New picture of the Beehive Cluster with unusual spikes around the brightest stars

[Steve Sells]

    

I took this picture May 5th with my recently acquired William Optics Fluorostar 91 refractor. Integration time was just over 2 hours. I've posted it on AstroBin at https://www.astrobin.com/9axcbr. There's more information to be found there, about what the Beehive Cluster is, plus details of the gear used and so forth.

In the picture, the five brightest stars each have two spikes. This interesting feature is an artifact of the picture-taking process. I don't know what the cause is. If you know of something that might explain these spikes, I'd be interested to hear about it. All theories welcome!

-Steve

[Michael Okincha]

The spikes are in slightly different directions for each star.  That makes me think the cause is something that varies locally in the image, like fingerprints or streaks on the optics or image sensor.   Sometimes lens coatings have ripples as well.   Mechanical supports or other large-scale physical features would create artifacts that are consistent across the entire image.

As a general observation, if you use a filter wheel there may be artifacts in some colors but not others.  This doesn't seem to be the case in your image, since the blue and yellow stars in the center have streaks in the same direction.

Mike

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[Steve Sells]

Thanks, Mike, for your thoughtful analysis, especially your suggestion that this might be a very local phenomenon. I'm wondering if it might be in the camera. I'm going to try to reproduce the problem and if I can then I'll do some experiments to try to isolate the problem. To see if it's in the camera, rotating the camera could be a good test. Shooting without the light pollution filter might be another thing to try. The Beehive Cluster season has ended for this year, but almost any star field might do. A really bright star like Arcturus might be more likely to demonstrate the spikes.

-Steve

[Michael Okincha]

Hi Steve,

Yes, any bright star would be fine for studying your optics.  You might consider capturing a series of images by panning the camera/scope around to put the star in different locations of the FOV.  You should be able to map the size/shape of any optical defects.  

That's pretty much exactly how production lens testers work in industry.  E.g. those by Tripotics.  They project a test pattern through a lens and systematically move the camera around, capturing images to build up a map of the lens' sharpness, distortions, etc.

Regards,

Mike

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