I have been a big of Focal Reducers and have been using one for over four years on every image I have taken with my telescope. When I first purchased the reducer all I knew was they increased the Field of View and the they lowered the Focal Ratio making them 'faster'. I did not know how they did this but believed some sites which erroneously said they do this by letting in more light. I also believed the benefit of more light/less exposure outweighed the loss of resolution.
Galaxy season was rapidly approaching so I wanted to get closer in on these objects without getting a bigger telescope so I did some research and opted for removing the reducer. This 'research' was very confusing because there was a lot of contradictory statements on the various discussion boards and forums I was reading.
I am no expert and this article is meant as a brief conceptual explanation of what is going on. I created some diagrams to aid in the explanations as a picture is worth 1000 words. Please feel free to add something if I missed it or stated something incorrectly or if you feel like you should.
YouTube Video
I have included links to more comprehensive articles by experts:
https://www.allaboutastro.com/the-focal-ratio-myth.html
http://www.stanmooreastro.com/f_ratio_myth.htm
http://www.clarkvision.com/photoinfo/f-ratio_myth/
https://www.davemorrowphotography.com/camera-sensor-size-guide#Camera_Sensor_Image_Quality_Basics
Discussion
The basic function of a Focal Reducer (or telecompressor) for a telescope is to increase the field of view of an optical system. It does this by decreasing the Focal length of the telescope which has the added effect of decreasing the Focal Ratio. So what is the confusion and why not leave it on and use it all the time? This is the question and where a lot of controversy is - the so called Focal-Ratio Myth, that is "the faster the scope is, the less time it takes to acquire an image". Jay Ballauer from allaboutastro.com presents a great discussion on this subject.
Inner Workings - My Telescope & Focal Reducer
Focal reducers provide an good way to increase your FOV without purchasing a new telescope and also flatten the image in some cases and they do indeed lower the Focal Ratio. Figure 1 shows is a conceptual light ray diagram of my AT115 refractor. As shown on the diagram the Focal Length is determined from the aperture, refractors use a lens for the aperture and reflectors use a mirror, to the focal plane, where focus is achieved. The top image is the of the telescope with no Focal Reducer.
Figure 1
The bottom image shows what happens when a focal reducer is used. I use the AT 0.8 Field Flattener/Focal Reducer combo designed for the telescope. Taking the Focal Length (FL) and dividing by the aperture gives you the Focal Ratio (F-ratio). In my case, 805mm/115mm = 7.0 without the FR and 644mm/115mm = 5.6 with the FR. Notice the aperture is constant. This is important as the aperture alone determines how much light or photons the telescope setup captures.
Note: Changing the F-ratio of a telescope with a FR is not equivalent to changing the F-stop on a
camera lens since changing the F-stop involves changing the aperture.
The bottom diagram shows how the FR concentrates the light sooner by reducing the FL resulting in a larger FOV. The new Focal Length is from the Focal Point to what I call the Virtual Objective Lens. It is at the position where the Objective Lens would be if the telescope were really F5.6 and the Aperture were 115mm.
Inner Workings - With a Sensor
Now we come to the confusing part - the sensor and how it relates to the system. The sensor is a rectangular grid containing millions of tiny square pixels. Pixels are the meet & bones of the sensor. Digital photographers Roger Clark and Dave Morrow both have extensive information on sensors and pixels. Each pixel is composed of a semiconductor material which absorbs photons and frees electrons (think Photoelectric Effect - Einstein). Those electrons are stored in potential wells. Roger Clark (Clarkvision.com) has a good analogy, think of a pixel as a "bucket of water holding rain drops, and the photons are the rain drops falling on the buckets" (see Figure 2). When the bucket (potential well) is filled, it is saturated and can't absorb any more. What happens next is the signal from the pixel is sent to an amplifier and then an analog-to-digital (A to D) converter so an image can then be produced.
https://www.davemorrowphotography.com/camera-sensor-size-guide#Camera_Sensor_Image_Quality_Basics
Discussion
The basic function of a Focal Reducer (or telecompressor) for a telescope is to increase the field of view of an optical system. It does this by decreasing the Focal length of the telescope which has the added effect of decreasing the Focal Ratio. So what is the confusion and why not leave it on and use it all the time? This is the question and where a lot of controversy is - the so called Focal-Ratio Myth, that is "the faster the scope is, the less time it takes to acquire an image". Jay Ballauer from allaboutastro.com presents a great discussion on this subject.
