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Unable to align secondary mirror to focuser tube

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#51 patrickt

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Posted 25 May 2022 - 08:52 AM

   This is what i have managed so far. I apologize for the poor picture quality. I just popped  a cheap webcam from  my desktop, then photograped the screen with my. cellphone.

 

   As i said earlier, the views through this arrangement pretty much gives the same views throughy my diy collimation cap. This time though i have the primary mirror in sight. You might have a hard time making out the diy central doughnut (colored red) on the primary mirror, so i encircled it in yellow. Two pics here, one withought the yellow circle, and one with the central doughnut encircled in yellow.

 

 

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Edited by patrickt, 25 May 2022 - 08:55 AM.


#52 Asbytec

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Posted 25 May 2022 - 09:55 AM

Hmm. Is this the same view through your collimation cap? If so, you can tell the diagonal should tilt upward toward the center of the focuser.

It really helps to have a reference for the focuser axis, like a site tube cross hair or collimated thin beam laser. Otherwise you have to eyeball it the old fashioned way using your webcam. The scope is f/10, right?

I can tell your focuser axis is not aligned to the primary center. Otherwise the primary reflection would be centered under the focuser draw tube. It's not. The primary reflection is almost centered on the diagonal which is, itself, not centered under the focuser. Both need to move upward toward the center of the focuser.

If the primary reflection is centered under the focuser, then it won't be completely visible in the diagonal mirror. But, if you can tilt the diagonal up toward the focuser center, then rotate the diagonal to recenter the primary reflection you'll be a lot closer.

Is the chip centered in the webcam or offset? You can see the reflection of the webcam. If it's pretty much centered in the camera, then the yellow circle should be over the chip for better primary alignment. Does the scope have primary collimation screws? (I don't recall, or it's a dumb question). Anyway, the camera and collimation cap should agree.

But first, tilt the diagonal upward toward the primary center. Then rotate it to recapture the primary reflection.

Edited by Asbytec, 25 May 2022 - 10:05 AM.


#53 Vic Menard

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Posted 25 May 2022 - 10:50 AM

This may help:

 

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#54 patrickt

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Posted 26 May 2022 - 09:16 AM

ASBYTEC - 

                 1) Yes, same view through the collimation cap. I made it a point to go back and forth from collimation cap to webcam view. Obviously the views will not be perfectly the same, but this being an eyeballing effort, should be close enough. 

 

                 2) I for now, only have the collimation cap, but plan on getting a combo Cheshire eyepiece sometime. Yep an f/10.

 

                  3)  Yes, the obvious thing to me is the diagonal not being centered under the focuser. I also realize both the diagonal and primary mirror reflection  need to move up. The green x-y axis lines of Vic Menard make it obvious.

 

                    4)  You said,  " If the Primary reflection is centered under the focuser, then it wont be completely visible in the diagonal mirror".  Is this because the diagonal mirror's axis itself is not centered under the focuser's central axis ?  But yes, as you recommended, i will work on the diagonal mirror's tilt and rotation.

 

                     5) Yes, the chip is centered in the webcam. And yes, the Primary does have the "Push-Pull" alignment screws.    I need to cover up the Time Tunnelish rings in front though, just because they emit a blue light that i think causes the "stained" view of the central doughnut...

 

                    6) I definitely will work on the diagonal tilt and rotation.   Thanks again for being thorough and clear.

 

 

VIC MENARD -

                           Thanks for the cross and circles. I am just not techie enough.. But i will learn one of these days...

 

 

 

    

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Edited by patrickt, 26 May 2022 - 09:17 AM.

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#55 DAVIDG

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Posted 26 May 2022 - 09:34 AM

 Everyone missed one very important point, the mechanical axis of the focuser MUST be aligned so it passes through the center of the tube. Your focuser is tilted and that is why your getting the miss alignment your seeing now.  It is easy to have this problem with a large focuser and smaller diameter metal tube were the tube is not perfectly round and can also flex. 

