Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

What does focal ratio really mean for the visual observer?

  • Please log in to reply
24 replies to this topic

#1 aatdalton

aatdalton

    Vostok 1

  • *****
  • topic starter
  • Posts: 143
  • Joined: 05 Feb 2019

Posted 09 October 2019 - 01:45 PM

I understand what focal ratio is, focal length / diameter, and I have a good understanding of how that affects photography and light gathering speed. I also understand it tells me a little bit about how demanding a scope is going to be on corrections needed by an eyepiece.

Does f/ ratio really tell you much else useful for a visual observer in the way it's a critical number for photography? Or is it just shorthand to tell you the specs of the scope?

#2 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 12214
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 09 October 2019 - 02:07 PM

Does f/ ratio really tell you much else useful for a visual observer in the way it's a critical number for photography? Or is it just shorthand to tell you the specs of the scope?

Yes. It tells us how a given eyepiece will behave on a certain telescope, in terms of image brightness. 

 

 

Clear skies!
Thomas, Denmark



#3 barbarosa

barbarosa

    Vanguard

  • -----
  • Posts: 2191
  • Joined: 11 Apr 2010
  • Loc: 139 miles W of the Awahnee Hotel

Posted 09 October 2019 - 02:08 PM

Field of view comes to mind and image scale likewise. The calculators at this site can be used to show the effect on an image of changing the aperture, the focal length or both. You can pick a scope from the list or create a custom scope to test the changes. 


  • desertstars, REC and outofsight like this

#4 aa6ww

aa6ww

    Vanguard

  • *****
  • Posts: 2064
  • Joined: 23 Oct 2011
  • Loc: Sacramento, Calif.

Posted 09 October 2019 - 02:26 PM

If you ask this same question in the Classic Telescope forum, you'll get the answers you are looking for from visual observers perspective.

...Ralph
  • Astrojensen likes this

#5 harbinjer

harbinjer

    Ranger 4

  • *****
  • Posts: 370
  • Joined: 17 Dec 2008
  • Loc: Southeastern Minnesota

Posted 09 October 2019 - 02:28 PM

Just the F/ ratio doesn't really tell you much, other than what you said about eyepieces. If I said it's an f/4 scope, you don't know if it's a small ST80, or a huge 30 inch dob, unless I give you another number, either the diameter or focal length. It much less important for visual than the F/ ratio for camera lenses.


  • Jon Isaacs likes this

#6 REC

REC

    Voyager 1

  • *****
  • Posts: 11476
  • Joined: 20 Oct 2010
  • Loc: NC

Posted 09 October 2019 - 02:30 PM

Field of view comes to mind and image scale likewise. The calculators at this site can be used to show the effect on an image of changing the aperture, the focal length or both. You can pick a scope from the list or create a custom scope to test the changes. 

 

Field of view comes to mind and image scale likewise. The calculators at this site can be used to show the effect on an image of changing the aperture, the focal length or both. You can pick a scope from the list or create a custom scope to test the changes. 

I just tried this site with a C - 8 and 10mm ep = 203x on Jupiter. It looked like the size of an eraser head!


  • desertstars likes this

#7 Soarrunwalk

Soarrunwalk

    Mariner 2

  • -----
  • Posts: 234
  • Joined: 17 Sep 2015
  • Loc: Canada

Posted 09 October 2019 - 02:40 PM

Another impact of f/ ratio is on the size of a given eyepiece’s exit pupil.

I’ve a small f/5 scope and like using a 15mm eyepiece when at public events. The resultant 3mm exit pupil is fairly generous for novice users at outreach. (15/5=3)

With an f/11 scope I would have to use a 32mm eyepiece to arrive at the same exit pupil.

Of course there are many other considerations when selecting an appropriate eyepiece. Exit pupil impact is just one criteria.
  • DesertScribe likes this

#8 REC

REC

    Voyager 1

  • *****
  • Posts: 11476
  • Joined: 20 Oct 2010
  • Loc: NC

Posted 09 October 2019 - 02:45 PM

Just the F/ ratio doesn't really tell you much, other than what you said about eyepieces. If I said it's an f/4 scope, you don't know if it's a small ST80, or a huge 30 inch dob, unless I give you another number, either the diameter or focal length. It much less important for visual than the F/ ratio for camera lenses.

