Widest Field Eyepiece-

[refractordude]

What size eyepiece will give me the widest view with a 120mm f/8 refractor? I will be using a 2" diagonal.

[SpaceWalker]

Any eyepiece with Field Stop of 46mm. For example, depending on your budget, 41mm TV Panoptic, 40mm ES68, 40mm WO Swan or similar.

[MarsG76]

Televue Ethos are 100 degree, TV Delos & Stellar Vue have 110 deg EPs, Explore Scientific have a 120 deg eyepiece.

 

[Pig]

Here you go...... see the true field column ? I have set the parameters of your scope.

There are some 120 degree eyepieces about but I am not sure of their specifications...

Design	Focal Length (mm)	Apparent Field (°)	Eye Rel. (mm)	Barrel Size	Mag. (x)	Exit Pupil (mm)	True Field (°)	True Field @ 1000 yards (feet)	Mag. w/2x Barlow (2x)	Mag. w/2x Big Barlow or Power- mate (2x)	Mag. w/2.5x Power- mate (2.5x)	Mag. w/3x Barlow (3x)	Mag. w/4x Power- mate (4x)	Mag. w/5x Power- mate (5x)
Panoptic	41	68	27	2"	23.4	5.1	2.7	143.8	--	46.8	--	--	93.7	--
Nagler 5	31	82	19	2"	31.0	3.9	2.5	131.3	--	61.9	--	--	123.9	--
Panoptic	35	68	24	2"	27.4	4.4	2.3	120.9	--	54.9	--	--	109.7	--

[SpaceWalker]

45 minutes ago, MarsG76 said:

Televue Ethos are 100 degree, TV Delos & Stellar Vue have 110 deg EPs, Explore Scientific have a 120 deg eyepiece.

 

AFOV has nothing to do with TFOV. An eyepiece with 46mm Field Stop, assuming that the OPs refractor FL= 960mm, will give TFOV=2.7*. The closest out of 100* AFOV eyepieces, a 25mm ES100, will give TFOV=2.4*. All the others suggested by you will give narrower view.

[alanjgreen]

Panoptic41 would give widest 2" as said above 2.7 degrees of sky in your scope. Nagler 31 comes in next at 2.5 degrees of sky in your scope with more magnification too.

televue have a calculator here 

http://www.televue.com/engine/TV3b_page.asp?NoBack=True&id=212&plain=TRUE&focal_length=&fl_unit=mm&aperture=120&a_unit=mm&fratio=8&_2in=yes&Ethos=TRUE&Nagler=TRUE&Panoptic=TRUE&title=&sortby=SortTrueField&SUBMIT1=CALCULATE#results

[Mr Spock]

With a field stop of 46.5mm the 42mm LVW will give you x24 and 3.02°. The 120mm has a focal length of 1000mm therefore f8.33.

I use a 120mm achro with this eyepiece and it works perfectly

 

[Ricochet]

5 hours ago, refractordude said:

What size eyepiece will give me the widest view with a 120mm f/8 refractor? I will be using a 2" diagonal.

What's your budget for this eyepiece and which country do you live in? This information will help people give more tailored suggestions.

[Charic]

5 hours ago, MarsG76 said:

Televue Ethos are 100 degree, TV Delos & Stellar Vue have 110 deg EPs, Explore Scientific have a 120 deg eyepiece.

 

Delos EP's are 72°, a step down of 28° from the Ethos, and  if you need smaller, DeLites have 62° which is the field I most prefer.

[Louis D]

8 hours ago, refractordude said:

What size eyepiece will give me the widest view with a 120mm f/8 refractor? I will be using a 2" diagonal.

All right, I see what's going on here.  Do you mean widest TRUE field of view or widest APPARENT field of view?  Some answers here assume the former, others the latter.

[Charic]

....and if you divide the AFOV by the MAGNIFICATION you will know your TFOV.

Example my scope has F-1200 and 32mm EP producing 37.5x power/mag. The EP  has  70° AFOV so 70/37.5 = TFOV 1.866° ( a full Moon is about 0.5°  to give you some idea of the scale of the TFOV )

My suggestion for a low power, wide field eyepiece is to choose one, not on the basis of how wide the field is ( one  can use binoculars for that )  but to make the most use of the scopes aperture, so  an eyepiece with a focal length that equals the scopes focal ratio multiplied by your eye entry pupil will be sufficient. 

[tuckstar]

Astronomy tools website has a calculator which has an extensive library of scope and eyepieces.