 

                          - Dave 

 

misaligned focuser.jpg


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#56 Asbytec

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Posted 26 May 2022 - 09:57 AM

Everyone missed one very important point, the mechanical axis of the focuser MUST be aligned so it passes through the center of the tube. Your focuser is tilted and that is why your getting the miss alignment your seeing now. It is easy to have this problem with a large focuser and smaller diameter metal tube were the tube is not perfectly round and can also flex.

- Dave

misaligned focuser.jpg

Yes, I was thinking the same thing. In my view, tilting the focuser to capture the secondary is the same as tilting the secondary to the focuser center. A lot depends on the mechanics as shown in your graphic. If one doesn't have enough travel, then tilt the other. So long as the spider and diagonal are reasonably well centered in the tube.

Is this because the diagonal mirror's axis itself is not centered under the focuser's central axis ?


Yes. As Jason and Vic might say, and I hope I understand well enough, think of the diagonal as a window on the primary. If the diagonal is centered under the focuser and tilted and rotated properly, then you can see the primary through the window.

This may be more than needs to be said, but your off center diagonal is properly tilted and rotated at its off center placement to see the primary. It's just not under the focuser. So, if you tilt the diagonal upward to focuser center, you will have to tilt and rotate the diagonal mirror again to see the primary again.

Probably self evident, but for every diagonal position there is a tilt and rotation solution to see the primary.

Edited by Asbytec, 26 May 2022 - 10:59 AM.

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#57 patrickt

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Posted 26 May 2022 - 04:21 PM

DAVIDG -

                  Hmm.. i admittedly / wrongly assumed the mechanical axis of the focuser would be aligned through the center of the tube, but your right of course, its not a perfectly round tube.  Thanks for mentioning this.

 

                   But i also see Asbytec's idea about tilting the secondary to the focuser to accomplish the same thing, or not (?). Regardless, i should probably / maybe check on the focuser tube central axis to center of tube alignment (for good measure).  So how do i go about that ? Feel free to just give me any links if you prefer. 

 

ASBYTEC -

                Your window illustration is clear enough. And as i said, i will work on the secondary tilt - rotation. As a newbie to Newt collimation, i can never really hear "too much", or be reminded of this and that,  so thank you.   

 

              Ok, i gotta get some shuteye, i've been up since 2:30 checking out the four wonderful planets, mostly Jupiter, just to see what my less than perfectly collimated 4 inch newt is capable of...while the weather permits... I am satisfied, but am really curious and excited to see what she can do when i get her collimation improved further .I'll get back ro you guys.


Edited by patrickt, 26 May 2022 - 04:22 PM.


#58 SteveG

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Posted 26 May 2022 - 05:33 PM

DAVIDG -

                  Hmm.. i admittedly / wrongly assumed the mechanical axis of the focuser would be aligned through the center of the tube, but your right of course, its not a perfectly round tube.  Thanks for mentioning this.

 

                   But i also see Asbytec's idea about tilting the secondary to the focuser to accomplish the same thing, or not (?). Regardless, i should probably / maybe check on the focuser tube central axis to center of tube alignment (for good measure).  So how do i go about that ? Feel free to just give me any links if you prefer. 

 

 

Do you have a laser? Most use a laser - Remove the secondary or spider assembly, put the laser in the focuser, and measure down the open tube to where the laser exits on the inside at the focuser. Now measure the opposite side of the open tube - it should be the same distance.



#59 apfever

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Posted 26 May 2022 - 06:01 PM

If the end of the tube is flat and squared to the center line. 

 

End rings can be crooked.

I've found main tube material tends to be fairly square, but sometimes the machinist has been watching the barber pole too long. 


Edited by apfever, 26 May 2022 - 06:09 PM.


#60 patrickt

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Posted 26 May 2022 - 08:50 PM

STEVEG - 

             No i dont have one. Not in my budget yet... Especially since its my understanding that some of the affordable lasers may not be properly aligned to its own eyepiece case (??) . 

