Cazz Posted July 11, 2012 Share Posted July 11, 2012 (edited) I'm asking this question in the beginners area, because a) I'm still a beginner, b ) it's probably a stupid question and this thread says nothing is stupid, c) I don't want to lower the tone of the eye piece thread. So here goes, Is it possible, if so, how do I work out the size in mm of our spotting scope eye piece marked in X's. The eye piece is from a 60mm spotting scope I believe, and it's marked from 15X to 45X. I'm guessing it works out at something like 7mm to 21mm, but this is more of a pot luck guess than any calculation, is there a formula or table showing the comparisons anywhere please? Many thanks for any pointers. Edited July 11, 2012 by Cazz Link to comment Share on other sites More sharing options...
Daz Type-R Posted July 11, 2012 Share Posted July 11, 2012 (edited) If I understand correctly you are after the mm size of you ep, is that correct? To find out the magnification of your ep's you normally divide the focal length of the scope by the focal length of the ep. so 60/4=15x zoom, so one ep is 4mm and applying the same to the 45x would give a mm of 1.3. So 15x ---> 45x = 4mm --->1.3mm. Have I got that right, seems a bit odd to me? Edited July 11, 2012 by Daz type-r Link to comment Share on other sites More sharing options...
oldfruit Posted July 11, 2012 Share Posted July 11, 2012 Hi Cazz, not a stupid question at all. To find out the magnification of an eyepiece you devide the focal length of the scope by the focal length of the eyepiece and the answer gives you the magnification. It works the same with the focal length of the scope devided by the magnification and the answer gives you the focal length of the eyepiece. Hope this helps. Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 If I understand correctly you are after the mm size of you ep, is that correct? That's correct. Link to comment Share on other sites More sharing options...
Daz Type-R Posted July 11, 2012 Share Posted July 11, 2012 Nice, then my answer stands, also, oldfruit confirms what I thought, your ep is a 1.3mm ----> 4mm. Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 Thanks guys, that is very useful information, I think I follow, I'm very easily confused until I get my head around something, then there's no stopping me!!! So my guess is clearly way off, well I did pluck the numbers from fresh air, lol. At least we have a start on our eye piece collection, it's very very cheap, and you have to start somewhere, it's a move up from our 1" fit 22mm one. It just proves everyone is welcome here. Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 Right, another question, I follow the workings and it all makes sense, but isn't the 60mm the aperture, not the focal length? I don't know what the focal length of the broken spotter scope was. I guess I'll have to take an average from focal lengths of finder scopes with 15X to 45X eye piece. Link to comment Share on other sites More sharing options...
oldfruit Posted July 11, 2012 Share Posted July 11, 2012 Yes it looks like 60mm was the size of your objective lens. It is the focal length that you need. What make of spotter scope was it? You may be able to look up the focal length of the scope if you no longer have it. Link to comment Share on other sites More sharing options...
Daz Type-R Posted July 11, 2012 Share Posted July 11, 2012 Hi Cazz, my bad. What I wrote is correct apart from the fact I missed that the 60mm was the apature not the focal length, so yes you are correct, it should be the focal length not the apature. That would explain why I thought it looked odd. Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 Yes it looks like 60mm was the size of your objective lens. It is the focal length that you need. What make of spotter scope was it? You may be able to look up the focal length of the scope if you no longer have it. The problem is, we haven't a clue, we've only ever seen the eyepiece. Perhaps my fresh air figures will have to do lol. Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 Hi Cazz, my bad. What I wrote is correct apart from the fact I missed that the 60mm was the apature not the focal length, so yes you are correct, it should be the focal length not the apature. That would explain why I thought it looked odd. I'll let you off, lol. But points for realising something wasn't quite right. Link to comment Share on other sites More sharing options...
Brantuk Posted July 11, 2012 Share Posted July 11, 2012 Do you have the focal ratio of the scope Cazz? We can work out the focal length from that combined with the aperture. It will be a number like f-10 or f7 or F/5 etc - usually marked somehwere on the scope (a plate or around the objective lense) Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 Do you have the focal ratio of the scope Cazz? We can work out the focal length from that combined with the aperture. It will be a number like f-10 or f7 or F/5 etc - usually marked somehwere on the scope (a plate or around the objective lense) Unfortunately not, we've only got the chunky heavy eye piece, it gave a nice sharp view of Saturn a few weeks ago. We'll have to bring it to the next meet so we can all play snap with size, lol. Link to comment Share on other sites More sharing options...
