10 replies to this topic

[Star Geezer]

I enjoy the low power wide field view filling my dark-adapted iris with as much light as it can hold as much as anyone else. When it comes to Dso’s, it seems to me the view gets better with a lower exit pupil. I have seen detail in some Dso’s with a 2mm exit pupil that probably would have gone unnoticed otherwise.

 

Is there anything behind this? Is it the dimming of the sky glow in my Bortle rated zone showing better contrast? Or is it my imagination?

 

Help!

Edited by Star Geezer, 24 January 2021 - 12:03 AM.

[TOMDEY]

Most people see the most detail around a 2mm pupil. Especially when using scopes that have a small central obstruction... 20% or smaller. There's a lot of discussion as to why, but the trend is quite consistent.    Tom

[Redbetter]

This is because you aren't changing just the exit pupil, when you choose an eyepiece that provides a smaller exit pupil than your eye can fully accomodate, you are also changing the image scale (magnification.)  Increasing the image's apparent size generally improves the detail seen, as long as the image doesn't become too dim in the process.  For most DSO's an image scale of 1 to 2mm can be quite good for revealing detail. Visual acuity tends to be best in this range.  (Some other stuff is going on for planetary, but we are talking about the fuzzy's at the moment.) 

 

Many of the faintest objects will not even be detected until the magnification is in the range provided by 1-2mm exit pupil.  Even with the 20" I use 1mm exit pupil when the seeing supports it to bring in the smallest/dimmest galaxies I can detect, but am mostly stuck at 1.4 or 1.8mm exit pupil because of seeing limitations, and sometimes 2.2mm is required or even that is mushy.  I will go even higher than this for DSO's if there is something in particular that requires additional scale, such as M87's jet, but the seeing rarely is adequate for such a larger scope.

 

Where larger exit pupil is helpful is on larger and or lower surface brightness objects.  The diffuse outer reaches of galaxies, the weakest nebulosity, the IFN, etc. are best seen with larger exit pupil.  There is still an optimum to be found for each object in terms of scale and brightness.  Another factor is the use of nebula filters, particularly with the most diffuse emission nebulosity.  For the latter the contrast increases greatly because the foreground glow disappears, leaving the glow of the nebula; but it can require a larger exit pupil to provide sufficient surface brightness to see it decently.

 

Even with small or very small galaxies I often observe the galaxy first at 3.2mm exit pupil (156x) where I am more likely to see low surface brightness aspects in averted vision.  Then I employ smaller exit pupils to improve the level of detail seen, but this sometimes results in the loss of the wider extent.  And there are a few rather large, low surface brightness dwarf galaxies that I typically observe at 6.2, 5.2, or 4mm. exit pupil, because that is how they are best seen with the aperture.

 

 

 

[TOMDEY]

Good points! My scope is 36 inches so I actually can blithely use bigger pupils to advantage and still have plenty of magnification. The 2mm pupil generally applies to smaller scopes, where you just got so much precious light to play with.    Tom

[gspeed]

...When it comes to Dso’s, it seems to me the view gets better with a lower exit pupil. I have seen detail in some Dso’s with a 2mm exit pupil that probably would have gone unnoticed otherwise...

Another way to see this: one should  compare different exit pupils at same magnification (i.e. with different scopes),  to really glance what is the exit pupil effect is - which turns to be driven by the diameter.

 

It also depends on the DSO object you're looking at: the more one increase magnification the more extended objects like the background sky become darker (brightness decreases with the inverse square of the magnification) vs stars that maintain the brightness, since they have no surface - so additional stars tend to appear on clusters.

[*skyguy*]

My "best" magnification for observing DSO's using my 12" SCT is 187.5X with a 1.6mm exit pupil. J.B. Sidgwick in his book ... Amateur Astronomer's Handbook ... states the "best" magnification for resolving all the detail in the focal plane of a telescope is determined by the equation ... M=13D, where D is the telescope diameter in inches. That would indicate the "best" magnification for my 12" SCT would be 156X, which ... coincidentally ... is a 2mm exit pupil.  However, there is such a great disparity between individuals in their ability to resolve fine details, this equation will give only very general results.   

Edited by *skyguy*, 24 January 2021 - 03:33 PM.

[Star Geezer]

Wow, I couldn't agree more with everything said. My scopes range from 70mm to 200mm in aperture. When I started this post, I was mostly considering the faint fuzzy's (Nebulae and Galaxies). However, it seems to hold true for any type of object as long and what you want to observe of the object fits the field of view of the eyepiece. I think, I have read here on Cloudy Nights, that Planets appear better between a 1 - 0.5mm exit pupil. I have compared the view of the Caroline’s Rose Cluster with my 4.5” StarBlast and 8” SkyQuest. I first located NGC 7789 with the StarBlast at 18x magnification and a 6.28mm exit pupil, at 30x with a 3.77mm exit pupil the Cluster really popped. With the Xt8, at 48x and a 4.23mm exit pupil the Cluster did not have that sparkle.

 

I have been contemplating my next scope. I may be in a position to afford a $3500 to $5000 scope. Originally, I had my eye on 12” f/8 Richie Criterion, then aperture fever started to set in (yes, I have finally caught the fever). For less than the optical tube assembly alone, I could get the Orion SkyQuest XX16g GoTo Truss Tube Dobsonian Telescope. Now, I am wandering how big I could go if I forgo the GoTo.

