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F Numbers????


Guest chaz2b

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Guest chaz2b

Okay, I`m a relative newcomer to Astronomy to a lot of you veterans on here ,2003 was the year of my first real scope, a Meade L200, I do have a general understanding of F numbers, but you can get lost in them.

As I see, there are many manufacturers producing many scopes with many F numbers! what I ask, is there a `sweet`  F number that deals with the many uses it can use, and why.

 

can of worms?....let them out!

 

chaz

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For visual astronomy, higher F numbers (i.e. f8 upwards) might be better for planetary observations as a 25mm eyepiece will produce more magnification than in a lower F number scope.

A lower F number (5 and below) tends to be better for wide-field scopes, as they will be more compact and you can use smaller eyepieces for them - as well as being easier to reach the widest possible view with.

Although, all-in-all I think F number doesn't matter so much as long as your scope is capable of both wide-field and narrow-field viewing. For visual this is most scopes on the market.

 

For deep-sky photography, you typically want the fastest f ratio you can get, since an f8 scope will produce the same image in 2 hours as an f4 scope will produce in 30 minutes. (assuming same camera). For planetary photography you might be using a longer scope (i.e. f8) and barlowing it to make an f16 or f24 in order to get the best pixel size.

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Focal ratio is a 'derived variable' and should be treated with caution. Aperture and focal length are primary qualities in an optical system.

A few things we can say for sure:

- fast (low F ratio) optics are harder to make. It is harder to rid them of field curvature, coma and false colour (in refractors.) If they are good they will be expensive.

- fast optics produce a steep light cone which gives a shallow depth of field, making focus critical and requiring a mechanical system free of tilt.

- In imaging, exposure times will be reduced as the square of the F ratio if the f ratio is lowered by increasing the aperture. If it is lowered by reducing the focal length then exposures will only be reduced because the light lands on fewer pixels.  (This is an example of a derived variable causing confusion!)

If money were no object and a brilliant optical engineer could make a perfect design then faster optics would be preferable because they would give you the option of a wider field of view if you wanted it. But, in reality, we tend to go for the compromise which suits our observing prefences and budgets.

Last year I had a look through a very rare beast indeed, a reflector with a variable focal length primary mirror.

Olly

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You need to be careful about using f number in reference to magnification, it makes no difference, or at least it tells you nothing without knowing the aperture. It is focal length that determines magnification. I could have an f2.5 30” dob with a focal length of 1900mm and an f10 4” frac with a focal length of 1000mm, the faster scope would still give a higher mag with the same eyepiece.

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This is very general and from a visual observers perspective but looking at what focal ratios play to the strengths of the various scope optical designs while avoiding most of their weaknesses, I'll risk putting forward the followng:

- F/6 for a newtonian,

- F/10 for a schmidt-cassegrain,

- F/12 for a maksutov-cassegrain,

- F/6 for a maksutov-newtonian,

- F/7 for an apochromat refractor,

- F/10 for an achromat refractor unless it's mostly for use as a low/medium power widefield in which case F/6

All to be taken as an "on or around" type spec :smiley:

Now everybody else can come forward with their preferences :smiley:

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3 hours ago, Peter Drew said:

"variable focal length primary mirror", care to elaborate Olly?     ?

It's another Ralf Ottow creation, Peter. A thin primary is separated from a rigid glass blank beneath it by a hermetic ring. When air is evacuated from the space between the two glass elements the curvature of the mirror is increased, so reducing its focal length. Remarkably, Ralf demonstrated this simply by sucking on a tube leading to the air space, though ultimately this will be done by a pump and connected to sensors which will preserve the selected pressure. He paid us only a flying visit so my chance to look through it was in the daytime, but it was very impressive - as are all the instuments he makes.

Olly

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1 hour ago, John said:

This is very general and from a visual observers perspective but looking at what focal ratios play to the strengths of the various scope optical designs while avoiding most of their weaknesses, I'll risk putting forward the followng:

- F/6 for a newtonian,

- F/10 for a schmidt-cassegrain,

- F/12 for a maksutov-cassegrain,

- F/6 for a maksutov-newtonian,

- F/7 for an apochromat refractor,

- F/10 for an achromat refractor unless it's mostly for use as a low/medium power widefield in which case F/6

All to be taken as an "on or around" type spec :smiley:

Now everybody else can come forward with their preferences :smiley:

I'd agree with that, though the F ratio of good apochromatic refractors is linked to aperture, so there are superb 4 inch F5 examples but once you're up to six inches it really has to be F7. I gather that the colour correction is the limiting factor. 

Olly

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@ollypenrice. Thanks Olly, I thought it had to be something like that. I'm familiar with the process of correcting a spherical primary by pulling it into a parabola which involves just a fraction of a wavelength but a whole change in focal ratio is a different ballpark. I have seen images of other telescopes by Ralf and they are most impressive.    ? 

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Guest chaz2b

Thanks for all your replies, that's added tremendously to my understanding of F numbers.... Just need to put it into more practice when i get the chance. Particularly more so as i have so many scopes of varying F ratios and configuration.

 

Chaz

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