Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Laser Collimation


Recommended Posts

I thought I was well and truly done with any collimation talk. I cleaned my dob primary mirror for the first time recently and whilst I started off with a Cheshire for all collimation, which I still think is best overall, I added a cheap laser that I collimated and have been using barlowed, for visual use. It's much quicker to get 99% of the way there and convenient with little available light. I hadn't gone looking before but did have a peek following the mirror clean as to whether the laser centred on the primary mirror, perfectly in the central ring. It doesn't, it sits just slightly outside the central circle. Here's the thing, I had never checked whether the spider vanes were as they should be (equal distance) out of the box before now and so found that they were a little out. Thinking that would be the answer, it didn't prove to be, following their adjustment. Is it simply that the slight tilt on the primary when aligned with the secondary (collimated well) does throw it off centre?

I use a concentre to set up the secondary, all is perfect there and the primary is simple to adjust from there. There does appear to be one axis that is not exactly the same as the other on the spider vanes, which could suggest the tube is slightly oval and not perfectly round. Maybe that has an effect. I'm thinking that it probably won't make a jot of difference for visual use, but I'd be interested to hear any reasoning around this. When I was using just a collimation cap and Cheshire, I would have been non the wiser, which almost confirms that in the real world, it doesn't matter?  But I'm interested anyway. 

Link to comment
Share on other sites

An interesting read on the subject, with useful illustrations, is New Perspectives on Newtonian Collimation by Vic Menard.

FLO used to stock it, maybe still do.

Following the read of that I now use Jim Fly's Catseye system on all my reflectors, much more accurate than a laser, the precisely machined autocollimator allows for very accurate collimation. 

Very few Newts are properly set up out of the box, quite often have to square up the focuser to the optical plane and so on, and don't forget that the optical plane does not necessarily have to be square to the tube 😜  

So many variables!!!

Tim

  • Like 2
Link to comment
Share on other sites

5 hours ago, Stardaze said:

Is it simply that the slight tilt on the primary when aligned with the secondary (collimated well) does throw it off centre?

No. Tilting the primary doesn't move the dot out of the doughnut at all. The dot location on the primary is determined by how the secondary is reflecting the laser beam.

To get accurate collimation one must always ensure the dot is in the centre of the primary prior to using the Barlowed laser technique for the primary alignment.

Having said that, sometimes a laser can have one believe the secondary is collimated by showing the dot in the centre of the primary. Under closer inspection one might find that the secondary is slightly rotated and tilted more on one side to account for it.

I was a big fan of using a laser for collimation too but after more experience I am now wary of them for the reasons listed above.

IMO a very accurate way to collimate is by using a Cheshire sightube for the secondary alignment, then ensure it is rotated properly and that the three primary clips are even in the secondary. Then simply use a collimation cap to ensure the primary is collimated. As you peer through the collimation cap everything should look completely concentric.

When you know everything is completely concentric, then pop your laser in to see if it agrees. If it does, there might have been a slight tilt in your secondary. If it doesn't, the problem is with the laser, and from experience it is possibly with how it is seated in the holder.

  • Like 1
Link to comment
Share on other sites

7 hours ago, Pitch Black Skies said:

No. Tilting the primary doesn't move the dot out of the doughnut at all. The dot location on the primary is determined by how the secondary is reflecting the laser beam.

To get accurate collimation one must always ensure the dot is in the centre of the primary prior to using the Barlowed laser technique for the primary alignment.

Having said that, sometimes a laser can have one believe the secondary is collimated by showing the dot in the centre of the primary. Under closer inspection one might find that the secondary is slightly rotated and tilted more on one side to account for it.

I was a big fan of using a laser for collimation too but after more experience I am now wary of them for the reasons listed above.

IMO a very accurate way to collimate is by using a Cheshire sightube for the secondary alignment, then ensure it is rotated properly and that the three primary clips are even in the secondary. Then simply use a collimation cap to ensure the primary is collimated. As you peer through the collimation cap everything should look completely concentric.

When you know everything is completely concentric, then pop your laser in to see if it agrees. If it does, there might have been a slight tilt in your secondary. If it doesn't, the problem is with the laser, and from experience it is possibly with how it is seated in the holder.

Thanks for taking the time here. The laser seating is possibly the cause yes, though I have a click lock. I prefer the concentre to a basic cap. The Cheshire I find to be most accurate for the primary. I’ll have a look again this morning, realise it’s the focuser or secondary where the issue is. 

