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DeejayP999

Why is "Fast" better for imaging?

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Faster focal ratios = shorter exposure times.

Lagoon Nebula in 50 seconds with a 12" F2.9 Newtonian

DSIR9690_1024_silly_zps54de9507.jpg

Edited by laser_jock99
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A "fast" scope is one with a lower "F" ratio. I.e. one with a short focal length compared to its aperture. There is no standard measure of what constitutes "fast".

The reason that some imagers favour fast scopes is that you get more light from the target object hitting each individual pixel in your camera in a given time. That allows for shorter exposure times to reach an arbitrary level of "brightness" compared to the inevitable background noise that all images contain.

The downsides are that the target image is smaller and therefore the amount of detail is less, compared to the size of the target in a "slower" (longer focal length / higher magnification) telescope. So if you used the same camera in a 100mm aperture telescope with a 500mm focal length as you did with a 100mm aperture and 1000mm telescope, the first image would show the target object as being both brighter and smaller (by a factor of 2 in the horizontal AND the vertical - so 4 times smaller in total)  than in the 1000mm focal length scope's image. The smaller image would also have more stuff in the field of view, around the object you were imaging - this can be retained or the final image can be cropped to remove it, depending on choice,

A second downside is that the telescope with the lower F-ratio will be more difficult to focus, since the range of focuser movement that results in a sharp image will be smaller. The faster telescope will also be more liable to losing focus due to temperature fluctuations during the imaging session.

When you get to very fast telescopes, you also find that certain types of narrow band filters don't work so well. This is because those filters are designed to work with a "light cone" (the light getting narrower as it nears focus, from the size of the objective lens / mirror to the camera) that narrows gradually - rather than quickly.

Edited by pete_l
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Hi DeejayP999,

In my opinion in order for your question to be more "proper" technically it has to be split down to two  :

1. Is "fast" imaging better ?

2. Is a "fast" scope better ?

Now my answers to these two two questions is in my opinion the following :

1. One simple way of "translating" <<fast>> imaging is to accomplish the required result (proper photon acquisition) in the less possible time. Weather conditions, available night hours and available/proper target positions are only a few of the reasons why would all prefer to end an imaging session as quickly as possible. So obviously if you "translate" the <<fast>> this way the answer is of course is better. 

2. Now if we translate technically the <<fast>> that means low focal length optics (for example f/2 to f/7)  in which  the light (photons) follow a faster "route" until they hit our sensor  in comparison with higher focal lengths (f/9-11 and beyond). Because telescopes in essence are optical instruments they follow this rule. So low focal length optics are "faster" telescopes while higher focal length optics are "slow" telescopes. This difference does not affect the quality of the data acquired from the telescopes (for example: final image) , it does mainly affect the time needed for completion of your exposures and the FOV (Field Of View) of your final "image".  So as you can understand there is no "better" telescope comparison between fast and slow ones which also have different design because fast telescopes are used for wide field AP and produce lower magnification while slow ones used for narrow field AP and produce bigger mags thus mainly the different design. For example to view or to make a photo of a wide nebula or galaxy you need a wide field designed fast scope (for example : refractors/newtonians) while for  planets and other "small" targets who need higher magnifications you need "lower" focal length scopes designed for that reason (for example: SCT's).

Conclusion of my answer/opinion : "Faster" is obviously better only when we compare optics/telescopes which are in the same category and used for the same purpose. 

Clear skies. 

Edited by seo

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With "fast" scope you DON'T need high accuracy of tracking, so a relatively cheaper mount can be used.

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2. Now if we translate technically the <<fast>> that means low focal length optics

My 12" F2.9 Newtonian is nearly 900mm focal length!!

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My 12" F2.9 Newtonian is nearly 900mm focal length!!

I didn't want to get into "specifics" and make more subcategories which would involve more designs and more sub-focal length categories (2-4, 4-6, 6-8 etc.) so I used a more "wide" categorization just to give a general picture of difference. My SW BD-120ED is also at 900mm focal length at f/7.5 but compared with my EdgeHD 14" which is at 3910mm is a "fast" and wide field telescope.

Clear skies. 

