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

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#1 smiller

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Posted 08 November 2018 - 11:10 PM

Hi,

 

I’m considering wide angle/low power eyepiece options from Explore Scientific, such as their 68 and 82 degree line of eyepieces for a f/5 12” Dob, 1500mm focal length.

 

I’m trying to get the widest actual FOV and also thinking about aparent image brightness at the

low magnification.  Here are the two contenders, the lowest power of the respective types:

 

30 mm 82 degree.  Actual FOV = 82/(1500/30) = 1.64 degrees

40 mm 68 degree.  Actual FOV = 68/(1500/40) = 1.86 degrees

 

so the 40 mm 68 wins, even though the apparent FOV is small.  However, the image should appear brighter and more compact, right?

 

But hold on.  The exit pupil of the 40mm is very large...

 

30 mm 82 degree: exit pupil is 30/5 = 6mm

40mm 68 degree: exit pupil is 40/5 = 8 mm

 

My understanding is young people have dark adjusted exit pupils of about 7mm and it‘s more like 5mm for older people like me at 55.

 

So with the 8mm exit pupil, is much of the light wasted and although the 40mm 68 will have a wider FOV but will be dimmer due to this for someone like me?

 

Regards,

 

Steven

 

 


Edited by smiller, 08 November 2018 - 11:12 PM.

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#2 J A VOLK

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Posted 08 November 2018 - 11:27 PM

I have a 10" f/5 Dob - have the same eyepieces, the 30mm 82 is much better because of the exit pupil & the higher magnification improves the view and contrast. I use the 40mm on slower scopes. FYI I am older than you and had my eyes dilated and measured, exit pupil over 7mm. Opthamologist didn't really subscribe to the age theory!
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#3 TOMDEY

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Posted 09 November 2018 - 12:27 AM

I have a 10" f/5 Dob - have the same eyepieces, the 30mm 82 is much better because of the exit pupil & the higher magnification improves the view and contrast. I use the 40mm on slower scopes. FYI I am older than you and had my eyes dilated and measured, exit pupil over 7mm. Opthamologist didn't really subscribe to the age theory!

But your chemically-dilated pupil is a LOT bigger than your darkness-dilated pupil, which will be pretty close to the textbook value:

 

age 15-30  5.9mm
age 31-45  5.0mm
age 46-60  4.5mm
age 60-      4.3mm

 

Very few labs are set up to measure the drug-free Scotopic (Fully Dark-Adapted) pupil. They chemically dilate because that opens FAR wider, and they are looking for diseases, etc... not recommending eyepieces for astronomy. My ophthalmologist and optometrist friends are near all blissfully ignorant re' astronomy eyes... like the general population.

 

Here's the results of a decent study, comprising large population of subjects >>>
Note also that illuminating one eye only also constricts the Other eye's pupil (somewhat). So, the eye patch idea is good, but not a complete panacea.

I'm 70, and got mine measured both ways: Chemically-induced ~9mm, Subdued Lighting 4.1mm, NO Lighting (aka NIR imagery) 4.4mm. I figure the 4.4 is probably closer to when I'm fully dark-adapted... because I couldn't see ANYTHING except a dim dot fiducial, when he was taking that measurement!

 

Most eye doctors will measure that dim light one, if you ask. They will park you in a subdued-lighting room for a while. Not what we astronomers call dark, but probably dark enough to get the pupil to open as far as it can. Then, (if they have it!) take a NIR "deer in the headlights" measurement.  Tom

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#4 Starlease

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Posted 09 November 2018 - 12:58 AM

Forget the exit pupil myth. You NEED a 40mm for lowest power searching. Not necessarily the widest view. I have a Bresser 40 with 60 degree field pinpoint stars to the edge. Out of focus edges hinder your search wasted field. Looking for all those tiny clusters in Cassiopia was really easy with the lowest power. Starhop right to them.


Edited by Starlease, 09 November 2018 - 12:59 AM.

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#5 havasman

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Posted 09 November 2018 - 01:01 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.


Edited by havasman, 09 November 2018 - 01:02 AM.

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#6 TieDyeAstronomer

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Posted 09 November 2018 - 01:07 AM

Hi Steven!

