Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Shortest Focal Length Useable Eyepiece

Eyepieces Observing Optics
  • Please log in to reply
29 replies to this topic

#1 woodsman

woodsman

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,535
  • Joined: 12 Mar 2008
  • Loc: Colorado Springs, CO

Posted 10 May 2021 - 11:02 AM

Just curious as to what other members have in the way of short focal length eyepieces.  I was always of the understanding that the Dawe's limit of Useful magnification ran to about 60X per inch of aperture.  I've heard some here say that they have eyepieces that are 3mm. Does any company make anything shorter in focal length?  

 

The shortest focal length eyepiece I ever used is 4mm. Every one of those small focal length eps has a very small exit (field stop) which is difficult to use as one needs to get the eye so close to even be able to see into it.

 

I noticed that Televue used to offer a 4mm plossl.  My guess would be that the field stop on the exit would be very small indeed.

 

On very short focal length telescopes I can see the possibility of using down to perhaps 2mm if the diameter of the diameter of the field stop was reasonable, however I'm not interested in purchasing an expensive eyepiece if the exit diameter is so tiny that one has to cram his eyeball right up against the eyepiece.

 

I did see that TV offers their Ethos down to 3.7mm, however that field stop is 7mm (not too big). 

 

Does anyone have experience with an eyepiece similar to that?

 

Is it comfortable to use?

 

Thanks! 

 

P.S.  I may be confusing field stop (FS) with exit pupil (EP) however on the TV website they don't list EP just FS


Edited by woodsman, 10 May 2021 - 11:05 AM.


#2 Dave Mitsky

Dave Mitsky

    ISS

  • *****
  • Moderators
  • Posts: 96,851
  • Joined: 08 Apr 2002
  • Loc: PA, USA, Planet Earth

Posted 10 May 2021 - 11:07 AM

My shortest focal length "eyepiece" is the 3mm setting on my 3-6mm Tele Vue Nagler zoom.  I only use the zoom with two of my short-focal-length refractors.



#3 cst4

cst4

    Viking 1

  • *****
  • Posts: 732
  • Joined: 06 Jun 2018

Posted 10 May 2021 - 11:10 AM

I have an SV 102 Access and use pretty short focal length EP's in it.  The shortest ones were a 3.2mm Paradigm and 3.2mm TMB Planetary at 223x with an exit pupil of around 0.45mm.  This corresponds to about 56x/inch of aperture.  I now regularly use 3.5mm Delos and 3.5mm APM XWA (will probably eventually sell one of them) at 202x and an exit pupil of 0.5mm, which is right at 50x/inch of aperture.  These are all very comfortable as they all have long eye relief of over 15mm.  The trick is just to avoid plossls, orthos, and other scaled designs where the eye relief is microscopic.  I recently picked up a 3mm Delite for Jupiter and Saturn once they are visible to me again.  


Edited by cst4, 10 May 2021 - 11:13 AM.


#4 dan_h

dan_h

    Gemini

  • *****
  • Posts: 3,109
  • Joined: 10 Dec 2007

Posted 10 May 2021 - 11:14 AM

There is a reason that exit pupil is not listed as an eyepiece spec.  

 

Exit pupil is an image of the objective lens. Since the eyepiece manufacturer doesn't know what scope will be used, exit pupil cannot be specified.   

 

Exit pupil = aperture / magnification,    So a 100mm scope at 100 X has a 1mm exit pupil.  

 

or 

 

Exit pupil = Eyepiece focal length / telescope f ratio,   A 12mm eyepiece in an f6 scope provides a 2mm exit pupil. 

 

However, it seems you are more concerned with eye relief and that is a function of the eyepiece design. There are short focal length eyepieces that have comfortable eye relief. 

 

dan


  • rodney j johnson jr likes this

#5 BlueMoon

BlueMoon

    Surveyor 1

  • *****
  • Posts: 1,504
  • Joined: 14 Jun 2007
  • Loc: South Central Idaho, USA

Posted 10 May 2021 - 11:18 AM

3mm DeLite with 20mm of eye relief so there is some "play" in getting one's eye placed just right with a tight exit pupil. 


