Reducer Myth revisited

[Rodd]

It has been said that it is a misconception that if one uses a reducer to lower the focal ratio of a telescope system then crop out a portion of the image and compare it with that same portion collected without the reducer, there will be a benefit as far as increased signal to the cropped area.   Reducers increase signal for a FOV as a whole, not for portions of images.  Well, I decided to run an experiment.  Here are two crops.  One is from a focal ratio of F7.7 and one is from a focal ratio of F3.   Both were cropped and not altered or resampled--these are the original resolutions--just cropped.  It seems to me that the F3 image is brighetr AND resolves more detail.  This is surprising bevuase the F3 was taken with 4" scope and the F7.7 with a 5: scope.  Help me make sense of this.  Oh, yeah--both images have the similar integration times 69 300 sec ssubs vs 60 300 sec subs.  I don't think teh 10 sub difference is to blame.

F7.7

 

F3

[celestron8g8]

To get a better comparison you need to take an image with the same scope  .  When you change scopes  your only changing the FL in the scope with the Focal Reducer attached  .  When I compare your images tho the f3 is slightly brighter than the f7 but since the f3 is smaller it's hard to compare detail properly . The only reason for a FR is to increase the speed of the exposure time and make the image brighter . Trade off is a smaller image due to a wider FOV but FRs' also can produce coma at the edge of FOV of the image . 

[Rodd]

11 minutes ago, celestron8g8 said:

To get a better comparison you need to take an image with the same scope  .  When you change scopes  your only changing the FL in the scope with the Focal Reducer attached  .  When I compare your images tho the f3 is slightly brighter than the f7 but since the f3 is smaller it's hard to compare detail properly . The only reason for a FR is to increase the speed of the exposure time and make the image brighter . Trade off is a smaller image due to a wider FOV but FRs' also can produce coma at the edge of FOV of the image . 

You can resample to make the same size.  I didn't do that because I do not have access to my software--just the ability to crop in windows.  In this case, the larger aperture was the one that appears dimmer--so it works in favor of the statement as opposed to against it (if the larger aperture was the F3 than you would have a point.  But the larger aperture did not seem to be enough to over come the longer FL.    And what the tenant of the myth states, is if you use a focal reducer and do not change aperture, it will not change the exposure time necessary to render a portion of an image (in this case the horse head).  It will reduce the time to get a desired SNR across the entire FOV--but cropping out targets, like galaxies or the horse head, thinking you can capture them in reduced time, will not work.  In that case you will not benefit form the use of a focal reducer with respect to needed exposure time.   This demostration seems to refute that.   Its not a perfect one as conditions on the different nights could be important.  But my point is an extra inch of aperture is not enough to overcome the deleterious effects of focal length.  if the two scope were the same aperture, the demonstration would be even more obvious, not less.

Rodd

[celestron8g8]

So basically your just going by a theory and not actual facts  . Sorry my bad for not understanding so i'll just bow out of the conversation . But I would like to mention a Focal Reducer is actually for this purpose and nothing else  : 

"QUOTE"   Focal reducers are primarily used in astrophotography to compress the light cone exiting the telescope down to a size closer to that of the sensor within your camera. This also results in a brighter image which reduces the exposure time. Focal reducers also reduce field curvature, providing the flat field necessary for astrophotography. "QUOTE" 

https://www.skiesunlimited.com/astro-imaging-accessories/focal-reducers/

[Rodd]

10 minutes ago, celestron8g8 said:

So basically your just going by a theory and not actual facts  . Sorry my bad for not understanding so i'll just bow out of the conversation . But I would like to mention a Focal Reducer is actually for this purpose and nothing else  : 

"QUOTE"   Focal reducers are primarily used in astrophotography to compress the light cone exiting the telescope down to a size closer to that of the sensor within your camera. This also results in a brighter image which reduces the exposure time. Focal reducers also reduce field curvature, providing the flat field necessary for astrophotography. "QUOTE" 

https://www.skiesunlimited.com/astro-imaging-accessories/focal-reducers/

That's the myth--they do not reduce exposure time for individual targets in the FOV--like galaxies.  You must apply what I am saying correctly--read above--I am NOT talking about the whole FOV.

