Need advice on secondary collimation

[jonathan]

Hi,

I'm collimating my Skywatcher 150p reflector and have got so far with aligning the secondary according to AstroBaby's guide, however I seem to be at a conflict with the positioning in the focuser tube.  It looks to be aligned up/down the main tube, but not left/right (looking into the main tube from the open end) or up/down when looking down the focuser tube with a collimating cap on.

i was told to align the secondary to the middle using the spider vane adjustments, but I can't see any other way to move the mirror than to adjust the spider vanes, which would result in the secondary no longer being precisely central in the main tube.  Am I missing something?  I hope these photos explain it better.  To my eyes there seems to be clearly a larger gap at the bottom than at the top.

 

Edited December 9, 2020 by jonathan

[NGC 1502]

There’s other adjustments available apart from the spider vanes.

The central screw on the spider hub can shift the secondary closer or further from the primary.  The secondary can be rotated on the central screw, and there’s the 3 screws around the central one.

It’s a matter of trying the above, often in combination to sort it.  It can be a bit of a fiddle, but you’ll get there.

Good luck, Ed.

[Matt61]

I didn't touch the spider vanes at all. Personally using a Cheshire collimator it was a case of adjusting the central philips head screw to get the secondary mirror under the focusser correctly ( this was out on my 2nd hand scope)and rotating it in tiny increments. Then balancing and tweaking the 3 Allen bolts to get the mirror as circular as you can . I'd say this is easier than concentrating on the 3 mirror clips.

[Jamgood]

Unless you know the spider vanes are off, leave them alone. You can measure them with a ruler or callipers.

To adjust the secondary mirror, you use the central screw and the 3 tilt adjusting hex screws. (You might have to back the hex's off a bit to move the centre screw.

I did mine earlier today. You want to get it to look like this (Image below) through you cap.

 

If you can get a good photo through the cap with your phone square to the cap, you can use Mire De Collimation on a PC to see how you're looking. 

I recommend using bright coloured card so you can see what you're looking at better.

 

Edited December 9, 2020 by Jamgood