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wimvb    2,111

M106_4Panel_l.thumb.jpg.e9d348421f63e697e85906823f73729e.jpg

4 panel mosaic of M106

Data: Liverpool telescope at La Palma (Canary Islands)

r (sdss-r), g (Bessel-V) and b (Bessel-B) datasets, 4 x 90 seconds per channel at f/10 (but a 2 m primary mirror)

My first time making a mosaic. The data was rather flat, with a very bright inner core and only faint outer arms.

Even though there is no Ha data, there is just a hint of Ha jets near the center.

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tooth_dr    1,184

Very impressive. What sort of size would the likes of this print out at? I've seen wallpaper you can design yourself and I always thought as astronomical theme would be great for a feature wall in work. 

Edited by tooth_dr

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wimvb    2,111
7 hours ago, tooth_dr said:

Very impressive. What sort of size would the likes of this print out at? I've seen wallpaper you can design yourself and I always thought as astronomical theme would be great for a feature wall in work. 

Thanks.

The mosaic isn't that large, actually. The camera's resolution is 0.15"/pixel. The mosaic wasn't downsampled and is about 3000x3000 pixels. The individual panels were 2000x2000 pixels. The data is too much oversampled to use drizzle.

I may try a Hubble mosaic sometime this summer, where I expect larger panels.

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ollypenrice    17,968

A nice change! 

Those who doubt the validity of the F ratio myth should look at those stats: 4 x 1.5 minutes per channel at F10. Incredible.

Olly

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Davey-T    9,701
6 minutes ago, ollypenrice said:

A nice change! 

Those who doubt the validity of the F ratio myth should look at those stats: 4 x 1.5 minutes per channel at F10. Incredible.

Olly

Be looking forward to something similar from your "new" SCT then.

Dave

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ollypenrice    17,968
21 minutes ago, Davey-T said:

Be looking forward to something similar from your "new" SCT then.

Dave

Don't expect too much because there's a difference between 0.25 metres and 2 metres! :eek: However, it will be interesting to image small targets with larger aperture and slower F ratio.

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wimvb    2,111
3 hours ago, ollypenrice said:

A nice change! 

Those who doubt the validity of the F ratio myth should look at those stats: 4 x 1.5 minutes per channel at F10. Incredible.

Olly

With 15 micron photon hoovers for pixels. Just too bad I couldn't find any Ha to go with the rgb.

Another problem that turned up is that the overlap between frames had considerably less noise after stretching than the 'corners'. This showed as a cross in the image during the final stages. I used Olly's histogram squeezing procedure to fix this.

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wimvb    2,111
7 minutes ago, PatrickGilliland said:

Nice work Wim, lovely fine details in there well done.

Thanks, Paddy.

Having data from a photon hoover (x 2, scope & camera) helps.

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gorann    1,656

Wow Wim, you pulled it off and found data for all panels. And with an impressive result! I gave up on digging out that data for M106 in the LT database (suspected it was not all there) and dug into M1 instead (so much so that I missed seing your post for 15 hours).

What's next?

Glad påsk!

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wimvb    2,111

Thanks, Göran

I've just downloaded data for M 51, but it's a very tight crop. I'm not sure yet if I will use it. Also found a lot of lum data on M 27, but no colour. I'll see if colour is available somewhere else.

Glad påsk.

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gorann    1,656
29 minutes ago, wimvb said:

Thanks, Göran

I've just downloaded data for M 51, but it's a very tight crop. I'm not sure yet if I will use it. Also found a lot of lum data on M 27, but no colour. I'll see if colour is available somewhere else.

Glad påsk.

Good luck in hunting LT data Wim!

It is a fishing trip but there are treasures in there waiting to be discovered and processed.

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wimvb    2,111
On ‎2017‎-‎04‎-‎13 at 10:27, ollypenrice said:

A nice change! 

Those who doubt the validity of the F ratio myth should look at those stats: 4 x 1.5 minutes per channel at F10. Incredible.

Olly

Had time to think this over ...poohthink.gif.051d516e6b67c62f68aaacb570d42d69.gif

*****************************************

            WARNING: math content

*****************************************

Here's how I see it:

Pixel resolution: r = 206 * p / f  (r = arcsecs/pixel, p = pixel size in micrometers, f = focal length in mm)

Each pixel receives light from r^2 arcsecs of sky that goes through the aperture (D^2). To compare the photon reveiving power of a system on pixel scale, that should give a number of

(D * p / f)^2

This looks like F-ratio is a winner, because F-ratio = f/D, so the above expression can be rewritten as (p/F)^2.  Lower F-ratio gives better photon gathering on pixel.

