Focal Length, Focal Ratio, and Aperture - Considerations

[Rodd]

This is a comparison of an FSQ 106 image with a C11 Edge image.   The FSQ was reduced .6x for a FL of318 mm and a focal ratio of F3.  the C 11 Edge was reduced .7x for a FL of 1,960 mm and a focal ratio of F7.  The images have roughly the same amount of exposure (13 300 sec for the FSQ and 19 300 sec for the C11Edge).  This comparison emphasizes a couple of things.  Firstly, it makes it quite clear that focal ratio alone does not determine signal, or speed.  It is commonly said that F3 is fast and F10 is slow, and people use reducers to bring down the focal ratio, thinking this will increase signal to a target on the sensor.  It does not.  As is clearly seen in these two images, the F7 1,960 mm FL image contains vastly more signal than the F3 318mm image.  This is evident buy the fact that in order to portray the two images at about the same level of brightness, I had to stretch the heck out of the FSQ image--look at the noise.  Look how clean the C11 Image is at the same level of stretch.  This makes it clear that aperture controls signal, not focal ratio.  In a slightly different way of looking at it, it can be said that aperture equates to speed.  Two scopes of the same aperture shooting at F5 will produce the same signal.  Change the focal ratio without changing the aperture of one scope and the signal will not change--it will be spread out differently on the sensor, but the same number of photons will be captured in a given time ( they will be spread over a wider FOV if a reducer is used, giving the illusion that more photons were collected).  However, change the aperture of one scope, even if focal ration goes up, and the scope will be faster.  Another way to look at it is even though the FSQ was F3, it requires at least 10x the data to match the signal of the C11Edge at F7--because of aperture.  In fact, as is indicated in the following point, under most conditions for many., aperture is directly equal to speed and nothing else (not resolution)

The other thing I find telling about this image is proof that seeing controls resolution.  Focal lengths of 318 mm and 1,960 mm are far enough apart that there should be absolutely no question as to which will yield a higher resolution image--especially if the 1,960 mm scope has an aperture of 11" vs 4.2" for the 318mm scope.  That is 2.46 arcsec/pix vs 0.4 arcsec/pix.  But what do we see?  Almost identical detail in the two images.  The C11Image certainly looks better--but I submit it is due to the difference in noise/signal.  If the FSQ image had 10x more data. it would look much better and the resolution would be pretty close.  This makes me thing a few things--it really says a lot about the capability of 4" refractors.  Its quite remarkable.  It also makes it clear that unless your sky can support the resolution of a large aperture (assuming it has a large focal length), then using a scope with bigger aperture will only do one thing--make data collection faster (and yield smaller stars, or course)

FSQ 106 with .6x reducer - 13 300 sec Ha

C11Edge with .7x reducer - 19 300 sec Ha

[vlaiv]

On first point - I agree completely.

It is namely "aperture at resolution" that determines speed of the system.

On second point - I don't completely agree, and in fact - there is proof hiding in these two images.

Yes, atmosphere dictates resolution for the most part, but aperture does play a role.

This is zoomed in C11 image on very close pair of stars:

There is no mistake in this image - those are two stars next to each other.

same region in FSQ image. Yes, SNR is much worse and noise is present, but still - I don't see two stars there. Resolution is clearly better in C11 image as it managed to resolve that stellar pair (see - resolution / resolve ).

[Rodd]

45 minutes ago, vlaiv said:

On first point - I agree completely.

It is namely "aperture at resolution" that determines speed of the system.

On second point - I don't completely agree, and in fact - there is proof hiding in these two images.

Yes, atmosphere dictates resolution for the most part, but aperture does play a role.

This is zoomed in C11 image on very close pair of stars:

There is no mistake in this image - those are two stars next to each other.

same region in FSQ image. Yes, SNR is much worse and noise is present, but still - I don't see two stars there. Resolution is clearly better in C11 image as it managed to resolve that stellar pair (see - resolution / resolve ).

Well, I would say that the noise is playing more of a role than you admit. But secondly, I think it is also due to the fact that stars are smaller in big scopes. But other features are not. At least not as much.  Finally, I did not say the resolution was identical. I said it’s about the same. If you have to zoom way in and pixel peep to see the difference, I think my point is made. Let’s look at it this way.  If someone was used to imaging with a 4” scope and decided to spend $50,000 on a 17” scope, I think if the improvement realized is the stars you pointed out, I think the person would be quite unsettled and regretful that he bought such a scope.  

[vlaiv]

1 minute ago, Rodd said:

But secondly, I think it is also due to the fact that stars are smaller in big scopes. But other features are not

If stars are smaller - then image is sharper.

These two are intimately interlinked. Star profile is the blur profile that is affecting the image, and tighter stars simply mean less blur.

In the end - as you say, most dominant factor is still the atmosphere, and there is small difference between images created with smaller and larger apertures as far as sharpness goes. This changes somewhat in very good seeing, so someone that has access to site with very good seeing will make the most out of larger aperture.

[Rodd]

Just now, vlaiv said:

If stars are smaller - then image is sharper.

These two are intimately interlinked. Star profile is the blur profile that is affecting the image, and tighter stars simply mean less blur.

In the end - as you say, most dominant factor is still the atmosphere, and there is small difference between images created with smaller and larger apertures as far as sharpness goes. This changes somewhat in very good seeing, so someone that has access to site with very good seeing will make the most out of larger aperture.

Absolutely. In fact, resolution is directly proportional and dependent on seeing. In good seeing, the opposite of this post is true. There is no question that if seeing is .2” and one images at 1.5” it will be far less sharp then someone imaging at .5”.  I reckon that is why they make $50,000 17” scopes 

[Rodd]

Also, remember the fsq image was upsampled 6x.  The stars look a lot smaller in the original image.  In fact, there are less pixels per star in the fsq image than the c11 image.