Edited by BlueMoon, 10 May 2021 - 11:19 AM.


#6 SeattleScott

SeattleScott

    Voyager 1

  • *****
  • Posts: 11,611
  • Joined: 14 Oct 2011

Posted 10 May 2021 - 11:24 AM

3.4 HR for me. Vixen made a 1.6 HR. I think the Tak TOE line goes down to 2mm or so. There are options out there for really short focal lengths without using a barlow.

Scott
  • Sarkikos and Bowlerhat like this

#7 Paul Skee

Paul Skee

    Ranger 4

  • -----
  • Posts: 377
  • Joined: 07 Jul 2016
  • Loc: Orange County, CA (near Disneyland)

Posted 10 May 2021 - 12:15 PM

The Celestron X-Cel LX series has a 2.3mm with 16mm of eye relief.

I have some of that line, not the 2.3, not interested in that level of magnification or exit pupil my scopes would yield.



#8 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 92,541
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 10 May 2021 - 12:40 PM

I have the 3.5 mm Nagler.  I use it with a 2X Barlow when I need shorter focal lengths. 

 

Every one of those small focal length eps has a very small exit (field stop) which is difficult to use as one needs to get the eye so close to even be able to see into it.

 

 

It seems like you are thinking about eye relief, the distance you view behind the eyepiece. (exit and field stop are something else. )

 

The 3.7mm Ethos has 15mm of eye relief, the 3.5 mm Nagler has 12mm.  

 

Jon


  • sevenofnine likes this

#9 csrlice12

csrlice12

    ISS

  • *****
  • Posts: 28,153
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 10 May 2021 - 12:56 PM

The 3.5 XW has 20mm eye relief and 70* afov.  There are used Vixen LVs down to 2.5mm with 20mm eye relief and 45* afov..



#10 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 32,366
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 10 May 2021 - 12:56 PM

1.6mm HR.  I've only ever used it for double stars.  If I used it for bright extended objects like planets or the Moon, I'd be in Floater Hell.  mrevil.gif

I suppose it could also be useful for planetary nebulae.

 

The eye relief on the 1.6 HR is 10mm, so not really so bad for such a short focal length.  For me, it's moderately comfortable.  I just have to take off my eyeglasses.  grin.gif

 

My next shortest focal lengths are the 2 HR, 2-4 Nagler Zoom, 2.5 Nagler, 2.58 XO, 3.4 HR and the 3.7 Ethos-SX.  

 

I'm waiting for Tele Vue to introduce a 2.7 Ethos-SX.  None of my mounts track.

 

Mike


Edited by Sarkikos, 10 May 2021 - 01:05 PM.


#11 woodsman

woodsman

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,535
  • Joined: 12 Mar 2008
  • Loc: Colorado Springs, CO

Posted 10 May 2021 - 01:06 PM

Thanks for all the responses so far.  I called the guys at Televue and was told that the exit pupil defined the brightness coming out of the ep.  I understood from his description that the eye relief was the comfortable distance between the eyeball and the eyepiece lens (upper glass). 

 

My reason for asking about the terminology and the various questions is that I don't want to have to smash my eyeball against the eyepiece to see anything if I'm going to purchase an eyepiece for fairly high magnifications, the way we all did in the old days when a 4mm Kellner or Ortho had a very small diameter glass on top. 

 

That is what I was thinking was the field stop.  I'm pretty sure that there is always a field stop on the inside of the ep that inserts into the scope in order to define the FOV of the ep, however I didn't know what to call the bezel on top that shrouds the glass on the top of the ep, where the eye is placed. 

 

Still doing research on the Actual FOV vs. the AFOV. 

 

Just need to get it all straight. 

 

I was told by the guy at Televue that with my long focus (3900mm) I would only need about 3 eyepieces to satisfy my viewing needs.