[Alien 13]

I am a complete novice in the theory of these things but for me it would make no difference to the number of photons and hence its brightness from a single star going down a 4 inch tube whether it had a focal length of a 10 foot or 6 inches, I expect extended objects might benefit from shorter focal lengths though with or without a reducer.

Alan

Edited October 17, 2019 by Alien 13

[Rodd]

Just now, Alien 13 said:

I am a complete novice in the theory of these things but for me it would make no difference to the number of photons from a single star going down a 4 inch tube whether it had a focal length of a 10 foot or 6 inches, I expect extended objects might benefit from shorter focal lengths though with or without a reducer.

Alan

Absolutely correct.   Using a reducer will not decrease the time it takes to collect data on a small galaxy in the FOV.  That's why it makes no sense to use a reducer to collect the data then crop out the galaxy and enlarge it--thinking you will take a shorter amount of time to collect the data then you would using a longer focal length  

The idea of changing focal ratios makes a lot of sense with camera lenses where one can simultaneously change the aperture.  But the aperture of a telescope is fixed.  The only real way of increasing the number of photons, hence increasing speed, is by increasing aperture.  Reducers are great for increasing your FOV so you can better frame a large target--but whatever speed you pick up imaging, you loose in resolution.  So you really are not even taking the same image of that galaxy--it will be a lower quality image even if you take all the time in the world to capture the data. 

[Laurin Dave]

Hi Rodd.  I think about it in terms of arc sec squared of sky per pixel, the more sky area per pixel for a given scope diameter the more photons arrive per pixel and hence SNR is higher..  ie using a reducer puts more photons  on each pixel with the obvious result ..  I estimate your 5” F7.7 has a pixel scale of .8 arcsec/pp and the reduced FSQ106 2.5 arcsec/pp ie almost 10x the sky area per pixel ..  light gathering capacity is 1.6 times in favour of the 5”..  so the reduced FSQ gets 6 times as many photons per pixel per unit time ..  (assuming same pixel size) although the resolution decreases..  could be completely wrong  on this 😬..  

 

Dave

[Rodd]

3 minutes ago, Laurin Dave said:

Hi Rodd.  I think about it in terms of arc sec squared of sky per pixel, the more sky area per pixel for a given scope diameter the more photons arrive per pixel and hence SNR is higher..  ie using a reducer puts more photons  on each pixel with the obvious result ..  I estimate your 5” F7.7 has a pixel scale of .8 arcsec/pp and the reduced FSQ106 2.5 arcsec/pp ie almost 10x the sky area per pixel ..  light gathering capacity is 1.6 times in favour of the 5”..  so the reduced FSQ gets 6 times as many photons per pixel per unit time ..  (assuming same pixel size) although the resolution decreases..  could be completely wrong  on this 😬..  

 

Dave

Thats right--but what I am saying is it will no longer be true if you crop out the hoursehead from the F3 image and equalize it to the horsehead from the other image.  One can't image a galaxy faster by throwing on a reducer, then enlarging the image and say--look at this galaxy image.  First off--its not teh same image as you intoned, the resolution will be less--so its not the same image. 

Consider this scenario.  You are imaging M82 with a 6" F/8 refractor.  You say "this is taking too long" so you put on a reducer, reduce your sub lengths accordingly, and get an image in 1/2 the amount bof time.  Then you crop out the galaxy and enlarge it and compare with the image taken with the same scope at F/8.  It will not be as good--even if you used the same amount of data.  that is the myth of reducers of which I speak.  Now--the full FOV will come to a particular SNR faster than teh full FOV of the F/8 configuration--but its not the same image at all--the image is different

[Laurin Dave]

Agree Rodd ..  shows the importance of getting the right pixel size for your scope.. or of binning .  If you'd binned the camera on the 5" I expect the result would be much closer..