The 40mm would win in my book. My reasoning follows below!

First, it's a myth that young people have 7mm pupils, and older people have 5mm max. I know multiple people older than you who have pupils larger than 7mm, and I'm young and have 9.7mm pupils! Here's a chart from a study which was done on ""dark adapted"" pupil diameter, showing that a noticeable amount of the participants had pupil diameters larger than 7mm, all the way up to ~60 years old:

O2wwjfi.png

However, there's something which is important to note: the measurements were taken after only two minutes of dark adaptation at a light level of 1 lux! 1 lux is as bright as twilight, not night, and every astronomer knows 2 minutes of dark adaptation isn't enough time. My measurements of my own eyes indicate that a "Dark Reflex Diameter" of 8.4mm is reached after the first second of exposure to darkness, and this does not change much over the course of 15 minutes, but after several hours of observing under dark skies, the 9.7mm diameter is reached. So the results from this study are very likely skewed toward the lower end.



But even if the 8mm exit pupil was bigger than your eye's pupil, I would still recommend the 40mm over the 30mm. To see the reason why, let's look at some of the "visual rules"; guidelines about how the visual system responds to faint light. I'm going to use the simple version here, because I tend to ramble on about this and I don't want to overwhelm, but if you're interested in this stuff let me know and I'll be happy to share what else I know!

Here are our guidelines about vision:

1.) The human eye is best at detecting things which are between 1-3 degrees in apparent size.

2.) The human eye can detect smaller contrasts against brighter backgrounds.



#1 means that, for a faint object where all we want to do is detect it (not see detail), we want to use a magnification that's in the range of 1/o to 3/o, where o is the diameter of the object in degrees. So for an object which is 1 arcminute in size, like a small galaxy, that's:

1/60' = 0.0166 degrees
1/0.0166 = 60x
3/0.0166 = 180x
use a magnification between 60x and 180x

That may sound like a wide range, but it helps to narrow things down- now we know that using 450x would not make it likely to see the very faint galaxy, even though it would still easily fit in the field of view. It would be overmagnified, and our visual system would have too tough of a time detecting it.

In this example of a small galaxy, the object would be under-magnified in a 40mm eyepiece in your telescope, but that's not true of all objects. Objects in the range of 1.6-4.9 arcminutes in size would be near optimum magnification in the 40mm. That's a lot of objects!

Magnification is a tool-- high magnification has its place, and so does low magnification. For this reason, I'd recommend a 40mm over a 30mm. Assuming this would be your lowest-power eyepiece, it would give you a good spread in magnifications available: more tools in your toolbox.



Now, let's look at guideline #2. "The human eye can detect smaller contrasts against brighter backgrounds." This one can sound contradictory at first- aren't we always trying to get away from bright backgrounds (light pollution)? Well... yes. The thing is, light pollution lowers the contrast with an object: what is 2x brighter than the sky under dark skies becomes 1.000001x brighter than the sky in the city (counting the light pollution in front of the object). So a better way to phrase this rule would be that given the same sky, and thus same contrast between object and background, brighter backgrounds are better.

That's all well and good.... but how do you get a bright background without changing the light pollution of the sky? The answer is simple: Use a larger exit pupil! When you use a small exit pupil, you are darkening the background of the sky artificially. The brightness of the image increases with the size of the exit pupil, until it's capped by the size of your own pupil. Fun fact: the brightest view you will ever see of an object is with the naked eye! The only reason we use telescopes (for resolvable objects) is magnification: to bring objects (or their details) to that 1-3 degrees in size, so that we can spot them, and see detail.

So, if your pupils are 8mm or over in size, the 40mm would be a clear winner: brighter background, ideal magnification for medium-large objects, what's not to like? Even if your pupils are 6mm or less in diameter, the 40mm would still win in my mind, due to offering you optimum magnification for a wide range of larger objects. Even though you're technically "losing light" by using an oversized exit pupil, your telescope is operating at maximum efficiency for that particular magnification: the background is still as bright as you can possibly get.



