Bubble Nebula rehashed L(Ha)SHO, Foraxx palette

[vlaiv]

3 minutes ago, CCD Imager said:

I have never heard or used AstroimageJ

Do look it up.

Here is a scientific paper written on how to use that software:

https://iopscience.iop.org/article/10.3847/1538-3881/153/2/77/pdf

[CCD Imager]

5 minutes ago, vlaiv said:

I'm sorry to say - that is complete nonsense and utter misunderstanding of Nyquist sampling theorem.

I don't understand why people insist relating x2 to something in spatial domain when it is clear in what it says:

So it is not twice FWHM, it's not twice Reyleigh criterion it is not twice Gaussian sigma - it is none of that.

You need to perform Fourier transform of the signal to find out where cut off frequency is and then use that (twice that value) in order to determine sampling rate.

As for 2D case, here is simplified proof that above x2 max frequency component still stands even in case of non optimum rectangular sampling grid (optimal sampling grid for 2d case is actually hexagonal, but that is different matter).

For sine wave either vertical or horizontal - we have reduction to 1d case. For any wave that is at an angle so not either horizontal nor vertical - it will be sampled at higher rate in X and Y then it's wavelength is suggesting:

So wavelength of any wave at an angle will produce sine wave on X and Y with longer wavelength, so if you sample at twice per green arrow in X and Y direction  - you'll produce more than two samples per blue arrow (or in fact along X axis).

 

 

This theory was born with telegram electrical signals where the signal was of significant duration. An image or stellar profile is completely different, a snapshot and requires Nyquist modification as Stan has pointed out. How can you ignore square pixels sampling round stars?

I suspect neither of us will change our thoughts, so as you said, maybe best to leave it here

 

[vlaiv]

1 minute ago, CCD Imager said:

How can you ignore square pixels sampling round stars?

Pixels are not squares! Pixels are point samples.

People get the idea that pixels are squares because of nearest neighbor sampling. But use any other resampling and you'll see that pixels are not squares.

Ok - here is simple exercise that you can perform in order to show if pixels are in fact little squares or not

- take image with one pixel lit up

- enlarge it by 1000% so you can "see the pixel" using nearest neighbor interpolation - you get "square" as you would expect "because pixel is square", right?

- rotate that image by 45 degrees

- enlarge that image by 1000% - you should see square rotated by 45%, right? What, still "regular" square?

What gives? Isn't pixel a square?

No - it's a point. It becomes a square when you enlarge it using "silly" interpolation method. Look what happens when you enlarge it using more advanced interpolation method:

It's now this rounded thing with some ringing around, what? I thought it was a square

Pixels are not squares, they are point samples - no size no width. On camera yes - but even there, they are not squares, they are more round like small glass windows (depends on technology and micro lenses applied):

 

[vlaiv]

13 minutes ago, CCD Imager said:

I suspect neither of us will change our thoughts, so as you said, maybe best to leave it here

agree

[CCD Imager]

6 minutes ago, vlaiv said:

Pixels are not squares, they are point samples - no size no width. On camera yes - but even there, they are not squares, they are more round like small glass windows (depends on technology and micro lenses applied):

They are squares, why do you think the spec sheet gives pixel dimensions like 3.8u x 3.8u??

You photographs show spheres on top of the pixels, these are micro lenses.

[vlaiv]

Just now, CCD Imager said:

They are squares, why do you think the spec sheet gives pixel dimensions like 3.8u x 3.8u??

You photographs show spheres on top of the pixels, these are micro lenses.

Because 3.8um x 3.8um is spacing between point samples that you will get - it is not actual physical size of pixel.

Physical size of pixel is smaller than that (as you have seen from images above - depending on sensor) and actual difference between surface of square for that pixel and actual surface of pixel (light gathering) is folded into QE - no pixels have 100% QE (even at peak sensitivity) because of this

Anyway - once you get your data - you only have data point, you no longer have dimensions and should not think in terms of square pixels - it will just confuse you.

They are samples without any physical size - they only have x and y location and measured intensity.

[CCD Imager]

I think you should read this:

CCD pixel size (stanmooreastro.com)

 

I reckon a sampling rate of 3.5x is going to drive you nuts, but Stan gives real data examples to back up his recommendations.