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Also if I wanted to be more "specific" and proper in favor of terminology then I'd have to mention the "focal ratio" which is exactly what characterize a telescope more fast or more slow in comparison with some other one
but then I'd have to explain about that too and this would lead to a "big" answer which I personally think wasn't necessary for the time. :laugh:

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To summarize the previous posts: faster is better because it allows you to gather signal faster, meaning you'll capture the desired amount of information in less time. Or put differently: when compared to exposing for the same amount of time through a slower scope, the faster scope will capture fainter details. But everything comes at a price and in this case the first thing is, well, price. Faster optics require more extreme curves so they're harder = more expensive to make. They're also less tolerant to collimation errors so more attention should be paid to that. And then there's focal length. If you're imaging something small like planetary nebulae you're going to need long focal lengths and at fast focal ratios you're going to end up with large apertures quite quickly: a 1000mm focal length scope at f/4 means 10 inch aperture, at f/2.8 you're already at 14 inch. There are practical considerations as well. Because of the "fat" light cone you're going to need large filters to avoid vignetting. The filters also need to be suitable for fast focal ratios. In a slow scope the light will pass through the filter at a fairly steep angle (closer to perpendicular) while in faster scope the angle will get shallower. It's like you're placing the filter in the light path at an angle and this can shift the spectral response of the filter with narrowband filters being the most sensitive to this.

So in conclusion a fast scope is definitely not for everyone and the trade offs have to be considered carefully.

Jarno

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This question easily generates a lot of misinformation, some of which is to be found above, or so I believe.

In my opinion we need to decide, first of all, on what picture we are going to take. This, for a given camera chip size, is governed by the focal length of the scope. If we start varying both focal length and focal ratio we will soon lose sight of which variable is responsible for which effect.

If we decide on a focal length first, and don't change it, then it is incredibly easy to see why a fast scope is fast. If the focal length is 1000mm and the aperture is 100mm then the F ratio is F10. That's slow. If the focal length is 1000mm and the aperture is 200mm then the F ratio is F5 which is fairly fast.

Why is it faster? For the perfectly simple reason that the area of  of a 200mm objective is four times larger than the area of a 100mm objective so it captures four times as much light. It really is that simple.

Why all the confusion? It is introduced by focal reducers, because focal reducers speed up the F ratio but do not increase the aperture and, in the simple explanation above, the whole point about increased speed is that it comes from increased aperture at a given focal length. A focal reducer takes a different picture, one with a wider field of view from a shorter focal length. Use a focal reducer to do that, by all means, if you want that wider field of view. But if your object will fit on the chip unreduced there is no point in using a focal reducer because you will get exactly the same amount of light from the small object as you got before. The object may look 'better but smaller' but it would also look 'better but smaller' if you just downsized the image taken without reducer. This is the highly contentious 'F ratio myth.'  If you reduce F ratio by increasing aperture then there is no myth. The myth appears when you reduce F ratio without increasing aperture.

Guiding? The accuracy of guiding required depends on the number of arcseconds per pixel at which you are imaging. The numbers that matter here are focal length and pixel size. F ratio has nothing to do with either, though it can be a minor factor. You will get more light in less time at a fast F ratio, sure, and that will make guiding easier if you have a certain kind guiding problem. But a single rotation of the mount's wheel typically takes about 8 minutes. Most autoguiding issues show up at much, much shorter time scales so cutting an exposure down from 8 minutes to four minutes will probably have little benefit. It would help unguided imagers but probably not help those working with an autoguider. If PA is bad then shorter subs also help. With a given camera a shorter FL images at a larger number of arcseconds per pixel and is, therefore, more tolerant of guiding error.

Nutshell: shedloads of confusion arise from muddling up both focal length and focal ratio in the same sentence. If you refuse to do this it is all dead simple. (Paragraph three.)

Olly

PS The bad habits of daytime photographers don't help either because, by tradition, they use the terms F ratio and aperture interchangeably. Whilst formally incorrect, this is because, unlike telescope imagers, they acutually do change the aperture of their lenses by opening or reducing the diaphragm which determines the clear aperture. This is never normally done by telescope imagers who want all the aperture they can get. (Users of the insanely fast Hyperstar system sometimes make life easier for themselves by stopping down to a slower F ratio/reduced aperture in the camera lens tradition.)

Edited by ollypenrice
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PS The bad habits of daytime photographers don't help either because, by tradition,

Argh ! I was just about to post a +1 for Olly's excellent analysis and add my own extra on how it was ok for landscsape and portrait photographers, Fox-Talbot, Box Brownies and all that,

and that 'fast' and 'focal ratio' had no useful place in what we are doing.

