Once this is done tighten only the adjustment screw that's aligned with the the focuser to tilt the secondary so that primary center-mark is centered with the secondary minor axis. While doing this I'll tighten the other 2 adjustment screws the same amount so that I can keep tightening the screw that's aligned with the focuser until finally I have primary mirror hotspot centered with the secondary major and minor axis.

 

Step 4. Insert laser and use the adjustment screws to fine tune laser within hotspot.

 

Step 5. Using tuBlug and laser (with 1mm aperture stop removed), align primary hotspot shadow.

 

Step 6. Fine tune primary and secondary with autocollimator.

 

Step 7. Check collimation results with sight tube.

 

Step 8. Go to fridge and pour a beer because the secondary looks like it has some decent tilt and rotation errors.

 

And just so I'm clear tilt is always performed using the collimation screws and rotation is always done by grabbing hold of the secondary and turning it? 

 

Here's pics of how steps 3-5 turned out. Upper cage is north and primary is south in the photos.

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#2 havieair

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Posted 23 May 2020 - 03:17 PM

Here's a racked in pic of step 5 along with 6 & 7. After using the autocollimator, and then verifying through the sight tube, it appears that the sight tube cross hairs neatly overlap the reflected sight tube cross hairs...but when I illuminate the hotspot with the clip on red light I can see another set of cross hairs that are offset. I can't get them to photograph so I marked up the photo to graphically show how it looks.

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#3 Vic Menard

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Posted 23 May 2020 - 03:31 PM

You're pretty darn close--just a small tilt rotation error--the secondary mirror is "bulging" toward 7 (maybe 7:30) o'clock (the green circle shows the optimal secondary mirror placement). I can't see the full reflection of the primary mirror, but the axial alignments are good. 

 

The collimation is certainly usable "as is".

 

 

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#4 havieair

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Posted 23 May 2020 - 03:45 PM

Thanks for the markups Vic! Great to know I'm close. For some reason I struggle to get the full primary reflection. At most I've been barely able to get 2 out of the 3 primary mirror clips in view.



#5 Vic Menard

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Posted 23 May 2020 - 03:48 PM

Here's a racked in pic of step 5 along with 6 & 7. After using the autocollimator, and then verifying through the sight tube, it appears that the sight tube cross hairs neatly overlap the reflected sight tube cross hairs...but when I illuminate the hotspot with the clip on red light I can see another set of cross hairs that are offset. I can't get them to photograph so I marked up the photo to graphically show how it looks.

It looks like a parallax error to me--although it could also be a small tilt/registration error between the TeleCat and the focuser drawtube--or both. That's why I normally use a good thin beam laser (or an autocollimator) in conjunction with an equally good sight tube--sight tube for secondary mirror placement, thin beam laser (or autocollimator) for focuser axial alignment.

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#6 Vic Menard

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Posted 23 May 2020 - 03:50 PM

Thanks for the markups Vic! Great to know I'm close. For some reason I struggle to get the full primary reflection. At most I've been barely able to get 2 out of the 3 primary mirror clips in view.

Shorten the TeleCat (about 1/2-inch?) so you can rack the focuser closer in.



#7 havieair

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Posted 23 May 2020 - 05:44 PM

I'll try shortening the the TeleCat and check if I can get the full reflection of the primary in view. If I understand correctly, I would need the reflected edge of primary, the secondary mirror edge, and sight tube to all be concenctric in order to see the full reflection of the primary. As of now I'm able to get snap focus so I'll take that as win.



#8 Vic Menard

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Posted 23 May 2020 - 08:34 PM

I'll try shortening the the TeleCat and check if I can get the full reflection of the primary in view. If I understand correctly, I would need the reflected edge of primary, the secondary mirror edge, and sight tube to all be concenctric in order to see the full reflection of the primary. As of now I'm able to get snap focus so I'll take that as win.

Just to clarify, making the three circles concentric (the bottom edge of the sight tube, the actual edge of the secondary mirror, and the reflected edge of the primary mirror) delivers centered and balanced field illumination. Getting the axial alignments corrected (to the prescribed tolerance) delivers image performance and snap focus--which is kind of what I said in post #3.  ubetcha.gif


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#9 Asbytec

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Posted 24 May 2020 - 08:10 AM

Havieair, 

 

As Vic said, your scope looks very well collimated to me in the one image I downloaded. Trying to figure out where that residual tilt came from. Now, I do defer to Vic on the problems with single axis collimation, because there are some small issues. But we can deal with them. 

 

Here's what I am thinking. You mentioned getting the primary reflection (hotspot) onto the secondary major axis, then tilt a single screw moving along the major axis until the hot spot was centered on the minor axis, too. Ideally that would be where the site tube crosshairs appear to intersect, too, with a perfectly placed secondary. But, what we're looking for at this stage is focuser axial alignment defined by the site tube crosshairs, not the secondary axes. So, I might suggest moving the hotspot onto the cross hairs instead of eyeballing the major and minor axes. As your are moving the hotspot along (or parallel to) the major axis, if you miss the hotspot you back off and refine your rotation until that single inline tilt brings the hot spot onto the crosshair as close as you like. That is focuser axial alignment regardless of secondary position which should be close because you put it there.

