Bubble Nebula rehashed L(Ha)SHO, Foraxx palette

[windjammer]

I imaged the Bubble about a year ago but was never that happy with the end result - so this last week of clouds was an opportunity to re-process with new tools and a better handle on processing images.  

Also, in a moment of chutzpah / hubris I will probably regret: a side by side comparison of a Hubble close up of the Bubble and my humble Startravel 150 achro refractor.. : )

Astronomik narrowband Type 2c 1.25" filters: Ha (656nm), Sii (672nm) and Oiii (501nm). Total exposures 8.6hrs.

Ha 2x2 bin - 16x600s (East side) + 600sx5 (West side) = 3.5hrs, 8-9 October 2022
SII 2x2 bin - 11x600s = 1.8hrs, 10-11 October 2022
OIII 2x2 bin - 20x600s = 3.3 hrs, 17-18 October 2022

Imaging scope: SW Startravel 150mm F5, 2.5x Celetron Luminos 2inch imaging barlow, Atik 460EX mono

Simon

and Hubble vs Startravel 150:

 

 

 

[Roy Foreman]

Very nice image. Who needs Hubble!

[kirkster501]

Wow, very nice.  They could have saved $10 billion on the JWST and gave it to you  

[simmo39]

Yep that just shows what modern processing technology can do, although we must all thank the first failure of Hubble's mirror for the processing programs we have today.

[Elp]

Imaging at F12.5?

[vlaiv]

7 minutes ago, Elp said:

Imaging at F12.5?

Why not? I mean in principle - in this particular case, it is way too over sampled even with pixel size of 460ex, but in general - it's not speed of optics that counts - it is aperture at resolution.

Hubble is F/24 if I'm not mistaken.

[Elp]

Just wondering, I've tried F6.3 on my C6, the detail was there but image was dim even after post process. I think I need to increase my per image sub lengths and use a bigger pixel camera, or bin. This result is certainly encouraging for me to try higher res higher FL imaging.

Edited September 1, 2023 by Elp

[windjammer]

1 hour ago, Elp said:

Imaging at F12.5?

Yes - F5 and 2.5x barlow.  I had to bin the camera 2x2 and even so 600s exposures.  Early on this year I realised (duh) a 2.5x barlow and then binning 2x2 was just snakes and ladders, so I've reverted to F5 and 1x1 binning on later projects.  You end up with the same number of pixels in the relevant object and in a wider fov, which you can always crop.  I might be in the market for a 1.5x barlow tho....

Simon

[vlaiv]

1 minute ago, windjammer said:

I might be in the market for a 1.5x barlow tho....

Do reality check first - examine your subs at F/5 and this pixel size and compare working resolution with actual FWHM you are able to achieve.

At F/5 or 750mm of focal length and 4.54um pixel size you are working at ~1.25"/px

In theory, with well corrected scope and good skies - this should be ok for Ha/SII narrowband with 150mm of aperture. OIII will struggle though as seeing is often poorer. You also have fast achromat which is not well corrected scope and probably suffers from quite a bit spherochromatism - so using NB filters won't remove that bit - there will be some spherical aberration present for sure.

In any case - if you are working at 1.25"/px - you should be having 1.25 * 1.6 = 2" FWHM stars in your subs.

You can check that in your old/current subs. If your star FWHM is larger than two arc seconds - you are already over sampling as is and adding barlow won't bring any new detail (it will only hurt your images by lowering SNR).

[CCD Imager]

Anything over 0.5 arc sec is under-sampled! Seriously, UK skies offer around 1.5 to 2.5 arc secs FWHM and you need to sample at a rate of 3x for optimal sampling. Not only that, if seeing is better than your optical resolution, you have missed out and if you end up over sampling, you can bin in post to suit your taste. And lastly, deconvolution algorithms just love an over sampled image, give Blur XTerminator a decent chance!

Adrian

[windjammer]

Well, the PixInsight FWHMEccentricity script reports the FWHM of Ha master lights over a few projects (at F5, no barlow) comes in at ~ 2.7 pixels.

The PI Image Solver script reports resolution is 1.25 asec per pixel, so FWHM comes in at 2.7 * 1.25 = 3.4 asec

The seeing website: https://www.meteoblue.com/en/weather/outdoorsports/seeing/london_united-kingdom_2643743  reports London UK seeing in range 1.5 to 3 asec.  Usually at the higher end TBH!

