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Rodd

Reducer Myth: Some data.

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These 2 images of the Gamma Cas nebula both contain 7 30 min subs collected through Astrodon 3nm filters.  1 was collected with the Televue np101is at F4.3 and 1 was collected with the TOA-130 at F7.7.  The only parameter that off is the F7.7 image was collected with 5.2 inches of aperture and the F4.3 image was collected with 4" of aperture.  But for the purpose of this inquiry, I do not think it relevant--especially since the extra inch of aperture was used with the F7.7 image--I'll explain.  In this demonstration, we are only interested in the FOV defined by the TOA image.  According to the myth, there are no more photons falling on this portion of the Televue image than are falling on the TOA image (I did not rotate the image so you will have to do that in your mind).  But I think it obvious that there ARE more photons falling on this part of the Televue nimage.  The stars are more numerous, and the nebulosity id brighter.  Now, if the extra inch of aperture was used with the Televue image, one might think it being brighter had something to do with the extra inch of aperture.  But the extra inch of aperture was used for the dimmer image--so it does not have much meaning (other than helping keep the TOA image from being even dimmer!).  The other interesting aspect of the images is that the FWHM values in the Televue image are around 3-3.5, while for the TOA image are 2.2-2.7.  But the stars look smaller in the Televue image....Hmm.

So....what am I missing?  It seems painfully obvious to me that more photons were collected in the "druid" in the Televue image then were collected in the "druid" in the TOA image.  Yes there is more detail visible in the TOA image, and for the druid, I think I find it more pleasing.  In fact, I like the TOA image better all around.  But that is another matter purely of  personal taste.  Please, all comments, criticisms, affirmations, questions, welcomed.

I should mention that both images are unprocessed-integrated using the same parameters, and stretched using identical STF settings (not the way I would do it for production).  Both were fully calibrated.

Televue:

TV-STF.thumb.jpg.094ad17b0cdfc924ca8bc43fd7ac4cc3.jpg

 

TOA:

TOA-STF.thumb.jpg.c6afb219686a801ce0e90bf75f8e7496.jpg

Edited by Rodd
forgot so mention calibration
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Interesting but given that all the photons that hit the chip have to come through the front of the telescope, where do you think these extra photons you talk about actually come from?

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Assuming the same camera was used for both imaged then photons per pixel are less in the TOA image due to the fact that the increase in aperture is too small to compensate for the difference in focal length (and corresponding  image scale).

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3 hours ago, Rodd said:

These 2 images of the Gamma Cas nebula both contain 7 30 min subs collected through Astrodon 3nm filters.  1 was collected with the Televue np101is at F4.3 and 1 was collected with the TOA-130 at F7.7.  The only parameter that off is the F7.7 image was collected with 5.2 inches of aperture and the F4.3 image was collected with 4" of aperture.  But for the purpose of this inquiry, I do not think it relevant--especially since the extra inch of aperture was used with the F7.7 image--I'll explain.  In this demonstration, we are only interested in the FOV defined by the TOA image.  According to the myth, there are no more photons falling on this portion of the Televue image than are falling on the TOA image (I did not rotate the image so you will have to do that in your mind).  But I think it obvious that there ARE more photons falling on this part of the Televue nimage.  The stars are more numerous, and the nebulosity id brighter.  Now, if the extra inch of aperture was used with the Televue image, one might think it being brighter had something to do with the extra inch of aperture.  But the extra inch of aperture was used for the dimmer image--so it does not have much meaning (other than helping keep the TOA image from being even dimmer!).  The other interesting aspect of the images is that the FWHM values in the Televue image are around 3-3.5, while for the TOA image are 2.2-2.7.  But the stars look smaller in the Televue image....Hmm.

So....what am I missing?  It seems painfully obvious to me that more photons were collected in the "druid" in the Televue image then were collected in the "druid" in the TOA image.  Yes there is more detail visible in the TOA image, and for the druid, I think I find it more pleasing.  In fact, I like the TOA image better all around.  But that is another matter purely of  personal taste.  Please, all comments, criticisms, affirmations, questions, welcomed.