Inner Workings - My Telescope & Focal Reducer
Focal reducers provide an good way to increase your FOV without purchasing a new telescope and also flatten the image in some cases and they do indeed lower the Focal Ratio. Figure 1 shows is a conceptual light ray diagram of my AT115 refractor. As shown on the diagram the Focal Length is determined from the aperture, refractors use a lens for the aperture and reflectors use a mirror, to the focal plane, where focus is achieved. The top image is the of the telescope with no Focal Reducer.
Figure 1
Note: Changing the F-ratio of a telescope with a FR is not equivalent to changing the F-stop on a
camera lens since changing the F-stop involves changing the aperture.
The bottom diagram shows how the FR concentrates the light sooner by reducing the FL resulting in a larger FOV. The new Focal Length is from the Focal Point to what I call the Virtual Objective Lens. It is at the position where the Objective Lens would be if the telescope were really F5.6 and the Aperture were 115mm.
Inner Workings - With a Sensor
Now we come to the confusing part - the sensor and how it relates to the system. The sensor is a rectangular grid containing millions of tiny square pixels. Pixels are the meet & bones of the sensor. Digital photographers Roger Clark and Dave Morrow both have extensive information on sensors and pixels. Each pixel is composed of a semiconductor material which absorbs photons and frees electrons (think Photoelectric Effect - Einstein). Those electrons are stored in potential wells. Roger Clark (Clarkvision.com) has a good analogy, think of a pixel as a "bucket of water holding rain drops, and the photons are the rain drops falling on the buckets" (see Figure 2). When the bucket (potential well) is filled, it is saturated and can't absorb any more. What happens next is the signal from the pixel is sent to an amplifier and then an analog-to-digital (A to D) converter so an image can then be produced.
Figure 2
What the F-ratio (Focal Length & aperture), sensor size, and pixel size determine is how many photons can be absorbed.
Inner Workings - With My Sensor
I have the ZWO ASI1600 which has a sensor size of 17.7 mm x 13.4 mm (Diagonal: 21.9 mm), pixel array of 4656 x 3520, and pixel size 3.8 um.
Figure 3
The following is not specific to my sensor, thus can be applied to any sensor. Figure 4 shows a highly idealized diagram of what is happening at the sensor with and without the Focal Reducer. As a result of the FR the image FOV is concentrated (see Figure 1) and the light is collected by fewer photons. The Medusa nebula takes up 5 pixels on the on the right model with the FR while it takes up 9 pixels on the left model without the FR. Because of this, the photons will fill the pixels with the FR sensor faster. However, it's not always a good thing to have all the photons hitting fewer pixels (something called "undersampling" the image) as the additional photons will be wasted.
Figure 3
The following is not specific to my sensor, thus can be applied to any sensor. Figure 4 shows a highly idealized diagram of what is happening at the sensor with and without the Focal Reducer. As a result of the FR the image FOV is concentrated (see Figure 1) and the light is collected by fewer photons. The Medusa nebula takes up 5 pixels on the on the right model with the FR while it takes up 9 pixels on the left model without the FR. Because of this, the photons will fill the pixels with the FR sensor faster. However, it's not always a good thing to have all the photons hitting fewer pixels (something called "undersampling" the image) as the additional photons will be wasted.
Figure 4
The aperture and the amount of light per unit time does not change. As Jay Ballauer puts it, "Changing f-ratio with a constant aperture puts the same # of photons on fewer pixels…that's all.".
Other Aspects
The quality of the reducer is important - cheaper reducers are really going to impact the optics, so the trade off for FOV has to be a strong desire. Also, adding the reducer will affect the collimation, so redoing the collimation is an added factor to consider.
