Darren27 Posted January 23, 2014 Share Posted January 23, 2014 HiCan 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. CheersDarrenSent from my LT26i using Tapatalk Link to comment Share on other sites More sharing options...
jambouk Posted January 23, 2014 Share Posted January 23, 2014 You need information on the eye piece(s) you will be using too:http://www.12dstring.me.uk/fov.htmJames Link to comment Share on other sites More sharing options...
jambouk Posted January 23, 2014 Share Posted January 23, 2014 Here's another one:http://www.csgnetwork.com/telefov.htmlThe FoV will vary with difference eye pieces.James Link to comment Share on other sites More sharing options...
rowan46 Posted January 23, 2014 Share Posted January 23, 2014 Another way is to put a telrad or rigel quickfinder on and use telrad charts http://www.atmob.org/library/member/skymaps_jsmall.html Link to comment Share on other sites More sharing options...
jnb Posted January 23, 2014 Share Posted January 23, 2014 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°. Link to comment Share on other sites More sharing options...
Stu Posted January 23, 2014 Share Posted January 23, 2014 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 Link to comment Share on other sites More sharing options...
Darren27 Posted January 23, 2014 Author Share Posted January 23, 2014 Sorry, what's the afov?Sent from my LT26i using Tapatalk Link to comment Share on other sites More sharing options...
jnb Posted January 23, 2014 Share Posted January 23, 2014 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. Link to comment Share on other sites More sharing options...
AlexB67 Posted January 23, 2014 Share Posted January 23, 2014 Dividing the apparent FOV of the eyepiece by the magnification is fine if you want a reasonable estimate and useTFOV = AFOV of eyepiece / MagnificationMagnification = focal length scope / focal length eyepieceBetter is to use the field stop measurements which sometimes may be known for an eyepiece and useTFOV = 57.3 * field stop / focal length scopeYou can refer to this page for the equationshttp://www.saharasky.com/saharasky/formula.htmlAFOV 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 Link to comment Share on other sites More sharing options...
John Posted January 23, 2014 Share Posted January 23, 2014 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 Link to comment Share on other sites More sharing options...
AlexB67 Posted January 23, 2014 Share Posted January 23, 2014 ... 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. Link to comment Share on other sites More sharing options...
John Posted January 23, 2014 Share Posted January 23, 2014 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 !. Link to comment Share on other sites More sharing options...
cantab Posted January 23, 2014 Share Posted January 23, 2014 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. Link to comment Share on other sites More sharing options...
Darren27 Posted January 25, 2014 Author Share Posted January 25, 2014 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 = 415mmEP1 20mmEP2 4mmThere 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. Link to comment Share on other sites More sharing options...
Stu Posted January 31, 2014 Share Posted January 31, 2014 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 Link to comment Share on other sites More sharing options...
Stu Posted January 31, 2014 Share Posted January 31, 2014 The second ep is 0.48 degrees, incase the arc min figure is not obvious.Cheers,Stu Link to comment Share on other sites More sharing options...
Darren27 Posted January 31, 2014 Author Share Posted January 31, 2014 Thanks Stu. Link to comment Share on other sites More sharing options...
lestergibson Posted February 2, 2014 Share Posted February 2, 2014 Here's a good read on eps from SGL member, Robin Wilkey:http://www.swindonstargazers.com/beginners/eyepieces.htm Link to comment Share on other sites More sharing options...
Darren27 Posted February 2, 2014 Author Share Posted February 2, 2014 Very good article on eyepieces by Robin, learnt a lot from that, thanks for the link. Sent from my MZ604 using Tapatalk Link to comment Share on other sites More sharing options...
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