1. As stated - focal ratio has not a thing to do with astro exposure time. It has everything to do with the TFOV. Photons are photons, and an electron volt is an electron volt.

A photon vs. well depth and energy - all that DOES NOT CHANGE - what does change is relative scale of image to pixel size... There is no possible way a photon somehow is 'more better' at one f ratio over another for A-P - they are boringly similar. To state that a fast ratio scope can image 'faster' than a low f ratio scope is just plain wrong. Imaging is about the pixel size relative to the f-ratio, it's a gaussion curve and efficiency depends on a LOT of things, not the least of which is the luminosity and desired resolution of the imaging target.

What becomes difficult at faster f ratios is the critical focus plane (and if using mirrors the collimation) and the dual speed focusers are very important on scope under f/6. What becomes difficult is also controlling field curvature in the focal plane at prime focus.. What becomes better is the maximum 'Total Field of View' (TFOV)for the given aperture. Sorry if it get's complicated but this also affected by choice of eyepiece FOV, or imaging chip and it's pixelsize/count.

Consider a focal ratio of f/7-8 as a sort of tipping point, or low center on a sort of pendulum function.

As the focal ratio gets smaller/shorter(lower F number) the 'potential for field curvature' increases (think of it as stressing the optic more per mm away from dead center) and the TFOV of the telescope optic increases.

As the focal ratio gets higher/longer(higher F number) the potential for field curvature gets smaller and the corresponding TFOV of the telescope optic decreases.

As a generality, longer focal ratios are more difficult to guide for imaging. This has a lot to do with the pixel size and 'arc second seeing' where you are, but also the greatly exaggerated relative scale definition of what is in the plane at prime focus.

From the point of projective geometry - think of it as a mapping function. A large TFOV will contain more 'information' at a lower resolution than a same aperture at a much longer (higher) focal ratio - which will have less TFOV but contain more resolution within that field. Fast(low f ratio number) a field corrected with a Field Flattener. High focal ratio scopes are field corrected (widened) with a Focal Reducer.

A notable exception to all this is the Petzval refractor desigh as seen in the TeleVue, Vixen, and Takahashi models. A wide flat TFOV is something some folks are willing to pay extra for? Edge HD and Tak FSQ, TV Paracor and Baader MPCC. There are others that try, but with added complexity 'can' come added difficulty in other ares.

Another consideration is in the visual use - eyepieces/magnification and exit pupil for a given aperture and object under scrutiny.. this a little more complicated by the fact that adding aperture cm's or inches and holding the f ratio constant will allow for a larger exit pupil at a desired size. Think of it as 'light cones' one extending from the telescope to space, and the other going from the eyepiece to your eye.

For visual astro think of it as the focal ratio tells you what the maximum TFOV of the optic is AND what eyepiece sizes will be practical for both magnfication and exit pupil. Design specific it tells you if a MPCC, ParaCORR, Focal Reducer or Field Flattener will impact the experience.

For astro imaging it tells you what the TFOV of the optic is and in turn what the arcsecond-seeing formula dictates as an optimal window for the pixel size of the CCD. Also if a Focal Reducer or Field Flattener will give advantage of flexibility - or mandatory.

I think focal ratio a more important number/concept to understand than the obvious aperture/light gathering often touted as king - and a great question to ask. It's all about light bending, but not about a photon somehow going faster or slower. Understanding focal ratio is fundamental to the overall experience for visual or imaging - and can help you buy 'smarter'.
Last edited by klaatu2u; 11-05-2010 at 06:24 AM.

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3. Originally Posted by Gappa
I have read about the science and definition about f#'s but what is better a lower or higher f#?

Neither is better for viewing the skies at night. Telescopes with higher focal ratios will tend to be a little better for viewing planets and the moon as well as deep space objects requiring magnification while telescopes with lower focal ratios will tend to be a little better for viewing deep space objects that cover a large area of space. However, you can use either for viewing everything.

First consider the aperture (diameter of the large objective lens on a refractor or largest mirror on a reflector). For equal apertures, the light gathering capability of a telescope is the same. For astronomical telescopes all the focal ratio does is give someone a vague ideal about the performance of the telescope. For two telescopes with equal aperture using identical eyepieces, the telescope with the higher focal ratio will show a larger and slightly dimmer image than the telescope with the lower focal ratio; on the other hand the telescope with the lower focal ratio will show a smaller but slightly brighter image than the telescope with the higher focal ratio.

