Green and Blue Dots

• by BenoitBoulet

  i noticed there are green and blue (hollow) dots on the right side of this image. I'm really not sure what i'm looking at here, as it could be a digital artifact. However they do look strange.

  Posted September 29, 2015 8:31 PM

• by Capella05 moderator

  Both of those dots are optical artifacts, so they are not real.

  I am more concerned that your image is incorrectly sized! #wrong_size

  Happy Hunting!

  Posted September 29, 2015 8:35 PM

• by sat666leg

  i've seen a few of these and when i switched from decals image to models there was a pair of purple edge on galaxies way back in the image..?

  Posted September 30, 2015 7:15 AM

• by Capella05 moderator

  The rings are caused by dust on the either the filter or the window of the CDD camera. The dust causes a shadow on the detector.

  Do you have an example of the purple edge on galaxies you saw?

  Posted September 30, 2015 11:03 AM

• by sat666leg

  il check

  Posted September 30, 2015 11:06 AM

• by klmasters scientist, admin

  That size is really way off. Lots going on there, so it looks like the automated pipeline indicating the size of the central galaxy is really wrong.

  Posted September 30, 2015 11:42 AM

• by sat666leg

  note on size i've had about 3/4 like that..

  Posted September 30, 2015 11:44 AM

• by Capella05 moderator

  Here is a sceenshot of the purple ring artifact in the DECaLS image explorer:

  

  and the image when switching to the DECaLS model examiner:

  

  It looks like the algorithms that are used to generated the model did not know how to interpret the artifact, so it assumed it was a pair of purple non-existent galaxies. I am sure @KWillett will be able to give more information on why the model did this.

  It would also be interesting to see how other artifacts get modelled!

  Hope this helps, and thanks for pointing it out 😃

  Posted September 30, 2015 11:47 AM

• by sat666leg

  look at the initial image right at the top on the left ..in decals model the dots are solid green and blue

  Posted September 30, 2015 4:49 PM

• by Capella05 moderator

  Can you give the RA / DEC? so I can have a closer look?

  Posted September 30, 2015 5:42 PM

• by sat666leg

  there in the image above that started this discussion..
  or right here to the left..
  RA, DEC= 318.3438, 0.9837.

  Posted October 1, 2015 10:24 AM

• by dstn

  These blue and green things are an asteroid. In these images, blue = g band, green = r band, and we take those images back to back. The asteroid moves that much in the time between exposures (though shouldn't they be streaked then?). They are "hollow" because of over-aggressive masking of pixels around saturated pixels. When given 'hollow' objects, the source detector can find multiple objects which then get fit as the closest thing it can find, such as these extended and strangely colored "galaxies".

  Posted October 1, 2015 3:25 PM

• by KWillett scientist, admin, translator

  Great to know - thanks, @dstn.

  Posted October 1, 2015 5:26 PM

• by ElisabethB moderator

  You learn something new everyday ! 😄 I would have gone with artefact in a heartbeat !

  Thanks !

  Posted October 1, 2015 5:41 PM