Collimation confusion

[bosun21]

3 minutes ago, PeterStudz said:

The spring on the secondary? That’s the one I cut down. Is the secondary mirror really that much heavier? 

PS - the 200p doesn’t have primary springs. It uses grommets instead. 

Sorry Peter I read it wrong 🙄. My apologies. That's twice on this thread. That's what I get for hurriedly trying to catch up with the thread after an absence for several hours.

[PeterStudz]

Just now, bosun21 said:

Sorry Peter I read it wrong 🙄. My apologies. That's twice on this thread. That's what I get for hurriedly trying to catch up with the thread after an absence for several hours.

No worries. I’ve done that before. And more than once! 

[bosun21]

1 minute ago, PeterStudz said:

No worries. I’ve done that before. And more than once! 

Same here, on numerous occasions. The only 8' reflector I have owned was the Stella Lyra  dobsonian which most certainly required the primary springs upgraded. They were like the springs you used to get inside pens.

[Kon]

We have a solution, I think.

I took the primary out, and it was all good but it gave me a peace of mind.

Put the spring back on the secondary.

Checked collimation but I could not get it to work.

The solution? The bloody Cheshire was the issue. For some reason I didn't check earlier but as I was rotating within the focuser, it seemed the central cross hair were a bit off!! As a result it was 'offsetting' the centre and subsequently the primary dot shadow. It was 3/4 centred and after I noticed and accounted for it bang it all fell in place! 

All clips visible, spring in secondary and plenty of room for wiggle.

Long and short Cheshire cross hair aligned, laser spot on. Star test nice concentric airy rings.

@PeterStudzworth checking your Cheshire.

@bosun21and @Mr Spockthanks for your advice and offers to help. The primary springs look quite strong too.

[Spile]

This is what I get from your earlier image. I couldn't see the edge of the sight tube in the last image so you might want to take a picture through a cap. Nevertheless the secondary looks good.

[bosun21]

13 hours ago, Spile said:

Deleted 

 

 

Edited February 24 by bosun21
Quoted wrong member.

[bosun21]

13 hours ago, Kon said:

We have a solution, I think.

I took the primary out, and it was all good but it gave me a peace of mind.

Put the spring back on the secondary.

Checked collimation but I could not get it to work.

The solution? The bloody Cheshire was the issue. For some reason I didn't check earlier but as I was rotating within the focuser, it seemed the central cross hair were a bit off!! As a result it was 'offsetting' the centre and subsequently the primary dot shadow. It was 3/4 centred and after I noticed and accounted for it bang it all fell in place! 

All clips visible, spring in secondary and plenty of room for wiggle.

Long and short Cheshire cross hair aligned, laser spot on. Star test nice concentric airy rings.

@PeterStudzworth checking your Cheshire.

@bosun21and @Mr Spockthanks for your advice and offers to help. The primary springs look quite strong too.

Well done Kostas. Great to hear that you found the culprit to your troubles. I'm also pleased to hear that you replaced the secondary mirror holder springs as they definitely serve a purpose. You would have had perpetual frustration trying to collimate with a dodgy cheshire had you not discovered it. I suspect you will like me be doing some lunar imaging now that the planets are gone. I may PM you if you don't mind regarding lunar imaging.

[lvan]

On 23/02/2024 at 11:42, Kon said:

I meant he had it out observing 6 times not the primary 🙂. He told me the collimation is very good but when I checked it was awful. He was using a laser only.  It was a genuine sale of somebody who never got into the hobby.

I wonder how many newcomers we’ve lost from the hobby due to a misaligned newtonian as a first scope.

[PeterStudz]

@Kon, that great that you’ve sorted out the issue and also good to know the reason.

I did check my Cheshire and it’s fine. In fact I have two and they’d both show the same problem, so two are unlikely to both be out in the same way! And if you look hard the little aline collimation cap that I have would show the offset problem too. As an exercise I once tried collimation from scratch (both mirrors out) with just the cap. It’s more of a faff and takes longer but I was surprised, when I check it with the Cheshire, how good it all was. 

Out of interest… I think you have a short Cheshire. Is there a reason for that? Maybe a low profile focuser? 

[bosun21]

1 hour ago, PeterStudz said:

Out of interest… I think you have a short Cheshire. Is there a reason for that? Maybe a low profile focuser? 

I think he thought it would enable him to see the primary mirror clips but I may be completely wrong about this assumption. I'm just glad that he now has good collimation and will now be imaging the moon. I want to pick his brains as I'll also be lunar imaging until the planets return. Darn it now you know why I was trying to help him so enthusiastically 😂.

[Kon]

3 hours ago, PeterStudz said:

Out of interest… I think you have a short Cheshire. Is there a reason for that? Maybe a low profile focuser? 

 

1 hour ago, bosun21 said:

I think he thought it would enable him to see the primary mirror clips but I may be completely wrong about this assumption. I'm just glad that he now has good collimation and will now be imaging the moon. I want to pick his brains as I'll also be lunar imaging until the planets return. Darn it now you know why I was trying to help him so enthusiastically 😂.

