Collimation Frustration, help :(

[jezhughes]

Hi everyone and apologies in advance as this may become quite a long post. Maybe take a loo break now or grab a coffee if you're here for the ride?

I've been having a nightmare over the last couple of weeks with my first full collimation of my newt. Until now i've been getting away with just tweaking the primary and getting away with it. The other week i started fiddling with the secondary.. (regrettable but at the time, needed), and have since undergone a full collimation minus focuser.

Last night i probably got it the closest so far but its still off, i've redone the whole process about 5 times now and sunk a good 8 hours in to trying to get it right

I was having issues with a pinched mirror, but have since sorted that. I'm now getting astigmatism on the focus and after several times stripping apart and redoing it, im still getting it. I'm using a Hotec laser collimator and a cheshire, they matching up almost perfectly and i've been meticulous with making sure i've followed each step exactly.

Here is the astigmatism i'm seeing through extra and intra focus. It clouded over last night so i had to take this of a air light on top of a industrial chimney about 2 miles away. This is pretty much the same effect as what i was seeing when star collimating though.

As you can see there's the shift in focal bloat as it goes in and out of focus. This was much worse but ive calmed it down a bit.

Here is a quick snap through my cheshire, from which i really can't see a misalignment. This is through a 250p so the secondary is offset. sorry its not a great shot.

So to my question, and sorry if it comes across vague, but where am i going wrong here?

What i have noticed / some notes:

- When turning my scope over on the mount i've noticed a bump in the collimation when checking with a laser, i can only assume this is where the weight of the primary is shifting a tiny bit. but the above photos were taken when everything had settled and i checked collimation.

- There is a slight collimation error with the focus travel, with the full focus travel i see the laser point travel about half and inch out of the doughnut. But does this explain the above? I collimated with the focus out, and when checking still got an error.

- FYI, the secondary spider is straight, one of the spines has a slight kink but its really very minute

- I followed the Astro baby collimation guide (so you know what process i took)

- The Hotec has a SCA, so i doubt this is down to snugging on the focuser.

I think that's about all i can think of mentioning right now, so i hope someone will be able to lend some experience / advise.

I'm thinking about investing in a new focuser, but unless someone says thats a blindingly obvious reason for the errors i'm not assuming that's at fault

Thanks so much in advance!

[andrew s]

If you are seeing astigmatism then I would suspect that one of the mirrors is not figured correctly - most probably the primary but a distorted secondary can cause it. Have you ever seen non astigmatic images? I say this because you would find coma dominating long before astigmatism from an off axis/poorly collimated parabolic mirror. Regards Andrew

[Astro_Baby]

Two things to check...

Primary mirror clips. These should be cinched down but not si much that you cant slip a piece of paper between them and the mirror surface. The clips should be just off the surface BUT the lateral asjustment should be snug. ie the sides of the mirror clips should be butted against the sides of the mirror. This most likley explains the bump when rotating the scope. With this present you wont get solid collimation.

Focuser.........i cant see why a focuser should cause astigmatism but the most common fault is focuser slop. Thats where the focuser tube wobbles on the way up and down focus. A solid focuser does make all sorts lf collimation problems go away but I cant say id it would help with your issues.

[Astro_Baby]

Looking at your pic down the tube with the ches. the seckndary mirror does seem way out from centre of the focuser tube. The seckndary should be centred with respect to the focuser and yours doesnt seem to be. Is that down to your picture or actually how it is ?

[NGC 1502]

Astigmatism is usually seen as an out of round oval when viewing an out of focus star, that flips orientation if you defocus in the other direction.

Pinched optics often shows as a triangular out of focus star.

These effects can be quite subtle if the problem is minor. Some online pics show major problems when they are easier to spot.

Star collimating is not the same as star testing - the former is for aligning the mirrors, the latter is to evaluate the quality of the optics, although they both use the out of focus star image.

When collimating using an out of focus star, you are looking for the secondary shadow to be concentric within the expanded star disc, when star testing to evaluate mirror quality you are checking how close the appearance of the expanded star looks either side of focus.

Most of the popular stuff sold today has decent quality mirrors, that when properly collimated, the scope cooled to ambient and good seeing conditions will give images that are fully good enough for the majority of people.

You mention the possibility of the mirror shifting when the scope is used. Of course, the mirror clips must not clamp the mirror, but just the tiniest clearance between the clips & mirror is fine, the oft quoted gap of 'the thickness of a credit card' is not needed, and could let the mirror shift around.

The kink in the spider won't cause much of a problem, you can still collimate accurately. It could perhaps cause a very tiny loss of contrast due to diffraction.

If there is a lot of slop in the focuser draw tube, that could cause a shift in collimation. Most focusers have adjustments to remove the slack.

You need to get the secondary collimated first, then tackle the primary. If you try to collimate by just doing the primary, then unless you are lucky and the secondary is ok, then you won't sort it.

Hope you get everything ok, regards, Ed.

