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New Concenter - Collimation Problems


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Hi All,

I've just taken delivery of my new concenter, bought to aid in lining up my secondary. It seems to be a great little piece of kit and makes centralising the secondary an almost trivial task but when I come to making sure that the secondary is aligned witht eh primary I find that the image of the primary is way off to the side. As a result I adjust the secondary so that I can see the primary clips but when I check the positioning of the secondary with the concenter again it needs adjustment to cenralise it and make it circular. Then I go back to the secondary alignment with the primary and it's well off again. I seem to be going around in circles so I was hoping someone might be able to shed some light on fixing my problem?

Many thanks,

Stu

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Sounds like your focuser may be tilted. Thee are threads on SGL about squaring up your focuser. 

But for now just do the following and see how you get on.

When using the concentre I adjust the secondary so it is properly centered. Then I level the primary mirror by adjusting the length of the adjusting screws to the same length using a little steel rule. This should result in the scope being very close to being collimated and only minor tweaks of the primary should be needed to get collimation spot on. 

I use a Hotech laser after the concentre for final,collimation.

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I have had this issue with more than one scope. What I do in this case is:

1) Set the secondary mirror boss central in the tube by adjusting the 4 spider vane screws around the edge of the OTA. Use a ruler or similar to measure the distance from the central screw to the tube wall along each of the 4 vanes and adjust so all 4 measurements are equal.

2) adjust the three secondary mirror tilt screws so that the secondary mirror holder is not tilted, I.e. there is the same amount of thread showing on each of the three screws between the secondary boss and the secondary holder (the bit that the mirror is stuck to). After doing these two steps the secondary mirror should be near as dammit bang in the centre of the tube and you now want to aim the focuser at it.  

3) look through the focuser with concenter in it and see if the secondary is centered or not in the up/ down direction (I.e. traversing the short axis of the OTA).

4) if it is out in the up down direction, rectify by tilting the focuser by whatever means necessary. In the past I've used washers between the focuser and tube to force a tilt. 

5) if it is out of centre in the other direction (along the long axis of the ota) then adjust this by loosening or tightening the central screw in the secondary boss to move the mirror back or forth.

Once the focuser and secondary are lined up, continue with the collimation as normal. Hopefully that should get you there.  The secondary collimation isn't that important really, just affecting the field illumination mainly. The primary collimation is the important one and will affect the sharpness of the views. 

 

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5 hours ago, Mark at Beaufort said:

when I collimate the secondary I always block off the primary.

The reason I do not do this is so that I can check the secondary reflection is offset in the plane of the focuser and towards the primary. 

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  • 3 weeks later...

Update:

I recently tried out some AP with my kit and the result seemed to show that the scope might be slightly out of collimation so went through the whole process again. I found that the secondary was very slightly off so I centred that using my concenter and checked with the cheshire:

2FA0FA3B-345F-4A41-981E-E0E764814FE6.thumb.jpeg.2f082b9146f66e3fb87c68facf979d31.jpeg
AB9DBB43-2E18-43CC-9BE7-09ED1DBB8865.thumb.jpeg.f93e55abab4a49f69b70f6358d82e4b6.jpeg

Then I found that the secondary wasn’t lined up with the primary - I dealt with that:

11F547D9-2937-4E08-AE57-B4DDCF3D3D20.thumb.jpeg.f691a4a65928f9d5f7405da0933b2ea7.jpeg

then I went back to check the secondary and found that it was, again, very slightly off according to the concenter but I chose not to make any further adjustments:

E7691412-CC86-4801-877B-FE67AD2A52E1.thumb.jpeg.6a2528991cc14fe8f4d831abd526f55e.jpeg

Finally I adjusted the primary:

9E182AC5-05E8-4B36-AA10-060E6300EE8C.thumb.jpeg.041cc7ae814450b8a494dd568769ef7b.jpeg

looking at the photos, can anyone tell me if there are any glaring errors please?

Many thanks,

Stu

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Here is my interpretation. If the axis runs as indicated (10 o'clock - 4 o'clock) then it would be good for me. However the other photos show 9 o'clock - 3 o'clock which means there would be an offset error.

 

5 Aug.jpg

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Hi All,

I was wondering if anyone could asvise me on whether this is good collimation or not - generally, when you look online everything seems to be absolutely perfect so it's difficult to compare. Also, I have had problems in the past with the focus tube being off-centre (now adjusted) but I can't seem to get the secondary/primary alignment exactly right while maintaining a spot-on secondary adjustment.