 

           You mention measuring from both sides of the tube from the laser, which i imagine entails using a flexible measuring tape, or something similiar. This i imagine can add to error in measurement, due to the measuring tape and/or the person measuring.         

 

             Is the "average degree of error" from a less than accurate laser collimator plus the "average degree of error" from doing the measuring considered negligible for proper aligning of the focuser central axis to the center of the tube. I asked because i absolutely have no experience with any laser collimator.

 

APFEVER -

                   Well, nothing much i can do about the End Rings (??) , or should i try something ?...



#61 Asbytec

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Posted 26 May 2022 - 10:56 PM

But i also see Asbytec's idea about tilting the secondary to the focuser to accomplish the same thing, or not (?). Regardless, i should probably / maybe check on the focuser tube central axis to center of tube alignment (for good measure).


Ok, i gotta get some shuteye, i've been up since 2:30 checking out the four wonderful planets, mostly Jupiter, just to see what my less than perfectly collimated 4 inch newt is capable of...while the weather permits... I am satisfied, but am really curious and excited to see what she can do when i get her collimation improved further.I'll get back ro you guys.

You can actually collimate your scope as is, it's just a matter of aligning to optical axes. You can do that without centering the diagonal under the focuser. All you have to do is tilt the diagonal (with a site tube or laser for reference) so the focuser axis hits the primary center.

Remember for every diagonal position there is a tilt and rotation solution that will allow you to "see the primary" to align the focuser axis. When that happens the reflection of the primary will be centered under the focuser, but the diagonal won't.

So you won't see the entire primary reflection in the diagonal, but the scope can deliver satisfying collimated images. The effect of not seeing the entire primary reflection is loss of light and the fully illuminated field will be somewhere else.

The main reason for centering the diagonal under the focuser is to center the fully illuminated field in the center of the eyepiece field of view. You can do that by tilting the diagonal under the focuser as mentioned or by tilting/shimming the focuser to "chase" the diagonal position. Or a little of both.

Edited by Asbytec, 26 May 2022 - 10:59 PM.

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#62 SteveG

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Posted 28 May 2022 - 02:57 AM

STEVEG - 

             No i dont have one. Not in my budget yet... Especially since its my understanding that some of the affordable lasers may not be properly aligned to its own eyepiece case (??) . 

 

           You mention measuring from both sides of the tube from the laser, which i imagine entails using a flexible measuring tape, or something similiar. This i imagine can add to error in measurement, due to the measuring tape and/or the person measuring.         

 

             Is the "average degree of error" from a less than accurate laser collimator plus the "average degree of error" from doing the measuring considered negligible for proper aligning of the focuser central axis to the center of the tube. I asked because i absolutely have no experience with any laser collimator.

 

APFEVER -

                   Well, nothing much i can do about the End Rings (??) , or should i try something ?...

The tolerance is very loose, so don’t worry. You can measure it with a standard tape measure. Raise the focuser so the tube is not entering the main telescope tube, then just extend the tape measure until the laser hits it. It’s probably 4” or so. Next measure down the other side and see if it’s the same. This of coarse is only one of two axis that need to be confirmed. For the other axis you would need a clothe tape to measure around the circumference  both directions.


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#63 Vic Menard

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Posted 28 May 2022 - 08:42 AM

...Raise the focuser so the tube is not entering the main telescope tube, then...

...then get your camera out and standing in front of the tube, align the camera so you're looking straight down the tube toward the primary mirror. You'll want to take the picture far enough from the front of the tube so you can see the extended focuser in the picture you're taking (it should look something like the diagram in post #55 above).

 

The image should show any significant tilt/decentering in this axis (I expect tilt). To view the other axis, with the focuser extended, take a picture from the side of the scope. The long, f/10, tube should work in your favor.

 

It's important to extend the focuser drawtube as much as possible (but not all the way where the drawtube might become torqued in the focuser body) so we can see a "long" drawtube to measure. Of course, you could also leave the drawtube slightly extending into the main tube, then we have the full length of the drawtube to measure...


