Reducer Myth revisited

[Rodd]

5 minutes ago, Laurin Dave said:

n LRGB the 150 combo is streets ahead..

That's interesting.   never considered the difference between NB and BB in this dicussion

Rodd

[vlaiv]

4 minutes ago, Adam J said:

If my SNR is already acceptable though would that not result in reduced detail as you cant get detail back by up samping? 

My current heart project is four frames with the 130PDS and ASI1600mm pro. I find that I need very little noise reduction after re-sampling so am recording true details at the pixel level, hence over four frames I end up with an image that will print to a 60cm x 60cm size on Alu very nicely. 

Would I be better off getting a 70mm F5 and getting it in one frame but four times the integration in that single frame....not sure but if you only have a hammer every problem is a nail.  

Adam

Yes, you are in fact right - I'm somehow always struggling with SNR (bortle 8 skies? ) and although I love max possible detail, sometimes better SNR just wins. If you already have plenty of SNR, then yes, simple resample to get stars tight enough is all that you need.

Another interesting topic is mosaics vs single shot with smaller scope. It is related to this, and if I'm not mistaken here it goes - Same speed scopes used with same camera will result in "same" SNR regardless if you do mosaic or shoot single frame (they will be of the same speed). In practice this is not true because small things that you need to account for - like changing FOV (takes time), and slightly smaller FOV due to overlap needed to piece together mosaic. Read noise also plays a part but not important one - for CCDs it is the same, for CMOS it is higher, but longer subs deal with that.

Here is simple "breakdown" of what happens. Let's observe simple case - same F/ratio, twice the focal length - consequently x4 collecting area. Bin x2 will result in same resolution.

Large scope will collect x4 more light than small scope for same sampling rate (difference in read noise only) if we bin x2. You can only spend 1/4 of the time with large scope per panel in comparison to full FOV of small scope. In the end collected signal is the same and hence SNR (except for things mentioned before).

[vlaiv]

13 minutes ago, Laurin Dave said:

Just read Craig Starks note..  my reading is that the f ratio myth would be a myth if it were not for read noise..  in my recent experience taking Ha at the same time through Esprit 150 SX-46 and Esprit 100 asi1600 the Esprit 100 stacks are better,(which is a trifle annoying..  added together though they are even better)  even though the Esprit 150sx-46 gets 1.5 times more photons per pixel.. read noise though is 9 vs 1.7 on the asi1600..  on LRGB the 150 combo is streets ahead..  

 

I see it the myth in couple of ways, and one is related to read noise.

1. Myth - faster F/ratio scope is faster to get to target SNR over slower scope

2. Myth - two scopes of same F/ratio are "equal in speed" (myth is myth even if we use same camera / pixel size).

While first one incorporates read noise part - it is not in the hart of it. For second myth - read noise is crucial part (and so is dark current noise). Second myth (or part of the myth) can be reformulated and is often used that way - scope of let's say F/5 ratio is x4 time as fast as F/10 ratio (or however those F/stops work, never could remember that).

This part is true only if there are noise sources dependent on aperture and no independent noise sources. Shot noise depends on aperture - because it is related to target signal, and LP noise depends on aperture - for the same reason, it is related to sky signal. Read noise depends on number of readouts and dark current noise on duration of exposure - not related to aperture / f/ratio and remain the same.

Once you are in low light regime - both read noise and dark current noise become important contributors and can't be neglected anymore - they change above statement to: F/5 scope will be at most x4 faster than F/10 scope, but in reality it will depend on how bright target is and can be only faster by small percentage.

[ollypenrice]

1 hour ago, Adam J said:

What you want it the lowest F-ratio possible for imaging at your chosen focal length / image scale. That also translates to the largest aperture possible for you chose focal length.  

Adam 

This has always been my position on F ratio as well. A good while ago I posted this, which I think makes precisely the same point:

With it I posted this:

This ignores the effect of object flux per pixel and really needs updating to include it. However, I stand by the optical point it makes and, in practice, I still think that resampling the left hand object image (M33) down to the size of the M33 in the reduced image will produce results only trivially different from each other.

Adam also said, 'There is no f-ratio myth,  just people who don't understand optics.' I would argue (with a smile!) that the myth arises from failing to understand optics. (In one of my other areas of interest I'd also upbraid myself for the use of 'myth' when the right term would be 'fallacy' but the term 'F ratio myth' was out there so I used it.) 

