FOV formula for Image at Prime Focus

[Startrek]

Would like to confirm if the following formula is correct to find out your FOV at prime focus when imaging targets

Formula
Width of camera chip in mm x 3460 / Telescope focal length in mm

Example: My 8” f5 newt with a Canon 600 D at prime focus

So width 22.3mm x 3460 / 1000mm = 77.2 arc minutes

and height 14.9mm x 3460 / 1000mm = 51.5 arc minutes

Therefore image FOV would be 77.2 arc minutes wide and 51.5 arc minutes high

Please advise if the above is correct?

Thanks in advance

[billdan]

Yes what you have is pretty close to what I calculated.

I haven't seen that formula that you used before.

The formula I use is, 206.3 / Focal length(mm) gives you arcsecs per micron.
So for a 1000mm focal length that would be 0.2063 arcsecs per micron.

You can then multiply 0.2063 by your pixel size to give arcsecs per pixel.

Or multiply 0.2063 by the width of your sensor (in microns) 22,300 * 0.2063 = 4600.49 arcsecs or 76.67 arc mins.

Or multiply 0.2063 by the height of your sensor (in microns) 14,900 * 0.2063 = 3073.87 arcsecs or 51.23 arc mins.

[astro744]

For eyepieces, Field of view in deg = field stop diameter in mm of eyepiece x 57.3 / telescope focal length in mm.

For cameras, replace field stop diameter with sensor dimensions.
E.g. Field of view in deg = sensor dimension in mm x 57.3 / telescope focal length in mm.

To work in arc min multiply 57.3 x 60 = 3438. You were close with 3460. If you want even more accuracy instead of 57.3 use 180/Pi = 57.2958 but you will find you 1000mm focal length of the telescope will have more variation so 57.3 is sufficient accuracy.

See http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=85

[Paullus]

https://astronomy.tools/ is your friend :)

[Startrek]

Astronomy tool seems to be a bit inaccurate in FOV
Maybe because they don’t have my telescope in the presets
I’m happy to use my formula and maths
Thanks all

[Merlin66]

I use CCDCalc.
You can enter your telescope/ lens/ camera combo and it will give the FOV, plate scale and a comparison image.
http://www.newastro.com/book_new/camera_app.html

Putting in your data gives 50.7 x 76.1 arcmin, at 1.17 arcsec/ pixel

[multiweb]

Hi Martin, check that spreadsheet. It's got a bit of everything in it but that's what I've used for a while now to figure out image scale, fov, etc... Check the formulas so you can make your own.

[Startrek]

Thanks All
The formula I found on Sky & Telescope 2018 are close to the mark , exactly what I wanted to double check !!
Appreciate the feedback and attached data
Many thanks
Clear skies

[Nikolas]

For those of us whose eyes glaze over by all the nerdtalk above (no offense I love that people can be so analytical)this is a visual version
https://astronomy.tools/calculators/field_of_view/

[multiweb]

That's a very neat site! Thanks for the link. :thumbsup:

[Startrek]

As I mentioned on this thread I’ve tried Astronomy Tools FOV calc and I don’t agree with outcome on the image displayed, maybe because they don’t have my telescope listed in the presets ??
M8 Lagoon Nebula is approximately 90 arc minutes x 40 arc minutes in size
My set up according to astro maths is 77 arc minutes x 51 arc minutes in FOV confirmed by a few IIS members on this thread but according to the AT image produced, my set up easily fits around M8 with heaps of room to spare
Somethings not quite right ?
So Im sticking to Astro maths, as I do for everything else in Astronomy both visual and AP
Proof in the pudding will be when I get a chance to image M8 end of May / early June
I will post the image and a brief note on the above posted topic

Cheers

[multiweb]

Do you have sky safari pro? The program shows you an accurate fov based on your telescope and camera specs.

[Startrek]

No but I’ll have a look
Thanks

[astro744]

You may also be interested in SkyTools 4 for imaging. See https://www.skyhound.com/skytools.html

Have a look at the video.

It is certainly not free but is very good. I have version 3 Pro but I use for visual only. In version 4 the developer split visual and imaging. Imaging has been released but visual is pending. There are different upgrade price options if you buy 3 now than if you bought but a couple of years ago. See Skyhound website for details.

[Merlin66]

Martin,
Using M8 to gauge your FOV is not accurate.
The most accurate method is to plate solve one of your images....
This will give exactly the field coverage and the plate scale.

I must say, CCDCalc has always given me very good results.

[Startrek]

Thanks Ken
I will have a look at CCDCalc as well
Everyone’s been so helpful on this thread , appreciate the feedback and great advice

[sil]

Precisely what I've always done. My gear rarely changes so I get one image and feed to astrometry to plate solve. Regardless the web specs on your gear the variayions in manufacturing tolerances etc mean in practice they are not that precise in the end. Your image is exactly the fov you get so why not measure just that?