From the "NGC 253 imaged as a dim Dwarf galaxy in Colour" thread.
Quote:
I wrote....I have never seen the halo extend deeply in any other amateur or professional image in the visible spectrum.
The pros image the halo in radio, UV and X-ray wavelengths.
One needs to be certain the halo is the real deal rather than a processing artefact.
I've sent the image to the European Southern Observatory for comment.
Given their involvement in the Carina Dwarf galaxy thread, hopefully they will resolve the issue.
ESO's response as follows. My image marked out by ESO and referenced in the response is in the attachment.
Quote:
Olivier Hainaut ESO wrote,
Hello,
Nice work!
Did you start from the original raw FITS files?
It's a little hard to judge on the JPG, so if you could make the final FITS (esp the R and the V ones) available to me, I could have a deeper look. If you work with the JPG or TIF, beware that these images are already heavily processes, and that the noise texture is affected, i.e. you cannot rely on it for further analysis.
What puzzles me is the fact that the disk profile is fairly flat and then drops sharply, but that could very well be the "liberation" curve (ie the response curve you use to go from the FITS to the photoshop or gimp stage -reference to the FITSliberator program) is enhancing that feature.
I don't know which processing you used for the _colour_halo image, but I suspect you used some fairly strong smoothing with some advanced rejection. This causes some strong artefacts around the bright stars: some dark disks centred on the star. So, at that distance, what you see is dominated by the artifacts.
Look at the galaxy, going to-right: there is a ~plateau, then a dark lane, then the background becomes brighter again with artifacts. My guess is that the real halo stops at the end of the plateau, and that at least what comes beyond is not real. Also, for the part within the plateau, I suspect that some of it results from you "boosting" the parametres of the method a little too much. I'd recommend you try again, but seeing what happens is you focus on the getting the region marked in green on my attachment -I marked in red some regions where the artifacts completely dominate, so you could aim at getting these in the background. Then we'll see if the halo extends.
Great work!
oli
Olivier is looking at this image in the cold hard light of the professional astronomer, it is encouraging he is prepared to invest further time in this image and hasn't rejected the extended halo out of hand.
Excellent discussion, Steven, and once again a stunning image.
Not thinking about your image specifically, but just thinking about general mechanisms, what might create a very wide artificial halo around any object? I can think of four. There might be others.
The first is actual physical smearing of light from the main galaxy by the optical path. Dust in the air, fingerprints on the optics, reflections off shiny bits. Perhaps only a thousandth part of the light from the galaxy needs to get spread to create a detectable false halo. You could perhaps check this experimentally by photographing Rigel, and seeing what the background looks like when very strongly stretched.
The second, as your ESO colleague mentioned, is any kind of low pass filtering operation (smoothing). For example, a wavelet filter with too many layers. To spread say 400 pixels, you'd have to use a wavelet filter with 8 or 9 layers. You could check by applying the same filtering to the Rigel shot, or by reprocessing with fewer wavelet layers.
The third general mechanism (unlikely to apply in the current case) is superb but not quite perfect flats. Residual directionality in the test light used to produce the flat (or not having exactly the same optical path) can produce quite complex gradients. We're only looking at an error of a fraction of one percent Rephotographing with the object off to one side would resolve that issue.
The final one I can think of is the use of a gradient removal tool. This can effortlessly produce a DARK halo around a galaxy. Perhaps a complicated gradient removal tool can produce a dark halo further out, causing an apparent light halo inside it. The only way I can think of to test this is to do the processing without a gradient removal tool.
Once again, a superb photo, and I don't believe I'm directly addressing your image, but this kind of 'systems engineering' approach might be useful to anyone exploring undocumented super-faint, extensive haloes.
The large halos in my image are mainly caused by the application of a star mask that was too large for the star. Stretching the image results in everything but the region underneath the mask being stretched.
Of greater concern was the alternating brightness variation as one moves away from the disk of the galaxy.
This was caused by an incompatibility between the skyglow subtraction routine and the non linear stretching.
I believe I have fixed this problem by modifying the routine.
At this stage I have sent Olivier a revised version without the skyglow subtraction routine. The halos and brightness variations are gone but the galaxy halo is not as conspicuous as in the original image.
I suspect ESO will measure the pixel values from the edge of the disk to the edges of the frame. A drop off in the pixel values down to the background noise is probably a strong indication the outer halo is real.
