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do focal reducers really make scopes faster?

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#1 adamphillips

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Posted Yesterday, 01:17 AM

ive always been a little skeptical. take for example my edge11. I have an optec lepus reducer .62. but according to image link its actually operating at about .65

 

sure it is reducing the focal length. for one, obviously adding glass is going to reduce some light transmission.

 

but here is what ive always wondered. on my SCT the light is already bouncing off two mirrors, and it already being transformed down through the f10 system, and then it hits the reducer.

im not an expert on optics so this has always been where my skepticism comes from. seems like the light has already been "slowed down", doesn't seem like you can "speed it back up" again, except by removing a mirror with hyperstar.

 

I know theres some experts out there. am I wrong or right? I really want to talk to somebody who knows what theyre talking about.



#2 Whichwayisnorth

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Posted Yesterday, 01:46 AM

Yes.

 

Point at something hard to see in the night sky.  Expose for 10 seconds without reducer. Do it again with reducer.  Compare.


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#3 Astrojensen

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Posted Yesterday, 01:47 AM

 

obviously adding glass is going to reduce some light transmission.

Yes, by 0.5%... 

 

 

sure it is reducing the focal length

If it reduces the focal length and the telescope is still working at full aperture, then the scope has also been "sped up". 

 

 

 

but here is what ive always wondered. on my SCT the light is already bouncing off two mirrors, and it already being transformed down through the f10 system, and then it hits the reducer.

im not an expert on optics so this has always been where my skepticism comes from. seems like the light has already been "slowed down", doesn't seem like you can "speed it back up" again, except by removing a mirror with hyperstar.

Yes you can. All that matters is the focal length and the aperture, as seen from the focal plane. The focal length governs the size of the image at the focal plane and the aperture determines how much light the scope captures. In other words, the aperture determines how much "paint" you have for your picture, while the focal length determines how large the painting is going to be. Long focal length = large painting, but since you only have a fixed amount of paint, the image is going to be faint and dim and the colors muted, while if you have the same aperture and thus the same amount of paint, but a much shorter focal length, the image is going to be small, but now the paint is concentrated in a smaller area, so the image is much brighter.

 

If you want to see first hand, how much brighter the image is at f/6.5, compared to f/10, simply use the scope visually with the same eyepiece in both configurations. 

 

 

Clear skies!
Thomas, Denmark 



#4 TOMDEY

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Posted Yesterday, 01:57 AM

Good question! The answer is that you are wrong.

 

The ~optical speed~ of an imaging system is a dimensionless number defined as the ratio of the focal length to the aperture diameter. This turns out to be a scaled approximation alluding to an aspect of a much more fundamental (and exact) metric defined by the physics of radiometry... what we call étendue. It is the denominator of the expression defining Radiance (the numerator being power). The way this plays out as electromagnetic radiation propagates anywhere, everywhere (including through telescopes) is fully-encompassed in Emmy Noether's Theorem. I understand that my explanation must come across as allusive... but it's the most universal explanation as to why optical speed morphs as one follows the light from object to image, through a passive optical imaging system. The beam indeed "speeds up" and "slows down" in strict adherence to Emmy's Gestalt. Unfortunately, to completely grasp that... one needs to study radiometry and electrodynamics in substantial detail... and hope to keep a firm grasp on the Tiger's Tail through that journey. Most fall by the wayside... including nearly all Optical Engineers and Scientists.

 

[Emmy's generalized theorem asserts the invariance or Radiance as flux propagates specularly through media where the imaginary part of the medium's complex index of refraction is identically zero. It's a profoundly ingenious gestalt, encompassing all such spaces, whether imaging or not. In that sense, it is entirely universal.]

 

Alas... there's no simpler exact way to put it.

 

Couple of images. Emmy, and my old T-shirt with the related posit.    Tom

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#5 sg6

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Posted Yesterday, 02:09 AM

Yes and No, well in my opinion.

The native focal length is not changed neither is the aperture so the focal ratio remains unchanged.

What changes is the size of the image created by the scope - it is made smaller, and in being smaller it is brighter and so the "reducer" size and brightness correlates to an f/6.3 scope not to an f/10 scope.

 

In AP it is often easier to refer to the focal ratio as that is the photographic term that many are interested in. Will me exposure be less time.

 

A Reducer or Barlow alters the image produced by the scope. In effect it is no longer the prime image as something is now in the light path causing changes to waht would have been the prime image.



#6 Astrojensen

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Posted Yesterday, 02:16 AM

A Reducer or Barlow alters the image produced by the scope. In effect it is no longer the prime image as something is now in the light path causing changes to waht would have been the prime image.

Wrong. The word "prime" here refers to the FIRST real image plane formed by the telescope and since both a barlow and a reducer are altering the light path BEFORE the first image is formed, the resulting image is still the prime image. 