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#64 patrickt

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Posted 29 May 2022 - 07:14 PM

STEVEG -

                 As i said, i dont plan on buying a laser anytime soon, but thank you for explaining how to go about it (for when i get a laser in the future).

 

VIC MENARD -

                        Just to be clear. Taking a photo from the side of the tube can show any significant tilting of the focuser along the length of the tube, and that it becomes even more obvious given the length of my tube ?  

 

                    Sorry, you lost me in your last paragraph. You mention focuser drawtube extension out of the tube, and then leaving a bit of it into the main tube. You speak of the full length of the draw tube to measure.     

                   Is this in reference to just visually "measuring" ("eyeball measuring") from pictures taken from the front and side of the tube, or is it in reference to using a laser ? Thank you.



#65 patrickt

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Posted 29 May 2022 - 10:31 PM

    I finally took pics that  showed an accurate enough view of what i in fact saw visually. As i said in a previous post, the secondary is mechanically centered in the tube.   

 

   My first look from the front of the tube rightaway told me that the focuser tube was grossly misaligned to the left. 

 

    I then took two pictures showing how i ended up having to also grossly tilt the mirror to the right (towards the focuser). One is a somewhat top view, and the other, a somewhat lower view to show this right tilt of the secondary mirror.  

 

    I then took pics from the side of the tube. Visually, the focuser doesnt seem to be grossly tilted on this axis. Also a picture from the front, which visually,  also did not show any tilt left or right on this axis.

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#66 Asbytec

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Posted 29 May 2022 - 10:54 PM

This image is telling. The diagonal holder seems to be pointed to the left. The spider hub might be twisted a little to the left, too. Your focuser looks pretty straight. Is the orthogonal screw on the bottom right screwed all the way down or at least more so than the one on the left? If so, loosen the screw on the right and tighten the screw on the left. Or are both screws at the bottom the same? If so, then you might be able to perform the same procedure to tilt the diagonal to the right under the focuser.

 

If you cannot do that, then check the vane mounting holes in the tube. You mentioned one was oversize and you pressed the mounting screw all the way to one side. If that corresponds to the leftward tilt of the diagonal, then pull that screw back to the other side of that over sized hole. Then use the two lower tilt screws to correct for any left or right tilt needed to center the diagonal under the focuser. The screw at top in line with the focuser tilts the diagonal toward or away from your eye looking down the focuser. 

 

Spider.jpg


Edited by Asbytec, 29 May 2022 - 10:55 PM.


#67 patrickt

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Posted 30 May 2022 - 01:29 AM

    On closer inspection of the secondary, you are right about the mirror being tilted to the left, and not to the right, as i previously stated (shame on me...).

 

     I think i assumed the secondary mirror was tilted to the right, because of how i unknowingly pivoted the secondary to the right.

 

    The pic with the arrow points to a screw  that i know to be exactly at the bottom of the secondary cell, and should be pretty much aligned with the bottom vane (that runs parallel to the focuser).   But the screw is postioned to the left of the vane, which to my eyes, made me think the mirror was tilted to the right...

 

       And yes, the bottom right screw is turned in more than the left screw (about 1.5 to 2mm more), which i realize tilts the mirror to the left  (see pic labeled "L" for left, and "R" for right).

 

       I cant really tell about the twisting to the left of the spider hub (?)   I just now remeasured the distance from 3 points on the front edge of the tube stiffener (points about 120 degrees apart) till the secondary holder central screw, and there was only a  1 mm. difference in length between the three points. I also meaured the distance of the secondary central screw along the vane, and till the inner tube wall, and got the same 1mm. difference in length

 

      Also,  the three vane attachment holes on the tube are actually three equally measured slots (see pic). So pushing all three attachment screws all the way forward should not have twisted the secondary hub one way or another.

