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# Eyepiece True field Of View

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### #1 ras

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Posted 07 December 2019 - 11:55 AM

As my question indicates I am a beginner.  If the formula for True Field Of View is eyepiece field stop diameter/telescope focal lengthX57.3, am I correct that the maximum TFOV for a telescope with a 1280" focal length is approximately 1.25 degrees for a 1.25" eyepiece and 2.10 degrees for a 2" eyepiece?

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Posted 07 December 2019 - 12:16 PM

Your numbers are about correct.  Remember that the barrel of the eyepiece might be 1 to 2 mm thick.

Wide field eyepieces generally use the barrel as field stop.

Why 1280? If you are considering getting a 100mm MCT (which has a 1300mm fl), it may not be able to accept 2 inch eyepieces.

### #3 StarBurger

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Posted 07 December 2019 - 12:20 PM

Well no. 1.25 " and 2 " eyepieces are only the size of the barrel so do not refer to the field stop dimension which of course has to be smaller.

Field stops in 1.25" can vary from the largest that is physically possible to the small.

Otherwise formula looks correct.

### #4 Shorty Barlow

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Posted 07 December 2019 - 12:20 PM

I just divide the magnification of the eyepiece by the AFOV. It probably isn't the most accurate way to calculate TFOV, but it suffices for my needs.

Example:

24mm Tele Vue Panoptic ~ 68° AFOV.

Magnification of Panoptic in 72mm Evostar ~ 17.5x.

68/17.5 = 3.885

3 arc degrees : 53 minutes : 6 seconds TFOV.

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### #5 SloMoe

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Posted 07 December 2019 - 12:20 PM

I use an online calculator for eyepiece tfov

http://www.csgnetwork.com/telefov.html

And a second one that others fine very useful,

http://astronomy.too.../field_of_view/

And yes, you're close enough.

Edited by SloMoe, 07 December 2019 - 12:22 PM.

### #6 WarmWeatherGuy

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Posted 07 December 2019 - 12:21 PM

Pretty much. 1.25" = 31.75 mm and 57.3 * 31.75 / 1280 = 1.42° and 2" = 50.8mm and 57.3 * 50.8 / 1280 = 2.27°.

This assumes that there is no other light restriction. For example, the light hole on the back of SCTs are often less than 2" so putting a 2" diagonal / eyepiece won't get the the full FOV you would otherwise calculate.

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### #7 Redbetter

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Posted 07 December 2019 - 12:47 PM

This assumes that there is no other light restriction. For example, the light hole on the back of SCTs are often less than 2" so putting a 2" diagonal / eyepiece won't get the the full FOV you would otherwise calculate.

This is an incorrect but common misconception about SCT's and Maks.  Even the 27mm or so baffle tube on the ES 127 Mak still allows full true FOV in the widest 2" field stops.  There is some vignetting, although it is not apparent at night even though it must be considerable in the outer field.  Field curvature is a more noticeable problem.

Going from 1.25" to 2" does change the focal length of these moving primary focusing systems though.  That is because the spacing between the primary and secondary set the focal length.  When a 2" diagonal is added the focal plane is moved out further, which requires considerable movement of the primary to achieve focus.  This somewhat increases the focal length, although only a fraction of that gained with the additional field stop.

Standard SCT's and Maks are only at their design focal ratio a specific focal plane position.  Historically the C8's were designed to achieve this with the stock visual back and 1.25" RA prism diagonal from what I understand.  I don't know what specific eyepiece focal plane position was required.  That is the thing, for eyepieces with different focal planes, the focal length is moving around depending on what eyepiece is being used.  The range with 1.25" is typically less, while the span from a 55 Plossl to a 31 Nagler or 21 Ethos is quite wide.

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### #8 sg6

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Posted 07 December 2019 - 12:58 PM

I suggest you get used to the "simple" one from Shorty Barlow.

Reason is simply that stood outside at 1:00 AM and maybe in a star party you are not going to be able to use a laptop or much else to get a more accurate idea. And likely the more accurate is not greatly more accurate.

If at outreach and someone asks if you can point at M45 so they can look, what eyepiece are you going to put in?

No time and likely no opertunity to pull up a laptop or whatever to determine the optimum eyepiece, you need to know how to get at the best one all yourself.

### #9 Shorty Barlow

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Posted 07 December 2019 - 01:24 PM

I suggest you get used to the "simple" one from Shorty Barlow.

Reason is simply that stood outside at 1:00 AM and maybe in a star party you are not going to be able to use a laptop or much else to get a more accurate idea. And likely the more accurate is not greatly more accurate.