I have a telescope which is virtually identical to yours. It's a Zhumell Z12, which is also a 12" with 1500mm focal length. I got a 55mm plössl for it this year, and I'm quickly finding it to be my favorite eyepiece under dark skies, getting nearly as much use as my 13mm Ethos. The field is not as good aesthetically, since the AFOV is smallish, and the large exit pupil reveals every aberration in my eye, giving the stars lots of spiking and flaring. However, I keep finding that I can see fainter large objects in the 55mm than the 13mm. Two examples from my latest outing to my astronomy club's dark site (SQM-L 21.2):

  • The very faint but rich open cluster IC 166 in Cassiopeia was totally invisible at all other magnifications, but appeared dimly in the 55mm at 27x.
  • While in pursuit of the small (but nice!) open cluster Pismis 27, I found that a large, round nebula appeared quite clearly around a bright star in the field! Despite its clearness in the 55mm, in the 13mm (which with its large AFOV should have framed it well) it completely vanished- well, except for a small bright knot on one side, which had gone unnoticed in the 55mm, but was magnified enough by the 13mm to become noticeable. The nebula turned out to be the Monkey Head Nebula, much more famous for astrophotography than visual observation!



Now that I've said all of this! I find that most- not all, but most- of the value of a long-focus eyepiece comes when doing low-contrast deep-sky work, and when under dark skies. If you are not interested in pushing for large extremely faint objects, or doing milky way sweeping from dark skies (gorgeous!), then you might not get nearly as much out of a very long focus eyepiece as I do. If you aren't interested in large objects and observe exclusively from the city, then you might consider the 30mm, if AFOV is a big factor for you. (I will say that in my experience, with eyepieces from 110deg AFOV to 30deg AFOV, I don't find 68deg to be noticeably small in terms of quality of views- the main reason to go wider in my mind would be to gain more TFOV for starhopping, but the 68deg already has more TFOV in this instance. 70deg eyepieces still feel wide to me, but that's just in my personal experience.)

By the way, some report that when using an eyepiece with an oversized exit pupil, they see a distracting secondary shadow imposed on the image. I notice no such issues with my 12" f/5 at 11mm exit pupil, but I have seen this in other's scopes with exit pupils as small as 7mm! (I once went back and forth between my friend Amelia's scope at 7mm exit pupil and mine at 11mm exit pupil. In hers I saw a shadow so black it almost totally blotted out the image, while in mine it was just a bright field which showed the object- the Pacman nebula- decently well.) I don't know exactly why it would be different between different scopes; I just figured I should mention that it can vary between scopes/observers.

Good luck with whichever eyepiece you choose!

Clear Skies!
Lauren Herrington


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#7 TieDyeAstronomer

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Posted 09 November 2018 - 01:51 AM

But your chemically-dilated pupil is a LOT bigger than your darkness-dilated pupil, which will be pretty close to the textbook value:

 

age 15-30  5.9mm
age 31-45  5.0mm
age 46-60  4.5mm
age 60-      4.3mm

 

Very few labs are set up to measure the drug-free Scotopic (Fully Dark-Adapted) pupil. They chemically dilate because that opens FAR wider, and they are looking for diseases, etc... not recommending eyepieces for astronomy. My ophthalmologist and optometrist friends are near all blissfully ignorant re' astronomy eyes... like the general population.

 

Here's the results of a decent study, comprising large population of subjects >>>
Note also that illuminating one eye only also constricts the Other eye's pupil (somewhat). So, the eye patch idea is good, but not a complete panacea.

I'm 70, and got mine measured both ways: Chemically-induced ~9mm, Subdued Lighting 4.1mm, NO Lighting (aka NIR imagery) 4.4mm. I figure the 4.4 is probably closer to when I'm fully dark-adapted... because I couldn't see ANYTHING except a dim dot fiducial, when he was taking that measurement!

