Reducer Myth: Some data.

[Rodd]

24 minutes ago, vlaiv said:

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 ).

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.

No the myth is that using a focal reducer will shorten exposure times needed for non extended targets.  People who believe in this believe that the focal reducer WILL NOT shorten the exposure time to achieve a certain signal strength in the nun extended target.

Rodd

[Adam J]

2 hours ago, ollypenrice said:

 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.

See my commend from a while back.

6 hours ago, Adam J said:

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.

Its because it does not work the same way with stars. When you are imaging a finite object such as a faint star reducing the focal length wont help as the peak brightness already only subtends a single pixel or a very small number of pixels and so with a small difference in F-number will not make any difference at all as all the photons from that point source end up going into a single pixel irrespective and with a larger aperture that means more photons into a single pixel.

On the other hand if its a large object like a nebula covering many many pixels you can reduce it down and concentrate more photons into less pixels.

2 hours ago, ollypenrice said:

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.

There are a number of factors at play here.

1) Such large pro telescopes such as the one you are talking about are not using the sensors that we mortals use. They use cryogenic cooling and have pixels the size of dinner plates with massive QE. So for example the Liverpool telescope wide field optical camera has 15um pixels for 0.15 arc-seconds / pixel. The QE at H-a is 97% and the dark current is < 0.002 e / pix / sec. This can lead to a perception that they are faster in comparison to amateur setups at a similar f-ratio when in fact is all down to the sensor technology being used.

2) Your example was a 500mm F10 and you have to perform the comparison using identical sensors, lets take the Liverpool telescope wide field optical sensor just for fun and bolt it to the 50mm F5 and your 500mm F10 then lets run those figures. (this works for any sensor).

You end up with 12.33 arc-seconds / pixel on the 50mm F5 finder in comparison to 0.62 arc-seconds / pixel on the 500mm F10.

50mm F5 = 12.33^2 = 152.03 arc-seconds^2 / pixel

500mm F10 = 0.3844 arc-seconds^2 / pixel

152.03 / 0.3844 = 395.5 so about 400

So light is collected from a angular area of space 400 times larger per pixel for the 50mm guide scope than for the 500mm F10 scope.

Now lets calculate the area of  the two objectives.

  500mm scope = 0.79m^2

  50mm scope = 0.00785m^2

  so 0.079 / 0.00785 = 100x funny old thing....

So its 100x more photons divided over 400x more pixels. (so thats your 4 to 1 ratio olly).

So lets assume that the ring nebula covers only 100 pixels in the 50mm scope (cant be bothered working out the size of the ring nebula. It follows that it will cover 40000 pixels in the 500mm scope (much better resolution) but each of the pixels on the 500mm scope has 1/4 the light entering it on average across all the pixels. So it takes longer to reach the same signal to noise ratio, it takes longer to fill the wells.

So there you go that is your explanation.

You chose an extreme example in the tiny ring nebula though and a big difference in scopes so in this case it would of course not be a good idea to image the ring nebula with a 50mm scope...you would just end up with a bright speck in the center of the image.

[Rodd]

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

 

 

 

 

You have me confused again Olly--mixing focal lengths and apertures.  I tried that and you warned me I would go mad.  

Rodd 

[Stub Mandrel]

6 hours ago, Adam J said:

So there you go that is your explanation.

I think Olly's question was rhetorical, but I enjoyed your answer anyway!

[gorann]

What puzzles me is that Gamma Cas looks so different in the two scopes. With the Televue you have something looking like small vane spikes, while with the TOA you get long unevenly distributed rays. What are causing these effects, or should I say artifacts? (God forbid in such expensive scopes)

[Freddie]

Another way to look at all of this is just a thought experiment to consider how this could be possible.

Take a photon, pass it down the telescope, it will hit the camera sensor with a certain energy and the sensor will respond to this and show a particular brightness.  Now, change nothing except add a bit of glass for the photon to pass through. Get that same photon and pass it back down the telescope, through the glass and it will hit the cam sensor again. For it to now make the cam show this as being slightly brighter, the photon will have to have more energy. The question would then be, how does passing a photon through some glass give it more energy? If it were possible to pass photons through glass and give them more energy I think we have got something of more importance than producing astro images more quickly!!

[Adam J]

14 minutes ago, Freddie said:

Another way to look at all of this is just a thought experiment to consider how this could be possible.