A 4" 100mm with a f/1000mm focal length and a 10mm eyepiece would give you 100x and a 1mm exit pupil. It's the exit pupil that will tell you how bright an image will be. Personally, I like a 2mm exit pupil for a lot of items, very comfortable. But on a bright object to start with like Jupiter, saturn, moon, you can afford to loose some exit pupil. In this case I drop down to around 1mm exit pupil to give me more magnification. Right now with Saturn and Jupiter out, I use a 1mm exit pupil with a 8-10mm eyepiece for more power and more detail on the planets.

 

And, while I'm at it, I must point out something on these descriptions above. When using higher power on objects like the moon and planets, there is also on big major player in all this. That would be seeing conditions. When the air is turbulent and you see stars twinkling, it is not a good sign for higher power observing. While some nights I can use a a 10mm EP for 100x in my 8" scope, when the seeing is poor, I might have to drop down to 150x or more. When seeing gets that bad, I'll observe star clusters ect. that are not affected so much as planets. I hope I did not run my mouth as much, but one thing after another came into my head as I was writing this :)


  • DesertScribe likes this

#9 vtornado

vtornado

    Vanguard

  • *****
  • Posts: 2025
  • Joined: 22 Jan 2016
  • Loc: Northern Illinois

Posted 09 October 2019 - 03:00 PM

In a perfect world focal ratio does not tell a visual observer much given that you also know the focal length and aperture.  However we don't live in a perfect world.

 

Fast optics (low f ratio) are generally harder to make then slow optics, so unless extra care is taken, the optic will

suffer from other abberations such spherical aberration, zonal errors etc.  (I'm not an optical expert).

 

In an Achromatic refractor fast optics  bend the light sharply the differences in focal length

with a fixed aperture, determine how far off each separate color comes to focus. 

 

In a reflector telescope, an aberration called coma comes into play.  I think??? that this same effect

is called field curvature in other scopes.  The focal plane is no longer flat the faster the optics are, the

more this effect comes into play.

 

As somone else noted, eyepieces most notably wide field eyepeices do not handle fast focal length

telescopes well.  Usually extra lenses or exotic glasses need to be added to have a sharp field

all the way to the edge.

 

I'm not sure what exactly you are driving at but if the aperture is fixed, the f ratio determines the focal length.

A slow telescope is a long telescope (cats excluded)  which in general is harder to mount.

 

Once again with fixed aperture, a slow telescope has a long focal length and you can use higher focal lenght

eyepieces to reach the same magnification as a fast telescope.  Classical eyepiece designs, like plossls and

orthos have eye relief proportional to eyepiece focal length.  Of course you can overcome the eye relief issue

in short focal length eyepieces by adding additional elements, but then again we are making the eyepiece more

complex.



#10 lsfinn

lsfinn

    Mariner 2

  • *****
  • Posts: 207
  • Joined: 28 Jan 2013
  • Loc: Santa Fe, NM

Posted 09 October 2019 - 04:10 PM

A telecsope's focal ratio gives the visual observer the range of eyepiece focal lengths that make full use of the telescope's aperture and average over typical imperfections in the observer's eye. 

 

A telescope's f/ratio, combined with the eyepiece focal length, tells you the size of the exit pupil when the eyepiece is used with the 'scope: in particular, xp = efl/fr, where xp is exit pupil diameter, efl is eyepiece focal length, and fr is telescope focal ratio. 

 

The exit pupil is the image of the telescope's aperture in the eyepiece. As pointed-out by @Astrojensen, the larger the exit pupil (up to the eye's entrance pupil) the brighter the image will appear to be.

 

The exit pupil size is also important for "matching" the eyepiece the observers eye. The pupil diameter of the dark adapted eye is typically in the range 5mm - 7mm (toward the upper end for young adults; toward the lower end for older adults). When observing with a telescope that has a central obstruction - e.g., a Newtonian reflector, a Schmidt-Cassegrain, a Maksutov-Cassegrain, etc. - the shadow of the central obstruction becomes a larger fraction of what you "see" when the exit pupil exceeds the eye's pupil. 

 

For example, my dob has a central obstruction 25% of the mirror aperture in diameter. When my eye is fully dilated (say 5mm) then for exit pupils up to 5mm the central obstruction is only 25% of the aperture that my eye sees. When the exit pupil is larger than 5mm, then the mismatch effectively reduces the aperture of the telescope - and, so, its effective resolution - while leaving the obstruction size alone, increasing the apparent size of the obstruction and dimming the image. 