[Moonshane]

I find it very difficult to interpret photos of collimation. I very much doubt you are seeing astigmatism. to my eyes anyway the images show that you are well out of the focus range required to see anything in terms of star testing/collimation.

I think what you are seeing is the secondary not being correctly rotated and therefore presenting a less than round 'face' to the focuser / primary. I also doubt that unless your scope is very fast that a very small movement in the position of the primary will generate large deterioration of image - a bit but not a lot. just put a piece of sticky felt onto one or more of your clips to hold the sides of the mirror to reduce lateral movement if needed.

surely the secondary is aligned accurately enough to the focuser as the primary donut sits pretty much in line with the cross hairs on the cheshire and the cheshire eye hole is in line with the donut on the primary. I think your problem is rotational re the secondary.

but as I say, I have great difficulty sometimes understanding the images!

what do the stars look like when you look with your eye and not your camera? do they look like this ?

[knobby]

Hi everyone and apologies in advance as this may become quite a long post. Maybe take a loo break now or grab a coffee if you're here for the ride?

I've been having a nightmare over the last couple of weeks with my first full collimation of my newt. Until now i've been getting away with just tweaking the primary and getting away with it. The other week i started fiddling with the secondary.. (regrettable but at the time, needed), and have since undergone a full collimation minus focuser.

Last night i probably got it the closest so far but its still off, i've redone the whole process about 5 times now and sunk a good 8 hours in to trying to get it right

I was having issues with a pinched mirror, but have since sorted that. I'm now getting astigmatism on the focus and after several times stripping apart and redoing it, im still getting it. I'm using a Hotec laser collimator and a cheshire, they matching up almost perfectly and i've been meticulous with making sure i've followed each step exactly.

Here is the astigmatism i'm seeing through extra and intra focus. It clouded over last night so i had to take this of a air light on top of a industrial chimney about 2 miles away. This is pretty much the same effect as what i was seeing when star collimating though.

As you can see there's the shift in focal bloat as it goes in and out of focus. This was much worse but ive calmed it down a bit.

Here is a quick snap through my cheshire, from which i really can't see a misalignment. This is through a 250p so the secondary is offset. sorry its not a great shot.

So to my question, and sorry if it comes across vague, but where am i going wrong here?

What i have noticed / some notes:

- When turning my scope over on the mount i've noticed a bump in the collimation when checking with a laser, i can only assume this is where the weight of the primary is shifting a tiny bit. but the above photos were taken when everything had settled and i checked collimation.

- There is a slight collimation error with the focus travel, with the full focus travel i see the laser point travel about half and inch out of the doughnut. But does this explain the above? I collimated with the focus out, and when checking still got an error.

- FYI, the secondary spider is straight, one of the spines has a slight kink but its really very minute

- I followed the Astro baby collimation guide (so you know what process i took)

- The Hotec has a SCA, so i doubt this is down to snugging on the focuser.

I think that's about all i can think of mentioning right now, so i hope someone will be able to lend some experience / advise.

I'm thinking about investing in a new focuser, but unless someone says thats a blindingly obvious reason for the errors i'm not assuming that's at fault

Thanks so much in advance!

No expert but if the laser spot is moving 1/2 an inch when the focuser is extended I'd say the first thing to do is to get that straight, maybe shimming or does yours have adjustment screws.

[bomberbaz]

If I can add my little bit here, I was scared to death about collination but once I saw a live demonstration that was explained, it made it sound so simple. I had read a lot of threads and advice posts about this but 10' mins with a man who can, well qorth a weeks reading.

My advice is try to find someone local to meet and speak with. Steve

[paul mc c]

Been following this,would a laser colllomator help to make sure the secondary is alligned properly or is a Cheshire best.

[Moonshane]

a laser cannot be used to correct the secondary being rotated incorrectly and most cheshire's don't allow enough field of view (obviously we need a nagler cheshire here ) to see it. this is where the collicap can be useful to get the correct position and rotation of the secondary before you start aligning the secondary or the primary with the laser or cheshire.

[jezhughes]

Thank you for everyone's reply, i appreciate the response!

I'll be trying out 3 things

1. Loosening the mirror clips
   
2. Collimating the focuser better
   
3. Re collimating the secondary (again)
   

I really don't think (1) and (2) are the issue here, although ive got no experience to go by; but i've seen a pinched mirror before and its not similar to what im seeing. I also had a really (really) tight factory fastening pre collimation but didn't have pinch problems then.

I'll update at the weekend when i have another bash!

If you are seeing astigmatism then I would suspect that one of the mirrors is not figured correctly - most probably the primary but a distorted secondary can cause it. Have you ever seen non astigmatic images? I say this because you would find coma dominating long before astigmatism from an off axis/poorly collimated parabolic mirror. Regards Andrew

Thank you, i will check that but im not sure if by 'figure' you mean a process i haven't done already?

Two things to check...

