What causes stars to look like little billiard balls in an integrated image?

[dmcclain]

I have an integrated stack of images from an ATIK 490 camera in 2x2 binning mode, through a Canon F/2.8 200 mm lens. The attached picture says it all...

[Dimitris Platis]

IS this image from a corner of your image?
It looks like coma to me

[dmcclain]

Heh! No, it is an image near the center of the frame. You should see the coma near the corners. Not as bad on the ATIK sensor at about 1/2 the dimensions of the Canon 6D sensor.

With the Canon camera body, I have F-stop control over the lens. But there is no manual F-stop ring on the Canon 200 mm F/2.8, and so I have to run it wide open with the ATIK camera.

The images aren't bad. The example image was magnified above 1:1, which is normally not done for viewing the images. I was just wondering what could cause the nice rounded little blobs with a highlight pixel always in the same place? Maybe it is residual coma? or slight defocus? The focus is very touchy at F/2.8, and can only be done manually by hand. But my image doesn't show the characteristic chromatic aberration that indicates out-of-focus conditions.

Coma actually looks like little comets with tails that splay outward along the radial direction, pointing away from image center. These "billiard balls" are nice and uniformly round.

[pfile]

you don't have to run it wide open.

put the lens on your camera, set the f stop you want, press the DOF preview button on the camera and remove the lens while holding the DOF preview button. the lens will stay stopped down to whatever you set it to.

if this was thru an L filter, what's happening is almost certainly that the camera is just not focusing all the different colors of light in the same place. stopping down will help with this (using a narrowband filter also helps). for wideband i'd stop this lens to at least f/4.

rob

[dmcclain]

Hi Rob,

Gee! Thanks for that advice about stopping down the lens. Who would have known?

The image was from the ATIK in 2x2 binning mode, so yes, a kind of L filter image. And your explanation certainly makes a lot of sense.

I also found while running with the Canon EOS 6D that stopping the lens down to at least F/4 was requisite for getting decent focused images, and a sort of best compromise between F-speed and corner coma. I don't look forward to losing the lens speed, but the image quality is certainly more repeatable.

Thanks!

[pfile]

no problemo, i've picked up a lot of random tidbits over the years...

in my 200L at f/2.8, focus (thru astrodon LRGB filters) looks pretty good in G kinda marginal in R. B and L look pretty bad. i made a comparison thru a b-mask:



rob

[chris.bailey]

Any filters at all in that image? If not you do really need a decent IR cutoff filter in the line somewhere.

[pfile]

again, these are chromatic aberrations... it is a fast lens not designed for astrophotography running wide open. even canon's MTF charts would indicate this is expected.

all the astrodon filters have IR cut built in. the sensor is an STT-8300M.

i don't have the diffraction patterns for narrowband filters, but the images look great wide-open when you restrict the frequency of the light.

rob

[chris.bailey]

all the astrodon filters have IR cut built in.

Sorry Rob, I was referring to the OP's posted picture.

[pfile]

oh sorry - good point though, that could have a lot to do with it. i had assumed he had a filter wheel... most filters have IR cutoff.

rob