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astrophotography. I want to know the focal length is to determine the field of view?

I am interested in astrophotography.  I want to purchase a camera for my telescope.  You can find the field of view if you know the focal length of the telescope.  But what is the focal length of a camera with a lens attached?  If I have a 50 mm lens, is the focal length 50 mm? or do you need to know the distance from the body camera to imaging sensor and add that to the focal length of the lens?  Thanks in advance..

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Re: astrophotography. I want to know the focal length is to determine the field of view?

If you have a 50mm lens... that is the focal length.  The focal length is the distance from the front of the lens to the focus plane.  If you look at a DSLR camera, there's a symbol on the body (usually the top of the body and often near the viewfinder) which resembles a small circle with a line drawn through it... that line marks the position of the focal plane inside the camera.

 

There's one more rub... if you had a "simple" lens, the this would be the focal length.  Many modern lenses are technically able to be physically shorter than the focal length that they provide ... because of the types of "glass" and configuration of the optics.  So the focal length is based on how long a simple lens would need to be in order to provide that same value (even if you were to physically measure it out and not get that value.)

 

All telescopes list their focal lengths as well as focal ratios among the telescope specs.  I have a 540mm refractor with a 101mm aperture... so that's "roughly" an f/5.4 scope.  I also have a larger 14" SCT with a focal length of 3556mm and it's an f/10 scope.

 

The "flange to focal plane" distance on a Canon camera is 44mm... in the grand scheme of things with a telescope, this will not significantly alter the focal length of the image.    

 

Depending on the scope, you can employ either a "focal reducer" or a "barlow" to decrease or increase the focal length.

 

One last thing... many "newtonian" type telescope designs are problematic for astrophotography.  This is because most refractors and catadioptric telescopes are designed with the intention that a persone would be using a 90º diagonal with their eyepiece.  The diagonal tends to add "roughly" 2" to the overal length of the focal path.  When you use a camera you remove that "diagonal" and just attach the camera for a "straight through" shot.  So while you lost the 2" of focal length created by the diagonal... the camera adds "roughly" the same amount back in.

 

BUT... when you use a Newtonian reflector design telescope, the eyepiece is near the front and there's no "diagonal" in use.  That means when you add a camera, the focal length gets longer than it would be with an eyepiece.    The focuser drawtube has some focus travel... but since the camera makes the focal length longer, you end up having to run the focuser all the way "in" to shorten the focal length to bring the image to focus.  And THIS is where you run into problems... very often on a Newtonian telescope, you hit the limit of the focuser drawtube (you can't shorten the focus any more than you have) and the image was just beginning to come to focus... but is still quite blurry.

 

This problem does not plague all newtonian reflectors... some can accommodate cameras and still come to focus.  You'd really have to check the specific scope. 

 

 

Tim Campbell
5D III, 5D IV, 60Da
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Re: astrophotography. I want to know the focal length is to determine the field of view?

More important than all that, is that the size of the sensor in the camera will affect your FOV.  I wouldn't worry about flange distance and all that noise, just use a simple online calculator if you want to find your FOV, like this:

 

http://www.howardedin.com/articles/fov.html

 

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Registered: ‎06-11-2013

Re: astrophotography. I want to know the focal length is to determine the field of view?


@Skirball wrote:

More important than all that, is that the size of the sensor in the camera will affect your FOV.  I wouldn't worry about flange distance and all that noise, just use a simple online calculator if you want to find your FOV, like this:

 

http://www.howardedin.com/articles/fov.html

 


Agree -- my point on the "flange to focal length" distance is that it's not enough to be of any conseuqence given the relatively long focal length of a telescope.  

 

I WOULD worry about using a "newtonian" type reflector telescope -- as many of these cannot come to focus with a DSLR camera attached.  (not all of them.)

 

For lenses field of view calculations, Skirball offers a good link.  Here's another and, this is the one I most often use because of it has so many types of calculations that it can do:  http://www.tawbaware.com/maxlyons/calc.htm

 

Lastly... when the camera is connected to a telescope, there are a number of applications which can show you the field of view overlaid on an image of the sky -- this indicates how much of the area will be captured by your camera when attached to your telescope.

 

I use Starry Night Pro Plus.  The Sky X is also bery popular.  Both are commercial and run on Windows & Mac.  On my iPad I use Sky Safari Pro.    While these are paid apps, the price of Sky Safari on an iPad is fairly low. 

 

But Stellarium is free and open source and it also supports these field of view overlays (Stellarium calls them "Occulars" and has a plug-in that does this.)  

 

You configure the specs for the scope (focal length of the scope) and also the specs for the camera (dimensions of the sensor in millimeters) and the appliation can then place a rectangle on your view of the sky showing the area that the camera would capture.  You can use this to help frame up a shot (you can also rotate the rectangular area.)

 

Tim Campbell
5D III, 5D IV, 60Da
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Re: astrophotography. I want to know the focal length is to determine the field of view?

Tim and Skirball,

 

Thanks for the feedback.  I use Sky Safari Pro to make my observing lists.  I have a 10 inch Newtonian with the orion intelliscope feature.  I connect the Intelliscope to an Orion Starseek Wi-Fi telescope module.  The Orion StarSeek then communicates to the Sky Safari on my Ipad.  As I move the scope,  the Ipad displays a circle that indicates what part of the sky the scope is loooking at. 

 

I did bookmark the FOV programs that you sent me.  Also, if the backfocus requirement for the canon camera is 44 mm, then I have a problem because my backfocus distance is 38 mm.   I measured this distance because Orion does not have a backfocus specification.  I may need to get a different focuser that can give me the backfocus distance to use a canon camera.  

 

What I want to do is to have a setup that provides live views of the night sky at star parties.  

 

Thanks for the feedback.  Butch

Esteemed Contributor
Posts: 3,809
Registered: ‎06-11-2013

Re: astrophotography. I want to know the focal length is to determine the field of view?

Sky Safari Pro ... if you're on version 4 then that has the field of views in it. Go into "Setup" -> "Equipment" and enter all the equipment you have. Then go into "Setup" -> "Display" and you can configure the combinations of scopes & cameras or scopes and eyepieces that you want to enable. While eyepiece field of view is round, since the camera field of view is rectangular you also get the option to set the orientation of the camera (rotation.)
Tim Campbell
5D III, 5D IV, 60Da
Esteemed Contributor
Posts: 3,809
Registered: ‎06-11-2013

Re: astrophotography. I want to know the focal length is to determine the field of view?

I have "heard" of people shimming the primary mirror on a newtonian scope just a few millimeters higher to push the focus plane back a little farther. A different focuser might also work if it can let the camera get a little closer. Use of a barlow usually also works but then that changes the focal ratio and effective focal length of the scope.

Some dedicated video cameras place the chip (focusing plane) much closer.
Tim Campbell
5D III, 5D IV, 60Da
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