  • Like 1
Link to comment
Share on other sites

You're welcome. To go a step further than levelling the focuser, align it so that it is perpendicular to the opposite side of the tube. In fact this should be done before ever collimating the telescope IMO. This gent does a great job at explaining it. It starts at 27:15.

Have you verified that the doughnut is centred while you had the primary out? It isn't really connected to the problem you're having with the off center laser dot, but it's good to know for that extra level of accuracy. I've had to recenter the primary spot on every new telescope I've bought so far.

The template included with the Cats Eye by Jim Fly is a useful tool for this.

BTW if the spider vanes are over tightened they can pull the OTA into an oval shape. Not a deal breaker, as long as the focuser is aligned and the secondary is concentric to it.

Keep at it, you will figure it out.

  • Thanks 1
Link to comment
Share on other sites

24 minutes ago, Pitch Black Skies said:

You're welcome. To go a step further than levelling the focuser, align it so that it is perpendicular to the opposite side of the tube. In fact this should be done before ever collimating the telescope IMO. This gent does a great job at explaining it. It starts at 27:15.

Have you verified that the doughnut is centred while you had the primary out? It isn't really connected to the problem you're having with the off center laser dot, but it's good to know for that extra level of accuracy. I've had to recenter the primary spot on every new telescope I've bought so far.

The template included with the Cats Eye by Jim Fly is a useful tool for this.

BTW if the spider vanes are over tightened they can pull the OTA into an oval shape. Not a deal breaker, as long as the focuser is aligned and the secondary is concentric to it.

Keep at it, you will figure it out.

I didn’t check whether the doughnut is centred I’ll admit whilst I had the primary out. Quite a few variables that I skipped early on. Didn’t realise how out the vanes were either, I won’t assume again. 

  • Like 1
Link to comment
Share on other sites

1 hour ago, Pitch Black Skies said:

You're welcome. To go a step further than levelling the focuser, align it so that it is perpendicular to the opposite side of the tube. In fact this should be done before ever collimating the telescope IMO. This gent does a great job at explaining it. It starts at 27:15.

Have you verified that the doughnut is centred while you had the primary out? It isn't really connected to the problem you're having with the off center laser dot, but it's good to know for that extra level of accuracy. I've had to recenter the primary spot on every new telescope I've bought so far.

The template included with the Cats Eye by Jim Fly is a useful tool for this.

BTW if the spider vanes are over tightened they can pull the OTA into an oval shape. Not a deal breaker, as long as the focuser is aligned and the secondary is concentric to it.

Keep at it, you will figure it out.

I think having watched that I could do with a full morning to take the mirrors out and ensure the focuser is perpendicular and the doughnut is centralised, looks a bit of a faff but peace of mind. Removing the central donut, if needed, looks a bit scary though. 😱

  • Like 1
Link to comment
Share on other sites

2 hours ago, Stardaze said:

Thanks for taking the time here. The laser seating is possibly the cause yes, though I have a click lock. I prefer the concentre to a basic cap. The Cheshire I find to be most accurate for the primary. I’ll have a look again this morning, realise it’s the focuser or secondary where the issue is. 

Loosen the clicklock slightly so that you can rotate the laser in the clicklock. Rotate the laser and watch the dot. If it stays on the same spot the laser is ok, if it draws a circle the laser needs collimating.

If the laser is OK move on to the clicklock. Loosen the 2" clamp a touch and rotate the clicklock. If the dot stays on the same point the clicklock is OK, if it draws a circle the clicklock clamping mechanism is tilting the laser. Also try unclamping the clicklock to check for repeatability. You should find that the laser always hits the same spot when clamped,

If the clicklock is OK, move on to your 2" clamp. You won't be able to check for rotation here but you can check repeatability. If you have a clamp with more than one thumbscrew try different screws and see what happens. If you've got the standard Bresser Hexafoc 2" clamp this is one of the places where your problem lies. The inner profile of the clamp is 2mm fixed - 8mm compression ring - 2mm fixed. That lower 2mm fixed section will always coincide with the location of an undercut, such as the one on the clicklock, so that the top fixed part of the clamp inner barrel becomes a pivot against which the compression ring tilts anything with an undercut. The solution here is either to change the 2" clamp or to change your 1.25"-2" adaptor to one that is smooth sided and never use any 2" eyepieces with undercuts.

