Appature and Astro Photography.

[Perkil8r]

Just a quick one, does the appature of the scope help bring more detail? By which I mean would a 12" for instance at f5, and exp of say 20 secs @ ISO 400, get more detail than a 8" f5 at the same settings? Or would the object just appear brighter with no increase in detail?

[Rusty Strings]

I think that it comes down to contrast. A slower scope will show greater levels of contrast. I guess it matters what your pointing at, DSO or planet.

[glider]

When imaging both images will look just as bright with the same contrast detail.

However, larger aperture will give more spatial resolution.

[Perkil8r]

What is "spatial resolution"?

[glider]

arcsec/pixel

[Perkil8r]

Ah, I see..... I think. I wondered if as with observing, if it collects more light then the image might be brighter.

[Tweedledee]

As a non-imager resurrecting this old thread, I would just like to try and get to the bottom of this and get the basic principles straight in my head. I'm sure an imager will confirm whether or not my thinking is anywhere near correct.

Going back to Mikes original question and breaking it down to numbers, sorry...

Let us assume that a 12" F5 is compared with an 8" F5 both using exactly the same size/specification chip.

The 12" F5 provides 1.5x the aperture of the 8" but the light grasp is 1.5 squared = 2.25x the 8"

The 12" F5 provides 1.5x the spatial resolution of the 8", but that light is spread out over 1.5 squared = 2.25x the area of the 8"

Therefore the chip in the 12" collects (1/2.25)x the total amount of light collected by it, but at a higher resolution.

It would follow that the 12" has 2.25x the light grasp of the 8" but uses only (1/2.25)x that light.

2.25 * (1/2.25) = 1

So the 12" F5 collects (1x) the same amount as an 8" F5 with the same chip in the same time.

Therefore, in simple terms F5 = F5 = F5 regardless of aperture. The difference is that a higher resolution image but smaller field of view is produced by the larger aperture but at the same brightness.

 

So, while achieving the same brightness of image the following would apply;

To reduce the exposure time, a shorter focal ratio is needed and/or a more sensitive chip.

To increase the field of view, a larger chip size and/or a smaller focal length is needed.

To increase the spatial resolution, a larger aperture and/or smaller pixel size is needed (the available resolution being ultimately limited by Dawes Limit and atmospheric conditions).

Am I on the right lines??

 

Thanks in advance to anyone who takes the time to ponder this one.

[andyboy1970]

Your bang on.

 

I use this calculator - http://www.12dstring.me.uk/fov.htm

 

A 300mm f5 scope with Atik314+ gives a resolution of 0.89"/pixel. Dawes limit 0.39"

A 200mm f5 scope with Atik314+ gives a resolution of 1.33"/pixel, Dawes limit 0.58"

[Tweedledee]

Thanks Andy, but do the rest of my ramblings stand up to any scrutiny?

[glider]

Brilliant bit of maths Pete, we should pin this thread for future reference.

[Tweedledee]

Thanks very much Noel.

 

Maybe I'm finally starting to catch on.

 

Cheers.

[Guest ollypenrice]

We are teetering on the brink of the dreaded 'F ratio myth' here, a subject which brings people to blows on lesser forums!

 

The F ratio myth arises from the incorrect idea that for a given aperture you can expose for less time on a given object for the same quality. The myth (though I've never met anyone who believed it) says 'An 8 inch F5 will need a quarter as long to capture M57 as an 8 inch F10.' (I've picked M57 because, being small, it will fit easily on both setups' chips.) This is wrong. Why is it wrong?

 

1) The photons from M57 (we'll call these the Object Photons) cannot possibly know the F ratio behind the 8 inch hole they are about to enter. So they have no way of becoming more numerous or less numerous based on that F ratio. An 8 inch hole is an 8 inch hole and all 8 inch holes pass the same number of object photons.

2) The F5 scope produces a 4x smaller M57 on the chip than the F10. This means that the M57 Object Photons are crammed onto a quarter as many pixels, so those pixels read a 4x brighter signal. But, BUT, it is also a 4x smaller image so when you stretch it out to the same size as the F10 image it becomes a quarter as bright. Just like the F10 image in fact. Not brighter. And there's the myth. To make it even worse, your F5 image only has a quarter of the pixel resolution so it will be blocky by comparison with the F10.

 

The OP asked about aperture's importance. The only way to think about this is to compare two apertures at the same focal length. If you do this you can avoid the myth. And you can easily see that the 8 inch with FL 'y' captures 4x as many object photons as the 4 inch of FL 'y' (area of aperture is Pi r squared) so F5 is 4x faster.

 

The moment you compare two scopes of different focal length you are in the mire. Don't do it!

 

Resolution does in theory go up with aperture but, sticking to our rule of only comparing constant focal lengths, the bigger the aperture the faster the scope. Now fast is good in imaging but fast is also difficult. Below F5 things start to get difficult for the telescope makers. Everything has to be more precise, optically and mechanically. The theoretical resolution of an ultra-fast instrument will be harder to acheive than with a smaller aperture slower one, so theory and practice tend to diverge... Only very, very rarely do Hyperstar images show anything like their theoretical resolution because, at F2, they are so hard to get right.

 

Olly

[fwm891]

Bravo Olly that has to be the best explanation going on the subject