Peter as Ian said it is so i can see the primary clips. I had a long that I still use with my 8".

Collimation is ok but during a quick imaging test last night, the results were not so good. You can see this awful flaring on both the planet and moon so I am still a long way from having it perfect. Only obvious on the very stretched image (I am quite picky)

I would love to start imaging rills on the moon as they are my favourite features.  You can message me whenever you want. I hope to be up and running soon.

Frustrating but I am enjoying and I am getting there slowly. At least I am not missing planetary imaging time so I have loads of time to sort the collimation.

[Kon]

So I am still having issues with collimation. This is the star test and something is not quite right, and I cannot point to it. Out of focus looks alright but the in focus is a bit odd. Ignore the bad seeing. To me it is the lack of symmetry; the primary was not locked so i do not think it is pinched optics (I had the primary out a few weeks ago and the retaining clips are loose too). Any thoughts?

 

[Mr Spock]

The oval shape is a sign of astigmatism.

[Kon]

26 minutes ago, Mr Spock said:

The oval shape is a sign of astigmatism.

 

if so, what's the remedy? Or nothing it can be done? But I wonder how much it is bad seeing. I need t orepeat the star test with my camera. Unless the primary had not cooled down although it was out for a good 3 hours when I did the star test.

Edited March 5 by Kon

[Ratlet]

What angle do you have the scope at when you collimate?  I made the mistake of collimating with it flat when I tilted it up to observe the mirror would shift slightly and affect the collimation.  Now I collimate at 45°.

I'm not doubting Mr Spock's diagnosis (he knows far more than me) and is probably correct, it's just afaik there is nothing to be done about astigmatism, so I'm grasping at straws.  Also I think the GIFs you post have a fair bit of seeing going on and there are bits in the second gif that makes the star look out of alignment from the diffraction rings.

[Mr Spock]

What you should do is repeat the test with better seeing to see if the distortion is consistent. If it is, -

First thing to check is if it is the primary or secondary. If it is the primary, the oval shape should change direction if you rotate the primary cell.

If it is the primary, check to see if the mirror is being distorted by being held too tight.

Same if it's the secondary - it's probably miss-mounted or being gripped too tight in the cell.

If either of the mirrors is astigmatic themselves and it's not mechanical, then the only cure is refiguring 

[bosun21]

Looks like a case of bad seeing and atmospheric turbulence to me. I have saw similar images while attempting a star test under such poor conditions only for them to resolve on a subsequent night with decent seeing. I would definitely wait for a better nights seeing before exploring the dodgy mirrors avenue. You said that the mirror clips are loose, but just how loose? They should still be able to hold the primary in position albeit without placing any undue pressure on the mirror surface.

[Kon]

5 hours ago, Mr Spock said:

What you should do is repeat the test with better seeing to see if the distortion is consistent. If it is, -

First thing to check is if it is the primary or secondary. If it is the primary, the oval shape should change direction if you rotate the primary cell.

If it is the primary, check to see if the mirror is being distorted by being held too tight.

Same if it's the secondary - it's probably miss-mounted or being gripped too tight in the cell.

If either of the mirrors is astigmatic themselves and it's not mechanical, then the only cure is refiguring 

I will give it a try with rotating the primary. Regarding the secondary, there is no cell just the glue at the back on the he holder, the standard skywatcher setup. I will rotate the primary and if it persists in the same direction, is it likely to be secondary?

4 hours ago, bosun21 said:

Looks like a case of bad seeing and atmospheric turbulence to me. I have saw similar images while attempting a star test under such poor conditions only for them to resolve on a subsequent night with decent seeing. I would definitely wait for a better nights seeing before exploring the dodgy mirrors avenue. You said that the mirror clips are loose, but just how loose? They should still be able to hold the primary in position albeit without placing any undue pressure on the mirror surface.

I agree seeing is not great but stars at the eyepiece are not pinpoint but have a bit of flaring. The clips are not touching the mirror as far as I can tell, but I can do the credit card check. There are 3 nylon screws between the retaining ring of the cell and primary that I have quite tight, these are on the side. Could that be it? I will loosen them and check.

[Kon]

5 hours ago, Ratlet said:

What angle do you have the scope at when you collimate?  I made the mistake of collimating with it flat when I tilted it up to observe the mirror would shift slightly and affect the collimation.  Now I collimate at 45°.

I'm not doubting Mr Spock's diagnosis (he knows far more than me) and is probably correct, it's just afaik there is nothing to be done about astigmatism, so I'm grasping at straws.  Also I think the GIFs you post have a fair bit of seeing going on and there are bits in the second gif that makes the star look out of alignment from the diffraction rings.

Not flat but at a high angle. My issue is flaring on Jupiter's moons thus my concern something is not quite right.

[Kon]

I think I have a solution. It was probably pinched optics; I took the primary out and although the retaining clips where loose (credit card could fit), it was the side nylon screws between the retaining ring and mirror that I had tightened quite a bit. Collimation and star test revealed nice concentric rings and no flaring at the stars. Out of focus star (the infocus was awful to make any judgments).

[Spile]

If I had to collimate my reflector at a particular angle I would be worried.