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Can't Get Secondary Mirror Where It Needs To Be

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#1 Cladinator

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Posted 25 September 2020 - 10:08 PM

My Skywatcher 8 Collapsible dob showed up and I put everything together. Time to collimate.

Now, let me start by saying I have collimated my Z12 plenty of times without issue. 

This secondary was tough from the beginning. It needed some adjusting but the screws were already really tight. 

Long story, short: my entire secondary has moved (due to the larger center screw) and now I have to figure out how to get it back in the correct position. 

I can't see a dot near the center donut anymore. I can't even really see it on the mirror at all. 

I am using a laser collimator. I just ordered some new batteries for it as I think they may be on the way out. I want to start fresh with some fresh batteries because it's possible that has something to do with it. 

Any help would be greatly appreciated. I'm basically a noob now as it's been a really long time since I observed so I am out of practice. But there was a time when I knew what I was doing. 

 

Thanks.



#2 wrnchhead

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Posted 25 September 2020 - 10:57 PM

I moved from a Z series (An Apertura, but same thing) to the SW Flextube also and collimation was definitely much more fiddly with the SW. Do you have the secondary aligned under the focuser? There are tons of how-tos on CN and the internet in general. That's not something you can do with the laser. If you don't have a cheshire/sight tube combo, get one. But for now you can put a piece of paper behind the secondary and eyeball it best you can. 

 

Something else that's important, make sure your laser is collimated itself. It's basically a laser pointer in an aluminum tube, and three allen set-screws control it's alignment. Rest it in something where you can spin it in place and see if the laser draws a circle on a wall a good 10 feet (more is better) away. If it does, it's out of alignment. Use the three screws to change it's position till you get it where it doesn't draw a circle on the wall. (if you've never done this, it takes some practice, be patient with yourself.) 

 

A good well recommended article on collimation- https://garyseronik....to-collimation/



#3 Cladinator

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Posted 25 September 2020 - 11:06 PM

I moved from a Z series (An Apertura, but same thing) to the SW Flextube also and collimation was definitely much more fiddly with the SW. Do you have the secondary aligned under the focuser? There are tons of how-tos on CN and the internet in general. That's not something you can do with the laser. If you don't have a cheshire/sight tube combo, get one. But for now you can put a piece of paper behind the secondary and eyeball it best you can. 

 

Something else that's important, make sure your laser is collimated itself. It's basically a laser pointer in an aluminum tube, and three allen set-screws control it's alignment. Rest it in something where you can spin it in place and see if the laser draws a circle on a wall a good 10 feet (more is better) away. If it does, it's out of alignment. Use the three screws to change it's position till you get it where it doesn't draw a circle on the wall. (if you've never done this, it takes some practice, be patient with yourself.) 

 

A good well recommended article on collimation- https://garyseronik....to-collimation/

 

Thanks for the response. I actually just mostly successfully aligned the secondary under the focuser. I just used some intuition and eyeballed it. The laser was right there near the donut so I'm getting closer now. The laser got dim over several minutes so I think it actually does need new batteries so I'm glad I ordered them. 

 

As far as the laser collimator itself, yes, it's been collimated. I learned that lesson back when I first got my Z12. 



#4 wrnchhead

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Posted 25 September 2020 - 11:16 PM

Ok great!



#5 Cladinator

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Posted 26 September 2020 - 06:38 AM

Ok, so I think I got it fairly dialed in but it's probably not 100%.

I did a star test. It was mostly a well-defined disk when taken out of focus except at the bottom of the disc it looked like a half-circle was notched out. 

I'm actually surprised I got that far. The laser battery was dying and there was a little slop in the focuser tube (I'm sure that's not the correct word for that) that I think may have affected it. I hastily aligned my Rigel Quick Finder and the right angle finderscope. I used a Hyperion Baader MK III 8mm-24mm zoom.

 

Not that bad of a night. I saw:

M1, M31, M33, M42, M45, Mars, possibly Uranus, and possibly some other things that I can't remember at the moment. 

 

All in all, I'm pretty happy. I'll need to take a little more time with collimation once I throw some new batteries in my laser. It wouldn't hurt to get the finderscope and the quickfinder aligned with the scope better. 

Mostly, I just wanted to get it out and make sure it's good-to-go. I'm satisfied. Huge bonus to see some objects that I haven't seen in a while. 


Edited by Cladinator, 26 September 2020 - 07:07 AM.


#6 Asbytec

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Posted 26 September 2020 - 07:38 AM

"I did a star test. It was mostly a well-defined disk when taken out of focus except at the bottom of the disc it looked like a half-circle was notched out."

Was the star tested near the center of the field? Notched or flattened? If it was flattened half circle, it sounds like some vignetting on the secondary related to its position.

Put a white paper background inside the tube opposite the focuser to allow us to see the secondary position. Focus down far enough to see the primary reflection in the secondary and take a picture.

Edited by Asbytec, 26 September 2020 - 07:41 AM.


#7 Cladinator

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Posted 26 September 2020 - 08:12 AM

"I did a star test. It was mostly a well-defined disk when taken out of focus except at the bottom of the disc it looked like a half-circle was notched out."

