Can someone please explain ...

[LukeSkywatcher]

If you take two scopes with the same aperture but different focal lengths, the image at the focal plane in the slower scope is stretched over a greater area, so it is dimmer per square mm of image.

Hence fast scopes (f5 etc) with a shorter focal length (shorter tube) are better for observing DSO's while slower scopes (f11 etc) with longer focal length (longer tubes) are better for oberseving bigger brighter objects such as the Moon and planets. Althought the faster scopes will also give give great views of the moon and planets.

[Ags]

Ha ha... The other day I finally figured out how exit pupils are formed by imagining cones inside cones... It's worth the effort to understand telescope optics.

[LukeSkywatcher]

I understand exit pupil stuff. On the other hand i am still a tad confused about eye relief. I really must make a point of reading about it more and taking it in rather then reading it and it going in one eye and out the other.

[brantuk]

Ok - think I'm up with the plot now lol - changed all my posts to reflect the correct position on this - which I think is now within my grasp - just - lol

[Dr. Phibes]

Just like to thank OPer and all those that responded.Another newbie here who couldn't figure out difference in fast and slow. Understand it now so thanks all, although I think my brain runs on f15

[great_bear]

If you take two scopes with the same aperture but different focal lengths, the image at the focal plane in the slower scope is stretched over a greater area, so it is dimmer per square mm of image.

Hence fast scopes (f5 etc) with a shorter focal length (shorter tube) are better for observing DSO's while slower scopes (f11 etc) with longer focal length (longer tubes) are better for oberseving bigger brighter objects such as the Moon and planets.

Not quite - I mean, it doesn't follow from the stretching / dimming issue: When it comes to observing (as opposed to imaging) then eyepieces come into the equation, and the brightness issue then balances itself out - i.e. for a given magnification, image brightness will be identical no matter what the F-Ratio of the scope.

The main reasons why fast/slow are preferred for dso/planets is because (a) in slow scope, focuser tube width limitations place a narrower limit on how wide the image can be, and ( in a fast scope, high magnifications require very short-focal-length eyepieces which are harder to manufacture to a high standard, and © high-magnification image contrast holds out better with a slower scope.

To the above, I would also add (d) that - manually - a long-tube scope is a lot easier to track the planets with than a short-tube scope is

[great_bear]

i am still a tad confused about eye relief.

It's a lot less exciting - basically, there's a vast number of beams of light, each beam being of exit-pupil width, which leave the eyepiece and all cross over at a central point, which is <N> millimeters from the top lens of the eyepiece. That distance <N> is the eye relief, and it needs to be about 17mm or more if people are to use the eyepiece whilst still wearing spectacles. However, that distance - 17mm - also means that people who don't wear spectacles would have to hover their gaze a few millimeters in front of the eyepiece and many people - including me - find this a really uncomfortable. Thus a roll-up or twist-up eyecup is desireable if the eyepiece has such generous eye-relief.

If you consider the width of the angle of beams entering the eye, it's not too hard to see that the further away the cross-over point, the larger the width of the eye-lens would have to be - but the maximum width of the eye-lens is dictated by its focal-length. Therefore, in simple eyepiece designs like Plossls and Orthoscopics, short focal-lengths dictate narrow eye-lenses which - in turn - dictates very short eye-relief. In more complex designs (such as Delos for example) the eye-lens has a much longer focal length, is correspondingly wider, and thus can provide much longer eye-relief.

If the beams of light leaving the eyepiece don't all cross in the correct place, this is known as "exit pupil aberration" and is what causes kidney-beaning and other blackout effects to occur when your eye isn't perfectly positioned.

It's a tricky thing in sophisticated eyepiece design to provide wide angle apparent fields of view, with long eye-relief, without distorting the exit-pupil.

[dph1nm]

What everybody forgets in this fast/slow imaging nonsense, is that image scale in mm, which people keep taking about, is only half the story. The other half is the pixel size of your detector. The two combined give you an effective focal length for your system.