 

      I also held on the vane lightly (as i was tightening the mounting screw outside the tube) to feel for any twisting of the vane, that might twist the hub as well.  But i will see if i can find a way to further check for the hub supposedly being slightly twisted to the left (maybe it was the angle i used taking the picture ??).

 

 

 

      

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Edited by patrickt, 30 May 2022 - 01:33 AM.


#68 patrickt

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Posted 30 May 2022 - 01:46 AM

    Ok, i just now pivoted the secondary to the left, such that the bottom screw (with the red arrow in my last post) is now at the bottom and aligned with the vane opposite the focuser.   

 

    The focuser and secondary hub still definitely look unaligned, especially obvious when seen visually (as in the focuser appears to be at the right of the secondary hub).

 

     Assuming tilting the left side of the focuser would have to be the next step, should i use circular washers under the two left screws ?

 

    And how about the incremental gaps that will be the result of using washers, shouldnt i try to make these gaps light tight, or might its effect on image contrast be acceptably negligible ?? I realize it will depend on the size of the gaps... 

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Edited by patrickt, 30 May 2022 - 02:03 AM.


#69 Asbytec

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Posted 30 May 2022 - 04:23 AM

A lot going on here, it seems. When you look down your focuser with the primary mirror to the right, the diagonal appears at the bottom edge of the focuser and would appear tilted to the right (no shame). But when we look down the tube it is obviously tilted to the left. I believe this is related to a graphic above and the reason we wanted pics of the focuser. Anyway, whatever you did to the red arrow screw holding the diagonal the diagonal mirror appears more flush with the diagonal holder. But, the diagonal and the holder still appear to point left of the primary center. It should point more or less toward the primary center.

 

You can see in your image below the spider hub and the diagonal are skewed from each other, with the holder and diagonal tilted to the left. This is probably because the R screw is screwed in further than the L screw. I believe the hub is okay, but the diagonal needs to tilt to the right using the L and R screws. Loosen R a little, then tighten L a little. Try to keep some tension on the diagonal holder as you walk the tilt to the right. I would turn both screws until they are pretty much equally screwed in or out putting the diagonal in a position of neutral tilt.

 

Spider.jpg

 

We can worry about what we see down the focuser in the next step. First, just put the spider in its neutral tilt position centered in the telescope tube. Now, as above, the diagonal was seen at the bottom of the focuser tube. Tilting it further to the right may make that situation as seen looking down the focuser worse. But that's okay. There are two ways to fix that. The one way I am leaning toward, as shown above in a recent graphic, is tilting and shimming the focuser to capture the diagonal's natural position centered in the telescope tube with neutral tilt. 


Edited by Asbytec, 30 May 2022 - 04:30 AM.


#70 Vic Menard

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Posted 30 May 2022 - 01:57 PM

                    ...Sorry, you lost me in your last paragraph. You mention focuser drawtube extension out of the tube, and then leaving a bit of it into the main tube. You speak of the full length of the draw tube to measure.     

                   Is this in reference to just visually "measuring" ("eyeball measuring") from pictures taken from the front and side of the tube, or is it in reference to using a laser ? Thank you.

Here's what I was looking for: a view down the tube where the primary mirror (red circle) appears centered under the secondary mirror (green circle w/3 vanes), and the focuser drawtube is visible above the tube and inside the tube.

 

That said, your recent images show the secondary mirror is tilted quite a bit to the left of the primary mirror (likely caused by your attempts to "center" the secondary mirror under the focuser. You might want to try to reset the secondary mirror tilt screws so they all appear equal (your first image in post #67 shows the left and right screws are different lengths).

 

(Edit: Sorry, I just realized I drew the spider upside down! In spite of my mistake, I hope the picture is self-explanatory.) 

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Edited by Vic Menard, 30 May 2022 - 06:10 PM.

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#71 patrickt

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Posted 30 May 2022 - 06:41 PM

ASBYTEC and VIC MENARD -

 

           Thank you both !  When i get a chance, I will check it out sometime today.


