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Eyepieces for 12” f/5 Dob: Any advantage of 40mm over 30 mm?

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#26 Achernar

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Posted 11 November 2018 - 04:15 PM

The 40mm will yield an exit pupil that is too large, you will see a dark blob in the field of view caused by the presence of the secondary mirror and far too much sky glow will also be present. The 30mm will not create these issues and will also work very well at darker sites. BOTH however are going to be large and heavy eyepieces and they might create balance issues. The 30mm is really the best choice for your telescope. For a slower reflecting telescope or a SCT, the 40mm would serve well but it is a very poor choice for an F/5 telescope.

 

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#27 areyoukiddingme

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Posted 11 November 2018 - 05:06 PM

I use a 40 XW in my 12.5" F5, but typically with paracorr (for F=5.75). 

 

The 40 is a luxury eyepiece for this scope. Useful and nice to have to max out the field, but not a necessity.

 

If I were choosing between the 40XW an a 31 Nagler, I'd take the Nagler.


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#28 Redbetter

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Posted 11 November 2018 - 11:54 PM

The 40mm will yield an exit pupil that is too large, you will see a dark blob in the field of view caused by the presence of the secondary mirror

This is actually unlikely from what I can tell/have experienced, and seems to depend more on the individual than a specific exit pupil.    Viewing the Moon with such a large exit pupil can produce the effect, but it is less likely in a normal star field. 

 

Most reports I see of such exit pupils are negative for seeing the central obstruction.  It might be useful to hear detail from those who do see the central obstruction at various large exit pupils when viewing a normal star field in dark/semi-dark conditions.


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#29 faackanders2

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Posted 12 November 2018 - 12:00 AM

40mm 70 AFOV University Optic's MK70 Koening and 41mm 68 AFOV TV Panoptic provide widest TFOV in a 2" eyepiece. See M31/M32/M110 all in one eyepiece, more of M44, M45, M41, M8, Viel, multiple objects, etc.

#30 faackanders2

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Posted 12 November 2018 - 12:02 AM

The 40mm will yield an exit pupil that is too large, you will see a dark blob in the field of view caused by the presence of the secondary mirror and far too much sky glow will also be present. The 30mm will not create these issues and will also work very well at darker sites. BOTH however are going to be large and heavy eyepieces and they might create balance issues. The 30mm is really the best choice for your telescope. For a slower reflecting telescope or a SCT, the 40mm would serve well but it is a very poor choice for an F/5 telescope.
 
Taras

Try before you buy. At night!

#31 Ernest_SPB

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Posted 12 November 2018 - 12:20 AM

See M31/M32/M110 all in one eyepiece, more of M44, M45, M41, M8, Viel, multiple objects, etc.

It might be interesting that TFOV 1 deg. is enough to to see M31, M32 and M110 in one field.

The same for M44 (formal size 70', but actual visible extent is less), main stars of M45 also could be covered by 1 deg. circle, M41 is relatively small (37') open star cluster, if you had in mind M42 - it extents within 45', M8 is 50'... So all these objects could be framed with 1.25" eyepiece like 24 mm Panoptic in focuser of discussed telescope. 31 mm Nagler shows them with large margins. 

 

Veil is rather large - 2°40' what requires for the scope FS diameter 70 mm. So it could not be seen whole in any 2" eyepiece limited by FS 46 mm in diameter.


Edited by Ernest_SPB, 12 November 2018 - 12:35 AM.

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#32 kfiscus

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Posted 12 November 2018 - 11:01 AM

I enjoy using my 41 Pan with my 12" f/5.
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#33 Achernar

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Posted 12 November 2018 - 05:37 PM

Try before you buy. At night!

Exactly. I have seen how the fuzzy dark blob appears glaringly even at night when the eyepiece creates an exit pupil too large and the telescope is obstructed. It is hard to miss especially when the sky 's light polluted.

 

Taras


Edited by Achernar, 12 November 2018 - 05:39 PM.


#34 Jon Isaacs

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Posted 12 November 2018 - 06:33 PM

Exactly. I have seen how the fuzzy dark blob appears glaringly even at night when the eyepiece creates an exit pupil too large and the telescope is obstructed. It is hard to miss especially when the sky 's light polluted.