Was the star tested near the center of the field? Notched or flattened? If it was flattened half circle, it sounds like some vignetting on the secondary related to its position.

Put a white paper background inside the tube opposite the focuser to allow us to see the secondary position. Focus down far enough to see the primary reflection in the secondary and take a picture.

Thanks for the response. I suspect you're probably right.
Hopefully, I figured out how to upload images here and it works. 
 
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#8 Asbytec

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Posted 26 September 2020 - 09:25 AM


Thanks for the response. I suspect you're probably right.
Hopefully, I figured out how to upload images here and it works.

Okay, nice images. To upload images in this forum, go to "More Reply Options" button at the lower right of your reply, then "Choose File" and "Attach This file". You can also "Add to Post".

You can see a little vignetting indicated by the "missing" part of the primary mirror not seen in the secondary. One or more clips are (or may be) missing as is the edge of the primary reflection. This is especially true if your focal plane is focused a little higher up and part of the apparent diameter of the primary extends beyond the edge of the secondary. (This is evident in your first image taken well outside the focuser, but your second image looks fine).

Your secondary is not really centered, you can see it appears to be tilted downward as seen in your image. The bottom edge is closer to the bottom of the draw tube. That's fine. Tilt it up in the direction of the blue arrow until it is closely centered on or near where the second inner red ring is seen. Tilting it up will require you rotate the secondary a little. That's okay, too.

You will also notice the secondary appears to be elliptical along 10 and 4 o'clock. After you tilt the secondary upward onto the second red ring, rotate your secondary so the primary reflection is centered "up and down" in the secondary. The secondary should be rotated so the primary reflection is also concentric with the second inner red circle. That rotation will help alleviate the elliptical appearance of the secondary and improve rotation "error".

Edit: To rotate your secondary I recommend loosening a single secondary adjustment screw in line with the focuser axis. Loosen it just enough you can rotate it be hand. I like the idea of rotating the secondary so the primary center mark falls on the secondary major axis. This ensures the primary reflection is centered "up and down". Then tighten that screw.

You could nudge the secondary a little to the right, if you need to. One way to do that is work slowly by slightly loosening the center bolt and tightening the three secondary adjustment screws. The screws should retain enough tension to hold the secondary as you gradually move it toward the primary. Repeat the small incremental movement until the secondary is equal distance from the focuser draw tube "left and right".

If you collimate your focuser axis at this point, you can evaluate your secondary position relative to the well collimated primary reflection which will necessarily be pretty much centered under the focuser. The primary reflection and secondary should be pretty much concentric with each other, and the entire primary and clips seen centered within the secondary. This means the secondary should also be pretty much centered under the focuser, too.

I cannot really make out what is happening in the dark elliptical secondary silhouette. If you primary is center marked, it should be visible in that dark area. You are using a laser, but what do you use to collimate your primary? The same laser or a collimation cap? Regardless, if you can better place and rotate your secondary, try your normal laser routine and take another picture. Or another star test.

Untitled.png

Edited by Asbytec, 26 September 2020 - 09:43 AM.

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#9 Cladinator

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Posted 26 September 2020 - 11:07 AM

Okay, nice images. To upload images in this forum, go to "More Reply Options" button at the lower right of your reply, then "Choose File" and "Attach This file". You can also "Add to Post".

You can see a little vignetting indicated by the "missing" part of the primary mirror not seen in the secondary. One or more clips are (or may be) missing as is the edge of the primary reflection. This is especially true if your focal plane is focused a little higher up and part of the apparent diameter of the primary extends beyond the edge of the secondary. (This is evident in your first image taken well outside the focuser, but your second image looks fine).

Your secondary is not really centered, you can see it appears to be tilted downward as seen in your image. The bottom edge is closer to the bottom of the draw tube. That's fine. Tilt it up in the direction of the blue arrow until it is closely centered on or near where the second inner red ring is seen. Tilting it up will require you rotate the secondary a little. That's okay, too.

You will also notice the secondary appears to be elliptical along 10 and 4 o'clock. After you tilt the secondary upward onto the second red ring, rotate your secondary so the primary reflection is centered "up and down" in the secondary. The secondary should be rotated so the primary reflection is also concentric with the second inner red circle. That rotation will help alleviate the elliptical appearance of the secondary and improve rotation "error".

Edit: To rotate your secondary I recommend loosening a single secondary adjustment screw in line with the focuser axis. Loosen it just enough you can rotate it be hand. I like the idea of rotating the secondary so the primary center mark falls on the secondary major axis. This ensures the primary reflection is centered "up and down". Then tighten that screw.

You could nudge the secondary a little to the right, if you need to. One way to do that is work slowly by slightly loosening the center bolt and tightening the three secondary adjustment screws. The screws should retain enough tension to hold the secondary as you gradually move it toward the primary. Repeat the small incremental movement until the secondary is equal distance from the focuser draw tube "left and right".

If you collimate your focuser axis at this point, you can evaluate your secondary position relative to the well collimated primary reflection which will necessarily be pretty much centered under the focuser. The primary reflection and secondary should be pretty much concentric with each other, and the entire primary and clips seen centered within the secondary. This means the secondary should also be pretty much centered under the focuser, too.

