vigneting not centre

[CedricTheBrave]

fitted a new steel track focuser to my 200pds!

i have spent a while collimating it using various methods and they all appear to match up and the star donut is good

however when i take some flats the visible vigneting is not centre it looks about 10mm towards the bottom of the frame when the top of the frame is the top of the scope

is this important? does it show that the collimation is still not correct?

[andrew s]

You can have a perfect star test and still have offset vignetting if the secondary is not centered under the focuser. You need a sight tube to do this. 

Also did you check the new focuser was squared  on to the tube?

Just thoughts. 

Regards Andrew 

Edited August 1, 2020 by andrew s

[tooth_dr]

5 minutes ago, andrew s said:

You can have a perfect star test and still have offset vignetting if the secondary is not centered under the focuser. You need a sight tube to do this. 

Also did you check the new focuser was squared  on to the tube?

Just thoughts. 

Regards Andrew 

Hi Andrew.
Is uneven illumination the only consequence of this?  But should be corrected with flats ok?

[andrew s]

1 minute ago, tooth_dr said:

Hi Andrew.
Is uneven illumination the only consequence of this?  But should be corrected with flats ok?

Yes for the vignetting.  Best for the focuser to be squared on as accurately as you can otherwise collimation  may change with focus position.

Regards Andrew 

[tooth_dr]

11 minutes ago, andrew s said:

Yes for the vignetting.  Best for the focuser to be squared on as accurately as you can otherwise collimation  may change with focus position.

Regards Andrew 

Thanks Andrew 👍🏻

[CedricTheBrave]

the way the steel track mounts on its difficult not to get it square unless you use different spacers

i will check the secondary again

thanks for your help

[newbie alert]

It's the focuser that's not square or central..it should have adjustment normally grub screws..flats will show it perfectly