I got involved in this debate because we very often see reducers advocated as providing benefits they do not, in fact, produce. As Adam says in the quotation with which I began, we must only compare different apertures at the same focal length. The fact that F stop and aperture are used as synonyms in the camera world is made possible because there is no change in FL. I think there is a myth and this is where it started.

Olly

[Rodd]

4 minutes ago, ollypenrice said:

I still think that resampling the left hand object image (M33) down to the size of the M33 in the reduced image will produce results only trivially different from each other.

But the question is the reverse--will upsampling the right hand image to be equal with the larger image show differences.  that is the direction of things (use a reducer, image is smaller, resample Up to reclaim the size so its equal to larger image).   

Rodd

[Rodd]

9 minutes ago, ollypenrice said:

What you want it the lowest F-ratio possible for imaging at your chosen focal length / image scale. That also translates to the largest aperture possible for you chose focal length.  

Adam 

But this makes no practical sense.  A person has a scope--the aperture is fixed. The focal length will then be dependent on the F-ratio (add a reducer and F-ratio goes up--FL goes down).  Image scale changes proportionally as well.  Most of us do not buy a new scope to shoot a new target.  We have a scope and can either use a reducer or not.  So with one scope there is only 1 F ratio for a given FL.  One can't remain constant as the other changes

[Rodd]

 

This is what I was talking about.  By no advantage I read "you can't put on a reducer image for 1/2 the time (or some fraction of the native resolution time) crop and enlarge and save time imaging".

Rodd

[Adam J]

11 minutes ago, Rodd said:

But this makes no practical sense.  A person has a scope--the aperture is fixed. The focal length will then be dependent on the F-ratio (add a reducer and F-ratio goes up--FL goes down).  Image scale changes proportionally as well.  Most of us do not buy a new scope to shoot a new target.  We have a scope and can either use a reducer or not.  So with one scope there is only 1 F ratio for a given FL.  One can't remain constant as the other changes

You chose your scope according to what you want to image though and according to the camera you intend to purchase. You dont just buy a scope based on its F-ratio like say a Samsung 135 F2 slap a Atik 4000 on it and expect it to image the crescent nebula, clearly a 130mm F7.7 would do a better job. At the same time if i want to image the Heart nebula I would not choose a 130mm F7.7 and a Atik 414EX.

You dont buy a scope to shoot every new target but you buy it having some idea of what size targets you want image and the appropriate image scale. 

Adam

Edited October 18, 2019 by Adam J

[ollypenrice]

19 minutes ago, Rodd said:

But the question is the reverse--will upsampling the right hand image to be equal with the larger image show differences.  that is the direction of things (use a reducer, image is smaller, resample Up to reclaim the size so its equal to larger image).   

Rodd

I don't think this is difficult. A key term, here, will be 'empty resolution.' If you upsize an image you make it bigger without increasing its resolution. A close double not quite split will remain not quite split however large the image is presented because the data does not contain the split.) The increase in size produces 'empty resolution.'  So upsizing an image will always produce empty resolution.

Imaging the same object at a longer focal length may or may not produce empty resolution. If the seeing and guiding don't support the image scale then the larger image will contain empty resolution. But if the seeing and guiding do support the longer focal length then the resolution will be genuine and will never be matched by the upsampled image.

However, the S/N on the object of interest will be better in the smaller image. If upsampled how would it compare with the normally sized image at its normal size. I can't say I greatly care because it's hard enough getting an image to hold up at full size anyway, though many of my TEC140 images are presented close to that. I would never post an upsampled image.

Olly

 

Edited October 18, 2019 by ollypenrice
Signed twice which always looks really stupid!!!

[Rodd]

1 minute ago, ollypenrice said:

I don't think this is difficult. A key term, here, will be 'empty resolution.' If you upsize an image you make it bigger without increasing its resolution. A close double not quite split will remain not quite split however large the image is presented because the data does not contain the split.) The increase in size produces 'empty resolution.'  So upsizing an image will always produce empty resolution.

Imaging the same object at a longer focal length may or may not produce empty resolution. If the seeing and guiding don't support the image scale then the larger image will contain empty resolution. But if the seeing and guiding do support the longer focal length then the resolution will be genuine and will never be matched by the upsampled image.

However, the S/N on the object of interest will be better in the smaller image. If upsampled how would it compare with the normally sized image at its normal size. I can't say I greatly care because it's hard enough getting an image to hold up at full size anyway, though many of my TEC140 images are presented close to that. I would never post an upsampled image.

