Help! UFO shaped stars!

[Black Knight]

Hello everyone

Some of you may know I'm fighting a battle against 'orrible shaped stars at the moment. This test image I took on Friday is a good example .

This sub is the last of 15 x 5 minute subs I took that night. For this image, my PA was good. Field rotation as measured by DSS after 15 x 5 minute images was 0.01%. However I did get image shift of -32.51 on the Dx axis and 26.73 on the Dy axis. So maybe my PA wasn't as good as I thought?

Anyway, the net result is 'flying saucer shaped stars' in the lower right of the image, gradually improving (although not perfect by any means), towards the upper left of the image.

I was guiding, I believe on Alnitak, the large star in the lower right. Dec guiding was OFF. RA agressiveness was 90, RA Hysteresis was 10. The guiding interval across different subs varied between 1-3 seconds. For this particular sub it was 3 seconds. This issue appears in all subs.

The pics show firstly the whole sub, then a closeup of the lower right area, then a closer-in shot of the lower right area, with the worst affected star.

Any idea what could be causing these odd shaped stars?

Yours hopefully

EDIT: Whoops, no pics!

[Black Knight]

Hopefully here are the pics!

The pics show firstly the whole sub, then a closeup of the lower right area, then a closer-in shot of the lower right area, with the worst affected star.

[Alternative4u]

From your pictures I can see what your saying, but this can happen if you use your scope through a glass window, the double glass in window will make lots of things odd shapes.

Also in your photo one item has a big flare out going all directions, this is a very common thing if your viewing through windows.

David

[lukebl]

As I've found with my own problems with funny-shaped stars, you need to work out if the star shapes are due to guiding errors or optical errors. Do you get the same shapes even with short exposures? In which case, it would indicate that the sensor isn't exactly square and the focuser alignment needs looking at. That's what I found with my (ongoing) issues.

[Black Knight]

Thanks Luke. I'm using the Skywatcher coma corrector which push fits into the focuser. I'm not convinced however that the camera and coma corrector is lying in the focuser correctly. I haven't tried short exposures to test this, but I will next time I'm out.

[Black Knight]

The other thing which may be relevant is that the RA line on the guide graph basically forms a sort of shallow sine wave shape. It spends some time tracking above the centre line (perhaps a couple of minutes), then dips below the centre line and tracks there for around the same amount of time.

I don't have a PHD graph to show unfortunately

[Black Knight]

I ran the stack through Nebulosity, doing an Align and Combine (fixed alignment) and it came up with this. It shows, well, I'm not really sure what it shows to be honest. Flexure or field rotation? I'm not sure.

[Shibby]

Certainly don't take my word for this, but from your original images it does look to me like it could be field rotation, presumably caused by poor polar alignment...

[Black Knight]

You may be right ... I thought I'd achieved a pretty good PA, although I didn't drift align intentionally this time. Perhaps my PA simply wasn't as good as I'd thought. It's all good though, in the sense of knowing how much I do actually need to do each time to get it right!

[Shibby]

I'd have thought you'd get away with 5minutes without drift aligning. Have you checked the reticle alignment recently?

I did read somewhere else that you could get field rotation if your guide star isn't near enough to the target you're imaging (in DEC).

[Black Knight]

I'd have thought you'd get away with 5minutes without drift aligning. Have you checked the reticle alignment recently?

I did read somewhere else that you could get field rotation if your guide star isn't near enough to the target you're imaging (in DEC).

Yeah, I have. It was off by quite a margin. It used to be OK, so I'm not really sure what happened to it. I do have a vague recollection of banging my head on the polar scope a little while ago ... perhaps that did it! .

Anyway, I re-aligned it before taking these images. I have spoken with a couple of other guys on SGL who also manage 5 minutes or more, and never drift align. So I don't know .... if it is FR, perhaps my reticle it still out or my PA still leaves a lot to be desired!

The guide star was Alnitak (I think). But the curious thing is, the UFO effect is worse there than it is further away, which is not what I expected .....

[terrym2002uk]

I think you need to go back to basics. Eggs in space have also been a recent or as Luke says ongoing problem. Your 500d is capable of hi iso's of 12800. Take a very short sub of seconds to see if you have an optical issue. If it doesn't show, then its almost definitely tracking or alignment ...or both!

Also may be worth checking your polar scope is correctly centered after your bump'd you nut on it.

[Black Knight]

A series of short and longer subs is all part of the plan - next time I'm out

[ollypenrice]

It may be more than one thing.

The distortion across the chip might well be tilt in to the optical train. The Saturn-like look of some of the stars looks like guiding to me. Most of the time the star is central and that gets the most light, so is largest. But sometimes it is off to one side and the other for significantly less time, so you get small, off centre images.

I wouldn't guide on a star like Alnitak. Its image will be heavily saturated on your chip making the centroid calculation very difficult for PHD. That could account for the guiding component in your problem.

In 5 min subs you should get away with murder in terms of PA at short focal lengths, especially if your guide star is fairly near the middle of your main image.

Olly

[Black Knight]

Thanks Olly, that's interesting what you say about bright stars like Alnitak. I'll bear that in mind for the future . I shall have to try and ensure the camera is flat in the focuser as well.

/writes list of reminders on a post-it note

[russellhq]

hi Black Knight, did you manage to fix your UFO shaped stars? I'm just starting out with guiding and am suffering from the same effect!