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Question about EPs and those tiny holes we're supposed to see through

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#1 Craven

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Posted 19 January 2021 - 08:57 AM

So, I'm still in a bit of a learning phase.   I'm looking at these 4MM plossls from Astrotech (or most 4MM for that matter) that have these insanely tiny holes that we're supposed to see through.  Is this the "exit pupil?"  And do they really expect us to see through that thing?

 

I see a 4MM from a much more expensive EP maker and the glass of the lens on the eye side is much larger, looks easier to use. 

 

Is this the exit pupiil?  And if so, does this mean that when the hole is really tiny (as in the 4MM plossl) that one must get up really really close to the EP to view, hence "short eye relief?"  And when the hole is larger, one need not be so close to the EP, hence greater eye relief?

 

Thanks all. Just trying to learn learn the lingo.


Edited by Craven, 19 January 2021 - 08:58 AM.


#2 Astrojensen

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Posted 19 January 2021 - 09:03 AM

 

Is this the exit pupiil?

No. 

 

 

does this mean that when the hole is really tiny (as in the 4MM plossl) that one must get up really really close to the EP to view, hence "short eye relief?"

Yes.

 

 

And when the hole is larger, one need not be so close to the EP, hence greater eye relief?

Generally, yes. 

 

 

Clear skies!

Thomas, Denmark


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#3 JohnnyBGood

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Posted 19 January 2021 - 09:23 AM

A 4mm Plossl requires you to jam your eyeball practically right up to the eyepiece to see the full field of view. You can hold your eye further away and still see through it, which is okay if you are looking at a small object (like a planet or small part of the moon) but you won't be able to see the whole "circle" the eyepiece image produces.

 

10mm is the shortest focal length Plossl I'm happy with. I can tolerate a 6mm but it's not comfortable. Typically you need a fancier (and more expensive) eyepiece design to provide more comfortable eye relief for shorter focal lengths.



#4 Barlowbill

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Posted 19 January 2021 - 09:32 AM

Regardless of whether you use a 4mm Plossl or a more expensive wide angle eyepiece,  that is considered a high power (magnification) eyepiece.  Your atmospheric (seeing) conditions might not allow you to use such high powers very often.  A slightly lower magnification eyepiece could probably be more useful more often.  Yes, I hate those tiny holes.  With practice you do get used to them but objects don't hang around very long in the view.  It is amazing how fast objects move across such a tiny view.  



#5 SloMoe

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Posted 19 January 2021 - 09:38 AM

I'm still wearing my 4mm plossl as a monocle,,,,,,,,


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#6 MickTaurus

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Posted 19 January 2021 - 09:56 AM

4MM plossls from Astrotech.

Hi Craven, it is a good idea to let others know what scope you intend to use those 4mm eyepieces in, as some scopes don't tend to benefit from using such high powered eyepieces. 



#7 Littlegreenman

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Posted 19 January 2021 - 10:43 AM

Hi Craven, it is a good idea to let others know what scope you intend to use those 4mm eyepieces in, as some scopes don't tend to benefit from using such high powered eyepieces. 

Agreed.

 

That said, there are two eyepiece lines that in the 4mm range that do not have this problem. Both are no longer made, but show up for sale on the used market frequently:

 

Orion 7-element Ultrascopic 5mm and 3.8mm

Here is an example of one that recently sold here on CN

https://www.cloudyni...rascopic-japan/

Note: several other brands sell very similar or exactly the same eyepieces as the Orion Ultrascopic line, except for the 5mm and 3.8mm. Those two have a 2 extra optical elements (glass lenses) that improve eye relief, and as far as I know no one else sold those eyepieces.

 

TMB "Planetary" eyepieces. Here is one sold under the Burgess name (I apologize; I thought this would be easier):

https://www.cloudyni...urgess-tmb-4mm/

 

Somebody speak up it there are currently new planetary eyepieces.

 

Another solution is to use a Barlow. Search for Barlow in the eyepiece forum here on CN.

I almost forgot, if disposable income is not an issue for you, Tele Vue makes a planetary zoom eyepiece. (It costs about 10 x the price of a basic Plossl). moneyeyes.gif



#8 SloMoe

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Posted 19 January 2021 - 11:02 AM

thinking about putting a dew strip on my monocle for when I'm in the shower, it fogs up pretty quick,,,,,,,,,, 


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#9 SeattleScott

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Posted 19 January 2021 - 12:42 PM

There are other much more expensive options around 4mm with good eye relief.

Scott
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#10 f74265a

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Posted 19 January 2021 - 01:03 PM

There are other much more expensive options around 4mm with good eye relief.

Scott


An outstanding but somewhat expensive option is the tele vue 4mm delite
Another more cost effective way to go is Barlow a longer focal length plossl. The Barlow will add eye relief.
There are people who still like the tiny holes and zero eye relief of small focal length orthos and plossls. I personally hate them and recently gave away my mid 1980s 4mm ortho to I somebody who appreciates them.

#11 Craven

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Posted 19 January 2021 - 01:51 PM

Thanks,.  I was really just using the 4MM as an example.  

 

 

Agreed.

 

That said, there are two eyepiece lines that in the 4mm range that do not have this problem. Both are no longer made, but show up for sale on the used market frequently:

 

Orion 7-element Ultrascopic 5mm and 3.8mm

Here is an example of one that recently sold here on CN

https://www.cloudyni...rascopic-japan/

Note: several other brands sell very similar or exactly the same eyepieces as the Orion Ultrascopic line, except for the 5mm and 3.8mm. Those two have a 2 extra optical elements (glass lenses) that improve eye relief, and as far as I know no one else sold those eyepieces.

 

TMB "Planetary" eyepieces. Here is one sold under the Burgess name (I apologize; I thought this would be easier):

https://www.cloudyni...urgess-tmb-4mm/

 

Somebody speak up it there are currently new planetary eyepieces.

 

Another solution is to use a Barlow. Search for Barlow in the eyepiece forum here on CN.

I almost forgot, if disposable income is not an issue for you, Tele Vue makes a planetary zoom eyepiece. (It costs about 10 x the price of a basic Plossl). moneyeyes.gif

 



#12 Dave Mitsky

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Posted 19 January 2021 - 01:55 PM

The 5mm Astro-Tech Paradigm Dual ED has 13mm of eye relief and costs only $60.  It would produce a 1mm exit pupil in an f/5 telescope.

https://www.astronom...gm-dual-ed.html


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#13 Sheol

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Posted 19 January 2021 - 02:01 PM

                                       This is where I can say, honestly, Barlow EPs are a blessing. Sure not everyone likes them, & they vary in quality according to manufacture. but not only do they allow you to double an EP collection without effort, but they help with that eye relief thing. With my eyes & at my age ( 58 Ack!) I cannot use anything much below a 10mm ( I do have 1 though). But my 2X Shorty Barlow transforms my 10mm into a 5mm Plossl. I do not think I could even nail a good focus with a REAL 5mm Plossl EP. Would not be equipment, it would be my dated Mark I Eyeballs.

 

 

        Clear Skies,

                Matt.


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#14 chrysalis

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Posted 19 January 2021 - 02:03 PM

Maybe I missed it, but EXIT PUPIL is the diameter of the light rays exiting the eyepiece. In the same telescope, every 4 mm eyepiece will have the same exit pupil size. They can have different AFOV (apparent field of view) depending on their design geometry - Plossls tend to be around 52° AFOV. Wider AFOV eyepieces generally require more elements, thus are more expensive.

 

I like the calculation OBJECTIVE DIAMETER/MAGNIFICATION to determine exit pupil because it makes sense physically to me.

 

So if you have a 200 mm objective lens and are at 100 power, the exit pupil will be 2 mm.



#15 Dave Mitsky

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Posted 19 January 2021 - 02:04 PM

The eye relief of Plössls and other simple designs decreases as the focal length decreases.  The eye relief of a Plössl is about 70% of its focal length.


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#16 SteveG

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Posted 19 January 2021 - 02:40 PM

No tiny eyepiece holes! 

 

You need a 3x barlow. I recommend the Orion TriMag. I use this with longer focus Brandons so that I don't have to look through those little eye-lenses.


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#17 sevenofnine

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Posted 19 January 2021 - 03:36 PM

If you want to have a wider view and not be looking through a peep hole get a Celestron 8-24mm zoom eyepiece or similar. The 24mm setting is a little narrow but as you zoom to 8mm the view widens considerably. Definitely a step up from the eyepieces that come with your scope and the zoom factor is pretty cool too. Good luck!



#18 chrysalis

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Posted 19 January 2021 - 05:53 PM

The eye relief of Plössls and other simple designs decreases as the focal length decreases.  The eye relief of a Plössl is about 70% of its focal length.

My first telescope (Edmund 3" f/10, 1965) came with just a 1/2" Ramsden eyepiece (well, and a variable Barlow)!

 

I was SOOOO excited to see the eye lens size of the 1" Kellner that came with my next telescope (Edmund 4.25" f/10, 1968)!


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