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Help, Newtonian secondary mirror collimation problem

Collimation Equipment
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#1 Midlands_Astro_96

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Posted 23 September 2022 - 11:37 AM

Hi everyone. New member here. Been dabbling in astrophotography for a few years, and have seen good advice on here but now I have a problem that I can't solve.

 

Yesterday I took delivery of an OOUK VX6 1/10pv and all looked good until I came to collimate it. I was going to start with my laser collimator and then fine tune with a star test when I got some clear skies but I haven't been able to get that far. (My laser collimator is itself collimated. When inserted into the focuser and rotated the laser dot on the primary stays in the same place)

My problem arose as soon as I tried to adjust the secondary mirror. Upon inserting the collimator the laser dot on the primary was initially towards the outer edge of the primary mirror. I began my adjustments and the dot was moving towards the centre of the primary mirror but then the collimation screws got harder to turn. It appeared that the secondary mirror had reached the limit of its adjustment and the laser dot was still about 2 inches from the centre of the primary mirror.

I tightened 2 of the collimation screws a little more and got the dot with and inch of the centre of the primary mirror but the screws seemed way too tight and I think they may have broken if I tried to tighten them any further. This was with the third collimation screw completely loose. Even tightening that third screw just a little (as I'm sure it's not supposed to be loose) moved the dot further from the centre of the primary mirror.

At this point I backed out the collimation screws until the secondary was just being held in place by the central screw. I then moved the secondary by hand to see if it was even possible to move the secondary enough to get the laser dot in the centre of the primary mirror. It was not possible. At one extreme of the secondary mirror travel the dot was about 2 inches from the centre. At the other extreme the dot was about an inch outside the edge of the primary mirror.

I am not an expert but it seems to me that there is something wrong with the secondary. Either that the secondary mirror is not mounted properly (properly being ~45 degrees) or the spider mounting holes are not positioned correctly (2 adjacent holes being to close to front or rear of scope tube) resulting in the entire secondary mechanism being tilted. The tilt being so severe that there isn't enough travel in the secondary mirror mechanism to correct it.

 

Any help would be appreciated. I have emailed OOUK (in detail as above) but the first reply I got seems that they only read up until "collimation problem" and then stopped reading, as they sent me a generic collimation instruction pdf and nothing more. I emailed again but am yet to receive a reply.

 

Regards

Will



#2 mmello

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Posted 23 September 2022 - 12:27 PM

Hi Will,

Apart from the mechanical issues you have described, the only other thing I can think of is whether your collimator's laser beam is misaligned. There are relatively simple ways to examine this and adjust it if necessary, and I would check this first.

 

Best,

Mike 



#3 KBHornblower

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Posted 23 September 2022 - 12:31 PM

You may need to rotate the mirror about the central screw and get the laser spot as close to the center as possible.  If it is rotated out of alignment enough it could be beyond the range of the outer screws to override it.  I would look into the focuser and rotate the secondary until the reflection of the primary is centered in the direction crossways to the tube.  It still may be off in the lengthwise direction, and that is adjusted with the outer screws.



#4 KBHornblower

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Posted 23 September 2022 - 12:32 PM

Hi Will,

Apart from the mechanical issues you have described, the only other thing I can think of is whether your collimator's laser beam is misaligned. There are relatively simple ways to examine this and adjust it if necessary, and I would check this first.

 

Best,

Mike 

He did check that.  He said the spot on the primary mirror stayed in one place when he rotated the laser in the focuser.



#5 Midlands_Astro_96

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Posted 23 September 2022 - 12:43 PM

You may need to rotate the mirror about the central screw and get the laser spot as close to the center as possible.  If it is rotated out of alignment enough it could be beyond the range of the outer screws to override it.  I would look into the focuser and rotate the secondary until the reflection of the primary is centered in the direction crossways to the tube.  It still may be off in the lengthwise direction, and that is adjusted with the outer screws.

I loosened the collimation screws (scope horizontal) so under gravity the secondary mirror was in its lowest possible position (just held by the central screw). Then rotated the secondary mirror. The laser dot on the primary moved in a relatively flat arc, the closest to the centre being about 1.5-2 inches away when directly above the centre. That appears to be the limit of travel. Can't get it any closer without excessive force on collimation screws.



#6 Vic Menard

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Posted 23 September 2022 - 01:12 PM

...Any help would be appreciated.