 

Taras

 

 

That is a function of the size of your dark adapted or not so dark adapted pupil as well as the exit pupil and the size of the CO .

 

I see the dark center with large exit pupils in daylight or when viewing the nearly full moon . Other wise I don't  see it .

 

Jon



#35 N3p

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Posted 13 November 2018 - 07:23 PM

I have a 7mm pupil maximum and I don't see the central obstruction with my 34mm on my 200x 1000 F5, the exit pupil should be 6.8mm. But I would not risk going lower then that obviously.



#36 Redbetter

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Posted 13 November 2018 - 08:08 PM

Exactly. I have seen how the fuzzy dark blob appears glaringly even at night when the eyepiece creates an exit pupil too large and the telescope is obstructed. It is hard to miss especially when the sky 's light polluted.

 

Taras

What combination have you seen this with and what do you estimate your own pupil is?  What we need is some information from those who see it:  their estimated pupil, eyepiece, scope aperture, central obstruction percent, and what sort of sky background they are looking at when it occurs.  And if they have tried the next increment smaller in those conditions and the effect is gone, it would be good to know what that is as well.

 

The problem does not seem to be as common as popularly believed.  Getting enough experiences documented can help provide better information to others.  


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#37 Achernar

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Posted 13 November 2018 - 08:51 PM

What combination have you seen this with and what do you estimate your own pupil is?  What we need is some information from those who see it:  their estimated pupil, eyepiece, scope aperture, central obstruction percent, and what sort of sky background they are looking at when it occurs.  And if they have tried the next increment smaller in those conditions and the effect is gone, it would be good to know what that is as well.

 

The problem does not seem to be as common as popularly believed.  Getting enough experiences documented can help provide better information to others.  

 

I tried a 40mm eyepiece on my 10-inch F/4.5 on one occasion on both the daytime sky and at night, and the dark fuzzy blob was glaringly obvious. I can still see it if I really look for it with a 32mm on the same telescope without the Paracorr, but it disappeared with the Paracorr in place. That was years ago, and now that I am in my 50's it has become somwhat more apparent. The sky glow however was much worse with those eyepieces than shorter focal length eyepieces at the same site during the same night.

 

Taras


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#38 CrazyPanda

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Posted 13 November 2018 - 09:16 PM

This is actually unlikely from what I can tell/have experienced, and seems to depend more on the individual than a specific exit pupil.    Viewing the Moon with such a large exit pupil can produce the effect, but it is less likely in a normal star field. 

 

Most reports I see of such exit pupils are negative for seeing the central obstruction.  It might be useful to hear detail from those who do see the central obstruction at various large exit pupils when viewing a normal star field in dark/semi-dark conditions.

I definitely notice a darker central region even in my 35 Pan in my 12" F/5 when viewing Andromeda from a Bortle class 4 site. It becomes distracting in my 40mm Plossl. 

 

I also see no difference in brightness between my 35 Pan and 40 Plossl, so my guess is I'm hitting the exit pupil limit for my eyes at *least* at the 35mm focal length. Likely at the 32mm focal length as well. 

 

To OP's original question, even if your eyes open up to 8mm, I don't see the point to anything above a 4mm exit pupil in that scope unless you've got a very big object to look at and you're in a very dark site. 


Edited by CrazyPanda, 13 November 2018 - 09:17 PM.


#39 Redbetter

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Posted 14 November 2018 - 01:29 AM

I tried a 40mm eyepiece on my 10-inch F/4.5 on one occasion on both the daytime sky and at night, and the dark fuzzy blob was glaringly obvious. I can still see it if I really look for it with a 32mm on the same telescope without the Paracorr, but it disappeared with the Paracorr in place. That was years ago, and now that I am in my 50's it has become somwhat more apparent. The sky glow however was much worse with those eyepieces than shorter focal length eyepieces at the same site during the same night.

 

Taras

That is useful information.  It may not match what I see because eyes differ, but it is helpful to others to get some idea of the range where things could become a problem.   Summarizing your results:

  • 8.9mm exit pupil was a major problem
  • 7.1mm exit pupil allowed you to detect the shadow if you looked for it.
  • 6.2mm exit pupil posed no problem.