If this is the case the next step would be to submit an image where the skyglow has been subtracted.
The interesting point in the discussions with Olivier is the "cultural differences" between amateur astroimagers and their professional counterparts.
We amateurs convey information through the visual presentation of our images, the professional on the other hand extracts quantitative information from the image.
The more we amateurs process an image, the less useful information is available. This was certainly the case when I performed a noise reduction on the image!
Hopefully this exercise will conclude where an image of an obvious extended halo is found to be real and is backed by scientific information.
I respect what you are doing here Steve.
I would be interested to know from Olivier if colour data using ubvri bands would be more useful (valuable)
I imagine it would be relatively straight forward to script regular calibration frames utilising standard reference fields during an imaging run with the data tagged to each exposure. They might even have the script to give us ex-gratis.
I doubt the throughput would be as high as you would get with high transmission lrgb filter sets, but it is a trade off that would be worth making in certain circumstances imho.
Anyway, there is no harm in putting the idea to the ESO guys.
At the very least, they will no doubt take you/us more seriously if we develop a more rigorous protocol with respect to our data acquisition.
I respect what you are doing here Steve.
I would be interested to know from Olivier if colour data using ubvri bands would be more useful (valuable)
I imagine it would be relatively straight forward to script regular calibration frames utilising standard reference fields during an imaging run with the data tagged to each exposure. They might even have the script to give us ex-gratis.
I doubt the throughput would be as high as you would get with high transmission lrgb filter sets, but it is a trade off that would be worth making in certain circumstances imho.
Anyway, there is no harm in putting the idea to the ESO guys.
At the very least, they will no doubt take you/us more seriously if we develop a more rigorous protocol with respect to our data acquisition.
Nice work btw.
Best
c
Thanks Clive.
I'll pass on your question when I get a response to the latest image submission.
Quote:
Originally Posted by LightningNZ
This is such cool work Steven. Thanks so much for sharing the process with us, it's fascinating.
-Cam
I respect what you are doing here Steve.
I would be interested to know from Olivier if colour data using ubvri bands would be more useful (valuable)
I imagine it would be relatively straight forward to script regular calibration frames utilising standard reference fields during an imaging run with the data tagged to each exposure. They might even have the script to give us ex-gratis.
I doubt the throughput would be as high as you would get with high transmission lrgb filter sets, but it is a trade off that would be worth making in certain circumstances imho.
Anyway, there is no harm in putting the idea to the ESO guys.
At the very least, they will no doubt take you/us more seriously if we develop a more rigorous protocol with respect to our data acquisition.
Nice work btw.]
Best
c
Hello Clive.
Here is the response from Olivier.
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WRT the question from the fellow amateur: indeed! that is typically what is done with the multi-colour data sets! calibrate the response of the telescope in each filters (taking into account the differences between your filters and the standard ones, the absorption of the atmosphere etc...), then look for interesting features in a colour-colour diagram. Normally, we prefer to do that with the individual resolved stars, but some work can be done on the non-resolved images of the galaxy itself.
Incidentally, it occurs to me that the variation in spectral sensitivity across the gamut of ccd sensors (available) renders strict adherence to Johnson filter curves a redundant and possibly even counter productive exercise if each exposure is going to be calibrated against reference stars anyway.
The good news is ESO is using science in evaluating whether the extended halo is real or not, instead of a subjective yes or no response based on looking at a pretty picture.
Their emphasis is on the noise in the background rather than the image itself.
The bad news is my processing efforts are hindering their efforts in making that evaluation.
I am becoming aware of the considerable differences in how amateurs and professionals process images.
Olivier Hainaut from ESO wrote...
Quote:
....Also, you have quite strongly smoothed the background with some clever noise reduction algorithm.
For us, that's a problem, as these algorithms tend to be non linear (i.e. if a star is 2x brighter than another one in the input, it will not be exactly 2x brighter in the output), and as the error analysis is screwed up ( *all* our measurements always come with error bars; to estimate these, we need a careful and detailed analysis of the noise, which the smoothing affects). So, definitely great for a nice image! dealing with the little glitches in the CCD would make it perfect.
This was the second image submission to ESO where I took pains to minimize the effects of noise smoothing.
Evidently a mild degree of wavelet processing in PI with the noise reduction option "on" was enough to wreck ESO's attempts in evaluating the noise in the regions as shown in the attachment.
