Post by Rick in NWArk on Feb 22, 2016 16:41:01 GMT
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Background
----------
First, lets get a few definitions out of the way. Field of View is just what it sounds like - how much of the sky is visible on the camera chip (or thru an eyepiece for visual observers). Field of View is commonly expressed in degrees, minutes, & seconds. The bigger the Field of View, the more space is captured - but, the smaller the objects in that space will appear. The smaller the Field of View, the smaller amount of space is captured but objects will appear larger.
------------
Dependencies
------------
Your Field of View will depend on your equipment. Your telescope's Local Length and the size of your Camera Chip are the two main factors that determine your Field of View. Adding Focal Reducers, Barlows, or Spacers will also change your Field of View.
---------------------
The Basic Calculation
---------------------
Let's start with a system that only has a telescope and a camera. To calculate your basic Field of View, follow these steps:
1. Get the Focal Length of your Telescope
This should come from your manual or you can look it up online. Worst case, you can multiply your aperture and your focal ratio to get the result. Make sure the result is in mm. Remember that 1 inch = 25.4 millimeters.
2. Get your Camera's Pixel Size
The first property of your Camera's Chip that you need is the size of the pixels in the chip. Pixel Size is shown in microns. Note that some chips have square pixels and have the same pixel size in both the horizontal and vertical directions. Some do not and will list different horizontal and vertical values. Sometimes you'll be given only one "diagonal" value to use.
3. Get your Camera's Chip Size
You now need to get the chip size, but in pixels. This information may be available in the information that came with your camera or on the manufacturer's website. Worst case, if you can find the manufacturer and model of the chip in your camera, you can go to the chip maker's website and look up their chip specifications. Make sure that you use the "effective pixels" values. These are the pixels that are used to capture the image. Chips often have more pixels than this but they are not part of the image capture.
Let's say that we have a Samsung SCB-2000 Camera. Here are the values we would use:
SCB-2000 Pixel Size: 6.35 x 7.40
SCB-2000 Chip Size: 768(W) x 494(H)
And say that we have a Stellarvue 102ED telescope:
Focal Length: 710mm
Now lets put them together into the calculation. The calculation will be performed for both the horizontal and vertical. The basic calculation is:
3.438 x (Pixel Size) x (Chip Size) / (Focal Length)
So using our setup:
Horizontal = 3.438 x 6.35 x 768 / 710 = 23.6 arc minutes
Vertical = 3.438 x 7.40 x 494 / 710 = 17.7 arc minutes
----------------------------------
Impact of Focal Reducers & Barlows
----------------------------------
Barlow lenses (focal extenders) and Focal Reducers change the effective Focal Length of the telescope by changing the focus point. These lenses typically range from 1.5x to 5x for Barlows (which make the Focal Length longer and the Field of View smaller), to .80x to .33x Focal Reducers (which make the Focal Length shorter and the Field of View bigger).
Let’s say we add a Focal Reducer to our setup that is a .5x reducer:
Horizontal = 3.438 x 6.35 x 768 / (710 x .5) = 47.2 arc minutes
Vertical = 3.438 x 7.40 x 494 / (710 x .5) = 35.4 arc minutes
------------------------------
Will my Object Fit on my Chip?
------------------------------
So you may be saying, hey thanks for the math exercise, but why do I care about this? I don't know about you, but I like to stick with one setup for a session. I will pick one camera to use and I'll also try to decide on what focal adjustment I want (usually Focal Reducer for DSOs). For me, it is very challenging to try to swap out equipment during a session. So if I am going with a certain setup, I want to know what my targets will look like. I want to know if some object will appear very, very small on my screen - or will I be so "zoomed in" that I don't get to see the entire object?
That's where Calculating the Field of View comes in. With the numbers above for my Stellarvue 102ED plus .5x focal reducer and the Sammy SCB-2000, I can now go look up individual objects.
Let's say I want to look at the Orion Nebula M42. Looking it up (Wikipedia), it is 65x60 arc minutes in size. Our setup gives us 47.2x35.4 - this means that we will not be able to see all of the nebula on the screen! I would either need a shorter focal length scope or more focal reduction or both!
What about the Ring Nebula? Well, its 1.5x1 arc minutes. So with our setup it’s going to be very small!
And Globular Clusters? - Omega Centauri is 36.3x36.3 which will be nice and big in our 47.2x35.4 (only a little cutoff). Hercules' M13 is 20x20 and will fill half the frame.
Galaxies? Well Andromeda M31 is right out - it's 178x63! The Pinwheel M101 is 22x22 which is nice and big. The Sombrer0 M104 is smaller at 9x4 - the same size as the Cigar M82. The Southern Pinwheel M83 is 11x10. Bode's M81 is 21x10. And that's just a few of the Messier objects.
---------------
Summary & Links
---------------
So, in summary, understanding what the Field of View of your setup will help set your expectations for the object(s) you intend to view. You can also plan ahead and create an object list that will work well with the setup you plan to use. And lastly, if there is an object that you want to focus on, you can determine what setup you need to most effectively view or image that object.
Although I have shown you above how to do the calculations, there are websites out there that will calculate it for you. Some will even let you view different objects to see if they would fit. These include:
www.12dstring.me.uk/fov.htm
astronomy.tools/calculators/field_of_view/
The other thing that would be helpful is a database of objects that can be searched by size. I have just discovered a website called DSO Browser, which allows you to search for objects and it provides the magnitude and size and much, much more. Check it out:
dso-browser.com/
Please let me know if this HOW TO has been helpful, and if you have any questions, please do not hesitate to ask. Also, feel free to add any additional online calculators or object databases that you use.
