Focal Ratio

[markuss1895]

Good Night Yall 

Got a couple questions

They say focal ratio is the speed of the telescope.

Having some difficulty understanding what they mean by the "speed"

Is there an ideal focal ratio for a NOOB telescope ?

All I know is that the focal ration affects zoom and the sharpness of the image. 

Is there an ideal focal ratio for a NOOB telescope that will allow me to see the moon in detail?

.http://www.refractingtelescopeworld.com

[Knighty2112]

Fast and slow are just terms brought into the mix from photography so for visual astronomy the terms fast and slow are redundant, but for astrophotographers it is important as a shorter focal ratio will capture an image quicker, or faster than a longer focal ratio scope which will capture the image slower. Fast telescopes are used by astrophotographers for wide views of deep sky objects like nebula or galaxies, whereas a slow telescope is better suited to planetary or moon images. If you don't plan on doing any astrophotography then it is not generally something to worry about.

What is more important about the focal length of a telescope is that the longer the focal length of the telescope the more powerful the scope will be. The shorter the length of the scope and it will have less magnification. A 20mm eyepiece in an f5 telescope will give a wider field of view and less magnification than the same 20mm eyepiece in an f10 telescope. Again, longer scopes will give higher magnifications of the moon and the planets, whereas shorter focal lengths give wider views of the sky more suited for dimmer objects such as nebula and galaxies. In short there is no ideal focal length for a NOOB telescope. The only general rule is get as large a diameter scope as you can afford so that it will gather more light so you can have brighter views of objects, especially those elusive nebula and galaxies.

[Alan64]

If you desire a good balance between higher-power observing within the solar system and wider, lower-power deep space observations, then an 8" f/5 or f/6 Newtonian will come closest to observing both; the gamut; a little of this and a little of that; and whilst favouring neither, especially in the case of an 8" f/6 Dobsonian.  I personally chose an 8" f/5 Newtonian as my "light bucket", and to lean slightly more towards wide-field deep-sky observations.

The focal ratio of a telescope, whether "slow" or "fast", most certainly has everything to do with observing, and through a static eyepiece.  Consider...

An 8" f/10 Schmidt-Cassegrain has a focal length of 2032mm, whilst an 8" f/5 Newtonian has a focal length of only 1000mm.  Insert a 15mm into either and you get magnifications of...

8" f/10: 135x, and ideal for lunar and planetary observations.  A 32mm ocular would result in a magnification of 64x; a bit high-powered for wide-field deep-sky.

8" f/5: 67x, and suited for general purpose observations of both the solar system and deep space.  A 32mm would offer 31x; slightly high in my opinion, but still an ideal per its 8" aperture for deep-sky, and for scanning the star-fields of the Milky Way even.  If you want higher power for viewing the shadow-transits of Jupiter's moons across the face of the planet, or a close-up of a lunar crater, then combine a 2x barlow with a 9mm eyepiece, and for a simulated 4.5mm: 222x. 

Hence, the overall versatility of an f/5.

[jambouk]

Focal ratio does not determine either the dimensions of the field of view, nor the magnification any given eye piece results in. Focal length is the factor which needs to be considered when determing these two.

James

[Alan64]

Focal ratio is nonetheless a determinant among like-apertured telescopes.

[jambouk]

If your interests are observing rather than astrophotography I would ignore the speed of the optics when considering your viewing pleasure. Go on focal length and the sort of field of views you desire, and aperture, balanced against portability, cool down times, storage, cost, sort of mount you have or can afford,,,

There are some considerations about some types of fast reflectors, but on the whole make your choice of scope based on other favtors. Once you have a hit list come back with a list against your astrobomy-plans and I think people will then be able to better advise one way or another.

Good luck, it is an exciting time buying a telescope. Ideally get to look through a few at a star party before making your final choice.

James

[jambouk]

I think it is confusing to use focal ratio when talking about field of view. Field of view is purely [nearly purely] a result of focal length.

My 11" f/10 SCT has a vastly narrower field of view than 5" f/10 SCT. The focal ratios are exactly the same but the focal lengths are worlds apart.

Your examples gives two scopes with the same aperture but with different focal ratios, mine gives two scopes with different apertures with the same focal ratio. So when comparing any telescopes, whether they are "similar" in terms of aperture or focal ratio, the only reliable way to calculate (or predict) their field of view is to look at the focal length.

In my second posting I hope I've suggested to the OP that I would ignore focal ratio when considering a first telescope, unless they are thinking of a £2000 astroimaging set up when it has more impact on decision making. For a first scope for a newbie to the hobby I think there are vastly more important considerations.

