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Are 2 inch eyepieces still useful?

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#26 havasman

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Posted 05 March 2021 - 03:06 PM

A larger field stop can be fitted into a 2" barrel than into a 1.25" barrel. So yes certainly there are advantages to 2" eyepieces. But the essential advantage can be said to be restricted to the field stop matter as it defines the TFOV for a given scope.


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#27 TOMDEY

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Posted 05 March 2021 - 03:11 PM

Exactly graphic Tom.  Although a minor point, you forgot the TV 55 mm Ploessl.

The TeleVue 55mm Plossl is right there on the top of the Yellow trajectory.    Tom



#28 Thomas_M44

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Posted 05 March 2021 - 03:56 PM

TV 55 Pl is at the 2" Ceiling limit (46mm).  The rest of the TV Pl are at or below the 1.25" Ceiling limit.

Except the 1.25" 40mm TV Plossl.

 

Notice it pays a reduced AFOV penalty because it's not 2-inch format.

 

****

 

BTW: The 55mm TV Plossl Rocks cool.gif

 

It's a fun, very nice eyepiece --particularly under darker skies and/or in conjunction with a good nebula filter


Edited by Thomas_M44, 05 March 2021 - 03:59 PM.

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#29 Starman1

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Posted 05 March 2021 - 04:21 PM

Maximum fields in 1.25" currently in the market place:

40mm--40°

32mm--50°

26mm--62°

24mm--68°

16mm--82°

13mm--100°

 

Compare that to 2":

55mm--50°

41mm--68°

31mm--82°

25mm--100°

 

As you can see, if you want a wide field in an eyepiece longer than 24mm, it'll be 2".


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#30 faackanders2

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Posted 05 March 2021 - 06:12 PM

I ask this question because, years ago when 2 inch EPs became available, most 1.25 inch EPs were 50-60 degrees or so. Now, though, it is common to have 1.25 inch EPs with 82 degree fields of view and even larger. In light of this, are 2 inch EPs of limited use over what can be gotten with wide-field 1.25 inch EPs? Is this what prompted the production of 3 inch and larger EPs?

2" provides wider TFOV with lower power eyepieces.  Most eyepiece manufactures optimize for weight, so they use 1.25" when they don't need 2".  ES is the exceptions and basically made all 100AFOV 2" eyepieces for a common barrel (similar to when eyepieces switch from 0.975" to 1.25" standard).

 

P.S.  The only way to go wider than 2" 40-41mm 68-70AFOV is to go 3", but it only works well for slow refractors with no central obstruction, and requires a 3" focuser.



#31 faackanders2

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Posted 05 March 2021 - 06:25 PM

Are there any 3" eyepieces that would be worthwhile in any telescopes besides large observatory telescopes or maybe big slow refractors or Cats?

 

Mike

Sieberts offers lots of 3" eyepieces for slow refractors.  ES offers just one 30mm 100AFOV.  Surprised TV does not offer any.


Edited by faackanders2, 05 March 2021 - 10:16 PM.

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#32 eblanken

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Posted 05 March 2021 - 06:31 PM

Exactly graphic Tom.  Although a minor point, you forgot the TV 55 mm Ploessl.

 

No he didn't, See where it says TV Plossl in Gold/Yellow  at the intersection of 55mm (up from the bottom) and 2 inch over from the left).

 

Cheers,

 

Ed (eblanken)



#33 SeattleScott

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Posted 05 March 2021 - 07:40 PM

Sieberts offers lots of 3" eyepieces for slow refractors ES offers just one 30mm 100AFOV. Surprised TV does not offer any.

There is a reason for that. Harry builds his eyepieces himself on an as needed basis. Televue outsources production. Televue needs to make a certain quantity of a single eyepiece in order for it to be economically viable for them. Harry can make one offs. Televue can’t. A three inch eyepiece is a specialty item that is going to sell very small quantities. Not really TV’s business model.

Granted ES outsources also, and they made the 30mm. Not sure how well that has gone. So I guess it isn’t inconceivable that TV could offer a 3” but it makes more sense for Harry.

Scott
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#34 JamesDuffey

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Posted 05 March 2021 - 07:47 PM

Maximum fields in 1.25" currently in the market place:

40mm--40°

32mm--50°

26mm--62°

24mm--68°

16mm--82°

13mm--100°

 

Compare that to 2":

55mm--50°

41mm--68°

31mm--82°

25mm--100°

 

As you can see, if you want a wide field in an eyepiece longer than 24mm, it'll be 2".

Alas, it wasn’t too long ago, when 50° was considered wide field in a well corrected eyepiece. And by not too long ago, I mean I can remember it. 


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#35 Avgvstvs

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Posted 05 March 2021 - 08:02 PM

For wide low power fields you need field stops larger than the 1.25" format.

