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Issues with 40mm eyepieces

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#1 Ray_T

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Posted 03 July 2020 - 06:01 PM

New to this forum.....

 

I was about to purchase a 40mm eyepiece for my 8" Sky-Watcher (FL=1200mm) when I came across the issues re field of view for 40mm eyepieces in Cloudy Nights. My goal was simple so I thought - to increase my field of view at 30x to approximately 2 degrees. Thus using the formula, apparent field of view/magnification, I assumed a standard 52 degree Plossl @ 30x woud give me close to 2 degrees of true field, a somewhat wider angle FOV eyepiece (say 60 degrees) would give me the desired 2 degrees with a few extra dollars expended. No problem and not too expensive!!

 

Then I read Cloudy Nights. It appears the Meade 40mm and the Celestron Omni 40mm  aren't much good for getting the 2 degrees I desire (apparently the apparent field of view is low). The experts say too narrow, aberrations, etc.!! What happened? No problem with my 26mm eyepieces (for a TFOV of about 1 degree). Where and when does the breakdown occur?

 

What eyepiece can you recommend to me that will give me my desired outcome at not too much cost?

 

Thanks for your assistance.

 

 



#2 The Ardent

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Posted 03 July 2020 - 06:06 PM

Use the Televue calculator to give the magnifications and field of view of plossl eyepieces vs other designs.

http://www.televue.c...=212&plain=TRUE

#3 slepage

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Posted 03 July 2020 - 06:13 PM

The 1 1/4 inch eyepieces that you are looking at will restrict your Apparent FOV.  Go for a 2 inch eyepiece such as the William's Optic 40mm SWAN here are the specs.

 

Focal Length: 40 mm
Barrel Size: 2-inch
Apparent FoV: 72-degrees
Eye Relief: 28 mm

 

Steve


Edited by slepage, 03 July 2020 - 06:14 PM.

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#4 eyeoftexas

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Posted 03 July 2020 - 06:30 PM

You might be careful with 40 mm eyepieces in your scope because the exit pupil will be very large, on order of 7 mm.


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#5 havasman

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Posted 03 July 2020 - 07:02 PM

The 40mm SWAN is great on a spreadsheet. I had one for the TFOV but it was not a good view. The outer field is not very clean at all at < f8 or 10. I do not recommend it and it is a 2" eyepiece. You probably don't actually want a 40mm eyepiece. The 40mm Plossls are almost w/o good application and an f5.9 Dob is not it.

If you want to stay with 1.25" ep's look at the ES68 24mm or at lower cost and poorer performance the 32mm Plossls.


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#6 whizbang

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Posted 03 July 2020 - 07:18 PM

First off, in 1.25 inch eyepieces, 32mm and 40mm have identical FOV.  Don't buy a 1.25 40mm.  You don't gain any Field of View and you lose magnification.

 

In 2 inch eyepieces, there are some affordable options that work okay in slow scopes (f8, F10, F12 etc).  Your scoope is F5ish and cheapie 40mm 2 inch eyepieces will blurr the outer field.  Nothing gained there either.

 

Forget about it.  40 or 40, both are wrong for you.


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#7 Jon Isaacs

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Posted 03 July 2020 - 07:57 PM

Ray:

 

Hello and :welcome: to Cloudy Nights.

 

To get a 2 degree Field of View, you will need to use 2 inch eyepieces. In general, entry level 2 inch wide fields are sharp in the center of the field but towards the edge, stars are no longer nice point like dots, they're astigmatic blurs.

 

Still such eyepiece's can provide enjoyable views and serve as effective finders if one has reasonable expectations. The Orion Q70s and AgenaAstro twins 70 degree eyepieces in the 32 mm and 38 mm focal lengths are under $100. The 32 mm would provide a 1.91° field at 37.5x, the 38 mm a 2.18° field at 31.6x.

 

One can spend large sums of money on 2 inch Widefields, the 31 mm Nagler is about $650.. it's a very nice eyepiece that weighs over 2 pounds.  I'd start with some modest and enjoy the view despite the imperfections.

 

Jon


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#8 Bill Weir

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Posted 03 July 2020 - 08:48 PM

Does your scope take 2” eyepieces? If so I’m going to give an answer different than the nay sayers. While I now have higher end eyepieces that can give me a similar FOV I’ve for a long time had a 2” 40mm Orion Optilux (similar to the OU Koenig). With my f/8 6” it gives a 2°FOV and it’s fabulous. Sure there’s a bit of light drop off at the edge due to the small secondary but you don’t notice it unless you go looking for it. Yes in your f/6 there will be a large exit pupil but if you are using it during no Moon periods then it won’t matter. Just don’t use it for very bright objects. 

