Things that make you go humm...

[Peter Ward]

I was gobsmacked by today's NASA APOD.

problem was they ran a similar image a couple of years back and it simply didn't mesh with what was posted today.

Image link of both here (http://www.atscope.com.au/BRO/gallery593.html)

I'm interested to hear explanations about why such a disparity.

[By.Jove]

Well, I can think of three possibilities which should be testable.

1. An artifact of the scope that took the image, and/or the processing. Easily confirmed by getting someone else to shoot same patch of sky independently with another rig. I doubt APOD would have published it without having it verified independently, which suggests....

2. The arcs have been there all along - but nobody noticed before. There are really deep wide field surveys such as the Palomar Schmidt plates that may show whether or not they were there years ago. The SDSS is searchable online...

Alternatively...

3. If it really wasn't there 2.5 months ago and really is there now, it would have to be wisps of interstellar fluff really close to, or maybe within the solar system. In which case another image in a few months would identify whether there is any apparent motion, and the parallax would give a distance. Even at the speed of light matter can't move so far as to suddenly be in the frame in just 2.5 months, even within our own galaxy, never mind the range of M31, so it can't be distant.

[Peter Ward]

Ah...I was not contesting the OIII arcs...The Ha however looks to have correlation in places but absolutely none in others.

[iborg]

Sensor sensitivity? Filter cutoff?



If I have found the correct information, one used a ZWO ASI2600MM Pro (https://www.astrobin.com/1d8ivk/) while the other used a Nikon D810a and ASI2600mm pro (https://www.flickr.com/photos/51743649@N07/52426245654/).


Given the mix of camera sensors, and differences in assembling the images, I suspect there is no surprise there is a significant difference in result.



Philip

[By.Jove]

Noticed that too, there are little disparities all over the place. While it could be the hardware I'm also wondering about the processing, as both look over-cooked to me.

[Bassnut]

I don't see a problem. The older image looks like its RGB only, the latter one is deeper and includes Ha.

[astroron]

I will leave the technical stuff to the expert astroimagers here.
But I do note the "Bridge" between M31 and M110 is none
existent in both images?
With both images taken over many hours, one would
think it would be quite pronounced but M110 is in
complete isolation?:shrug:
Cheers:thumbsup:

[Peter Ward]

Both images are a combination a deep Ha and RGB. What should happen...if the image is showing real flux...its Ha features should develop further with extended exposures regardless of the sensor.

Just the opposite is happening in some areas of these images. Hence I suspect
some sort of calibration artefact is being promoted as "real" when in fact it is just noise.

[Stefan Buda]

I completely agree that there are big differences in some areas while in other areas there is a good match of the H-alpha nebulosity, which doesn't make any sense. It must be some dodgy use of masks in the processing of one of the images, I guess.

[OICURMT]

https://iopscience.iop.org/article/10.3847/2515-5172/acaf7e

[N1]

I'm not a DSO photographer, but -

It seems to me that the brighter objects and their surroundings had to be dialled back significantly in the more recent APOD so they don't appear completely blown out, and it was during that step that things went astray, for example by arbitrary selection of which areas/features to apply this to, and by what amount. I've seen this in some aurora photos, where parts of the aurora ended up being darker than the surrounding sky.

Secondly, we don't know that what's being shown in red in either image (but particularly the 1st one) is actually Hα nebulosity as opposed to a galaxy halo, for example, where the disparity is especially pronounced. Maybe a bigger effort was made in the 2nd photo to isolate actual narrowband Hα from other stuff. There are full-colour images online (https://www.flickr.com/photos/lmnosunsetdeluxe/31103940040/) that show the same (https://www.deepskycolors.com/archive/2017/01/01/Clouds-Of-Andromeda.html)feature but much less red. So I'd say no, the H-alpha dataset, particularly in the earlier image, is not accurate.

Thirdly, this article (https://www.sciencedaily.com/releases/2017/04/170418111504.htm) suggests that while hydrogen is possibly abundant in halos around galaxies, it's not necessarily emitting at the H-alpha line, so mapping the whole thing to the same colour (assuming its H content can even be detected in the first place) as H-alpha would be taking significant liberties.

[Stefan Buda]

I'm not a DSO photographer eighter so I'm not sure what all the red nebulosity represents, but the bright star towards the top middle of the image has a lot of red stuff around it, while on the other image it has none, and just a bit lower, both images show an equally prominent red cloud. How is that possible?

[Peter Ward]

I am confident that the red nebulosity is supposed to be h-alpha emissions.
(neutral hydrogen would be undetectable with a H-alpha filter)

The Ha-halo around also disappears in the second image.

While I suspect the OIII data is likely to be real (i.e. verifiable) in this case
I have serious misgivings about the veracity of the red regions being ultra faint "h-alpha" emissions.

No doubt other amateurs with wide-field fast instruments will re-visit this space with uber-deep images in time

[rustigsmed]

https://www.youtube.com/watch?v=Ul048rfPtOE
Anton has done a video on it.

[Kal]

Here are the full details of both images as descibed by the astrophotographers who captured them:


https://www.flickr.com/photos/51743649@N07/52426245654/


https://www.astrobin.com/1d8ivk/


As to why the discrepancy, I would assume it has to do with this comment on the more recent photo that was APOD:


"IMPORTANT NOTE

The [OIII] emission arc appears very bright in the images - but it is an extremely faint object that can only be adequately visualized by special subtraction techniques, since the signal in the unprocessed condition is almost completely outshone by the light from the galactic halo of M31."


If it is heavily processed through special subtraction techniques then it would surely distort alot of the details in the image you would normally expect to see and correlate with the more naturally processed earlier APOD image.

[AdamJL]

The subtraction technique involved the O3 and blue filters from memory. I'm with Peter on this one... it's odd that some Ha regions have significantly dialled back in the newer image.