What I suspect ESO were investigating is the amount of Poisson noise in the region of interest. This is "good noise" related to the statistical variation of real data. If "bad noise" or Gaussian noise related to factors such a read out noise and noise produced by dark and flat field subtraction dominates, then the halo is nothing more than an artefact that has been enhanced by processing.
The next submission if ESO are still prepared to tolerate me, is an image where there has been no processing except stretching including the pixel mapping process.
Hopefully ESO will also provide more information on how they evaluate the background noise.
I have been able to isolate and measure the Gaussian noise contributions in the areas highlighted by ESO for investigation.
The Gaussian noise was measured in the darks and flats used in the image reduction process for the luminance image. The darks also include bias noise.
The Gaussian noise from stacking and stretching the image was also evaluated. No sharpening was performed.
Using this information the percentage of Poisson noise in the luminance image was calculated.
Poisson noise from extraneous sources such as light pollution, vignetting, hot spots and field flooding were eliminated as possibilities.
Therefore the Poisson noise was from the external halo itself.
In region 1 (the area above ngc 253 in the attachment), the percentage of Poisson noise in the total noise was 79.6%.
In region 2 (below ngc 253) the percentage was 81.8%.
As these percentages are high I believe the external halo is real.
Hopefully ESO will confirm this by their own noise evaluations on the same image.
Having look at your excellent rendition, I stretched the hell out of my effort (Lum). The halo is certainly real, its huge!. I didnt apply any processing apart from uber stretch. Noisey as hell, not as smooth as yours.
Having look at your excellent rendition, I stretched the hell out of my effort (Lum). The halo is certainly real, its huge!. I didnt apply any processing apart from uber stretch. Noisey as hell, not as smooth as yours.
That's a good effort Fred.
What was the total exposure time for your luminance?
I had no doubt the halo is real but convincing an astronomer without quantifiable data is futile.
What was the total exposure time for your luminance?
Regards
Steven
6 hrs, 20min subs. I may have strectched it too far. Given the halos around big stars, which arnt real I suspect, the main halo might also mostly be fake, although I dont know how that happens.
Steven, this is great work your doing and I find this thread very interesting. Watching and waiting as I'm sure are many others.
Thanks Rex.
Olivier has been somewhat quiet lately.
I know one of his projects is the Earth based observations of comet 67P/Churyumov–Gerasimenko which Philae landed on recently, so perhaps he has been occupied with much more important events.
Quote:
Originally Posted by alpal
Hi Steven,
I doubt that such a large halo is real - I rely only on skeptical scrutiny.
Yes - the halo does extend much further than most amateur pictures
but is no where near the size of your picture.
cheers
Allan
Alan there is absolutely no doubt the extended halo is real.
It's been shown in Mike's negative image (with the mysterious S-bend), Octane's and now Fred's.
It shows up in professional images in radar, X-ray and FUV/NUV as shown in the attachment.
Since the halo emits synchrotron radiation in NUV and is a continuous rather than an emission spectrum, the radiation spills over into the visible range.
ESO's role in this is to play the devil's advocate, they know the halo is there, but they can't make a judgement call until they examine an image where the background noise has not been altered in any way by processing.
Quote:
Originally Posted by Bassnut
6 hrs, 20min subs. I may have strectched it too far. Given the halos around big stars, which arnt real I suspect, the main halo might also mostly be fake, although I dont know how that happens.
My luminance exposure was only 1.5hrs exposure total.
I think that has added to the complications.
If it was a 30 hr exposure like the Carina dwarf galaxy, I don't think ESO would have any doubts.
Allan there is absolutely no doubt the extended halo is real.
It's been shown in Mike's negative image (with the mysterious S-bend), Octane's and now Fred's.
It shows up in professional images in radar, X-ray and FUV/NUV as shown in the attachment.
Since the halo emits synchrotron radiation in NUV and is a continuous rather than an emission spectrum, the radiation spills over into the visible range.
ESO's role in this is to play the devil's advocate, they know the halo is there, but they can't make a judgement call until they examine an image where the background noise has not been altered in any way by processing.
there are very faint galaxies showing up that are not in your image yet
you have a halo showing instead?
Surely if the Chilean telescope picked up such faint galaxies it would pick up the halo too?
nice to have some pro feedback - very interesting work as well.