 

The 2° Is about as close to refractor for wide fields as my little dob can do. I can’t think of any low cost way for me to have done this sketch.  https://rascvic.zenf...01c9d4#h201c9d4

 

Look used you might find something.

 

Bill


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#9 Ray_T

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Posted 03 July 2020 - 09:35 PM

Thanks everyone. I very much appreciate your replies. Some technical things I don't quite understand but I get the gist - standard 52 degree Plossl eyepieces don't work well for a 40mm eyepieces, yet they work well for 25mm eyepieces. I'll guess "the why" will remain a mystery to me.

 

A note to Bill Weir: Bill, I just live down the road from you in Cobble Hill. Tell me, do we ever get any clear night skies on Vancouver Island? I don't remember the last one!! Take care.


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#10 Bill Weir

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Posted 03 July 2020 - 10:49 PM

Thanks everyone. I very much appreciate your replies. Some technical things I don't quite understand but I get the gist - standard 52 degree Plossl eyepieces don't work well for a 40mm eyepieces, yet they work well for 25mm eyepieces. I'll guess "the why" will remain a mystery to me.

 

A note to Bill Weir: Bill, I just live down the road from you in Cobble Hill. Tell me, do we ever get any clear night skies on Vancouver Island? I don't remember the last one!! Take care.

Hey, yes we do and they are coming. It’s all twilight now anyway. I had a great night out on the 23rd. 

 

You a member of the RASC Victoria?  https://www.victoria.rasc.ca

How about do you know of this bunch? They are a great bunch right in your neighbourhood.   https://starfinders.ca  They have a nice affiliation with the Shawnigan Lake School and its observatory.

 

Hope to meet you sometime.

 

Bill



#11 Dave Mitsky

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Posted 04 July 2020 - 01:43 AM

Some technical things I don't quite understand but I get the gist - standard 52 degree Plossl eyepieces don't work well for a 40mm eyepieces, yet they work well for 25mm eyepieces. I'll guess "the why" will remain a mystery to me.

The why is the fact that the diameter of the barrel ultimately limits the apparent field of view possible as the focal length of the eyepiece increases.  A 1.25" eyepiece has a maximum field stop of about 27mm.  A 2" eyepiece has a maximum field stop of 46mm.

 

A 24mm 68-degree AFOV eyepiece produces the same true field of view as a 32mm 50-degree AFOV eyepiece or a 40mm 43-degree AFOV eyepiece.


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#12 Tony Flanders

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Posted 04 July 2020 - 04:34 AM

Thanks everyone. I very much appreciate your replies. Some technical things I don't quite understand but I get the gist - standard 52 degree Plossl eyepieces don't work well for a 40mm eyepieces, yet they work well for 25mm eyepieces. I'll guess "the why" will remain a mystery to me.


There is no mystery. It is perfect possible to make a 40-mm Plossl with a 52-degree apparent field of view, but its lenses would be too wide to fit into a 1.25-inch barrel.

The standard 40-mm Plossls sold by many vendors have been tailored to fit into 1.25-inch barrels, for people whose telescopes cannot accept 2-inch eyepieces. That is achieved by reducing their apparent field of view.


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#13 phillip

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Posted 04 July 2020 - 05:25 AM

Nice info well covered.

 

My simplicity report as what I see thru the eyepiece.

 

I do have and use a standard 40mm eyepiece in my XT8i and the outside perimeter view is certainly not sharp, also I get an unwanted less contrast as  perhaps scattered light, what does happen is things get abit lit up in the background.

 

But do enjoy it's increased field, used often when I lose an object in high power and zero things back in. Infact used just today on a 300X ortho look at Mars, put things back in the high power view. Works beautifully. 

 

Wouldn't be without it, just not the serious pleasing view. Great search eyepiece use.

 

My pleasing look is a Baader 30mm eyepiece, all sharp and nice view. 

 

May still consider it realizing it's limitations. 

 

XT8i, XT10 Dobs

 

Pentax 7.5mm, Clave 8mm, Baader 10mm, recent orthos 6mm, 4mm for Mars!



#14 Paul Sweeney

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Posted 04 July 2020 - 09:35 AM

I have had a Celestron "Halloween" 40 mm for decades. I like it because it gives me a low power view that helps me find stuff. I had a 2" 32mm, but it was only sharp in the middle, so it was junk to me. I gave to someone who loves it. Hmmm. I also didn't like switching the 2/1.25 adapter in and out all the time. So if 2° is your goal, go for the 2" eyepiece. Just make sure that the quality is good. It would be best to actually try the eyepiece out in your scope so you can see if it is performing like you want. A club or star party would help here.