BUT: resolution changes with focal length, so a better way to express the photon gathering power would be:

(rD)^2

For any camera / scope resolution (r), it's only the aperture (D) that increases photon capture. The focal length is not relevant.

 

pooh131.gif.b84d397cacb72cbe3503c646a170fb5b.gif Stan Moore in 'Lessons from the Masters', doesn't cover this part. He only explains the SNR of it all.

 

Or am I completely wrong here?

6273305_orig.jpg.36b59c5d38cb99fc55a35a278c3e0588.jpg

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Adam J    621
On 13/04/2017 at 09:27, ollypenrice said:

A nice change! 

Those who doubt the validity of the F ratio myth should look at those stats: 4 x 1.5 minutes per channel at F10. Incredible.

Olly

As wimvb says its probably got more to do with the 15um pixels, very high QE, cryogenic cooling and a read noise along the same lines as a ASI1600. That is enabling outstanding performance even at F10.

By my reckoning that is 11 x more collecting area per pixel than say a Atik460EX with 60% less read noise and thanks to the extreme cooling probably next to no thermal noise in a 1.5min exposure (all that is without factoring in that the image was clearly 2x2 binned). Am going to take a guess at it having a per pixel S/N ratio of something along the lines of 20 to 30 times that of the 460EX if placed on the same scope....of course the 460EX would be massively over sampled and only capture 1/30th of the image shown above. So in terms of signal to noise I would guesstimate that is equivalent to about 2.5 hours of data from a Atik 460EX (150hours for it to make the equivalent mosaic).

In my opinion there never really was an F-ratio 'myth' its more a f-ratio misconception. Talking about F-ratio in terms of signal to noise per pixel, is fine and correct, just so long as you and your audience understand the limitations of the argument, the argument being that s/n ratio will stay the same for a given F number irrespective of focal length. You just need to accept that you will lose resolution and understand that the argument breaks down as the angle subtended but the object being imaged approaches the angle subtended by a single pixel. All the stuff floating about concerning less information being gathered is a valid but totally separate argument.

On 17/04/2017 at 13:10, wimvb said:

Here's how I see it:

Pixel resolution: r = 206 * p / f  (r = arcsecs/pixel, p = pixel size in micrometers, f = focal length in mm)

Each pixel receives light from r^2 arcsecs of sky that goes through the aperture (D^2). To compare the photon reveiving power of a system on pixel scale, that should give a number of

(D * p / f)^2

This looks like F-ratio is a winner, because F-ratio = f/D, so the above expression can be rewritten as (p/F)^2.  Lower F-ratio gives better photon gathering on pixel.

BUT: resolution changes with focal length, so a better way to express the photon gathering power would be:

(rD)^2

For any camera / scope resolution (r), it's only the aperture (D) that increases photon capture. The focal length is not relevant.

 

 

 

If we are really saying that some people actually think that their little amateur scope is equivalent to a 2 meter scope because they share the same f-ratio? If so then I am not sure what that is but I would not call it a 'myth' more a fundamental misunderstanding of the measure. If you are not saying that then I am not sure I understand what you are trying to convey?

I dont know maybe what I am not actually understanding is the nature of the myth itself??? I just think that the short exposure performance at F10 in this case is everything to do with the camera and that the resolution of the image is to do with the size of the primary mirror. But however you put it the slow f-ratio is not helping and a larger primary with the same focal length for a faster f-ratio would give a superior result in an even shorter exposure.

What you are doing above is comparing two totally different measures both of which have a place but mean totally different things. The F-ratio of a optical system is a useful piece of information to have as it tells you the light gathering power you have for a given focal length at a quick glance nothing more nothing less.  

 

Edit: I SHOULD PROBABLY SAY NICE IMAGE ! :)

Edited by Adam J

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Rodd    2,272
On ‎4‎/‎19‎/‎2017 at 17:36, Adam J said:

As wimvb says its probably got more to do with the 15um pixels, very high QE, cryogenic cooling and a read noise along the same lines as a ASI1600. That is enabling outstanding performance even at F10.