I should mention that both images are unprocessed-integrated using the same parameters, and stretched using identical STF settings (not the way I would do it for production).  Both were fully calibrated.

Televue:

TV-STF.thumb.jpg.094ad17b0cdfc924ca8bc43fd7ac4cc3.jpg

 

TOA:

TOA-STF.thumb.jpg.c6afb219686a801ce0e90bf75f8e7496.jpg

According to the myth--there should not be extra photons in the druid itself.  But there seems to be.

Rodd

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1 hour ago, Ikonnikov said:

Assuming the same camera was used for both imaged then photons per pixel are less in the TOA image due to the fact that the increase in aperture is too small to compensate for the difference in focal length (and corresponding  image scale).

Focal length is irrelevant.  When you use a reducer, you change the focal length by definition, so it is not in consideration.  Only aperture and F ratio.  According to the myth, the druid itself should be the same brightness in the unreduced and the reduced image.  We are only considering the FOV of the unreduced image

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1 hour ago, Freddie said:

Interesting but given that all the photons that hit the chip have to come through the front of the telescope, where do you think these extra photons you talk about actually come from?

The myth says that the druid itself should be the same brightness in an unreduced and a reduced image.  That does not seem to be the case.  The druid IS brighter in the educed image

Edited by Rodd

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Looks brighter in the reduced image exactly as I would have expected. Same number of photons spread over less sensor area / pixels due to the increased field of view. You increase signal to noise at the expense of reduced resolution. 

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Ah, that would rather be the focal ratio myth ;-)

As said in many other threads, and as debatable as it may be, focal ratio wins. The focal length is as much relevant as it's linked to the focal ratio.

And IMHO Ikonnikov is right : depending on focal ratio (at fixed aperture) the object image is more or less stretched in the focal plane, so that ends in more or less photons per pixel. Yes, the overall photon quantity is the same, but you only capture a part of them corresponding to your FoV.

Edit: BTW, I didn't grasp your setup, but the reducer should be irrelevant as it's only a way how to achieve shorter focal length and lower ratio; Again what seems to count is the effective focal ratio (just as with lenses after all).

Edited by rotatux
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8 minutes ago, Adam J said:

Looks brighter in the reduced image exactly as I would have expected. Same number of photons spread over less sensor area / pixels due to the increased field of view. You increase signal to noise at the expense of reduced resolution. 

No--we are only concerned with partr of the image.  When cropoped and enlarged it should not be brighter--but it is.  Thats teh myth

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42 minutes ago, Rodd said:

No--we are only concerned with partr of the image.  When cropoped and enlarged it should not be brighter--but it is.  Thats teh myth

There are no myths only people who don't understand optics. In this case I am not really sure what the myth is supposed to be. But every part of that image will appear brighter once reduced as every part of that image is spread over less pixels. In the non reduced image those photons don't vanish they just miss the sensor. It's like taking a flash light with an adjustable beam and shining it onto a wall. The smaller the spot the brigher it appears. The number of photons falling onto the wall is the same. It just looks brigher because they are concentrated onto a smaller part of the wall. It's all about photon flux. 

Edited by Adam J
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Not sure what the myth is, but if something looks brighter - that does not mean a thing! Each image can be made to look brighter by simply adjusting the contrast.

If we want to establish actual difference in brightness of two images, we need to look at photon / electron count per sky area in same places of two images.

So we need to know:

1. resolution at which each image was recorded

2. QE efficiency of each sensor at wavelength of recording (H alpha is it?) - if different cameras were used.

3. e/ADU relationship

Then we can say first image has N photons per arcsec squared, while second image has M photons per arcsec squared - hence first is "brighter" (N>M). On the other hand, that tells us nothing about quality of data in image. Brighter image can have lower SNR - just throw ligth pollution into mix (even when doing narrow band) and your measured photons per arcsec squared will be off - enlarged by amount of LP in that band. Nebula signal stays the same, but noise will increase. Image is brighter but of a less quality.

