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Eyepiece suggestion for Widest Field of View AT102EDL

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#1 MichaelJB

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Posted 28 January 2022 - 02:37 AM

I do not own this telescope yet.  Hypothetically, could someone suggest an eyepiece that would give me the widest field of view in the AT102EDL and maybe explain the math to me.  Still new at this, I am not sure if I am getting it right.

 

The AT102EDL:

F/7

711 focal length

 

 



#2 ABQJeff

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Posted 28 January 2022 - 03:05 AM

Eyepiece: Pentax XW 40mm, 70 degree Apparent Field of View (AFOV) or Televue 41mm 68 degree AFOV Panoptic. Both 2” EPs.

True Field of View (TFOV) = Apparent Field of View/Magnification = (Apparent Field of View x Eyepieve Focal Length)/Telescope Focal Length

Note how numerator in second equation is completely agnostic of telescope, so you can now quickly compare which eyepiece gives a bigger field of view just by multiplying eyepiece AFOV x eyepiece focal length. On CN someone gave it the units “jumbos”, which I think is great! So the Pentax XW 40 has 2800 jumbos, the TV 41-68, 2788 jumbos, so the Pentax XW 40 should (see comment below) have a slightly larger field of view.

Using the Pentax XW 40 in your 102 f/7 with 711mm FL then gives:

TFOV = 2800/711 = 3.94 degrees for your maximum field of view.

Note: the most accurate way to determine true field of view is from knowing the field stop of the eyepiece, as numbers like I just stated above for eyepiece focal length and Apparent Field of View (AFOV) can be off by as much as 5% from manufacturing tolerances. But those are hard to find and for a lot eyepieces not even published. So for a casual observer where a tenth if a degree here or there really doesnt make a difference, I just use “jumbos” formula to determine my FOV.
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#3 MichaelJB

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Posted 28 January 2022 - 03:11 AM

Eyepiece: Pentax XW 40mm, 70 degree Apparent Field of View (AFOV) or Televue 41mm 68 degree AFOV Panoptic. Both 2” EPs.

True Field of View (TFOV) = Apparent Field of View/Magnification = (Apparent Field of View x Eyepieve Focal Length)/Telescope Focal Length

Note how numerator in second equation is completely agnostic of telescope, so you can now quickly compare which eyepiece gives a bigger field of view just by multiplying eyepiece AFOV x eyepiece focal length. On CN someone gave it the units “jumbos”, which I think is great! So the Pentax XW 40 has 2800 jumbos, the TV 41-68, 2788 jumbos, so the Pentax XW 40 should (see comment below) have a slightly larger field of view.

Using the Pentax XW 40 in your 102 f/7 with 711mm FL then gives:

TFOV = 2800/711 = 3.94 degrees for your maximum field of view.

Note: the most accurate way to determine true field of view is from knowing the field stop of the eyepiece, as numbers like I just stated above for eyepiece focal length and Apparent Field of View (AFOV) can be off by as much as 5% from manufacturing tolerances. But those are hard to find and for a lot eyepieces not even published. So for a casual observer where a tenth if a degree here or there really doesnt make a difference, I just use “jumbos” formula to determine my FOV.

Thank you so much for the quick response!  I will try to figure some out on my own.  really appreciate the math!



#4 ABQJeff

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Posted 28 January 2022 - 03:20 AM

And while you didnt ask, it is the logical next question:

Exit Pupil (the size of light beam hitting your eye) = Telescope Objective Aperture/Magnification = Telescope Objective Aperture x Eyepiece Focal Length/Telesxope focal length = Eyepiece Focal Length/f/# of telescope.

That last equation then allows you to quickly calculate your exit pupil for a given eyepiece and telescope. So a Pentax XW 40mm then would give you an Exit Pupil of 40/7 = 5.7 mm exit pupil. That is a nice size for large diffuse nebula viewing in dark skies.

Some rules of thumb, totally varies by person, as seeing and eyesight permits: 0.3-0.5mm exit pupil (planets/double stars/moon) to 1-3 mm (galaxies, PN, GC) to 5mm+ for big gaseous nebula (Veil/North American/California, etc.).

The maximum exit pupil you will genealky want is about 7mm exit pupil because that is maximum of human eye (note ok to go above this of trying to achieve a certain field of view).


Edited by ABQJeff, 28 January 2022 - 03:25 AM.

