TheSkySearchers

Bigzmey wrote: ↑Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.

notFritzArgelander wrote: ↑Thu Sep 22, 2022 4:01 pm

Bigzmey wrote: ↑Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.

This raises an interesting point that has a subtlety of optical design buried in it: How is this exaggeration done? In many case to the observing eye at the exit pupil the AFOV is as advertised and specified. But the formula isn't strictly applicable since the wide apparent field of view is achieved by distorting the field of view using aberrations!

https://www.telescope-optics.net/eyepie ... tion_2.htm

Angular magnification distortion is used to increase the AFOV above what the formulas would give based on field stop size. So it's not so much advertising hype as it is using an optical trick to exaggerate the image of the TFOV at the exit pupil.

Bigzmey wrote: ↑Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.

Example: ES 68 deg 24mm EP has a field stop of 27.2mm and advertised apparent FOV =68 deg.

However, if we calculate AFV from the field stop it comes to 57.3 x (27.2/24) = 64.9 deg.

In 102mm F7 refractor ES 68 deg 24mm EP will produce TFV = 57.3 (27.2/714) = 2.18 deg.

Maximum possible true FOV in any scope is determined by the telescope focal length and max field stop of either 1.25" or 2" EP

1.25" EP max field stop is ~27mm
2" EP max field stop is ~46mm

Thus, max TFV for 102mm F7 refractor is ~2.2 deg for 1.25" EP and ~3.7 deg for 2" EP.

Bigzmey wrote: ↑Thu Sep 22, 2022 5:29 pm

notFritzArgelander wrote: ↑Thu Sep 22, 2022 4:01 pm

Bigzmey wrote: ↑Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.

This raises an interesting point that has a subtlety of optical design buried in it: How is this exaggeration done? In many case to the observing eye at the exit pupil the AFOV is as advertised and specified. But the formula isn't strictly applicable since the wide apparent field of view is achieved by distorting the field of view using aberrations!

https://www.telescope-optics.net/eyepie ... tion_2.htm

Angular magnification distortion is used to increase the AFOV above what the formulas would give based on field stop size. So it's not so much advertising hype as it is using an optical trick to exaggerate the image of the TFOV at the exit pupil.

Interesting point nAF!

Don Pensack wrote: ↑Wed Oct 19, 2022 7:50 pm

Bigzmey wrote: ↑Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.

Example: ES 68 deg 24mm EP has a field stop of 27.2mm and advertised apparent FOV =68 deg.

However, if we calculate AFV from the field stop it comes to 57.3 x (27.2/24) = 64.9 deg.

In 102mm F7 refractor ES 68 deg 24mm EP will produce TFV = 57.3 (27.2/714) = 2.18 deg.

Maximum possible true FOV in any scope is determined by the telescope focal length and max field stop of either 1.25" or 2" EP

1.25" EP max field stop is ~27mm
2" EP max field stop is ~46mm

Thus, max TFV for 102mm F7 refractor is ~2.2 deg for 1.25" EP and ~3.7 deg for 2" EP.

You misunderstand the effect of distortion on the apparent field.
The 24mm eyepiece you mention does have a 68° apparent field. It is not exaggerated.
You can measure it yourself using the flashlight test.
How is that possible?
The outer field is stretched by positive rectilinear distortion (pincushion), so the apparent field is larger than the field stop would suggest.
An eyepiece with negative rectilinear distortion (barrel), like the Nobles 12.5mm, has an apparent field that is smaller than the field stop would suggest because of compression at the edge of the field.

Unless you know the distortion characteristics and %, you cannot successfully derive an apparent field from the field stop or true field measurements.

You can use the field stop or true field dimension to calculate the effective apparent field (eAFOV), which allows you to use the simple formula TF = AFOV / M and get an accurate true field
(otherwise, that formula is worthless since distortion is ignored), but you have to recognize eAFOV is not a real physical measurement, but merely a construct
to make a poor formula work.

As for maximum field stop in a given size, there is a 1.25" eyepiece with a 28.5mm field stop, and there was one in the past (a couple, actually) with 29.0mm field stops.
They did it by putting the field stop above the barrel and allowing some vignetting of the edge.
Likewise, there are a couple 2" eyepieces with 46.5mm field stops,

Bigzmey wrote: ↑Wed Oct 19, 2022 8:09 pm

Don Pensack wrote: ↑Wed Oct 19, 2022 7:50 pm

Bigzmey wrote: ↑Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.