If at outreach and someone asks if you can point at M45 so they can look, what eyepiece are you going to put in?

No time and likely no opertunity to pull up a laptop or whatever to determine the optimum eyepiece, you need to know how to get at the best one all yourself.

Yeah, you can't beat simple.

I'm with Kelly Johnson on this principle: https://en.wikipedia.../KISS_principle

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### #10 howardcano

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Posted 07 December 2019 - 02:24 PM

It's interesting applying KISS to an SR-71.

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### #11 Shorty Barlow

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Posted 07 December 2019 - 02:30 PM

It's interesting applying KISS to an SR-71.

Good point.

### #12 grzesznypl

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Posted 07 December 2019 - 02:51 PM

Does the same rules of calculating TFOV apply to zooms? For example my Baader Hyperion Mark IV Zoom gives me following

52X = 24mm   BHZ at 24mm    Eye Pupil = 4.89mm   AFOV - 48°   TFOV - 0.92°
62X = 20mm   BHZ at 20mm    Eye Pupil = 4.08mm   AFOV - 52°   TFOV - 0.83°
78X = 16mm   BHZ at 16mm    Eye Pupil = 3.26mm   AFOV - 58°   TFOV - 0.74°

While magnification, eye pupil and AFOV are accurate is TFOV correct?  How about zooms +  barlow???

236X = 5.3mm  (BHZ at 12mm + 2.25X barlow)   Eye Pupil = 1.12mm   AFOV - 63°   TFOV - 0.27°
357X = 3.5mm  (BHZ at   8mm + 2.25X barlow)   Eye Pupil = 0.71mm   AFOV - 68°   TFOV - 0.19°

Edited by grzesznypl, 07 December 2019 - 03:50 PM.

### #13 Redbetter

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Posted 07 December 2019 - 03:43 PM

I suggest you get used to the "simple" one from Shorty Barlow.

Reason is simply that stood outside at 1:00 AM and maybe in a star party you are not going to be able to use a laptop or much else to get a more accurate idea. And likely the more accurate is not greatly more accurate.

If at outreach and someone asks if you can point at M45 so they can look, what eyepiece are you going to put in?

No time and likely no opertunity to pull up a laptop or whatever to determine the optimum eyepiece, you need to know how to get at the best one all yourself.

Apparent FOV is anything but simple since it is wrong for determining true field of view.    I see that a lot on the forums, people stating a field of view that is greater than what their instruments can provide.

Simple is calculating true FOV from the field stops and ignoring the misleading AFOV.  Afterall, you have to use the focal length anyway to calculate magnification (which is what is used for AFOV approximations)...so why not go ahead and calculate true field of view at the same time, rather than using value that is almost always inflated?

Here is what I call simple:  I keep a sheet with calculated TFOV and magnification for each of the various scopes I use.  For the eyepieces I typically use I know magnification off the top of my head with most of the scopes.

But I don't see how this "concern" applies to outreach anyway.  Outreach is the bright and beautiful, the familiar.   And if one can't be bothered to look up field of view for an eyepiece and scope they use regularly, then what makes them more likely to know what size the object is anyway?  You gotta look that up as well unless you already know...and in that case you should have some idea what eyepiece works for that scale.  If a person doesn't know what eyepiece to use on M45 with a given scope then they have bigger problems doing outreach.

For things like M45 most scopes will be using their widest true field eyepiece or similar.  That is the eyepiece that it is most important to know the actual true field of view for, since it anchors the end.

For outreach I don't ever recall having trouble deciding what eyepiece to use based on field of view.

### #14 Shorty Barlow

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Posted 07 December 2019 - 03:55 PM

I still prefer simple. My 24mm Pan gives roughly 4 arc degrees in my ED72. That's about 8 Full Moons. No need to be an anorak about it if you ask me.

Simples!

Edited by Shorty Barlow, 07 December 2019 - 04:03 PM.

### #15 lphilpot

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Posted 07 December 2019 - 04:11 PM

From the "proof is in the pudding" department...

Find a couple of identifiable stars that will just barely fit in the field of view. Measure their separation on a chart, or with planetarium software, etc. Regardless of what the formulas say, that's your "true" field of view.

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### #16 izar187

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Posted 07 December 2019 - 04:37 PM

From the "proof is in the pudding" department...

Find a couple of identifiable stars that will just barely fit in the field of view. Measure their separation on a chart, or with planetarium software, etc. Regardless of what the formulas say, that's your "true" field of view.