I assume the central obstruction diameter was somewhere around 25%?



#40 Jon Isaacs

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Posted 14 November 2018 - 07:53 AM

To OP's original question, even if your eyes open up to 8mm, I don't see the point to anything above a 4mm exit pupil in that scope unless you've got a very big object to look at and you're in a very dark site.

 

It is not so much the size of the object as the fact that an 8mm exit pupil is 4X brighter than an 4mm exit pupil.  For me, I find that very large exit pupils are most useful with H-Beta and O-III filters.  With these filters, the sky is about 3 magnitudes darker so if one is at a sight that is 21+ mpsas already, with the filter, it's 24+ mpsas.  

 

Often such objects are often very large not always.  

 

As I said previously, I do have a 41mm Panoptic and occasionally use it at F/5 to get the large exit pupil but more often I will remove the Paracorr and drop down to F/4.4 and use either the 31mm Nagler (7mm exit pupil), the 32mm TV WF (7.3mm exit pupil) or the 35mm Panoptic (8mm exit pupil.)  With an aggressive filter, coma is not an issue.  

 

The TV Widefield is my preferred eyepiece for the Horsehead, the narrower field of view helps keep Alnitak out of the field of view.  

 

Jon


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#41 Sarkikos

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Posted 14 November 2018 - 08:49 AM

I will be 68 before the end of the year and my pupils dilate to 6.8mm in a DARK (SQM-L 22.8) room. No chemicals. MEASURED in dim red light after allowing time for adaption. Charts be hanged. Some folks older than me dilate even more.

 

And I say get the ES82 30mm. It's more useful in more circumstances. I have an ES68 40mm and a 31T5 Nagler. The 31T5 is more often useful, by far. The 40's a nice ep but the ES82 30 is a better one with better edges than the ES68 40 and a flatter field than my big Nagler. I have owned it too. Max TFOV is wonderful but it's not as simple as the arithmetic. You're close enough w/the ES82 30 that you'll never notice and the field will be really nicely rendered.

 

I have other ep's that render exit pupils up to 9.3mm in some of my scopes. Now & then I give 'em some focuser time to see the effect of very large exit pupils and also to enjoy the crisp presentation despite the lower efficiency.

Yep, I think those charts are wrong.  Or do amateur astronomers tend to be the outliers?  In any case, I'm 62 and my exit pupils are about at 7mm as well.

 

Mike


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#42 bremms

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Posted 14 November 2018 - 09:06 AM

If you are over 40 and certainly over 50 an 8mm exit pupil is just too large. Your exit pupil at 50 is 6mm if you are lucky. You are effectively stopping your scope down to 9" with a large central obstruction.  The outer mm or so of your pupil tends to have more aberrations than the rest of the eye. I try and keep the lowest power to 5-6mm exit pupil. A 31MM  works very well at F5 or F5.5. The tiny difference in TFOV is more than offset by the proper exit pupil and slightly higher magnification.


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#43 CrazyPanda

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Posted 14 November 2018 - 09:34 AM

It is not so much the size of the object as the fact that an 8mm exit pupil is 4X brighter than an 4mm exit pupil.  For me, I find that very large exit pupils are most useful with H-Beta and O-III filters.  With these filters, the sky is about 3 magnitudes darker so if one is at a sight that is 21+ mpsas already, with the filter, it's 24+ mpsas.  

 

Often such objects are often very large not always.  

 

As I said previously, I do have a 41mm Panoptic and occasionally use it at F/5 to get the large exit pupil but more often I will remove the Paracorr and drop down to F/4.4 and use either the 31mm Nagler (7mm exit pupil), the 32mm TV WF (7.3mm exit pupil) or the 35mm Panoptic (8mm exit pupil.)  With an aggressive filter, coma is not an issue.  

 

The TV Widefield is my preferred eyepiece for the Horsehead, the narrower field of view helps keep Alnitak out of the field of view.  

 

Jon

Well from my experience at a Bortle class 4 site, anything wider than a 4mm exit pupil in a 12" scope *without* filters just looks washed out, and the magnification is too little for most filterable objects. 