 

At the other end of the scale - small exit pupils - one becomes sensitive to imperfections in the eye. Large exit pupils illuminate a large fraction of the eye's lens and the vitreous humor that fills the eye. This leads to an averaging out of imperfections in the eye and floaters in the vitreous humor, etc. With small exit pupils one loses this averaging effect and the visual observer becomes much more sensitive to imperfections in the eye. Where this becomes a problem also varies by person; however, the rule of thumb is that it is somewhere between 0.5mm and 1mm. 

 

With these limits in mind a telescope's f/ratio gives you the bounds on the eyepiece focal length that such that you take full advantage of the telescope's aperture and are not limited by imperfections of your eye. E.g., for an f/4 scope and good young adult eyes eyepieces you'll want eyepieces with focal length between 2mm and 28mm (i.e., 0.5mm and 7mm exit pupil): larger than that and the eye is "stopping-down" the telescope's aperture (and magnifying the shadow of any central obstruction), and smaller than that and the image will be distorted by imperfections in the eye itself (lens imperfections, floaters, etc.)


  • zleonis likes this

#11 39.1N84.5W

39.1N84.5W

    He asked for it

  • *****
  • Posts: 3965
  • Joined: 24 Oct 2006
  • Loc: cincinnati

Posted 09 October 2019 - 04:14 PM

It tells me how long the refractor is going to be and if I want to deal with that.
  • Freezout likes this

#12 Keith Rivich

Keith Rivich

    Surveyor 1

  • *****
  • Posts: 1700
  • Joined: 17 Jun 2011
  • Loc: Cypress, Tx

Posted 09 October 2019 - 05:41 PM

Visually the f/ratio is not a factor.  Take five scopes of the same diameter with different f/ratios, put in eyepieces that give all five a similar magnification and they will all look pretty much the same.

 

As others state, though, it does tell you about the telescopes dimensions. 


  • Jon Isaacs likes this

#13 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 78967
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 October 2019 - 06:04 PM

My two cents:

 

In a camera focal ratio and focal length are the two important factors.  The focal ratio determines the exposure time, the focal length determines the image scale.  

 

A telescope is an Afocal device, to a first order, it can be described by two parameters, exit pupil and magnification.  Exit pupil determines the image brightness and magnification determines the image size.  

 

This basically says a telescope is a black box.  If all I can determine is the exit pupil and the magnification, then I can determine the Aperture because aperture = exit pupil x magnification.  But I cannot determine the focal ratio of the telescope or the focal lengths of the eyepiece and the telescope.  A 2mm exit pupil at 100x could be a 200mm F/5 telescope with a 10mm eyepiece or a 200mm F/10 telescope with a 20mm eyepiece.  

 

As others have said, it comes down to second order, practical factors.  One of these is aberrations.  Faster focal ratios have more off-axis aberrations.  In a Newtonian, coma (coma is not field curvature) is inversely proportional to the cube of the focal ratio.. In a refractor, field curvature is inversely proportional to the focal length, chromatic aberration is proportional to aperture and inversely proportional to focal ratio.  

 

Beyond that, there are even more practical considerations.  Field of view is inversely proportional to the focal length, so shorter focal lengths mean a wider possible field of view.  For a given aperture, a faster scope has a shorter focal length so it can provide a wider field of view than a slower scope of the same aperture.

 

And of course the focal length or the objective or primary mirror translates into the length of the telescope translates into the length/height of the telescope.  A 25 inch F/3 will have far more coma that a 25 inch F/5 but coma can be corrected and the 25 inch F/5 focuser will be 10 feet off the ground at the zenith while the 25 inch F/3 will be a comfortable 6 feet.

 

Jon 


  • Dave Mitsky, peleuba, harbinjer and 1 other like this

#14 aatdalton

aatdalton

    Vostok 1

  • *****
  • topic starter
  • Posts: 143
  • Joined: 05 Feb 2019

Posted 09 October 2019 - 06:25 PM

Thanks for all the replies everyone. I think the key I was missing or didn't completely place value on was f/ and exit pupil. More and more I've been planning my eyepieces around exit pupil, especially as I want to get into sketching DSOs.

Seems like it generally gives a good hint at physical dimensions as well, especially in refractors. Not so much for SCT, MCT, or RC without more info.

