Startrails top right and bottom left ?

[Scott]

Hi all,

I've posted this image on another thread and its been pointed out to me (and rightly so, it's how we learn) that there is a fair bit (read lots) of trailing in the tr and bl corners. this was taken with a dslr on a camera tripod, stacked in dss and processed in ps. can anyone tell me why this is so and how it can be countered. thanks for taking the time to read this.

edit: Oh, and it's 40 lights x15 or 20 secs and 20? x darks if that makes any difference. I have noticed that it seems to get worse the more lights i stack

Scott

[Matt Scunthorpe]

I think thats the way it goes wjth an DSLR. Its the same on my images. From what ive learnt is you will need a field flattener to correct it. But im not 100%. Jm sure other people will know for sure though.

[StarRaver]

If your using lenses on a fixed tripod then your exposure is to long depending on what lenses your using

[StarRaver]

Field flatteners are used with scopes to correct the corners of the image and make the stars the same across all the image, but not sure what kit you are using

[Scott]

If your using lenses on a fixed tripod then your exposure is to long depending on what lenses your using

To be honest StarRaver, I haven't got a clue. it was shot at 18mm which fits well within the 600 rule. also, if the exposure times are too long wouldn't the star trails be over the whole image? like I said, I'm clueless and just trying to get my head around it . thanks for your response thought .

Scott

[Scott]

Field flatteners are used with scopes to correct the corners of the image and make the stars the same across all the image, but not sure what kit you are using

canon 1100d on camera tripod for this one. I was overseas so it was more just a chance to record what i was seeing

[StarRaver]

I could be wrong like yourself im still learning, maybe its dss as the image moves ever so slightly across the sensor with each exposure, and when dss aligns and stacks it works the main part of the image and maybe the edges don't line up properly, im sure someone with alot more experience will put me in my place if im wrong

[StarRaver]

just had a proper look at you image on the computer ( was looking on phone earlier ) And it is a stunning image with lots of depht and detail, Did you use flats ? If not they will help with the vignetting at the corners of the image and will make a huge difference to the final outcome.

If your not to sure what darks bias and flat frames are here is a link to help explain http://deepskystacker.free.fr/english/index.html

[StarRaver]

you will have to click on the FAQ's as that linkdidnt work properly for some reason

[ollypenrice]

I don't think this is mainly star trailing so much as coma. The elongations, which actually affect all corners, are orientated radially - which is the classic sign of coma. Look carefully at the very faint stars in the upper left corner. They are elongated into streaks at ninety degrees to the elongations in top right and bottom left. Slight elongation in the bottom right is not perfectly radial but is not in the same orientation as the two worst corners. Trailing has to follow a single orientation across the whole image. (Well, no, not around Polaris but you are clearly not imaging the Milky Way anywhere near the pole, by definition!!)

I don't see any diaphragm induced spikes on bright stars so I wonder if you were running the lens wide open? If so it would be an almighty good lens that could deliver coma free stars into the corners.

I suspect that what then happens is that the stacking softwate has to handle coma, field rotation and shift in translation and simply throws up its hands! A very sophisticated alignment programme like Registar might do a better job though I just don't know.

Olly

http://ollypenrice.s...39556&k=FGgG233

[Scott]

StarRaver, to be honest, at the time I took this I didn't even know what flats were . I will be using them now I've had a chance to read M.E.P.C.

Olly, from memory I had the lens set at the lowest possible f/stop. 18-55mm @ 18mm f3.5. thought I was doing the right thing (my bad). just has to put it down to experience I guess. i hadn't notice the stars in the top left and bottom right until you pointed them out.

Thanks everyone for your time and helpful advice.

Scott