[vlaiv]

1 minute ago, CCD Imager said:

I think you should read this:

CCD pixel size (stanmooreastro.com)

 

I reckon a sampling rate of 3.5x is going to drive you nuts, but Stan gives real data examples to back up his recommendations.

And he uses bicubic resampling.

I've already linked and explained how important the choice of resampling method is.

I can perform exactly the same experiment with different algorithms used to get completely different results

[CCD Imager]

1 minute ago, vlaiv said:

Because 3.8um x 3.8um is spacing between point samples that you will get

Pixels are separated by drain and reset gates, but they are small columns compared with the imaging area. Furthermore, micro lenses increase the surface area of the pixel

[andrew s]

Lots of misunderstanding of each other. 

For example,  a cameras are areal detectors normally square but give a single "point" output per pixel.

Another would be a signal varying in time compared with one in space with 1/time frequency compared to 1/distance a spatial frequency. 

Best to follow your own proposal and call it an unsatisfactory no score draw. 😊

Regards Andrew 

[CCD Imager]

1 minute ago, vlaiv said:

And he uses bicubic resampling.

I've already linked and explained how important the choice of resampling method is.

Although more modern re-sampling methods may give more reliable results, the differences are really quite small. This afternoon, I ran all 9 algorithms in Pixinsight, they all basically gave the same result

[vlaiv]

Just now, CCD Imager said:

Although more modern re-sampling methods may give more reliable results, the differences are really quite small. This afternoon, I ran all 9 algorithms in Pixinsight, they all basically gave the same result

Ok, so here is very simple example to show FWHM vs sampling rate.

This is baseline:

left is high resolution image and right is Fourier transform of it. Now I'm going to apply blur of different FWHM size to image so we can se results of this.

FWHM of 2 (2 pixels per FWHM)

FWHM of 3 (3 pixels per FWHM):

FWHM of 6:

As we increase FWHM - we simply shrink frequency response to smaller area.

But look what happens if I resample above image that was blurred to 6px FWHM to 1/3.75 of its size  (or 6/1.6 = 3.75 so x3.75 times smaller):

It again has "full" frequency spectrum and data of frequency goes to the edge.

If I however reduce that image by factor of 6/3.3 (so having 3.3 samples per FWHM instead of 1.6 per FWHM)

we again have empty space - just no data around at high frequencies.

Ok, sure, I'm done explaining.

Math is out there, for anyone interested - what ever I'm saying I can back up with actual sources for further understanding. I simply don't want to spend any more time on this subject.

[windjammer]

Lots of v. interesting stuff in this discussion - I will archive it and give it a long read!  I might start giving my subs the once over with an FT package to get a better feel of what is going on.

What I would give my eye teeth for is two cameras, pixel dimensions differing by a factor of two, on the same scope, same night, same target and compare the subs through the wringer discussed above!

And then stand back.... : )

Simon

 

[vlaiv]

5 minutes ago, windjammer said:

What I would give my eye teeth for is two cameras, pixel dimensions differing by a factor of two, on the same scope, same night, same target and compare the subs through the wringer discussed above!

That is very easy to achieve - simply rig your system to sample at say 0.6"/px and then use the same data binned x2 for 1.2"/px and examine all aspects of the data - FWHM, SNR after stacking, compare FTs of both images and of course compare final product of processing at both resolutions to get the feel (enlarge smaller image to match size of larger and vice verse - reduce larger image to match the size of smaller).

[ollypenrice]

I'll become really interested when I'm shown an amateur image of M51 which is significantly better than those shot at around an arcsecond per pixel. I don't want to be shown calculations and measurements, I want to be shown a picture in which I can see a worthwhile difference.

Please do rattle my cage when this image is available!

Olly

[CCD Imager]

1 hour ago, ollypenrice said:

I'll become really interested when I'm shown an amateur image of M51 which is significantly better than those shot at around an arcsecond per pixel. I don't want to be shown calculations and measurements, I want to be shown a picture in which I can see a worthwhile difference.

Please do rattle my cage when this image is available!