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The only way to get more pixels is to use a larger aperture.

The key here is 'appearance of brightness' in the digital image not absolute brightness, you cant change the number of photons collected without more aperture.

What you do change is the photon density / flux at the sensor which results in more photons being directed into less pixels everything else being equal. So in terms of digital image signal to noise, then for non finite targets (ie not stars) the nebulocity will have the appearance of being brighter and will have a better signal to noise ratio when using a reducer. 

However, if you have a bright target with very good signal to noise it wont make much difference as it will have a good signal to noise both before and after reduction, the Sombrero galaxy that i may correctly remember Olly sighting as a good example of the "f-ratio myth" is in fact such an instance, its a very bright target. Its with the faintest stuff that you get the gains in terms of signal to noise due to more arc seconds per pixel.

 

Edited by Adam J

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1 hour ago, Rodd said:

Olly......this thread needs you!

Rodd

@ollypenrice, your thoughts please :) 

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1 hour ago, Rodd said:

Focal length is irrelevant.  When you use a reducer, you change the focal length by definition, so it is not in consideration.  Only aperture and F ratio.  According to the myth, the druid itself should be the same brightness in the unreduced and the reduced image.  We are only considering the FOV of the unreduced image

Oh, ok, I finally get what the "Myth" is and where are the roots of it.

So, let's state it clearly:

Object should have same brightness on reduced image (using same scope with reducer) as in unreduced image (using only scope without reducer) given that whole object fits on both images.

This statement is both true and false :D

Let me explain. If by brightness we mean integrated / total brightness or magnitude of object then yes - it will stay the same. Provided that whole object fits inside both reduced and normal frames, total amount of light from object is just function of aperture (in general it is function of many more things, like atmospheric extinction, transparency, ...., but let's keep it simple and just look at examples where only difference is using focal reducer vs not using focal reducer).

So it is true for integrated brightness / total amount of light from object - object itself will not change brightness.

It is also true for surface brightness - again object will not change because we used reducer - it will give off same amount of light per surface area, and given same aperture we will capture same amount of light per surface area.

If we look at individual "signal" levels of each pixel in reduced and unreduced image - these will change, and not because F/ratio but because of change of Focal length (hence the name focal reducer :D ) which has direct impact on how much sky surface is covered by single pixel - so sky area / pixel area ratio change, and we just saw that brightness per sky area remains the same - hence levels recorded by pixel must be different.

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41 minutes ago, vlaiv said:

Oh, ok, I finally get what the "Myth" is and where are the roots of it.

So, let's state it clearly:

Object should have same brightness on reduced image (using same scope with reducer) as in unreduced image (using only scope without reducer) given that whole object fits on both images.

This statement is both true and false :D

Let me explain. If by brightness we mean integrated / total brightness or magnitude of object then yes - it will stay the same. Provided that whole object fits inside both reduced and normal frames, total amount of light from object is just function of aperture (in general it is function of many more things, like atmospheric extinction, transparency, ...., but let's keep it simple and just look at examples where only difference is using focal reducer vs not using focal reducer).

So it is true for integrated brightness / total amount of light from object - object itself will not change brightness.

It is also true for surface brightness - again object will not change because we used reducer - it will give off same amount of light per surface area, and given same aperture we will capture same amount of light per surface area.

If we look at individual "signal" levels of each pixel in reduced and unreduced image - these will change, and not because F/ratio but because of change of Focal length (hence the name focal reducer :D ) which has direct impact on how much sky surface is covered by single pixel - so sky area / pixel area ratio change, and we just saw that brightness per sky area remains the same - hence levels recorded by pixel must be different.

This is exactly correct, everything is the same but you are getting more signal per pixel and so the pixel values are higher making it appear brighter within the digital image and improving signal to noise ratio. Same amount of light emitted by the source, same amount of light collected by the scope. The only thing that matters is arcsec / pixel and that is what you are changing.