 

Now your focuser axis is collimated, but not necessarily the best secondary position you can achieve. However, interestingly and importantly, now that the focuser axis is pointing at the well centered hotspot, then the reflected edge of the primary should (necessarily) be concentric with the focuser draw tube regardless of the secondary position. This is true because the center of the site tube (cross hair) is on the center of the primary (hotspot). These are two of our collimation signatures, as well: draw tube and reflected primary edge. So we can use these signatures to refine the secondary position (also a signature) to them from its initial position set above. This is the beauty of refining your collimation at the apex, any error becomes very apparent.  

 

With the focuser axis collimated, focus out to the apex where the reflected edge of the primary appears the same size (or slightly smaller) than the secondary mirror. Any deviation in your secondary position will become very apparent against the - now collimated - reflected mirror edge (and focuser draw tube). This is why we sometimes mention seeing the edge clips. They are not a collimation signature, but they are a proxy for the reflected edge of the primary. While your focuser axis is still collimated to the hotspot, you can see any deviation between the secondary (one signature) and the primary edge which is also concentric with the focuser draw tube (the other two signatures). Refine your secondary position using fore and aft and rotation - and even a little tilt - as needed until the edge of the secondary coincides with the edge of primary reflection when the focuser axis is, again, collimated to the hotspot. Your mirror clips will be easily visible all around and you should have better secondary position. 

 

Of course adjusting the secondary position will mess up your initial collimation. That's okay. We'll bring it back after refining the secondary position (as above) to the other two signatures. This time with a little better secondary position rotate the hotspot back onto the secondary centerline. Loosen the one screw, refine rotation, and repeat the single axis secondary alignment at the apex. Tilt the hotspot onto the crosshair along the secondary major axis. Tighten one screw, loosen the other two or visa versa. If you miss it. Back off, refine your rotation a little, then tilt the secondary along that axis, again, until you hit the center mark with the crosshair.

 

This time, with better secondary position (relative to the other two signatures) you will notice a beautiful thing as you bring the hotspot back onto the crosshair. All three collimation signatures will come together at the same time. Draw tube, (refined position of the) secondary, and lastly the reflected primary edge as the focuser axis is collimated. It's like magic when you see it happen and your secondary offset will point at the primary mirror, too. It'll look as good as it performs. smile.gif

 

So, yea, I think the one step you are missing (probably my fault) is refining your signatures at the apex. When the focuser axis is collimated to the primary center, both the draw tube and the primary reflection will be concentric with each other. We can use those concentric signatures to refine the secondary position so it is also concentric with both (when the focuser axis is collimated). I think that is an important step you missed above. I only briefly mentioned earlier. This is the magic part...and how I refine collimation in 10 minutes prior to each observing session. You can too. It really is easier than it sounds. lol.gif

 

Step 4. Insert laser and use the adjustment screws to fine tune laser within hotspot.

 

The other thing that catches my eye, after all the trouble holding onto your signatures above, is using the secondary collimation screws again to adjust the tilt of the secondary in step 4 may have induced some unwanted tilt and rotation. A small amount. If you used the three screws independently, then I am sure of it. The two orthogonal screws are responsible as they induce off axis tilt of the secondary and they also tend to rotate the secondary a little bit with off axis torque applied to the secondary.

 

This was my biggest problem using a site tube, too, I kept retweaking the secondary as I was trying to collimate the focuser axis. I could see the tilt and rotation looking down the site tube, then I'd try to correct it in a seemingly endless battle fighting to hold the secondary position, rotation, and signature while trying to achieve collimation. You cannot see this happening with a laser, but you can with a site tube. The result is frustration.

 

This is why I prefer the single axis collimation, so much easier. The single screw on axis greatly reduces those errors. You can do it with a laser, too, but it's hard to avoid the temptation of using all three screws independently coming at collimation form three different directions. One direction is actually easier and it really helps hold onto your collimation signatures we worked so hard to achieve. 

 

Sorry for the long read...


Edited by Asbytec, 24 May 2020 - 08:29 AM.

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#10 havieair

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Posted 24 May 2020 - 11:54 AM

I appreciate the response Asbytec. It'll take me a minute to digest your info and then later today I'll give this another shot to see if I can clean up the residual tilt.


Edited by havieair, 24 May 2020 - 11:57 AM.


#11 Asbytec

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Posted 24 May 2020 - 08:43 PM

I appreciate the response Asbytec. It'll take me a minute to digest your info and then later today I'll give this another shot to see if I can clean up the residual tilt.

I apologize if it sounds difficult. It's really not, just difficult to explain. Often refining collimation and secondary position at the apex is mentioned in the collimation steps. I'm just describing a little different way to finish that step (unfortunately) in great detail the best I could.

The take away when stripped from all the verbiage is to understand when the site tube crosshair is centered on the primary center (hot-spot), the focuser draw tube and primary edge are concentric because their centers coincide.Those are two of the collimation signatures you are trying to achieve. 