Olly

Olly

So this basically is in agreement with my original post--that one can not throw on a reducer, image a galaxy, then crop it out and upsample and save time imaging.  Thank you

Rodd

[Rodd]

5 minutes ago, Adam J said:

You chose your scope according to what you want to image though and according to the camera you intend to purchase. You dont just buy a scope based on its F-ratio like say a Samsung 135 F2 slap a Atik 4000 on it and expect it to image the crescent nebula, clearly a 130mm F7.7 would do a better job. At the same time if i want to image the Heart nebula I would not choose a 130mm F7.7 and a Atik 414EX.

You dont buy a scope to shoot every new target but you buy it having some idea of what size targets you want image and the appropriate image scale. 

Adam

But you said What you want it the lowest F-ratio possible for imaging at your chosen focal length.  What I am suggesting is for each scope, the FL and FR change together--only aperture is fixed (unless you are imaging the sun and use a mask).  Most people buy a scope to image a wide range of targets (unless you have a lot of money and can buy many different scopes).   Most people but a scope and then decide to use a reducer or not--in this case the FL and FR both change together. 

[ollypenrice]

7 minutes ago, Rodd said:

 

This is what I was talking about.  By no advantage I read "you can't put on a reducer image for 1/2 the time (or some fraction of the native resolution time) crop and enlarge and save time imaging".

Rodd

You can use the reducer and crop, yes. Use it, crop and enlarge? No. Certainly not in my view.

One might use a focal extender to improve resolution if aperture isn't the limiting factor in resolution, as it might not be in a fast, short FL scope. Or you can use a reducer to get a wider FOV. (You'll get a satisfactory S/N ratio faster on that wider FOV image as well but I argue that this is only worthwhile if you actually want all that FOV - as often you might.)

I find that there isn't much I can't do with two scopes and two cameras, short of camera lens style widefield. My FLs are 530 and 1015mm.

At 530mm and 3.5"PP I can do widefield.

At 1015mm I can do high res imaging at 0.9"PP in one camera or semi-widefield, decent resolution imaging at 1.8"PP.

We discussed the implications of 3.5"PP in a recent thread, Rodd. This is a crop from my Witch Head shown at full size. I can live with this sampling rate. Obviously I'd like four times as many pixels but I've got what I've got!

Olly

 

[Adam J]

28 minutes ago, Rodd said:

But you said What you want it the lowest F-ratio possible for imaging at your chosen focal length.  What I am suggesting is for each scope, the FL and FR change together--only aperture is fixed (unless you are imaging the sun and use a mask).  Most people buy a scope to image a wide range of targets (unless you have a lot of money and can buy many different scopes).   Most people but a scope and then decide to use a reducer or not--in this case the FL and FR both change together. 

You kinda looking at it the wrong way around. 

Lets say I have a 102mm F7 and I want to image medium galaxies ~M101. 

I can do one of two things. 

I can image at native focal length with an ASI 1600mm pro and get around 1.1" / pix.

Or I can use a reducer and get F5.6 and 1.4" / pix

The objects fits in the FOV in both cases. 

I would most likely chose to reduce the scope with a 0.8x reducer and go with 1.42 / pix. I would do that because I dont believe that with my skys and mount I can acheive 1.1" / pix. 

In this situation I would use the reducer because I wont loose real world detail in my image but I will gain some SNR. 

But here is the thing. I can most likely software re-sample the 1.1"/ pix image down to 1.4"/pix or even less in software and get a similar SNR improvement. Hence the reducer is not really adding much in this case. Its marginally better than software re-sampling. 

 

On the other hand. 

If with my camera sensor size I needed a 550mm focal length to fit my target into the FOV and I had a  80mm F7.5 then I would need to use my 0.85x reducer to fit the object into the frame. Hence software re-sampling is not going to help me and I must use the reducer. 

My other options would be to use a 100mm F5.5 scope. If I use the 100mm F5.5 scope then I will also fit my target in, but it will be at a faster f-ratio than the reduced 80mm F7.5 and so I will image in a shorter time. 

Or I could use a 70mm scope at F8. Thats probably not the best option but all of these give allow me to fit my target perfectly into my FOV. 

 

Lots of this come down to affordability. Its more affordable to own a 80mm F7.5 and reduce it than it is to own a 100mm F5.5 and operate it at native focal length.  By a factor of about 3x the cost.  

 

That help?