That is, given the choice between a filter at 4mm exit pupil (which is 76x in a 12" F/5), and a filter at 7mm exit pupil (which is 43x in the same scope), the filter at 4mm wins virtually every time.

 

4mm is still plenty bright for even the most aggressive of line filters, and the extra magnification often means the difference between an object being visible and invisible. Many Abell planetaries are invisible with OIII at 35mm, but are just at the edge of detectability with an OIII at 21mm. 

 

The Veil looks better at 21 than 35, as does M42 and a whole host of other objects. The only object that seems to like the brighter exit pupil of 35mm is the Gulf of Mexico region of the North America nebula. Since this object is extremely large and doesn't contain any meaningful detail (from my location anyway), extra magnification is not helpful here like it is on virtually every other object.

 

A larger scope or a darker sky would be a different story. Darker sky means the brighter exit pupil won't result in washed out looking objects, and a bigger scope means that brighter exit pupil can still maintain a reasonable magnification for those smaller objects that benefit from strong line filters. 

 

Basically, I think the main problem with too much exit pupil in a 12" class scope or smaller is that it cannot produce sufficient magnification for the vast majority of objects. That is, the extra brightness becomes counter-productive against the loss of magnification. Again, this may be a different story at a darker site, but that's been my experience with my 12" from my site.


Edited by CrazyPanda, 14 November 2018 - 09:38 AM.


#44 Redbetter

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Posted 14 November 2018 - 02:24 PM

If you are over 40 and certainly over 50 an 8mm exit pupil is just too large. Your exit pupil at 50 is 6mm if you are lucky. You are effectively stopping your scope down to 9" with a large central obstruction.  The outer mm or so of your pupil tends to have more aberrations than the rest of the eye. I try and keep the lowest power to 5-6mm exit pupil. A 31MM  works very well at F5 or F5.5. The tiny difference in TFOV is more than offset by the proper exit pupil and slightly higher magnification.

Wrong. The above that I have crossed out is simply not accurate.  How many times must folks post that they are using 7 to 8mm exit pupils and nearly all over 50, before people stop perpetuating this myth?   A more accurate rephrasing of the above might be "Your exit pupil at 50 is 6mm or greater unless you are unlucky."   

 

The TFOV comment misses the point.  The larger pupil at the extreme typically isn't for TFOV; it is for image brightness, particularly when using filters.  Maximizing the effective surface brightness of diffuse nebulae pays dividends with filters.  A 41 Pan vs. a 31 Nagler will yield ~75% higher image brightness if one's eye can accommodate it.  And with an f/5 even if the eye only dilates to 7mm the 41 Pan will be about 27% brighter than the 31 Nagler.  The other targets that I use max exit pupil for are some dwarf galaxies (Sculptor, Fornax, Ursa Minor, Draco, sometimes Barnard's.)

 

This doesn't mean that the absolute largest exit pupils are best for all things.  Instead they tend to be niche uses when not being used as general finder eyepiece.  Even Bortle 2 skies are bright naked eye, and at max exit pupil the sky is just as bright in the eyepiece as it is naked eye.  Magnification/image scale is most often more appealing and useful for the average target.  Even the 31 Pan is bright to my eye at f/5 in very dark skies and the 41 Pan is brighter still.  I tend to prefer exit pupils of around 5mm in dark sky conditions.  My ST80 used as a finder is operating at 4.8mm and I often find 5.2mm preferable in the main scope for large targets if they are not so wide that I want to use the 31 or 41. 


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#45 Sarkikos

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Posted 14 November 2018 - 02:41 PM

If you are over 40 and certainly over 50 an 8mm exit pupil is just too large. Your exit pupil at 50 is 6mm if you are lucky. You are effectively stopping your scope down to 9" with a large central obstruction.  The outer mm or so of your pupil tends to have more aberrations than the rest of the eye. I try and keep the lowest power to 5-6mm exit pupil. A 31MM  works very well at F5 or F5.5. The tiny difference in TFOV is more than offset by the proper exit pupil and slightly higher magnification.

It depends on the individual.  Some 70-year-old's have 8mm exit pupils.  I'm 62 and my exit pupil is around 7mm.  

 

:grin:

Mike



#46 Redbetter

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Posted 14 November 2018 - 03:20 PM

Well from my experience at a Bortle class 4 site, anything wider than a 4mm exit pupil in a 12" scope *without* filters just looks washed out, and the magnification is too little for most filterable objects. 

That is, given the choice between a filter at 4mm exit pupil (which is 76x in a 12" F/5), and a filter at 7mm exit pupil (which is 43x in the same scope), the filter at 4mm wins virtually every time.

 

4mm is still plenty bright for even the most aggressive of line filters, and the extra magnification often means the difference between an object being visible and invisible. Many Abell planetaries are invisible with OIII at 35mm, but are just at the edge of detectability with an OIII at 21mm. 

 

The Veil looks better at 21 than 35, as does M42 and a whole host of other objects. The only object that seems to like the brighter exit pupil of 35mm is the Gulf of Mexico region of the North America nebula. Since this object is extremely large and doesn't contain any meaningful detail (from my location anyway), extra magnification is not helpful here like it is on virtually every other object.

 

A larger scope or a darker sky would be a different story. Darker sky means the brighter exit pupil won't result in washed out looking objects, and a bigger scope means that brighter exit pupil can still maintain a reasonable magnification for those smaller objects that benefit from strong line filters. 

 

Basically, I think the main problem with too much exit pupil in a 12" class scope or smaller is that it cannot produce sufficient magnification for the vast majority of objects. That is, the extra brightness becomes counter-productive against the loss of magnification. Again, this may be a different story at a darker site, but that's been my experience with my 12" from my site.

The sky is still bright naked eye from a dark site.  Using maximum exit pupil will make the background equivalently bright in the eyepiece.  I remember it as being bright on Mauna Kea even, and that was a very good night.  The difference at such a very dark site is that the contrast of the dimmer objects against the background is improved, so they stand out.  The sites I use typically run Bortle 2 to Bortle 3 and even on the best nights with 21.8+ MPSAS overhead the sky is bright both naked eye and through a large exit pupil eyepiece combination.  Filters dim the background greatly, leaving much of the nebulosity.

 

Max exit pupil should not be confused with optimum.  Maximum exit pupil has specific/niche uses in my estimation.  The objects you describe above are bright ones so the optimum for them will tend to be smaller exit pupil.  The Veil is bright enough that on a good night I can observe both ends of it in the red/orange zone suburban backyard with an OIII filter using only a 60mm scope.   The core of M42 can be seen unfiltered with small refractors as the sky is turning blue in the morning and only the brightest planets and a few stars are visible to the west.  The North America Nebula is fairly bright as well although less than the others and is complicated by the bright Milky Way star field that merges into it.

 

The max exit pupil targets are usually quite large and of low surface brightness.  They also include fainter portions of larger objects.  How far do you want to trace out the Pleiades nebulosity?  The Rosette is of similar size and extent.  Cygnus has quite a few H-beta emission knots around Sadr that do best with max field and max pupil and you will need to do a lot of panning to find them.  Barnard's loop is so vast that it extends beyond the field of any scope I have (maxing at 7.3 degrees.)  M42 has much wider extent that what people usually observe.  The California nebula is quite large and of modest surface brightness.  How many of the fainter portions of the Veil have you sought out?   The Horsehead benefits from the largest exit pupils.

 

By all means experiment with various fields of view and exit pupil with or without filters.  That is how I came to appreciate going all the way out to the 8mm range.  Others' eyes and skies might limit them to 6 or 7 or whatever, but by experimenting some with different brightness and various sized objects one can learn what specific situations benefit from different approaches.


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#47 bremms

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Posted 14 November 2018 - 04:57 PM

Wrong. The above that I have crossed out is simply not accurate.  How many times must folks post that they are using 7 to 8mm exit pupils and nearly all over 50, before people stop perpetuating this myth?   A more accurate rephrasing of the above might be "Your exit pupil at 50 is 6mm or greater unless you are unlucky."   

 

The TFOV comment misses the point.  The larger pupil at the extreme typically isn't for TFOV; it is for image brightness, particularly when using filters.  Maximizing the effective surface brightness of diffuse nebulae pays dividends with filters.  A 41 Pan vs. a 31 Nagler will yield ~75% higher image brightness if one's eye can accommodate it.  And with an f/5 even if the eye only dilates to 7mm the 41 Pan will be about 27% brighter than the 31 Nagler.  The other targets that I use max exit pupil for are some dwarf galaxies (Sculptor, Fornax, Ursa Minor, Draco, sometimes Barnard's.)

 

This doesn't mean that the absolute largest exit pupils are best for all things.  Instead they tend to be niche uses when not being used as general finder eyepiece.  Even Bortle 2 skies are bright naked eye, and at max exit pupil the sky is just as bright in the eyepiece as it is naked eye.  Magnification/image scale is most often more appealing and useful for the average target.  Even the 31 Pan is bright to my eye at f/5 in very dark skies and the 41 Pan is brighter still.  I tend to prefer exit pupils of around 5mm in dark sky conditions.  My ST80 used as a finder is operating at 4.8mm and I often find 5.2mm preferable in the main scope for large targets if they are not so wide that I want to use the 31 or 41. 

That is rather obnoxious. I'm done here. I would explain why a slightly more conservative exit pupil is better in every other way.. But, with answers like that. It's a waste of time.



#48 Jon Isaacs

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Posted 14 November 2018 - 06:29 PM

Well from my experience at a Bortle class 4 site, anything wider than a 4mm exit pupil in a 12" scope *without* filters just looks washed out, and the magnification is too little for most filterable objects.

 

That is, given the choice between a filter at 4mm exit pupil (which is 76x in a 12" F/5), and a filter at 7mm exit pupil (which is 43x in the same scope), the filter at 4mm wins virtually every time.

 

4mm is still plenty bright for even the most aggressive of line filters, and the extra magnification often means the difference between an object being visible and invisible. Many Abell planetaries are invisible with OIII at 35mm, but are just at the edge of detectability with an OIII at 21mm.

 

I think Red's reply was right on target.  I have had my 12.5 inch F/4.06 since about the year 2000 and together, we have been all over the southwest. Last year I spent about about 110 nights in skies that probably are classed Bortle 2-4. Some very dark skies, some not so dark.  It's not that an 8mm exit pupil is always better than a 4mm exit pupil but there are certainly times when it is.  

 

As a point of reference, I consider the Veil relatively bright.  My San Diego backyard measures about 18.6mpsas directly overhead.  With an O-III filter on a good night, I have seen the eastern Veil in an 80mm refractor, I see quite a bit of detail in the Veil with the 13.1 inch. 

 

If you are over 40 and certainly over 50 an 8mm exit pupil is just too large. Your exit pupil at 50 is 6mm if you are lucky. You are effectively stopping your scope down to 9" with a large central obstruction.  The outer mm or so of your pupil tends to have more aberrations than the rest of the eye. I try and keep the lowest power to 5-6mm exit pupil. A 31MM  works very well at F5 or F5.5. The tiny difference in TFOV is more than offset by the proper exit pupil and slightly higher magnification.

 

I have told this story several times here on CN but I think it is worth repeating one more time. I bought my 25 inch F/5 about 8 years ago and sold it almost 2 years ago.  Being a 100% star hopper, the relative narrow field of view, 0.66 degrees with the Paracorr and the 31mm Nagler represented something of a challenge.  Dealing with the ladder and pointing the scope and all, a bit wider field I felt would make things easier.  Without the Paracorr, the 41mm Panoptic would provide a 0.84 degree field of view and I figured with 25 inches to work with, even if my dark adapted pupil was 6 mm, I would still have an effective aperture of 18 inches.   And so when a local astronomer who I knew advertised a 41mm Panoptic at a good price, I decided to buy it.  

 

Well, it turned out that the added field wasn't really much help, the 31mm Nagler without the Paracorr provided 0,76 degrees so the 41mm Panoptic wasn't such a big help.  But what happened was that I noticed that some objects were clearly better seen with the 8.2mm exit pupil of the 41mm even when compared to the 7mm exit pupil of the 35mm Panoptic.  I just figured it was question of brightness gradient, poorly defined vague boundaries are better defined at a lower magnification.  