The thing that is often overlooked is that by selective use of eyepieces the images in either telescope are identical. Here is a graph for three telescopes each having an aperture of 130 mm. One telescope has a focal ratio of f/10, the second has a focal ratio of f/6.9, and the third telescope has a focal ratio of f/5. The graph shows the magnification (image size) of each telescope vs eyepiece focal length. As you can see the performance of all three telescope overlaps considerably. The upper magnification of a telescope is generally limited by atmospherics while the lower magnification is limited by the diameter of your eye's pupil.

What does this all mean? Not much for a telescope used for visual viewing. For visual viewing a telescope with a lower focal ratio will offer somewhat better views of objects that cover a wide area of the sky and the telescope with the higher focal ratio will offer a better view for objects that require magnification. A telescope with the higher focal ratio will also be less sensitive to eyepiece quality and refractors with higher focal ratios will tend to have less color aberration.

The two most popular types of telescopes today are the Schmidt Cassegrain Telescope (SCT) and a Newtonian reflector on a Dobson mount (DOB).

DOBs generally have a focal ratio of f/5 and are used for viewing only. The use of a focal ratio of f/5 is not done for any optical advantages, in fact there is a host of disadvantages, but for a simply practical reason. The tube length of a Newtonian telescope starts becoming very long when apertures start exceeding 150 mm (6 inches). An f/8 150 mm Newtonian has a tube length of about 1200 mm (about 4 feet) and a 250 mm (12 inch) f/8 Newtonian has a tube length of 2400 mm (about 8 feet). The eyepiece on a Newtonian is at the end pointing toward the stars so a DOB with a reasonable aperture and a high focal ratio required the use of a ladder in order to view through the telescope. By using a focal ratio of f/5 the tube length is shorter making use of a DOB far more practical. However, even today, DOBs much bigger than around 12 inches in aperture require a ladder for viewing.

All mass produced SCTs today have a focal ratio of f/10. The design of a SCT produces very short tubes. A f/10, 200 mm SCTs will have a tube length of only 400 mm (16 inches) quite a contrast to a f/10 Newtonian or refractor that have tube lengths of 2000 mm (about 80 inches). SCTs are used by people who want a compact telescope that is suitable for both viewing and/or for photography.

Refractors are found in two versions and are becoming increasingly popular as manufacturing costs decline. Typically refractors have a high focal ratio that produces tube lengths similar to that of Newtonian telescopes. However unlike the Newtonian, the refractor has its eyepiece at the end away from the night sky, thus, far more accessible for viewing. Its long tube length requires a higher tripod than a similar SCT. Refractor size is essentially limited by aperture as manufacturing a refractor's objective lens larger than 6 inches becomes very expensive and the telescope becomes very heavy and bulky requiring a very expensive mount. However, recent advances in manufacturing technology has made economical refractors available having lower focal ratios which has reduced their length thus increasing their portability. Refractors are preferred by a very sizable number of astrophotographers and the new short tube refractors with low focal ratios are becoming popular as grab and go telescopes.

Does focal ratio make a difference in photography? Yes. Unlike visual viewing where focal ratio has little impact, for photography the focal ratio is an important parameter, but not the only one. A telescope with a high focal ratio will be called a slow scope and one with a low focal ratio will be called a fast scope.

For two telescopes, each with the same aperture, a f/10 telescope will need twice the exposure time as needed by a f/5 telescope. Digital photography has lessened the impact of this characteristic quite a bit but has not eliminated it. However, many more factors are involved. Many low focal ratio telescopes have difficulty with photography because of the lack of sufficient back focus needed for cameras, etc. This is one reason why refractors and SCTs with higher focal ratios are most often used for photography.

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5. Sxinias - you might want to take a look at this and consider you information about imaging?

6. After 50 + years in this hobby (all as a visual observer0 I look at a scoped focal ratio for a couple of reasons...

1. the lower the number the wider the FOV will be.
2. The wider the FOV the better my eyepieces will have to be
3. The higher the number the more magnification any eyepiece will deliver
4. The higher the number the less critical the eyepiece becomes...