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#72 Asbytec

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Posted 30 May 2022 - 07:07 PM

Patrick, I agree with Vic and the post above on the same subject. It looks like someone really tried to force the diagonal hard to the left to center it under the focuser. Without success. In this case, it's better to tilt and shim the focuser to capture the diagonal instead of forcing the diagonal under the focuser.

Edited by Asbytec, 30 May 2022 - 07:08 PM.

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#73 patrickt

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Posted 31 May 2022 - 09:33 AM

 Ok, lets try this again...

 

      I first set the secondary holder in the "neutral" position. 

 

      1) I adjusted the two Tilt screws until they were of the same length. I measured this with a depth gauge, such that the length only differed by a tiny fraction of a mm. See one PIC.

     2)  I then rotated the secondary holder until its longer portion was aligned with the lower vane (which is why the lower screw visible in my previous post, circled in red, is now hidden by the lower vane). See one PIC.

 

    A]  I proceeded to place a white sheet between the primary and secondary, to make the secondary holder more visible.    I then raised the focuser tube all the way up and just positoned my cellphone over the focuser tube. See one PIC.

 

NOTE: All the pics i took will of course suffer from some parallax error, but should approximate my actual view. 

 

B] I then put on a Collimation cap and took off the white sheet to show the reflection of the Primary on the secondary mirror. These are the two smaller pics,    One overexposed one ( thus not revealing the vanes reflection), with the focuser fully racked UP.   The other, revealing all three vanes, but with the focuser fully racked DOWN. See two PICS.

 

C]  The view from the front of the tube show the secondary hub and cell holder combo now looking better aligned under the focuser. See two two PICS.

 

       And thats all for now... So what do you guys think ?

 

     

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Edited by patrickt, 31 May 2022 - 09:42 AM.


#74 patrickt

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Posted 31 May 2022 - 09:39 AM

Pics showing how the secondary holder and hub "combo" look much better aligned now under the focuser, after merely adjusting the tilt screws...

Attached Thumbnails

  • Screenshot_20220531-212508_Gallery.jpg
  • Screenshot_20220531-214244_Gallery.jpg


#75 Asbytec

Asbytec

    Guy in a furry hat

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Posted 31 May 2022 - 09:54 AM

Patrick, that's a dramatic improvement. The diagonal mirror is much closer to focuser center.

Stop for a second and study your third image. You can clearly see the primary reflection is a little "high" in the diagonal. At this point, a neat trick I learned was to rotate the diagonal so the primary reflection is centered on the diagonal long axis. To my way of thinking, doing so will give you better rotation than eyeball rounding, and doing so bring the single screw in line with the focuser onto play during tilt alignment of the focuser axis/diagonal mirror. There are other benefits, too, but we can discuss those later.

But first, we have to finish step one for secondary placement and rotation. You're at the point you should be able to get a little better diagonal centering by using those same two L and R screws to nudge the diagonal "up" or "down" and the center bolt "left" or "right" (toward or away from the primary). Once you get good centering of the diagonal, then rotate as described above. Then you're ready for step to aligning the focuser axis.

Much better! You may not have to tilt and shim the focuser after all.

I am curious, though, because focused all the way down your diagonal mirror looks to be about the same size or smaller than the primary reflection. When your focused all the way down, a properly sized diagonal should appear larger than the primary reflection. As you focus outward, the apparent diameter of the distant primary mirror will remain pretty much the same size. However, the apparent diameter of the nearby diagonal will appear to become smaller. Thus, at some point, the entire primary reflection will appear larger than the diagonal. But that should happen around mid focuser travel beyond the focal plane, not when you're focused all the way down.

Well see, though, once you center and rotate the diagonal mirror. (rotation will center the primary reflection on the diagonal *major* axis, that single screw in line with the focuser will center the primary reflection on the diagonal *minor* axis). We still need some way to see the focuser axis, like a cross hair, but you'll be close to collimation.

Edited by Asbytec, 31 May 2022 - 10:11 AM.

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