Background
----------
First, lets get a few definitions out of the way. Field of View is just what it sounds like - how much of the sky is visible on the camera chip (or thru an eyepiece for visual observers). Field of View is commonly expressed in degrees, minutes, & seconds. The bigger the Field of View, the more space is captured - but, the smaller the objects in that space will appear. The smaller the Field of View, the smaller amount of space is captured but objects will appear larger.
------------
Dependencies
------------
Your Field of View will depend on your equipment. Your telescope's Local Length and the size of your Camera Chip are the two main factors that determine your Field of View. Adding Focal Reducers, Barlows, or Spacers will also change your Field of View.
---------------------
The Basic Calculation
---------------------
Let's start with a system that only has a telescope and a camera. To calculate your basic Field of View, follow these steps:
1. Get the Focal Length of your Telescope
This should come from your manual or you can look it up online. Worst case, you can multiply your aperture and your focal ratio to get the result. Make sure the result is in mm. Remember that 1 inch = 25.4 millimeters.
2. Get your Camera's Pixel Size
The first property of your Camera's Chip that you need is the size of the pixels in the chip. Pixel Size is shown in microns. Note that some chips have square pixels and have the same pixel size in both the horizontal and vertical directions. Some do not and will list different horizontal and vertical values. Sometimes you'll be given only one "diagonal" value to use.
3. Get your Camera's Chip Size
You now need to get the chip size, but in pixels. This information may be available in the information that came with your camera or on the manufacturer's website. Worst case, if you can find the manufacturer and model of the chip in your camera, you can go to the chip maker's website and look up their chip specifications. Make sure that you use the "effective pixels" values. These are the pixels that are used to capture the image. Chips often have more pixels than this but they are not part of the image capture.
Let's say that we have a Samsung SCB-2000 Camera. Here are the values we would use:
SCB-2000 Pixel Size: 6.35 x 7.40
SCB-2000 Chip Size: 768(W) x 494(H)
And say that we have a Stellarvue 102ED telescope:
Focal Length: 710mm
Now lets put them together into the calculation. The calculation will be performed for both the horizontal and vertical. The basic calculation is:
3.438 x (Pixel Size) x (Chip Size) / (Focal Length)
So using our setup:
Horizontal = 3.438 x 6.35 x 768 / 710 = 23.6 arc minutes
Vertical = 3.438 x 7.40 x 494 / 710 = 17.7 arc minutes
----------------------------------
Impact of Focal Reducers & Barlows
----------------------------------
Barlow lenses (focal extenders) and Focal Reducers change the effective Focal Length of the telescope by changing the focus point. These lenses typically range from 1.5x to 5x for Barlows (which make the Focal Length longer and the Field of View smaller), to .80x to .33x Focal Reducers (which make the Focal Length shorter and the Field of View bigger).
Let’s say we add a Focal Reducer to our setup that is a .5x reducer:
Horizontal = 3.438 x 6.35 x 768 / (710 x .5) = 47.2 arc minutes
Vertical = 3.438 x 7.40 x 494 / (710 x .5) = 35.4 arc minutes
------------------------------
Will my Object Fit on my Chip?
------------------------------
So you may be saying, hey thanks for the math exercise, but why do I care about this? I don't know about you, but I like to stick with one setup for a session. I will pick one camera to use and I'll also try to decide on what focal adjustment I want (usually Focal Reducer for DSOs). For me, it is very challenging to try to swap out equipment during a session. So if I am going with a certain setup, I want to know what my targets will look like. I want to know if some object will appear very, very small on my screen - or will I be so "zoomed in" that I don't get to see the entire object?
That's where Calculating the Field of View comes in. With the numbers above for my Stellarvue 102ED plus .5x focal reducer and the Sammy SCB-2000, I can now go look up individual objects.
Let's say I want to look at the Orion Nebula M42. Looking it up (Wikipedia), it is 65x60 arc minutes in size. Our setup gives us 47.2x35.4 - this means that we will not be able to see all of the nebula on the screen! I would either need a shorter focal length scope or more focal reduction or both!
What about the Ring Nebula? Well, its 1.5x1 arc minutes. So with our setup it’s going to be very small!
And Globular Clusters? - Omega Centauri is 36.3x36.3 which will be nice and big in our 47.2x35.4 (only a little cutoff). Hercules' M13 is 20x20 and will fill half the frame.
Galaxies? Well Andromeda M31 is right out - it's 178x63! The Pinwheel M101 is 22x22 which is nice and big. The Sombrer0 M104 is smaller at 9x4 - the same size as the Cigar M82. The Southern Pinwheel M83 is 11x10. Bode's M81 is 21x10. And that's just a few of the Messier objects.
---------------
Summary & Links
---------------
So, in summary, understanding what the Field of View of your setup will help set your expectations for the object(s) you intend to view. You can also plan ahead and create an object list that will work well with the setup you plan to use. And lastly, if there is an object that you want to focus on, you can determine what setup you need to most effectively view or image that object.
Although I have shown you above how to do the calculations, there are websites out there that will calculate it for you. Some will even let you view different objects to see if they would fit. These include:
www.12dstring.me.uk/fov.htm
astronomy.tools/calculators/field_of_view/
The other thing that would be helpful is a database of objects that can be searched by size. I have just discovered a website called DSO Browser, which allows you to search for objects and it provides the magnitude and size and much, much more. Check it out:
dso-browser.com/
Please let me know if this HOW TO has been helpful, and if you have any questions, please do not hesitate to ask. Also, feel free to add any additional online calculators or object databases that you use.