Tweedledee Posted July 11, 2012 Share Posted July 11, 2012 Hi Cazz, If your scope looks like this one on ebay then the focal length is 500mm as stated. />http://www.ebay.co.uk/itm/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l2557&item=330586506126&nma=true&rt=nc&si=jQbAUEndFI2Fz6Q7P0LA9vAnekg%253D&orig_cvip=true&rt=nc The eyepiece you have was probably not originally supplied with your telescope. So the 15x to 45x written on the eyepiece is probably not the magnifications it will provide in your scope. If it was, then the eyepiece focal length would calculate to 500/15 = 33.33mm and 500/45 = 11.11mm. Never heard of a zoom with those focal lengths. Now if your scope is the 500mm focal length then the magnification and focal length of the eyepiece can be worked out by a different method as follows. First find the focal ratio of your scope. That will be focal length/aperture = 500/60 = 8.33. So the focal ratio is F8.33. During daylight, point the scope at a bright window with the eyepiece set to its lowest magnification. Get a ruler and measure the diameter of the exit pupil. That is the diameter of the circle of light exiting the eyepiece. You will see this best from several inches away from the eyepiece. The diameter will be somewhere between about 3mm and 6mm. Lets assume you measure it at 3mm. Divide the aperture by this number and you will have the true magnification. aperture/exit pupil = magnification Say 60mm/3mm = 20x magnification Now to get the focal length divide the focal length of the scope by the magnification. eg: 500/20x = 25mm The higher end magnification will be 3x that and the focal length will then be one third. Hope you can follow that. Cheers, Pete. Link to comment Share on other sites More sharing options...
Brantuk Posted July 11, 2012 Share Posted July 11, 2012 You can get a very rough idea of the focal length with a tape measure from the objective lens to about half way along the draw tube. Divide that by 60mm to get the rough F-ratio. Or divide it by the eyepiece size to get magnification Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 Our scope is a 60mm, 900mm focal length 1" fit. The eye piece is a 1.25" fit from someones random unknown broken spotter scope, we have an adapter thing so it fits. We were just trying to be all technical and work out what mag we were getting the other week, we will give the suggestions above a go and see how we go, prepare to be amazed by my skills, lol. PS, I've only ever seen the following once, the moon, Jupiter and Saturn. Link to comment Share on other sites More sharing options...
Brantuk Posted July 11, 2012 Share Posted July 11, 2012 Ok - that's f-15 (900/60) and the ep's are 900/15=60mm and 900/45=20mm Link to comment Share on other sites More sharing options...
Brantuk Posted July 11, 2012 Share Posted July 11, 2012 Woops - I just realised it's from the 60mm spotting scope so I'll revise the eyepieces: It's 60/15=4mm and 60/45=1.3mm. So in your astro scope it'll be 900/4=225x mag and 900/1.3=692x mag. I'd forget the 1.3mm - they work totally different in spotting scopes (at least I got the f-ratio right lol) Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 (edited) Does that work if the aperture is 60mm, the focal length is known. Edited July 11, 2012 by Cazz Link to comment Share on other sites More sharing options...
Brantuk Posted July 11, 2012 Share Posted July 11, 2012 Oh rats!! Ignore me - I'm getting all confused myself now lol. You're quite correct - we don't know the fl of the spotter so I'm right out of line now..... think I'll go and take some meds lol Link to comment Share on other sites More sharing options...
Tweedledee Posted July 11, 2012 Share Posted July 11, 2012 Same trap as Daz fell into Link to comment Share on other sites More sharing options...
Brantuk Posted July 11, 2012 Share Posted July 11, 2012 Yup - not paying attention to detail - I must be tired or getting old lol Link to comment Share on other sites More sharing options...
Tweedledee Posted July 11, 2012 Share Posted July 11, 2012 I reckon the 15x and 45x is not the magnifications on this scope, since the eyepiece is from a different spotting scope. The only real way to sort it is with exit pupil sizes, and focal length. I bet it is something in the region of 8mm to 24mm cos that is a standard size. Nice puzzle Link to comment Share on other sites More sharing options...
Cazz Posted July 11, 2012 Author Share Posted July 11, 2012 I'll think I'll join you, lol. Sorry to cause everyone grief. Perhaps we're best just looking and enjoying, and not be technical. We'll bring it in July, then stand in the corner. Link to comment Share on other sites More sharing options...
Tweedledee Posted July 11, 2012 Share Posted July 11, 2012 Yup - not paying attention to detail - I must be tired or getting old lol I know the feeling Hey, it's not quite as difficult as finding the Higgs Boson . Link to comment Share on other sites More sharing options...
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