So... Open clusters.... well I need a wide FOV so I'll take a low Focal Ratio since I will be using low magnification anyway.

On the moon and planets I could care less about the FOV but I want as much magnification as possible so I want a scope with a higher Focal ratio ..

Those who cross over to the dark side and image have their own needs, LOL

Bob G

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8. I have a related question regarding f-ratio:
Background: I have designed a simple spectroscope that consists of a very small slit (3mm x 50um) followed by a diffraction grating. This works better than a diffraction grating alone because the slit helps spread the spectra into readable divisions. However, the small slit also makes the spectroscope light deprived, so I want to use the device with a telesope that returns as much light as possible.
Question: Would there be any difference for my purposes between a telescope with an aperture of 120mm and a smaller 80mm scope if they both have the same f-ratio? Yes, I understand that the larger scope collects more light but wouldn't it be the same on a unit basis for both telescopes given that the slit is the same size and the f-ratio is the same for both? It seems to me that the amount of light entering that slit is going to be the same with either scope. Is there an error in this logic?

9. Originally Posted by klaatu2u
As stated - focal ratio has not a thing to do with astro exposure time. It has everything to do with the TFOV. Photons are photons, and an electron volt is an electron volt.
I hate to disagree with you, since you're a much more accomplished imager than I am, but I don't think that as this is worded is correct.

The image scale at the focal plane is determined by the focal length, and doesn't directly depend on focal ratio. Image scale is just 1/FL (in radians/m, or whatever units you use for FL). Aperture (and therefore focal ratio) will determine how steep the light cone is and how many photons there are, but doesn't affect the FOV on the sensor.

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11. Originally Posted by illuminite
I have a related question regarding f-ratio:
Background: I have designed a simple spectroscope that consists of a very small slit (3mm x 50um) followed by a diffraction grating. This works better than a diffraction grating alone because the slit helps spread the spectra into readable divisions. However, the small slit also makes the spectroscope light deprived, so I want to use the device with a telesope that returns as much light as possible.
Question: Would there be any difference for my purposes between a telescope with an aperture of 120mm and a smaller 80mm scope if they both have the same f-ratio? Yes, I understand that the larger scope collects more light but wouldn't it be the same on a unit basis for both telescopes given that the slit is the same size and the f-ratio is the same for both? It seems to me that the amount of light entering that slit is going to be the same with either scope. Is there an error in this logic?
I believe it depends on where the slit is ..If the slit is in front of the primary mirror or lens then yes the light gathering ability is what the slit will allow to pass weather it be 80mm or 120mm , but if the slit is in back of the scope behind the eyepiece where it can project the light on to the slit then no .. A larger mirror or lens gathers more light ...But if you mask that with a slit it can gather but so much ..

12. Rich,
Thanks for your response. The equipment order is Telescope-slit-diffraction grating-DSLR camera. No question that the larger aperture equates to more light collection, but if the two scopes have the same f-ratio, then I think the amount of light passing through that slit is going to be the same... but I am not sure and that is my question. With the same f-ratio, are the same number of photons reaching that open slit that is the same size in both cases?

13. Originally Posted by Gappa
I have read about the science and definition about f#'s but what is better a lower or higher f#?
Much depends on what you want to use it for. If you are thinking about deep sky, wide-field astrophotography you will probably want to look at f/7 or faster scopes (preferably apochromatic for the color correction) due to their ability to take images more quickly. Faster f ratios are also desirable if you want to do lots of wide field viewing as well. If you are interested in lunar/planetary and/or double star observing; a longer f ratio will allow higher magnification (but that has its practical limits!), often with lower power eyepieces which tend to provide better eye relief (and are less fatiguing to use). Should you be looking at 4" to 6" achromatic refractors you may want go for the longer focal lengths...and I mean over f/12 to f/14 on up if you want to fight chromatic aberration (the infamous purple fringing), these scopes can provide amazing depth of field and terrific views. So think about what you are likely to be using the scope for, and keep doing your home work. If you are looking for a great jack-of-all-trades OTA you might want to consider either the 100ED f/9 or 120ED f/7.5 refractors with Synta made optics like the Skywatcher Pro/Black Diamond models...these tubes are brilliant performers and some of the best value for money in amateur astronomy today. I hope this helps...

Clear Skies, Brian

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