I cannot really make out what is happening in the dark elliptical secondary silhouette. If you primary is center marked, it should be visible in that dark area. You are using a laser, but what do you use to collimate your primary? The same laser or a collimation cap? Regardless, if you can better place and rotate your secondary, try your normal laser routine and take another picture. Or another star test.

attachicon.gifUntitled.png

 

Thanks for all that. It's a lot to take in. 

I'm definitely going to try all that. It'll definitely be good for the practice. I just ordered some Bob's Knobs so I'll be doing this all over again once they arrive. I'm not really a fan of those little hex screws.

 

Yes, I use the laser for collimating the primary as well.


Edited by Cladinator, 26 September 2020 - 11:31 AM.

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#10 wrnchhead

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Posted 26 September 2020 - 03:06 PM

Dang, mine came with Bob's knobs (which I do not care for) so I put hex ones in. Haha we could have just traded!


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#11 Cladinator

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Posted 26 September 2020 - 08:57 PM

Dang, mine came with Bob's knobs (which I do not care for) so I put hex ones in. Haha we could have just traded!

Haha, no way.

I think the locking and collimation screws for the primary aren't bad at all. But I figured I'd get them since I was definitely replacing the secondary screws. 

Those are pretty bad. One in particular bites into the allen wrench too much and I have to practically yank it out. That makes aligning the secondary pretty difficult.



#12 Cladinator

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Posted 27 September 2020 - 12:13 AM

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Ok, so the batteries for the laser arrived and I can say with certainty that's where my biggest problems stem from. 

Collimating after replacing the batteries was a breeze. I could actually see a bright laser dot. 

Had I used fresh batteries from the start I wouldn't have thrown my secondary out of whack so much. So, lesson learned.

 

I realigned my secondary and I'm fairly confident it's as good as you can get it without a precision instrument (I eyeballed it). Using the set screws was still a pain but it was much easier with a bright dot. 

I made a few adjustments to the primary and then locked it down. Very quick, very simple 

I can't go out and test it tonight. The weather has seen to that. But I'm confident it will be more dialed in than my first attempt with the old laser batteries. Part of that problem was thinking it was way more out of whack than it was. A dim, fuzzy laser will do that.


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#13 Vic Menard

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Posted 27 September 2020 - 10:57 AM

Looking at your latest images, it's pretty obvious that none of them have the camera precisely centered over the focuser. I'm guessing you adjusted the camera "centering" to show the full reflection of the primary mirror in the secondary mirror. That would seem to indicate that while your outgoing laser beam alignment (secondary mirror tilt) may be correct, your secondary mirror placement is probably not. The good news is that secondary mirror placement errors have little or no impact on visual image performance. So I wouldn't worry too much about that. 

 

IF you're using the laser's return beam to align the primary mirror tilt, it's very important that the outgoing laser beam alignment should be near perfect (laser dot carefully centered in the primary mirror center marker). Then the return beam has some validity, and as long as it comes back to the center of the laser target, your primary mirror alignment should be good enough to deliver good image performance.

 

(When you're ready to tackle the secondary mirror placement, I would suggest including a collimation cap like this: https://agenaastro.c...t-eyepiece.html  and with your laser, this procedure:

https://www.cloudyni...ment/?p=5260727  )


Edited by Vic Menard, 27 September 2020 - 10:59 AM.


#14 Cladinator

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Posted 27 September 2020 - 09:19 PM

Looking at your latest images, it's pretty obvious that none of them have the camera precisely centered over the focuser. I'm guessing you adjusted the camera "centering" to show the full reflection of the primary mirror in the secondary mirror. That would seem to indicate that while your outgoing laser beam alignment (secondary mirror tilt) may be correct, your secondary mirror placement is probably not. The good news is that secondary mirror placement errors have little or no impact on visual image performance. So I wouldn't worry too much about that. 

 

IF you're using the laser's return beam to align the primary mirror tilt, it's very important that the outgoing laser beam alignment should be near perfect (laser dot carefully centered in the primary mirror center marker). Then the return beam has some validity, and as long as it comes back to the center of the laser target, your primary mirror alignment should be good enough to deliver good image performance.

 

(When you're ready to tackle the secondary mirror placement, I would suggest including a collimation cap like this: https://agenaastro.c...t-eyepiece.html  and with your laser, this procedure:

https://www.cloudyni...ment/?p=5260727  )

 

Thanks for the response. I suspect you're right. Although, I think my secondary is probably really close to where it should be after adjusting, I think ultimately I'll want to get a collimation cap like you suggested. I think it's probably as good as it can get without some type of instrument at the moment.

The camera definitely isn't centered over the focuser. That's more difficult than it looks. 

I am using the laser for the primary as well. The weather isn't cooperating tonight but I think tomorrow should be good enough to test out the collimation.

 

I really like the step by step process described in the link you posted. I think I might actually get a sight tube and cheshire for future collimation but it's probably a good idea to know how to use all of these methods. I think I've relied solely on a laser for too long. 


Edited by Cladinator, 27 September 2020 - 09:26 PM.



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