 

Most eye doctors will measure that dim light one, if you ask. They will park you in a subdued-lighting room for a while. Not what we astronomers call dark, but probably dark enough to get the pupil to open as far as it can. Then, (if they have it!) take a NIR "deer in the headlights" measurement.  Tom

Hi Tom,

In my case, chemical dilation dilated my pupils to the same diameter as I measure using calipers and dim red light after observing at a dark site for hours. I had my mom take a photo of my eyes after chemical dilation the last time I went to the opthamologist, then I used that photo to measure my pupils after the fact by taking the ratio to the known measurement of my iris. My left pupil came out to be 9.3mm on the short axis and 9.9 on the long axis (For some reason, at full dilation my pupils have a flat notch cutting into the pupil diameter on the ear side). I have measured my left pupil after observing for hours at a dark site, by looking into a mirror with calipers and dim, grazing red light- not enough to actually shine into the eye, just touch the iris- and come up with 9.7mm (short axis, but I didn't measure vertically). I have not repeated that measurement, because it's rare for me to be out at that dark site during the time of the year when there are enough leaves on the trees to block the many mag 0 to -8 lights visible from my vantage point, which seem to constrict my pupils somewhat.

I have also been able to make several measurements in the day. I was truly shocked when I made my very first pupil measurement and found out that I had a pupil diameter of 7.7mm in a room lit by large windows and noon-time cloud-light! I mean, gobsmacked! That went against everything I thought I knew about pupil sizes. I've since gotten fairly good at estimating pupil size to the nearest millimeter visually, after making more measurements and starting to pay a whole lot more attention to my pupils! As it turns out, 7mm is a fairly typical size for me in moderate lighting conditions, and I've not yet seen my pupils smaller than about 4mm (though I try to wear sunglasses in bright situations, so I can't usually see my eyes in a mirror then). 

Anecdotally, just a couple weeks ago my grandmother went to the opthamologist, and came back with her eyes dilated, complaining of visual effects due to the dilation. Her pupils appeared to be only ~3-4mm in diameter. So I definitely don't argue that all seniors have large pupils- just that the spread in pupil sizes is very large, and many don't realize that their pupils are actually much larger than they think.

I believe that part of the prevalence of the myth is due to the natural spherical aberration within the human eye, which sabotages common indirect self-measuring pupil diameter tests like the "allen wrench" or "paper strip" methods. My theory is that the spherical aberration causes the results to appear smaller than they should, because light past a certain diameter is deflected off-axis, instead of into the tell-tale star pattern. The central, unaberrated portion of the eye can be masked completely while still allowing off-axis light to form a very thin glare. In my case, the effects of spherical aberration kick in at around the 7mm diameter, meaning that at maximum dilation fully half of the light entering my pupil is directed into glare and halos instead of star images. Indeed, when I performed the allen-wrench test, I got the result that my pupils were between 7-8mm in diameter, closer to the 7mm mark. (Note: I was doing the test at a site which is only moderately dark- SQM 21.1ish- and I still have yet to directly measure my pupils there; so while based on my measurements in varying light conditions I believe my pupils would have dilated to my "dark reflex diameter" of 8.4mm at that dark site, I can't be certain.)

Clear Skies!
Lauren Herrington


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#8 J A VOLK

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Posted 09 November 2018 - 02:03 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.

Pretty much agree - I have the ES 30mm 82, and one of the best corrected 40mm eps - the TMB Orthoscopic Super-Wide. I just don't use the 40mm on my f/5 Dob. It just sits right there in my case, the 30mm just plain gives a better view with close to the same field. As stated the 31mm T5 would get you closer, but really breaks the bank. On classic SCTs (F/10 - f/11) the 40mm TMB gives the best edge correction of any 40mm I have tried (many).

Edited by J A VOLK, 09 November 2018 - 02:05 AM.

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#9 TOMDEY

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Posted 09 November 2018 - 02:34 AM

Lauren... you are sure convincing me! When I got my implants, they are corrected for spherical aberration... the "natural" spherical aberration of the human eye. Hmmm... My chemically-dilated pupils are 9mm, each side. Now I'm questioning what my chart there, from the literature claims... Maybe something in their set-up... Geesh!

 

It would indeed be true that a retro "deer in the NIR headlights retroreflection" would report a smaller (effective) pupil, in the presence of eye's spherical aberration.

 

My implanted lenses are 6.5mm physical diameter, but appear to be bigger, (and therefore effectively bigger) because they are significantly posterior to the cornea.

 

Think I will try ... must have it here somewhere (?!) It's a bar-scale that measures the functional pupil diam... covering a bright star until it can be entirely occluded by the bar. Then just read off the bars to see which one barely retains the star and next one makes it invisible. If I can't find that thing... I'll use drill bits or something!