Adam 

 

Edited October 18, 2019 by Adam J

[Rodd]

3 minutes ago, ollypenrice said:

You'll get a satisfactory S/N ratio faster on that wider FOV image as well but I argue that this is only worthwhile if you actually want all that FOV

This is the crux--if you do not want that FOV--there is no benefit of using the reducer to image the galaxy. 

BTW--when you say full size, is that "full resolution" on Astrobin--or SGL?  Or, 1:1 when I am processing in PI?  As an aside, My lowest resolution with the ASI 1600 is 2.46 arcsec/pix .  Since I cant use the STT 8300 I have at F3 (back focus issues)-the closest resolution to 2.46 I can get with that camera is 2.58.  The interesting thing is the 2.46 has a much wider FOV than the 2.58. 

You also have to consider the sky quality and processor ability.  I am quite confident that if I take an image at 3.5 and view it at 1:1--it will look far worse than yours for these 2 reasons. 

The thing I hate about all this is after all this, I am more frustrated than ever!

Rodd

[Rodd]

2 minutes ago, Adam J said:

hat help?

I get it....

[Adam J]

5 minutes ago, Rodd said:

This is the crux--if you do not want that FOV--there is no benefit of using the reducer to image the galaxy. 

BTW--when you say full size, is that "full resolution" on Astrobin--or SGL?  Or, 1:1 when I am processing in PI?  As an aside, My lowest resolution with the ASI 1600 is 2.46 arcsec/pix .  Since I cant use the STT 8300 I have at F3 (back focus issues)-the closest resolution to 2.46 I can get with that camera is 2.58.  The interesting thing is the 2.46 has a much wider FOV than the 2.58. 

You also have to consider the sky quality and processor ability.  I am quite confident that if I take an image at 3.5 and view it at 1:1--it will look far worse than yours for these 2 reasons. 

The thing I hate about all this is after all this, I am more frustrated than ever!

Rodd

There is a slight benefit over software re-sampling but only if your under sampled at the native focal length. In that case a focal reducer is marginally better in terms of increasing SNR than software re-sampling, but it is probably too marginal for you to see the difference. 

[vlaiv]

1 hour ago, ollypenrice said:

This has always been my position on F ratio as well. A good while ago I posted this, which I think makes precisely the same point:

With it I posted this:

This ignores the effect of object flux per pixel and really needs updating to include it. However, I stand by the optical point it makes and, in practice, I still think that resampling the left hand object image (M33) down to the size of the M33 in the reduced image will produce results only trivially different from each other.

Adam also said, 'There is no f-ratio myth,  just people who don't understand optics.' I would argue (with a smile!) that the myth arises from failing to understand optics. (In one of my other areas of interest I'd also upbraid myself for the use of 'myth' when the right term would be 'fallacy' but the term 'F ratio myth' was out there so I used it.) 

I got involved in this debate because we very often see reducers advocated as providing benefits they do not, in fact, produce. As Adam says in the quotation with which I began, we must only compare different apertures at the same focal length. The fact that F stop and aperture are used as synonyms in the camera world is made possible because there is no change in FL. I think there is a myth and this is where it started.

Olly

Ah, yes, now I remember.

You are in fact right saying that SNR per unit sky area can't be increased with focal reducer. No question about it - it is equivalent to my statement above that regardless of how much FOV is occupied by object - same aperture will gather same number of photons for that target.

However, I do think that you should be careful when saying:

1 hour ago, ollypenrice said:

I got involved in this debate because we very often see reducers advocated as providing benefits they do not, in fact, produce.

as in your own words:

1 hour ago, ollypenrice said:

This ignores the effect of object flux per pixel and really needs updating to include it.

For most people that don't contemplate what happens to such depths as we do in this and similar discussions, focal reducers provide real benefit.

They won't be resampling / binning their data, and if data is not altered in such way post acquisition - it will indeed have higher SNR with focal reducer. Target SNR will be reached in shorter time and image will look less noisy / smoother when observed at 1:1 magnification. Target will of course be smaller due to coarser sampling rate.

Ultimately, as seen from this example and earlier discussions, coming from you as well known and accomplished imager - such words have weight and people might get confused because they don't read "the fine print".

Edited October 18, 2019 by vlaiv

[vlaiv]

52 minutes ago, Rodd said:

So this basically is in agreement with my original post--that one can not throw on a reducer, image a galaxy, then crop it out and upsample and save time imaging.  Thank you

Rodd

In fact you can

There is a case where such action will result in virtually same image quality, and there is a case when such action will result in very small change in image sharpness.

