FOV

[Darren27]

Hi

Can anyone help explaining how to calculate the FOV of a scope?

I am using an old 60mm refractor and I was trying to plan using my star atlas how to find an object. The guide said to plot a series of moves from a know point to the destination but I don't know what the FOV for my scope is.

Cheers

Darren

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[jambouk]

You need information on the eye piece(s) you will be using too:

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

James

[jambouk]

Here's another one:

http://www.csgnetwork.com/telefov.html

The FoV will vary with difference eye pieces.

James

[rowan46]

Another way is to put a telrad or rigel quickfinder on and use telrad charts http://www.atmob.org/library/member/skymaps_jsmall.html

[jnb]

Another way is to use your scope and eyepiece combination and time an object as it crosses the field of view. One minute = 15' FOV, one second = 15" FOV. To do that pick objects at or near 0° declination, preferably at local meridian.

An even simpler way is to look at an object of known size and simply estimate the FOV. For that the moon is the easiest target as it's a nice easy 0.5°.

[Stu]

Do you know the focal length of the scope and eyepiece? You also need the afov of the eyepiece then it's quite easy to calculate.<br />

<br />

Stu

[Darren27]

Sorry, what's the afov?

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[jnb]

Apparent field of view (ie how big the field of view looks through the eyepiece). If you buy a new eyepiece then it should have a stated AFOV. The true FOV is that divided by magnification. For standard eyepieces that you might get with a scope that might be about 50° so if the magnification is 80 then the true field of view is about 37.5'. If you don't know those numbers then it might be easiest to just measure it.

[AlexB67]

Dividing the apparent FOV of the eyepiece by the magnification is fine if you want a reasonable estimate and use

TFOV = AFOV of eyepiece / Magnification

Magnification  = focal length scope / focal length eyepiece

Better is to use the field stop measurements which sometimes may be known for an eyepiece and use

TFOV = 57.3  * field stop / focal length scope

You can refer to this page for the equations

http://www.saharasky.com/saharasky/formula.html

AFOV should be supplied by the eyepiece, if not google the make of it, look on the side of it, or look at the info or manuals that came with the scope. Same for the focal length of the telescope. I you can tell us the exact make and model of scope and eyepiece, likely there will be some info printed on the eyepiece somewhere, we can help further if need be

The method of watching and timing an object drift across is not really needed unless you want to know it for other reasons like verifying the reported apparent FOV or determine the true FOV experimentally. The above methods are more than likely what you need and good enough, much simpler, unless it is something specific you are after I am not aware of.

Good luck

[John]

Sometimes physically measuring the field stop diameter (if it is accessible) or performing drift timing demonstrate that the apparent and true fields of view are not quite what the manufacturer claims 

[AlexB67]

... like the BST starguiders for example, I recall a review  where the the 8 or 15mm, (can't recall which) was measured, it is sold as a 60 degree eyepiece though reported to have 57 degrees in that article.  Not that it ever bothered me or measured it myself, but some may demand their money back   If I bought a 100 degree eyepiece and it only has 85 or less I may feel a bit niffed though.

[John]

I seem to recall the Hyperion 21mm is more like 60 degrees than 68 and the Hyperion zoom Mk III has been measured at 42 degrees at 24mm growing to 70 degrees at 8mm. A case of "swings and roundabouts" with that one !.

[cantab]

If I'm using a new-ish EP and I come across a pair of stars that just fit in the view, or just don't fit in, I note down the stars in question. Then I can measure the distance between them in Stellarium to get the true field, and ensure that Stellarium's ocular plugin is set up right. It probably isn't as precise as timing a star on the equator, but it's good enough for me.

[Darren27]

Thanks all for the replies.

I don't have much info on my current scope, but I have managed to find the following,

60mm refractor,

F = 415mm

EP1 20mm

EP2 4mm

There are no other markings on any parts and I dont have any of the orignal documentation.

If the weather ever clears I am going to try to find a couple of stars that almost fill the view and them try to measure of Stellarium to get a rough figure.

[Stu]

I put this info into Luminos, and assuming the eyepieces have a 50 degree fov then the figures are in this attachment. Should be fairly close.

Stu

[Stu]

The second ep is 0.48 degrees, incase the arc min figure is not obvious.

Cheers,

Stu

[Darren27]

Thanks Stu.

[lestergibson]

Here's a good read on eps from SGL member, Robin Wilkey:

http://www.swindonstargazers.com/beginners/eyepieces.htm

[Darren27]

Very good article on eyepieces by Robin, learnt a lot from that, thanks for the link.

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