Focal Lengths and Ratios

[daz]

Have posted this here to save hijacking other threads, and so it can be easily followed....

OK, I understand a telescope has a focal length, an aperture and a focal ratio, and that the focal ratio is the focal length divided by the aperture:

1000 / 200 = f/5

I also understand (from Rob's excellent terminology topic) that the focal length is the distance from the objective lens to the point at which the focus is reached.

Question 1: So, do I understand then, that this is the distance from my objective lens, back up the scope to the secondary, through the focusser and out to a measureable point? This point should then be the focal length?

Question 2: What is the impact of the focal ratio number? For example, my 200mm scope has a ratio of f/5 (1000 / 200). If I stop down the aperture to 100mm, this produces a focal ratio of f/10 (1000 / 100). What is the reason for doing this, when surely the point of the bigger aperture is to gather more light?

Have been trying to follow other topics on focal lengths and ratios, and my head has melted, so I thought I start again!!!! :?

Daz

[Ant]

Hi Daz,

I beleive that the reason that a lot of people stop down the aperature (like in your example) is that smaller aperature scopes are less affected by atmospheric turbulance. So when viewing the sun for example you get better results when the scope is "stopped" down to 100mm F/10 or 50mm F/20.

I, however, has always worked on the principle that the bigger the aperature the better - but it appears that I may be wrong

Ant

[GazOC]

Question 1 is bang on daz!

Question 2...well.....There are several reasons to stop down the aperture. On a scope like yours (a Newt) people stop down the aperture to remove the effect of the secondary mirror and the vane that holds it in place (ie just have a hole in one of the 'quarters', it's called an 'off axis mask'), that way no contrast is lost due to these two. It's claimed that a Newt stopped down like this is as good as the equilivant sized Apo, but I'm not going to go down that road....... .

Refractors are usually stopped down 'on axis' ie in the middle, this reduces the CA (false colour)in achros as a 100 f5 can be stopped down to an (effectively) 50mm f10. The size of the primary lens and the focal length determine the amount of CA a scope will show (and how well made the scope is obviously). This isn't the full story as the stopped down 100mm lens will still be thicker in the middle than a natural 50mm lens and so have to refract the light at a sharper angle, but thats splitting hairs really.

In general scopes are usually stopped down when light gathering isn't an issue, like the Moon, the Sun and, if you have a BIG NEWT, the planets.

Gaz

[daz]

(ie just have a hole in one of the 'quarters', it's called an 'off axis mask')

Ah, my end cap has just such a hole and cover. I wondered if this was for solar filters and the like.

Understand the point on stopping down.

OK, now suppose I have two scopes - one is 1200mm focal length, 250mm aperture and therefore a focal ratio of 4.8. The second one has a focal length of 240mm, 50mm aperture and a focal ratio of 4.8. Is the only difference the light-gathering ability - less aperture, less photons?

Finally (you hope!), eyepieces. An eyepiece has a focal length - 15mm for example - and the lens of the eyepiece intersects the light from the secondary mirror before its focal point. The light is then refocussed by the eyepiece. The net effect is that the focal length of the optical path is changed by the focal length of the eyepiece. Is this correct?

Thanks for your patience!!

Daz

[Astroman]

OK, now suppose I have two scopes - one is 1200mm focal length, 250mm aperture and therefore a focal ratio of 4.8. The second one has a focal length of 240mm, 50mm aperture and a focal ratio of 4.8. Is the only difference the light-gathering ability - less aperture, less photons?

Finally (you hope!), eyepieces. An eyepiece has a focal length - 15mm for example - and the lens of the eyepiece intersects the light from the secondary mirror before its focal point. The light is then refocussed by the eyepiece. The net effect is that the focal length of the optical path is changed by the focal length of the eyepiece. Is this correct?

Thanks for your patience!!

Daz

First:No. The difference is light gathering, but also the ability to focus at higher magnifications. The light cone of the linger scope is narrower and easier to focus. Also, it is less susceptible to bad seeing.

Second:No. Well, yes and no. Light comes to focus at the field stop of an ep. If this point is the same in all your eypieces, they are said to be "Parfocal". That means, you don't have to refocus even slightly when changing them. The focal point of the lens is the same regardless, but the overall focal ratio may change ever so slightly, due to the physical position of field stop in the ep.

[FLO]

Great thread Gaz.

Three more things to consider:

1) Optical physics dictates that a telescopes aperture effects its ability to resolve detail; the greater the aperture, the more detail can be resolved. 

2) The focal ratio effects the depth of focus (the amount in focus in front and behind the actual point of focus) resulting in a fast telescope (ie: f5 and below) having less depth of focus than a slow one (f8 and above) making the focussing altogether more fussy.   

3) Slower telescopes (long focal length and/or smaller aperture) have more contrast. 

Steve

[Astroman]

Steven: Your #2 above is exactly what I was trying to say. Well done!

[daz]

Thank you chaps!

Now I understand a little more about this, I can go back and re-read other posts to see if they make more sense. If not, I'll be back!!

Daz