Why are my stars bad?

[Z3roCool]

Hi all, I wonder if you could give me some of your knowledge and possible reasons for my stars being bad. What I am trying to work out is why are my stars in the corners out of focus and badly shaped.

This is an image I took tonight. I am using an ASI 1600MM and 1.25" Ha filter (ZWO). I am using an adapter connected to a Canon 70-200 IS camera lens. 

The focus is not perfect but in the top left of the image it is very bad and stars are not pinpoint (Not many are!) Is the problem with the camera lens or the filters, or is it not possible to know? I also use a 400mm lens and the problem is also there but less obvious. Stars however still seem to have a flare off to one side.

 

Thanks for any input.

Tom.

[symmetal]

This is a problem primarily with coma caused by the lens itself, and is visible in varying degrees when using normal photographic lenses for astrophotography. The lenses were not designed with astrophotography in mind and controlling the coma of point light sources off axis is not a high priority, particularly in lower cost lenses. Zoom lenses, having more elements than prime lenses are more likely to show more coma too. The lenses not being perfectly centred on the optical axis can also cause this effect so two lenses of the same type may exhibit different coma characteristics depending on how well they were aligned in manufacture. This is probably why your image shows it worse at the top.

There are a number of lenses mentioned in various topics here that are noted as having very good coma characteristics, though some people have had to send the lens back to have it swapped for another, to find one that is good.

The only way to reduce the lens coma shown is to reduce the lens aperture, by stopping the lens down by a stop or two. This reduces the maximum angle of off axis light rays. This does mean longer exposures to compensate so is not always a practical solution.

Alan

[Z3roCool]

6 hours ago, symmetal said:

This is a problem primarily with coma caused by the lens itself, and is visible in varying degrees when using normal photographic lenses for astrophotography. The lenses were not designed with astrophotography in mind and controlling the coma of point light sources off axis is not a high priority, particularly in lower cost lenses. Zoom lenses, having more elements than prime lenses are more likely to show more coma too. The lenses not being perfectly centred on the optical axis can also cause this effect so two lenses of the same type may exhibit different coma characteristics depending on how well they were aligned in manufacture. This is probably why your image shows it worse at the top.

There are a number of lenses mentioned in various topics here that are noted as having very good coma characteristics, though some people have had to send the lens back to have it swapped for another, to find one that is good.

The only way to reduce the lens coma shown is to reduce the lens aperture, by stopping the lens down by a stop or two. This reduces the maximum angle of off axis light rays. This does mean longer exposures to compensate so is not always a practical solution.

Alan

Thanks Alan, that makes good sense! Had not thought about stopping the lens down a bit. Will give that a go next time I am out. 

My 400mm L is not too bad but the 70-200 2.8 L (Image above) is awful. 

Thanks for the detailed feedback

Tommy.

[symmetal]

3 hours ago, Z3roCool said:

My 400mm L is not too bad but the 70-200 2.8 L (Image above) is awful. 

I would have expected better from a Canon 'L' zoom from what yours shows. I have a Canon 100-400 L zoom and it has noticeable coma too but not as bad as yours. I've not had much luck using lenses for astro. Even my Samyang 14mm which is widely regarded as giving good astro images has bad coma like aberration over the whole frame, even when stopped down. The only lens I have which doesn't show noticeable coma is a Canon 40mm 'pancake' lens. 

Alan

[johngm]

+1 for Alan's comments. Drop the Aperture down a couple of stops, especially when using zoom lenses.

John 

[Z3roCool]

20 hours ago, symmetal said:

I would have expected better from a Canon 'L' zoom from what yours shows. I have a Canon 100-400 L zoom and it has noticeable coma too but not as bad as yours. I've not had much luck using lenses for astro. Even my Samyang 14mm which is widely regarded as giving good astro images has bad coma like aberration over the whole frame, even when stopped down. The only lens I have which doesn't show noticeable coma is a Canon 40mm 'pancake' lens. 

Alan

I think I will be looking to get my first scope at some point! Just need to save up and then get by the other half  I love the Canon lens nicknames!