By my reckoning that is 11 x more collecting area per pixel than say a Atik460EX with 60% less read noise and thanks to the extreme cooling probably next to no thermal noise in a 1.5min exposure (all that is without factoring in that the image was clearly 2x2 binned). Am going to take a guess at it having a per pixel S/N ratio of something along the lines of 20 to 30 times that of the 460EX if placed on the same scope....of course the 460EX would be massively over sampled and only capture 1/30th of the image shown above. So in terms of signal to noise I would guesstimate that is equivalent to about 2.5 hours of data from a Atik 460EX (150hours for it to make the equivalent mosaic).

In my opinion there never really was an F-ratio 'myth' its more a f-ratio misconception. Talking about F-ratio in terms of signal to noise per pixel, is fine and correct, just so long as you and your audience understand the limitations of the argument, the argument being that s/n ratio will stay the same for a given F number irrespective of focal length. You just need to accept that you will lose resolution and understand that the argument breaks down as the angle subtended but the object being imaged approaches the angle subtended by a single pixel. All the stuff floating about concerning less information being gathered is a valid but totally separate argument.

If we are really saying that some people actually think that their little amateur scope is equivalent to a 2 meter scope because they share the same f-ratio? If so then I am not sure what that is but I would not call it a 'myth' more a fundamental misunderstanding of the measure. If you are not saying that then I am not sure I understand what you are trying to convey?

I dont know maybe what I am not actually understanding is the nature of the myth itself??? I just think that the short exposure performance at F10 in this case is everything to do with the camera and that the resolution of the image is to do with the size of the primary mirror. But however you put it the slow f-ratio is not helping and a larger primary with the same focal length for a faster f-ratio would give a superior result in an even shorter exposure.

What you are doing above is comparing two totally different measures both of which have a place but mean totally different things. The F-ratio of a optical system is a useful piece of information to have as it tells you the light gathering power you have for a given focal length at a quick glance nothing more nothing less.  

 

Edit: I SHOULD PROBABLY SAY NICE IMAGE ! :)

What the myth is supposed to say (I am one that is not convinced yet) is that if you take a 10 min sub of m51 with a 10" scope at f10, and a 10 minute sub of m51 with the same scope and camera but with a reducer at F5, then you crop the reduced sub so that m51 is the same scale as it is in the unreduced sub--M51 will look identical in each sub.  Same data, when scaled to be the same.

But take alook at the Celestron RASA M51 still on page 1 and you might also question the validity of the myth.

Rodd

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alacant    1,300
56 minutes ago, Rodd said:

take alook at the Celestron RASA M51

Phew, f2.8. That must be fast. And indeed it is. In an hour he got what most of us would take several nights over...

I don't think any of us are going to be installing our own 2000mm f10s any time soon, so how about we dispel parts of the myth? How about something along the lines; ...in telescopes we own, f10 is dim? OWTTE.

BTW, nice processing wim. 

Edited by alacant

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Rodd    2,272
13 minutes ago, alacant said:

Phew, f2.8. That must be fast. And indeed it is. In an hour he got what most of us would take several nights over...

I don't think any of us are going to be installing our own 2000mm f10s any time soon, so how about we dispel parts of the myth? How about something along the lines; ...in telescopes we own, f10 is dim? OWTTE.

BTW, nice processing wim. 

I suppose Thats not really a good one to use though--as he also used an ASI 1600mmc--which is a fast camera.  But--according to the myth, at f10, M51 WILL NOT be dimmer than m51 shot at F2.8 (same camera and scope with reducer) after you crop and enlarge (resample).  That's the myth--thinking that using a reducer to get a faster focal ratio will enable you to shoot m51 faster than at F10.  You will be able to shoot the entire FOV faster--but M51 itself will be the same once you rescale.  So, they say, if you want to shoot m51 and are not interested in the larger FOV you get with a reducer--using a reducer is pointless. 

Rodd

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Adam J    621
8 hours ago, Rodd said:

I suppose Thats not really a good one to use though--as he also used an ASI 1600mmc--which is a fast camera.  But--according to the myth, at f10, M51 WILL NOT be dimmer than m51 shot at F2.8 (same camera and scope with reducer) after you crop and enlarge (resample).  That's the myth--thinking that using a reducer to get a faster focal ratio will enable you to shoot m51 faster than at F10.  You will be able to shoot the entire FOV faster--but M51 itself will be the same once you rescale.  So, they say, if you want to shoot m51 and are not interested in the larger FOV you get with a reducer--using a reducer is pointless. 