This.

### #17 Jon Isaacs

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Posted 07 December 2019 - 04:42 PM

As my question indicates I am a beginner.  If the formula for True Field Of View is eyepiece field stop diameter/telescope focal lengthX57.3, am I correct that the maximum TFOV for a telescope with a 1280" focal length is approximately 1.25 degrees for a 1.25" eyepiece and 2.10 degrees for a 2" eyepiece?

I think you have it right.

The maximum possible field stop for a 1.25 inch eyepiece is between 27mm-28mm unless one is willing to accept some vignetting.  Plugin 28mm in to the equation:

TFoV (1.25 inch) = 180 deg/Pi x 28mm /1280mm = 1.253 degrees

For a 2 inch eyepiece, the maximum field stop diameter is 46mm:

TFoV (2inch) = 180 deg/Pi x 46mm/1280mm = 2.053 degrees.

Jon

### #18 Jon Isaacs

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Posted 07 December 2019 - 04:48 PM

This is an incorrect but common misconception about SCT's and Maks.  Even the 27mm or so baffle tube on the ES 127 Mak still allows full true FOV in the widest 2" field stops.  There is some vignetting, although it is not apparent at night even though it must be considerable in the outer field.  Field curvature is a more noticeable problem.

Have you actually measured to the TFoV to see?

A C-5 which has a rear port of 1 inch = 25.4  I measured the TFoV at 1.85 degrees where as the field stop calculation suggested 1.97 degrees.  There was noticeable vignetting.

Jon

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### #19 Jon Isaacs

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Posted 07 December 2019 - 05:01 PM

Does the same rules of calculating TFOV apply to zooms? For example my Baader Hyperion Mark IV Zoom gives me following

52X = 24mm   BHZ at 24mm    Eye Pupil = 4.89mm   AFOV - 48°   TFOV - 0.92°
62X = 20mm   BHZ at 20mm    Eye Pupil = 4.08mm   AFOV - 52°   TFOV - 0.83°
78X = 16mm   BHZ at 16mm    Eye Pupil = 3.26mm   AFOV - 58°   TFOV - 0.74°

While magnification, eye pupil and AFOV are accurate is TFOV correct?  How about zooms +  barlow???

236X = 5.3mm  (BHZ at 12mm + 2.25X barlow)   Eye Pupil = 1.12mm   AFOV - 63°   TFOV - 0.27°
357X = 3.5mm  (BHZ at   8mm + 2.25X barlow)   Eye Pupil = 0.71mm   AFOV - 68°   TFOV - 0.19°

With a zoom, you don't really know the field stop so you can only estimate the TFoV based on AFoV or measure the TFoV.  Measuring TFOV is best done by drift timing.  You measure the time it takes for a star to drift across the field and then depending on the declination of the star, you can compute the TFoV.

As far as the AFoV of the Baader zoom:  It is possible to actually measure the AFoV of an eyepiece. I use an projection technique that David Knisely developed. Basically the eyepiece is illuminated by a light and the beam is projected on a wall.  The distance and the diameter of is measured and the AFoV calculated.

I measured the Baader Mark IV zoom:

24mm 43.8 degrees
20mm 48.9 degrees
16mm 54.0 degrees
12mm 59.4 degrees
8 mm 68.4 degrees

When you add a barlow, the magnification is uncertain, it can vary 20% or more.  With a zoom, the TFoV is only estimated.

Jon

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### #20 Jon Isaacs

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Posted 07 December 2019 - 05:26 PM

I still prefer simple. My 24mm Pan gives roughly 4 arc degrees in my ED72. That's about 8 Full Moons. No need to be an anorak about it if you ask me.

Simples!

Some thoughts on TFoV:

- For what it's worth:

The Evostar 72ED has a focal length of 420mm.  The 27mm Panoptic has a field stop of 27.0 mm  Using the field stop equation, the TFoV is 3.68 degrees.  The 4.0 degree number overestimates the TFoV by about 9%.  The AFoV equation estimates it at 3.87 degrees. That overestimates the TFoV by 5%.

-For most purposes 5% is accurate enough.  But Ras is interested in the more exact calculation using the field stop diameter and the focal length of the scope. If he want's to do it right, I say let's do it right.

The reason the field stop calculation is more precise is this:

The field stop determines the diameter of the image at the focal plane that the eyepiece sees. It is independent of the focal length of the eyepiece, the AFoV of the eyepiece, the distortion of the AFOV of the eyepiece.