Example: ES 68 deg 24mm EP has a field stop of 27.2mm and advertised apparent FOV =68 deg.

However, if we calculate AFV from the field stop it comes to 57.3 x (27.2/24) = 64.9 deg.

In 102mm F7 refractor ES 68 deg 24mm EP will produce TFV = 57.3 (27.2/714) = 2.18 deg.

Maximum possible true FOV in any scope is determined by the telescope focal length and max field stop of either 1.25" or 2" EP

1.25" EP max field stop is ~27mm
2" EP max field stop is ~46mm

Thus, max TFV for 102mm F7 refractor is ~2.2 deg for 1.25" EP and ~3.7 deg for 2" EP.

You misunderstand the effect of distortion on the apparent field.
The 24mm eyepiece you mention does have a 68° apparent field. It is not exaggerated.
You can measure it yourself using the flashlight test.
How is that possible?
The outer field is stretched by positive rectilinear distortion (pincushion), so the apparent field is larger than the field stop would suggest.
An eyepiece with negative rectilinear distortion (barrel), like the Nobles 12.5mm, has an apparent field that is smaller than the field stop would suggest because of compression at the edge of the field.

Unless you know the distortion characteristics and %, you cannot successfully derive an apparent field from the field stop or true field measurements.

You can use the field stop or true field dimension to calculate the effective apparent field (eAFOV), which allows you to use the simple formula TF = AFOV / M and get an accurate true field
(otherwise, that formula is worthless since distortion is ignored), but you have to recognize eAFOV is not a real physical measurement, but merely a construct
to make a poor formula work.

As for maximum field stop in a given size, there is a 1.25" eyepiece with a 28.5mm field stop, and there was one in the past (a couple, actually) with 29.0mm field stops.
They did it by putting the field stop above the barrel and allowing some vignetting of the edge.
Likewise, there are a couple 2" eyepieces with 46.5mm field stops,

Thanks for the clarification Don! How common are positive or negative rectilinear distortions in the EP design? What about 52 deg 32mm Plossls from Meade, Celestron etc? Do they really incorporate positive rectilinear distortion in their design to get extra 2 deg compared to the premium Plossls like TV 50 deg 32mm?

Bigzmey wrote: ↑Wed Oct 19, 2022 8:25 pm Looking at the graphs of pincushion and barrel distortions on line, it seems that they stretch and shrink the apparent FOV, respectively, but did not capture additional data, i.e. if a star is hidden by the edge of the field stop, then it would be still invisible if pincushion is applied. Do I read it correctly?

notFritzArgelander wrote: ↑Wed Oct 19, 2022 10:19 pm

Bigzmey wrote: ↑Wed Oct 19, 2022 8:25 pm Looking at the graphs of pincushion and barrel distortions on line, it seems that they stretch and shrink the apparent FOV, respectively, but did not capture additional data, i.e. if a star is hidden by the edge of the field stop, then it would be still invisible if pincushion is applied. Do I read it correctly?

No it would not be visible. The angular magnification distortion applies only to what is inside the field stop.

Bigzmey wrote: ↑Wed Oct 19, 2022 8:25 pm Looking at the graphs of pincushion and barrel distortions on line, it seems that they stretch and shrink the apparent FOV, respectively, but did not capture additional data, i.e. if a star is hidden by the edge of the field stop, then it would be still invisible if pincushion is applied. Do I read it correctly?

Bigzmey wrote: ↑Wed Oct 19, 2022 10:36 pm

notFritzArgelander wrote: ↑Wed Oct 19, 2022 10:19 pm

Bigzmey wrote: ↑Wed Oct 19, 2022 8:25 pm Looking at the graphs of pincushion and barrel distortions on line, it seems that they stretch and shrink the apparent FOV, respectively, but did not capture additional data, i.e. if a star is hidden by the edge of the field stop, then it would be still invisible if pincushion is applied. Do I read it correctly?

No it would not be visible. The angular magnification distortion applies only to what is inside the field stop.

So, if we go back to ES68 24mm, it has 64.9 deg image stretched to 68 deg, right?

notFritzArgelander wrote: ↑Wed Oct 19, 2022 10:17 pm

Bigzmey wrote: ↑Wed Oct 19, 2022 8:09 pm

Don Pensack wrote: ↑Wed Oct 19, 2022 7:50 pm

You misunderstand the effect of distortion on the apparent field.
The 24mm eyepiece you mention does have a 68° apparent field. It is not exaggerated.
You can measure it yourself using the flashlight test.
How is that possible?
The outer field is stretched by positive rectilinear distortion (pincushion), so the apparent field is larger than the field stop would suggest.
An eyepiece with negative rectilinear distortion (barrel), like the Nobles 12.5mm, has an apparent field that is smaller than the field stop would suggest because of compression at the edge of the field.