Take a photon, pass it down the telescope, it will hit the camera sensor with a certain energy and the sensor will respond to this and show a particular brightness.  Now, change nothing except add a bit of glass for the photon to pass through. Get that same photon and pass it back down the telescope, through the glass and it will hit the cam sensor again. For it to now make the cam show this as being slightly brighter, the photon will have to have more energy. The question would then be, how does passing a photon through some glass give it more energy? If it were possible to pass photons through glass and give them more energy I think we have got something of more importance than producing astro images more quickly!!

Its really does not work like that.

[rotatux]

Wow what a passionate and rich debate

I like AdamJ explanations very much: clear, demonstrated, common sense.

I was about adding that as it all boils down to arcsec²/pixel, and so a more pixelized sensor would get less signal / ADU level / SNR (given same OTA, target, and sensor overall size), but AdamJ brilliantly demonstrated the case.

Another thing to note: the argument about point / "non-extended" sources. This is a view of mind, only useful to caracterize the physical behaviour of optics. In reality, so many things will scatter stars light: optics (whether refracting or reflecting) and filters, atmosphere, and even maybe a bit of inter-stellar gas and dust. So for us amateurs stuck on earth (not talking about space telescopes nor interferometry!) star light is indeed scattered over several fractions of arcsecs², and hence several pixels (provided they are small enough). Those pixels will obey the previously explained behaviour. Only for the stars which scattered image fits in a single pixel, is the aperture the only determinant factor (so maybe you should see more faint stars with the TOA at the same exposure time, depending on weather conditions).

[Freddie]

28 minutes ago, Adam J said:

Its really does not work like that.

Why not? Photons go down telescopes and hit camera sensors and the energy is turned into a brightness value.

[Filroden]

20 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’m really confused.

You're comparing images from two different scopes that have different apertures and different focal lengths. There is no myth to expose in this case.

The myth (or fallacy) carries over from the use of camera lenses where changing the f stop changes the exposure needed to achieve equal brightness. This makes sense. The lens has a fixed focal length so you are changing the aperture. Of course this affects exposure.

The myth in astrophotography is to carry over that conclusion - that changing f ratio changes the exposure needed to achieve the same brightness. But we normally image with the same telescope so in this case aperture is fixed and the only way to change f ratio is to change focal length, e.g. use a reducer. In this case, so long as the area of interest falls entirely in both fields of view, you are capturing the same number of photons - because aperture is fixed. You do affect over how much of the sensor those photons are collected, so the shorter focal length could appear brighter as more photons are hitting each sensor. But the total integrated photon count over the same imaged area will be the same (ignoring things like atmospheric effects, etc).

But in your test you changed both aperture (so collected more photons) and focal length (affecting the concentration of those photons on the sensor). No myth to test here.

You’re comparing two effects. The TOA is collecting more photons as it has a larger aperture but also has a much longer focal length so those extra photons are spread over more of the sensor area. The Televue collects fewer photons But is much shorter in focal length so concentrated them in a much smaller area. So it could appear the brighter if the ratio of focal lengths are greater than the ratios of the clearing areas.

Or am I missing something?

[Stub Mandrel]

35 minutes ago, Freddie said:

Why not? Photons go down telescopes and hit camera sensors and the energy is turned into a brightness value.

Quantum physics. The workings of the system rely on the photo giving an atom a big enough 'kick' to transfer one electron into a 'storage well'. Each photon increases the count by one subject to (a) it getting through any filter and (b) the probability of the electron being kicked and captured, called the quantum efficiency.

So brightness is a count of the  number of photos not their energy.

The energy determines their wavelength (and therefore colour), our sensors typically work from IR via visible to UV with an energy range of about 1:3. The wavelength/colour affects what filters they will pass through.

[Rodd]

2 hours ago, gorann said:

What puzzles me is that Gamma Cas looks so different in the two scopes. With the Televue you have something looking like small vane spikes, while with the TOA you get long unevenly distributed rays. What are causing these effects, or should I say artifacts? (God forbid in such expensive scopes)

I wondered the same thing.  I am told that the "lighthouse" beacon is a common TAK artifact.  But I agree--way different look to the star, different size, and teh TAK doesn't really look like a lighthouse  beacon anyway.

Rodd

[ollypenrice]

1 hour ago, Filroden said:

I’m really confused.

You're comparing images from two different scopes that have different apertures and different focal lengths. There is no myth to expose in this case.

The myth (or fallacy) carries over from the use of camera lenses where changing the f stop changes the exposure needed to achieve equal brightness. This makes sense. The lens has a fixed focal length so you are changing the aperture. Of course this affects exposure.