The quality of the reducer is important - cheaper reducers are really going to impact the optics, so the trade off for FOV has to be a strong desire. Also, adding the reducer will affect the collimation, so redoing the collimation is an added factor to consider.
I did NOT mention or discuss the Signal to Noise Ratio (SNR), Read Noise, Total Noise (Total Noise = sqrt [signal + (read noise)^2]), Quantum Efficiency, Oversampling, or Pixel Scale (Pixel Scale = Pixel Size*206.3/Focal Length) as I wanted to keep things simple and I don't have enough experience discussing these issues. Ideally you should also take into account these aspects. However, some of those factors will be just as affected by sky conditions, light pollution, and weather. If you are good with the math, by all means do all the calculations to find out if using a FR will or will not produce a better image. However, I also recommend you do your own experimentation - try using one and then not using one. I suspect in many cases it won't matter.
For a more in depth discussion of the above use the following Links:
https://www.allaboutastro.com/the-focal-ratio-myth.html
http://www.clarkvision.com/photoinfo/f-ratio_myth/
https://www.davemorrowphotography.com/camera-sensor-size-guide#Camera_Sensor_Image_Quality_Basics
Example
Conclusions
https://www.davemorrowphotography.com/camera-sensor-size-guide#Camera_Sensor_Image_Quality_Basics
Example
The following comparison image was kindly provided by Gary Imm. This comparison was made the following telescopes: Takahashi 130, Celestron C11R (with reducer), and Celestron C11 (without reducer). Doing comparisons such as this are extremely difficult as the sky conditions are never exactly the same. For example the seeing is not the same, the quality of focus may be slightly different, the moonlight is different, and etc. Although this is not a true apples to apples comparison (maybe apples to pears), it is very useful information.
From his own words:
"I have concluded that the only way to discern this for a given system is to carefully make a comparison. Attached is my image comparison between my Tak130, my C11R (with reducer), and my C11 (without reducer)., with everything made equal the best I could. I did not have any prior agenda here. The results are close, but I like the native C11 a bit better."
I also like the C11 results better. Although it may hard to see in this web post, the C11R image is a bit brighter but under close observation it is not as detailed and a bit noisier.
Figure 5
Conclusions
What is the reason for a Focal Reducer? The correct answer that everyone agrees to is that it increases the FOV. If you were buying for the lower F-ratio to increase the speed so less exposure time is needed? Nothing is free in this world. So why are objects captured with a FR brighter and appear to need less exposure time? Resolution was the price!
If you use a focal reducer on very small objects and then crop the image, the decreased resolution may be more apparent then if you did not use the focal reducer in the first place.
My personal current take on this whole thing is for large DSO's such as nebula which takes up the majority of the FOV, use a Focal Reducer and for small objects such as galaxies, don't use one. What about mid sized objects? I plan to go with how I happen to have the telescope configured or what I plan to image in the future. This is not the most scientific or technical method but it is the most practical.
Additional Acknowledgements:
In addition to the websites and individuals all ready mentioned, I also want to thank Gary Imm for his fruitful suggestions. Gary's input made this article possible.
(https://www.astrobin.com/users/GaryI/collections/)
Other Related YouTube Videos:
AstroBackyard
https://www.youtube.com/watch?v=46WoTDhuAZc
Slymin
https://www.youtube.com/watch?v=sFYD4aR8WQs
AstroForum
https://www.youtube.com/watch?v=k4VRaqT759E&t=73s
In addition to the websites and individuals all ready mentioned, I also want to thank Gary Imm for his fruitful suggestions. Gary's input made this article possible.
(https://www.astrobin.com/users/GaryI/collections/)
Other Related YouTube Videos:
AstroBackyard
https://www.youtube.com/watch?v=46WoTDhuAZc
Slymin
https://www.youtube.com/watch?v=sFYD4aR8WQs
AstroForum
https://www.youtube.com/watch?v=k4VRaqT759E&t=73s
No comments:
Post a Comment