First I used a concenter to make sure the secondary was in a good position and orientated properly:

IMG_0283.thumb.jpg.a462f651ed3edfbd908bf8ef6d477547.jpg

 

Then I checked with my cheshire:

IMG_0286.thumb.jpg.f54719d4aa7ed58b98344b953c18b7b5.jpg

 

Then I had to adjust the secondary to see all of the clips on the primary (it's slightly off but the picture is slightly off to so it seems to be a bit exaggerated:

IMG_0287.thumb.jpg.e980224cf0fdcd8e519437e9ab0dc202.jpg

 

Then I checked the secondary again with the concenter. It isn't as good as the first image but not quite as bad as the third:

IMG_0288.thumb.jpg.be7980ddac8c9e492c555c14c0ef4e79.jpg

 

Finally I lined up the primary:

IMG_0290.thumb.jpg.117bb61d28309a32d313c0a21c00cfc4.jpg

 

Does this seem to be ok for a collimation? In other words - am I expecting the almost impossible?

Many thanks,

Stu

Edited by Sabalias
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Looks pretty good to me, but I cant really see the 'dot-in-the-donut' cheshire view in the last image though as it is out of focus - that is the primary collimation and is the most critical one to get right.

It's worth doing a few iterations from start to finish.... so when you get to your last photo (primary collimation), start again with the concenter and go right through up to primary collimation again, the errors at each stage will tend to get smaller with each iteration I find.

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3 minutes ago, CraigT82 said:

Looks pretty good to me, but I cant really see the 'dot-in-the-donut' cheshire view in the last image though as it is out of focus - that is the primary collimation and is the most critical one to get right.

It's worth doing a few iterations from start to finish.... so when you get to your last photo (primary collimation), start again with the concenter and go right through up to primary collimation again, the errors at each stage will tend to get smaller with each iteration I find.

Thanks Craig,

Yes, the joys of taking a photo with your phone - just couldn't get it to focus on the mirror under the crosshair.

Stu

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On 03/08/2021 at 18:44, Sabalias said:

recently tried out some AP with my kit and the result seemed to show that the scope might be slightly out of collimation

What was it in the images that suggested a collimation issue? 

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8 minutes ago, CraigT82 said:

What was it in the images that suggested a collimation issue? 

Just the fact that the secondary alignment has moved after bring the primary into view. I don't really have a frame of refererence so I don't know what I don't know (trite answer perhaps).

Stu

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2 hours ago, Spile said:

Here is my interpretation. If the axis runs as indicated (10 o'clock - 4 o'clock) then it would be good for me. However the other photos show 9 o'clock - 3 o'clock which means there would be an offset error.

How do you mean? This is something I haven't come across before.

Thanks,

Stu

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  • Sabalias changed the title to Collimation - anything wrong (duplicate to this topic: https://stargazerslounge.com/topic/379973-new-concenter-collimation-problems/?tab=comments#comment-4120546)
5 minutes ago, Sabalias said:

Just the fact that the secondary alignment has moved after bring the primary into view. I don't really have a frame of refererence so I don't know what I don't know (trite answer perhaps).

Stu

Sorry I meant the AP images you captured with the scope, what what it in those that suggested collimation was out? That might guide you in what to look for

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Just now, CraigT82 said:

Sorry I meant the AP images you captured with the scope, what what it in those that suggested collimation was out? That might guide you in what to look for

Ah, I follow. It was suggested here that I might want to check the collimation as the stars are a little egg-shaped.

Stu

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37 minutes ago, Sabalias said:

Ah, I follow. It was suggested here that I might want to check the collimation as the stars are a little egg-shaped.

Stu

Ok so to me, looking at your image on the other thread, the field illumination looks pretty good, even without flats, maybe a little darker in the top corners but flats will sort that and I'd leave the secondary alone.

The stars showing signs of coma all pointing away from the lower right, and it gets worse as you move towards the upper left.  How I'd interpret this is that the light cone from the primary is not centred on the sensor (i.e. the primary collimation is out) and the centre of the light cone is somewhere off to the bottom right.  You will need to tweak the primary collimation to bring the centre of the light cone back to the centre of the sensor. There may be some issues with CC spacing too but that will be easier to diagnose once collimation is sorted.

The best way to do the primary collimation is to aim the scope/camera at a star which is close to your intended target in the sky, defocus the image of the star slightly so you can see the classic donut shape and tweak the primary collimation screws to get the donut concentric. Keep the image of the star in the middle of the sensor whilst you do this (each adjustment to a collimation screw will move the star and you'll have to move the scope to bring it back to the centre). By dong it this way you are collimating through the imaging train, i.e. aligning your primary to your camera sensor, which is exactly what you want for imaging. You can also do it with a cheshire eyepiece but there may be some error creeping back in when you swap out the cheshire for the CC/camera.

