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?
cheers
Allan
Are you sure about that Allan?
Perhaps we can narrow down the comparison by looking at the region around the stars HD4555 and HD4572 (the two bright stars above the disk).
You also have to also consider that three other astroimagers in this forum have produced more or less the same structure and the halo has been observed at other wavelengths.
The best method for comparison is to reprocess the raw data set from the Chilean scope so that the only differences are in the processing methods.
This was done in the Carina Dwarf Galaxy thread.
ESO provided me with their raw data which I reprocessed and sent back to them.
The comments were most encouraging claiming the reprocess resulted in an image with greater depth.
The only negative was the non linear stretching prevented them from using the image for photometric purposes.
It is very well known that NGC 253 has an extended halo, I am unsure of what all the fuss is about? Is it that you feel it is bigger and more extensive in your data Steve?
It is very well known that NGC 253 has an extended halo, I am unsure of what all the fuss is about? Is it that you feel it is bigger and more extensive in your data Steve?
Mike
Mike,
I think even it's even larger than the FOV of my ST-X10ME.
I'll revisit this one with my STXL-11002 and a much deeper exposure.
Perhaps we can narrow down the comparison by looking at the region around the stars HD4555 and HD4572 (the two bright stars above the disk).
Hi Steven,
Ok but maybe at the top right hand corner at 1 o'clock where you still have a halo would be better.
I have made a comparison image between yours & the Chilean one.
I hope they don't mind - it's just a tiny section.
I haven't equalised the background brightness - it's as is.
Can you see all the little smudged galaxies coming through on the right hand Chilean photo
but not on your left hand one?
The 32" is used in seeing conditions that can be as good as 0.3 arcseconds.
Compared to my site, this is about 10X better then what I can achieve.
Also the 32" has an aperture 3X larger than my own scope.
The small galaxies visible in the 32" are probably beyond the resolution limits of my scope and location site and can therefore be treated as point sources.
As point sources their visibility in an image is dependant on the aperture and exposure time.
The halo on the other hand is not a point source but an extended object and depends on the f/ratio and exposure time.
So you are not comparing apples with apples.
The other issue is how each image is processed.
Clearly the 32" hasn't been processed with the idea to bring out faint detail as illustrated by the inky black background and high contrast.
The 32" is used in seeing conditions that can be as good as 0.3 arcseconds.
Compared to my site, this is about 10X better then what I can achieve.
Also the 32" has an aperture 3X larger than my own scope.
The small galaxies visible in the 32" are probably beyond the resolution limits of my scope and location site and can therefore be treated as point sources.
As point sources their visibility in an image is dependant on the aperture and exposure time.
The halo on the other hand is not a point source but an extended object and depends on the f/ratio and exposure time.
So you are not comparing apples with apples.
The other issue is how each image is processed.
Clearly the 32" hasn't been processed with the idea to bring out faint detail as illustrated by the inky black background and high contrast.
Regards
Steven
What if I equalise the background of the Chilean pic with yours?
What if I equalise the background of the Chilean pic with yours?
I Al, I think the point Steven has been raising throughout the thread is the imaging processing techniques used. ie making a pretty pic vs linear stretching. unless you have the raw files from the Chilean image you can't compare data sets.
I Al, I think the point Steven has been raising throughout the thread is the imaging processing techniques used. ie making a pretty pic vs linear stretching. unless you have the raw files from the Chilean image you can't compare data sets.
cheers
rusty
Yes - you have a point.
I try however as attached.
I removed all the colour & gave a 5 pixel blur to the Chilean image.
The faint galaxies still stand out better:
What I suspect ESO were investigating is the amount of Poisson noise in the region of interest (of my image). 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.
Olivier Hainaut ESO replied.
Quote:
For us, the background noise is very important:
-the noise characteristics allow us to evaluate the data processing: the noise should be pure poisson, with a FWHM corresponding to what is expected from the sky photon noise. If it is not the case (typically it is not perfectly), we can then see if the flatfield went wrong/imperfect, or if there are some additional issues, like an electronic pick-up noise.