#15 SeattleScott

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Posted 04 July 2020 - 12:15 PM

Ideally a 30mm 82 AFOV would be better, with darker background sky, although a 40mm 70 AFOV could get some use at a dark sky site. A compromise would be a 30-32 70 AFOV. Wouldn’t get two degrees but maybe 1.7 or so, and these might be cheaper and lighter than a 30mm 82 AFOV.

As for paying $200+ for a high performance eyepiece or $99 for something with considerable edge distortion, it kind of comes down to whether you are serious about stargazing and it is a primary hobby for you that you want to invest in, or if stargazing is more of a side hobby and you want to save money for other expensive hobbies that you care more about.

Scott
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#16 Ray_T

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Posted 04 July 2020 - 12:36 PM


 

 

 

  A 1.25" eyepiece has a maximum field stop of about 27mm.  A 2" eyepiece has a maximum field stop of 46mm.

 

 

 

Field stop?  Not familiar with the meaning of the this term.  I'm learning, so some of the technical language is difficult. So my understanding is that the best I can do without losing true field of view with my 1.25" eyepiece is to go no higher than 27mm. BUT, I can go to 40mm with a 2" eyepiece and not lose any field of view according to the formula - apparent field of view/mag. = true field of view. Thanks. 



#17 SteveG

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Posted 04 July 2020 - 01:01 PM

Field stop?  Not familiar with the meaning of the this term.  I'm learning, so some of the technical language is difficult. So my understanding is that the best I can do without losing true field of view with my 1.25" eyepiece is to go no higher than 27mm. BUT, I can go to 40mm with a 2" eyepiece and not lose any field of view according to the formula - apparent field of view/mag. = true field of view. Thanks. 

 

The field stop is a metal ring mounted inside the barrel of an eyepiece at the focal plane. You see it when you look through the eyepiece. It’s the round border on the edges of your fov.


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#18 Tony Flanders

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Posted 04 July 2020 - 02:03 PM

As for paying $200+ for a high performance eyepiece or $99 for something with considerable edge distortion, it kind of comes down to whether you are serious about stargazing ...


I don't see it like that at all! I certainly consider myself serious about stargazing; after all, as a Sky & Telescope editor, it was my livelihood for many years. But I just don't care a whole lot about edge distortion in low-power wide-field eyepieces -- especially when I'm operating at oversized exit pupils, which is a very conscious and major sacrifice of detail in favor of field of view. If I really want to get a good view of something, I center it up. Or use higher magnifications.

 

This is a matter of degree, of course. There is a limit to exactly how much distortion I'm willing to accept. But I think that $99, wisely spent, should satisfy me.

 

For me, being serious about stargazing is a matter of time and effort, not money.


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#19 LDW47

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Posted 05 July 2020 - 06:24 PM

The 1 1/4 inch eyepieces that you are looking at will restrict your Apparent FOV.  Go for a 2 inch eyepiece such as the William's Optic 40mm SWAN here are the specs.

 

Focal Length: 40 mm
Barrel Size: 2-inch
Apparent FoV: 72-degrees
Eye Relief: 28 mm

 

Steve

That is a great ep ! Looking at the Double Cluster through it on a black nite is exceptionally awesome ! Clear skiys !


Edited by LDW47, 05 July 2020 - 06:32 PM.


#20 LDW47

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Posted 05 July 2020 - 06:25 PM

I don't see it like that at all! I certainly consider myself serious about stargazing; after all, as a Sky & Telescope editor, it was my livelihood for many years. But I just don't care a whole lot about edge distortion in low-power wide-field eyepieces -- especially when I'm operating at oversized exit pupils, which is a very conscious and major sacrifice of detail in favor of field of view. If I really want to get a good view of something, I center it up. Or use higher magnifications.

 

This is a matter of degree, of course. There is a limit to exactly how much distortion I'm willing to accept. But I think that $99, wisely spent, should satisfy me.

 

For me, being serious about stargazing is a matter of time and effort, not money.

Very well said !  Clear Skies !



#21 LDW47

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Posted 05 July 2020 - 06:28 PM

Field stop?  Not familiar with the meaning of the this term.  I'm learning, so some of the technical language is difficult. So my understanding is that the best I can do without losing true field of view with my 1.25" eyepiece is to go no higher than 27mm. BUT, I can go to 40mm with a 2" eyepiece and not lose any field of view according to the formula - apparent field of view/mag. = true field of view. Thanks. 

Part of your education in this great hobby, you should familiarize yourself with the basic terminology of  it all ! And I am sure you will !  Clear black skies now and in the future !




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