Knighty2112 Posted April 17, 2017 Share Posted April 17, 2017 (edited) Silly question maybe, but what is the advantage of having a 3 inch focuser on some scopes? I am not aware of EPs or diagonals that come in this size range, so what is the point of this larger focuser really, if you then need to use either a 2" or 1.25" adapter to use any equipment with it? Edited April 17, 2017 by Knighty2112 Link to comment Share on other sites More sharing options...
Alan White Posted April 17, 2017 Share Posted April 17, 2017 I am sure there is a valid reason, but the synic in me says 'so folks upgrade and buy one' but I clearly could be wrong ? 1 Link to comment Share on other sites More sharing options...
ollypenrice Posted April 17, 2017 Share Posted April 17, 2017 It will avoid vignetting for imagers using large format chips. Olly 3 Link to comment Share on other sites More sharing options...
Knighty2112 Posted April 17, 2017 Author Share Posted April 17, 2017 1 minute ago, Alan White said: I am sure there is a valid reason, but the synic in me says 'so folks upgrade and buy one' but I clearly could be wrong ? But what would the upgrade achieve? Do you get better/wider views or something? Are 3 inch focusers better for astrophography or what? Not seeing a real obvious reason for them myself! ?? Link to comment Share on other sites More sharing options...
Knighty2112 Posted April 17, 2017 Author Share Posted April 17, 2017 2 minutes ago, ollypenrice said: It will avoid vignetting for imagers using large format chips. Olly OK. So this is purely something required just certain kinds of astrophotography then. Thanks! Link to comment Share on other sites More sharing options...
Alien 13 Posted April 17, 2017 Share Posted April 17, 2017 There are eyepieces that are four inch or greater which would be good for me, I like widefield even if it means huge exit pupils, it is also more rigid which always helps with AP. Alan 4 Link to comment Share on other sites More sharing options...
John Posted April 17, 2017 Share Posted April 17, 2017 ES do a 30mm 100 degree eyepiece in the 3" fitting and a matching diagonal: https://www.explorescientific.co.uk/en/Eyepieces/Explore-Scientific-100-Ar-Eyepiece-30mm-3-76-2mm.html https://www.explorescientific.co.uk/en/Optical-accessories/Explore-Scientific-Diagonal-Mirror-76-2mm-3.html 3 Link to comment Share on other sites More sharing options...
Knighty2112 Posted April 17, 2017 Author Share Posted April 17, 2017 3 minutes ago, Alien 13 said: There are eyepieces that are four inch or greater which would be good for me, I like widefield even if it means huge exit pupils, it is also more rigid which always helps with AP. Alan Really? Wasn't aware of that at all. Thanks! Bet they cost quite a bit of dosh though! ?? Link to comment Share on other sites More sharing options...
Knighty2112 Posted April 17, 2017 Author Share Posted April 17, 2017 2 minutes ago, John said: ES do a 30mm 100 degree eyepiece in the 3" fitting and a matching diagonal: https://www.explorescientific.co.uk/en/Eyepieces/Explore-Scientific-100-Ar-Eyepiece-30mm-3-76-2mm.html https://www.explorescientific.co.uk/en/Optical-accessories/Explore-Scientific-Diagonal-Mirror-76-2mm-3.html Nice prices there John! Hehe! Link to comment Share on other sites More sharing options...
Alien 13 Posted April 17, 2017 Share Posted April 17, 2017 1 minute ago, Knighty2112 said: Really? Wasn't aware of that at all. Thanks! Bet they cost quite a bit of dosh though! ?? I think the rigidity and lack of vignetting are the big plus points, you can actually use camera lenses as eyepieces if the focus tube was wide enough. Alan 1 Link to comment Share on other sites More sharing options...
John Posted April 17, 2017 Share Posted April 17, 2017 There are 4" eyepieces as well (the small one in this pic is a 2" Pentax 40mm): 2 Link to comment Share on other sites More sharing options...
AndyH Posted April 17, 2017 Share Posted April 17, 2017 Some more insanely massive goodies ...... Japanese monster 3" ep + 3" diagonal. http://www.kkohki.com/English/kkohkiparts.html#Masuyama 3 inch eyepiece Andy 2 Link to comment Share on other sites More sharing options...
Knighty2112 Posted April 17, 2017 Author Share Posted April 17, 2017 1 hour ago, John said: There are 4" eyepieces as well (the small one in this pic is a 2" Pentax 40mm): Wow! Just.......Wow! ??? Link to comment Share on other sites More sharing options...
Knighty2112 Posted April 17, 2017 Author Share Posted April 17, 2017 1 hour ago, AndyH said: Some more insanely massive goodies ...... Japanese monster 3" ep + 3" diagonal. http://www.kkohki.com/English/kkohkiparts.html#Masuyama 3 inch eyepiece Andy For such big EPs the FOV seems very small? Also, they do taper in quite a lot at the top like a volcano? Link to comment Share on other sites More sharing options...
John Posted April 17, 2017 Share Posted April 17, 2017 At 80mm focal length, even in a 3" barrel, the FoV will max out at 52 degrees. The field stop is probably close to the full internal width of the 76.2mm barrel to get that. The tapering top makes the eye lens more accessible. 1 Link to comment Share on other sites More sharing options...
AndyH Posted April 17, 2017 Share Posted April 17, 2017 It's a simple-ish 5 element ep, so 52 deg is about right, in what I think is a modified plossl design. To get 52 deg, a 70mm field stop is needed, hence the 76.2mm barrel size. Perhaps the tapered volcano top is to aid in binoviewer use........... ahem 2 Link to comment Share on other sites More sharing options...
DaveS Posted April 17, 2017 Share Posted April 17, 2017 The 3.7" focuser on my 130 f/7 allows the 0.75x APM Ricardi reducer to fit inside, limiting the overhang. 1 Link to comment Share on other sites More sharing options...
Knighty2112 Posted April 17, 2017 Author Share Posted April 17, 2017 25 minutes ago, DaveS said: The 3.7" focuser on my 130 f/7 allows the 0.75x APM Ricardi reducer to fit inside, limiting the overhang. OK, not quite sure why that's a good thing, but great! Link to comment Share on other sites More sharing options...
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