So 2" is required for low power wide fields, ie 31mm Nagler Type 5

If you want to use all 1.25" your AFOV will get smaller with wider fields

due to the field stop size. Yes 2' is well worth it.

The only 1.25" EP's I use are 6.7mm and shorter.

I think it feels more solid to use 2" anyway


Edited by Avgvstvs, 05 March 2021 - 08:07 PM.


#36 eyeoftexas

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Posted 05 March 2021 - 09:15 PM

No he didn't, See where it says TV Plossl in Gold/Yellow  at the intersection of 55mm (up from the bottom) and 2 inch over from the left).

 

Cheers,

 

Ed (eblanken)

Yeah, my mistake.  I flipped the axes when I looked at it.  



#37 faackanders2

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Posted 05 March 2021 - 10:23 PM

There is a reason for that. Harry builds his eyepieces himself on an as needed basis. Televue outsources production. Televue needs to make a certain quantity of a single eyepiece in order for it to be economically viable for them. Harry can make one offs. Televue can’t. A three inch eyepiece is a specialty item that is going to sell very small quantities. Not really TV’s business model.

Granted ES outsources also, and they made the 30mm. Not sure how well that has gone. So I guess it isn’t inconceivable that TV could offer a 3” but it makes more sense for Harry.

Scott

Economies of scale would reduce cost and price, but as you mentioned if demand is low, inventory turnover would be slow.  I would think there would be demand for a 40mm 100AFOV(or 82+AFOV) 3" for refractors, or slow/long focal length scopes.


Edited by faackanders2, 05 March 2021 - 10:25 PM.


#38 Echolight

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Posted 05 March 2021 - 11:23 PM

I sure hope so! Because all of my fixed focal length eyepieces are 2 inch.

 

But lately I've been considering a C5 and not so sure if the fit is right for 2 inch diagonal and eyepieces where size, weight, and balance are concerned. So possibly I'll have to add a low power widefield in the 1.25 format. We shall see.


Edited by Echolight, 05 March 2021 - 11:28 PM.


#39 Starman1

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Posted 06 March 2021 - 01:01 AM

Economies of scale would reduce cost and price, but as you mentioned if demand is low, inventory turnover would be slow.  I would think there would be demand for a 40mm 100AFOV(or 82+AFOV) 3" for refractors, or slow/long focal length scopes.

Is it possible?

3" is 76.2mm.

If the barrel wall is 1.5mm thick (counting threads), the I.D. would be 73.2mm

A 40mm 100° would have around a 69.6mm field stop.

That might be possible, but it might not due to the negative/positive design.

It would also weigh around 6 lbs or more.

40mm 82° would be more practical.

But, selling that eyepiece (at $2000?) would be somewhat difficult, so I don't think it's very likely.


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#40 RedzoneMN

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Posted 06 March 2021 - 06:15 AM

Maximum fields in 1.25" currently in the market place:

40mm--40°

32mm--50°

26mm--62°

24mm--68°

16mm--82°

13mm--100°

 

Compare that to 2":

55mm--50°

41mm--68°

31mm--82°

25mm--100°

 

As you can see, if you want a wide field in an eyepiece longer than 24mm, it'll be 2".

Don, could you pencil in the respective make and models on the above list. 



#41 junomike

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Posted 06 March 2021 - 08:51 AM

Yep, I knew that. waytogo.gif   I was simply pointing out that it was missing.  Very helpful plot!  A picture indeed is equal to a thousand words.

???? None of the individual EP's are marked, just the shortest and longest (as I see it).

In regards to the TV PL's, the dot for the longest 1.25" PL  (40mm) is marked as is the dot for the 55 PL at the 46mm Field Stop.

 

Not sure what I'm missing?



#42 junomike

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Posted 06 March 2021 - 08:53 AM

AFAIK, there are 2" eyepieces with longer focal lengths than the TV Plossl, but they will have less than 50 degree AFOV.  Or will they?  I've owned some of these.  Owned.

 

Mike

There are several but all have the same 46mm (some 46.5mm) Field Stops so the FOV is the same



#43 Sarkikos

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Posted 06 March 2021 - 10:51 AM

There are several but all have the same 46mm (some 46.5mm) Field Stops so the FOV is the same

I said AFOV, not TFOV.  I try to be specific.  Just throwing FOV out there can be problematic.  I try to avoid ambiguity.  In a thread like this, you shouldn't assume the context will make things clear, when the context is fluid.

 

As the focal length increases, won't the AFOV (Apparent Field of View) decrease, even though the field stop remains constant?  The TFOV (true field of view) should remain the same.  But at a longer focal length - and lower magnification - the AFOV should decrease if the TFOV stays constant.  

