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#1




FOV and f ratio
Hi:
What kind of FOV can I get from an f 8.75 80mm refractor? What is the relation that tells you this? 
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#2




hat kind of FOV can I get from an f 8.75 80mm refractor? What is the
relation that tells you this? Hi Matt, Have a look at the eyepiece chart on the Tele Vue site... http://televue.com/engine/page.asp?ID=3D107 The formulae are listed and there is a calculator you can use to get such info for all Tele Vue eyepieces. Florian 
#3




Hi:
What kind of FOV can I get from an f 8.75 80mm refractor? What is the relation that tells you this? 1.25 inch focuser? I believe the internal barrel diameter is around 28 mm for a 1.25 inch eyepiece. This is the maximum field stop diameter which determines the maximum possible Fielf of view. The focal length of the scope in question is something like 8.75*80mm=700mm The field of view will be 28mm/700mm = 0.04 radians x 57.3 deg/rad = 2.3 degrees. 2 inch focuser will be significantly greater.. jon 
#4




I believe the internal barrel diameter is around 28 mm for a 1.25 inch eyepiece. This is the maximum field stop diameter which determines the maximum possible Fielf of view. The focal length of the scope in question is something like 8.75*80mm=700mm The field of view will be 28mm/700mm = 0.04 radians x 57.3 deg/rad = 2.3 degrees. 2 inch focuser will be significantly greater.. Thanks for the answer. It is 1.25". If I upgraded to a 2" focuser with a 1.25" adapter, would I still get the increase in FOV? According to that formula, I would. Is that the case? Do I need 2" EP's to get the increase in FOV? Even if I don't, 2.3 degrees isn't too bad. 
#5




Matt St. Helens wrote:
What kind of FOV can I get from an f 8.75 80mm refractor? What is the relation that tells you this? Well, if it uses an 1.25" eyepiece focuser (eyepiece outside diameter is 1.25"), the maximum field stop (the opening at the front of the field lens of the eyepiece) diameter for such eyepieces is around 27mm or so. The field stop formula will yield the true field: TFOV = (180/Pi)*(EFSD/Fl), where EFSD is the eyepiece field stop diameter, Fl is the telescope's focal length, and 180/Pi is the number of degrees in a radian (about 57.296 degrees). Thus, and 80mm f/8.75 refractor (700mm Fl) will yield a maximum true field of view on the sky of about 2.2 degrees. A 24mm Tele Vue Panoptic eyepiece would give you 29x and that maximum 2.2 degree field, which would be good for examining large objects at low power. If you replaced the focuser with one for 2" OD eyepieces, the maximum you might hope for would be about 3.8 degrees (47mm field stop). Clear skies to you.  David W. Knisely Prairie Astronomy Club: http://www.prairieastronomyclub.org Hyde Memorial Observatory: http://www.hydeobservatory.info/ ********************************************** * Attend the 11th Annual NEBRASKA STAR PARTY * * July 1823, 2004, Merritt Reservoir * * http://www.NebraskaStarParty.org * ********************************************** 
#6




Thanks for the answers. It seems the EP field stop is the main limiting
factor. 
#7




On Fri, 01 Oct 2004 21:56:59 GMT, Matt St. Helens
wrote: Hi: What kind of FOV can I get from an f 8.75 80mm refractor? What is the relation that tells you this? There is a Telescope Optical Parameters Calculator available for free download at: http://www.vvm.com/~piscescs/telecalc/telecalc.html Cheers, Larry G. 
#8