Unless you know the distortion characteristics and %, you cannot successfully derive an apparent field from the field stop or true field measurements.

You can use the field stop or true field dimension to calculate the effective apparent field (eAFOV), which allows you to use the simple formula TF = AFOV / M and get an accurate true field
(otherwise, that formula is worthless since distortion is ignored), but you have to recognize eAFOV is not a real physical measurement, but merely a construct
to make a poor formula work.

As for maximum field stop in a given size, there is a 1.25" eyepiece with a 28.5mm field stop, and there was one in the past (a couple, actually) with 29.0mm field stops.
They did it by putting the field stop above the barrel and allowing some vignetting of the edge.
Likewise, there are a couple 2" eyepieces with 46.5mm field stops,

Thanks for the clarification Don! How common are positive or negative rectilinear distortions in the EP design? What about 52 deg 32mm Plossls from Meade, Celestron etc? Do they really incorporate positive rectilinear distortion in their design to get extra 2 deg compared to the premium Plossls like TV 50 deg 32mm?

Any eyepiece which is not orthoscopic suffers (by definition) from angular magnification distortion. That extra 2 degrees? It might just be different glass. There are a lot of variables. I wouldn't worry about that unless there was a complete prescription of the design. Now wide field eyepieces like the Pentax XW and TV Panoptics and Naglers are not orthoscopic and employ the angular magnification distortion trick. (Trick: i.e. technique.)

Don Pensack wrote: ↑Wed Oct 19, 2022 11:13 pm And, as an aside, most Plössl eyepiece lines vary from maybe 47° to 52°, so the number chosen for the apparent field is done by the advertising department.

SkyHiker wrote: ↑Thu Oct 20, 2022 3:56 pm The AFOV formula that you used to compare with the advertised AFOV (64.9 vs 68 degrees) is only approximate. For an ideal lens of focal length f, when viewing an object d of diameter d (say, the field stop), the angular FOV is given by alpha=2*atan(0.5*d/f) radians (or 180/pi=57.3 times that in degrees). See Wikipedia under "Eyepiece" and "Angle of View", where it is named AAOV (Apparent Angle Of View).

..........
The author of that thread says it is much more accurate to use aa telescope and measure the time of a star crossing the FOV - and apply math to that number to get the AFOV. But even that number is subjected to the distance of the eyeball to the eyepiece, isn't it? Because no matter what, the field stop will be in focus and that's what determines it, but its perceived AAOV depends on it.

So how can we get a number that depends on the eyepiece only and not the eyeball of individuals (with different eye relief / near sightedness, etcetera)? Wikipedia names a standard that supposedly describes this, ISO 14132-1:2015. Unfortunately, you have to spend a lot of money to get it. Thoughts, anyone?

SkyHiker wrote: ↑Thu Oct 20, 2022 3:56 pm The AFOV formula that you used to compare with the advertised AFOV (64.9 vs 68 degrees) is only approximate. For an ideal lens of focal length f, when viewing an object d of diameter d (say, the field stop), the angular FOV is given by alpha=2*atan(0.5*d/f) radians (or 180/pi=57.3 times that in degrees). See Wikipedia under "Eyepiece" and "Angle of View", where it is named AAOV (Apparent Angle Of View).

In your example for the case of f=24 mm and d=27.2 mm we would have AFOV=64.9 degrees and AAOV=59.1 degrees. So, when comparing ideal lens formulas with the vendor specified AFOV of 68 degrees you should be using the AAOV. Both are worthless due to distortion the AAOV formula is just "less worthless" than the AFOV formula. If the 68 degrees is correct, the AAOV gives a better number of how much distortion the lens has than the AFOV formula.

Even the AAOV does not account for the human eyeball that could have glasses in front of them when performing the "flashlight test" that Don mentioned. Near-sighted persons like me would move their eyeball closer to the eyepiece than the average person, vice versa for far-sighted. This will affect the outcome. I just tried this with a 24 mm eyepiece. At large distances the FOV remains constant but around 10 mm from the eye the FOV changes quite a bit.

