Field of view from Skywatcher explorer 130pds

[Rustang]

Has anyone got, or can post a link to an exact field of view that I would get from a full frame DSLR mounted on a skywatcher explorer 130pds!? I have founds slight differences/confusion from researching on the web so an image straight from someone who has this set up would be great. I mainly would like to know in regards to the FOV for M42. And is it going to be narrower (closer into the object) than a 400mm camera lens!?

[jjosefsen]

http://astronomy.tools/calculators/field_of_view/

 

Select imaging mode and input camera and telescope.

[Rustang]

Done that, It looks like you get nice and close to the Orion Neb but from real life images Ive seen it doesnt look much closer than a 400mm lens, hence the confusion and what I would like to confirm as the actually field of view. The tool is also different in FOV to another similar online tool so even more confusion comes there!

[jjosefsen]

I have found it to be quite accurate, it is what i usually use..

Make sure you input any reducers / barlows you might use though.

[Rustang]

See now between the tool and an image posted on here using the 130pds, they dont seem to match, the tool seems to fill the middle of the frame nicely, the second isant far off my 400mm camera lens, that was taken on a crop sensor DSLR so should have been "closer in" than my full frame!

 

 

[Rustang]

The yellow box is apparently what a Cannon 600D should see which doesnt match the photo taken! Hopefully you now see my confusion!

[vlaiv]

600D pretty much matches the field, doesn't it?

It is different framing/orientation and extent of nebula represented is different between FOV tool and the image, but pixel scale looks exactly as it should.

There might be slight difference between the two - if image was taken with use of coma corrector that is also reducing or extending focal length.

There are several coma correctors out there that change focal length, and since there is no info supplied with the image - we can only guess.

Field of view depends on two things only - focal length of scope and diameter (or sides) of sensor - and there is very straight forward way to calculate it - a bit of trigonometry and you can have it solved without need of any online calculators. Once you have that, you can compare it to particular tool and see if they agree (and they should).

Here is diagram that can help:

As you see, focal length and distance on sensor form a triangle, and tangent of that angle is sensor_distance / focal length

So if your sensor is full frame - or 36mm x 24mm then

18 / 650 = tan(alpha) => alpha = 1.586247 degrees so 36mm equates to 2 x 1.586247 = 3.1725 degrees = 3 degrees 10' 20"

12 / 650 = tan(alpha) => alpha = 1.057648 degrees so 24mm equates to 2 x 1.057648 = 2.1153 degrees = 2 degrees 6'  55"

So your fov will be 3° 10' 20" x 2° 6' 55"

If you observe your screen shot, D750 is listed as having 3.16 x 2.11 FOV - which differs from above 3.17x2.11 FOV simply because Nikon D750 is not 36mm x 24mm but 35.9mm x 24mm.

[Rustang]

Thanks for your help trying to clarify things, I think I understand a bit better. Cheers.