Camera Labs

I know how to find a camera lens’ FOV.
I know how to find my telescopes FOV with a specific ocular (eye piece).
I haven’t a clue how to compute a camera’s FOV mounted on prime focus.
I’ve downloaded this wonderful program- Carte de Ciel, a computerized sky chart – I find it much more helpful than any planetarium software that I’ve came across in helping finding specific objects in the sky with a scope. And it’s free!
One of its features is the option to plot a circle(s) or rectangle(s) that encompasses a specific FOV.
It’s very helpful, if you don’t have a go-to system, in plotting a “star hopping” way by drawing circles with your finder’s FOV.
I’d like to use this feature to plot my image rectangle on the chart so I’d know what to expect, but I need the vertical and horizontal FOV in arc seconds and I don’t know how to find it for a specific sensor in prime focus and scope focal length and aperture.

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Radu
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
Canon580EXII
http://www.errre.net

You can use one of the many calculators out there. I particularly like this set. Having said that, the FoV is returned to a resolution of 1/10 of a degree so you might want to look for one with more precision elsewhere at long focal lengths.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

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Of course there's always those wonderful old-fashioned things called tangents. Not everything they tried to teach you at school should be forgotten...

Bob.

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OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

@popo, thanks for the link
@Bob. Although I’ve never forgot my tangents, geometric optics are much more blurry. I’ve thought that a telescope is similar to a camera lens in this respect, but I couldn’t be sure that a Newtonian reflector would not introduce some other factors to consider. Sadly, in my experience, things are more complex than they seem. But if you say that tangents are enough, I’m satisfied that they are. Thanks for the help

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Radu
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
Canon580EXII
http://www.errre.net

Hi Radu,

I guess the trouble with modern optics, especially DSLR lenses actually, is that they consist of multiple elements and it's not always obvious where the center of an equivalent single lens would be. But actually that doesn't matter because all lenses, telescopes included, have a headline focal length.

And I'm not just theorizing here as I'm definitely putting my money where my keyboard is. I have a new 'scope on order and it has a focal length of 980mm. A ray of light passing through the center of the aperture can be pretty much assumed to pass through undeflected (same deal with a reflector apart from the fact that the ray doubles back on itself and angle of incidence equals angle of reflection) so I've used that together with tangents to calculate the field of view from the center of the sensor out to the sides and then doubled that figure to get the total field of view. Those calculations did prove expensive, however, as I rapidly convinced myself I needed to specify a CCD camera with a bigger sensor!

Update: Usable FOV might be another matter, though. Newtonians can have a limited FOV over which everything is really sharp. The telescope's documentation should help determine if that's a factor for you.

Bob.

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OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

I find Stellarium is ideal for this (http://www.stellarium.org/). Is free and the interface is pretty nice.

You can get a plugin for it called Ocular. In the plugin you can put into all the scopes you own and all the eye pieces and all the cameras (and even camera lenses). You can then choose any combination of scope and camera (or eyepiece) or camera and lens and it will project a rectangle of the FOV onto the sky.

I find it great for seeing what a certain lens will capture for widefield AP and great to know what targets are suited when using my scope.

This screenshot shows the scope and lenses where the focal length and apertures are added:



This shows the camera info required:



Both shots show how saturn would look on my D200 on my SW250p DS which has a focal length of 1200mm.

HTH

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Lee Diggle
My Astronomy Blog | My Photography Gallery

Hi Bob, sorry for causing you trouble. I have managed to compute the FOV, but I’ve done something stupid . I’ve computed resolution too and reached about 1.3 seconds /pixel. That’s disappointing too, I need more (i.e. more $$$$)

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Radu
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
Canon580EXII
http://www.errre.net

Hi Digz.
Thank you for the heads up. I didn’t know about the plug in. I’ll surely try to get it. BTW, I’m waiting for the skies to clear, so I may get a shot at Porrima and Saturn too.

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Radu
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
Canon580EXII
http://www.errre.net

Hi Radu,

No problem. So far as resolution is concerned a lot depends on your best seeing as well as the optical resolution of the 'scope. As I'll be mainly interested in relatively wide-field deep imagery of about 2° edge to edge I'm happy enough to settle for a sensor limited resolution of 1.85 arcseconds. I don't live on a mountaintop so although the Dawes Limit suggests a figure of 0.85 arcseconds as the telescope limited resolution I somehow doubt I'll miss out too much on most nights. Of course as the camera will be almost a permanent fixture I'll never know any better anyway! If I were interested in planetary imaging I'd certainly want better resolution as well as "video" capability and for that I'd want to swap out the expensive cooled CCD and fit a cheap webcam chosen especially for its small pixels (an Orion StarShoot Solar System Color Imaging Camera IV with its 3.6µm pixels maybe???) but that would be another project altogether.

Bob.

_________________
OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.