If you oversample when imaging at native focal length to such extent that using focal reducer provides proper sampling or oversampling (but not undersampling) for given conditions, you will get the same image in less time.

As explained above - SNR per unit sky area does not change with use of focal reducer. What changes is mapping of that sky to pixels, and per pixel SNR does in fact change. Image recorded with focal reducer will have better SNR for same time, or will reach same SNR in less time than your oversampled image at native focal length.

I've already shown in this thread that upsampling of the image does not reduce SNR. So image recorded with focal reducer will keep SNR when enlarged.

Only thing that can happen in this process is that you potentially loose some of the information, and only in the case that image with focal reducer is undersampled. If that is not the case - you will get the same image.

 

[Rodd]

4 minutes ago, vlaiv said:

For most people that don't contemplate what happens to such depths as we do in this and similar discussions, focal reducers provide real benefit.

They won't be resampling / binning their data, and if data is not altered in such way post acquisition - it will indeed have higher SNR with focal reducer. Target SNR will be reached in shorter time and image will look less noisy / smoother when observed at 1:1 magnification. Target will of course be smaller due to coarser sampling rate.

This is contrary to my original post.  My contention was that using a reducer to image a galaxy and crop the galaxy---i.e throw away all that extra FOV--there would be no benefit.  My statement assumed that people would manipulate the data (by cropping or resampling--what ever).  I agree, that if you want teh FOV reducers are the bomb.  But if you dont--there is no benefit.

[Rodd]

2 minutes ago, vlaiv said:

In fact you can

There is a case where such action will result in virtually same image quality, and there is a case when such action will result in very small change in image sharpness.

If you oversample when imaging at native focal length to such extent that using focal reducer provides proper sampling or oversampling (but not undersampling) for given conditions, you will get the same image in less time.

As explained above - SNR per unit sky area does not change with use of focal reducer. What changes is mapping of that sky to pixels, and per pixel SNR does in fact change. Image recorded with focal reducer will have better SNR for same time, or will reach same SNR in less time than your oversampled image at native focal length.

I've already shown in this thread that upsampling of the image does not reduce SNR. So image recorded with focal reducer will keep SNR when enlarged.

Only thing that can happen in this process is that you potentially loose some of the information, and only in the case that image with focal reducer is undersampled. If that is not the case - you will get the same image.

 

That is not what I read by Olly's statement "no added benefit"!

[Rodd]

I am out----and around and around we go.   Even with Olly's statement--we seem to be right back where we started.  So I am going to bow out and get off this ride.  Circular rides ruin my day (literally)

Rodd

[vlaiv]

Just now, Rodd said:

That is not what I read by Olly's statement "no added benefit"!

Yes, that is because Olly omitted effects of pixel scale in his argument, or rather per pixel SNR.

[Rodd]

11 minutes ago, vlaiv said:

Yes, that is because Olly omitted effects of pixel scale in his argument, or rather per pixel SNR.

As olly stated--I do not think those effect are enough to negate his claim

" I stand by the optical point it makes and, in practice, I still think that resampling the left hand object image (M33) down to the size of the M33 in the reduced image will produce results only trivially different from each other."

Edited October 18, 2019 by Rodd

[vlaiv]

8 minutes ago, Rodd said:

As olly stated--I do not think those effect are enough to negate his claim

" I stand by the optical point it makes and, in practice, I still think that resampling the left hand object image (M33) down to the size of the M33 in the reduced image will produce results only trivially different from each other."

In principle I agree with that statement, however it is missing important piece of information to make it complete.

What type of resampling are we talking about? There are different resampling algorithms with different properties.

For example - nearest neighbor resampling will make 0 improvement in SNR - it will leave SNR exactly the same. Binning as a form of resampling has very predictable improvement in SNR. Other forms of resampling will have SNR improvement somewhere in between (or even more due to pixel to pixel correlation).

Like I mentioned before, difference between binning and using equivalent focal reducer is in read noise contribution and covered FOV. Other forms of resampling (those that are worth using) will have slightly smaller SNR benefit compared to binning / equivalent reducer.

On the other hand upsampling behaves differently.

For most people that don't resample and have no clue about binning and such - focal reducer will provide real benefit in SNR, especially if they are oversampling at native focal length.

[Rodd]

20 minutes ago, vlaiv said:

or most people that don't resample and have no clue about binning and such - focal reducer will provide real benefit in SNR, especially if they are oversampling at native focal length.

Here is they key component. that you are missing......only if they want the extra FOV.  My original scenario was if  they do not.