The Rayleigh criterion would be 0.8 asec at 500nm, so we are nowhere that.

With modest enhancement parameters (no artefacts) in BXT the FWHM comes down to ~ 1.6 pixels, or 1.6 * 1.25 = 2 asec.

I agree with the point that deconvolution tools need extra pixels to work with (no evidence, just a gut feel!).  

So my thinking is working at F5 with 1.25 asec/pxl sensor is probably about right, and a little bit higher F number might work as well if the light flux doesn't take exposure times to extreme lengths.  600s to 900s is my practical limit, just in terms of getting a reasonable number of exposures in a session.  Another optical element in the image train is a bit of a downer as well.

Simon

[CCD Imager]

I dont know how Meteoblue calculates their FWHM, I tried to find a reference to it, but alas nothing. However, they may not be far off. Nearly 30 years ago, I performed seeing analysis on the majority of clear nights over one year, using an SBIG STV, utilising the DIMM method. Seeing was mostly sub 2.0 arc secs and often less than 1.5, occasionally less than 1.0

This is the raw base seeing, but you then have to add the effects of optical quality, focus, guiding etc that blurs the FWHM, so users usually end up with a much higher FWHM. Over the many years since I measured raw FWHM, I have been working to reduce additional blurring components, Should you manage to own a high quality triplet refractor and a mount that tracks to less than 0.5 arc sec, your resultant images will have a much better FWHM.

 

Most astro Imagers aim for a sampling of 1.0 arc sec/pixel, a good compromise for the majority of users

Adrian

[vlaiv]

6 hours ago, CCD Imager said:

Anything over 0.5 arc sec is under-sampled! Seriously, UK skies offer around 1.5 to 2.5 arc secs FWHM and you need to sample at a rate of 3x for optimal sampling.

Please don't spread misinformation.

x3 times FWHM is very far away from optimal sampling.

 

[CCD Imager]

You havent heard of Nyquest sampling?
I have used a sampling rate of 0.47 arc sec/pixel and been under sampled.

[vlaiv]

Just now, CCD Imager said:

You havent heard of Nyquest sampling?

Of course I have, and for the record - it states following:

"Given band limited signal, you need to sample at twice highest frequency component of that signal in order to be able to perfectly restore it".

Can you tell me how would you justify "x3 per FWHM" being twice highest frequency component of band limited signal?

[ollypenrice]

Just now, CCD Imager said:

You havent heard of Nyquest sampling?
I have used a sampling rate of 0.47 arc sec/pixel and been under sampled.

This is a deep sky imaging thread, so not connected with 'lucky imaging.'

If you have been under-sampled at 0.47"PP you have some astonishingly detailed images to show us and I'll look at them with interest - not to say astonishment. In the absence of such images I'll have to put this claim down to a triumph of theory over practice.

Olly

[CCD Imager]

5 minutes ago, vlaiv said:

Of course I have, and for the record - it states following:

"Given band limited signal, you need to sample at twice highest frequency component of that signal in order to be able to perfectly restore it".

Can you tell me how would you justify "x3 per FWHM" being twice highest frequency component of band limited signal?

Because the Nyquest theorem was applicable to continuous audio signal and astronomical measurements are 3D and pixels are rectangular not circular

Edited September 1, 2023 by CCD Imager

[CCD Imager]

In fact the precise calculation from memory is more like 3.2x

[CCD Imager]

5 minutes ago, ollypenrice said:

This is a deep sky imaging thread, so not connected with 'lucky imaging.'

If you have been under-sampled at 0.47"PP you have some astonishingly detailed images to show us and I'll look at them with interest - not to say astonishment. In the absence of such images I'll have to put this claim down to a triumph of theory over practice.

Olly

There was a discussion on CN about am image I took with 1.2 arc sec FWHM. The debate centered on insufficient pixels to properly sample in addition to insufficient aperture! I used the SW Esprit 150. However, I have regularly captured images around the 1.5 arc sec FWHM

[vlaiv]

1 minute ago, CCD Imager said:

Because the Nyquest theorem was applicable to continuous audio signal and astronomical measurements are 3D and pixels are rectangular not circular

Pixels are not 3d and are certainly not rectangular. For purpose of this discussion pixels are point samples.