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If one wishes to talk about contrast, then focal ratio may be one of the factors to consider, but I would suggest more aperture would be the way forwards rather than worrying about an f/8 vs an f/10 scope for example.

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As with many questions about telescopes and the like, there is no one telescope which does everything brilliantly, and each observer has different desires, different sky conditions, different budgets and different health issues which all impact upon choice of scope.  I stick by my original assertion that for visual observing I would ignore focal ratio.

Good luck with it all markuss

James

[Charic]

For visual observing only, using the Skywatcher Skyliner 200P f/6, I keep it really simple!

F/6 tells my that my scope is 6 times longer that its diameter ( no compact Jones-bird here ? ) and using the focal ratio to determine my high power EP, I chose the 6mm providing a power of 200x, to match the design of the telescope.

Its just a numbers game, how you interpret them , is up to you?

[jambouk]

This is a great website:

http://www.12dstring.me.uk/fov.htm

You may need to click "switch to visual view" which appears in red on the top right area.

Select a scope you are interested in, don't select a barlow or focal reducer, and on the eyepiece just pick something like a Skywatcher Super 20 (a stock eye piece which comes with many Skywatcher scopes). Just click submit to check it has all been registered, then go to objects menu, solar system, moon and you should get a view of how the Moon would look through that scope and eye piece - the phases of the Moon appear here so if you only see a sliver of Moon it is because it is using the correct phase, not because you've got an error. If the field of view calculation just above the box readers less than about 30', you won't be able to fit the whole moon into the FoV, and you'll need to reduce the magnification of the eyepiece you are using (try a 10mm eyepiece) or just change scope.

It's a fun website to play around with, and will hopefully give you a bit more idea about FoV and magnification.

James

[jambouk]

Here is a recent ish thread about focal ratios which may or may not help clarify issues, or just muddy the waters further:

http://stargazerslounge.com/topic/255904-having-difficulty-understanding-f-ratio/?hl=%2Bcontrast+%2Bfocal+%2Bratio#entry2794421

James

[Knighty2112]

Whithout getting into the technicalities and the math used to determine FOV or magnification, as a general rule all my three 70mm refractors (too many I know!) if I use the exact same 20mm eyepiece in each of them give different magnification powers for any of the three focal lengths of these scopes (400, 500 & 700mm), which give diffent sizes of the images of the same objects when I use them. The 400mm scope gives a magnification of x20 the image, the 500mm scope x25 the image and the longest focal length scope at 700mm gives a view at x35 the image.

So in general terms I will get a higher magnification with a longer focal length telescope, over a shorter focal length telescope with any given eyepiece if we use the exact same eyepiece in both telescopes.

Hope that hasn't muddied the waters anymore as I am trying to keep it relatively simple without any of the boring formula or maths used to actually determine these things when looking through any actual telescope.

[Stu]

I guess the point there is that it's the focal length which is determining the magnification. The focal ratio is not relevant to that particular calculation. I think it's been said before but if you have 100mm, 200mm or 400mm aperture scopes, all with say 1600mm focal length, they would have focal ratios of f16, f8 and f4 respectively. Regardless of this they would all give the same magnification with the same eyepiece eg x100 with a 16mm eyepiece.

Where aperture and focal ratio does become relevant is in the exit pupil achieved in each scope. This is the diameter of the beam of light exiting the eyepiece and entering your eye. The quickest way to calculate this is to divide the focal length of the eyepiece by the focal ratio of the scope. In our three example scopes this would give 1mm, 2mm and 4mm respectively. This is related to the brightness of image you see, so the larger aperture scope is able to deliver the same image scale at higher brightness, or the same brightness at larger image scale.

There are various other effects of faster scopes, not always good. The steeper light cone tends to mean that the focus point is more critical and that can make finding focus more fiddly, especially when seeing is poor. Faster scopes are more sensitive to accurate collimation, and finally they also require more expensive eyepieces (if using widefield EPs), again because the steeper light cone requires more complex designs to control the light correctly. Regardless of the expense of eyepiece used, fast scopes (below about f4.5 or perhaps f4) definitely benefit from a coma corrector to avoid stars appearing as little comets towards the edge of the field.

No simple answers necessarily, but two things to finish with. Firstly 'speed' of scope is irrelevant for visual observing, and secondly I tend to agree that somewhere around f6 is a good place to start as a new observer. You avoid many of the issues I've described, plus the tube length remains quite manageable.

[laudropb]

I totally agree with the last post. Beginners not looking to do AP are better to avoid fast scopes and the inherent collimation, focusing and choice of eye pieces they bring.

[YKSE]

Excellent explanations, Stu  !