My OTA is also slightly oval and to compensate I have added a couple of washers under two of the focuser faceplate fixing screws to get the focuser pointing closer to the correct point and then tweaked the secondary position by simultaneously shortening and lengthening the up/down spider vanes to get the secondary perfectly under the focuser. Having the secondary perfectly centred in the OTA is just a starting point, and not required for collimation.

  • Like 1
  • Thanks 1
Link to comment
Share on other sites

9 minutes ago, Stardaze said:

Removing the central donut, if needed, looks a bit scary though. 😱

Just sit the mirror in a bowl of water and it will loosen. Mine loosened and moved the first time I washed my mirror and had to be replaced.

  • Like 1
Link to comment
Share on other sites

1 hour ago, Ricochet said:

Loosen the clicklock slightly so that you can rotate the laser in the clicklock. Rotate the laser and watch the dot. If it stays on the same spot the laser is ok, if it draws a circle the laser needs collimating.

If the laser is OK move on to the clicklock. Loosen the 2" clamp a touch and rotate the clicklock. If the dot stays on the same point the clicklock is OK, if it draws a circle the clicklock clamping mechanism is tilting the laser. Also try unclamping the clicklock to check for repeatability. You should find that the laser always hits the same spot when clamped,

If the clicklock is OK, move on to your 2" clamp. You won't be able to check for rotation here but you can check repeatability. If you have a clamp with more than one thumbscrew try different screws and see what happens. If you've got the standard Bresser Hexafoc 2" clamp this is one of the places where your problem lies. The inner profile of the clamp is 2mm fixed - 8mm compression ring - 2mm fixed. That lower 2mm fixed section will always coincide with the location of an undercut, such as the one on the clicklock, so that the top fixed part of the clamp inner barrel becomes a pivot against which the compression ring tilts anything with an undercut. The solution here is either to change the 2" clamp or to change your 1.25"-2" adaptor to one that is smooth sided and never use any 2" eyepieces with undercuts.

My OTA is also slightly oval and to compensate I have added a couple of washers under two of the focuser faceplate fixing screws to get the focuser pointing closer to the correct point and then tweaked the secondary position by simultaneously shortening and lengthening the up/down spider vanes to get the secondary perfectly under the focuser. Having the secondary perfectly centred in the OTA is just a starting point, and not required for collimation.

Thanks. I’ll reread and digest that again later. The laser is fine as I’ve knocked up a little rig to collimate it.

The hexaloc fixings are clearly different to the Quattro focuser, only two grub screws with small allen keyed screws. A basic measure isn’t so straightforward measuring 4 points around, I found earlier.

Did you strip out and add a donut opposite the focuser to ensure the focuser is perpendicular?

Edited by Stardaze
Link to comment
Share on other sites

2 hours ago, Stardaze said:

I think having watched that I could do with a full morning to take the mirrors out and ensure the focuser is perpendicular and the doughnut is centralised, looks a bit of a faff but peace of mind. Removing the central donut, if needed, looks a bit scary though. 😱

Yeah, it's definitely not something you would want to be rushing. Give yourself plenty of time, or maybe do an hour here and there over a few days. It can be a bit painstaking but I was also going to mention like you said, you have peace of mind. It's also very satisfying to know everything is squared up. It's a good opportunity to get to know your telescope better too, us Newton owners are tinkerers at heart.

If my memory serves me correctly I think the chap accidentally broken his secondary mirror in that video I shared. Be super careful 😬😊

Ricochet's advice above is really good, it should really help narrow it down.

Be sure to update when you've found the culprit!

  • Thanks 1
Link to comment
Share on other sites

12 minutes ago, Pitch Black Skies said:

Yeah, it's definitely not something you would want to be rushing. Give yourself plenty of time, or maybe do an hour here and there over a few days. It can be a bit painstaking but I was also going to mention like you said, you have peace of mind. It's also very satisfying to know everything is squared up. It's a good opportunity to get to know your telescope better too, us Newton owners are tinkerers at heart.

If my memory serves me correctly I think the chap accidentally broken his secondary mirror in that video I shared. Be super careful 😬😊

Ricochet's advice above is really good, it should really help narrow it down.

Be sure to update when you've found the culprit!

Will do. I bought this dob really to get my head around everything. He did break the secondary in the video. I’ve had both mirrors out now so I’m not too bothered about dismantling, but definitely need some time for it all. I’ll have to get it something like for tomorrow night as it looks to be clear at last.
I still want to add a cooling fan at the bottom and he had a great mounting plate for that in the vid, which I think I’ll copy. 