Unfortunately, you can't (correctly) align your secondary mirror using only a simple thin beam laser. You'll need another tool to visually assess (and correct) the secondary mirror placement. You can sort out the secondary mirror with a thin beam laser and a collimation cap, like this:  https://www.rigelsys.com/telecoll.html


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#7 Midlands_Astro_96

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Posted 23 September 2022 - 01:21 PM

Unfortunately, you can't (correctly) align your secondary mirror using only a simple thin beam laser. You'll need another tool to visually assess (and correct) the secondary mirror placement. You can sort out the secondary mirror with a thin beam laser and a collimation cap, like this:  https://www.rigelsys.com/telecoll.html

It's not that I can't get a correct alignment, it's that I can't get close to even a poor/rough alignment. Is the difference between a correct and rough alignment getting the laser in the centre of the primary vs being able to only get it 2 inches from the centre?

 

I'm not anywhere near trying to get a perfect collimation yet.



#8 Vic Menard

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Posted 23 September 2022 - 01:29 PM

It's not that I can't get a correct alignment, it's that I can't get close to even a poor/rough alignment. Is the difference between a correct and rough alignment getting the laser in the centre of the primary vs being able to only get it 2 inches from the centre?

 

I'm not anywhere near trying to get a perfect collimation yet.

I understand. But you can't see what's going on with the secondary mirror when there's a laser in the focuser, and with nothing in the focuser it's almost impossible to center your eye sufficiently to clearly see what's going on. A collimation cap will help you to see what's going on (and it will allow you to post an image of your alignment so we can help you).

 

And you're right, we're not anywhere near trying to get a "perfect" collimation... At f/5 (and I assume you're coma corrected), you'll need to get the axial alignments within tolerances for imaging performance and the secondary mirror placement close enough to keep the illumination profile centered--but that's after you get the secondary mirror sorted...


Edited by Vic Menard, 23 September 2022 - 01:30 PM.


#9 KBHornblower

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Posted 23 September 2022 - 02:10 PM

Midlands, you are in good hands now.  Vic is our expert in all of the booby traps that can trip us up when collimating a Newtonian reflector.


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#10 KBHornblower

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Posted 23 September 2022 - 03:55 PM

Midlands, you are in good hands now.  Vic is our expert in all of the booby traps that can trip us up when collimating a Newtonian reflector.

Addendum:  I jumped the gun with my previous advice.  Vic is right.  Make sure the secondary is properly located first.  He will have more details on how to do it if you need it.



#11 Spile

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Posted 23 September 2022 - 04:27 PM

I find a Cheshire eyepiece and sight tube combination tool the best way to align the secondary mirror https://astro.catshi...limation-guide/


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#12 MellonLake

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Posted 23 September 2022 - 04:43 PM

With a collimation cap you can tell if the secondary position is right it is important. If the rotation is off and/or the position towards/away from the primary is off you won't get the beam to the spot with the tilt screws.

Vic's advice above is good. He wrote the book on collimation. Literally.

Also while you are waiting for a tool, measure the spider vanes to the centre of spider and adjust the nuts to make them equal length.

When you get the collimation cap, take a cell phone picture through it and we can help solve your problem better.

Rob.

#13 MellonLake

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Posted 23 September 2022 - 04:45 PM

I find a Cheshire eyepiece and sight tube combination tool the best way to align the secondary mirror https://astro.catshi...limation-guide/


If you go with a combo/Cheshire, get an SVBony or Celestron. They slide farther into the focuser so you can see the whole secondary.
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#14 Asbytec

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Posted 23 September 2022 - 11:33 PM

My problem arose as soon as I tried to adjust the secondary mirror. Upon inserting the collimator the laser dot on the primary was initially towards the outer edge of the primary mirror.

Good collimation begins with step 1, secondary placement (including rotation). As others are explaining, that's impossible to do with a laser in the focuser. You need a tool, a site tube is ideal or a collimation cap, to look down your focuser to properly center and rotate your diagonal mirror under the focuser.

Also be sure your spider is mounted near the center of your telescope tube. Centering the spider in the telescope tube and centering and rotating the diagonal under the focuser is good "front end geometry" from step 1. Be sure to finish step 1, diagonal placement, before you do step 2, focuser alignment. That makes collimation so much easier.

With the diagonal centered in the tube and under the focuser,you should have enough tilt leeway to put the laser on the primary center mark. If for some reason you cannot tilt the diagonal to the focuser center, you can tilt or "level" the focuser, instead, to chase the diagonal position centered in the telescope tube.