Dave

[Rodd]

5 minutes ago, Laurin Dave said:

Agree Rodd ..  shows the importance of getting the right pixel size for your scope.. or of binning .  If you'd binned the camera on the 5" I expect the result would be much closer..

Dave

Well--its CMOS so no hardware binning.  I know software binning is supposed to be nearly the same--but I must not know how to do it--when I do an integer resample it just looks like I zoom out--which of course I do not want to lose the scale, so then I upsample to original size at end--and it looks the same.  Maybe its me.  All I know is I am unsatisfied with the ASI 1600 and the TOA 130 native......just can't quite get there.  I am wondering about throwing teh reducer on--would that help?  I would lose the scale--so I might as well go back to the FSQ at F3.  i never seem to be satisfied.  I think the TOA 130 is better suited for the STT-8300.  The next time I use it will be with that camera.  But I dont know.  All that research to make sure I got decent stuff was all a waste it seems. 

Edited October 17, 2019 by Rodd

[celestron8g8]

1 hour ago, Rodd said:

That's the myth--they do not reduce exposure time for individual targets in the FOV--like galaxies.  You must apply what I am saying correctly--read above--I am NOT talking about the whole FOV.

I would like to see you argue with manufactures of Focal Reducers and convince them otherwise cause all brands of FRs’ claim exactly what i copied/pasted above then gave a link to the quote . So just to let you know i wont argue with you but i usually go by manufactures description of their product . I know what your trying to say relating to resolution but resolution will be determined by the camera you use and the size of it’s sensor . I understand the more pixels the bigger the resolution where you cannot increase the image size without degrading it as where you can reduce an image without degrading it . What I don’t understand is why your so concerned with that theory over a FR ?? Especially if your not using one  .  What you should have done is reduce the larger image to the same size as the smaller image then do your comparison . 

Edited October 17, 2019 by celestron8g8

[Kev M]

All a focal reducer does if effectively change the focal length ( so we are into the focal ratio myth....do we want to go there ? ).

All else being equal aperture wins....size does matter !

However all else is not equal.......

There is a limit to aperture size, where atmospherics rule out any linear increase in benefit above a certain size.

A shorter focal ratio ( for the same aperture ) will effectively place a larger area of sky onto a pixel so it will fill up quicker, this will be at the expense of resolution.

This balances out exactly ( with ideal sensors ) so there is no benefit to be gained, other than the ability to get a wider field of view....this may be important !

Sensors however are not ideal....and there may be sweet spots where the maximum photons can be gathered at a required resolution for any sensor at a particular focal length/ratio/aperture combination.

It is not as simple as it would first appear.....

 

[Rodd]

54 minutes ago, celestron8g8 said:

I would like to see you argue with manufactures of Focal Reducers and convince them otherwise cause all brands of FRs’ claim exactly what i copied/pasted above then gave a link to the quote . So just to let you know i wont argue with you but i usually go by manufactures description of their product . I know what your trying to say relating to resolution but resolution will be determined by the camera you use and the size of it’s sensor . I understand the more pixels the bigger the resolution where you cannot increase the image size without degrading it as where you can reduce an image without degrading it . What I don’t understand is why your so concerned with that theory over a FR ?? Especially if your not using one  .  What you should have done is reduce the larger image to the same size as the smaller image then do your comparison . 

They perpetuate the myth because they want to sell you stuff

[Rodd]

27 minutes ago, Kev M said:

All a focal reducer does if effectively change the focal length ( so we are into the focal ratio myth....do we want to go there ? ).

All else being equal aperture wins....size does matter !

However all else is not equal.......

There is a limit to aperture size, where atmospherics rule out any linear increase in benefit above a certain size.

A shorter focal ratio ( for the same aperture ) will effectively place a larger area of sky onto a pixel so it will fill up quicker, this will be at the expense of resolution.

This balances out exactly ( with ideal sensors ) so there is no benefit to be gained, other than the ability to get a wider field of view....this may be important !

Sensors however are not ideal....and there may be sweet spots where the maximum photons can be gathered at a required resolution for any sensor at a particular focal length/ratio/aperture combination.