[Moonshane]

Personally, I feel that the choice of scope should really depend of a number of factors other than focal ratio. Assuming visual astronomy, far more important in my experience are the weight of the telescope, the types of objects you want to observe and viewing comfort. Whilst yes, the speed of a scope will affect the views (especially the edges of field) through less expensive eyepiece options, you will still be able to see everything whatever eyepiece you use and whatever scope you use assuming it fits in the field.

If the scope is too heavy to be bothered with or you need to do yoga/gymnastics every time you use it then it will soon get boring. Also, if the objects you want to see are either too dim for the aperture or to large for the maximum field of the scope, or need night tight airy disks then it will soon get frustrating.

So choose a scope that will be a good all rounder and approximately do what you think you want currently. A good example of this is a 200mm f6 dobsonian. It will have a decent field. a decent aperture and is also reasonably light with great optics.

I have to say that no single scope will provide everything you need to see all objects although the scope mentioned comes close. Eventually you will either buy an additional scope that complements the first purchase or buy a scope that is more specialised. I have six telescopes which do more or less cover all avenues of astronomy (two of which are solar scopes). I could probably get away with three though. Large aperture dob, 6" slow dob and small wide field refractor.

[Moonshane]

One slight advantage of slower scopes is that in my experience the apparent depth of focus seems higher with them compared with fast scopes - i.e. the depth of field is more with slower than faster scopes. this can help get focus quickly at higher magnifications with slower scopes.

[Alan64]

On this first page alone... http://agenaastro.com/telescopes/refractor-telescopes.html

...most every telescope has its focal-ratio listed within its respective thumbnail.  One has to dig deeper, and into the specifications to find the focal-length.  When beginners and "newbies" begin searching for a telescope, it's the focal-ratio that they see advertised, hence the title of this thread.  Apertures that are considered by those first entering into our hallowed pasttime range from 60mm to 8"(200mm).  Within the subset ranging from 4"(102mm) to 8"(200mm), knowing the focal-ratio alone is quite helpful in determining that which is capable of low-to-medium magnifications for deep-sky, and that which is capable of medium-to-high magnifications for lunar and planetary observations, and at a glance as one flits from one page, from one site, to another.  However, it is recommended that beginners delve into the specifications once they zero in on a telescope or telescopes of keener interest, and for the focal-length in determining if the telescope would be best suited for the planned observing agenda.

If one wants the odd look at the Moon or Saturn and, hopefully, at a not-quite-so-twinkling star, then a 4" f/10-f/15 would serve, and capably under light-polluted skies.  Conversely, under much darker Texan skies, a 6" or 8" f/5-f/6 might be the ticket, and for observing beyond the solar system into deep space, along with satisfactory views of the solar system in addition.  

Some observe double and variable stars, exclusively, and may favour a 3" or 4" f/16 refractor.  Others desire only to observe the Moon and the planets, and then might choose a 4" or 5" f/9-f/12 refractor.  Still another prefers observing only galaxies, nothing more, and picks a 12" f/4-f/5 Newtonian for the task.

Then, to complicate matters, there's the budget.  Beginners are generally reluctant or unable to sink a lot of cash into a first telescope, but they want their money's worth, and yes, "the best bang for the buck", hence...

"May I interest you in an 8" f/6 Dobsonian, sir?"

[Charic]

"May I interest you in an 8" f/6 Dobsonian, sir?"

I've not yet had the pleasure of an astro club visit to try any other scope/eyepiece combination, although I did try to attend the Regents Park  venue in July, and with only the  Celestron 127EQ to compare, I will always recommend an  8" -  200P as a reliable and easy  to use, first time/life time telescope.

My eyepieces are  not the most expensive in the World either, in fact quite the opposite,  ranging from £9 to £66,  this system works well  for my uncorrected eyes?

The system gets better when at a darker site, although that's been so long, I've almost forgotten the feeling.

markuss1895 asks is there an ideal?. f/6 is just fine.  With a 200P 8" telescope and a 6mm eyepiece or smaller, the Moon will totally amaze you. I have to keep up with the Moon, because at this magnification, with Planet Earth  cruising around at just over 1000 mph? it takes a little practice to maintain track on the smaller rocks/hills in the valleys?  But when the conditions are right,  and looking along the terminator ( the line between dark/light) it feels like I'm flying through the Mountain peaks, its really that good. I just wish I could see more detail of the surface structures on Jupiter (my one and only issue with my system!) the detail is there, just  too small for my eyes! which means I have to stare and study for much longer, whereas a bigger aperture would gather more light, showing more detail.  But as Jupiter is so far away, and the Moon is so close, one scope can't do it all, but  this one does very well as it stands, or sits, as its on a Dobsonian