The myth in astrophotography is to carry over that conclusion - that changing f ratio changes the exposure needed to achieve the same brightness. But we normally image with the same telescope so in this case aperture is fixed and the only way to change f ratio is to change focal length, e.g. use a reducer. In this case, so long as the area of interest falls entirely in both fields of view, you are capturing the same number of photons - because aperture is fixed. You do affect over how much of the sensor those photons are collected, so the shorter focal length could appear brighter as more photons are hitting each sensor. But the total integrated photon count over the same imaged area will be the same (ignoring things like atmospheric effects, etc).

But in your test you changed both aperture (so collected more photons) and focal length (affecting the concentration of those photons on the sensor). No myth to test here.

You’re comparing two effects. The TOA is collecting more photons as it has a larger aperture but also has a much longer focal length so those extra photons are spread over more of the sensor area. The Televue collects fewer photons But is much shorter in focal length so concentrated them in a much smaller area. So it could appear the brighter if the ratio of focal lengths are greater than the ratios of the clearing areas.

Or am I missing something?

Exactly my own view so I don't think you're missing anything at all.

What we might add is that if we shoot our 'area of interest' with a focal reducer we get a smaller image with better S/N ratio than we do without the reducer. But see what happens to the image quality when you resample it up to the size of the unreduced image of the same exposure. It will look remarkably like the unreduced image. Or do the experiment the other way round, resampling the unreduced image downwards.

When I want to enhance a widefield by blending in some higher res data from a larger, longer, slower, scope I am always amazed by how little data I need from the bigger scope. Reducing it to a quarter by area of its full sized self, the SN ratio soon proves to be more than adequate in quite a short run but the resolution is still better than that of the widefield, which is the whole point.

Anyone still addicted to the idea that F ratio is the be-all and end-all of exposure time should look at Wim and Gorann's Liverpool Telescope data processing jobs, noting the shortness of the exposures and the slowness of the telescope.

(Like Filroden I would be happier to call the F ratio myth the F ratio fallacy but I am just sticking to the conventional term out of idleness!.)

Olly

[Rodd]

1 hour ago, Filroden said:

I’m really confused.

You're comparing images from two different scopes that have different apertures and different focal lengths. There is no myth to expose in this case.

The myth (or fallacy) carries over from the use of camera lenses where changing the f stop changes the exposure needed to achieve equal brightness. This makes sense. The lens has a fixed focal length so you are changing the aperture. Of course this affects exposure.

The myth in astrophotography is to carry over that conclusion - that changing f ratio changes the exposure needed to achieve the same brightness. But we normally image with the same telescope so in this case aperture is fixed and the only way to change f ratio is to change focal length, e.g. use a reducer. In this case, so long as the area of interest falls entirely in both fields of view, you are capturing the same number of photons - because aperture is fixed. You do affect over how much of the sensor those photons are collected, so the shorter focal length could appear brighter as more photons are hitting each sensor. But the total integrated photon count over the same imaged area will be the same (ignoring things like atmospheric effects, etc).

But in your test you changed both aperture (so collected more photons) and focal length (affecting the concentration of those photons on the sensor). No myth to test here.

You’re comparing two effects. The TOA is collecting more photons as it has a larger aperture but also has a much longer focal length so those extra photons are spread over more of the sensor area. The Televue collects fewer photons But is much shorter in focal length so concentrated them in a much smaller area. So it could appear the brighter if the ratio of focal lengths are greater than the ratios of the clearing areas.

Or am I missing something?

But when you zoom in and make the televue image the same size--its still brighter.  Besides every time you put a reducer on you change the focal length so using 2 scopes with different focal lengths should not be a problem.  1" does not really matter.  We are talikn F7.7 and F4.3--far enough apart to be able to ignore the 1"--especially, as I said in my post, that the extra inch seems to work against the TOA in this example.

Rodd

[Rodd]

25 minutes ago, ollypenrice said:

Exactly my own view so I don't think you're missing anything at all.

What we might add is that if we shoot our 'area of interest' with a focal reducer we get a smaller image with better S/N ratio than we do without the reducer. But see what happens to the image quality when you resample it up to the size of the unreduced image of the same exposure. It will look remarkably like the unreduced image. Or do the experiment the other way round, resampling the unreduced image downwards.

When I want to enhance a widefield by blending in some higher res data from a larger, longer, slower, scope I am always amazed by how little data I need from the bigger scope. Reducing it to a quarter by area of its full sized self, the SN ratio soon proves to be more than adequate in quite a short run but the resolution is still better than that of the widefield, which is the whole point.

Anyone still addicted to the idea that F ratio is the be-all and end-all of exposure time should look at Wim and Gorann's Liverpool Telescope data processing jobs, noting the shortness of the exposures and the slowness of the telescope.