 

My biggest "usable" pupil would probably be around 6-7mm... IF my iris is actually opening enough...

 

ANECDOTAL: I indeed DO use low power eyeps on my giant Dobsonian:

 

31mm 7.2mm pupil @ 127x

20mm 4.6mm pupil @ 197x

17mm 3.9mm pupil @ 232x

13mm 3.0mm pupil @ 303x

 

Those 1st two indeed deliver magnificent images... I figured I was clipping my eyes pupil... maybe NOT, or maybe not as much as I think! And That would be GOOD!  Tom


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#10 sg6

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Posted 09 November 2018 - 03:17 AM

Strongly suspect that your pupil will be bigger then is often quoted. One person here had theirs measured and it result was actually quite large. Long time back but I seem to recall they were mid 50's and got a result of 8.3mm.

 

Also your dark adaption is more then just the pupil, time will be a factor as I would expect the pupil still opens a fair amount but likely does the final bit slower.

 

Any way eyepiece: The simplest answer is not much of an advantage. Th e resulting field will be similar, magnification a little different, background is something I rarely think of as I only go as far as good or not good. Also I suppose the question only really arises with plossl eyepieces. I would like to compare a 30mm Paradigm to a 40mm Paradigm, just such things do not exist lol.gif lol.gif 

 

Assuming the question concerns plossl eyepieces then you may need to buy a reasonable one as at f/5 aspects become more critical and the larger diameter lens used will lead to a few additional aberrations that can come through.

 

Minor aspect of that is I bought an inexpensive 40mm simply as I did not overly care about quality, I wanted width of field for goto alignment of a Mak.


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#11 otocycle

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Posted 09 November 2018 - 05:04 AM

I have both eyepieces under consideration for use in several scopes including a similar 12" f/5 Z12 and the 30mm ES82 gets used far more with the Dob.   The 40mm shares finder duty that is duplicated by a Meade 56mm Super Plossl.

 

I have never cared about exit pupil optimization for eyepieces...just use what I like and cover the bases with many choices.   But if I had to choose in this instance, it would be the 30ES82.



#12 Tony Flanders

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Posted 09 November 2018 - 05:26 AM

The human eye can detect smaller contrasts against brighter backgrounds.

Precisely where did you get that dictum? My own experience is precisely the opposite.

Like you, I think that eye pupil size is somewhat of a red herring -- but in the opposite direction. Let me tell you my own experience.

My pupils open to 5.5 mm, and the main eyepiece that I use with my f/5 Dob is a 27-mm 68-degree, yielding an almost identical 5.4-mm exit pupil. I occasionally use a 40-mm eyepiece that yields a somewhat larger true field of view. However, the loss in what I can see moving from 27 to 40 mm is huge, across the board, with every object I have ever looked at. Not only can I see much more detail with the 27-mm eyepiece, I can also see much fainter objects, and much more broad-stroke, low-contrast detail such as the dust lanes of M31.

Moreover, smaller exit pupils continue to show me more well below that 5.5-mm figure. In fact, I find a pretty dramatic increase in ability to see faint, low-contrast objects down to an exit pupil around 4 mm. Below that, things get more complicated. With a few very large objects with fairly ill-defined edges, 4 mm is the optimum exit pupil for me. But with the overwhelming majority, I see much more at a 2.8-mm exit pupil than at a 4-mm exit pupil. I have never yet encountered an object where an exit pupil bigger than 4 mm is optimal for me.

 

Put another way, in response to the original question -- if you can only get one eyepiece, the 30-mm eyepiece seems like a no-brainer. And that's from someone who actively dislikes apparent fields of view bigger than 68 degrees. Even so, the much greater power of the 30-mm 82-degree EP outweighs the slight advantage in true field of view of the 40-mm 68-degree EP. I can almost guarantee that you will be able to see much fainter objects as well as more detail in every object


Edited by Tony Flanders, 09 November 2018 - 06:13 AM.

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#13 smiller

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Posted 09 November 2018 - 10:36 AM

Wow, excellent discussion.  I’m leaning towards the 30mm to start with but I expect I’ll have opportunities to test drive lower power eyepieces to see what my personal experience is.