 

 

Clear skies!
Thomas, Denmark


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#7 Asbytec

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Posted Yesterday, 03:49 AM

I do not understand entendue nearly as much as most scientists don't, apparently. Instead, it's easier to think in terms of relative aperture as it has to do with the angle of the light cone. As Glenn Le Drew might say, think of a bug looking up from the focal plane at the aperture. If the angle to the apparent aperture overhead is very wide, the relative aperture is very large (and fast relative to it's distance above you, i.e., the focal length). There is a lot of light coming down even though the aperture is the same if it were much higher above you.

 

A focal reducer changes the steepness of the narrow slow light cone (with a small relative aperture) into a wider cone (with a larger relative aperture). This is true even if the aperture is the same, but the focal length to the aperture is different. Thus, for some reason, a steeper light cone and larger relative aperture provides higher entendue or radiance. A focal reducer changes the light cone from a narrow one to a steep one, that is what matters. 

 

Think of a 20' skylight in a roof 200' (20 stories) above you. That's f/10. The skylight from your perspective on the floor looks small and the light spread out on the floor is dim. Now, consider the same 20' skylight 80' above you. That's f/4. The skylight looks larger even though it's the same aperture. The angle from your eye to the edge is wider and the floor is also brighter. Higher radiance. The cone is steeper, and that cone is somehow related to entendue. 

 

So, at f/6.5, you have a larger relative aperture than at f/10. The faster the scope, the larger the relative aperture. F/4 is relatively larger than f/10, even with the same aperture. Thus higher luminance and a "faster" image. So, yes, focal reducers make the scope "faster" by creating a larger relative aperture and a steeper light cone. 


Edited by Asbytec, Yesterday, 03:57 AM.

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#8 imtl

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Posted Yesterday, 04:52 AM

Reducers do not change aperture. They create an effective focal length of the entire optical system that is shorter than the native focal length. That gives a steeper light cone which in turn gives more photons PER UNIT AREA. Meaning that more photons are going to hit each pixel, for example, per given time. In that sense it is a "faster" system. That means that every pixel will need less time to accumulate the same amount of photons once you put a reducer in. Or, it will accumulate more photons if you keep the same exposure time as before. Giving a brighter image.

The trade off is that your image scale is going to change as well.


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#9 Alex McConahay

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Posted Yesterday, 08:00 PM

Scopes themselves are not faster or slower.

 

Gathering a certain amount of light over a given area can take less time (faster) or more time (slower). 

 

If we can gather that amount of light faster (in less time), then we call the equipment that gathered that lighter "Faster." Now, in this sense, we can call one scope faster than another. 

 

The amount of light you can gather is determined by the overall aperture of the prime optic. Let's say light is water. In one minute you can get one "bucket" of light (water) through the aperture, say. 

 

Now, empty that bucket into a pan that is 12 inches by 12 inches, you will get, say 3 inches deep water. 

 

Empty that bucket into a pan that is 8x8 inches, you will get almost seven inches of water. 

 

Have you gathered more light? No

 

Have you gathered it faster? No. The speed at which you can gather light is fixed by the aperture of the Prime Optic. 

 

Have you gathered more in the given area of interest (in our case---is there more depth to the water in the one pan than the other)? YES. You did not gather any more light (water), but the intensity of the light (depth of the water) is greater. It was faster. 

 

Changing from from F10 to F6.3 reduces the size of your pan. All the light that used to spread over X area, now spreads over the smaller .63 * X area. And each part of that smaller area has more light, more water, is faster. 

 

Of course, the distance from one part of the scene to the other has also been reduced. Instead of covering the area that was covered at F10, it has to be rearranged into the area covered by F6.3. So, things are smaller (not as magnified). 

 

The primary optic has not changed. It is still gathering as much light as it had been. And it is converging the light at a rate of 1 to 10. It is a light cone ten times longer than it is wide.  But, someplace along the way, you will stick in a focal reducer that will change the angle of the sides of that cone so that it is only 6.3 times as long as it is wide. This brings the sides of that cone together sooner, and into a smaller area. Because the same amount of light is hitting a smaller area, any given part of that area will see more of the original light. It will reach a given exposure point more quickly. It will be "faster." 

 

The scope will now have a final light cone, and an APPARENT focal length of a scope with a F6.3 focal ratio. You will gather your images (get them to a given exposure level, depth of the water, etc. ) FASTER> 

 

Alex



#10 Alex McConahay

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Posted Yesterday, 08:05 PM

Oh, since you have an SCT, your primary is probably at F2.0 or something. It hits the secondary, and lengthened to F10, and then the reducer, which shrinks it back to F6.3 or whatever. 

 

Alex



#11 adamphillips

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Posted Yesterday, 10:22 PM

Scopes themselves are not faster or slower.

 

Gathering a certain amount of light over a given area can take less time (faster) or more time (slower). 