Edited by Adam J
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Sorry, I'm right in the thick of the moonless time and with clear skies and guests to consider. I'll be delighted to join in if we get a cloudy night!!!

Olly

 

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3 minutes ago, ollypenrice said:

Sorry, I'm right in the thick of the moonless time and with clear skies and guests to consider. I'll be delighted to join in if we get a cloudy night!!!

Olly

 

I would much rather be doing that too, as ever its cloudy here in Lincolnshire uk.

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The myth is actually more about time than brightness...but the two can be interchanged.  According to what Olly has said, it makes no sense to use a reducer to shorten imaging time if one is going to crop the image and only display a portion of it--in this case the druid.  However, as is clear from the images, if you crop the druid from the Televue image and enlarge it, which I have done--it is brighter than the TOA 130 druid by itself.  Therefore, less time would be required to capture the druid at the same signal strength using the reducer.  This is contrary to what the purveyors of the myth believe.    But we need to hear from those individuals who believe there is a myth.   

Rodd

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1 hour ago, Rodd said:

The myth is actually more about time than brightness...but the two can be interchanged.  According to what Olly has said, it makes no sense to use a reducer to shorten imaging time if one is going to crop the image and only display a portion of it--in this case the druid.  However, as is clear from the images, if you crop the druid from the Televue image and enlarge it, which I have done--it is brighter than the TOA 130 druid by itself.  Therefore, less time would be required to capture the druid at the same signal strength using the reducer.  This is contrary to what the purveyors of the myth believe.    But we need to hear from those individuals who believe there is a myth.   

Rodd

You would lose resolution but gain in signal to noise ratio. So yes in effect to match the signal to noise of the un-reduced image you would need less time.

I think that it all had something to do with the amount of information captured....which to me missed the point as I would be more concerned with the quality of information captured. Also it depends on the camera as you may be over sampling and so using a reducer will increase your arcseconds per pixel and so there would be no down side in that case.

Edited by Adam J

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Rodd, you say that there are more stars in the smaller aperture image than the larger. They may just be smaller in the larger aperture image, and so less visible. Or the nights may have been different. It's hard to be sure. However, I frequently do composite images using 106mm aperture and 140mm aperture images, overlaying the larger aperture data onto the smaller. I always find that there are more (and smaller) stars from the larger aperture so, if you're not finding that, I don't know what to say other than that you had a less transparent night for the bigger scope.

What I would like the people who believe only in F ratio to do is explain why a 50mm F5 scope with an objective area 100x smaller than that of a half meter F10 can, with only a hundredth of the incident photons, take an image of the Ring Nebula in a quarter of the time needed by the big scope.

Olly

 

 

 

 

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1 hour ago, Rodd said:

The myth is actually more about time than brightness...but the two can be interchanged.  According to what Olly has said, it makes no sense to use a reducer to shorten imaging time if one is going to crop the image and only display a portion of it--in this case the druid.  However, as is clear from the images, if you crop the druid from the Televue image and enlarge it, which I have done--it is brighter than the TOA 130 druid by itself.  Therefore, less time would be required to capture the druid at the same signal strength using the reducer.  This is contrary to what the purveyors of the myth believe.    But we need to hear from those individuals who believe there is a myth.   

Rodd

Ah, so using reducer does not decrease exposure time - that is the myth?

Again, some truth to it but in general it is quite wrong (hence it is the myth :D ).

I'll be talking about extended sources (surface brightness, not stars, for stars argument is a bit different). So here is first fact: Using focal reducer with same scope and same camera always increases level of signal recorded by a single pixel. This is true for both extended sources and stars. Argument is the same in that both extended sources emit certain level of light per area unit, and stars emit certain level of light (no area here - point sources). If we use focal reducer - we simply increase area that one pixel is covering - for extended sources this means more area - more light. With point sources story is a bit different and we are looking at Gaussian profile of star caused by combination of airy disk, seeing smear and guiding error. But story is pretty much the same - for a given FWHM of a star in arc seconds - with focal reducer less pixels will be used to cover star profile hence more signal per individual pixel.