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#5 Jon Isaacs

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Posted 28 January 2022 - 03:50 AM

To add to Jeff's explanations:

 

The. more accurate way to calculate the TFoV AFoV is to use the field stop.  The field stop is the ring you see at the edge of field.

 

.it's an actual ring at the focal plane. The larger the ring, the wider the field of view.. the field of view is also inversely proportional to the focal length of the telescope, the longer the focal length, the narrower the field of view.

 

TFoV = field stop / focal length telescope.

 

That provides the angle in radians. To convert to degrees, you multiply by 180°/π radians = 180/3.14159= 57.3 deg /rad 

 

TFoV = 57.3 deg/rad x FS / To scope.

 

The 41 mm Panoptic has a 46mm field stop, the 40 mm XW a 46.5 mm.  I.n your scope.

 

TFoV pan = 57.3 x 46/711 = 3.71 degrees. Using AFoV /Mag = 3.92 degrees, the error is about 6% this is an error in the accuracy of the equation..

 

For the XW:: actual TFoV, 3.75 degrees. 3.95 degrees estimated, error, about 5%.

 

Jon


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#6 SeattleScott

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Posted 28 January 2022 - 08:09 AM

The 42 Vixen LVW also has a 46.5mm field stop and is a mere 19oz. It is also a bit cheaper than the others. I get the impression the others are a little better at the edge, although the Vixen does well at F8. Noticeably struggles at F5. I’m sure it would be fine at F7 but likely detectably not as sharp at the edge? Another midrange option is the TS Optics 35mm 68 which is similarly light, considerably cheaper, and also pretty well corrected but not as good as Pentax or TV. The TS Optics won’t max out the view though, but it gives a bit more magnification and a nice 5mm exit pupil. I don’t think anyone has compared it to the LVW so I don’t know if it can truly rival the LVW for considerably less money or not.

Something to consider. I almost feel like exit pupil is a little too bright with 42LVW in my ED103S in light pollution. I suspect I would be happier with a 35mm 68 (ideally Panoptic!) in that scope. Granted you want to go as wide as possible, but how wide do you need? Pleiades, Veil still fit in around a 3 degrees FOV. And for sweeping the sky, sky cruising, I like a reasonably dark sky background. What is the point of randomly scanning the Milky Way if everything looks washed out? So this low power Nirvana thing needs some degree of balance. You want a wider view than SCT, but a small enough exit pupil that sky is relatively dark, or else it kind of kills the aesthetics. And aesthetics are about 110% of the point of surfing the Milky Way. You know, not actually intentionally trying to find something but just scanning a region to see what pops up. Now a 5.7mm exit pupil isn’t bad under dark skies, but if you have light pollution that could be pushing it. 5.7mm exit pupil gives a 30% brighter sky background than a 35mm 68. The 35mm 68 only goes 85% as wide, but it will be noticeably darker. Taking that a step further, a 31 Nagler will go 90% as wide as a 40XW/41 Panoptic/42LVW, however the sky will be 75% brighter in the 41 Panoptic. Granted the 31 Nagler will mess with balance and cost you a kidney. The Meade 28PWA would be a more reasonable cost and weight option. Granted the view is getting even narrower, but a nice dark 4mm exit pupil.

Something to consider anyway.

Scott

#7 SeattleScott

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Posted 28 January 2022 - 08:22 AM

Ok some specific examples. I have a 42LVW and ES 24/82 for 2” eyepieces. In my 4” F7.7 I found I prefer the wider view of the LVW, even in light pollution. In my 4” F6.5, I found I preferred the darker sky of the 24mm. It was only three degrees instead of four degrees wide, but the sky was much darker, and hey it was still three degrees wide. Now you have a F7 so this might not be terribly helpful, plus we are all different. There are the wide field junkies who feel like life begins at 4+ degrees. Generally they are in reasonably dark skies though.

Scott

#8 Jon Isaacs

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Posted 28 January 2022 - 10:12 AM

Something to consider. I almost feel like exit pupil is a little too bright with 42LVW in my ED103S in light pollution.

 

 

My two cents:

 

Don't chose your lowest power, widest field of view eyepiece based on it's effectiveness under light polluted skies, chose it for it's performance under darker skies because that's where it will really come to life..

