oval and round stars in the same image

[alacant]

Hi everyone. At the top of the frame I get elongated starts. At the bottom they're OK. It's a 400mm guided by a 60mm finder under phd2. Here are two snaps with magnified top and bottom. North is left. Any ideas anyone? TIA. 

[Davey-T]

400mm what ?

Dave

[ChrisLX200]

Many possibilities but the favourite in this case is likely tilt - the focuser sagging.

ChrisH

[alacant]

18 minutes ago, Davey-T said:

 

400mm

 

Sorry. Focal length.

[Davey-T]

42 minutes ago, alacant said:

Sorry. Focal length.

No I meant what scope / lens etc

Dave

[alacant]

Ah, OK. It's a refractor with a Canon 700d. Thanks.

[Davey-T]

How are you focusing ? try focussing on the top stars to see if the the ones at the bottom go eggy.

Also try the trick of focusing a third from the top edge to see if it makes any difference.

Try imaging straight up to see if it eliminates it, if it does it's probably focuser sag as Chris said.

Dave

[alacant]

1 hour ago, Davey-T said:

 

How are you focusing ?

 

I'm using a Bahtinov Mask with APT live view, but it only works on bright stars; I don't have the only-on-the-top-stars option I'm afraid. Trying to picture whether APT will allow me to zoom onto say Capella a third of the way from the top. I don't think it will, will it? It will only zoom on what's central (?). The straight up is doable though. Would a light frame help diagnose? Thanks.

[Davey-T]

You could you use the Canon utilities and live view ?

Dave

[alacant]

OK. Never tried it. Does that allow me to zoom anywhere in the frame?

[Thalestris24]

APT will let you zoom in. Just take a short exposure, hi iso sub and double click on any part of the preview to display the image at 1:1 or select 1:1 with scroll for the preview.

Louise

[alacant]

Thanks. I was thinking more of being able to scroll to a bright star on a third within live view to enable me to use the Bahtinov.

[Thalestris24]

31 minutes ago, alacant said:

Thanks. I was thinking more of being able to scroll to a bright star on a third within live view to enable me to use the Bahtinov.

You could simply slew to a star that would put a bright star in the right place. In any case, a short exposure will confirm if you have tilt. You probably need to do that first.

Louise

[ollypenrice]

Tilt is favourite villain. It is likely to be from the focuser draw tube but the entire focuser may be tilted relative to the light path. Some of the more budget refractors have a rather crude way of attaching the entire focuser assembly to the tube. Ideally they should screw together but sometimes the entire focuser pushes in and is held by three radial screws. This gives some wiggle room for aligning focuser with light path but increases the chances of needing that wiggle room. One trick is to put a laser collimator into the focuser and see where its beam hits the objective. It should be in the middle, of course. Using a compass you can make a paper mask for the objective and the pinhole from the compass will define the centre of the objective nicely for this test.

Olly

[carastro]

Canon Utility you can zoom in on a star to focus.

Carole