 

Please keep the information coming, I'm learning something here!.   laugh.gif



#12 havasman

havasman

    Voyager 1

  • *****
  • Posts: 12,712
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 10 May 2021 - 01:19 PM

Below 5mm I have 4.7E, 3.7E, 3.5T6 and 2.5T6. In my main scope those yield 446x with 0.9mm exit pupil, 567x with 0.7mm exit pupil, 600x with 0.7mm exit pupil and 839x with 0.5mm exit pupil. There's also a 9mm TAO that I Barlow to effectively get 4.5mm performance from with good eye relief. I am very glad when conditions allow use of the shortest focal lengths and consider the rare night when the 4.7E is not useful to have poor conditions for the site. I have a new scope with shorter focal length and I am testing to see whether I'll be advised to add a 3mm Delite between the two T6's to fill a gap between 490 and 686x.

 

I do not care for long eye relief eyepieces and find all of these quite comfortable with the entire FOV useful. I admit to having some trouble keeping my eye properly located with the very small exit pupil of the 2.5mm if I'm observing while standing instead of seated.

 

Yes, I think you have conflated the terminology and concepts of field stop and exit pupil. See figure 209 here  -  https://www.telescop...t/eyepiece1.htm


Edited by havasman, 10 May 2021 - 01:21 PM.


#13 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 32,366
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 10 May 2021 - 01:27 PM

There were two eyepieces that I recall actually having uncomfortably short eye relief for me:

 

4mm Celetron Plossl.  A throw-in with some inexpensive telescope I bought.  Terribly uncomfortable eye relief.

 

BK7 Ball Eyepiece.  I made this one myself.  Zero eye relief.

 

I also have a 4 Aspheric, which I'd forgotten about.  I bought two for binoviewing, but never got around to it.  The eye relief is comfortable compared to the 4 Plossl.

 

Mike



#14 AstroVPK

AstroVPK

    Messenger

  • -----
  • Posts: 469
  • Joined: 12 May 2019
  • Loc: Sunnyvale, CA

Posted 10 May 2021 - 02:36 PM

I have the Vixens HRs and regularly use the 3.4 mm for observing Jupiter and Mars (when in the sky and when seeing permits). I use the 2.4 mm, 2 mm, and 1.6 mm HRs for splitting very close doubles. All 4 are fairly comfortable to use IMO, though the TFOV on the 2 mm and 1.6 mm HRs gets small enough that keeping the star in the FOV becomes tiring... Still, I do find good use for all 4.
I also have the 4.7 mm and 3.7mm E-SXs and those are very comfortable and easy to use - both are some of my all time favorite eyepieces!
  • Sarkikos likes this

#15 John Huntley

John Huntley

    Gemini

  • ***--
  • Posts: 3,063
  • Joined: 16 Jul 2006
  • Loc: SW England

Posted 10 May 2021 - 03:11 PM

I have the Nagler 4mm-2mm zoom. I don't use the 2mm setting much with my scopes but I have occasionally. The eye relief with this eyepiece is 10mm across the zoom range and the apparent field of view a constant 50 degrees. Considering the focal lengths covered it is a reasonably comfortable eyepiece to use. I don't wear glasses when observing.


  • Sarkikos likes this

#16 SeattleScott

SeattleScott

    Voyager 1

  • *****
  • Posts: 11,611
  • Joined: 14 Oct 2011

Posted 10 May 2021 - 03:58 PM

Thanks for all the responses so far. I called the guys at Televue and was told that the exit pupil defined the brightness coming out of the ep. I understood from his description that the eye relief was the comfortable distance between the eyeball and the eyepiece lens (upper glass).

My reason for asking about the terminology and the various questions is that I don't want to have to smash my eyeball against the eyepiece to see anything if I'm going to purchase an eyepiece for fairly high magnifications, the way we all did in the old days when a 4mm Kellner or Ortho had a very small diameter glass on top.

That is what I was thinking was the field stop. I'm pretty sure that there is always a field stop on the inside of the ep that inserts into the scope in order to define the FOV of the ep, however I didn't know what to call the bezel on top that shrouds the glass on the top of the ep, where the eye is placed.