 

Jon


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#9 aeajr

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Posted 28 January 2022 - 11:10 AM

I do not own this telescope yet.  Hypothetically, could someone suggest an eyepiece that would give me the widest field of view in the AT102EDL and maybe explain the math to me.  Still new at this, I am not sure if I am getting it right.

 

The AT102EDL:

F/7

711 focal length

 

Is this your first scope or one of many?   Do you have a set of eyepieces or are you buying everything for the first time?

 

What is your budget?  Since you are looking at an EDL, I presume a $300 to $600 eyepiece would not be unreasonable.  I typically work at a lower budget.

 

Is this primarily for visual or AP?  If visual,  you will want to plan your eyepiece set.

 

I have the same scope, but I have the ED, not the EDL version.  Lots of good advice above.  I will share how I have approached this.  I always want 1 eyepiece that will max out the field of view. 

 

 

 

Understanding Telescope Eyepieces- There are recommendations, based on budget, and some are no longer available,

but the meat of the article is about understanding the math, considerations, language and specifications to know when selecting eyepieces.
https://telescopicwa...cope-eyepieces/

 

 

For my AT102ED, and my Apertura AD12 Dob, I have an Agena Astro 38 mm 70 degree 2" Eyepiece.  This is the same as the Orion Q70 38 mm.  Around $100

This is a good low priced eyepiece, but it is not as well corrected at the edges as many of the higher priced eyepieces.  It serves me well for scanning the sky, star hopping and for viewing low power wide view targets.  But, at F7, it is not clean to the edge.  I have a second low power wide view to complement it as a next step eyepiece, shown here. 

https://agenaastro.c...a-eyepiece.html

 

 

Astro Tech AT102ED Refractor 102 mm/714mm F7

AA SWA 38 mm/70                              19X and   3.7 degrees FOV    EP 5.4 mm  2”
Meade  20 mm/82                                36X and   2.2 degrees            EP 2.8      2”

 

If you are interested I can share the chart of all of my eyepieces for this scope.

 

 

 

If I were to upgrade from this eyepiece I would likely go to the Explore Scientific 40 mm 68 degree for better edge correction.  Around $520 

https://www.astronom...2-eyepiece.html

 

 

The ES 34/68 would give up a little FOV at $419, but save some money

https://www.astronom...2-eyepiece.html

 

 

At only a little higher price, the best choice might be the Tele Vue Panoptic 41mm / 68 degree  Around $580

https://www.astronom...2-panoptic.html

 

 

The Panoptic 35 mm, at around $425 would give up a little FOV but save a lot of money

https://www.astronom...2-panoptic.html


Edited by aeajr, 28 January 2022 - 11:27 AM.


#10 Starman1

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Posted 28 January 2022 - 05:05 PM

Eyepiece: Pentax XW 40mm, 70 degree Apparent Field of View (AFOV) or Televue 41mm 68 degree AFOV Panoptic. Both 2” EPs.

True Field of View (TFOV) = Apparent Field of View/Magnification = (Apparent Field of View x Eyepieve Focal Length)/Telescope Focal Length

Note how numerator in second equation is completely agnostic of telescope, so you can now quickly compare which eyepiece gives a bigger field of view just by multiplying eyepiece AFOV x eyepiece focal length. On CN someone gave it the units “jumbos”, which I think is great! So the Pentax XW 40 has 2800 jumbos, the TV 41-68, 2788 jumbos, so the Pentax XW 40 should (see comment below) have a slightly larger field of view.

Using the Pentax XW 40 in your 102 f/7 with 711mm FL then gives:

TFOV = 2800/711 = 3.94 degrees for your maximum field of view.

Note: the most accurate way to determine true field of view is from knowing the field stop of the eyepiece, as numbers like I just stated above for eyepiece focal length and Apparent Field of View (AFOV) can be off by as much as 5% from manufacturing tolerances. But those are hard to find and for a lot eyepieces not even published. So for a casual observer where a tenth if a degree here or there really doesnt make a difference, I just use “jumbos” formula to determine my FOV.

Pentax has a 46.5mm field stop.  Panoptic has a 46mm field stop.

With 711mm focal length, true fields are:

Pentax: (46.5/711)x572958=3.75°

Panoptic: (46/711) x 57.2958 = 3.71°

 

A 35mm Panoptic is lighter and smaller and yields (38.7/711) x 57.2958 = 3.12° field.