Primary mirror clips. These should be cinched down but not si much that you cant slip a piece of paper between them and the mirror surface. The clips should be just off the surface BUT the lateral asjustment should be snug. ie the sides of the mirror clips should be butted against the sides of the mirror. This most likley explains the bump when rotating the scope. With this present you wont get solid collimation.

Focuser.........i cant see why a focuser should cause astigmatism but the most common fault is focuser slop. Thats where the focuser tube wobbles on the way up and down focus. A solid focuser does make all sorts lf collimation problems go away but I cant say id it would help with your issues.

Thanks for that insight, its possible that my clips are still a bit tight so i'll definately loosen up and see how that differs.

Looking at your pic down the tube with the ches. the seckndary mirror does seem way out from centre of the focuser tube. The seckndary should be centred with respect to the focuser and yours doesnt seem to be. Is that down to your picture or actually how it is ?

Sorry, that's the picture. Its just shot off angle down the first sight view making it look off; when in fact when looking down the cheshire, it's all aligned up.

Astigmatism is usually seen as an out of round oval when viewing an out of focus star, that flips orientation if you defocus in the other direction.

Pinched optics often shows as a triangular out of focus star.

These effects can be quite subtle if the problem is minor. Some online pics show major problems when they are easier to spot.

Star collimating is not the same as star testing - the former is for aligning the mirrors, the latter is to evaluate the quality of the optics, although they both use the out of focus star image.

When collimating using an out of focus star, you are looking for the secondary shadow to be concentric within the expanded star disc, when star testing to evaluate mirror quality you are checking how close the appearance of the expanded star looks either side of focus.

Most of the popular stuff sold today has decent quality mirrors, that when properly collimated, the scope cooled to ambient and good seeing conditions will give images that are fully good enough for the majority of people.

You mention the possibility of the mirror shifting when the scope is used. Of course, the mirror clips must not clamp the mirror, but just the tiniest clearance between the clips & mirror is fine, the oft quoted gap of 'the thickness of a credit card' is not needed, and could let the mirror shift around.

The kink in the spider won't cause much of a problem, you can still collimate accurately. It could perhaps cause a very tiny loss of contrast due to diffraction.

If there is a lot of slop in the focuser draw tube, that could cause a shift in collimation. Most focusers have adjustments to remove the slack.

You need to get the secondary collimated first, then tackle the primary. If you try to collimate by just doing the primary, then unless you are lucky and the secondary is ok, then you won't sort it.

Hope you get everything ok, regards, Ed.

Thank you Ed. I've seen a pinched optic before as i had that problem initially, but what i'm seeing is slightly different. Perhaps the secondary is still out so i will have to take a much closer look at that. But i'd be surprised at such a small offset potentially having this impact on collimation.

I find it very difficult to interpret photos of collimation. I very much doubt you are seeing astigmatism. to my eyes anyway the images show that you are well out of the focus range required to see anything in terms of star testing/collimation.

I think what you are seeing is the secondary not being correctly rotated and therefore presenting a less than round 'face' to the focuser / primary. I also doubt that unless your scope is very fast that a very small movement in the position of the primary will generate large deterioration of image - a bit but not a lot. just put a piece of sticky felt onto one or more of your clips to hold the sides of the mirror to reduce lateral movement if needed.

surely the secondary is aligned accurately enough to the focuser as the primary donut sits pretty much in line with the cross hairs on the cheshire and the cheshire eye hole is in line with the donut on the primary. I think your problem is rotational re the secondary.

but as I say, I have great difficulty sometimes understanding the images!

what do the stars look like when you look with your eye and not your camera? do they look like this ?

Thanks for your insight! To your question, that's correct (roughly what i'm seeing). As i mentioned though, i spent a good few hours on the secondary and especially the rotation. I would be surprised if such a small offset would cause this. Then again... everything surprises me with collimation!

No expert but if the laser spot is moving 1/2 an inch when the focuser is extended I'd say the first thing to do is to get that straight, maybe shimming or does yours have adjustment screws.

I'll definately collimate the focus as a next step to try and reduce this. I want to get a new focuser anyway (at some point) so hopefully the slop in travel will reduce then.

If I can add my little bit here, I was scared to death about collination but once I saw a live demonstration that was explained, it made it sound so simple. I had read a lot of threads and advice posts about this but 10' mins with a man who can, well qorth a weeks reading.

My advice is try to find someone local to meet and speak with. Steve

Thanks! I can't imagine how i'd go about that i've watched a tonne of videos, i wouldn't have got anywhere without them; but they're not that good for specific problems and trouble shooting

a laser cannot be used to correct the secondary being rotated incorrectly and most cheshire's don't allow enough field of view (obviously we need a nagler cheshire here ) to see it. this is where the collicap can be useful to get the correct position and rotation of the secondary before you start aligning the secondary or the primary with the laser or cheshire.

That's correct, i used a collicap for the secondary, not the cheshire

[knobby]

Good luck