Rodd

I am not convinced that is entirely incorrect to be honest as you are deciding the photon flux between less pixels the s/n will improve at the expense of resolution at the same exposure length. The problem with that is when your object is only subtending one or two pixels prior to reduction as in that case you don't gain anything....so dim stars will remain the same leading you to believe that the limiting magnitude is unchanged....however if you had something like a wide field nebula that is uniform on small to medium scales then as you reduce the focal length it will increase the pixel photon flux and hence improve s/n...but that's not strictly the same as making something brighter...but it will  improve the image in some cases as in your m51 example exactly the same number of photons are being collected by the scope but are being focused onto fewer pixels. Forget about the overall wider fov those photons were always being collected by the scope it's just without the reducer they were falling off the edge of the sensor. So I still say there is no myth! 

 

 

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Rodd    2,272
24 minutes ago, Adam J said:

I am not convinced that is entirely incorrect to be honest as you are deciding the photon flux between less pixels the s/n will improve at the expense of resolution at the same exposure length. The problem with that is when your object is only subtending one or two pixels prior to reduction as in that case you don't gain anything....so dim stars will remain the same leading you to believe that the limiting magnitude is unchanged....however if you had something like a wide field nebula that is uniform on small to medium scales then as you reduce the focal length it will increase the pixel photon flux and hence improve s/n...but that's not strictly the same as making something brighter...but it will  improve the image in some cases as in your m51 example exactly the same number of photons are being collected by the scope but are being focused onto fewer pixels. Forget about the overall wider fov those photons were always being collected by the scope it's just without the reducer they were falling off the edge of the sensor. So I still say there is no myth! 

 

 

Talk to Olly about it--if he can't convince you, no one can! (I am still questioning).

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Adam J    621
50 minutes ago, Rodd said:

Talk to Olly about it--if he can't convince you, no one can! (I am still questioning).

I would have to be missing something big. I would be interested to see what he has to say on the subject though mind you.

Another way to look at it is a badly focused star...its dimmer right....well that is because the photon flux per pixel is less because you have spread it over more pixels than if (in the ideal case) it was all falling on ~ 25 pixels or so when focused. Now if you have huge massive pixels, one covering the entire unfocused star....the unfocused star will still be exactly as 'bright' or having the same signal to noise ratio as if it was focused...it would make no difference...but when we image that is not a circumstance that happens very often?

The scope is always (always) collecting exactly the same number of photons from a target through its aperture. How many pixels of a fixed size you are spreading that over depends on the focal length and hence F-ratio. Same noise in a pixel, same ratio of signal to sky glow irrespective of aperture, but with a reducer more photons hitting a single pixel. 

You are correct to question it i think.

Easy to prove if someone has two scopes of the same aperture and a flat field panel at quite some die distance from the scope. 

Edited by Adam J

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wimvb    2,111

This may be relevant to your discussion.

 

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Rodd    2,272
2 hours ago, Adam J said:

I would have to be missing something big. I would be interested to see what he has to say on the subject though mind you.

Another way to look at it is a badly focused star...its dimmer right....well that is because the photon flux per pixel is less because you have spread it over more pixels than if (in the ideal case) it was all falling on ~ 25 pixels or so when focused. Now if you have huge massive pixels, one covering the entire unfocused star....the unfocused star will still be exactly as 'bright' or having the same signal to noise ratio as if it was focused...it would make no difference...but when we image that is not a circumstance that happens very often?

The scope is always (always) collecting exactly the same number of photons from a target through its aperture. How many pixels of a fixed size you are spreading that over depends on the focal length and hence F-ratio. Same noise in a pixel, same ratio of signal to sky glow irrespective of aperture, but with a reducer more photons hitting a single pixel. 

You are correct to question it i think.

Easy to prove if someone has two scopes of the same aperture and a flat field panel at quite some die distance from the scope. 

What you say about pixel efficiency and pixel dynamics may be true--but the claim is that when you use a focal reducer and go from F10 to F7--it is 4x faster.  I don't think the pixel dynamics you describe can account for that much of a difference.  Maybe some but not that much.

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