Star Test for Collimation - Forums

Topic starter

Nov 22, 2023

Dear Astromasters good Day!

One of the exceptional nights came and I tried to collimate the scope with metaguide! I think that I got it pretty close with the remote support of a master astronomer. However, the infocus star had that triangular shape which raised some eyebrows for pinched optics. However, the scope is a Tak 210 as received from the factory and I doubt our Japanese masters did dispatch the scope with overtigthened optics.

I have a raw video which I stacked 5 % of the frames to show the star shape.

Can you please advise what might be the issue with it? Did i overtightened the secondary? But I am pretty sure that most of the collimation adjustments were done by loosening the collimation alens and slightly tightening the opposite ones.

Your input will be greatly appreciated.

Clear skies.

andrea tasselli

Frequent contributor

Nov 22, 2023

It is not unheard of Mewlons having their primary cell overtightend (for shipment, maybe?). And yes, it looks trifoil to me. Another possibility is that the cell is tight enough to have issue until the primary is cooled down, so a transient pinching in a way.

Freestar8n

Nov 23, 2023

This is fairly common to see in 'scopes with an internal baffle or secondary mirror, and it's likely due to a heat plume and the 'scope isn't fully equilibrated yet. So I would just try again and make sure it's fully equilibrated.

This is showing trefoil and it could be due to pinching - but I would rule out thermal issues first.

If there is only a small amount of trefoil you can still judge how collimated it is based on how uniform the three lobes are. But when it's fairly strong like this it's harder to tell. The top lobe of that pattern is thinner than the others - but it may just be due to the way the heat is flowing.

For best results use a bright star near the zenith - and a red or IR filter to make the pattern as large and well-defined as possible.

Good luck!

Frank

Well Written Helpful Insightful

Omiros Angelidis

Topic starter

Nov 24, 2023

andrea tasselli:
It is not unheard of Mewlons having their primary cell overtightend (for shipment, maybe?). And yes, it looks trifoil to me. Another possibility is that the cell is tight enough to have issue until the primary is cooled down, so a transient pinching in a way.

Thanks Andrea! Heat currents are definitely excluded as the scope sits in a permanent installation outside and it was already 3-4 hours with an open Roof!

Omiros Angelidis

Topic starter

Nov 24, 2023

This is fairly common to see in 'scopes with an internal baffle or secondary mirror, and it's likely due to a heat plume and the 'scope isn't fully equilibrated yet. So I would just try again and make sure it's fully equilibrated.

This is showing trefoil and it could be due to pinching - but I would rule out thermal issues first.

If there is only a small amount of trefoil you can still judge how collimated it is based on how uniform the three lobes are. But when it's fairly strong like this it's harder to tell. The top lobe of that pattern is thinner than the others - but it may just be due to the way the heat is flowing.

For best results use a bright star near the zenith - and a red or IR filter to make the pattern as large and well-defined as possible.

Good luck!

Frank

Thank you Frank for your response. It was tested with an 850nm filter with Capella (if my memory doesn't lie) at zenith and I am certain that heat plumes were not causing this as it was already sitting outside with an open roof for long. The atmosphere was already one of the steadiest I had for long. I shall try to slack all 3 collimation screws of the secondary and recheck.