A 31mm Nagler yields (42/711)x57.2958 = 3.38°

 

The TFoV = AFoV / M formula yields true fields that are too large (for a variety of reasons I won't go into here).

Better to use the formula TFoV = (field stop/Tel.FL) X 57.3

The last figure is really 180/pi, so what figure you use depends on simplicity.  I used 57.2958, but I could have used 57.3 or 57.2957795131.grin.gif


Edited by Starman1, 28 January 2022 - 05:06 PM.

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#11 ABQJeff

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Posted 28 January 2022 - 10:44 PM

“The TFoV = AFoV / M formula yields true fields that are too large (for a variety of reasons I won't go into here)”…true that!

 

Unfortunately not all EP brands (especially lower cost brands that beginners gravitate towards) share their field stops.  Don’s annual eyepiece list is one of the best resources available (especially for beginners that don’t own a large number of EPs to do their own field stop measurements).



#12 aeajr

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Posted 28 January 2022 - 11:17 PM

The field stop method is the most accurate approach, as I understand it. However, not everyone needs the EXACT true FOV.

For many of us a close approximation is good enough.

TFOV / magnification gives a close approximation that is good enough.

Whether my 38mm/70 degree gives me 3.7 degree TFOV or 3.6 or even 3.5 degree, the simple calculation is close enough for me.
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#13 Jon Isaacs

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Posted 29 January 2022 - 09:13 AM

Pentax has a 46.5mm field stop.  Panoptic has a 46mm field stop.

With 711mm focal length, true fields are:

Pentax: (46.5/711)x572958=3.75°

Panoptic: (46/711) x 57.2958 = 3.71°

 

A 35mm Panoptic is lighter and smaller and yields (38.7/711) x 57.2958 = 3.12° field.

A 31mm Nagler yields (42/711)x57.2958 = 3.38°

 

The TFoV = AFoV / M formula yields true fields that are too large (for a variety of reasons I won't go into here).

Better to use the formula TFoV = (field stop/Tel.FL) X 57.3

The last figure is really 180/pi, so what figure you use depends on simplicity.  I used 57.2958, but I could have used 57.3 or 57.2957795131.grin.gif

 

My calculator uses an internal value of:

 

57.295779513082320876798154814105

 

The error using 57.3 is less than 0.0001 = 0.01%

 

Since the field stop is known only to 0.2% at best and the focal length is probably even less, 180°/ π = 57.3 degrees / radian is more than sufficient.

 

Jon


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#14 Jon Isaacs

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Posted 29 January 2022 - 09:32 AM

The field stop method is the most accurate approach, as I understand it. However, not everyone needs the EXACT true FOV.

For many of us a close approximation is good enough.

TFOV / magnification gives a close approximation that is good enough.

Whether my 38mm/70 degree gives me 3.7 degree TFOV or 3.6 or even 3.5 degree, the simple calculation is close enough for me.

 

Using TFoV = AFoV /Mag, for your AT 102 ED and the 38 mm 70 deg spec, you'd get 3.73 degrees, actual using the 45.7 mm field stop is 3.66 degrees.  Quite close. 

 

If you were trying to decide between your 38 mm and the old TeleVues 40 mm 65 degree Wide Field eyepiece you might want to use the field stop equation..

 

AFoV/Mag = 3.66 degrees, the field stop equation provides 3.31 degrees.

 

But whether or not one thinks it's important to use the more accurate equation, when someone asks about an eyepiece that provides the widest field of view, providing the full story is part of answering the question..

 

Jon


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#15 Procyon

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Posted 29 January 2022 - 09:46 AM

57.295779513082320876798154814105

Jon


Lol!

#16 rexowner

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Posted 29 January 2022 - 11:25 AM

I have an AT 102EDL, and compared the 40mm XW and the 41mm Panoptic viewing the

Pleaides last night.  With this scope to my eyes the stars were a little sharper at the across the

edges of the field with the 41mm Panoptic.

 

I've also compared the two during daylight looking at details of foliage a few hundred

yards away.  The Panoptic looked sharper across the field at that time as well.

 

The Pentax 40mm is an excellent eyepiece, and the XWs are a great choice if you

want to build a set of comfortable eyepieces, but individually, the Panoptic seems

sharper to me.  Of course, the Pan's heavier and more expensive.  IMO you wouldn't

really go wrong either way.


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