Secondary mirror adjustment

[sedm1809]

I have a skywatcher 150pds Newtonian. I have never collimated it before but obviously there's a first time for everything.... As I think is normal, there are three secondary mirror adjusters with allen key heads in the center of the spider, and a phillips-head screw right in the middle of yhe whole thing. Trouble is that neither the adjusters nor the central screw yield immediately to any turning force. I don't want to break anything so can I check - is the central screw supposed to be loosened before I touch the adjusters? Sorry for the potentially daft question, but it did almost seem like I was going to strip one of the bolt heads if I twisted any harder, and online search has not proved fruitful so far.

Many thanks

[Louis D]

The center one generally is for adjusting height along the optical axis and rotation of the holder along that same axis.  There's generally no reason to change either from the factory setting.

The three on the back of holder need to be loosened in a counter-clockwise manner before the holder can be tipped at all.  There's nothing spring loaded in there.  All three are snugged up tight against the holder.

Before mucking about with them, first check to see if the secondary is already centered and square to the focuser with a sight tube.  It should be circular and centered in the tube.

After that, verify that the secondary is pointing straight at the primary.  I generally use a laser collimator for this.  If the beam hits the center of the primary mirror center mark, you're done.

Last, check that the primary is aligned with the optical axis.  I generally use an Aline for this.  Once the secondary shadow is perfectly centered within the primary mirror center mark (easiest if it's a ring mark), you're done.

[Spile]

The three outer bolts need to be loosened slightly before you can turn the central screw clockwise. Doing so will move the secondary away from the primary. It’s the same process when moving the secondary towards the primary except the centre screw is turned anti-clockwise and the outer bolts will need to be turned further. Think of the outer bolts as taking up slack when moved significantly and followed by subtle adjustments when applying tilt or rotation. For significant movements,  hold the assembly with your hand cupped so you don’t touch the surface of the secondary.

https://astro.catshill.com/through-the-eyepiece-tube/

Edited August 13, 2021 by Spile

[CraigT82]

24 minutes ago, Spile said:

The three outer bolts need to be loosened slightly before you can turn the central screw anti-clockwise. Doing so will move the secondary away from the primary. It’s the same process when moving the secondary towards the primary except the centre screw is turned clockwise and the outer bolts will need to be turned further.

This isn't quite right. Loosening the centre screw moves the secondary towards the primary and the three adjusters will then need tighetning. Tightening the centre screw moves the secondary away from the primary and the three adjusters will need loosening.

However the OP will certainly need to loosen the three adjusters a tad before the centre screws can be adjusted. Put a clean (new) microfiber cloth around the mirror and grab hold of it to prevent too much turning force bending the vanes out of shape. 

 

[Alan64]

I have a 150mm f/5 Newtonian, and manufactured by the same company, Synta.  My own is one of Synta's basic models, and with a plastic 1.25" focusser...

But the interior is nigh the same as your own.  One thing that can help make collimating that much easier, is to replace the three secondary set-screws with thumbscrew-like screws...

The set-screws are at M4 or other, I'm not certain.  You simply take one to your local hardware, have it sized, and get three of the other type.  You'll then have finger-tip control whilst adjusting them, and in using a tool only to tighten them afterwards.

In that the secondary-assembly gives the most fits, we want to make adjusting same as easy as possible.

[alacant]

11 hours ago, sedm1809 said:

online search has not proved fruitful

Hi

Unfortunately, there is a lot of mis-information out there. Try Seronik and Telia. Both tell it just as it is.

Cheers

[alacant]

3 minutes ago, Alan64 said:

the three secondary set-screws with thumbscrew-like screws

+1

They're m4.

[sedm1809]

Many thanks everyone! I was getting distracted by the central screw which I didn't need to touch at all. All done now. Great idea to  get longer screws so can finger-adjust before tightening - that's definitely a hack. Thanks again.

[Alan64]

I just checked the width of my own, and they are M4 screws.

In theory, you can have them as long as you want, a kilometre long even, as long as they don't flex or sway into the view.

[Spile]

2 hours ago, CraigT82 said:

This isn't quite right. Loosening the centre screw moves the secondary towards the primary and the three adjusters will then need tighetning. Tightening the centre screw moves the secondary away from the primary and the three adjusters will need loosening.

Which bit do you believe is not right?  In order to adjust the centre screw the three outer bolts first need to be loosened in order to allow movement. That applies if the secondary is moved towards the primary (centre screw is turned anti-clockwise, outer bolts turned clockwise ) or away from the primary (centre screw is turned clockwise, outer bolts turned anti-clockwise).

[alacant]

1 hour ago, Spile said:

Which bit do you believe is not right? 

I believe this bit:

4 hours ago, Spile said:

turn the central screw anti-clockwise. Doing so will move the secondary away from the primary. It’s the same process when moving the secondary towards the primary except the centre screw is turned clockwise

That is incorrect. It is the opposite of what is written;)

Cheers.

@CraigT82 please accept my apology for answering. I thought it may save the OP some hair pulling if he were in the act;)

Edited August 13, 2021 by alacant

[Spile]

5 hours ago, CraigT82 said:

The three outer bolts need to be loosened slightly before you can turn the central screw anti-clockwise. Doing so will move the secondary away from the primary.

 

2 hours ago, alacant said:

That is incorrect. It is the opposite of what is written;)

Yes! sorry about that  - now corrected. I should have just used the diagram 🙂