Then he added his PS and thus ruined the need for this post ! LOL !

Focal ratio is a 'dependent variable', maths & science experiments require as inputs 'independent variables' (aperture and focal length)

Edited by Ptarmigan

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Argh ! I was just about to post a +1 for Olly's excellent analysis and add my own extra on how it was ok for landscsape and portrait photographers, Fox-Talbot, Box Brownies and all that,

and that 'fast' and 'focal ratio' had no useful place in what we are doing.

Then he added his PS and thus ruined the need for this post ! LOL !

Focal ratio is a 'dependent variable', maths & science experiments require as inputs 'independent variables' (aperture and focal length)

The phrase 'dependent variable' is very sweet and encapsulates exactly what is wrong with F ratio as a term. If we are wealthy we choose our scope to fit our target. If we are not wealthy we choose our target to fit our scope. The choice is based upon focal length. Once that is chosen we can, if we are still wealthy :grin: , have a lot of aperture at that focal length and so a fast F ratio.

If we are wanting a focal length of 2.5 metres and we want a fast F ratio we are, alas, going to need to be 'beyond wealthy...'

:embarrassed: lly

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Focal ratio is a 'dependent variable', maths & science experiments require as inputs 'independent variables' (aperture and focal length)

Ummm, I think you mean it's a derived variable :laugh:

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As a non mathematician they sound like the same thing to me. To derive from or to be dependent upon...  Linguistically I'd have thought that either would do...?

Olly

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If we are wealthy

'beyond wealthy...'

Nice one ! Two more variables !!

Dependent - upon a good job or good marriage

Independently wealthy,

me ? I retired years ago, , , ,  :)

but, however, back to the Op and op and the derived propositions :

One reason for 'fast' mirrors ( ie, less than f8ish or f5ish) in Newts is to get away from the need of ladders to get to the eypiece. The consequent problem is the cost of eyepieces to do justice to those f-ratios

my contemplation is a (almost)  Lord Ross type solution ?

ie. is it practicable to adopt a cheaper  f8 (say) 24" mirror  and accept a limited ( almost meridian) mount where  the bulk of the scope can be in a pit, OR the observer platform can be raised up a bit ? In other words wait for objects to 'come by' in the field of view say +/- 20deg eiter side of south and maybe 40deg to zenith ?

(with which size of mirror there should be plenty of such objects to entertain a night ? )

Edited by Ptarmigan

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I think something that hasn't been mentioned here and is worth mentioning is that the term "fast" comes from the photography world where a lens is at it's "fastest" when the iris is wide open and the full aperture of the lens is available.  f/ratio is just a ratio.  It measures the relation of the aperture to the focal length.  "Fast," in photography lens terms, literally means APERTURE. Since camera lenses don't change their focal length, the only way for them to change their f/ratio is to increase or decrease their aperture with the iris.  So changing from f/1.8 to f/4 would be to "stop" the lens down (or cover it) until the relationship of the aperture to focal length is 4. Since this effectively makes the lens smaller in diameter, it decreases the amount of light that will hit the CCD sensor, making it take longer to expose the shot.  Hence, a "faster" CAMERA lens has a larger aperture, which has a smaller f/ratio.  So, for photography, f/ratio = aperture.

 

For telescopes, its a bit different, but some of the concepts still apply.  With telescopes, you rarely change the aperture (stop the lens) to change the f/ratio.  Most of the time an astrophotographer/astronomer will add a focal reducer to their imaging chain to "decrease" their focal length, or a barlow to increase it.  This does not change aperture and so does not increase the TOTAL amount of light.  So in the term of "fast" as it's used in photography, an 8" scope will always have the same "speed" unless you purposefully stop down the lens yourself [Remember photographic speed, or f/ratio, equals aperture].  However, when you increase or decrease the focal length of the scope, you will affect the "density" of photons (or photon flux) hitting the each pixel.  The field of view is determined by the focal length of your telescope (not aperture).  Shorter focal lengths "see" more than do longer focal lengths.  Imagine taking a magnifying glass and focusing the light from the sun to a 1" circle.  That 1" circle is the image of the sun.  If your CCD were capturing the light from that 1 inch circle, the photon density would be some number of photons hitting each pixel per second.  If you then move the magnifying glass so that the image circle is smaller (say 1/2 inch), you have just increased the photon flux 4x. ie. quadrupled the number of photons hitting each pixel per second thereby decreasing imaging time for the same amount of signal.