I tried to find the "flashlight test" that Don mentioned. I can't find a reference but this thread on CN mentions a way to measure the angle with daytime object distances - which I presume a flashlight test would be. It talks about putting the eyepiece at eye relief distance to get a sharp image. At eye relief distance though, the field stop is blurry, so you could not get an accurate measurement. Moreover, the FOV changes radically at those distances. Sometimes I view with glasses on, sometimes without. The FOV is much larger without glasses than with (I am very near-sighted).

The author of that thread says it is much more accurate to use aa telescope and measure the time of a star crossing the FOV - and apply math to that number to get the AFOV. But even that number is subjected to the distance of the eyeball to the eyepiece, isn't it? Because no matter what, the field stop will be in focus and that's what determines it, but its perceived AAOV depends on it.

So how can we get a number that depends on the eyepiece only and not the eyeball of individuals (with different eye relief / near sightedness, etcetera)? Wikipedia names a standard that supposedly describes this, ISO 14132-1:2015. Unfortunately, you have to spend a lot of money to get it. Thoughts, anyone?

Don Pensack wrote: ↑Thu Oct 20, 2022 7:42 pm

SkyHiker wrote: ↑Thu Oct 20, 2022 3:56 pm The AFOV formula that you used to compare with the advertised AFOV (64.9 vs 68 degrees) is only approximate. For an ideal lens of focal length f, when viewing an object d of diameter d (say, the field stop), the angular FOV is given by alpha=2*atan(0.5*d/f) radians (or 180/pi=57.3 times that in degrees). See Wikipedia under "Eyepiece" and "Angle of View", where it is named AAOV (Apparent Angle Of View).

In your example for the case of f=24 mm and d=27.2 mm we would have AFOV=64.9 degrees and AAOV=59.1 degrees. So, when comparing ideal lens formulas with the vendor specified AFOV of 68 degrees you should be using the AAOV. Both are worthless due to distortion the AAOV formula is just "less worthless" than the AFOV formula. If the 68 degrees is correct, the AAOV gives a better number of how much distortion the lens has than the AFOV formula.

Even the AAOV does not account for the human eyeball that could have glasses in front of them when performing the "flashlight test" that Don mentioned. Near-sighted persons like me would move their eyeball closer to the eyepiece than the average person, vice versa for far-sighted. This will affect the outcome. I just tried this with a 24 mm eyepiece. At large distances the FOV remains constant but around 10 mm from the eye the FOV changes quite a bit.

I tried to find the "flashlight test" that Don mentioned. I can't find a reference but this thread on CN mentions a way to measure the angle with daytime object distances - which I presume a flashlight test would be. It talks about putting the eyepiece at eye relief distance to get a sharp image. At eye relief distance though, the field stop is blurry, so you could not get an accurate measurement. Moreover, the FOV changes radically at those distances. Sometimes I view with glasses on, sometimes without. The FOV is much larger without glasses than with (I am very near-sighted).

The author of that thread says it is much more accurate to use aa telescope and measure the time of a star crossing the FOV - and apply math to that number to get the AFOV. But even that number is subjected to the distance of the eyeball to the eyepiece, isn't it? Because no matter what, the field stop will be in focus and that's what determines it, but its perceived AAOV depends on it.

So how can we get a number that depends on the eyepiece only and not the eyeball of individuals (with different eye relief / near sightedness, etcetera)? Wikipedia names a standard that supposedly describes this, ISO 14132-1:2015. Unfortunately, you have to spend a lot of money to get it. Thoughts, anyone?

The flashlight test:
with illustrations:
https://www.cloudynights.com/topic/5744 ... ?p=7958975
https://www.cloudynights.com/topic/5744 ... ?p=7959408
Glasses/no glasses: doesn't matter as long as your tape measure is accurate and you know a tiny bit of trigonometry.

This is a CHEAP operation and can be accurate to ~0.1 to 0.2° of apparent field, which is close enough.

One note: the AFOV of the eyepiece in question is not 64.9° No instrument will measure that. A device like the one that has been used by S&T to measure apparent fields (accurate to 0.1°) would measure 68°.
The angle that is subtended from edge to edge IS the apparent field of the eyepiece. The smaller number is calculated from the field stop and is only used so the TF = AF/M formula is accurate.
Why use an inaccurate formula if you have an accurate one: TF = (field stop / telescope focal length) x 180 / pi or (FS/TFL) x 57.2958 ?
That formula doesn't even require knowledge of apparent field or distortion, yet yields an accurate number for field stop and true field.

Timing a star yields an accurate true field and a field stop dimension. It does NOT yield an apparent field.