Edited by Asbytec, 23 September 2022 - 11:48 PM.


#15 Midlands_Astro_96

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Posted 24 September 2022 - 11:10 AM

I have heard back from the guys at Orion Optics UK and they have said I can take the scope in and they'll have a look at it. Luckily I'm only about an hours drive from them.

I appreciate the advice but I think you guys are a few steps ahead of where I'm at with this scope. I understand that an accurate collimation can't be done with a laser alone but to not even be able to get the laser dot within 1.5-2 inches of the centre of the primary mirror, to me indicates that there is something very wrong with the secondary mirror assembly. I have tried loosening the collimation screws and then by hand moving the secondary back, forward, left, right, up, down, rotating it. In doing so I have tried to explore all combinations of secondary mirror positioning within it's adjustment range yet the laser dot is never within 1.5-2 inches of the centre of the primary mirror. It is always off to one side as if the secondary mirror is at the wrong angle to some extent. That strikes me as a major defect with the scope not just something that can be fixed with a proper collimation routine.

 

Will report back when I've hopefully popped in to see the guys at OOUK.


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#16 MellonLake

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Posted 24 September 2022 - 11:54 AM

Get them to run through collimation with you. After you learn how to do it, have them screw up the collimation and you fix it with their guidance.

Nice that you are so close.

#17 cuzimthedad

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Posted 24 September 2022 - 12:03 PM

Welcome to Cloudy Nights Will!



#18 SteveG

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Posted 24 September 2022 - 03:35 PM

I have heard back from the guys at Orion Optics UK and they have said I can take the scope in and they'll have a look at it. Luckily I'm only about an hours drive from them.

I appreciate the advice but I think you guys are a few steps ahead of where I'm at with this scope. I understand that an accurate collimation can't be done with a laser alone but to not even be able to get the laser dot within 1.5-2 inches of the centre of the primary mirror, to me indicates that there is something very wrong with the secondary mirror assembly. I have tried loosening the collimation screws and then by hand moving the secondary back, forward, left, right, up, down, rotating it. In doing so I have tried to explore all combinations of secondary mirror positioning within it's adjustment range yet the laser dot is never within 1.5-2 inches of the centre of the primary mirror. It is always off to one side as if the secondary mirror is at the wrong angle to some extent. That strikes me as a major defect with the scope not just something that can be fixed with a proper collimation routine.

 

Will report back when I've hopefully popped in to see the guys at OOUK.

The secondary mirror is fully adjustable for any angle. That’s what the 3 little screws do. Follow Vic’s advice carefully.

 

Sadly, taking it to the store might not really result in good collimation, but we’ll be interested in hearing the results.



#19 Midlands_Astro_96

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Posted 25 September 2022 - 09:39 AM

IMG_20220924_164727070 smaller.jpg

The secondary mirror is fully adjustable for any angle. That’s what the 3 little screws do. Follow Vic’s advice carefully.

 

Sadly, taking it to the store might not really result in good collimation, but we’ll be interested in hearing the results.

It is supposed to be fully adjustable for any angle. That's the problem. It's not. I took the secondary mirror assembly out today to take a closer look. With the collimation screws backed out and just held by the centre screw to let it move freely. When looking from the side the two pieces appear parallel. I can tilt the mirror upwards (rotating anticlockwise in the image) with very little upwards pressure but if I try to move it downwards it does not move. Its range of movement goes from horizontal to tilted slightly upwards when viewed from the side. It can't tilt downwards at all. I think this is the problem as when using the laser collimator the dot on the primary is always about 1.5-2 inches above the centre of the primary mirror and it won't move any lower.



#20 kathyastro

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Posted 25 September 2022 - 09:42 AM

If the focuser was not square to the optical axis, it could do that.  It would make the laser hit the secondary off-centre, at an angle.



#21 Vic Menard

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Posted 25 September 2022 - 10:58 AM

...It is supposed to be fully adjustable for any angle. That's the problem. It's not. I took the secondary mirror assembly out today to take a closer look.

OK--we can start here with some basics. I've annotated your image below. The three green parallel lines show the edges and center of your secondary mirror stalk/holder. The holder is normally centered (note the center line goes through the center of the center mounting screw) and the face of the secondary mirror is usually centered on the stalk/holder or slightly offset away from the focuser side of the tube assembly (yours is offset a small amount toward the focuser side of the tube assembly).