Just select an object, your scope from the list or enter your scopes details then select an eyepiece and it will show you the fov and give you other useful info.

https://astronomy.tools/calculators/field_of_view/

[Charic]

....clicking the RESOURCES tab on the menu bar at the top of any page from Stargazers Lounge also takes you there!

[Mr Spock]

...and like everywhere else it gives the LVW 42mm as 65° when it's actually 72°...

[michael.h.f.wilkinson]

I gather the 65° figure is actually correct if you consider the field stop, but 72° is what it looks like through the lens, due to pincushion distortion. 

[refractordude]

7 hours ago, Pig said:

Here you go...... see the true field column ? I have set the parameters of your scope.

There are some 120 degree eyepieces about but I am not sure of their specifications...

Design	Focal Length (mm)	Apparent Field (°)	Eye Rel. (mm)	Barrel Size	Mag. (x)	Exit Pupil (mm)	True Field (°)	True Field @ 1000 yards (feet)	Mag. w/2x Barlow (2x)	Mag. w/2x Big Barlow or Power- mate (2x)	Mag. w/2.5x Power- mate (2.5x)	Mag. w/3x Barlow (3x)	Mag. w/4x Power- mate (4x)	Mag. w/5x Power- mate (5x)
Panoptic	41	68	27	2"	23.4	5.1	2.7	143.8	--	46.8	--	--	93.7	--
Nagler 5	31	82	19	2"	31.0	3.9	2.5	131.3	--	61.9	--	--	123.9	--
Panoptic	35	68	24	2"	27.4	4.4	2.3	120.9	--	54.9	--	--	109.7	--

Thank you.

[SpaceWalker]

13 hours ago, Mr Spock said:

With a field stop of 46.5mm the 42mm LVW will give you x24 and 3.02°. The 120mm has a focal length of 1000mm therefore f8.33.

I use a 120mm achro with this eyepiece and it works perfectly

 

Nice catch  I missed it. Indeed, the 42mm LVW with FS of 46.5mm provides the widest view possible in 2" focuser. In 120/80 refractor it should give ~2.8*TFOV assuming no distortion.

 

8 hours ago, Mr Spock said:

...and like everywhere else it gives the LVW 42mm as 65° when it's actually 72°...

BTW, have you performed timing test to be sure it really shows 3.02*TFOV? I suspect that 72* at 42mm is too optimistic for a 2" focuser even though if the EP would be capable, e.g. in 2.5" focuser 

[Mr Spock]

I've never really looked into it. It doesn't have any vignetting and is nice and sharp. I'm happy with the view it gives and that's enough for me

[Ruud]

18 hours ago, Mr Spock said:

...and like everywhere else it gives the LVW 42mm as 65° when it's actually 72°...

You may have overestimated the 72° apparent field of view (afov) of this eyepiece

With a 65° afov and a 46.5 mm field stop total pincushion distortion would be 13.1%, not bad at all. With a 72° afov and a 46.5 mm field stop total pincushion would be very strong: 23.8%.  You would notice that as extreme pincushion, and the eyepiece is not known for its strong distortion.

The field stop required to have zero angular magnification distortion (AMD) in a 65°, 24 mm focal length eyepiece is 47.7 mm.
If the eyepiece were 72° the field stop needed for zero AMD is 52.8 mm.
The actual field stop of this eyepiece is 46.5 mm.
A 42 mm eyepiece with a field stop of 46.5 mm and zero AMD would have an afov of 63.44° (ideal)

Considering this, and the reputation of the eyepiece, an afov of 65° seems more likely than 72°. 

Try  ScopeCalculatorV2.xlsx

I used these formulas: ScopeCalculator v2 .pdf

---

I need to brush up the nomenclature in the spreadsheet. Some terms are a little clumsy. Here is some clarification:

The eyepiece's afov itself cause pincushion distortion because the eyepiece presents a flat focal plane as a spherical view. In the spreadsheet I call this "Rectilinear Distortion from AFOV".

Angular magnification distortion occurs when the magnification at the edge of the afov is larger than in the centre. When present, positive AMD adds to the overall pincushion.  When the magnification at the edge is smaller than in the centre, AMD is negative (barrel distortion) and it reduces the overall pincushion. Ideally for astronomy, AMD is zero. With zero AMD circles look like circles, not just in the centre but also at the edge of the field.

