Example of a Saturated star
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gavinrider
Some objects are noticeably bigger in the blue images than in the near IR. Does this indicate that they are more likely to be nebulae or galaxies than stars? There is no "rejection criterion" for this kind of object, if they should be rejected. How to deal with them?
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lrebull
scientist, translator, admin
No, it's just the stretch, I think. If it's noticeably asymmetric, then it's a worry.
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gavinrider
in response to lrebull's comment.
I'm not sure what you mean by "the stretch"? Surely the imaging resolution is better at short wavelength so a star should look sharper in the blue than in the red part of the spectrum? Some of these objects are definitely larger in the blue than in the near infra red. The first two images in the sequence can be twice the size of the third and fourth, then the WISE images get progressively large again (as in this example). I have seen many like this.
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jdebes
scientist, admin
lrebull is talking about the scaling used to assign color/intensity to the image. The size of the objects is related to the fact that the different surveys, DSS, 2MASS, and WISE all had slightly different "sharpness" to their images, which depends on the size of the telescope, the wavelength of light you're looking at, and whether your telescope is in space or on the ground. We are working on putting up a FAQ that explains this better, but you are basically looking for things that don't look round--the size is not as important. That will mean that some round galaxies might get through our initial screening, but we hope to address that in the future!
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gavinrider
in response to jdebes's comment.
Yes, I would understand what you mean about this being due to the relative resolution of each different telescope if all the image sequences showed the same basic trend, but there are some very small, circular objects in the blue, while others are larger than in the near IR. This suggests to me that they have different characteristics than stars would have - see image AWI0000aex as an extreme example of this.
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jdebes
scientist, admin
You are very right! This comes around to the temperature of the star you're looking at. Sun-like stars will be bright in the visible, and slowly get dimmer and dimmer in the IR. Cool stars, like an M dwarf, will be faint in the blue then very bright in the 2MASS images. I refer you to here for a rough idea of what's going on:
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jdebes
scientist, admin
Sometimes our targets will be bright in the visible and present a shape that looks like this, or perhaps a large circular blob. These still might be good candidates! In this case you will have to rely only on the 2MASS images to determine whether the object is "not round". But odds are good that if the object is very bright in the DSS, it is a nearby star, or a very bright giant star.
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jdebes
scientist, admin
Feel free to tag these as #saturated
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gavinrider
OK, got that (I'm a physicist). So the question is whether or not you want objects that are extremely bright in the UV/blue part of the spectrum, like AWI0000aex, included in the search as "good candidates" or not?
If you want them included, that's OK but I just wanted confirmation because I was unsure about exactly what you are looking for. If not, then I estimate about 20% of the objects should be rejected but there is no criterion relating to this on which to reject them at the moment. They aren't galaxies so don't fit the "not round in DSS2" condition.
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jdebes
scientist, admin
Oh great, welcome fellow scientist! We take all kinds of stars. Most of these objects are unknown, or unremarkable in the past so no one's looked at them very closely. We expect debris disks to be around both hot and cool stars. If you look at the SEDs and they look very flat, that is cause for some concern, but at this point we are visually narrowing down the list--for our first pass we want a list of objects that unresolved and not contaminated by visual companions. If we end up with a huge list of "good candidates", then we'll probably have to do a second phase where people select objects based on SED. But a week into this project, we still don't have hard numbers on how that will go.
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Artman40
If a star fully saturates one pixel, is it going to start saturating neighboring pixels equally or just one pixel next to it?
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lrebull
scientist, translator, admin
It depends on how bright it is. It can saturate just one pixel or a clump of pixels. It will be brightest in the center, though; it won't saturate one px in the center and one way off to the side.
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