Still doing research on the Actual FOV vs. the AFOV.

Just need to get it all straight.

I was told by the guy at Televue that with my long focus (3900mm) I would only need about 3 eyepieces to satisfy my viewing needs.

Please keep the information coming, I'm learning something here!. laugh.gif

3900mm FL and you are asking about 3mm eyepieces? 1000x with your 4mm isn’t enough magnification??
  • woodsman likes this

#17 woodsman

woodsman

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,535
  • Joined: 12 Mar 2008
  • Loc: Colorado Springs, CO

Posted 10 May 2021 - 04:13 PM

3900mm FL and you are asking about 3mm eyepieces? 1000x with your 4mm isn’t enough magnification??

lol.gif

 

I started my posting with this:   Just curious as to what other members have in the way of short focal length eyepieces.

 

And I have more than the C14 telescope

 

A good portion of my posting was to find out some information about eyepieces and the terminology associated with them.  I was a bit confused about exit pupil vs. eye relief vs. field stop.

 

For one thing, I'm thinking of possibly  getting a few expensive eyepieces and I don't want to run into a problem of having to cram my eye against the eyepiece in order to see anything and especially if I can see the FOV as designed into the optical device.

 

I've had my C14 to 5mm, 780x and I'm interested in getting that view out of some other designs, not just the Barlowed 10mm that I have. 

 

Does that make more sense?  laugh.gif
 


  • eblanken likes this

#18 turtle86

turtle86

    Mr. Coffee

  • *****
  • Posts: 5,942
  • Joined: 09 Oct 2006

Posted 10 May 2021 - 06:11 PM

Just curious as to what other members have in the way of short focal length eyepieces.  I was always of the understanding that the Dawe's limit of Useful magnification ran to about 60X per inch of aperture.  I've heard some here say that they have eyepieces that are 3mm. Does any company make anything shorter in focal length?  

 

The shortest focal length eyepiece I ever used is 4mm. Every one of those small focal length eps has a very small exit (field stop) which is difficult to use as one needs to get the eye so close to even be able to see into it.

 

I noticed that Televue used to offer a 4mm plossl.  My guess would be that the field stop on the exit would be very small indeed.

 

On very short focal length telescopes I can see the possibility of using down to perhaps 2mm if the diameter of the diameter of the field stop was reasonable, however I'm not interested in purchasing an expensive eyepiece if the exit diameter is so tiny that one has to cram his eyeball right up against the eyepiece.

 

I did see that TV offers their Ethos down to 3.7mm, however that field stop is 7mm (not too big). 

 

Does anyone have experience with an eyepiece similar to that?

 

Is it comfortable to use?

 

Thanks! 

 

P.S.  I may be confusing field stop (FS) with exit pupil (EP) however on the TV website they don't list EP just FS

 

I have both the 3.7mm Ethos and the Nagler 3-6mm zoom.  Both are plenty comfortable for me.  I use the 3.7mm in my 18" on good nights for planets and high power views of some planetary nebulae. More than once I've seen the central star in M57 with it.

 

I used to have a Pentax 2.5mm XO.  I found the eye relief on it to be unpleasantly tight so that one is gone now.


Edited by turtle86, 10 May 2021 - 06:12 PM.


#19 woodsman

woodsman

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,535
  • Joined: 12 Mar 2008
  • Loc: Colorado Springs, CO

Posted 10 May 2021 - 06:59 PM

I have both the 3.7mm Ethos and the Nagler 3-6mm zoom.  Both are plenty comfortable for me.  I use the 3.7mm in my 18" on good nights for planets and high power views of some planetary nebulae. More than once I've seen the central star in M57 with it.

 

I used to have a Pentax 2.5mm XO.  I found the eye relief on it to be unpleasantly tight so that one is gone now.

Thanks, I was looking at that. 


Edited by woodsman, 10 May 2021 - 07:00 PM.


