eggy corners

[blink138]

hello members this is my first pic for some time
it is afflicted with stretched stars near the edges in a circular pattern as opposed to radially if it was coma
anyhow all of my other pics have not been like this at all..... nice round stars all the way
the centre of the piece seems to be focussed fine with round stars and no stretching
38 @ 180s @ 1000iso / darks and flats too but i do not know where all of the vignetting came from!
f6.3 c11 60da
thanks in advance
pat

[rcheshire]

As no one has responded. Pat. Are you certain of the fstop. I would expect to see this effect with some camera lenses, wide open - noted, you are using a telescope.

[RickS]

Field rotation from a poor polar alignment? Were you guiding on a star towards the centre of the field?

[cometcatcher]

Looks like field rotation to me also.

[blink138]

yes at prime focus rowland
rick and kevin i did not think it was possible to get field rotation in a GEM?
1365 was straight zenith when started though, and looking a little more closely my other post of the flame nebula has it a little too though not near as obvious
i learnt a new word about a year ago from this sight, non orthogonal, i know that my guide scope is not quite centred on my subject, but i would of thought that there would be also some rotation on the main subject
pat

[cometcatcher]

You can get field rotation on anything if it's not closely polar aligned.

One way to test, point the guide scope off to one side of the field of the main scope. With field rotation the stars will rotate around where the guidescope is pointed.

[RickS]

You get field rotation with a GEM if your polar alignment is out. Autoguiding will keep you on the target, but the field will rotate around the guide star. Otherwise there would be no need to polar align at all!

Cheers,
Rick.

[blink138]

gotcha rick, but my g11 always says i am between 0 and +/- 6(?) on both axis and i have never noticed it before
however it does not take long to go over the calibration
pat

[RickS]

Could be something else, Pat, but the shape of the stars looks like it is rotation. Another experiment you could try is to do a shorter and/or longer exposure and see if the effect is reduced or increased (if it is field rotation then it will show less in a shorter exposure and more in a longer one).

Cheers,
Rick.

[blink138]

good idea rick, i will have to give that a go
pat

[rcheshire]

On second look, yes, field rotation. New to me as well. Good emulation of one of my Fuji lenses..

[Tony_]

Hello Pat,

I use a c9.25 on a CGEM, I've never seen field rotation - and it seems like quite a lot for only 3 minute exposures? I only use the mount bubble level and the all star polar alignment - so I'm sure my polar alignment isn't that good (+/- 5' according to the hand controller).
Sometimes I get good guiding - other times I get drift - possibly due to differential flexure?
I don't know how often you have used the f6.3 reducer but I haven't had much success with mine. I get excessive vignetting (as in yours) and distortion/coma of stars, especially in the corners. I only have a cheap Hirsch brand - maybe the Celestron one is better?

Regards,
Tony.

[blink138]

thanks tony i too get good guiding and then sometimes drift on different nights or different parts of the sky
i always use the reducer flattener tony and i have not had any drama with it and the vignetting is fixed with darks and flats
here is another galaxy i imaged that shows good round stars to the edge with the same length subs
pat

[Tony_]

Nice image Pat.
What brand is your reducer? Actually I have had a few decent images using the reducer - I should give it another try. The last time I used it the stars looked like pacman in the corners. It would be easier to guide at f/6.3 than f/10 and it is faster.

Tony.

[blink138]

thanks tony
it is the celestron reducer flattener and it never leaves my moonlite!
f6.3 is the only way to go for me mate and 3min iso 800 appears to be the sweet spot in my backyard
pat