Field of View

[Qualia]

I was wondering if anyone could help me?

I'm wanting to work out the Field of View I see through my OTA, each of my EPs and the two different finderscopes. I'll then be able to draw up some little rings on plastic sheeting to overlay on the sky atlas I have. The trouble is I'm completely confused. For example, the Telrad's outer ring has a FOV of 4º, the 9x50 finderscope 5.5º (how that is worked out I have no idea) and then I read that, say, the Tal 25mm plossl has a FOV of 52º and the X-Cel LXs 60º, but in my head that's impossible. I'm 'seeing' a lot less space through a 7mm than my finderscope, for example, but the statistics are saying I have a FOV of 60º in the EP, 12x that of the finderscope

I'm completely lost and confused

Can anyone advise me on how I work out FOV, that is, the field of space I'm seeing and if possible tell this dumb Spanish fool what exactly is a FOV if it isn't the heaven-space I'm seeing?

[Freddie]

Try this website

http://www.12dstring.me.uk/fov.htm

Hope it helps

[Stephen]

True FOV = AFOV (the 58/60 degree figure quoted) divided by Magnification

Magnification = Focal Length (scope) divided by Eyepiece size

So for a 52deg 25mm plossl in a 1000mm scope

Magnification = 1000 / 25 = 40x

True FOV = 52 / 40 = 1.3deg

[Steve Ward]

Try the Ocular plug-in in Stellarium .

You can load all your eye-piece info and then see on screen what you wlil see through ep.

Good tutorial here ,

Hope this helps.

[Qualia]

Thanks Freddie, but it's not working for me. I'm still at base. For example, I plug in Hyperion 5mm and it tells me I have a FOV of 20.4' which sounds an awful lot more width of space than what my finderscope is 'seeing', but as you'll appreciate, that's just impossible.

- - - - -

Wow!! You guys posted just as I was scripting to Freddie. Thank you so much for the swiift reply, chaps. That's just brilliant, especially that little equation Stephen has offered! That's just what I was looking for. Thank you and I'm sorry for taking up your time.

[euphony]

I found this really helpful too. Great stuff

@Qualia: Where in Spain are you based? I'm on my way over there next week, I can't wait!

[AndyH]

Regarding simpler eyepiece designs like plossls orthos etc, ie: ones that have a machined in field stop in the barrel - measure it as accurately as possible. Multiply it by 57.3 then divide the result by the scopes focal length in millimetres. That should result in a pretty accurate TFOV.

Another way, perhaps the best and one that is quite fun - google (or similar), 'true field of view star drift test'. I've done this to a few of my pieces. Found out that my Tal 2" UWA 24mm, rated as 80 degrees is actually 95 degrees !!

I often do it when waiting for a scope to cool down

Andy.

[glowjet]

This is another interesting way you can do it to give quite an accurate measurement of your EP www.umanitoba.ca/science/astronomy/cbrown/16207/fov.htm this is done at the at the telscope with a stop watch and calculator

John.

[Qualia]

Thanks for the great replies! I did some calculations at work this afternoon and have it more or less sussed. I'll make a little wire ring or two this night and will try to use it with the star atlas. Hopefully, I'll get a better grips of where exactly I am in space.

Please forgive my ignorance, but I have a question. Imagine I have a FOV 52º 25mm plossl in a 1000mm scope, (1000 / 25 = 40x) that means I have a FOV of about 1.3º (52 / 40). So what does the former FOV mean? Does it say that with my 1.3º FOV I should except about 50% of that being sharp and clear? That is, of the 1.3º FOV potential I should be expecting to 'see' about 52º of that?

As you can see, I'm a tad confused with all this.

[Mike73]

True FOV = AFOV (the 58/60 degree figure quoted) divided by Magnification

Magnification = Focal Length (scope) divided by Eyepiece size

So for a 52deg 25mm plossl in a 1000mm scope

Magnification = 1000 / 25 = 40x

True FOV = 52 / 40 = 1.3deg

Its a question which I've been meaning to ask for a while so thanks for bringing it up Qualia and thanks Stephen for the straight forward answer.

[glowjet]

Instead of a wire ring, why not try some clear plastic sheet upon which you can draw waterproof ink circles, you can then overlay your star charts with it

John.

[AndyH]

Thanks for the great replies! I did some calculations at work this afternoon and have it more or less sussed. I'll make a little wire ring or two this night and will try to use it with the star atlas. Hopefully, I'll get a better grips of where exactly I am in space.

Please forgive my ignorance, but I have a question. Imagine I have a FOV 52º 25mm plossl in a 1000mm scope, (1000 / 25 = 40x) that means I have a FOV of about 1.3º (52 / 40). So what does the former FOV mean? Does it say that with my 1.3º FOV I should except about 50% of that being sharp and clear? That is, of the 1.3º FOV potential I should be expecting to 'see' about 52º of that?

As you can see, I'm a tad confused with all this.

Hi.

The FOV that is stated with an eyepiece's focal length, is referred to as the apparent field of view or AFOV. This is how wide the field looks when you actually look into the eyepiece.

The true field of view(TFOV) is how much of the sky you'll see WITH the scope. You get that by dividing the AFOV of the EP by the power that the EP gives you in your scope(as long as the manufacturer gives correct info). It should be pretty close to reality. To get it more accuarate, use the field stop or star drift test method.

You might find this interesting........

http://www.astrosurf.com/luxorion/reports-epsuggestions.htm

Cheers,

Andy.