-the noise distribution allows us to quantify the limits on out detections. For instance, for very faint objects, we can then evaluate if the variations we see are compatible with the noise, or if there is some intrinsic variability.
We can not beat the noise, so we use it!
Cheers!
oli
The noise in my image has become more important than the image itself, particularly the Poisson noise from the sky background.
As yet my last submission has not been evaluated.
Quote:
Hi,
I'm travelling right now, so I cannot really get the images.
This has been a real eye opener in how professionals deal with astro images.
Since flat fields are a source of Gaussian noise (in particular when poor flats are used), the subject of how professional astronomers perform flat fields cropped up.
Olivier Hainaut wrote.
Quote:
Typically we use twilight sky flats - that's the standard.
Dome flat are used only when TwiFF are not available or as part of a more advance FF technique.
The best is to combine Dome FF, TwiFF and night sky FF (ie using deep images on ~empty fields) -
the Dome FF have ~infinite signal-to-noise ratio, but very poor representation of the illumination; the TwifFF have decent S/N; the night FF have the best representation of the illumination, but have poor S/N. Using some techniques like wavelet filtering, one can extract the S/N from the DoFF, and combine it with the illumination from the TwiFF and the NiFF. Using that, you can flatfield at the 99.99% level. Of course, one needs that level only for specific cases; in most situation, 99% is more than enough.
Cheers
oli
Since flat fields are a source of Gaussian noise (in particular when poor flats are used), the subject of how professional astronomers perform flat fields cropped up.
Olivier Hainaut wrote.
Typically we use twilight sky flats - that's the standard.
Dome flat are used only when TwiFF are not available or as part of a more advance FF technique.
The best is to combine Dome FF, TwiFF and night sky FF (ie using deep images on ~empty fields) -
the Dome FF have ~infinite signal-to-noise ratio, but very poor representation of the illumination; the TwifFF have decent S/N; the night FF have the best representation of the illumination, but have poor S/N. Using some techniques like wavelet filtering, one can extract the S/N from the DoFF, and combine it with the illumination from the TwiFF and the NiFF. Using that, you can flatfield at the 99.99% level. Of course, one needs that level only for specific cases; in most situation, 99% is more than enough.
Cheers
oli
Wow! That's incredibly useful info. Worthy of a thread of its own
Makes perfect sense when you think about it. Probably also emphasises the increasing difficult in obtaining good flat fields with increasing focal length most people encounter.
Wow! That's incredibly useful info. Worthy of a thread of its own
Makes perfect sense when you think about it. Probably also emphasises the increasing difficult in obtaining good flat fields with increasing focal length most people encounter.
I read a paper where Dome flats are considered to have an all round superiority to twilight and sky flats when LEDs are used as the illuminating source and a screen material that efficiently reflects light over a wide range of wavelengths.
Quote:
Flattening Scientific CCD Imaging Data with a Dome Flat Field System
ABSTRACT
We describe the flattening of scientific CCD imaging data using a dome flat field system.
The system uses light emitting diodes (LEDs) to illuminate a carefully constructed dome flat field screen. LEDs have several advantages over more traditional illumination sources: they are available in a wide range of output wavelengths, are inexpensive, have a very long source lifetime, and are straightforward to control digitally. The circular dome screen is made of a material with Lambertian scattering properties that efficiently reflects light of a wide range of wavelengths and incident angles. We compare flat fields obtained using this new system with two types of traditionally-constructed flat fields: twilight sky flats and nighttime sky flats.
Using photometric standard stars as illumination sources, we test the quality of each flat field by applying it to a set of standard star observations. We find that the dome flat field system produces flat fields that are superior to twilight or nighttime sky flats, particularly for photometric calibration. We note that a ratio of the twilight sky flat to the nighttime sky flat is flat to within the expected uncertainty; but since both of these flat fields are inferior to the dome flat, this common test is not an appropriate metric for testing a flat field. Rather, the only feasible and correct method for determining the appropriateness of a flat field is to use standard stars to measure the reproducibility of known magnitudes across the detector.
Thanks Steven. Will have a read.
I subscribe to any information that reinforces the notion there is a scientific reason why the flats were so bad for my old 8" Newt (any reason except "myself" that is )
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