It is not as simple as it would first appear.....

 

Yeah.....what he said!!!!

[coatesg]

1 hour ago, Rodd said:

Well--its CMOS so no hardware binning.  I know software binning is supposed to be nearly the same--but I must not know how to do it--when I do an integer resample it just looks like I zoom out--which of course I do not want to lose the scale, so then I upsample.

Well, not really the same. CCD binning occurs on chip pre read out so for a 2x2 bin, there's one lot of read noise but 4x the signal (ish) - effectively the read noise is 1/4 as much (ish).

CMOS binning is post read out, so you've got 4x signal, but only half as much read noise (sqrt(4) - errors add in quadrature). It seems a little pointless to upsample again - all you're doing is losing resolution...

It is worth noting that the binning only really helps when your error terms are dominated by read noise (eg short exposure, or exposures from very dark sites/narrowband). If thermal noise or sky background dominate, then it makes not a lot of difference at all...

Aperture is the important term all round - it dictates how many photons arrive from a given source. Comparing the brightness of images is not sufficient either - you need to measure the signal to noise ratio in the images. To me, the comparison images look like the second is noisier, even if it is brighter...

[vlaiv]

I'm not sure why is this causing so much headache to people - it is in fact simple.

Using focal reducer will in fact increase the speed needed to reach target SNR regardless of the fact that:

1. whole target fits both unreduced and reduced FOV

2. same aperture is used and number of total captured photons for the target remains the same.

When we talk about SNR in terms of imaging it is rather simple - one is interested in number of photons per pixel - that is what counts towards the SNR. It is signal level per each pixel that goes into that equation. Using focal reducer means that (from above two points) - same amount of light from a target is divided by smaller number of pixels (coarser sampling rate). Each pixel there fore receives more signal and that means that it has better SNR (for same total imaging time - or system is faster to reach target SNR).

Although common belief is that this holds only for extended targets it is actually true for stars as well to some extent. Star profile is about the same expressed in arcseconds (FWHM or other measure) and when we decrease sampling rate - again star profile is spread over fewer pixels. Stars are almost never single pixel unless image is hugely undersampled - so they are in fact spread over number of pixels. Relation between resolution and spread is not as straight forward as with extended targets, but in principle that is what happens - increase sampling rate - you decrease SNR. Decrease sampling rate and you increase SNR.

Using focal reducer is not part of F/ratio myth. It works and speeds up things. F/ratio myth is about something else - it is about saying that fast scope will always be faster than slow scope. That is not true because it does not take into account pixel size (again how much light is reaching individual pixels). Slow scope with large pixels can be faster than fast scope with smaller pixels. That is F/ratio myth. When using same camera / same pixels - reducer does raise the speed of reaching target SNR.

 

Edited October 17, 2019 by vlaiv
slow, flow, all the same :D

[vlaiv]

1 minute ago, coatesg said:

It is worth noting that the binning only really helps when your error terms are dominated by read noise (eg short exposure, or exposures from very dark sites/narrowband). If thermal noise or sky background dominate, then it makes not a lot of difference at all...

Not true.

Given certain SNR unbinned, regardless of dominant noise source, bin x2 will provide SNR increase by factor of 2. It acts on total noise, so dark noise, shot noise, LP noise and read noise combined.

[coatesg]

15 minutes ago, vlaiv said:

Not true.

Given certain SNR unbinned, regardless of dominant noise source, bin x2 will provide SNR increase by factor of 2. It acts on total noise, so dark noise, shot noise, LP noise and read noise combined.

Sorry - yes, I hadn't put it well - I meant CCD binning only works in comparison to CMOS when read limited. 

If you aren't read dominated then the SNR doesn't increase by factor 4 (but yes, the other noise terms add in quadrature). It is important to state which realm you're working under - if you're dominated by read noise, the R squared term in the ccd equation dominates (and leads to the 4x reduction for ccds, but 2x for CMOS).

You have to cope with the corresponding loss of resolution though.