Larry G wrote in message ...
On Fri, 01 Oct 2004 21:56:59 GMT, Matt St. Helens wrote: What kind of FOV can I get from an f 8.75 80mm refractor? Although this is a completed thread, you may find the following recent article helpful. The eyepiece field stop size is a physical constraint, but other characteristics of commonly available eyepieces  the combination of the apparent field and focal length  may be something to consider before filling out your eyepiece collection. See  Seronik, G. Oct. 2004. Exploring Low Power Limits. Sky & Telescope. 108(4):125128  reciting (at page 126) many of the basic equations discussed in this thread: Longest useful eyepiece focal length = telescope f/ratio * your maximuim exit pupil size Telescope magnfication = focal length objective / focal length eyepiece Exitpupil diameter = Objective aperature size / magnification true FOV = fieldstop diameter*57.3/telescope focal length true FOV = apparent FOV / magnification When filling out your eyepiece collection, this note runs you through some eyepiece math to determine what FOV you will see in each eyepiece, supplementing the math behind the great online calculator that Larry G referred you to at TeleVue  http://www.vvm.com/~piscescs/telecalc/telecalc.html Although the largest physically obtainable fieldofview is limited by the fieldstop diameter, the combination of the apparent field and focal length of commonly available eyepieces may be the asthetic constraining factor you encounter. For example, using the 29mm eyepiece field stop in a 1.25" eyepiece barrel, a 700mm objective focal length and an objective size of 80mm, discussed in this thread, your maximum theoretically useable FOV, based on the longest useful eyepiece focal length and your (human) eye's exitpupil diameter, is: telescope f/ratio * your maximuim exit pupil size: (700mm/80mm) * 7mm = (8.75f/ratio * 7mm) = ~61 mm: which yields a theoretical maximum FOV of: (61mm/700mm)*57.3= ~ 5.0 degrees As noted in this thread, the 29mm eyepiece fieldstop does constrain the 61mm theoretically useful limit: (29mm/700mm)*57.3= ~2.4 degrees  and can be worked around by "upgrading" to a 2" inch eyepiece size  something that isn't an option for a smaller 80mm refractor. For example, I have a 52mm Anatres Erfle 2" lens with a 46mm eyepiece fieldstop: (52mm/700mm)*57.3= ~4.2 degrees (46mm/700mm)*57.3= ~3.8 degrees  a bit closer to the 5.0 degree theoretical limit. But with the commonly available larger focal length 1.25" barrel eyepieces today, let's say  a 32mm eyepiece with a 44deg apparent field, and a 40mm eyepiece with a 43deg apparent field you only get  AF / M = TF where M = Obj_fl/EyeP_fl 44 degrees / (700mm/32mm) = 44deg / 21.9x = about a maximum of 2.0 degrees 43 degrees / (700mm/40mm) = 43deg / 17.5x = about a maximum of 2.4 degrees  which are pushing up against the 29mm eyepiece fieldstop constraint limit at ~2.4 degrees. These limits can be seen by running the Televue online eyepiece calculator and sorting the results by "True Field". Although David Knisely's summary is righton, on a practical daily level, you also might consider what eyepieces are commonly available and what are the rated focal length and apparent field of each individual eyepiece before buying. As Seronik's article in the October issue of _Sky & Telescope_ points out, there are inherent tradeoffs in eyepiece design between the size of the apparent field and the focal length. As a result, the 43 degree AF, 40mm eyepiece yielding the 2.4 deg true field represents about the maximum FOV that you can buy with a 1.25" barrel. Expensive ultrawide eyepieces with higher apparent fields inherently have shorter focal lengths. Thus, with the ultrawides, the you end up with the same maximum true FOV, just at a higher magnification. Examples include:  a Meade Superwide 24.8mm with 67 deg AF gives a FOV = ~2.4 deg at 28x  a Meade Ultra wide 14m with a FOV of 84 deg gives a FOV = ~1.7 deg at 50x The main reason for fillingout your eyepiece collection by trying to go as low as you can go is for starhopping, particularly for nonGOTO altaz mounts. It's easier to star hop looking through a 2.5 degree "toilet tube" binocularsized fieldofview, than with an ~1.01.5 degree "soda straw" true FOV using medium a focal length eyepiece around 15mm20mm with a 52 apparent FOV. The lowpower eyepiece gives you a starhopping FOV bridge between your scope's finder and higherpower observing magnifications. Expensive specialty ultrawide eyepieces are nice for better "framing" of large extended objects in the eyepiece at higher magnifications, but IMHO, something to put off buying until you are really seriously addicted to the hobby. In summary, for a 1.25" barrel, 29mm eyepiece fieldstop, 80mm objective, 700mm fl, refractor, the FOV limits a Theorectical human exit pupil limit  ~5.0 degrees Eyepiece fieldstop limit  ~2.4 degrees Available eyepieces asthetic limit  ~2.02.4 degrees at 22x and 18x, depending on what magnification you are most comfortable starhopping with.  Canopus P.S.  Hope this post isn't too repetitive. 
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