 

Mike


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#44 Tamiji Homma

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Posted 06 March 2021 - 11:25 AM

I don't know of any commercial telescopes that come with 3" focusers to accept a 3" ocular.  But, since 3" oculars tend to be very low magnification (long focal length), the exit pupil will most likely be very large in any scope.

 

Explore Scientific used to offer 3" diagonal connection to their 6"-ish refractor.  I don't know they do today, though.

 

Here is prototype 3" 100 degree eyepiece on ES 6" f/8 refractor from ASAE (Arizona Science & Astronomy Expo) floor in 2013.

 

large.jpg

 

I have several refractors (92mm f/5.5, 100mm f/9, 102mm f/8, 130mm f/7, 130mm f/9.2, 152mm f/8) that can attach 3" diagonal with stock/custom adaptors.

 

Here is the smallest refractor that I have to mate with Explore Scientific 3" diagonal and 3" eyepiece.

It is challenging to balance scope due to short tube.

 

large.jpg

 

Tammy


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#45 Starman1

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Posted 06 March 2021 - 11:39 AM

Don, could you pencil in the respective make and models on the above list. 

Suggest you download this:

https://www.cloudyni...ide/?p=10917573

because there are multiple answers to each listing.

 

Maximum fields in 1.25" currently in the market place:

40mm--40° all 40mm Plössls

32mm--50° most 32mm Plössls

26mm--62° ES 62x26

24mm--68° most 24mm widefields

16mm--82° TV Nagler, KUO clones (OVL, et al.)

13mm--100° most 100° 13mm eyepieces

 

Compare that to 2":

55mm--50° includes 56mm, such as TeleVue, Meade, etc.

41mm--68° TV Panoptic

31mm--82° TV Nagler

25mm--100° ES 100x25


Edited by Starman1, 06 March 2021 - 12:06 PM.

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#46 JamesDuffey

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Posted 06 March 2021 - 11:55 AM

Don, could you pencil in the respective make and models on the above list. 

See Don’s post above, which crossed mine in the internet for the latest version of his buyers guide < https://www.cloudyni...ide/?p=10917573 > It is a very useful resource and you should have the latest version. Sorry for the confusion.

 

I’m not Don, and this isn’t pencil, but the eyepiece buyers guide post 28 here: < https://www.cloudyni...rs-guide/page-2 > sorted by focal length and AFOV, or a few minutes in the eyepiece section of Astronomics, Agena Astro, Eyepiecesetc, or almost any astronomical equipment supplier will get you the information. 

 

To start with, off the top of my head:

1.25" 
40mm--40° - almost any modest or better quality Plössl: GSO, Astro-Tech, Televue
32mm--50° - almost any modest or better quality Plössl: GSO, Astro-Tech, Televue
26mm--62° - Explore Scientific
24mm--68° - Explore Scientific 
16mm--82° - Explore Scientific, Nagler Type 5
13mm--100°

Compare that to 2":
55mm--50° - almost any modest or better quality Plössl: GSO, Astro-Tech, Televue
41mm--68°
31mm--82°
25mm--100°

 

I don’t have a telescope with  2” focuser. It seems like I am missing out, so my next one will.


Edited by JamesDuffey, 06 March 2021 - 12:42 PM.


#47 Starman1

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Posted 06 March 2021 - 12:07 PM

I’m not Don, and this isn’t pencil, but the eyepiece buyers guide post 28 here: < https://www.cloudyni...rs-guide/page-2 > sorted by focal length and AFOV, or a few minutes in the eyepiece section of Astronomics, Agena Astro, Eyepiecesetc, or almost any astronomical equipment supplier will get you the information. 

 

To start with, off the top of my head:

1.25" 
40mm--40° - almost any modest or better quality Plössl: GSO, Astro-Tech, Televue
32mm--50° - almost any modest or better quality Plössl: GSO, Astro-Tech, Televue
26mm--62° - Explore Scientific
24mm--68° - Explore Scientific 
16mm--82° - Explore Scientific, Nagler Type 5
13mm--100°

Compare that to 2":
55mm--50° - almost any modest or better quality Plössl: GSO, Astro-Tech, Televue
41mm--68°
31mm--82°
25mm--100°

 

I don’t have a telescope with  2” focuser. It seems like I am missing out, so my next one will.

James,

That's a link to the older 2020 guide.

I posted a link to the 2021 guide in the post before yours.



#48 JamesDuffey

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Posted 06 March 2021 - 12:36 PM

James,

That's a link to the older 2020 guide.

I posted a link to the 2021 guide in the post before yours.

Thanks Don. Sorry, our posts crossed in the internet. I wasn’t aware the new one was out. It is a useful resource, I will update my post. 




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