#20 John Huntley

John Huntley

    Gemini

  • ***--
  • Posts: 3,063
  • Joined: 16 Jul 2006
  • Loc: SW England

Posted 10 May 2021 - 07:32 PM

As well as the Nagler 2-4mm zoom I mention above I also have a Pentax XW 3.5mm. This is a comfortable short focal length eyepiece to use. Very large eye lens and 20mm of eye relief. Great optical performance as well.


  • woodsman likes this

#21 woodsman

woodsman

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,535
  • Joined: 12 Mar 2008
  • Loc: Colorado Springs, CO

Posted 10 May 2021 - 08:04 PM

I have the 3.5 mm Nagler.  I use it with a 2X Barlow when I need shorter focal lengths. 

 

 

It seems like you are thinking about eye relief, the distance you view behind the eyepiece. (exit and field stop are something else. )

 

The 3.7mm Ethos has 15mm of eye relief, the 3.5 mm Nagler has 12mm.  

 

Jon

Yes, you are correct.  I'm starting to get a handle on the terminology and so maybe you or another can correct me if I'm wrong. My inkling now is that if I look down on the eyepiece and I see the size of the glass, the defined diameter is the exit pupil.  I have some vintage eps, Galoc Saturn ones, and the glass on the 0.24 inch also labeled 6.0mm is about 3/16 in diameter. 

 

It would make sense to call that diameter the exit pupil and so I think I was on the right track with the field stop, and now I think I get it that the eye relief is the distance one's eye is away from the eyepiece glass when focus is achieved. 

 

However I'm still a bit lost on AFOV vs. FOV.  In speaking with the guy at Televue this morning, he explained the AFOV or as the angle that the incident light is exiting the eyepiece.  Some eyepieces are as much as 100 degrees.  Typically my old Celestron Volcano tops had an  AFOV of 50 degrees. That is what was listed in their catalog.

 

His example was to indicate that the full moon takes up fully 1/2 degree of the entire night sky.  If I have an eyepiece that takes in the entire full moon and it is encompassing the enitire Field of view in the eyepiece I'm observing through, then the True field of view is 1/2 degree, however it depends upon the eyepiece to define the AFOV.  I can figure the magnification by dividing the AFOV by the TFOV. 

 

Is that correct? 

 

Thanks. 



#22 barbie

barbie

    Surveyor 1

  • *****
  • Posts: 1,736
  • Joined: 28 Jan 2013
  • Loc: Northeast Ohio

Posted 10 May 2021 - 08:20 PM

I have a Takahashi TOE 3.3mm for both of my refractors. This is as short a focal length as I can use with my 60mm F8.3 and 76mm F7.5 Tak fluorite apos and my typically average skies. If I get a rare night of excellent seeing, I can 2x Barlow my AT 5mm Paradigm but that doesn't happen very often.


  • woodsman likes this

#23 SeattleScott

SeattleScott

    Voyager 1

  • *****
  • Posts: 11,611
  • Joined: 14 Oct 2011

Posted 10 May 2021 - 09:08 PM

Yes that helps 🙂

#24 rowdy388

rowdy388

    Skylab

  • *****
  • Posts: 4,306
  • Joined: 09 Apr 2013
  • Loc: Saratoga County, NY

Posted 10 May 2021 - 10:11 PM

My current shortest is the 4.77mm Lunt110. Nice to have an extra wide field for very high power. I used to have shorter but they almost never got used.

I can still use barlows if I want to go deeper.



#25 sevenofnine

sevenofnine

    Viking 1

  • -----
  • Posts: 922
  • Joined: 16 Apr 2016
  • Loc: Santa Rosa, California

Posted 10 May 2021 - 10:56 PM

The Baader zoom with it's dedicated barlow is a very comfortable high power option. The view is very wide at 4mm and fun to examine lunar craterlets. It's also a kick to watch suspected double stars split as you zoom from 12-4mm. waytogo.gif


Edited by sevenofnine, 10 May 2021 - 10:57 PM.

  • AJK 547 likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Eyepieces, Observing, Optics



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics