How do I make the primary mirror clips appear?

[Synq]

I try to collimate telescope because Jupiter and Saturn looked bad and I center primary in secondary and I make secondary look as circular and centered in focuser as I could with my equipment (home-made collimation cap). Then I align secondary in primary with the back nobs at the far end of the telescope. When I look in focuser I can only see one of the primary mirror clips fully, then other one by a half and last one, I can't see. I tried rotating secondary mirror rotators to no avail. Help.

[Pixies]

I think your secondary is rotated. It looks like the top of the secondary (in the picture) is rotated away from the focuser. You can see this in the reflection, too.

Can you post a picture of the above view, rather than a video. Also, how big is the hole in the collimation cap?

[Synq]

59 minutes ago, Pixies said:

I think your secondary is rotated. It looks like the top of the secondary (in the picture) is rotated away from the focuser. You can see this in the reflection, too.

Can you post a picture of the above view, rather than a video. Also, how big is the hole in the collimation cap?

Half a centimetre

[Tiny Clanger]

Are you sure you solved your previous problem ?

https://stargazerslounge.com/topic/366620-secondary-mirror-keeps-tilting-and-not-staying-in-position-after-first-attempt-in-collimation/

[Pixies]

Hi,

You might want to have a smaller eye-hole. On a Cheshire or collimation cap, the hole is usually around 2mm in diameter. You really need to make sure your view is dead-centre of the focuser.

Can I suggest you google "Astro Baby Collimation Guide", it'll really help. But I can see a few things that should help too.

Before you start, you really need to make sure that the secondary assembly is central in the tube. Measure the distance from the middle of the centre screw to the outside of the tube in several directions, it should be the same distance in all directions. Also - which only applies to these types of secondary assemblies that have a single arm attaching them to the tube - make sure the outer surface is completely parallel with the outside edge of the tube end. I hope you understand what I mean - that it hasn't twisted around the rod.

I can see that the secondary mirror holder is tilted away from the other part (with the screws). Here: 

and here:

This has happened because you have tried to align with the primary, when the secondary was twisted.

I would suggest that you try and get these 2 parts parallel right at the start. The 3 secondary adjuster bolts should all be at the same depth to start with, before you try to rotate the secondary to get it looking circular in the collimation cap view.

 

Does that make sense? Drop me a PM if you want a hand

 

 

[Synq]

41 minutes ago, Pixies said:

Hi,

You might want to have a smaller eye-hole. On a Cheshire or collimation cap, the hole is usually around 2mm in diameter. You really need to make sure your view is dead-centre of the focuser.

Can I suggest you google "Astro Baby Collimation Guide", it'll really help. But I can see a few things that should help too.

Before you start, you really need to make sure that the secondary assembly is central in the tube. Measure the distance from the middle of the centre screw to the outside of the tube in several directions, it should be the same distance in all directions. Also - which only applies to these types of secondary assemblies that have a single arm attaching them to the tube - make sure the outer surface is completely parallel with the outside edge of the tube end. I hope you understand what I mean - that it hasn't twisted around the rod.

I can see that the secondary mirror holder is tilted away from the other part (with the screws). Here: 

and here:

This has happened because you have tried to align with the primary, when the secondary was twisted.

I would suggest that you try and get these 2 parts parallel right at the start. The 3 secondary adjuster bolts should all be at the same depth to start with, before you try to rotate the secondary to get it looking circular in the collimation cap view.

 

Does that make sense? Drop me a PM if you want a hand

 

 

So first of all, let’s say I measure from the secondary mirror centre screw to the outer edges in different places and the distances are not the same, how exactly do I centralise it?
 

“Make sure the outer surface is completely parallel with the outside edge of the tube end.” Would you mind explaining this further? I don’t understand.

I will adjust the 3 bolts so they are the same length.

Lastly, how do I rotate the secondary mirror to get it circular? Do I loosen the 3 adjustment bolts and rotate the mirror manually?

Thanks.

