Another Collimation Question

[richyrich_one]

Hi

(All directions are looking into the focuser)

The spider is centred in the tube and the focuser is squared.

When centering the secondary under the focuser the left and right is straight forward using the central screw, done.

But what if it needs to move up or down(needs to move up in my case) to be centred? Do I tilt the secondary using the 3 screws to achieve this? Do I re-adjust the focuser to centre it? Adjusting the spider vanes would do it but won't it cause issues with the difraction spikes?

Puzzled what is the correct method.

Any help much appreciated

 

 

 

[AKB]

The centre screw is just to hold the whole assembly together and tensioning the spring.  You can slacken it off a bit if the whole secondary needs rotation.  The majority of the secondary collimation is done with the three adjusting screws to get the centre spot of the primary onto the Cheshire crosshairs (or whatever you're using.)  Don't worry about what that does to the image of the secondary, or collimation eyepiece.  That's fixed later with the primary collimation bolts.

I was busy collimating scopes today... nothing else to do in this weather!

 

[BXRO]

Provided your secondary is centered in the OTA by adjusting the length of the spiders (measured mine with a set of callipers) and your secondary mirror is correctly placed under the eyepiece and square to it your final aiming of the secondary is by tightening up and  releasing the three recessed alignment (grub) screws. If a lot of movement is required it may be necessary to slacken off the centre screw on the primary a little to achieve this

[billyharris72]

Hi Rich:

Here's my take. If the spider is centred in the tube then that should take care of your left and right and back-forward movement, other than the left-right tilt of the mirror which as you say is easily done with the secondary collimation screws.

Up and down (i.e. up and down the tube) can be adjusted slightly, by loosening off the central retaining bolt in the secondary mirror cell and then tightening all three collimation screws - this will force the mirror down into the tube - vice versa will bring it up.

Don't worry if it very slightly off centre - half a milimetre is not really going to make any difference, so long as all the light from the primary is being reflected straight out along the optical axis of the focuser (in a Newt the secondary is a flat, passive element). That will be the case if the primary is centred in the secondary and the secondary tilt and focuser alignment are such that the secondary mirror appears circular.

No not try to correct the secondary being vertically off centre by tilting the secondary or changing the focuser alignment. That's much more liable to make your collimation worse, as the mirror then won't be aligned with the focuser. If it is really bad you could try to modify the secondary cell or physically move the distance between the spider vanes and the secondary, but that seems like a very drastic measure.

Hope this helps.

Billy.

[richyrich_one]

So, how I undersand it then is that if the spider is centred then the secondary SHOULD be centred up/down under the focuser.

But it's not!

[faulksy]

if you rack your focuser in and out with your sight tube in place you can find if your focuser is not square

[richyrich_one]

11 minutes ago, faulksy said:

if you rack your focuser in and out with your sight tube in place you can find if your focuser is not square

With the XLKP fitted there is no deviation so all good there.

[JOC]

I hope you had more success that I did - I've given up on collimating mine until I find someone more experienced than me to help get the screws undone!!

[richyrich_one]

4 hours ago, JOC said:

I hope you had more success that I did - I've given up on collimating mine until I find someone more experienced than me to help get the screws undone!!

On that note, start with the smaller 3 and loosen at least one of them which will make turning the centre one easier/possible whithout bending something.

[cletrac1922]

Rich, I use a laser collimator. There should not be any need to adjust the spider holding the secondary mirror. Have found with cheaper supermarket scopes, if loosen the centre screw, the whole secondary mirror will shift, then you in all kinds trouble getting the secondary mirror aligned. Just use the 3 outer screws to adjust the secondary mirror, onto the centre dot of the primary mirror. Loosening the locking screws of the primary mirror, then adjust the primary mirror. Should take no longer than 5 minutes to do. I find that I occasionally have to do a collimation every couple of months, if been transporting the scope around doing field displays with my astronomy club. If just using scope at home, then very rarely will need to do another collimation

[Moonshane]

It could be that one of your secondary adjuster screws has been wound in or out too far. Check from side with a little mirror they should all be about the same length.

Could be your tube Is slightly out of round. In which case you need to have vanes one axis different to other.

Could be rotation too.

All that said the secondary has a very minor effect on the views so I'd not worry too much if close.

[richyrich_one]

Thanks, I'll wind the the whole lot back and check it again.

[richyrich_one]

Thanks for all your replies