Odd star shapes

[blinky]

I am getting the below shaped stars tonight from my F4 newt. I suspect it to be collimation but as far as I can tell it is pretty much OK, collimation wise - now 'OK' may not be good enough when it comes to F4 or it is something else? This is a 100% crop from somewhere near the centre (Tonights target is the cocoon nebula).

[blinky]

Should also add I double checked the focus and it was bang on.

[garethmob]

its not trailing is it? iv occasionally had some widefield shots similar shapped and was told it was ever so slight trailing

[blinky]

Dont think so - guiding has worked perfectly tonight....

[martin_h]

Could it be a bit of flex somewhere? The PHD graph would be ok but your imaging scope might be doing something different and it wouldnt show on the graph...just a thought for what its worth..

[blinky]

I get the same shape with a 20s exposure (of Deneb) as I do in the image above.

[blinky]

I am wondering if it's a pinched mirror in the cell? I did go and tighten everything up to try and stabilise the collimation, prior to this I did have nice round stars, although I do somehoe suspect it to be collimation based, it would be good to get some feedback

[SlipperySquid]

If the stars are like a pear shape(you know slightly triangular) I'd go for pinched optics as I had the same on my Newt after cleaning the main mirror and then over tightening the mirror clips!

Worth a punt!

Matt.

[terrym2002uk]

Im asuming you are using a coma corrector? Have you got a full frame to show us? Do the distrotions all run in the same direction or to they point in different directions. Also are they different in the centre of the frame as apposed to the edges?

Also some quick snaps of a bright defocused star at various potions in the fov may help to diagnose it.

[blinky]

It's all in the same direction and i am using a coma corrector. I do have a full frame but it will be shrunnk to fit on SGL and wont really show it very well but here goes!

[terrym2002uk]

Well i dont think that is coma. Is your secondary alignment correct, especially the rotation?

[RobH]

What it looks like is a doubling caused by a slight mount movement.

Does this happen all the time, or is it just on this occasion?

I've seen the same effect when a cable has been caught and then suddenly freed itself, however, you'de expect to see a big jump on the PHD graph, and it doesn't look like you have one from the graph you posted.

Rob

[blinky]

Nah, it's not that - I had it on all subs and even on a 20s test exposure of Deneb

[blinky]

Here's a 200% crop, does this give any more clues?

[RobH]

Try rotating the camera (filter wheel etc.....can't remember what kit you have) and see if the angle follows the rotation....if it does, it's a camera issue, if not, then you've eliminated that side of things.

Rob

[blinky]

Good idea Rob! Will try that next time and see.

[great_bear]

Sorry if this sounds obvious but from the photo itself, surely you can see if the compass-direction of the smear is a movement in R.A. ?

If so, it's odds-on a tracking issue.

[blinky]

Well I discarded the movement idea as a 20s exposure is the same as a 300s exposure movement wise, if it was a guiding issue surely it would have been greater in the 5min exposure?

[shaunster]

Yes it would.

[ncjunk]

What shape do you get on an out of focus star...out of interest..

[blinky]

Dunno, as usual in Scotland it's now raining! Will check next time i'm out.

[MickyWay]

Had very similar star shapes myself. Thought it might be due to bad seeing, but after reading this: Backyard Astronomy,

I'm beginning to think it's differential flexure.

[RobH]

No, it's not differential flexure.

That manifests as a gradual movement of the stars across the image over the course of several exposures, and on long enough single exposures is seen as an elongation of the stars. The elongation is dependant on exposure time, and in this case the abberation is the same independant of exposure time.

Rob

[MickyWay]

What about seeing then Rob?

[RobH]

What about seeing then Rob?

Seeing wouldn't have such a marked and predictable effect.....what you get with bad seeing is star shapes that aren't round, but are different on every sub.

Rob