f stops and scope speeds.

[ollypenrice]

Sorry Charic but I still don't agree with you. You say, 'f/4 to f/6 providing wider fields of view (FOV) making them more suitable for deep space observations.'  WIder than what? Than a scope of the same aperture and a slower F ratio. Sure, this would be correct but it seems confusing to me to introduce an unspecified constant in the form of the aperture. I agree with your underlying point but since I run courses in AP in which there are so many possible areas of confusion, most notably the F ratio myth, I like to keep aperture, F ratio, focal length and object photons nicely separated for clarity.

Olly

[Charic]

Olly........Im still learning too. You mention, "It's focal length which determines field of view"..... that is totally true for CCD devices and imaging. I agree with that.

My understanding is that a fast f/ratio say, f/4 will have a much wider field of view than say that of an f/6, from reading information on the web, hence my original post.

I`m still studying my notes and information available on the web.

Note to OP: this is not a hijack or argument. We can all learn from the outcome!

[ollypenrice]

Olly........Im still learning too. You mention, "It's focal length which determines field of view"..... that is totally true for CCD devices and imaging. I agree with that.

My understanding is that a fast f/ratio say, f/4 will have a much wider field of view than say that of an f/6, from reading information on the web, hence my original post.

I`m still studying my notes and information available on the web.

Note to OP: this is not a hijack or argument. We can all learn from the outcome!

No problem. The answer is very simple. What you have read on the web about FOV and F ratio is true provided the aperture is constant. Field of view in both visual and photographic situations is determined by the effective focal length and, of course, by the size of the sensor or the apparent FOV of the eyepiece. Let's assume that we change neither the sensor size nor the eyepiece AFOV in the following discussion.

The effective focal length is the combined focal length of all the optics in the system. In visual this will mean the scope, EP, Barlow or reducer if used. In imaging it will mean the scope, plus any reducer, extender or eyepiece. This effective focal length alone defines the FOV.

The effective focal length in visual use can be made up of a long FL, slow telescope and a long FL eyepiece or it can be made up of a short FL, fast telescope and a short FL eyepiece. If the numbers give the same magnification then the FOV will be the same.  And there will be precious little to choose between the views. On very expensive kit that means, in effect, no difference. On budget kit the slower scope version might be better. In terms of physics they are equivalent.

Popular misconceptions;

I will get a brighter view of object X in a fast F ratio scope of a certain aperture. I will need shorter exposures on Object X in a fast F ratio scope of a certain aperture. Both are wrong. This is 'The F ratio myth.' In visual use the brightness is, for a given aperture, governed by the same numbers as magnification. More magnification means dimmer. Whether the scope itself is fast or slow has no effect if the aperture and magnification remain the same as on a different scope.

In imaging and visual the number of photons detected from Object X is governed by only one thing, aperture. So why does F ratio matter in imaging? It matters when 'Object X' is bigger than your chip. If Object X fits on your chip at, say, F10 then there is no point in using a reducer to bring you down to F6. All you do is make X appear smaller and brighter. But you could image it at F10 and then make it smaller by zooming out on your PC screen. However, if Object X is a big nebula you use a reducer to pull its outer parts onto your chip. The original FOV is now only the inner part of the picture and the light from that inner part is fed onto fiewer pixels, making them 'fill' quicker. Plus light from the outer parts 'fills' the outer pixels on the chip.

The same happens in visual when higher magnification distributes the light across more rods and cones in the eye. Each rod or cone gets less light.

Olly

[Christopher Davenport]

Popular misconceptions;

I will get a brighter view of object X in a fast F ratio scope of a certain aperture. I will need shorter exposures on Object X in a fast F ratio scope of a certain aperture. Both are wrong. This is 'The F ratio myth.' In visual use the brightness is, for a given aperture, governed by the same numbers as magnification. More magnification means dimmer. Whether the scope itself is fast or slow has no effect if the aperture and magnification remain the same as on a different scope.

In imaging and visual the number of photons detected from Object X is governed by only one thing, aperture. So why does F ratio matter in imaging? It matters when 'Object X' is bigger than your chip. If Object X fits on your chip at, say, F10 then there is no point in using a reducer to bring you down to F6. All you do is make X appear smaller and brighter. But you could image it at F10 and then make it smaller by zooming out on your PC screen. However, if Object X is a big nebula you use a reducer to pull its outer parts onto your chip. The original FOV is now only the inner part of the picture and the light from that inner part is fed onto fiewer pixels, making them 'fill' quicker. Plus light from the outer parts 'fills' the outer pixels on the chip.

The same happens in visual when higher magnification distributes the light across more rods and cones in the eye. Each rod or cone gets less light.

Olly

Thanks Olly,

Somebody should pinn this observation on the imaging board! How true, exactly what I knew subconsciously.

I am struggling to image at F10 on a 127Mak and was thinking of introducing a reducer to increase the speed.

But logically I could not reconcile how this would help as the object would just be smaller, no more light enters.

And using a reducer add comma around the edges so I would need to crop alot, so almost back to where I started.

So only way to get it brighter is to increase my exposure time and more subs or get a larger aperture. 

So I am back out trying to capture more light and saving for a larger aperture.

Edit:

As a side note with a longer focal length it is harder get long subs, but the same is true for a large bulky telescopes.

But what about binning is 2x2 the same as reducing your focal length?

[ollypenrice]

Thanks Olly,

Somebody should pinn this observation on the imaging board! How true, exactly what I knew subconsciously.

I am struggling to image at F10 on a 127Mak and was thinking of introducing a reducer to increase the speed.

But logically I could not reconcile how this would help as the object would just be smaller, no more light enters.

And using a reducer add comma around the edges so I would need to crop alot, so almost back to where I started.

So only way to get it brighter is to increase my exposure time and more subs or get a larger aperture. 

So I am back out trying to capture more light and saving for a larger aperture.

Edit:

As a side note with a longer focal length it is harder get long subs, but the same is true for a large bulky telescopes.

But what about binning is 2x2 the same as reducing your focal length?

We need to be mindful of thread creep but I still think F ratio is important in imaging because 8 times out of 10 I'm imaging things that don't fit on the chip, so focal reducers do speed things up.

Binning 2X2 isn't the same as reducing focal length but it is a way of working faster, certainly. Expect a gain of about 1.6 to 2x, but not the 4x you might hope for! If your unbinned pixels are trying to resolve details finer than the seeing allows then you'll fail and will lose no resolution by binning. The gain in speed comes at no expense, in other words.

To put it all another way, if you want the same FL as your Mak you will need more aperture to speed it up. As you say, it all starts to get a bit top heavy at this point! 

Olly

[Charic]

olly......I should have asked you first!