OK Olly, show us your best image of M51 at 1 arc sec/pixel, I would be interested to see it. I'm guessing you'll have an advantage of excellent S/N from your skies

[Elp]

1 hour ago, ollypenrice said:

Please do rattle my cage

Cage rattled (225hrs)?

www.galactic-hunter.com/amp/m51-the-whirlpool-galaxy

 

[CCD Imager]

29 minutes ago, Elp said:

Cage rattled (225hrs)?

www.galactic-hunter.com/amp/m51-the-whirlpool-galaxy

Interesting that you should link that collaboration image. As discussed earlier in this thread, there are two major aspects of an image I look for to assess its overall quality. The sheer amount of signal that has been acquired is unrivalled for a ground based telescope with new nebulosity apparent. This is clearly what has struck a chord with many viewers. But look a little closer and assess the resolution/detail in the image, it is really quite low. Take a look at the detail in the spiral arms in this thread and compare.

I guess its what floats your boat. 

Adrian

[Elp]

It's not the highest res for sure but it's a great collaboration result that you don't see too often. For more detail or deeper field, that's what space telescopes are for.

[ollypenrice]

11 hours ago, Elp said:

Cage rattled (225hrs)?

www.galactic-hunter.com/amp/m51-the-whirlpool-galaxy

 

It's a really great image, we can agree. Now let's ask what's great about it. The resolution of small scale detail? No, that is unremarkable. Plenty of M51s have that resolution, or even better, but it is representative of what I regard as roughly 'what you'll get' out of an amateur system, seeing-limited.

What is remarkable, very remarkable, is the depth of signal on the faint stuff. The outer halo shows modelling and structure which I have never seen before and the Ha feature just beside the pair is also new to me.  These don't require high resolution.

Indeed, this image supports the thrust of my argument perfectly: as amateurs we can bang our heads on a wall in search of scarcely perceptible improvements in resolution or we can go for what matters, what will really allow us to show something new, and go for depth of signal.  The headline of this image is 225 hours. Exactly, and it shows. It doesn't show in resolution, it shows in depth.

Space telescopes are not primarily, or even significantly, created for making pictures. Their use is primarily spectroscopic. Amateur scopes are for whatever the amateur wants to do with them. Hubble and James Web could not/cannot take widefield images but amateurs can. There are PNs out there, still being discovered by amateurs but new insights into the night sky, in the amateur domain come, as it seems to me, from images which combine depth with breadth of field.

Olly

Edit: Ironically you have brought Adrian and me into agreement! 

Edited September 4, 2023 by ollypenrice

[CCD Imager]

10 hours ago, Elp said:

It's not the highest res for sure but it's a great collaboration result that you don't see too often. For more detail or deeper field, that's what space telescopes are for

Its a collaboration effort predominantly from Americans and was started from scratch. It leads me to think how many M51 images have we seen presented on SGL. We wouldnt need to start from scratch and ask members to start imaging, surely the data is already there. How many hours would we have?

I'll nominate Olly to do the processing  

Adrian

[Elp]

I'm guessing his internet speed will say, no. Funnily enough it's one I haven't imaged yet but on here collectively they'd be far more than 200hrs.

[tomato]

Nice to see this lively thread maybe ending on a remarkable image of M51, although I have to say it is not to my tastes. The faint nebulosity looks a bit over processed to me and has a painted on look but perhaps that how it is in reality. 
I prefer images with (dare I say it) more detail in the actual galaxies. 
I agree with Olly that amateurs can really only break new imaging ground by going deep and off the beaten track, but if your favourite objects are galaxies then you can only strive to get the best detail you can with the kit and location you have.

And I’ve obviously not got the message as I have just ordered a camera with 2 micron pixels….😉

[CCD Imager]

22 minutes ago, Elp said:

I'm guessing his internet speed will say, no. Funnily enough it's one I haven't imaged yet but on here collectively they'd be far more than 200hrs.

It would need commitment of around 20-40 imagers who have the data. The laborious part would be calibrating, registering and combining, all in the latest and greatest PC. It doesnt have to be M51, there was also a recent very deep M31 image showing an arc not seen previously.

[CCD Imager]

19 minutes ago, tomato said:

And I’ve obviously not got the message as I have just ordered a camera with 2 micron pixels….😉

Oh no, not another resolution freak 🤣

So which camera??

Adrian