Even if you are referring to camera pixels not being perfect point sampling device - effect of that is convolution with pixel blur which is much smaller contributing factor than telescope aperture and seeing, and after all - it only adds blur and reduces resolution rather than enhance it.

2d sampling case is the same as 1d case in case of rectangular sampling grid - except we must break down to X and Y direction - in which case it holds that X sampling rate must be twice max wavelength in X direction and Y sampling rate must be twice sampling rate in Y direction.

Optimum 2d sampling case is hexagonal grid - but no one is making such sensor, and since sensor is square grid and X and Y sampling rates are the same and any other wavelength in direction other than pure X and Y will have longer X and Y wavelength (vector projected on unit vector basis will have smaller length in those basis than length of vector itself) - original requirement still stands - we need to sample at twice highest frequency component regardless of wave orientation.

 

[vlaiv]

2 minutes ago, CCD Imager said:

There was a discussion on CN about am image I took with 1.2 arc sec FWHM. The debate centered on insufficient pixels to properly sample in addition to insufficient aperture! I used the SW Esprit 150. However, I have regularly captured images around the 1.5 arc sec FWHM

That is something I'd like to see. Most amateur setups are limited to 1.6" FWHM or higher and this is with twice as much aperture. I would love to see sub with 1.5" FWHM

[ollypenrice]

4 minutes ago, CCD Imager said:

Because the Nyquest theorem was applicable to continuous audio signal and astronomical measurements are 3D and pixels are rectangular not circular

I couldn't care less about the Nyquest theorem (or the Nyquist ), I want to see these images which were undersampled at 0.5"PP!

1 minute ago, CCD Imager said:

There was a discussion on CN about am image I took with 1.2 arc sec FWHM. The debate centered on insufficient pixels to properly sample in addition to insufficient aperture! I used the SW Esprit 150. However, I have regularly captured images around the 1.5 arc sec FWHM

And I couldn't care less about discussions on CN or about FWHM, neither of which I can see when I look at an astrophoto. I want to see an actual image which was undersampled at 0.5"PP because it will be the most detailed image of the object in question that I have ever seen from an amateur system.

Post the image and win the argument!

Olly

[CCD Imager]

3 minutes ago, vlaiv said:

Pixels are not 3d and are certainly not rectangular. For purpose of this discussion pixels are point samples.

Even if you are referring to camera pixels not being perfect point sampling device - effect of that is convolution with pixel blur which is much smaller contributing factor than telescope aperture and seeing, and after all - it only adds blur and reduces resolution rather than enhance it.

2d sampling case is the same as 1d case in case of rectangular sampling grid - except we must break down to X and Y direction - in which case it holds that X sampling rate must be twice max wavelength in X direction and Y sampling rate must be twice sampling rate in Y direction.

Optimum 2d sampling case is hexagonal grid - but no one is making such sensor, and since sensor is square grid and X and Y sampling rates are the same and any other wavelength in direction other than pure X and Y will have longer X and Y wavelength (vector projected on unit vector basis will have smaller length in those basis than length of vector itself) - original requirement still stands - we need to sample at twice highest frequency component regardless of wave orientation.

 

You are misinterpreting, it is the stellar profile that you are measuring and is 3D - x, y and intensity

[vlaiv]

Just now, CCD Imager said:

You are misinterpreting, it is the stellar profile that you are measuring and is 3D - x, y and intensity

Not misinterpreting anything. It is a 2d function.

It's like saying sine is 2d function because it has height / intensity - it is not, it is 1d function.

[CCD Imager]

1 minute ago, ollypenrice said:

I couldn't care less about the Nyquest theorem (or the Nyquist ), I want to see these images which were undersampled at 0.5"PP!

And I couldn't care less about discussions on CN or about FWHM, neither of which I can see when I look at an astrophoto. I want to see an actual image which was undersampled at 0.5"PP because it will be the most detailed image of the object in question that I have ever seen from an amateur system.

Post the image and win the argument!

Olly

Lol Olly
apologies about the spelling, I forgot you were a teacher!
The image in question is a single sub and the final image had a FWHM of 1.4 arc secs. I don't think you would be enamoured with a single sub?

Adrian