How to calculate FOV

[PatrickO]

Can anyone help me with working out the FOV with a ZWO ASI224MC camera attached to a 500mm Nikon lens.

For example, what would be the view of M31 Andromeda galaxy.

Many thanks, Patrick

Edited March 21, 2022 by PatrickO

[Grant93]

https://astronomy.tools/calculators/field_of_view/

Just put in your camera and Lens/Telescope (Might have to do the lens FL and Aperture manually as it usually doesnt have lens info stored)

Then put in the target you wanna shoot and it will show you an image of the FOV

Edit: But at a guess, with a 224mc and a 500mm FL lens, you will be imaging core only on Andromeda

Edited March 21, 2022 by Grant93

[ollypenrice]

Yes, it's as Grant says; you'll have less than half a degree FOV with that setup and M31 is about 3 degrees. You need a smaller target!

Olly

[vlaiv]

Couple of ways:

- you can use arc second per pixel and camera resolution.

With 3.75um pixel size of ASI224 and 500mm FL you will have 206.4 * 3.75 / 500 = 1.54725"/px

(which is by the way - waaay over sampled for any lens).

now you have 1304 x 976px with said resolution produces -

1304 * 1.54725 x 976 * 1.54725 = 2017.6" x 1510.1"

which then you can convert to 2017.6" / 60 = 33.637 arc minutes or 1510.1" / 60 = 25.02 arc minutes

(dividing with another 60 will give you degrees).

- second way to calculate it is using trigonometry - 2 * arctan ( half_width / focal_length). Width of sensor is 4.9mm and focal length is 500 so you need 2 * arctan of 2.45mm / 500mm.

Arctan of 2.45 / 500 is 0.2807

2 * 0.2807 degrees = 0.5615 degrees or if you want that in arc seconds - multiply with 60 to get 0.5615 * 60 = 33.67 arc minutes

Using first method we have 33.63 arc minutes and using second method we have 33.67 arc minutes. That is simply because width of sensor is not precisely 4.9mm and pixel size is not precisely 3.75um otherwise we would have following match

3.75 * 1304 = 4890 and not 4900 (or 4.89 vs 4.9 in mm), so there is small difference due to rounding of sensor specifications.

And of course - you can use online tool if you don't want to remember all the math involved (which is just basic trigonometry really).

 

 

[vlaiv]

1 minute ago, ollypenrice said:

Yes, it's as Grant says; you'll have less than half a degree FOV with that setup and M31 is about 3 degrees. You need a smaller target!

Olly

I would phrase it like this:

You need bigger sensor .

This holds true regardless of the target as lenses are usually not sharp enough to justify such small pixels and resolution will suffer. Since this sensor is 1304 x 976 - it will produce images in 640 x 480 px range and that is very low res by today's standards.

[ollypenrice]

Just now, vlaiv said:

I would phrase it like this:

You need bigger sensor .

This holds true regardless of the target as lenses are usually not sharp enough to justify such small pixels and resolution will suffer. Since this sensor is 1304 x 976 - it will produce images in 640 x 480 px range and that is very low res by today's standards.

Smaller targets are free!

🤣lly

[vlaiv]

4 minutes ago, ollypenrice said:

Smaller targets are free!

🤣lly

So is bigger sensor

Well, at least if you don't subscribe to that "Time is money" thing :D

Mosaics do wonders ...

[ollypenrice]

2 hours ago, vlaiv said:

So is bigger sensor

Well, at least if you don't subscribe to that "Time is money" thing :D

Mosaics do wonders ...

Let's ask PatrickO if he wants to do an 18 panel mosaic of M31!

lly

[vlaiv]

1 minute ago, ollypenrice said:

Let's ask PatrickO if he wants to do an 18 panel mosaic of M31!

lly

Sooner or later everyone wants to do panel of M31.

It starts with 2 pane, then it grows with every new season

By the way - @PatrickO alternative would be to use shorter FL lens as well. I would still use something like 2x2 mosaic and bin data you get from camera to make pixels larger. You can get old M42 lens like Helios / Biotar 58mm very cheap - it would have very usable FOV at that FL (about 5 degrees - that would frame M31 nicely) and stopped down to F/4 would not have any aberrations.

Here is what M31 looks like with 4.8um pixel size and Samyang 85mm lens (this is actually crop - FOV is very nice with this combination and ASI178). Again - I had to bin the pixels as 2.4um pixels of ASI178 are way too small for this lens, and even when binned x2 - image still is not as sharp as it could be (in part because I left lens wide open at F/1.4 so there is a lot of chromatic aberration - btw, this is green channel only rendered as mono image - stretched very hard just to see how much data there really is).

 

[ollypenrice]

Yes, a shorter FL lens would serve you well with this camera.

Olly

[bosun21]

I thought the ASI224 MC was strictly a planetary camera that had a high frame rate and would be too noisy on longer exposures?. This is what I have been led to believe. Is this not the case? Thanks 

[vlaiv]

7 minutes ago, bosun21 said:

I thought the ASI224 MC was strictly a planetary camera that had a high frame rate and would be too noisy on longer exposures?. This is what I have been led to believe. Is this not the case? Thanks 

Image does not get noisier with exposure time - only less noisy. This is true for any sensor.

DSOs can be imaged with planetary cameras - same as with any other camera - except the fact that sensor size is very small - which limits its use.

This image was taken with ASI185mc guide / planetary camera.

camera is 1944x1224 and I was using long focal length instrument so I had to do mosaic of 2x2 panels - seam can be noticed in the image and total exposure divided on 4 panels was not enough to go very deep - but I did manage to get NGC7331 and small neighboring galaxies.

[inFINNity Deck]

6 hours ago, Grant93 said:

https://astronomy.tools/calculators/field_of_view/

Just put in your camera and Lens/Telescope (Might have to do the lens FL and Aperture manually as it usually doesnt have lens info stored)

Then put in the target you wanna shoot and it will show you an image of the FOV

Edit: But at a guess, with a 224mc and a 500mm FL lens, you will be imaging core only on Andromeda

An alternative is using Stellarium for that. There it also is possible to define scopes, cameras and reducers/barlows. It directly shows you a nice red window over the heavens (here is SkyWatcher Esprit 80ED @ native focal length with ZWO ASI1600MM Pro Cool):

It also provides information like pixel scale and actual FOV. Here is a way too short exposed image using that same scope/camera combination:

Appears to be spot-on. 🙂

Nicolàs

[Grant93]

21 hours ago, inFINNity Deck said:

An alternative is using Stellarium for that. There it also is possible to define scopes, cameras and reducers/barlows. It directly shows you a nice red window over the heavens (here is SkyWatcher Esprit 80ED @ native focal length with ZWO ASI1600MM Pro Cool):

It also provides information like pixel scale and actual FOV. Here is a way too short exposed image using that same scope/camera combination:

Appears to be spot-on. 🙂

Nicolàs

Oh nice! Everyday is a school day. 🤓😃

Whilst I'm back here that stellarium suggestion also reminded me of https://telescopius.com/

Another good website for FOVs

[gilesco]

If you are really lazy, take one of your pictures, and upload it to astrometry.net: https://nova.astrometry.net/upload

Once submitted and processed you get lots of information (e.g.):

You can additionally just plate solve your picture locally and a lot of information will be present in the logs. This is quicker, but using astrometry.net online, while it takes a while, you get lots of information about your picture, annotated versions, overlays on the World Wide Telescope etc...