 

So changing the focal length to shorter means you are cramming more photons into a pixel per second than for the same aperture scope with a longer focal length.  This has two effects... increased field of view for the shorter focal length, and decreased imaging time for the same amount of signal due to increased photon flux per pixel.

 

So now that I have explained that, your question was, why is faster better for imaging?  It's not.  You should choose a camera and telescope to image what you want to achieve.  Wide field is better achieved through a shorter focal length scope with a matched CCD to give the "impressive" views you see online so much, but at the expense of reduced resolution (ie. arcsecs / pixel resolution; not camera resolution).  Longer focal length scopes will give you a closer view of your target, but the photon flux per pixel will be reduced and so imaging time may increase depending on your goals.

 

By the way: formulas for calculating this stuff is as follows....

To calculate the factor of photon flux per pixel by focal length, use:  (FL1 / FL2) ^2  

So a scope with 1000mm focal length compare to a scope with 500mm focal length gives (1000/500)^2 = 4; or a factor of 4x more photons per pixel for the 500mm scope.

 

For aperture simply calculate the aperture area and compare, but be sure to include an secondary obstruction in the case of Newtonian/reflecting systems.

Light Collecting Area = pi x radius^2

A scope with an aperture of 204mm (~8") would have a light collecting area of 32,685 sq mm; which is 4x greater light collecting than a 4" scope (102mm), which has 8,171 sq mm.

 

I personally think using f/ratio as a means of comparing photographic speed is a bit misleading.  However, if you understand the change of photon flux per pixel given certain focal lengths, then it is easy to compare different scopes and how quickly they will capture "signal" vs other scopes.

Clear as mud huh?  :)

 

Jason

 

 

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On 3/4/2016 at 11:37, Jaymocha said:

I think something that hasn't been mentioned here and is worth mentioning is that the term "fast" comes from the photography world where a lens is at it's "fastest" when the iris is wide open and the full aperture of the lens is available.  f/ratio is just a ratio.  It measures the relation of the aperture to the focal length.  "Fast," in photography lens terms, literally means APERTURE. Since camera lenses don't change their focal length, the only way for them to change their f/ratio is to increase or decrease their aperture with the iris.  So changing from f/1.8 to f/4 would be to "stop" the lens down (or cover it) until the relationship of the aperture to focal length is 4. Since this effectively makes the lens smaller in diameter, it decreases the amount of light that will hit the CCD sensor, making it take longer to expose the shot.  Hence, a "faster" CAMERA lens has a larger aperture, which has a smaller f/ratio.  So, for photography, f/ratio = aperture.

 

For telescopes, its a bit different, but some of the concepts still apply.  With telescopes, you rarely change the aperture (stop the lens) to change the f/ratio.  Most of the time an astrophotographer/astronomer will add a focal reducer to their imaging chain to "decrease" their focal length, or a barlow to increase it.  This does not change aperture and so does not increase the TOTAL amount of light.  So in the term of "fast" as it's used in photography, an 8" scope will always have the same "speed" unless you purposefully stop down the lens yourself [Remember photographic speed, or f/ratio, equals aperture].  However, when you increase or decrease the focal length of the scope, you will affect the "density" of photons (or photon flux) hitting the each pixel.  The field of view is determined by the focal length of your telescope (not aperture).  Shorter focal lengths "see" more than do longer focal lengths.  Imagine taking a magnifying glass and focusing the light from the sun to a 1" circle.  That 1" circle is the image of the sun.  If your CCD were capturing the light from that 1 inch circle, the photon density would be some number of photons hitting each pixel per second.  If you then move the magnifying glass so that the image circle is smaller (say 1/2 inch), you have just increased the photon flux 4x. ie. quadrupled the number of photons hitting each pixel per second thereby decreasing imaging time for the same amount of signal.

 

So changing the focal length to shorter means you are cramming more photons into a pixel per second than for the same aperture scope with a longer focal length.  This has two effects... increased field of view for the shorter focal length, and decreased imaging time for the same amount of signal due to increased photon flux per pixel.

