Can someone please explain ...

[Dunkster]

So it's how steep this cone is that matters as the steeper cones (shorter focal lengths) produce smaller brighter images - all other things being equal. We of course put a detector at this point such as a CCD with a fixed size and so it usually only intercepts part of this image. Bigger sensors capture more field of view but unless the pixels are also bigger will not make the system "faster".

If I'm following this correctly (and I'm not assuming for a second that I am!), then the size of the pixels inherently introduces a compromise since for a given "practical" size for a CCD, the bigger pixels are more desirable for faint objects because they capture more photons per pixel BUT this limits resolution (image is smaller but brighter because of the steeper cone).

Conversely, if we wanted higher resolution, we'd need smaller pixels each capturing fewer photons per pixel and requiring longer exposures for the same signal. Or a shorter focal length. Or more sensitive pixels!

Aaaaahhhhhh! It's dob vs Mak all over again

[robbieince]

Hi dunkster

That's pretty much it. Of course, the pixel size is a given for the camera.

We can actually specify an optimum focal length to illuminate the specific camera and get a resolution within local seeing conditions. If the pixels are too small or the focal length too long, the star images are spread over many pixels and this is inefficient (called over sampled), conversely too short a focal length or too big pixels could mean the star would only be recorded on 1 pixel and would not have any structure (called undersampled).

So whilst big pixels are more sensitive there is a practical limit. Equally small pixels (such as found on the many megapixel dslrs) are not very sensitive and need high gain electronics and so tend to be noisy.

Regards

Rob

[Luke]

Interesting thread!

Like the OP, what got my head spinning about this was that I wanted to understand why my SCT was slow, especially considering its big aperture. As the OP has seen, the SCT is plenty bright enough when observing.

What helped me get my head around it all was to keep thinking about the relationship between focal ratio, focal length and aperture:

focal ratio = focal length / aperture

I did some number crunching to compare my 80mm F6.25 refractor and 280mm F10 SCT, and without going into all the maths, what it boils down to is that for prime focus imaging:

Although the SCT lets in about 12 times more light, it scales up the image by about 5.6 times more, which means an object covers about 30 times more pixels in the SCT than in the refractor! (doubling up the image scale covers four times as many pixels, as shown in the picture, I think this is an easy point to miss).

So to sum it up, and maybe someone's got a better way of saying it, the reason why the SCT is slower is that the increase in brightness due to the bigger aperture is less than the loss of brightness (per pixel) due to image scaling.

But keep in mind that the image is much more scaled than in the refractor, so the scopes are doing different things.

When it comes to observing, it's a very different matter because you change the magnification of the image by using eyepieces. I would view the object at about the same size in both scopes, so that would cancel out loss of brightness in the SCT due to image scaling.

The SCT lets in about 12 times more light, so the object should look about 12 times brighter in the SCT if the magnification is the same in both scopes.

Ultimately, they are very different scopes that do different things, and all scopes have their benefits. I love both these scopes!

[ollypenrice]

I think the question you asked is interesting and some of the science behind it is counter-intuitive. I'm also fascinated by the business of writing explanations because, as anyone who has ever done any teaching knows, it is easy to think you understand something and you only find out that you don't when you try to explain it! How many times have I been there and done that!? My own policy was to own up straight away and say, 'I'll be back next week...'

There are some great bits of explaining on here. The more ambitious explanations tried to cover the effects of focal length and focal ratio at the same time, looking at the idea of spreading the light over larger or smaller areas. This is a worthy thing to try and do. My attempt copped out and fixed focal length in order to tap into the essential feature of F ratio alone.

As Bob Hoskins said, 'It's good to talk.'

Olly