 

I have out my copy of “Your Guide To The Sky” by Rick Shaffer, getting ready to read more about exit pupil. Of all my amateur astronomy publications it is my belief anyone considering their first Telescope or Binoculars should read this first. Why is this book not still in publication?

[TOMDEY]

Another way to see this: one should  compare different exit pupils at same magnification (i.e. with different scopes),  to really glance what is the exit pupil effect is - which turns to be driven by the diameter.

 

It also depends on the DSO object you're looking at: the more one increase magnification the more extended objects like the background sky become darker (brightness decreases with the inverse square of the magnification) vs stars that maintain the brightness, since they have no surface - so additional stars tend to appear on clusters.

Yep... I came up with this graphic to illustrate that >>>  Click on the image to get the big view.  Tom

 

~click on~ >>>

Attached Thumbnails

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[sanbai]

My "best" magnification for observing DSO's using my 12" SCT is 187.5X with a 1.6mm exit pupil. J.B. Sidgwick in his book ... Amateur Astronomer's Handbook ... states the "best" magnification for resolving all the detail in the focal plane of a telescope is determined by the equation ... M=13D, where D is the telescope diameter in inches. That would indicate the "best" magnification for my 12" SCT would be 156X, which ... coincidentally ... is a 2mm exit pupil. However, there is such a great disparity between individuals in their ability to resolve fine details, this equation will give only very general results.

Not that coincidentally. The formula can be written as D/M =1"/13.
D/M is also the formula for exit pupil, and 1"/13 is 1.95 mm. So "lSidgwick is actually saying that the best exit pupil is 2 mm.

In the end, I think it's misleading to the novice (put me in that group!) telling which is the best magnification or best exit pupil. It depends so much on the object, the sky condition, the optical system, and (not least) one's aesthetic preferences.

Try your eyepieces and stay with the one that delivers the best view for what you want to see.

In my last session the sky delivered and I enjoyed some planetary nebulae at very high magnification, with to me surprising result. It's not rare to see people in CN saying they went father than 2xD (mm) for those. I now believe it's true (at least for some cases).

[Jon Isaacs]

This is because you aren't changing just the exit pupil, when you choose an eyepiece that provides a smaller exit pupil than your eye can fully accomodate, you are also changing the image scale (magnification.)  Increasing the image's apparent size generally improves the detail seen, as long as the image doesn't become too dim in the process.  For most DSO's an image scale of 1 to 2mm can be quite good for revealing detail. Visual acuity tends to be best in this range.  (Some other stuff is going on for planetary, but we are talking about the fuzzy's at the moment.) 

 

Many of the faintest objects will not even be detected until the magnification is in the range provided by 1-2mm exit pupil.  Even with the 20" I use 1mm exit pupil when the seeing supports it to bring in the smallest/dimmest galaxies I can detect, but am mostly stuck at 1.4 or 1.8mm exit pupil because of seeing limitations, and sometimes 2.2mm is required or even that is mushy.  I will go even higher than this for DSO's if there is something in particular that requires additional scale, such as M87's jet, but the seeing rarely is adequate for such a larger scope.

 

Where larger exit pupil is helpful is on larger and or lower surface brightness objects.  The diffuse outer reaches of galaxies, the weakest nebulosity, the IFN, etc. are best seen with larger exit pupil.  There is still an optimum to be found for each object in terms of scale and brightness.  Another factor is the use of nebula filters, particularly with the most diffuse emission nebulosity.  For the latter the contrast increases greatly because the foreground glow disappears, leaving the glow of the nebula; but it can require a larger exit pupil to provide sufficient surface brightness to see it decently.

 

Even with small or very small galaxies I often observe the galaxy first at 3.2mm exit pupil (156x) where I am more likely to see low surface brightness aspects in averted vision.  Then I employ smaller exit pupils to improve the level of detail seen, but this sometimes results in the loss of the wider extent.  And there are a few rather large, low surface brightness dwarf galaxies that I typically observe at 6.2, 5.2, or 4mm. exit pupil, because that is how they are best seen with the aperture.

 

I think Red pretty much nailed it.  There is no one perfect exit pupil, some objects are best at large exit pupils because they are relatively large, dim and low contrast, some objects are small and bright, something like NGC-1535 (Cleopatra's eye) or the Eskimo Nebula will probably be best at an exit pupil under 1mm.  

 

But in general, the range from 2.6mm on down is best to see smaller, relatively faint objects.  I often use a 13mm Ethos as my finder eyepiece when I am looking for galaxies, it's a 2.6mm exit pupil and is 160x in the 16 inch and 216x in the 22 inch.  Some small diffuse galaxies are best seen in the 13mm but generally increasing the magnification is helpful and generally my 72 year old eyes like the range from 2.6mm to 1.6mm exit pupils for galaxies.  I just depends on the galaxy.  Sometimes there are two or more galaxies near one another, and the optimal exit pupil will be different for each one.

 

But the main thing here is this:

 

Star Geezer is doing this right.  He was paying attention to what he was seeing rather than following some guideline he read somewhere.  This is the key to observing..  Experiment, each object is an experiment to see what is visible, swap eyepieces, zoom in and out. Look carefully and learn not only about the object itself but also about what provided the best possible view. 

 

I call this:  Listening to your eye.

 

Jon 

[REC]

I like others seem to find the 2mm exit pupil is the most comfortable range.I usually start off with my 19mm Pan in my scopes or a pair of 20mm in my binoviewer for general observing. I'll change them around for other objects to get the bet views and detail like planets. They need 1to 1.5mm to get the best detail. Some very large nebula I jump up to a 3mm exit pupil.