 

At risk of expanding the discussion into a can of worms, this is what I was thinking, all 82 degree for general observing of deep sky and planets:

 

- 30mm, 11mm, 8.8mm

- high quality 2”, 2x Barlow to give effective 15mm, 5.5mm, and 4.4mm

 

resulting magnification list:

 

50x, 100x, 136x, 170x, 273x, 341x (for those rare seeing nights)

 

Not totally evenly spaced, but the best I could configure.  I expect over time to fill in with additional optimized eyepieces as I learn my preferences based on actual usage and viewing conditions.

 

Cheers,

 

Steven



#14 star drop

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Posted 09 November 2018 - 11:23 AM

I use a 41mm Panoptic eyepiece on my 25" f/5 (with a Paracorr) most of the time. The views are brighter, colors more evident and image stability due to poor seeing is improved.


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#15 rowdy388

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Posted 09 November 2018 - 11:59 AM

In my f/5 dobs I can see the secondary shadow with a 40mm and longer eyepiece. That

annoys the tar out of me. My max pupil entrance is 5.5mm with a fair amount of natural

astigmatism.  My lowest power eyepieces are 32 and 31mm but I strongly prefer 20mm

and under for pinpoint stars and higher magnification.



#16 smiller

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Posted 09 November 2018 - 12:47 PM

And then there are high dollar options for us older folks:  ES 100 degree at 25mm:  60x power, 1.67 degree FOV and 5.08 exit pupil.  $700!

 

 

BTW, I used a “narrow slit method” with different slit widths looking at a point light source in a dark room to do a first pass at my dark pupil diameter.  I got from 4.9mm to 5.6mm, so I’m beginning to doubt that I’m a 55 year old 7mm outlier.

 

Cheers,

 

Steven



#17 Dave Mitsky

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Posted 09 November 2018 - 03:09 PM

I also agree that the 30mm ES 82 degree eyepiece, which I happen to own, would be the better choice.

 

Dave Mitsky


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#18 Dave Mitsky

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Posted 09 November 2018 - 03:27 PM

And then there are high dollar options for us older folks:  ES 100 degree at 25mm:  60x power, 1.67 degree FOV and 5.08 exit pupil.  $700!

 

 

BTW, I used a “narrow slit method” with different slit widths looking at a point light source in a dark room to do a first pass at my dark pupil diameter.  I got from 4.9mm to 5.6mm, so I’m beginning to doubt that I’m a 55 year old 7mm outlier.

 

Cheers,

 

Steven

The 25mm ES has been reported to be less than ideal optically.

Reviews can be found at https://www.cloudyni...-eyepiece-r2813 and https://www.cloudyni...iece/?p=5490194

 

Dave Mitsky


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#19 Jon Isaacs

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Posted 09 November 2018 - 07:14 PM

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.

Precisely where did you get that dictum? My own experience is precisely the opposite.

Like you, I think that eye pupil size is somewhat of a red herring -- but in the opposite direction. Let me tell you my own experience.

 

 

My experiences agree with both Dick and Tony.  My scopes are mostly around F/5, the 12.5 inch operates at F/4.67 with the Paracorr, the 16 and the 22 inch are at F/5.06 with a Paracorr, the NP-101 is at F/5.4.  My longer focal length eyepieces include the 21mm Ethos, 31mm Nagler and the 35mm and 41mm Panoptics.  I play around with large exit pupils.  I am 70 and about 18 months ago I photographically measured my pupil at about 7.7mm.

 

About the only time I find a near maximum exit pupil to be useful is when using an H-Beta or maybe an O-III filter for large dim objects.  Good examples are Barnard's Loop or the HorseHead with an H-Beta.  Otherwise, I almost always see more with the 31mm and 21mm Ethos.  A smaller exit pupil means a larger image scale so details are more easily seen. The contrast of an extended object is unchanged but the contrast of a star, open cluster or to a certain point, a globular cluster is dramatically improved by increasing the magnification.  Doubling the magnification does not affect the brightness of the star but it dims the background sky by a factor of 4,  that's 1.5 magnitudes greater contrast.  Increasing the magnification can make cluster come alive as those faint tiny stars pop out.