In the spreadsheet, "Combined Rectilinear Distortion" is an awkward term. I should have called it something like Total Pincushion (+)  or Barrel Distortion (-). You'd think you would want total pincushion to be zero: Straight lines would look straight anywhere in the field. But round off-axis object would look squeezed against the edge of the field because of the negative AMD needed to reduce the total pincushion to zero. We don't see many straight lines in astronomy and we prefer round objects to look round rather than elongated no compressed. Hence we want AMD close to zero. (It is not possible to get both undistorted lines and perfect circles at the same time, but with a small afov this condition can be approximated. Eyepieces that do this are called orthoscopic, eyepieces that don't are pseudoscopic, according to the original definitions.)

Many astronomical eyepieces have a few percent positive AMD. That stretches out circles at the edge of the field and exacerbates pincushion, but it also happens to allow for easier control of astigmatism.

 

 

[Mr Spock]

No estimations at all - it says 72° on the eyepiece

[michael.h.f.wilkinson]

32 minutes ago, Mr Spock said:

No estimations at all - it says 72° on the eyepiece

I know it does, I have one, but as I mentioned earlier in this thread, Vixen did issue a statement that 65° is more accurate, in terms of true AFOV (i.e. computed from the field stop). The view through the EP does look as wide as my Delos EPs, so if you factor in some pincushion distortion, that does explain the discrepancy

 

[Louis D]

Just shine a light through it and measure the projected circle and do the trig to figure out the AFOV directly to end all this arguing about 65 vs 72.

@RuudI directly measured the AFOV of my 40mm Meade 5000 SWA to be 71 degrees rather than 68 degrees as claimed.  I always thought it looked wider than my 27mm Panoptic.  What does this say about its distortion percentages?  It's reported to have a 46mm field stop.  I don't have it handy right now to measure it directly.

[Ruud]

2 hours ago, Louis D said:

I directly measured the AFOV of my 40mm Meade 5000 SWA to be 71 degrees rather than 68 degrees as claimed.  I always thought it looked wider than my 27mm Panoptic.  What does this say about its distortion percentages?  It's reported to have a 46mm field stop.  I don't have it handy right now to measure it directly.

Hello Louis,

For a 40mm focal length eyepiece with 46mm field stop you get this:

5% AMD is a value you find for many eyepieces. Some Ethos, Delos and Morpheus are 1% or under, that is excellent.

Rectilinear distortion from the afov only depend on the afov. Every 71° eyepiece has 13.1% pincushion distortion from the afov.

The Combined Rectilinear Distortions are the total pincushion. You want hem to be close to the Distortion from AFOV values.

We want AMD close to zero. 3.1% AMD is good. 7.2% is reasonable. The AMD follows from the field stop, focal length and apparent field of view. Remember that for instance 46 mm actually means something between 45.5 and 46.5, which would make a difference. Afov and focal length will also introduce rounding or measuring errors.

Do you happen to know the field stops of the other Mead 5K SWAs? I've tried to find them, but didn't succeed. I did find calculated values, I think, because they all corresponded to a field stop for zero AMD.

I want to know because I have the Maxvisions 68° 20, 24, 28 and 34mm, and by comparing their image circles  side by side I see different sizes. The 34 has the widest afov, the 24 the narrowest. These eyepieces are the same as the Meade 5K SWAs, so knowing more about the Meades would help.

The AMD in the Maxvisions seems to be around 5%. That's only an estimate.

I should buy a calliper and devise a smart method to measure afovs precisely.

 

[Louis D]

22 minutes ago, Ruud said:

Do you happen to know the field stops of the other Mead 5K SWAs?

I don't have any Meade 5000 SWA except for the 40mm.  The rest have diminishing eye relief or were redundant with something I already had at the time of the blowout sale.  You should be able to get a rough estimate without removing the lower barrel by affixing a ruler over the field end and moving way back to reduce parallax error.  Hmm, a telescope might be handy for this.

25 minutes ago, Ruud said:

I should buy a calliper and devise a smart method to measure afovs precisely.

This requires no fancy equipment at all.  I rig up a miter box with wood blocks to hold an eyepiece in a 2" extension tube (possibly with 1.25" adapter) with a bright flashlight shining into the open end of the tube.  I then measure the eye relief point distance, the distance to the projected circle, and the diameter of the projected circle.  The longer the projection, the less measurement error.  Using trig, it's pretty simple to calculate the AFOV from these three measurements.  I've found that most Televue, Pentax, and ES eyepieces are spot on or slightly larger in AFOV than claimed.  Meade eyepieces are all over the place.  Some are bigger than claimed, others smaller.  Most claims are close, though, regardless of brand.

[Pig]

On 04/02/2018 at 05:32, refractordude said:

What size eyepiece will give me the widest view with a 120mm f/8 refractor? I will be using a 2" diagonal.

You see the trouble you have caused ??