So in both cases we increase signal per pixel for given time, or for equal signal per pixel we need less integration time. If we look at it from this perspective - yes it is a myth, using focal reducer shortens the time needed to achieve certain level of signal per pixel in both extended sources and in stars (given same camera and scope).

Now, there is second part to this story, and it concerns SNR. Here things are not quite straight forward. It really depends if one is shooting in dark skies or LP skies. For dark skies above argument still stands, you will lessen time to achieve certain level of signal and certain level of SNR (in less time one achieves certain signal level, but it takes more time than that to achieve certain SNR). In LP skies story is a bit different. Here using focal reducer also somewhat shortens integration time for target SNR but far, far less than one might expect. If by using focal reducer we increase level of target signal per pixel, we also increase level of sky background per pixel, and sky background in LP skies is really bringing in a lot of shot noise with it. So for cases where dominant source of noise is LP, focal reducer does not do much in terms of SNR. I've done some calculations, for following parameters: 8" scope, 18mag skies (that is about level of LP I do my imaging in), target around mag 24 (that is 24mag/arcsec squared), F/8 scope, x0.67 reducer giving F/5.4 (0.5"/pixel vs 0.7"/pixel in terms of resolution, but I would bin both of them so effectively 1"/pixel vs 1.4"pixel), for 4h of 1m exposures (240 exposures in total) increase in SNR when using focal reducer vs not using one is order of few percent - far less then one would expect for going from F/8 to F/5.4.

Side note to whole discussion: Comparing images by signal level needs great care to make sure images are indeed comparable. Air mass alone can be responsible for loss of signal of order of 0.1-0.2 mags (shooting at zenith vs shooting at 60deg alt for example). Transparency also comes into play - AOD (aerosol optical depth) can be responsible for knocking off another 0.2-0.3 mags of target brightness. So we can end up with difference of 0.4-0.5 mags between two nights - and that is factor of x1.4 in signal - enough to confuse with increase in signal level due to use of focal reducer.

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As the only way of comparing f-ratio alone is to use an aperture mask (which no-one ever does with scopes) and keep EVERYTHING else the same there are always too many confounding variables for a meaningful direct comparison.

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1 hour ago, ollypenrice said:

Rodd, you say that there are more stars in the smaller aperture image than the larger. They may just be smaller in the larger aperture image, and so less visible. Or the nights may have been different. It's hard to be sure. However, I frequently do composite images using 106mm aperture and 140mm aperture images, overlaying the larger aperture data onto the smaller. I always find that there are more (and smaller) stars from the larger aperture so, if you're not finding that, I don't know what to say other than that you had a less transparent night for the bigger scope.

What I would like the people who believe only in F ratio to do is explain why a 50mm F5 scope with an objective area 100x smaller than that of a half meter F10 can, with only a hundredth of the incident photons, take an image of the Ring Nebula in a quarter of the time needed by the big scope.

Olly

 

 

 

 

Olly--I was wrong about the stars--I had a relook and there are not more in the Televue image--just looked that way before I really looked.  However--if you look closely at the images above--the stars ARE smaller in the first image (Televue), no question.  Agree?  And I still am confused as to why the Televue druid has a stronger signal than the TOA druid.

Rodd

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1 hour ago, Adam J said:

You would lose resolution but gain in signal to noise ratio. So yes in effect to match the signal to noise of the un-reduced image you would need less time.

I think that it all had something to do with the amount of information captured....which to me missed the point as I would be more concerned with the quality of information captured. Also it depends on the camera as you may be over sampling and so using a reducer will increase your arcseconds per pixel and so there would be no down side in that case.

Both images were taken with the STT-8300 with 5.4um pixels.

Rodd

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