 

So now that I have explained that, your question was, why is faster better for imaging?  It's not.  You should choose a camera and telescope to image what you want to achieve.  Wide field is better achieved through a shorter focal length scope with a matched CCD to give the "impressive" views you see online so much, but at the expense of reduced resolution (ie. arcsecs / pixel resolution; not camera resolution).  Longer focal length scopes will give you a closer view of your target, but the photon flux per pixel will be reduced and so imaging time may increase depending on your goals.

 

By the way: formulas for calculating this stuff is as follows....

To calculate the factor of photon flux per pixel by focal length, use:  (FL1 / FL2) ^2  

So a scope with 1000mm focal length compare to a scope with 500mm focal length gives (1000/500)^2 = 4; or a factor of 4x more photons per pixel for the 500mm scope.

 

For aperture simply calculate the aperture area and compare, but be sure to include an secondary obstruction in the case of Newtonian/reflecting systems.

Light Collecting Area = pi x radius^2

A scope with an aperture of 204mm (~8") would have a light collecting area of 32,685 sq mm; which is 4x greater light collecting than a 4" scope (102mm), which has 8,171 sq mm.

 

I personally think using f/ratio as a means of comparing photographic speed is a bit misleading.  However, if you understand the change of photon flux per pixel given certain focal lengths, then it is easy to compare different scopes and how quickly they will capture "signal" vs other scopes.

Clear as mud huh?  :)

 

Jason

 

 

You also have to distinguish in AP between the photons you do want (from the object) from those which you don't much care about (empty sky around the object.) This can totally change the usefulness or otherwise of focal reducers.

Olly

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Very interesting thread, Then add in the variables of different sensitivities of

cameras, CCD /  Slr camera for instance can change there  aperture to f4  giving x amount

of photons hitting the sensor at a given speed say 100th of a second , but then

they can drop it down to f5,6 at still the same speed of 100th of a second and maintain the

same result of brightness of the image captured by altering the iso value ( chip gain ) 

with still the same amount of photons hitting the censor.

Paul.

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On 26 September 2015 at 08:15, laser_jock99 said:

Faster focal ratios = shorter exposure times. Absolutely beautiful image!  Now, where do I get a 12: F2.9 from? 

Lagoon Nebula in 50 seconds with a 12" F2.9 Newtonian

DSIR9690_1024_silly_zps54de9507.jpg

 

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You buy a 12 inch F/4 and add a 0,73 reducer

Or just buy one of these 12 inch F/3

p.s. If you do go for the F/3, let me know what you think of it !

Edited by pete_l
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On 29/04/2016 at 09:13, pete_l said:

You buy a 12 inch F/4 and add a 0,73 reducer

Or just buy one of these 12 inch F/3

p.s. If you do go for the F/3, let me know what you think of it !

As Pete says, the 'cheap' way is to buy a GSO (or Skywatcher make one now) 12" F4 scope:

http://www.telescopehouse.com/telescopes/telescopes-by-brand/brand-revelation-telecopes/revelation-12-f-4-m-lrn-optical-tube-assembly-ota.html

And then add an ASA Keller 2-inch Newton Coma Corrector and 0.73x Reducer

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4685_ASA-2--Newton-Korrektor-und-Reducer-0-73x.html

Interestingly enough the ASA Keller could be used with the Orion 12" F3 to give an F2.19 scope!!! 

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The OP is simply asking if fast is better.

Well, fast is just faster.

If it's going to be judged as better you need to warrant the extra cost vs available imaging time. You will also need to read up on optical problems that arise as you push lenses or mirrors towards a faster design. Problems can pop up, not least the issue of accurate focus. The faster the setup the narrower the band of 'spot on focus' becomes, so more money has to be spent to combat temperature shifts.

I recently swapped SW ED80s for reduced APM LZOS 105s with the general thinking that the new OTAs will be roughly twice as fast. Well, that is true but they are also more than twice as tricky to focus....

/Jessun

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Just to queer your pitch - just to be awkward I have an x2-x3 auto-teleconverter for ordinary photography which has the effect of reducing the f-ratio pretty drastically!

Neil

Just a thought - it's effectively a barlow - but seems to be of very high quality (at least it used to take good pictures back in the day) muct try it with my 135mm Zeiss lens, that could make a very nice lens for large objects at 270mm f7.

Edited by Stub Mandrel
Just a thought...

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