collimation

[spillage]

Sorry for the really daft question but I am pulling my hair out.

SW quattro 8"

If when running a star test the outer of the star is not a perfect circle is this just due to the secondary not being square? I have a funny feeling that the focuser is not square and may need re-aligning, but want to be sure before tearing it apart.  To me it seemed the secondary is to close to  one side of the scope wall (not on the primary opening side)

Last night I tried a star test but for the life of me could not get the secondary to sit centrally in the star.

I normally use a baader lazer to get it roughly right then use my cheshire.

To be honest I swapped over my ed80 last saturday when I knew we had clear skies, this was my first mistake. I then got in a panic trying to get everything up and running and got myself into spin over it all.

 

Cheers,

 

 

 

 

[Ricochet]

6 hours ago, spillage said:

If when running a star test the outer of the star is not a perfect circle 

What shape is it? Is it the same on both sides of focus? How much are you defocusing to see this? 

6 hours ago, spillage said:

To me it seemed the secondary is to close to  one side of the scope wall (not on the primary opening side)

In a fast scope the secondary is offset away from the focuser. This offset is probably built into the secondary support so if you measure from the centre screw of the secondary to the walls of the OTA and get it centred it will probably be good enough. 

6 hours ago, spillage said:

Last night I tried a star test but for the life of me could not get the secondary to sit centrally in the star.

This sounds like you are defocusing enough to see the secondary shadow, which is too much. You should only be defocusing enough to see diffraction rings as far as I know. 

[spillage]

Thanks for you input @Ricochet.

It appeared that the secondary was not perfect so making the star elongated. I was aware of the offset but it was the actual secondary mirrors position using the cheshire although this was make easier using my sight cap only. 

I eventually stripped it back and took my time. To make things easier I connected my handset to the mount to ensure the star was central and things really do look much better.

I tried to run a full test last night but after rebooting 6 times and it being 1am before platesolve started to work I gave up without a single frame.  Looking at the platesolve images things are looking okay.

[Ricochet]

On 26/04/2019 at 11:17, spillage said:

It appeared that the secondary was not perfect so making the star elongated. I was aware of the offset but it was the actual secondary mirrors position using the cheshire although this was make easier using my sight cap only. 

That's interesting. In what way was the secondary incorrectly positioned to give this elongation?

[spillage]

I just need to take my time and adjust the rotation of the secondary. Using the cheshire was making it harder to see the secondary correctly and by using my collimation cap really helped. I think I was trying to rush it in order to get some imaging time in. I still have to do a real test and even though I had clear skies last night it was very gusty.

I did try last Thursday but I took me till 1am to get platesolve to actually solve an image and when it just decided to start to work guiding was playing up so I went to bed.