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The problem I've found with relying on AFOV number to translate to TFOV numbers is that they don't take into account distortion.  Field stop is the number that really needs to be used for accuracy.

For instance, the 35mm Panoptic has a 68 degree AFOV (might be 69 degrees), but given its 38.7mm field stop (provided by TV, which tends to be very accurate on these measurements), it has an effective AFOV (eAFOV) of only 63.4 degrees, assuming no distortion.  In other words, the Panoptic is stretching the outer part of the AFOV by an extra 4.6 degrees.  Thus, you need to use 63.4 degrees in these calculators get an accurate TFOV calculation.

In comparison, the 35mm Aero ED also claims 68 degrees, but actually shows 73 degrees of AFOV (from my projection measurements).  I measured the field stop to be 44.4mm yielding an eAFOV of 73 degrees, indicating practically no distortion across the field.  That extra 5.7mm of field stop width translates into a much wider TFOV despite both 35mm eyepieces claiming to be 68 degrees.

Take these online TFOV calculations given by using advertised AFOV with a healthy dose of skepticism.

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5 hours ago, Louis D said:

The problem I've found with relying on AFOV number to translate to TFOV numbers is that they don't take into account distortion.  Field stop is the number that really needs to be used for accuracy.

For instance, the 35mm Panoptic has a 68 degree AFOV (might be 69 degrees), but given its 38.7mm field stop (provided by TV, which tends to be very accurate on these measurements), it has an effective AFOV (eAFOV) of only 63.4 degrees, assuming no distortion.  In other words, the Panoptic is stretching the outer part of the AFOV by an extra 4.6 degrees.  Thus, you need to use 63.4 degrees in these calculators get an accurate TFOV calculation.

In comparison, the 35mm Aero ED also claims 68 degrees, but actually shows 73 degrees of AFOV (from my projection measurements).  I measured the field stop to be 44.4mm yielding an eAFOV of 73 degrees, indicating practically no distortion across the field.  That extra 5.7mm of field stop width translates into a much wider TFOV despite both 35mm eyepieces claiming to be 68 degrees.

Take these online TFOV calculations given by using advertised AFOV with a healthy dose of skepticism.

Well, me as a newby, for me it made a lot clear, what is what, and what is it doing, you have to remember, we newbies, well I newby wants to learn but a lot is FOV and 82° and AFOV TFOV and and and, Let me tell you, I am buying my hardware now and days of clouds here, I was fun typing my eyepieces in and see "sort of" what they do, I saw that my 2x Barlow will not be enough to see Saturn with my Scope, need at least a 3x, just fun to see if what the program sais will be correct, I for one can't wait, can I have some clear sky please. It is what is is.... -Pim- ? 

p.s. I was thinking tho, I don't know that type of hardware they used, probably better than I have so I will not gonne get what is shown there, I wish it well be that good, I me all this is fun, so shoot me ? but be prepared, shoot back when you miss, I an a Sports shooter too ? -P-

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Posted (edited)
16 minutes ago, Roadrunr said:

Well, me as a newby, for me it made a lot clear, what is what, and what is it doing, you have to remember, we newbies, well I newby wants to learn but a lot is FOV and 82° and AFOV TFOV and and and, Let me tell you, I am buying my hardware now and days of clouds here, I was fun typing my eyepieces in and see "sort of" what they do, I saw that my 2x Barlow will not be enough to see Saturn with my Scope, need at least a 3x, just fun to see if what the program sais will be correct, I for one can't wait, can I have some clear sky please. It is what is is.... -Pim- ? 

p.s. I was thinking tho, I don't know that type of hardware they used, probably better than I have so I will not gonne get what is shown there, I wish it well be that good, I me all this is fun, so shoot me ? but be prepared, shoot back when you miss, I an a Sports shooter too ? -P-

These calculators are a great visualization tool for newbies.  I'm just saying to temper your expectations based on them.  If you were to splash for that 35mm Panoptic because it looked like it would frame the entire Pleiades in your scope based on the tool, and then you plonk it into the focuser and find out it leaves a bit out at the edges, that would be a bad time to learn about eyepiece distortion.

Have fun playing around with these tools.  I did exactly the same thing 22 years ago starting out, except I had to write my own program to churn out the numbers since the internet was in its infancy and these tools didn't exist.  It was only after owning multiple eyepieces of the same or near same focal length that I started to discover that distortion was an important variable not well documented in marketing literature (if at all).

Edited by Louis D
typo

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1 hour ago, Louis D said:

These calculators are a great visualization tool for newbies.  I'm just saying to temper your expectations based on them.  If you were to splash for that 35mm Panoptic because it looked like it would frame the entire Pleiades in your scope based on the tool, and then you plonk it into the focuser and find out it leaves a bit out at the edges, that would be a bad time to learn about eyepiece distortion.

Have fun playing around with these tools.  I did exactly the same thing 22 years ago starting out, except I had to write my own program to churn out the numbers since the internet was in its infancy and these tools didn't exist.  It was only after owning multiple eyepieces of the same or near same focal length that I started to discover that distortion was an important variable not well documented in marketing literature (if at all).

Thanks for your input, 22 years,? what you must have seen already . 

-Pim-

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