 

But after a while, I got to really looking and I realized that an 8mm exit pupil was brighter to my eye than a 7mm.  The skies in question are reasonably dark and most often but not always, this was with an H-Beta filter.  By conventional wisdom, it just didn't make sense, I was within a year of being 70 and it was difficult for me to imagine that my dark adapted pupil was significantly greater than 7mm.  I discussed it via PM with Glenn LeDrew and I ended up deciding to measure my dark adapted pupil.  

 

I decided to do it photographically.  A dark closet with a camera imaging my eye with a calibration standard held next to it.  Getting good focus was a bit tricky because it had to be done manually, trial and error and every trial meant that I had to readapt.  But I was able to get a good image or two.  When I measured the diameter of my dark adapted pupil, it turned out to be 7.7mm-7.8mm.  This was not a long dark adaptation, no more than a minute in the dark closet.  

 

So now every time I see mention that one should consider themselves lucky if they're over 50 and have a dark adapted pupil of 6mm, I cringe.  I think of all those years I figured I was just like everyone else and that I was lucky if my pupil was 7mm.  If it's 7.8mm now at age 70, what was it 20 years ago when I was 50?

 

My take away is this:  For most purposes a large pupil like mine is not an advantage, in general, one sees more at smaller exit pupils rather than larger exit pupils.  Hunting down small, faint galaxies is better with a 1mm-2mm exit pupil than with a 4mm exit pupil or larger exit pupil.  But there are situations, moments, when it does come in handy.  This is probably enhanced by the fact that at my age, I almost certainly have cataracts which dim the image.  

 

I believe this has been posted in this thread already but it is worth looking at.

 

https://www.ncbi.nlm...pubmed/20506961

 

20 to 29 years (n=66), 7.33 mm (range: 5.7 to 8.8 mm)

30 to 39 years (n=50), 6.64 mm (range: 5.3 to 8.7 mm)

40 to 49 years (n=51), 6.15 mm (range: 4.5 to 8.2 mm)

 

50 to 59 years (n=50), 5.77 mm (range: 4.4 to 7.2 mm)

60 to 69 years (n=30), 5.58 mm (range: 3.5 to 7.5 mm)

 

The important thing is not the average, we are individuals not averages.  What is striking is the wide range of pupil diameters.. 3.5mm to 7.5mm, that is more than a factor of two.  It is best not to make that same mistake I did and assume I was average.  

 

==========

 

If I were putting together another set of eyepieces for a 12.5 inch F/5 and I were forced to choose between a 31mm Nagler/30mm ES and a 41mm Panoptic/40mm ES, I would definitely go with the 31mm.  I said that early on, it's just a much more useful eyepiece.  As Red said, at F/5, the 41mm Panoptic is really a niche eyepiece, one to add on when other more commonly used exit pupils have been taken care of.

 

Jon


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#49 Redbetter

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Posted 14 November 2018 - 06:48 PM

That is rather obnoxious. I'm done here. I would explain why a slightly more conservative exit pupil is better in every other way.. But, with answers like that. It's a waste of time.

Really?  I kept it civil despite the fact that I considered the comment I responded to obnoxious in the light of the examples above of people proving it demonstrably false.   I do reserve the right to point out that many of us have discovered it is incorrect.  How do we know?  Because we actually either tested our eyes or tested things in the field rather than accepting the myth at face value.   

 

As I have noted there is a difference between the maximum useful and optimum.  In my experience there are some objects for which the maximum is also the optimum and it just so happens to be around 8mm for me.  It might be more than that or 7mm or 6mm or 5mm or whatever for others.  The point is that these "one-size-fits-all" correlations aren't worth diddly. 

 

For smaller and often higher brightness objects, more conservative exit pupils and greater image scale will be better, but there are things that maximum exit pupil can do that those conservative exit pupils can not.  Once you see it, your perspective changes.


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#50 AxelB

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Posted Yesterday, 08:33 PM

Lots of long winded posts around here lol.gif

 

Just get the ES82 30mm, you can’t go wrong with this excellent eyepiece and it’s well matched to your f5 scope.

 

I may buy the ES68 40mm some day but it would be used at f6.5 for a 6,15mm exit pupil.


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