(Like Filroden I would be happier to call the F ratio myth the F ratio fallacy but I am just sticking to the conventional term out of idleness!.)

Olly

Thats my point Olly--I did reasmple it upward and it still looks allot brighter.

Rodd

[Filroden]

28 minutes ago, Rodd said:

But when you zoom in and make the televue image the same size--its still brighter.  Besides every time you put a reducer on you change the focal length so using 2 scopes with different focal lengths should not be a problem.  1" does not really matter.  We are talikn F7.7 and F4.3--far enough apart to be able to ignore the 1"--especially, as I said in my post, that the extra inch seems to work against the TOA in this example.

Rodd

Let's compare the numbers on your two configurations.

	f ration	Aperture diameter	Aperture area	Focal length
Teleview	4.3	101	8,011.85	434.30
TOA	7.7	130	13,273.23	1,001.00
		Ratios	1.66	2.30

The TOA captures 1.66x as many photons as the Teleview (1" really does matter) but the TOA also spreads them over 2.3x as much of the sensor. So the Teleview will "appear" brighter as it's putting its 60% photons into 43% of the sensor (the inverse of the ratios above). If the focal length of the Teleview was about 600mm or longer, then the TOA would have "appeared" brighter.

Forget the f ratio. You're varying aperture AND focal length between images so you need to look at both those factors independently and see which has the bigger effect.

Your TOA captures more light but you Teleview makes what it captures appear brighter by pushing them into a much smaller area on the camera sensor. I don't think resampling will help, as the re-sampling algorithm might try to restore total brightness and therefore be applying a hidden multiplier.

[Rodd]

1 minute ago, Filroden said:

Let's compare the numbers on your two configurations.

	f ration	Aperture diameter	Aperture area	Focal length
Teleview	4.3	101	8,011.85	434.30
TOA	7.7	130	13,273.23	1,001.00
		Ratios	1.66	2.30

The TOA captures 1.66x as many photons as the Teleview (1" really does matter) but the TOA also spreads them over 2.3x as much of the sensor. So the Teleview will "appear" brighter as it's putting its 60% photons into 43% of the sensor (the inverse of the ratios above). If the focal length of the Teleview was about 600mm or longer, then the TOA would have "appeared" brighter.

Forget the f ratio. You're varying aperture AND focal length between images so you need to look at both those factors independently and see which has the bigger effect.

Your TOA captures more light but you Teleview makes what it captures appear brighter by pushing them into a much smaller area on the camera sensor. I don't think resampling will help, as the re-sampling algorithm might try to restore total brightness and therefore be applying a hidden multiplier.

But the one inch did not help the TOA!  If I used a 4 inch TOA (don't exist but imagine), the question remains--and the example would be even more pronounced in the Televue's favor.  What you explain above is true if you just use a reducer (spreading the light).  Olly says upsampling will make them look nearly the same--the extra inch should ENSURE this--but it does not.  

Rodd

[Filroden]

1 minute ago, Rodd said:

But the one inch did not help the TOA!  If I used a 4 inch TOA (don't exist but imagine), the question remains--and the example would be even more pronounced in the Televue's favor.  What you explain above is true if you just use a reducer (spreading the light).  Olly says upsampling will make them look nearly the same--the extra inch should ENSURE this--but it does not.  

Rodd

For the images to "appear" of equal brightness in the same exposure time EITHER the Teleview's focal length would need to be 600mm OR the TOA's aperture would need to be 153mm (or some combination of those two changes that achieve the same effect). But in either case, the TOA is always capturing more photons. You're not just spreading light in your example; you're also collecting different amounts of light too. Two variables that have two different effects on appearance of the image. You cannot just ignore the effect of one to demonstrate the effect of the other.

[vlaiv]

Few points to note:

Down sampling the image to match resolution of system with focal reducers can be done in number of ways:

1. Binning - both average and sum

2. Filtered resampling using one of the algorithms (bilinear, biquadratic, bicubic, and many more ...)

Each of these techniques has different result on "brightness" and on SNR.

I would personally avoid using any filtered resampling and always use SuperSampling technique (not sure if it is implemented in any of current astro image processing software) for any change in image resolution. Software binning is kind of SuperSampling applied with only integer multiplications of original resolution of image. SuperSampling can deal with fractional multiples of resolution, and it can behave the same way as binning in terms that it can both average and sum.

Now lets just consider binning - if we use average method we will gain SNR but we will not gain any "brightness". If we use sum binning - we will gain both "Brightness" and SNR.