#15 csrlice12

csrlice12

    ISS

  • *****
  • Posts: 24733
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 09 October 2019 - 10:03 PM

It tells me I can't use my 2.5mm in my f10 scope.  The seeing usually tells me I can't use a 5mm either.


Edited by csrlice12, 09 October 2019 - 10:04 PM.

  • gnowellsct likes this

#16 sg6

sg6

    Fly Me to the Moon

  • *****
  • Posts: 5911
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 10 October 2019 - 03:19 AM

It says much the same as a camera lens - how bright the final prime image should be. In itself specifically you can extract little else.

 

Try the question: I have the chance of an f/6 scope, how good will it be?

Really you can say little, other then ask  a whole load of questions.

 

To me it conveys the likely characteristics of the scope.

An f/6 achro will show a degree of CA, an ED almost no CA but likey a little and an apo triplet I would expect none.

As the ratio gets slower then the requitements on the glass is less demanding and so things "improve".

 

Diameter is as much or more relevant as the surfaces are spherical and so non-ideal, a big f/6 will be "poor" compared to a medium/small f/6.

 

It is useful, but it is in a way just another factor to be considered.



#17 Tony Flanders

Tony Flanders

    Hubble

  • *****
  • Posts: 17035
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 11 October 2019 - 04:45 AM

Thanks for all the replies everyone. I think the key I was missing or didn't completely place value on was f/ and exit pupil. More and more I've been planning my eyepieces around exit pupil, especially as I want to get into sketching DSOs.

Exactly. The exit-pupil rule of thumb (0.5 to 7 mm) translates into this with respect to eyepieces:
 
The shortest focal length likely to be useful often with an f/x scope is x/2, yielding an 0.5-mm exit pupil.
 
The longest focal length likely to be deliver the full aperture to your eye is 7*x, delivering a 7-mm exit pupil.
 
Most of my scopes are around f/5, so my eyepiece set ranges from about 2.5 to 35 mm. That's equally true for my 80-mm refractor and my 317-mm Dob.
 
When the focal length gets really long, like f/10, the ideal wide-field eyepiece (70-mm focal length) is likely not to exist due to the huge physical size that would be needed to implement it.

 

To my mind, the only other really general thing you can glean from focal ratio is that for achromatic refractors, it tells you pretty clearly how much false color you will experience. In general, expect lots of false color with any achromat f/6 or faster. Though this also depends on aperture.


  • Dave Mitsky likes this

#18 desertstars

desertstars

    ISS

  • *****
  • Posts: 44739
  • Joined: 05 Nov 2003
  • Loc: Tucson, AZ

Posted 11 October 2019 - 07:42 AM

barbarosa, on 09 Oct 2019 - 12:08 PM, said:

Field of view comes to mind and image scale likewise. The calculators at this site can be used to show the effect on an image of changing the aperture, the focal length or both. You can pick a scope from the list or create a custom scope to test the changes.

 

I just tried this site with a C - 8 and 10mm ep = 203x on Jupiter. It looked like the size of an eraser head!

Tried that with an 8" f/5 Newtonian and an 8mm eyepiece (the last combination I used on Jupiter) and saw pretty much the same thing. Except that Carte du Ciel apparently has better seeing conditions than I ever experience. wink.gif

 

Which is to say, it looks to me like an otherwise accurate representation of the result. That FOV calculator would be a good thing to recommend to new telescope owners so they are better prepared for what they'll actually see at first light.



#19 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 78967
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 October 2019 - 07:55 AM

I just tried this site with a C - 8 and 10mm ep = 203x on Jupiter. It looked like the size of an eraser head!

 

Jupiter is small, about 40 arc-sseconds. Magnify it 200 times, it's about 2.2 degrees in diameter.  Compare that to the 50 degree Afov of a Plossl.. 

 

Jon


  • REC likes this

#20 Karl Fabian

Karl Fabian

    Messenger

  • *****
  • Posts: 488
  • Joined: 02 Nov 2011
  • Loc: Illinois

Posted 11 October 2019 - 09:10 AM

To be clear, image brightness is ultimately determined by telescope aperture regardless of the f ratio. For example, a 6" f10 telescope operating at 60x with a 50 degree 1 inch focal length eyepiece will provide the same brightness and field of view as a 6 inch f5 at 60x with a 50 degree 1/2" eyepiece. For stars which are essentially point sources aperture alone is the determining factor. In fact low powers can limit star visibility by brightening the skyglow reducing contrast.