The other thing to note is that the amount of light per square arcsecond in your image is not dependent on focal length, only aperture.

NigelM

[Ags]

Yes image brightness (for extended objects) depends on the ratio of aperture to magnification, aka the exit pupil. I thought I made that clear in my earlier response but obviously not.

It is worth adding that the brightness of point sources like stars depends only on aperture and is not affected by magnification (visual) or speed (astrophotography).

[Cornelius Varley]

What everybody forgets in this fast/slow imaging nonsense, is that image scale in mm, which people keep taking about, is only half the story. The other half is the pixel size of your detector. The two combined give you an effective focal length for your system.

The other thing to note is that the amount of light per square arcsecond in your image is not dependent on focal length, only aperture.

NigelM

Can you explain this a bit more.

Peter

[Dude]

Think of it like this.

Imagine you have a film projector projecting an image on an A4 sheet of paper, the image on the paper is the field of view (FOV).

If you move the paper further away from the projector the image gets bigger and at the same time dimmer and the FOV gets smaller because as the image gets bigger less of the whole image stays on the piece of paper. This is the same as increasing the focal length such as an f10 telescope.

Now if you move the paper closer to the projector the image gets smaller and brighter and the FOV gets bigger, because as the image gets smaller more of the whole image stays on the piece of paper.

This is the same effect as a shorter focal length such as F4.8.

Hope that makes scene and helps.

Mike.

This explains it perfectly for me, thanks Mike

[robbieince]

I guess the confusing bit of that for a given aperture with increasing focal length is that the same quantity of light (photons per angular arc) is spread over a bigger physical area so whilst the native image gets bigger it gets fainter. This is because the physical area has changed but the angular area has not.

Regards

Rob

[Tim]

Think of it like this.

Imagine you have a film projector projecting an image on an A4 sheet of paper, the image on the paper is the field of view (FOV).

If you move the paper further away from the projector the image gets bigger and at the same time dimmer and the FOV gets smaller because as the image gets bigger less of the whole image stays on the piece of paper. This is the same as increasing the focal length such as an f10 telescope.

Now if you move the paper closer to the projector the image gets smaller and brighter and the FOV gets bigger, because as the image gets smaller more of the whole image stays on the piece of paper.

This is the same effect as a shorter focal length such as F4.8.

Hope that makes scene and helps.

Mike.

OK, only now will I admit to not properly understanding why f this or that was faster or slower than another f!

Mike, your explanation is perfect

Cheers

Tim

[brantuk]

Thank goodness it wasn't just me Tim lol

Mikes explanation is perfect agreed !!

[ollypenrice]

There are some entirely erroneous explanations on here and some that are bang on the money! Let me try my two penn'orth...

1) Let's fix the focal length at a metre in two scopes.

2) Let's give scope one an aperture of 100mm so it is F10.

3) Let's give scope two an aperture of 200mm so it is F5.

4) The surface area of aperture (Pi r squared) in scope two is four times larger than that of scope one so it pours four times as much light onto the chip and takes one quarter of the time to achieve the same result. It really is that simple.

There are two sources of confusion in this discussion. The first is not comparing scopes of the same focal length. This leads us into the famous 'F ratio myth' which we can do without! (If you keep the same aperture but reduce the focal length in order to boost the F ratio, do you gain any time on an object entirely framed by both systems? No, and you lose resolution.) The second is the camera world's habit of referring to F stop as if it were unconnected with aperture, but when you stop a lens down you are simply reducing its aperture without changing its focal length, so it collects less light and gets slower as a result.

It is true that pixel size comes into the true speed of aquisition but that lies outside the remit of the question.

Long focal lengths are not slow and short focal lengths are not fast. Either can be either. It is the ratio of aperture to focal length that matters.

The speed of light is a constant. The f ratio of your scope does not affect it.

Olly

[Stu]

I must confess, I was thinking 'Where's Olly when you need him?'