 

The vertical green line represents the focuser axis. If you stand in front of the tube assembly, the focuser axis should line up with one of the three secondary mirror tilt adjustment screws. From your picture, I believe this screw (green arrow) is the one in the green circle. If you tighten this screw (and loosen the other two screws equally), it pushes the bottom of the secondary mirror outward and upward. If you loosen this screw (and tighten the other two screws equally), it pulls the bottom of the secondary mirror inward and downward.

 

Similarly, if you loosen all three tilt adjustment screws and tighten the center mounting screw, it pulls the secondary mirror toward the spider hub, and if you loosen the center mounting screw and tighten all three tilt adjustment screws, it pushes the secondary mirror away from the spider hub.

 

With these tilt adjustments, fore and aft adjustments, and rotation adjustment, you can usually find the optimal placement for the secondary mirror.

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#22 Midlands_Astro_96

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Posted 25 September 2022 - 11:16 AM

OK--we can start here with some basics. I've annotated your image below. The three green parallel lines show the edges and center of your secondary mirror stalk/holder. The holder is normally centered (note the center line goes through the center of the center mounting screw) and the face of the secondary mirror is usually centered on the stalk/holder or slightly offset away from the focuser side of the tube assembly (yours is offset a small amount toward the focuser side of the tube assembly).

 

The vertical green line represents the focuser axis. If you stand in front of the tube assembly, the focuser axis should line up with one of the three secondary mirror tilt adjustment screws. From your picture, I believe this screw (green arrow) is the one in the green circle. If you tighten this screw (and loosen the other two screws equally), it pushes the bottom of the secondary mirror outward and upward. If you loosen this screw (and tighten the other two screws equally), it pulls the bottom of the secondary mirror inward and downward.

 

Similarly, if you loosen all three tilt adjustment screws and tighten the center mounting screw, it pulls the secondary mirror toward the spider hub, and if you loosen the center mounting screw and tighten all three tilt adjustment screws, it pushes the secondary mirror away from the spider hub.

 

With these tilt adjustments, fore and aft adjustments, and rotation adjustment, you can usually find the optimal placement for the secondary mirror.

I understand fully what the collimation screws do. The problem is that they are not doing what they are supposed to do.

When I try to tighten the top 2 collimation screws with the bottom one you circled backed out a bit(to try and move the secondary mirror down) the secondary DOESN'T tilt downwards. That is what I've been trying to explain to you. That's what the problem is. If I do the opposite (tightening the bottom screw with the top 2 backed out a bit) then the mirror tilts upwards as it is supposed to. But it will NOT tilt downwards.


Edited by Midlands_Astro_96, 25 September 2022 - 11:18 AM.


#23 MellonLake

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Posted 25 September 2022 - 11:22 AM

Put something (pen, pencil, etc) the same diameter in the centre hole. My guess is that the hole is not perpendicular to the spider. This should show this.

The other option is that the hole in the secondary holder is not parallel to the holder axis. Try checking this as well.

Rob

#24 Vic Menard

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Posted 25 September 2022 - 11:35 AM

I understand fully what the collimation screws do. The problem is that they are not doing what they are supposed to do.

When I try to tighten the top 2 collimation screws with the bottom one you circled backed out a bit(to try and move the secondary mirror down) the secondary DOESN'T tilt downwards.

Does it move downward if you remove the bottom screw? If not, the center mounting screw might be jammed in the hub (it should be loose). In fact, in your recent picture, I assumed you were holding the center mounting screw flush to the hub with your index finger. If you weren't holding the center mounting screw in place, the secondary mirror should have tilted downward in the image you posted. (I'm assuming the center mounting bolt is threaded into the secondary mirror holder, not the hub...)


Edited by Vic Menard, 25 September 2022 - 11:40 AM.


#25 Midlands_Astro_96

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Posted 25 September 2022 - 11:58 AM

ezgif.com-gif-maker.gif

 

The centre screw turns freely and easily where it enters the piece of metal the secondary is attached to and it is free to move/tilt in the spider mechanism.

I think rob has hit on the issue. I think the hole in the metal attached to the secondary hasn't been drilled straight. I don't have anything that fits the hole exactly so I put an Allen key in there and took pictures with it held in opposite orientations and then aligned the images in photoshop. The Allen key sits noticeably lower in one orientation than the other. Looking from the front of the scope it also appears that without moving the secondary mirror with the collimation screws it is also tilted to one side as well as the upward tilt. 

 

 

 




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