 

Something to consider:  Even under dark skies, the sky glow is bright enough to affect the dark adaptation of the eye.  Dimming the sky glow and the object with a smaller exit pupil means that the eye can be better dark adapted, this would be the photo-chemical aspects which are the important aspects of dark adaptation. I think this is why I really only notice an improvement of dim objects using an 8mm exit pupil if I use an H-Beta filter or possibly an O-III filter.  The sky is so much darker with a narrow band filter, my eye can more fully dark adapt.  An H-Beta or O-lll filter might have a bandwidth of 12nm, the brighter part of the visual spectrum is around 200nm, this means the sky is dimmed by about 3 magnitudes, a sky that is 21 mpsas is now about 3 magnitudes dimmer at 24mpsas, a factor of about 17.  

 

And too, as much as I enjoy wide field viewing with the big, bright exit pupils, and as much as I am a vocal proponent of large exit pupils and wide fields of view,  the vast majority of the objects out there are small, making them larger is probably the most important aspect of seeing them better.   

 

As a finder eyepiece, I primarily use the 21mm Ethos, at F/5, the 4mm exit pupil is plenty bright but the increased magnification means that I can see small objects like galaxies etc much better than with the 31mm Nagler.  The 13mm Ethos provides a 2.6mm exit pupil and it is even better for picking out small objects, the field of view gets somewhat narrow, less than 1/2 degree in the 22 inch but I can certainly pick out the faint fuzzies.  

 

As far as measurements: 

 

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

 

"RESULTS:

Two-hundred sixty-three individuals participated.

 

For participants aged 18 to 19 years (n=6), the mean dark-adapted pupil diameter was 6.85 mm (range: 5.6 to 7.5 mm);

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);

70 to 79 years (n=6), 5.17 mm (range: 4.6 to 6.0 mm);

80 years (n=4), 4.85 mm (range: 4.1 to 5.3 mm).

 

These values were consistent with studies using infrared photography. The standard deviation was >0.1 mm in 10 (3.8%) participants, all of whom were younger than 55 years."

 

However, in my experience, the main takeaway is that very large exit pupils, very large dilated pupils are rarely an advantage and one almost always sees more with a 6 mm exit pupil rather than with an 8mm exit pupil as well as with a 4mm exit pupil rather than a 6mm exit pupil.  There are specific circumstance when I will choose large exit pupils, the Heart and Soul nebular complex is large and dim, low contrast, a 7+mm exit pupil with a narrow band filter is about optimal.  

 

Bottom line:  Get the 31mm Nagler/30mm ES 82 degree.  Get the 41mm Panoptic/40mm ES 68 degree after you purchase the 21mm Ethos/ 20mm ES 100 degree if you want to see what it looks like.  I think you will find most folks working at F/5 rarely use a 40mm eyepiece. Even for those with large dilated pupils, it's just rarely an advantage. 

 

Jon Isaacs


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#20 Ernest_SPB

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Posted 10 November 2018 - 01:32 AM

Interesting topic...

 

Let me add my cents.

 

(1) I am 56 and my own measurements show adapted pupil diameter 7.2 mm for left my eye and 7.5 mm for right 

(2) I have 30+ years experience in astronomical observing and focus of my interests is mostly shifted from popular to dim and challenging DSO

(3) I observe most time from dark places (21.20 m/sec2 and darker)

(4) In DSO observing with F5 12" Dobson most time I use 11 mm Nargler (50%), then 16 mm Nagler (33%), then 20 mm ES100 (15%) and 8.8 mm ES82 (5%) sometimes 6.7 mm ES82.

(5) I have also 30 mm ES82, 35 mm Paragon, 40 mm XL. But in the Dobson they were used just several times. 

 

Pretty compact 1.25" Naglers with FL 11 mm and 16 mm are my favorites - they easy to handle, have small weight, effective in search for dim DSO. Bulky and massive long focal eyepieces are inconvenient in use and effectively limit my observations with only relatively bright well known for me objects.