You took your time coming to the rescue while we were all floundering around :-) :-) :-)

Stu

[ollypenrice]

I must confess, I was thinking 'Where's Olly when you need him?'

You took your time coming to the rescue while we were all floundering around :-) :-) :-)

Stu

Phone lines went down! The price you pay for living in rural France. That plus working flat out on a new observatory before the winter gets here...

Olly

[Stu]

Well, am pleased you are sorted now. Thanks for clarifying the whole fast/slow thing :-)

Good luck getting the observatory finished, look forward to seeing some pics

Stu

[AlistairHowie]

Thanks, Olly - clear to me now!

[dph1nm]

It is true that pixel size comes into the true speed of aquisition but that lies outside the remit of the question.

I would dispute this. I think this is the whole point about imaging in the age of CCDs (and lies behind the f-ratio myth, which as Olly says we had better not go into!). Otherwise you end up with the ridiculous assertion that you could image just as deep with an 8inch scope as an 8metre, providing the 8inch had a small enough f-ratio (probably impractically small, but one could do a thought experiment!).

NigelM

[dph1nm]

It is worth adding that the brightness of point sources like stars depends only on aperture and is not affected by magnification (visual) or speed (astrophotography).

This is only true if you are not limited by seeing. If you are, then stars behave just like little extended sources.

NigelM

[Astrosurf]

This is a great thread!

I'm trying to understand this visually. So, as the light radiates out, the shorter focal length catches a narrower 'cone' of light than the longer focal length where the light has spread out more? Here's a rubbish diagram to try to help me visualise this:

Fast and Slow Scopes.bmp

[robbieince]

Nope - as far as makes no difference, all light rays from your target are parallel. They only form a cone after your primary mirror or lens try to bring them to focus at its natural focal length.

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".

Of course when visual we also magnify this image with an eyepiece which has its own focal length, aperture etc to add in to the equation.

Fast / slow only really makes sense when photographic with a simple focal length situation.

Regards

Rob

[harry page]

Hi

This was on another site , but I thought was very good

There's a number of factors operating here. All 8" primary scopes gather the same number of photons if we exclude the difference caused by different sized obstructions. A 10" primary will gather 100/64 more photons than the 8".

Once we've gathered the photons, the f/ ratio determines what we do with them. This becomes critical when imaging extended objects. The longer the focal ratio (for a given objective), the larger the image. An f/10 lens produces an image that is (linearly) twice as big as that produced by an f/5 lens. An image that's 2x as large occupies 4x the area as the smaller image. The difference for imaging is that the same number of photons will be illuminating 4x as many pixels, giving 1/4 the number of photons/pixel. We normally compensate for this by using an exposure that's 4x as long.

Another way of viewing this phenomenon is that the f/10 objective has a smaller field of view that the f/5 one. Using a focal reducer concentrates the available photons into a smaller area, illuminating less pixels, and putting more photons into each pixel. This lets us use shorter exposures for the target.

Image size is actually solely a function of focal length. That's one of the reasons why planetary telescopes are f/10, f/15, or longer focal rations. These scopes' long focal lengths, with the consequential narrow field of view and larger image make it easy to get high magnification with longer focal length eyepieces.

The next problem we encounter in this journey is the effects of seeing. The larger the objective; the worse the effects of seeing become. I've attended star parties where the owners of 14" scopes had poor viewing of planets while the 6" scopes had clear ones.

We all use our scopes for different purposes. It's up to us to learn enough before making a purchase to understand what will work best for our intended use.

Harry

[AlistairHowie]

Thanks, Harry - I find that explanation very helpful.

I will say, I'm intrigued by how many replies this thread has attracted! I posted what I thought was a pretty basic, newbie question, but I guess it goes to show that not everybody knows what they are talking about. I don't mean that to sound rude, we can't all be experts in everything. It just goes to show that you can enjoy the hobby while not necessarily understanding all of it ... even if you think that you do!