Collimation question

[Richard N]

My secondhand Starwatcher 127/900 had its second outing last night. I did the star defocussing test and it seemed like its perhaps a bit out of alignment. The difficulty that I have is that the primary mirror does not have a centre mark so I am guessing where it is. Do I need to remove it and mark the centre (sounds fraught with risk)? I'm using a laser collimator - I have done some basic tests on that and it seems resonably collimated itself. Visual astronomy only at this stage,

Edited July 9, 2021 by Richard N
Typo

[Alan64]

I've a couple of reflectors, one a Newtonian and the other a catadioptric, with spherical, primary mirrors.  I've centre-spotted the latter...

https://garyseronik.com/centre-dotting-your-scopes-primary-mirror/

For durability, use a reinforcement-ring of polyvinyl(plastic), instead of paper.

It has been said that a spherical primary-mirror does not need centre-spotting, since it's a perfectly round bowl, a portion of a perfect sphere, but that's a technical point.  I like to know where everything is, particularly the centre of the primary-mirror, within the scenes exhibited through my sight-tube and collimation-cap.

You're at perfect liberty to use a laser-collimator, but given the quality of entry-level units I strongly urge the use of passive tools, a Cheshire/sight-tube and a collimation-cap, instead...

  

[wimvb]

AfaIk, collimation isn't as critical for visual use as it is for AP, and as long as collimation isn't too far off, you can just enjoy the scope as is. But if you want to improve collimation, you can use a defocused star (polaris doesn't move as much in your fov, so is easier to use) to collimate your scope in the field. No center spot needed.

[Spile]

Some useful guidance on collimating this Mak...

https://www.telescope.com/assets/product_files/instructions/29183_02-09.pdf

https://www.cloudynights.com/topic/413766-skywatcher-127-mak-collimation/ (see post #8)

 

 

[CraigT82]

I don't think the OPs scope is a Mak, it's an f/7 newt like this one...

Being f/7 it shouldn't be too fussy about collimation as the 'sweet spot' is 7.6mm in diameter, so pretty close will be good enough. 

Without a centre spot the best way to collimate is on a defocused star. If the central hole of the star appears squashed over to one side then hold a finger up to the front tof the scope so you can see the shadow of it in the defocused star, then move the finger around the OTA until the shadow of it is aligned with the squashed side of the donut. Then adjust the collimation screw that is nearest the position of your finger on the OTA to move the cental hole of the shadow back towards the centre.

I would disregard the laser collimator for now and just use a sight tube or collimation cap to get the secondary in the correct position beneath the focuser with all primary mirror clips equally seen in the reflection, then use the defocused star method to fine tune the primary collimation. 

 

[Spile]

Aha! Thanks for clarification CraigT82