 

[vlaiv]

4 minutes ago, coatesg said:

Sorry - yes, I hadn't put it well - I meant CCD binning only works in comparison to CMOS when read limited. 

If you aren't read dominated then the SNR doesn't increase by factor 4 (but yes, the other noise terms add in quadrature). It is important to state which realm you're working under - if you're dominated by read noise, the R squared term in the ccd equation dominates (and leads to the 4x reduction for ccds, but 2x for CMOS).

You have to cope with the corresponding loss of resolution though.

 

I think it is better to look at it this way:

CCD camera with 5e read noise and certain pixel size - let it be for example 4um, with hardware binning - produces effectively camera that has 5e read noise and 8um pixel size.

CMOS camera with 2e read noise and certain pixel size - let it be the same as above 4um, with software binning - produces effectively camera that has 4e read noise and 8um pixel size.

Regardless of the resulting read noise - one can deal with it by increasing single exposure duration (less subs for same total imaging time) - as it again adds like you said when stacking subs, and by increasing sub length you can make it be much less than other noise sources (a bit harder for narrow band but doable).

[don4l]

I remember when you brought up this subject last year, Rodd.  I've been looking for a new camera, and this question seemed very pertinent.

I am not a mathemetician, so the following might be nonsense.

My new camera has a 16200 CCD, and I have an FSQ106 which, if I remember correctly,  is the same as the scope that you used on your Heart image.

To my simple mind, there will be no difference in exposure time when you are imaging at F3 compared to my setup imaging at F5.  We will both have the same FOV (roughly).  We will have a similar resolution.  The pixels in my camera are bigger, but at F5 they will be collecting the same number of photons from the same area of the sky.

So, any differences in exposure time will be down to QE and noise.

[Thalestris24]

Um, dare I ask about the converse i.e. adding a Barlow vs imaging small targets with an equivalent longer focal length scope? I have a feeling that using even a high quality Barlow for imaging is never a good idea...

Louise

ps Think I've raised this before but I've forgotten the answer, d'uh

Edited October 18, 2019 by Thalestris24

[Rodd]

1 hour ago, don4l said:

I remember when you brought up this subject last year, Rodd.  I've been looking for a new camera, and this question seemed very pertinent.

I am not a mathemetician, so the following might be nonsense.

My new camera has a 16200 CCD, and I have an FSQ106 which, if I remember correctly,  is the same as the scope that you used on your Heart image.

To my simple mind, there will be no difference in exposure time when you are imaging at F3 compared to my setup imaging at F5.  We will both have the same FOV (roughly).  We will have a similar resolution.  The pixels in my camera are bigger, but at F5 they will be collecting the same number of photons from the same area of the sky.

So, any differences in exposure time will be down to QE and noise.

Absolutely--in fact you will have an advantage of not having to work at F3, where things have to be absolutely perfect as far as othgnality and collimation and all that stuff.  Plus--you can always throw the .6x reducer on someday and wow--mosaic time!

[Rodd]

1 hour ago, Thalestris24 said:

Um, dare I ask about the converse i.e. adding a Barlow vs imaging small targets with an equivalent longer focal length scope? I have a feeling that using even a high quality Barlow for imaging is never a good idea...

Louise

Never say never--I have seen some great images.  Takahashi makes a 1.6x extender for the FSQs.  granetd, it only bumps them up to F8 or so--but that is significant for a 4-5" scope.  But--you are right, its more diffiult, things tend toward the dim, unless you are imaging a very brightbtarget--like the ring nebula.

rodd

[Rodd]

By the way--I gave it a bit of a stretch.  I think I was being too conservative.  It still needs several hours of data--but I see some promise now.  In the end--its not teh camera--its the photagrapher.  As far as quality goes--FOV and pixel size are camera.  Hosey is a bit dark--just a quicky to see how teh data is.  Also--i will definitely take a cloer look at teh subs--I havent even looked at them yet!  I am sure there are some nastis in there.

 

 

Edited October 18, 2019 by Rodd