This is definitely weird! What is it?


Looking at dust patterns I came across this feature which looks similar to what one might see in a radio galaxy - a central bright point with two lobes emanating from it. It is in the middle of the “ripples” reported by Mr_Universe in January last year, which is why I was studying the area. the odd thing is that there seems to be no optical counterpart to this structure. Any idea what is going on here?

to show the correlation with the visible stars, I have made an overlay. It shows that this feature is not aligned with the galaxy slightly to the left.

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That’s odd!! If you flip over to the unWISE W1/W2 layer (from which the dust map is derived) you can see a diffuse bar in that region. No idea what it is!

It’s hard to tell without knowing how much processing has gone on to make the images we see, but it has a structure that doesn’t look much like an artefact to me. But that diffuse bar in the W1/W2 layer certainly does look a bit odd. I can’t see anything in any of the other layers to match this feature.

Here is an overlay showing that the diffuse bar in W1/W2 does correspond with this feature in the dust image. Unfortunately when I zoom out to try and get the wider context of these features, the dust map does not display - it only appears at high magnification. Is this a fault?

I asked Aaron Meisner, author of the WISE Dust maps and master of the unWISE coadds, and he said:

“(ra, dec) = (258.5330, 64.6409) is an example of what happens to
bright star latents near the ecliptic poles, due to spread in the
angle at which WISE scans are approaching the pole.”

So unfortunately, a WISE artifact!

What on Earth is a “bright star latent” and where is the bright star that this description applies to?

Any idea why I can’t display the dust maps at low magnification settings?

And did you ask Aaron about those ripples that Mr_Universe spotted? Are they artefacts too?

Unfortunately, the whole viewer is going to be down now, for about 5 days, due to power maintenance work at NERSC, where it lives. You can use
https://imagine.legacysurvey.org/
meanwhile.

A “bright star latent” is an effect in the WISE detectors – basically an afterglow in the detectors.

There’s an example shown a little further down the page here,

PS, my guess is it’s the very bright red star below and to the right,

Hmmm. They certainly have a lot of artefacts with these detectors, but in this field there are plenty of bright stars that would potentially imprint images - how come there is just the one weird feature visible in the W1/W2 image that correlates with this feature in the dust map? Usually these imprints would have a sharp ring like this one from the catalog of artefacts, no? This feature is diffuse, with no signs of a radially uniform dark ring.

Screen Shot 2020-12-15 at 16.51.55

This location is close to the ecliptic pole, so imagine taking that artifact and smearing it out along a line in ecliptic longitude.

I think I would need to understand the structure of the telescope and camera to understand how that affects the imaging quality! I thought these cameras scanned the sky, taking a series of small snapshots that get stitched together, so what difference would it make which direction it was pointing in for the camera to be able to produce a well resolved rectilinear image?

And does this “latent image” problem with the sensors mean that all the fine structure in the dust image is caused by latent image artefacts too? (because if they are, why are they not smeared out too?)

PS: I am a physicist with some experience of optics, and I really am not convinced that this feature matches any of the artefact types reported in the catalog. I’d like to see more data on this before dismissing it.

The WISE mission is describe here: https://arxiv.org/abs/1008.0031

The WISE telescope scans along lines of constant ecliptic longitude. The telescope scans smoothly, but there is a mirror that swings back and forth to hold an image steady on the sensors. (This doesn’t really matter to what follows, but it’s fun.) There is a persistence effect when you put a bright star on the sensor. Those pixels on the sensor will have an artifact in the next few images. Due to the scan strategy, that means the artifacts will appear at roughly the same ecliptic longitude and one step in ecliptic lat. The scans converge at the ecliptic pole, so that artifact gets spread out in an arc.

Ah yes, now I am able to zoom out with the dust map I can see the star you are referring to, and another anomaly equidistant on the other side, which is pretty convincing proof that it is an artefact. Does Aaron have an explanation for those ripples in the dust map reported previously by “Mr_Universe”?

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You mean this one?

No, the star at 257.1075, 64.3185. Look at it in the dust map and you see the anomalies on either side, but oddly enough that particular dust map isn’t listed in the current selection bar…???

here is a screen shot showing the Cat’s Eye Nebula top left (the one you referred to) and the ripples in the dust map at about 4 o’clock from there, and in among the ripples you can see the bright star with the WISE anomalies to either side of it.

Blinking between the WISE 12-micron dust map and the SFD dust map is informative.

https://www.legacysurvey.org/viewer/?ra=260.5957&dec=64.4917&layer=sfd&zoom=5&blink=wssa

Indeed - the WISE 12 micron dust map is full of data processing anomalies…

To such an extent that one would be UNWISE to use that particular data set, methinks!

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yes, those are moon artifacts in the original unWISE maps. (Should be improved in the later, eg “neo6”, unWISE reductions, but I don’t think the WISE 12-micron maps have been updated to use those inputs.)