Filtered resampling usually behaves as average bin. When we resize "normal" image in any of editing programs - it does not become "brighter".

 

Similar argument can be used for enlarging image - up sampling. Except in this case if we don't want to get blocky / pixelated result we need to use some sort of filtering. Only case where this might not be the case is if we up sample subs prior to stacking and use sub pixel precision for alignment.

So enlarging image by means of filtered resampling in any available software will not "dilute" pixel values - brightness will remain the same (although if we observe laws of physics and want to be correct about it - it should go down).

 

 

[Rodd]

5 minutes ago, Filroden said:

For the images to "appear" of equal brightness in the same exposure time EITHER the Teleview's focal length would need to be 600mm OR the TOA's aperture would need to be 153mm (or some combination of those two changes that achieve the same effect). But in either case, the TOA is always capturing more photons. You're not just spreading light in your example; you're also collecting different amounts of light too. Two variables that have two different effects on appearance of the image. You cannot just ignore the effect of one to demonstrate the effect of the other.

Hdere are the equalized images.  The televue is much brighter.  The scope had Less aperture.  There should be no difference between using 2 different scopes with the same aperture and different focal ratios compared with the same scope with a reducer.  As long as the scopes are of comparable quality and design.  According to the Fable (not sure its real so lets change its name again), these 2 images should look very similar.  The only difference between this comparison and a straight focal reducer on 1 scope experiment is 1 inch of aperture.  It did not seem to help.  I still am not able to equalize the tenants of the Lay (once again lets change its name), and the appearances of these images.  I will at some point do the experiment with 1 scope and a focal reducer--but that is a pain and I will lose imaging time so I decided to try this way.  I stil maintain there is an irregularity somewhere in here based upon what I have been told by those who know more--which is just about everyone--including those above.  The words (above and elsewhere) do not seem to jibe with the images below. 

Televue-upsampled

 

TOA to tefresh memory and reduce scroll time

[Rodd]

6 minutes ago, vlaiv said:

Few points to note:

Down sampling the image to match resolution of system with focal reducers can be done in number of ways:

1. Binning - both average and sum

2. Filtered resampling using one of the algorithms (bilinear, biquadratic, bicubic, and many more ...)

Each of these techniques has different result on "brightness" and on SNR.

I would personally avoid using any filtered resampling and always use SuperSampling technique (not sure if it is implemented in any of current astro image processing software) for any change in image resolution. Software binning is kind of SuperSampling applied with only integer multiplications of original resolution of image. SuperSampling can deal with fractional multiples of resolution, and it can behave the same way as binning in terms that it can both average and sum.

Now lets just consider binning - if we use average method we will gain SNR but we will not gain any "brightness". If we use sum binning - we will gain both "Brightness" and SNR.

Filtered resampling usually behaves as average bin. When we resize "normal" image in any of editing programs - it does not become "brighter".

 

Similar argument can be used for enlarging image - up sampling. Except in this case if we don't want to get blocky / pixelated result we need to use some sort of filtering. Only case where this might not be the case is if we up sample subs prior to stacking and use sub pixel precision for alignment.

So enlarging image by means of filtered resampling in any available software will not "dilute" pixel values - brightness will remain the same (although if we observe laws of physics and want to be correct about it - it should go down).

 

 

I use Pixinsight--not sure what it does.  Look at the images above and see if you can tell

Rodd

[Filroden]

6 minutes ago, Rodd said:

Hdere are the equalized images.

In your opinion, would you say the Teleview looked about 35% brighter?

[vlaiv]

6 minutes ago, Rodd said:

I stil maintain there is an irregularity somewhere in here based upon what I have been told by those who know more--which is just about everyone--including those above.  The words (above and elsewhere) do not seem to jibe with the images below. 

Disregard "brightness" for the moment on these two images - due to methods employed to produce them (processing wise) brightness is not comparable here, but do pay attention to noise levels in both images. Which one seems to have more noise?

To my eyes - Televue one.

[Rodd]

Just now, Filroden said:

In your opinion, would you say the Teleview looked about 35% brighter?

I don't know (I overshot the upsample a bit I think--its close but not exact).  I am just thinking of what Olly said and can't seem to get to that point. 

Rodd

[Rodd]

3 minutes ago, vlaiv said:

Disregard "brightness" for the moment on these two images - due to methods employed to produce them (processing wise) brightness is not comparable here, but do pay attention to noise levels in both images. Which one seems to have more noise?

To my eyes - Televue one.

To answer--the Televue.  But they were not processed at all.  They were given precisely the same alignment, stacking and stretch (pre-set STF).   But what does that mean?

Rodd