At the apex, the reflected primary edge and secondary appear the same size. Their edges should be concentric too. But if not, then you can reposition the secondary to the primary edge so all three edges are concentric (when the focuser axis is collimated). The secondary edge is the other signature you are trying to achieve.

Then align the focuser axis in a way to hold the three edge signatures. I prefer rotating the secondary and single directional approach to focuser alignment because we induce almost no unwanted tilt and rotation error caused by the other two screws working independently and orthogonal to the axis. (You can use a little orthogonal tilt, if needed, without much tilt or rotation error. No worries).

 

You seemed to do good getting some secondary placement and collimation, then used just about every tool in the book to refine your collimation. In the process of laser collimation, you probably tilted the secondary a little causing it to move away from focuser center without seeing it happen until you looked down the focuser.

 

The result is you achieved great collimation, which is what you want, but the secondary probably tilted and rotated a bit. This is caused by using the two orthogonal screws independently as we all have a tendency to do. It's intuitive to do so. A Vic explained above, residual rotation and tilt error does not affect axial alignment. It only affects the less critical fully illuminated field. 


Edited by Asbytec, 24 May 2020 - 09:26 PM.


#12 havieair

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Posted 24 May 2020 - 10:27 PM

Ah thanks for the breaking it down like that - it's starting to click. Feeling pretty good about this one. I'm not 100% sure I can get the full primary view reflected in the secondary though. Maybe a 3.1" secondary for a 16" f4 is cutting it close. Even with an empty focuser it's hard to see the full primary view.  You can see the 2 mirror clips @ 10:30 & 1:30...the other one is just slightly out of view.

 

If only it wasn't raining tonight.

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#13 Asbytec

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Posted 25 May 2020 - 03:18 AM

I'm not 100% sure I can get the full primary view reflected in the secondary though. Maybe a 3.1" secondary for a 16" f4 is cutting it close. Even with an empty focuser it's hard to see the full primary view.

 

Ah, okay, I see you're using the Telecat Site tube, probably tuned to f/3 to f/5, I presume. That must be it reflected in the primary mirror. I believe you can still center the hotspot and know the primary edge is concentric with the focuser draw tube. It might be more difficult to see the actual edge of the primary, then, to place the secondary to the difficult to see primary edge. Especially when you are focused all the way down (the primary reflection should be smaller than the secondary in most applications), and all you can see is barely the edge of the clips(?). I presumed the secondary would allow it for full field illumination. Sorry about that. 


Edited by Asbytec, 25 May 2020 - 03:19 AM.


#14 Starman1

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Posted 25 May 2020 - 01:33 PM

I hate to point out the obvious, but if the view through the pupil of the TeleCat does not show the entire primary when the focuser is racked in,

then neither does an eyepiece.  That means the secondary mirror is too small.

My calculations show the smallest secondary for a 16" f/4 with a typical secondary-to-focal plane of 12.5" is a 3.5" mirror.

[8" radius + 0.75" clearance + 1" to bottom of focuser + 1.75" for a low profile focuser + 1" to focal plane]

So if it is 3.1", you'll be well-served to replace the secondary with something larger.

Probably one of those "I read it needs to be 20% or less" construction protocols.  Sigh.


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#15 Vic Menard

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Posted 25 May 2020 - 03:07 PM

...I'm not 100% sure I can get the full primary view reflected in the secondary though. Maybe a 3.1" secondary for a 16" f4 is cutting it close. Even with an empty focuser it's hard to see the full primary view.

Three questions:

Is your focuser fully racked in?

Is your TeleCat mounted flush (fully inserted) to the drawtube shoulder?

Did you shorten the TeleCat?

 

(For the record, your axial alignments look good and your tilt/rotation error seems to be mostly corrected!)


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#16 havieair

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Posted 25 May 2020 - 06:53 PM

Three questions:

Is your focuser fully racked in?

Is your TeleCat mounted flush (fully inserted) to the drawtube shoulder?

Did you shorten the TeleCat?

 

(For the record, your axial alignments look good and your tilt/rotation error seems to be mostly corrected!)

Yes, to questions 2 & 3 but no to your first question -- wow I should've realized this. I have a filter slide mounted to the inside of the upper cage which prevents the last 5mm or so of inward travel.  



#17 Vic Menard

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Posted 26 May 2020 - 08:56 AM

I've done a cursory search online for your scope's specifications and all I can find is a 3.5-inch minor axis secondary mirror. Are you sure yours is 3.1? Also, on the NMT website, the NMT 16 is shown with a MoonLite focuser that appears to be mounted on a riser block--is yours configured this way?

 

If your focuser is fully racked in, the TeleCat is fully inserted to the drawtube shoulder, you shortened your TeleCat by 1/2-inch, and you still can't see the entire primary mirror reflection on the face of the secondary mirror, I think Don Pensack may have drawn the right conclusion.

 

Do you know the distance from the secondary mirror to the focal plane?




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