 

If we can gather that amount of light faster (in less time), then we call the equipment that gathered that lighter "Faster." Now, in this sense, we can call one scope faster than another. 

 

The amount of light you can gather is determined by the overall aperture of the prime optic. Let's say light is water. In one minute you can get one "bucket" of light (water) through the aperture, say. 

 

Now, empty that bucket into a pan that is 12 inches by 12 inches, you will get, say 3 inches deep water. 

 

Empty that bucket into a pan that is 8x8 inches, you will get almost seven inches of water. 

 

Have you gathered more light? No

 

Have you gathered it faster? No. The speed at which you can gather light is fixed by the aperture of the Prime Optic. 

 

Have you gathered more in the given area of interest (in our case---is there more depth to the water in the one pan than the other)? YES. You did not gather any more light (water), but the intensity of the light (depth of the water) is greater. It was faster. 

 

Changing from from F10 to F6.3 reduces the size of your pan. All the light that used to spread over X area, now spreads over the smaller .63 * X area. And each part of that smaller area has more light, more water, is faster. 

 

Of course, the distance from one part of the scene to the other has also been reduced. Instead of covering the area that was covered at F10, it has to be rearranged into the area covered by F6.3. So, things are smaller (not as magnified). 

 

The primary optic has not changed. It is still gathering as much light as it had been. And it is converging the light at a rate of 1 to 10. It is a light cone ten times longer than it is wide.  But, someplace along the way, you will stick in a focal reducer that will change the angle of the sides of that cone so that it is only 6.3 times as long as it is wide. This brings the sides of that cone together sooner, and into a smaller area. Because the same amount of light is hitting a smaller area, any given part of that area will see more of the original light. It will reach a given exposure point more quickly. It will be "faster." 

 

The scope will now have a final light cone, and an APPARENT focal length of a scope with a F6.3 focal ratio. You will gather your images (get them to a given exposure level, depth of the water, etc. ) FASTER> 

 

Alex

I see I guess it really just is the final focal length that matters.

the funny thing is to me in your pan analogy, its almost paradoxical or counterintuitive. because at f10 your field of view is smaller. makes me think the pan is smaller. reduce it and the field of view is bigger. seems like the pan is bigger, spreading out the water.

 

im sure there is a more technical explanation to make it make sense.

 

because I also understand even when doing visual, the more you zoom in the dimmer it gets, the ring nebula looks great at 200x, I tried to blow it up to 600x and I could just barely make out the ring.



#12 Asbytec

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Posted Yesterday, 10:28 PM

"...the funny thing is to me in your pan analogy, its almost paradoxical or counterintuitive. because at f10 your field of view is smaller."

The image is larger at f/10 than f/6. The larger pan has less water per unit area.

Edited by Asbytec, Yesterday, 10:29 PM.


#13 Alex McConahay

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Posted Yesterday, 11:14 PM

In a simple world the the usable target field of view in the F10 is as wide as the field of view of that scope with an F 6.3 reducer. But the image circle is not the same.  (An image circle is the illuminated area of an in-focus image.) 

 

In the real world, there are field stops, and optical and mechanical considerations that limit the entire "Image Circle." You scope is designed to have an image circle that completely fills (with focused light) a certain size sensor at a given F ratio (Focal length and aperture).  You can change that F ratio (by putting in a reducer). But, when you do that, you no longer have the same size image circle. 

 

If I take my F 10 telescope, with a full frame chip in it, I get an image circle sufficient to cover the sensor from edge to edge, corner to corner. The chip is evenly illuminated. If I add an F6.3 reducer, I reduce the image circle so that it is illuminated only in the middle, with not enough light around the edges. I get vignetting in the corners of the frame. I simply do not have light getting into the corners.  All the light (one corner of the target to the other corner of the target) still gets to the chip. That is, all the stuff coming from the target (the target Field of View) is still coming through the scope and hitting the sensor.  In the F10 configuration, my entire chip is illuminated. But, at F6.3 the outer perimeter of my full size chip is not getting much light. The whole chip is not usable. If I cut out the "unusable" parts of the what hit the sensor, I am back to the same TARGET (including target field of view) dimensions as the original F 10. (That is an overgeneralization, and contains a lot of judgement calls, but is generally right. )

 

Now, of course if you had a smaller sensor in the first place, everything would have been usable. But, you would have had an even larger Apparent Field of View had you a proper size sensor that matched your scope's image circle. Using a smaller sensor, you simply waste the photons that fall within your image circle but outside the edges of your sensor chip. 

 

It is always dangerous to argue from analogy, because parallels eventually break down. But, remember the original bucket of water analogy was about pouring into different size pans. Focused photons do not flow like water. If they did, they would be free to flow anywhere. In a telescope, they must be in a specific spot relative to other spots of photons. So, let's not over work the analogy.

 

What misleads you is your assumption that the image circle is the same. 

 

Alex




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