  • Like 1
Link to comment
Share on other sites

3 hours ago, Stardaze said:

The hexaloc fixings are clearly different to the Quattro focuser, only two grub screws with small allen keyed screws. A basic measure isn’t so straightforward measuring 4 points around, I found earlier.

I'm not entirely sure what you're talking about here but I suspect that you may be trying to do something you've seen in the video that isn't possible with the Bresser focuser. If you can wind the focuser though the full range of travel with it always pointing at the same spot then as far as I know there aren't any grub screws you should be touching. I've shimmed mine with washers because there are no "squaring collimation" screws. The small grub screw (hole) that you can see in the side of the focuser base plate on the left of the photo just holds the focuser to the plate as far as I can tell and the grub screw further up the focuser to the right of the photo controls how the drawtube runs in the focuser.

DSC_2830.thumb.JPG.33a65b8930d88d5be080a621a092d04d.JPG

3 hours ago, Stardaze said:

Did you strip out and add a donut opposite the focuser to ensure the focuser is perpendicular?

No, I just drew a cross on the side with a pencil. I may have even drawn it on some masking tape that I later removed as squaring the focuser only ever needs to be done once. If there is only a slight error you don't ever have to do it as you can just shift the secondary a bit. So long as the secondary is under the focuser that's all that matters.

Edited by Ricochet
  • Like 1
Link to comment
Share on other sites

1 hour ago, Ricochet said:

I'm not entirely sure what you're talking about here but I suspect that you may be trying to do something you've seen in the video that isn't possible with the Bresser focuser. If you can wind the focuser though the full range of travel with it always pointing at the same spot then as far as I know there aren't any grub screws you should be touching. I've shimmed mine with washers because there are no "squaring collimation" screws. The small grub screw (hole) that you can see in the side of the focuser base plate on the left of the photo just holds the focuser to the plate as far as I can tell and the grub screw further up the focuser to the right of the photo controls how the drawtube runs in the focuser.

DSC_2830.thumb.JPG.33a65b8930d88d5be080a621a092d04d.JPG

No, I just drew a cross on the side with a pencil. I may have even drawn it on some masking tape that I later removed as squaring the focuser only ever needs to be done once. If there is only a slight error you don't ever have to do it as you can just shift the secondary a bit. So long as the secondary is under the focuser that's all that matters.

The skywatcher focuser had 3 screws for aligning but the bresser, as you say, doesn’t seem to have any adjustment that I can see? So the only thing you can do is to make sure the tube sits squarely on the plate and check from there it seems. If it has no proper way of aligning, surely most people don’t bother? 
I did wonder if just tilting the secondary to ensure the laser sits in the middle of the donut is all that is needed. It does sit under the focuser.

Link to comment
Share on other sites

31 minutes ago, Stardaze said:

If it has no proper way of aligning, surely most people don’t bother? 

Exactly. As the optical axis and the mechanical axis of the telescope don't need to coincide, all you have to do is to be able to get the secondary mirror under the focuser and then collimate from there. However, the squarer that the focuser is, the easier it is to collimate the secondary, which is why I shimmed mine a bit.

  • Like 1
Link to comment
Share on other sites

Just now, Ricochet said:

Exactly. As the optical axis and the mechanical axis of the telescope don't need to coincide, all you have to do is to be able to get the secondary mirror under the focuser and then collimate from there. However, the squarer that the focuser is, the easier it is to collimate the secondary, which is why I shimmed mine a bit.

Interesting. I did have a few minutes rolling the laser around in the click lock to see how that looked, wasn’t perfect. I’ve checked the laser and it was a tiny smidge out, so after half hour faffing, it’s probably slightly worse. I’ll sort that with a fresh head but led me to think, what if the click lock doesn’t centralise? 
 

Link to comment
Share on other sites

1 minute ago, Stardaze said:

Interesting. I did have a few minutes rolling the laser around in the click lock to see how that looked, wasn’t perfect. I’ve checked the laser and it was a tiny smidge out, so after half hour faffing, it’s probably slightly worse. I’ll sort that with a fresh head but led me to think, what if the click lock doesn’t centralise? 
 

Centralisation isn't strictly necessary. If it clamps squarely and repeatedly in the same position then you can just make sure to always put the clicklock in the 2" clamp at the same angle and use collimate to that point. If it isn't repeatable or it is tilting the laser and/or eyepieces then I would try a different 1.25"-2" adaptor and if confirmed as the issue talk to Baader about getting it replaced.

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.