#21 Redbetter

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Posted 10 November 2018 - 03:23 AM

My own results have been like many of those above.  My pupils seem to dilate past 7mm and I am on the wrong side of 50.  The brightest views I have seen came from exit pupils in the 8mm range with reflectors or refractors.  Examples have been a 41 Pan in the 20" f/5 and 55 Plossl in the f/7 110ED.  This is aesthetically less attractive even in dark or very dark skies (Bortle 2 to 3 at worst) because it is quite bright to me.  Therefore I tend to reserve these largest exit pupils for large very low surface brightness objects and particularly for use with narrowband or line filters.   

 

I prefer to sweep/find in dark skies using an eyepiece closer to 6mm exit pupil, and in bright skies something around 3-4mm is more comfortable.  Even for large objects with filters, intermediate exit pupils can be the optimum depending on the brightness of the object, size (framing), and benefits from increased scale.

 

An interesting aside is that while I really prefer the views through the 31T5 in the big scope, I have recently begun using the 41 Pan most of the time for the low power work.  It gives slightly more field and it is better on the largest nebulae, but the primary reason is that I don't have to run the primary mirror collimation as high to allow the 31T5 to focus--running the collimation to the end makes the sling position less than optimal and results in more collimation shift.  Instead I find myself jumping from the 41 Pan to the 26T5 or 20T5 to find the sweet spot for an object. 


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#22 chrysalis

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Posted 10 November 2018 - 07:01 AM

I will be 64 at the end of the month. I use a 40 mm TMB Paragon 69* EP all the time. I do not perceive issues with this EP when used at night. If used in daytime (like Venus Occultation by Moon), there is a shadow of the secondary that is a bit bothersome.



#23 N3p

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Posted 10 November 2018 - 11:27 AM

I was in the exact same quest of getting the widest TFOV possible for my telescope. I finally went for the ES 34mm 68d 6.8mm exit pupil, then a couple of months later I ordered the ES 24mm 68d. In all honesty, the 34 makes things complicated with my telescope on equatorial mount and I take the 24mm most of the time instead. Both give me a very nice TFOV.. and I could probably live without the 34mm today.

 

BUT putting aside the weight of the 34, I prefer it's optics over the 24 by a slight margin and i find it even more comfortable, both give me great satisfaction.

 

I would not go with 40mm with an exit pupil over 7mm using my 200 x 1000 Newtonian. I don't think it's necessary to push the limits like that. 



#24 MitchAlsup

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Posted 11 November 2018 - 02:59 PM

Very few labs are set up to measure the drug-free Scotopic (Fully Dark-Adapted) pupil. 

Very few labs can afford to let you wait in complete darkness for 45 minutes for your eye pupils to dilate as much as possible.

 

Nor would the patients be happy doing absolutely nothing for 45  minutes.



#25 TOMDEY

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Posted 11 November 2018 - 03:46 PM

Very few labs can afford to let you wait in complete darkness for 45 minutes for your eye pupils to dilate as much as possible.

 

Nor would the patients be happy doing absolutely nothing for 45  minutes.

Yeah, I know. Here's an anecdote to go with that: >>>

 

At work, we did optical metrology... thirty guys in our department, and that was ALL we did! We were building another lab, this one mostly for photometry/radiometry. I went all-out and had the walls and ceiling blackened and even the floor tiles replaced with black. But houses and buildings have light leaks just like they have air leaks... almost unavoidable, except for vacuum chambers.

 

I got a couple of techs to help look for light leaks. We got all comfortable, then turned off all the lights and just waited, discussing our favorite topic, Zernike Circle Polynomials... like you say... well over half an hour. Just as I was quizzing, "what is the angle between orthogonal dodecafoils?" [correct answer is 15 degrees], one of the techs offered, "Let's look for Light Leaks!" The place was FULL of leaks! Even the blackened dropped ceiling panels were not opaque. We redid ALL of those and plugged the other leaks. Our next test session, we armed ourselves with Night Vision and Thermal viewers... and found some NIR/IR signatures from power supplies and the like. Fixed those, then a third session... and opened the Lab for business.

 

The techs are now steeped in the use of NV equipment, familiar with Dark Adaptation... and quite expert re' ZCPs. Very few people can explain what "orthogonal on the unit circle" means, or why it is so profoundly useful!  And to avoid clothing or makeup that glows in the dark...  Tom

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