The key thing is that the primary mirror can shift, as can the focus drawtube (droop/sag) and the primary collimation will change slightly as you point the scope to different parts of the sky as the mirror moves slightly in the cell or the focus tube hangs differently.  It is very difficult to engineer a mirror cell that holds a mirror lightly enough to not change the figure (pinching) but firm enough to prevent this mirror shifting during changes in scope position.  This is why you ideally want to collimate the scope whilst it is pointing at your intended target.

 

Edited by CraigT82
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17 minutes ago, CraigT82 said:

Ok so to me, looking at your image on the other thread, the field illumination looks pretty good, even without flats, maybe a little darker in the top corners but flats will sort that and I'd leave the secondary alone.

The stars showing signs of coma all pointing away from the lower right, and it gets worse as you move towards the upper left.  How I'd interpret this is that the light cone from the primary is not centred on the sensor (i.e. the primary collimation is out) and the centre of the light cone is somewhere off to the bottom right.  You will need to tweak the primary collimation to bring the centre of the light cone back to the centre of the sensor. There may be some issues with CC spacing too but that will be easier to diagnose once collimation is sorted.

The best way to do the primary collimation is to aim the scope/camera at a star which is close to your intended target in the sky, defocus the image of the star slightly so you can see the classic donut shape and tweak the primary collimation screws to get the donut concentric. Keep the image of the star in the middle of the sensor whilst you do this (each adjustment to a collimation screw will move the star and you'll have to move the scope to bring it back to the centre). By dong it this way you are collimating through the imaging train, i.e. aligning your primary to your camera sensor, which is exactly what you want for imaging. You can also do it with a cheshire eyepiece but there may be some error creeping back in when you swap out the cheshire for the CC/camera.

The key thing is that the primary mirror can shift, as can the focus drawtube (droop/sag) and the primary collimation will change slightly as you point the scope to different parts of the sky as the mirror moves slightly in the cell or the focus tube hangs differently.  It is very difficult to engineer a mirror cell that holds a mirror lightly enough to not change the figure (pinching) but firm enough to prevent this mirror shifting during changes in scope position.  This is why you ideally want to collimate the scope whilst it is pointing at your intended target.

 

Thanks Craig - I'll follow that to the 'T'.

Stu

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When you are adjusting the secondary with a Cheshire, to get the primary centred in it, don't worry about the mirror clips: just adjust it so that the crosshairs are over the doughtnut. It's the same thing.

Anyway, I think your secondary might be tilted slightly, even though it appears a concentric circle. May I offer some advice? I found that the best start to collimation is to ensure your secondary mirror assembly is completely parallel (flat) to the spider assembly. You can do this by making sure that the 3 secondary mirror screws are protruding by the same amount, but this assumes everything is made to equal precision. This is what I did to fix my collimation woes:

 

Keep loosening the 3 adjuster screws and tightening the centre screw. This brings the mirror back up the tube, away from the primary. Keep doing this until the mirror has been pulled all the way back to the top of the tube and is flush against the vane-assembly, with the adjusters just barely loose. This means that the base of the secondary holder is now flush with the assembly.

Then - bring it back down the tube bit-by-bit, by loosening the centre bolt and taking up the slack with the adjusters. It's important to turn the adjusters by the same amount each, so that they all keep the same distance. If they are thumbscrews (like Bob's Knobs) use a sticker on each one with a line to show their positions. Then you know you are turning each one by exactly the same amount. Keep bringing it down the tube with the adjusters always equal. When you reach the correct position under the focuser, you know the whole thing is straight and level.

 Now, all you need to do is make sure it is rotated and facing the focuser correctly - so that the image of the secondary is concentric as above. Then you can take out the paper and make a final adjustment of the secondary tilt, to get the doughnut centred in the Cheshire.

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8 hours ago, Sabalias said:

How do you mean? This is something I haven't come across before.

Thanks,

Stu

Look at the dark circle which is the reflection x2 of the offset secondary. It should be “pointing” away from the focus tube - 6 o’clock in these diagrams . 

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  • 3 months later...
On 15/07/2021 at 16:28, johninderby said:

Sounds like your focuser may be tilted.

John you just made my day having spent quite a few hours fitting my steeltrack , I could never get on with the concentre it never looked right , low and behold I could clearly see focuser on the wonk hence probably why using  old stock focuser the secondary never looked right through the concentre, so two shims Under one side of the verabase and one shim on the other focuser now sitting 90 degrees aligning a metal ruler along spider vanes and eyeing up the concentre barrel to edge of ruler all now parallel and secondary looking good through concentre 😁👏👏👏👏

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