 

[Synq]

53 minutes ago, Tiny Clanger said:

Are you sure you solved your previous problem ?

https://stargazerslounge.com/topic/366620-secondary-mirror-keeps-tilting-and-not-staying-in-position-after-first-attempt-in-collimation/

Thanks for the concern. I lodged a bit of folded paper between the rod and mirror as a temporary fix. It doesn’t move and will only move if I push or pull it.

 

I just don’t know which way I point it. Do I point it where the primary mirror is centred within it?

[runway77]

if you have a f/5 or faster telescope you might not necessarily be able to see the clips. this is the case for my 150 pds. In my f/6 dob I can see all three clips in the cheshire eyepiece. 

[Pixies]

9 hours ago, Synq said:

So first of all, let’s say I measure from the secondary mirror centre screw to the outer edges in different places and the distances are not the same, how exactly do I centralise it?

The only way you can adjust it is along the axis of the arm holding it. I would expect that it will be central perpendicular to that - unless the arm has been bent. 

9 hours ago, Synq said:

“Make sure the outer surface is completely parallel with the outside edge of the tube end.” Would you mind explaining this further? I don’t understand.

I mean - make sure this bit: 

is not twisted - ie. the top face of it is parallel with the plane of the end of the tube. It does sound like you have had issues with this. How is it fixed to the arm normally? Is it screwed in?

9 hours ago, Synq said:

I will adjust the 3 bolts so they are the same length.

Assuming the above is OK, a good way of doing this is to wind out the 3 adjusters and tighten the central screw, which will gradually bring the mirror back up the tube. Keep going until you have brought it all the way up and it is flush with the bit arrowed above. Then, bring it back down the tube by slackening the central screw and taking up the slack by tightening the 3 adjusters by equal amounts each time. The mirror will move back down the tube but remain parallel with the top bit. Keep going until it is centered under the focuser.

9 hours ago, Synq said:

Lastly, how do I rotate the secondary mirror to get it circular?

yes - loosen the 3 adjusters, or slacken the central screw slightly. This open design makes it easy to see the orientation of the mirror. You are trying to rotate it so that it is 'facing' the focuser. The elliptical shape should be aligned so that the 'long' direction is in line with the focuser. It is shaped so that it appears as a circle in the focuser, when it is facing it like that.

 

This all appears very complicated at first, but once you get your head around what you are trying to achieve, it should make sense.  The fact that your secondary assembly could rotate on the arm is a problem that needs sorted though. Perhaps another heritage owner can comment?

Edited December 3, 2020 by Pixies

[Synq]

5 hours ago, runway77 said:

if you have a f/5 or faster telescope you might not necessarily be able to see the clips. this is the case for my 150 pds. In my f/6 dob I can see all three clips in the cheshire eyepiece. 

I have an f/5 scope.

[Synq]

2 hours ago, Pixies said:

The only way you can adjust it is along the axis of the arm holding it. I would expect that it will be central perpendicular to that - unless the arm has been bent. 

I mean - make sure this bit: 

is not twisted - ie. the top face of it is parallel with the plane of the end of the tube. It does sound like you have had issues with this. How is it fixed to the arm normally? Is it screwed in?

Assuming the above is OK, a good way of doing this is to wind out the 3 adjusters and tighten the central screw, which will gradually bring the mirror back up the tube. Keep going until you have brought it all the way up and it is flush with the bit arrowed above. Then, bring it back down the tube by slackening the central screw and taking up the slack by tightening the 3 adjusters by equal amounts each time. The mirror will move back down the tube but remain parallel with the top bit. Keep going until it is centered under the focuser.

yes - loosen the 3 adjusters, or slacken the central screw slightly. This open design makes it easy to see the orientation of the mirror. You are trying to rotate it so that it is 'facing' the focuser. The elliptical shape should be aligned so that the 'long' direction is in line with the focuser. It is shaped so that it appears as a circle in the focuser, when it is facing it like that.

 

This all appears very complicated at first, but once you get your head around what you are trying to achieve, it should make sense.  The fact that your secondary assembly could rotate on the arm is a problem that needs sorted though. Perhaps another heritage owner can comment?

I’ll try to follow the guide and your steps.

 

Thanks so much.