The focal length of the telescope determines the image scale at the focal plane.  These two create a triangle with the diameter of the field stop as height and the focal length of the telescope as the base length.  Ideally this is an isosceles triangle but the angles are small enough so one can use the small angle approximation.  With a 5 degree TFoV, the error is 0.06%.

TFoV = Field stop /focal length telescope.   The result is in radians.  To convert to degrees multiply by 180 degrees/Pi radians = 57.3

TFoV = 57.3 deg/radian x FS/FLscope.

Jon

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### #21 grzesznypl

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Posted 07 December 2019 - 05:51 PM

With a zoom, you don't really know the field stop so you can only estimate the TFoV based on AFoV or measure the TFoV.  Measuring TFOV is best done by drift timing.  You measure the time it takes for a star to drift across the field and then depending on the declination of the star, you can compute the TFoV.

As far as the AFoV of the Baader zoom:  It is possible to actually measure the AFoV of an eyepiece. I use an projection technique that David Knisely developed. Basically the eyepiece is illuminated by a light and the beam is projected on a wall.  The distance and the diameter of is measured and the AFoV calculated.

I measured the Baader Mark IV zoom:

24mm 43.8 degrees
20mm 48.9 degrees
16mm 54.0 degrees
12mm 59.4 degrees
8 mm 68.4 degrees

When you add a barlow, the magnification is uncertain, it can vary 20% or more.  With a zoom, the TFoV is only estimated.

Jon

I took those AFOV numbers from Baader manual, you would think they are relatively accurate, right?

### #22 Asbytec

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Posted 07 December 2019 - 06:12 PM

I just divide the magnification of the eyepiece by the AFOV. It probably isn't the most accurate way to calculate TFOV, but it suffices for my needs.

Example:

24mm Tele Vue Panoptic ~ 68° AFOV.

Magnification of Panoptic in 72mm Evostar ~ 17.5x.

68/17.5 = 3.885

3 arc degrees : 53 minutes : 6 seconds TFOV.

On a spread sheet, I'll be more accurate. But on the fly, this is the calculation I use. One or two significant digits and a few percentage points is fine.

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### #23 Shorty Barlow

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Posted 07 December 2019 - 06:47 PM

On a spread sheet, I'll be more accurate. But on the fly, this is the calculation I use. One or two significant digits and a few percentage points is fine.

It suits me, but I'm not a telescope designer or manufacturer. In fact, I usually translate field into Full Moons. My 24mm Pan gives nearly eight of them in the ED72. The 19mm Pan gives six. My 6mm AH orthoscopic will give just over half a Full Moon.

Hold your little finger out at arm's length and up to the sky. The tip of the finger (across) is roughly one arc degree.

I just like to look at stuff from my back garden.

I'm not a cosmonaut. lol

Edited by Shorty Barlow, 07 December 2019 - 06:48 PM.

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### #24 Redbetter

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Posted 07 December 2019 - 09:32 PM

Have you actually measured to the TFoV to see?

A C-5 which has a rear port of 1 inch = 25.4  I measured the TFoV at 1.85 degrees where as the field stop calculation suggested 1.97 degrees.  There was noticeable vignetting.

Yes, I have for both the 127 Mak and the 8" SCT.  What I found is reasonable agreement between the TFOV and the change in focal length of the 2" eyepiece based on field stop positions.  (Using 1.25" eyepiece field stops with a 2" diagonal confirmed it.)  I don't have a C5 so its smaller baffle tube might be a factor, but it isn't for a 5" Mak from what I have seen.   Can't speak for the C5 which is in a range that I really don't consider that important for SCT's.  I sort of see a C6 as a minimal size and going below that favors a Mak.

The other aberrations in the outer field, primarily field curvature related, were the biggest negative impact.  I don't spend much time looking at the field edge, other than drift timing.  It is more of the frame rather than the picture--necessary, but not what I am concentrating on in the image.   While the far outer edge vignetting is seen in the drift timing tests, it doesn't really impact the image noticeably in practice.

### #25 Redbetter

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Posted 07 December 2019 - 11:19 PM

I still prefer simple. My 24mm Pan gives roughly 4 arc degrees in my ED72. That's about 8 Full Moons. No need to be an anorak about it if you ask me.

The full Moon varies in size of course.  My understanding is that the average is about 31 arc minutes with a range of ~29.3 to 33.5.  This is an example of one source of error compounding on another.  So while you are assuming 8 moons wide, the actual field is much closer to 7 moon widths on average.

Generally conservative estimates are preferred/more useful than overly optimistic ones.  This is particularly true when it comes to framing an object.

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