#21 csrlice12

csrlice12

    ISS

  • *****
  • Posts: 24733
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 11 October 2019 - 10:04 AM

All scopes are the exact same till you remove the caps.


  • Dave Mitsky likes this

#22 GilATM

GilATM

    Messenger

  • *****
  • Posts: 456
  • Joined: 04 Aug 2007
  • Loc: Ventura, CA

Posted 13 October 2019 - 10:23 PM

These are the areas that come to mind, and most have already been mentioned.

 

1.  exit pupil calculations and their relevant observing issues, (as mentioned in good detail above).

2.  increases chromatic aberration in refractors that is increasingly (and more expensively) controlled as f ratio becomes smaller.  

3.  increases coma in reflectors as f ratio becomes smaller.

4.  is increasingly difficult and more expensive to create accurate parabolic mirrors as f ratio becomes smaller.

5.  smaller f ratios require increasingly larger secondary mirrors in Newtonians, increasing the cost of the secondary and decreasing contrast in extended objects (such as planets.)

6.  more complicated eyepieces are needed with shorter f ratios.

7.  longer tubes, of course, with larger f ratios with the same objective size, which can become unwieldy for both refractors (taller mounts) and reflectors (may need a ladder).

8.  in refractors short focal ratios can create vignetting issues with standard sized focusers if extra care isn't taken.

9.  long f ratios can make low magnifications and consequently larger fields of view difficult to achieve.

 

(There is reason for the old time standard 6" f8 Newtonian:   It worked well for many objects without much trouble.)

 

 

That said, all telescopes are miraculous!  

 

Gil


  • Freezout and aatdalton like this

#23 gnowellsct

gnowellsct

    Hubble

  • *****
  • Posts: 15732
  • Joined: 24 Jun 2009

Posted 13 October 2019 - 10:36 PM

It tells me I can't use my 2.5mm in my f10 scope.  The seeing usually tells me I can't use a 5mm either.

Yes indeed.  Because at f/10 a 10mm gives you a 1 mm exit pupil.  And it would be very rare to get conditions permitting as little as a .5 mm exit pupil, which would be a 5 mm eyepiece.  

 

The focal ratio is also an important variable in assessing your h-alpha Daystar Quark.  You really want to be f/7 to f/8 for optimal results with the 2x and 4x power mates that are recommended.   

 

It's hard looking at Saturn or Jupiter in a 2.5mm or shorter eyepiece.  But that's what you're going to end up doing with an f/6.3 or faster apo. 

 

Greg N



#24 gnowellsct

gnowellsct

    Hubble

  • *****
  • Posts: 15732
  • Joined: 24 Jun 2009

Posted 13 October 2019 - 10:37 PM

On the other hand, getting that f/6.3 apo finally gave me a solid use for the XO 2.5 I had acquired in a moment of ocular greed.   "Well using tiny glass isn't very satisfying, but thank goodness I have this here XO 2.5 with its generous eye relief."  Greg N



#25 aatdalton

aatdalton

    Vostok 1

  • *****
  • topic starter
  • Posts: 143
  • Joined: 05 Feb 2019

Posted 13 October 2019 - 11:33 PM

These are the areas that come to mind, and most have already been mentioned.

 

1.  exit pupil calculations and their relevant observing issues, (as mentioned in good detail above).

2.  increases chromatic aberration in refractors that is increasingly (and more expensively) controlled as f ratio becomes smaller.  

3.  increases coma in reflectors as f ratio becomes smaller.

4.  is increasingly difficult and more expensive to create accurate parabolic mirrors as f ratio becomes smaller.

5.  smaller f ratios require increasingly larger secondary mirrors in Newtonians, increasing the cost of the secondary and decreasing contrast in extended objects (such as planets.)

6.  more complicated eyepieces are needed with shorter f ratios.

7.  longer tubes, of course, with larger f ratios with the same objective size, which can become unwieldy for both refractors (taller mounts) and reflectors (may need a ladder).

8.  in refractors short focal ratios can create vignetting issues with standard sized focusers if extra care isn't taken.

9.  long f ratios can make low magnifications and consequently larger fields of view difficult to achieve.

 

(There is reason for the old time standard 6" f8 Newtonian:   It worked well for many objects without much trouble.)

 

 

That said, all telescopes are miraculous!  

 

Gil

Great summary. 




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics