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Eyepiece Framing A Target-Based Approach

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#1 Charlie Hein

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Posted 13 July 2013 - 09:05 AM

Eyepiece Framing A Target-Based Approach

By Jeff Morgan

#2 jrbarnett

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Posted 13 July 2013 - 02:11 PM

What's the most popular commercial telescope on the planet? The C8; 8" f/10 SCT with a 2000mm focal length.

How does a ~20mm, 100-degree AFOV eyepiece perform in that scope? 2mm exit pupil, 100x magnification and 1-degree TFOV.

Bingo!

Next?

:grin:

(Nice, thought-provoking article, by the way. However, anyone that has used a 20mm ES 100 or 21mm Ethos in an 8", 16" or 20" scope with a 2000mm focal length, knows that it is an eyepiece you can use under any conditions (bad seeing, light polluted environment, etc.) and never feel compelled to switch eyepieces during a session. 100x, 1-degree, and 2mm, 4mm or 5mm exit pupil = magic as a set-it-and-forget-it eyepiece.)

- Jim

#3 Mak2007

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Posted 13 July 2013 - 07:21 PM

Great article Jeff! I've learned a lot. I like your novel approach.

#4 Starman1

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Posted 15 July 2013 - 10:33 AM

When there is a deep sky object in the field, there is more than mere framing going on.

Personally, I find Jeff's 75% rule to be unaesthetic. I would say an object width of 50% is more pleasing for the view because it gives more context for the object.

Context is the nature of the immediate neighborhood of the object. Whether or not there are other nearby objects that form nice pairings or groups with the object is not only nice to know--sometimes it is nice to see those other objects at the same time.

But just because it is nice to see those other objects at the same time and in the context of closeness to the object in study doesn't mean we necessarily want to use such a low power that the details of the object disappear with the low power.

It can be nice to see a couple of close objects in the same field at the same time but with sufficient magnification to actually see them well--at the correct power to observe details in those objects. To not only see the objects in context but to also see them with the right amount of magnification, like M13 and NGC6207 in the same field at 180X.

Now if you want to view the objects in context and with enough power to observe them well, where does that lead? To large apparent fields of view.

But this doesn't only apply to groups of objects. Say I want to view an edge-on galaxy like NGC891, which is an edge-on galaxy in Andromeda. This object's maximum length is about 13'. If I stay with my 50% rule (and with this object the field is magnificent), I will want a field about 26' wide. But, experience with this object tells me that a magnification over 200X is necessary to show details in the dark lane and show irregularities in the interior of the galaxy. I can easily achieve that with an 8mm eyepiece having a 100 degree apparent field. It would take a 16mm eyepiece with a 50 degree field to see the same size field, but then the magnification would be too low to see the details that can be seen at higher power.

OK, NGC891 is a larger object. What about, say, M75, the globular in Sagittarius? It's only 6.8' wide. Well, like most globulars, the stars are faint. The brightest members hover around magnitude 14.6 and the horizontal branch is clustered around magnitude 17.5. How do I see as many stars in this cluster as possible? The answer: high power. At 300X, in good seeing, this cluster appears to resolve at least partially in my 12.5" and provide a view of stars all over the cluster. It begins to resemble a globular and not a faint nebula. How can I use sufficient power to see the faint stars and still see sufficient field to note the shape of the outlying stragglers and to see a nice context for the object?
A wide apparent field.

Jeff's article points out that most deep-sky objects aren't that large. That's true. But you only need a 30' field to see the whole Moon, while a 1 degree field gives a much more pleasing view of the full moon. In fact, it could be argued an even larger field is more aesthetically pleasing.

Advertising hyperbole aside, there is a valid reason for wider apparent fields with excellent correction. They do a better job of framing the objects we view, and of showing the field around the object (its context) with sufficient magnification.

It's not a matter of "drinking the Kool-Aid" or "falling for advertising hype", its a practical matter of wanting to use higher powers to study and view those, as Jeff points out, largely small objects and still see them in a field size sufficient to show context if they are grouped with other objects (and many, if not most, deep-sky objects are). Yes, a shorter focal length telescope (likely to have a smaller aperture) would suffice to yield the wider fields of view, but at the sacrifice of magnification necessary to see details in those small objects.

And the wider the apparent fields of view, the larger the spread in the magnifications that can be tolerated because the field size doesn't seem to shrink so dramatically when shortening the focal length. Three eyepieces and a barlow can yield all the magnifications you need.
Narrower fields have to have their magnifications closer together to be able to come up with a sufficient field size for the objects. Paradoxically, expensive hyper-wide field eyepieces may form a cheaper collection than a case filled with narrow field eyepieces, looked at from that standpoint.

To be fair to Jeff, he is not advocating for the use of only narrow field eyepieces. And, whereas we differ in how much context we'd like to see when framing an object being observed, we agree on the idea of framing. I spend a lot of time with other observers at a popular observing site, and this is a lesson that is not well-known. I see most observers observing at too low a magnification to see details in the object being viewed--a 6' galaxy in a 1 degree field. Certainly there is a place for this (as when trying to see several galaxies in Markarian's Chain in Virgo), but not if one wants to see all that can be seen in the scope.

So kudos, Jeff. Thanks for bringing up the idea of framing and explaining it. I hope it becomes a commonly-used technique of observing.

#5 semiosteve

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Posted 16 July 2013 - 02:56 PM

Jeff,

Thank you for capturing something I've tried to articulate informally in various posts over the years.

For me, what I am looking for is a sort of maximum aesthetic experience for each object. Framing is certainly part of that. So is darkness/blackness and tight, tiny stars. And it varies by class of object, so globulars seem to glory in larger FOV's more than other DSO's for example.

Bottom line is I find I need a variety of EP's that optimize different parts of that experience so I can go from wide framing to tight framing or darker sky, tighter stars, etc.

I think the correct parallel here would be to how photographers view their arsenal of camera lenses.

sv

#6 Bill Steen

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Posted 16 July 2013 - 05:55 PM

I almost grew up in a portrait studio and spent a number of years persuing photography as a passion and a sometimes profession. I am personally burned out on the photo side of things and do not to astrophotography, but I cannot keep from wanting to get each object framed in the eyepiece just like I would have with a camera. Personally, I might differ with the author on exactly what size I might want in the eyepiece, due to differences in personal taste, but I think he is spot on with what he writes.

Again, others will disagree with him...personal taste. But, to me, each eyepiece and each telescope is a tool that is used to get some particular view just right. To me, a really wide angle eyepiece certainly has its place and can be wonderful and versitile. But all the others have their place also, depending on the circumstances.

I think this is a good article that makes me think a little more about what I am doing. I enjoyed reading it.

Bill Steen

#7 Matt Wallin

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Posted 16 July 2013 - 06:21 PM

One failure of this analysis is the short shrift given to the value of observing multiple object in the same field, for easy comparison and contrast or just aesthetic reasons. Whether you are doing variable star comparisons, or just want to see multiple DSO's in context, without having to use a large exit pupil eyepiece, hyperwides have their place. While I appriciate your attempt to do a data-driven analysis regarding some aspects of the usefulness of these eyepieces, I do not consider the wowed and immersive feeling that I get using these eyepieces to be marketing hype or internalized groupthink.

#8 faackanders2

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Posted 16 July 2013 - 08:44 PM

When there is a deep sky object in the field, there is more than mere framing going on.

Personally, I find Jeff's 75% rule to be unaesthetic. I would say an object width of 50% is more pleasing for the view because it gives more context for the object.

Context is the nature of the immediate neighborhood of the object. Whether or not there are other nearby objects that form nice pairings or groups with the object is not only nice to know--sometimes it is nice to see those other objects at the same time.

But just because it is nice to see those other objects at the same time and in the context of closeness to the object in study doesn't mean we necessarily want to use such a low power that the details of the object disappear with the low power.

It can be nice to see a couple of close objects in the same field at the same time but with sufficient magnification to actually see them well--at the correct power to observe details in those objects. To not only see the objects in context but to also see them with the right amount of magnification, like M13 and NGC6207 in the same field at 180X.

Now if you want to view the objects in context and with enough power to observe them well, where does that lead? To large apparent fields of view.

But this doesn't only apply to groups of objects. Say I want to view an edge-on galaxy like NGC891, which is an edge-on galaxy in Andromeda. This object's maximum length is about 13'. If I stay with my 50% rule (and with this object the field is magnificent), I will want a field about 26' wide. But, experience with this object tells me that a magnification over 200X is necessary to show details in the dark lane and show irregularities in the interior of the galaxy. I can easily achieve that with an 8mm eyepiece having a 100 degree apparent field. It would take a 16mm eyepiece with a 50 degree field to see the same size field, but then the magnification would be too low to see the details that can be seen at higher power.

OK, NGC891 is a larger object. What about, say, M75, the globular in Sagittarius? It's only 6.8' wide. Well, like most globulars, the stars are faint. The brightest members hover around magnitude 14.6 and the horizontal branch is clustered around magnitude 17.5. How do I see as many stars in this cluster as possible? The answer: high power. At 300X, in good seeing, this cluster appears to resolve at least partially in my 12.5" and provide a view of stars all over the cluster. It begins to resemble a globular and not a faint nebula. How can I use sufficient power to see the faint stars and still see sufficient field to note the shape of the outlying stragglers and to see a nice context for the object?
A wide apparent field.

Jeff's article points out that most deep-sky objects aren't that large. That's true. But you only need a 30' field to see the whole Moon, while a 1 degree field gives a much more pleasing view of the full moon. In fact, it could be argued an even larger field is more aesthetically pleasing.

Advertising hyperbole aside, there is a valid reason for wider apparent fields with excellent correction. They do a better job of framing the objects we view, and of showing the field around the object (its context) with sufficient magnification.

It's not a matter of "drinking the Kool-Aid" or "falling for advertising hype", its a practical matter of wanting to use higher powers to study and view those, as Jeff points out, largely small objects and still see them in a field size sufficient to show context if they are grouped with other objects (and many, if not most, deep-sky objects are). Yes, a shorter focal length telescope (likely to have a smaller aperture) would suffice to yield the wider fields of view, but at the sacrifice of magnification necessary to see details in those small objects.

And the wider the apparent fields of view, the larger the spread in the magnifications that can be tolerated because the field size doesn't seem to shrink so dramatically when shortening the focal length. Three eyepieces and a barlow can yield all the magnifications you need.
Narrower fields have to have their magnifications closer together to be able to come up with a sufficient field size for the objects. Paradoxically, expensive hyper-wide field eyepieces may form a cheaper collection than a case filled with narrow field eyepieces, looked at from that standpoint.

To be fair to Jeff, he is not advocating for the use of only narrow field eyepieces. And, whereas we differ in how much context we'd like to see when framing an object being observed, we agree on the idea of framing. I spend a lot of time with other observers at a popular observing site, and this is a lesson that is not well-known. I see most observers observing at too low a magnification to see details in the object being viewed--a 6' galaxy in a 1 degree field. Certainly there is a place for this (as when trying to see several galaxies in Markarian's Chain in Virgo), but not if one wants to see all that can be seen in the scope.

So kudos, Jeff. Thanks for bringing up the idea of framing and explaining it. I hope it becomes a commonly-used technique of observing.


+1 Agree 100%. Multiple objects are more enjoyable with wider TFOV - the more you can see in the same FOV the more the merrier. Wider TFOV also allow more days planet conjunctions and or comet/DSO/planet conjunctions are in same TFOV. Those who don't have tracking and find objects manually, wider TFOV and AFOV make it more likely you will actually find the objects, and not miss it while sweeping/hunting the sky. Lastly, manually tracking the ISS is far easier with wider TFOV; once you lose it it may be gone.

Although field stop may be an input yielding to TFOV, many eyepieces only list AFOV and eyepiece focal length. These can back out to TFOV althogh don't know what the discrepancy/error is.

I have tasted the wide AFOV, and am hooked. Unlike photography where framing and cropping with zoom lenses is a big plus; visual astronomy is all about finding these dim objects and wider AFOV and TFOV makes them easier to find (and then you can increase power to the limit, and wider AFOV makes it less likely you will lose the object and have to hunt/search again for it. ;)

#9 russell23

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Posted 16 July 2013 - 10:09 PM

Last night I decided to push my 140 mm refractor to 200x on M-13 using an 8mm Ethos and a 2x ES Barlow. So the TFOV was 0.5 deg and the cluster was well framed. The Ethos provides twice the area of a 68/70 deg AFOV eyepiece which is really beneficial at high magnifications.

I actually find a 1.0 deg TFOV to be a good general purpose field. It is interesting to compare my 30mm Silvertop plossl at 53x, 24mm ES at 67x, and 8mm Ethos at 100x in my refractor - all of which are 1.0 deg and the first two achieved with a 2x ES barlow. The aesthetics change with each magnification and sometimes the lower magnification is preferable - usually with resolved open clusters.

Dave

#10 Jon Isaacs

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Posted 17 July 2013 - 10:28 AM

Jeff:

An interesting read and some interesting stuff, you mentioned some of your findings in various forum posts, I have thoughts:

- Many objects are best viewed in small portions. This is where the framing in not applicable and wide field eyepieces show their value. I am looking at M7 with my 16 inch Dob. Sky Tools 3 tells me M7 is about 1.3 degrees across, greater than the TFoV of the 16 inch with a 31mm Nagler and Paracorr. I can still pretty much see the entire object but I want to use greater magnification because inside M7s boundries are small open clusters as well as a globular cluster. To show these at their best, I need to increase the magnification to a point where I can only see a small portion of M7. The appropriate magnification is a function of exit pupil, a wider AFoV allows me to see a great portion of M7 in that single field of view.

- It's true that most objects are small but larger objects are in general more interesting, there is more to see. A statistical approach might look at the total area of an object rather than the simply counting the number. M31 has a lot more to see than NGC6768.

- The objects on the NGC list were discovered by observers without access to modern short focal length telescopes and eyepieces with large fields of view. The size scale of what you find is directly related to the size scale of what you can see.

- Putting it all together, the largest object we look at has no NGC or Messier number, it's called the Milky Way... it's quite interesting on any scale, be it naked eye or a tiny exit pupil. It cannot be framed except naked eye. For the advocate of the widest possible field of view, observing the large scale features of the Milky Way is of great interest but most are missed in any tabulation based on NGC discoveries.

- I think of eyepieces as matched sets, much like the gears in a transmission. I want a gear that will allow me to climb the steepest hill I can possibly climb, I want a gear that will allow me to go as fast I can possibly go and I was a reasonable number of gears in between. That way I have the right gear for every imaginable situation.

For my eyepieces, I have a number of telescopes, I want a set of eyepieces that works nicely with them all. So I want an eyepiece that will provide the widest possible field of view, I want an eyepiece that will provide the maximum possible magnification/smallest exit pupil I will ever use and I want enough eyepieces in between so that I will have the right eyepiece for any given situation.

Just some thoughts...

Jon

#11 Jeff Morgan

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Posted 18 July 2013 - 01:24 AM

I feel like I had my say on the topic via the article, but a new consideration has come along, and I wanted a few days to think it through - the Multiple Object Field. Here is the perfect segue:

+1 Agree 100%. Multiple objects are more enjoyable with wider TFOV - the more you can see in the same FOV the more the merrier. Wider TFOV also allow more days planet conjunctions and or comet/DSO/planet conjunctions are in same TFOV. Those who don't have tracking and find objects manually, wider TFOV and AFOV make it more likely you will actually find the objects, and not miss it while sweeping/hunting the sky. Lastly, manually tracking the ISS is far easier with wider TFOV; once you lose it it may be gone.


I'm with you to a point on this. That being, I have always advocated one should start building an eyepiece collection around the anchor of the set - the low power eyepiece. This of course is the eyepiece that maximizes field stop in a given barrel size (and unfortunately, usually costs the most). Something with a field stop of at least 38 mm in a 2" barrel, which leaves some allowance for considerations of expense, weight, exit pupil and of course optical performance. For a given scope this is your peak framing performance - everything else is a big step down!

So on to the Multiple Object Field. If I am understanding the claim correctly, it would be that you have found and enjoyed a multiple object field (M65, M66, and NGC3628 in Leo perhaps?) with the max true field eyepiece (for sake of example, let's say a 31 Nagler). Now you want more detail. More magnification. Smaller exit pupil. Really, three ways of saying the same thing.

So, you have two choices in your eyepiece case. A 20 Nagler and 21 Ethos. You try the Nagler first and find that you can't squeeze all three objects into the field. Sadness. But wait! You have the 21 Ethos. So you pop in the magical 100 AFOV eyepiece and viola! There are all three objects!

If I am understanding your claim, you have succeeded in seeing them better. My comments:

1) Parking any object at or near the field stop to see it better is a rather unorthodox observing strategy. If you're telling me you have it in the center and can see the other two on the field edge, I'm telling you that you could have succeeded with the Nagler.

2) Unless you have supra-human physiology, you are only going to see one of those three galaxies better at one time. Sure, you can "see" the others separated by 80+ degrees of apparent field (remember, the Nagler didn't work!). But only as peripheral objects without fine detail. Seeing all three (or any two) sharply at the edges of such a wide field is not supported by the eye. Unless they are close together, to see detail in any single one you must shift your vision to them individually - at which point you might as well be using the Nagler - or a Plossl. IIRC, one of the selling points of Dobs is that they nudge pretty easily.

Now, a better example of what you seek would be an object like NGC7000 (the North American Nebula). Here you have a large object with small imbedded objects (two open clusters and numerous dark nebula). In this case the extra true field gives you the black surrounding sky such that the nebula does not "disappear" for lack of context when you start getting interested in resolving the clusters.

Bottom line? Liking an eyepiece is enough of a reason to buy it in my book. It really is. But under the marketing influence of the manufacturers, take a cold hard look at the nature of the targets you look at - and your own physiology.

#12 russell23

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Posted 18 July 2013 - 07:49 AM

I've seen people express different approaches on when the want the 82/100 deg AFOV. Some people want the arrest AFOV at low mag and are ok with smaller AFOV at high mag. I prefer the opposite approach. I like the ultra wides at high mags but am good with the 48 deg AFOV of my 48mm Brandon at low mags.

Dave

#13 Jeff Morgan

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Posted 18 July 2013 - 08:54 AM

I've seen people express different approaches on when the want the 82/100 deg AFOV. Some people want the arrest AFOV at low mag and are ok with smaller AFOV at high mag. I prefer the opposite approach. I like the ultra wides at high mags but am good with the 48 deg AFOV of my 48mm Brandon at low mags.

Dave



As I started doing this project it got me to thinking that a short Ethos (perhaps the SX) would be a very important tool. In the high magnification regime the numbers become compelling. That is to say, when you hit the peak of the DSO size distribution chart. On the high side of the peak the performance difference is rather small.

#14 galexand

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Posted 19 July 2013 - 10:35 AM

Jeff, thanks for the great article! I really enjoyed your approach.

I happen to have taken issue with nearly everything you said, though. :p My two day-to-day instruments are 400mm and 750mm, rich-field and multiple-object framing are important to me, and I'm with Jon Isaacs that I like very wide (say, 50%) framing. And you say we always increase the magnification if the object fits and the seeing isn't atrocious, and it is like reading a letter from my penpal on Venus...the thought bubble opened over my head, "when is seeing *not* atrocious?!"

And of course probably the biggest nitpick is that even though most objects are quite small, my favorites are often large, some are even binocular objects.

But I'm a cheap eyepiece aficionado, I accept a lower magnification to get a wider view (and I don't mind squinting through a short plossl when the seeing does line up), and I think you're spot on that finding fun ways to look at it is more important than finding new equipment to look through. Thanks for something fun to think about.

#15 Jeff Morgan

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Posted 19 July 2013 - 03:28 PM

Jeff, thanks for the great article! I really enjoyed your approach.

I happen to have taken issue with nearly everything you said, though. :p My two day-to-day instruments are 400mm and 750mm, rich-field and multiple-object framing are important to me, and I'm with Jon Isaacs that I like very wide (say, 50%) framing. And you say we always increase the magnification if the object fits and the seeing isn't atrocious, and it is like reading a letter from my penpal on Venus...the thought bubble opened over my head, "when is seeing *not* atrocious?!"

And of course probably the biggest nitpick is that even though most objects are quite small, my favorites are often large, some are even binocular objects.

But I'm a cheap eyepiece aficionado, I accept a lower magnification to get a wider view (and I don't mind squinting through a short plossl when the seeing does line up), and I think you're spot on that finding fun ways to look at it is more important than finding new equipment to look through. Thanks for something fun to think about.


Glad you enjoyed the article Greg. With those two telescopes I should think you can "frame" with just about any eyepiece!

I wouldn't get to hung up on the percentage (width) of the frame. It's just a personal choice with no set answer. I never even really thought about specific numbers until I started the article. Thinking back to how I observe, if the target is 50% of the field I usually want at least one more jump - but that is just me.

Changing your preferred framing percentage does not affect the marginal results until you get into the "fat" area of the size distribution curve - call it 10 arc minutes give or take a little bit. At that point instead of the Ethos SX providing the best "bang for the buck" that title might shift upwards to perhaps the 6 or 8 Ethos. In the medium and long eyepiece focal lengths its likely that a Nagler will still be about 5% "better" than a Plossl and an Ethos about 3% "better" than a Nagler.

#16 astrophile

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Posted 20 July 2013 - 11:54 AM

Interesting and thought-provoking article Jeff, thanks. It has made me consider more closely why or why not I might wish to spend more money for more FOV at a given focal length.

That said, I think a serious drawback to your data-driven assessment is that the numerical results rest on an assumption that all objects have equal attraction and viewing time/frequency. This is most certainly not true for me; it's precisely those "few" large and medium-expanse outliers (e.g. Veil, North American, Rosette, M42/43, Epsilon Lyrae, Double Cluster) that I want to come back to time & time again, and for longer viewing periods. Without a personalized weighting of each underlying data point (or categories thereof), IMHO the pure numbers don't tell an accurate story.

Thank you again, though, for taking the time for such an interesting study.

#17 derangedhermit

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Posted 20 July 2013 - 10:05 PM

Of the 2405 objects, three groups (OCs, BNs, DNs) total about 1050 of those objects, or about 44% of the total. Each of the three groups average 24 arcmin in size. When I look at the top chart, it does not look like it contains 1000+ objects of 24 arcmin average size.

And when I look at your tables, the "frame field" for that average would be 32 arcmin. Closest to that is the low power 2800mm fl scope with the 21 Ethos, giving a frame field of 33.3. The chart says that includes 2,215 objects. That leaves max 190 objects above that 24 arcsec to weight things to get to the average.

I'm no mathematician, and I'm sure it's possible; but it seems worth asking about. Could you please clear this up for me?

Lee

#18 Jeff Morgan

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Posted 22 July 2013 - 04:06 PM

Of the 2405 objects, three groups (OCs, BNs, DNs) total about 1050 of those objects, or about 44% of the total. Each of the three groups average 24 arcmin in size. When I look at the top chart, it does not look like it contains 1000+ objects of 24 arcmin average size.

And when I look at your tables, the "frame field" for that average would be 32 arcmin. Closest to that is the low power 2800mm fl scope with the 21 Ethos, giving a frame field of 33.3. The chart says that includes 2,215 objects. That leaves max 190 objects above that 24 arcsec to weight things to get to the average.

I'm no mathematician, and I'm sure it's possible; but it seems worth asking about. Could you please clear this up for me?

Lee


I'm no mathematician either and I'm not sure I understand the question, but I'll take a shot at it.

Your numbers are close. Remember, on the nebula list I combined the small NGC designations that make up the Rosette and Veil because we all think of them as a large target, not several small components. Here is the breakdown:

- 541 open clusters, average size 24.5
- 150 bright nebula, average size 24.6; and
- 349 dark nebula, average size 24.3

These total 1040 which is 43.2% of 2405.

So you're asking if a scope with a 33.3 arc minute field frames 2215 objects, how can the remaining 190 objects pull the average up so much? Let's talk the case of two open clusters: M7 (one of my favorites) at 80 arc seconds and NGC1931 at 1 arc seconds. M7 gets to "vote" 80 times for single "vote" that poor NGC1931 gets. (If you are from Chicago, I'm sure you can get this concept immediately and instinctively.)

Averages work well with "normally" distributed data. However, the size distributions of the bright DSO's are strongly skewed. An oversight on my part, it would have been better to talk about Median values. The median divides (or bins) the population into two groups - the 50% above and 50% below. Looking at those numbers:

- 541 open clusters, median size 11.0
- 150 bright nebula, median size 9.5; and
- 349 dark nebula, median size 13.0

One could further divide for more "resolution" such as quartiles, quintiles, etc. What I did to generate the graphs was give the Frequency function in the spreadsheet a resolution of 1 (one arc second).

To keep the article length reasonable and avoid repetition I did not provide a graph for each object class. I'll attach a combined graph here for your examination. Each category essentially looks the same with the notable exception of Dark Nebula. Here one can see an interesting spike every 15 arc seconds. No doubt an artifact of the method professional astronomers used to calculate the sizes from photographic plates. Aside from this oddity, each object class is quite similar in distribution, all have quite low median values.

There has been some suggestion that the objects should have been weighted by time spent observing. That is to say, something like M42 should have 10x (or more) the weight of little-known IC 922. I understand the sentiment totally - I usually start and end the night on a few showpieces. If you do outreach, it might be all you do, and even then the General Public loses interest quickly once you get past Saturn, M42, or M13. The issue is, the weighting would be different for all of us. How do you determine it? To some extent, I did "weight" the sample with a magnitude 12 cut-off.

There is nothing proprietary in the data - astro planning software, databases, and spreadsheets are there to be had by all. Try it. If you want the Best and Brightest (and larger sizes) all you need to do is adjust the magnitude cut-off. Try magnitude 8. The resulting size distribution curves will start shifting where you want them. Your observing list will also get very small very fast. IIRC, one of the criticisms that people using DSC or GOTO systems have of traditional star-hoppers is that they look at the same 50 (or so) objects all the time.

Attached Files



#19 derangedhermit

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Posted 22 July 2013 - 09:43 PM

Thanks for the clarification. I figured the median was much lower than the mean, but it wasn't apparent it was so by inspection.

Try magnitude 8. The resulting size distribution curves will start shifting where you want them.



I don't "want" them to be anywhere in particular. I try to not make assumptions about what others are thinking.

If you add in the LDN catalog, you get different results; but I wouldn't, since I don't think many people want to observe 2,000 dark nebula.

Some specific things I notice in the tables:
Low power
1700mm + 35Pan ff = 34.6 and 2800mm + 35 Pan ff = 34.6?
High power
1000mm + 3.7E of = 2021 and 2800mm + 3.7E of = 2021?
There are some more that seem worth double-checking.

Not that it changes anything, but I think the most common f.l. are:

400mm-750mm (many 4" and under refractors, a few 4.5" f/4 Newtonians)
750mm (6" f/5)
1000mm (4" f/10, 4.5" f/9, 120mm f/8)
1200mm (6" f/8, 8" f/6, 10" f/4.8)
1500mm (12" f/5)
1800mm-2000mm (8" f/10, 15" f/5, 16" f/4.5)
2800mm (11" f/10)

I think that a low percentage of telescopes in use except SCTs have focal lengths of 2000mm or more.

Some consideration of exit pupil should be included in this. I am not sure that a 3.7mm focal length ep in an f/10 system is useful very often (0.37mm exit pupil). (This improves the strength of your argument for this case).

If you exclude the LDN catalog, then 90%+ of the objects available to view in moderate or large apertures are smaller than half a degree (30 arc min). Even then, about half the objects over 30 arcmin are Barnard objects.

Over 50% of objects are smaller than 5 arcmin. Over 75% are smaller than 10 arcmin. About 10% of those 2400 objects are 1 arcmin or less (according to SkyTools). So a case could be made, and I think has been made, that your main instrument should have a useable field of view of 30 arcsec.

It makes sense to me to buy one eyepiece that gives you the widest field that your telescope can provide and that matches well with your eye; the latter part largely determines how "wide field" that eyepiece needs to be. For an f/10 SCT, that 55mm Plossl may be ideal. That same eyepiece is not ideal for an f/5 Newtonian; a 26mm or 31mm Nagler or 35 Pan may be, depending on your eye.

I want an eyepiece that gives me the maximum magnification under the best skies I anticipate, and one that gives me the maximum magnification under my typical conditions. These need to be around the 1mm exit pupil range, probably one somewhat below and one above. Wide fields and good eye relief are very nice, here.

Other than those 3, it seems more a matter of taste and funds; that's 3 eyepieces, and either one or two more seems like enough to make a complete set.

The attraction for a lot of people for premium ($$$) eyepieces may be the very wide AFOV. For me, the fundamental appeal is that it seems that things like excellent performance at low focal ratios and availability of consistent adequate or long eye relief, independent of the focal length of the eyepiece, are only available in these lines of eyepieces, and are, for me, more important factors.

Lee

#20 Jeff Morgan

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Posted 23 July 2013 - 11:56 PM

Some specific things I notice in the tables:
Low power
1700mm + 35Pan ff = 34.6 and 2800mm + 35 Pan ff = 34.6?
High power
1000mm + 3.7E of = 2021 and 2800mm + 3.7E of = 2021?
There are some more that seem worth double-checking.


:foreheadslap:

Yep, you found a couple of copy and paste errors. (Linking spreadsheets to tables in word processing docs is a little advanced for me).

1) The frame field of the 35 Panoptic barlowed down to 21 mm should read 21.0 arc seconds. That was the only error in that row, number and percentage of objects framed is correct.

2) The 3.7 Ethos SX in the Large scope should have a frame field of 6.5 arc minutes, 1343 objects framed, for 55.8% of all objects. Despite the lower number, it still does largely better than a Nagler barlowed to 3.7 mm focal length (14.5% better on the object count).

You're right, things didn't change much but I'd rather the errors not be there. I'll take another check and see if the mods will let me submit an addendum or corrected version.

Not that it changes anything, but I think the most common f.l. are:

400mm-750mm (many 4" and under refractors, a few 4.5" f/4 Newtonians)
750mm (6" f/5)
1000mm (4" f/10, 4.5" f/9, 120mm f/8)
1200mm (6" f/8, 8" f/6, 10" f/4.8)
1500mm (12" f/5)
1800mm-2000mm (8" f/10, 15" f/5, 16" f/4.5)
2800mm (11" f/10)

I think that a low percentage of telescopes in use except SCTs have focal lengths of 2000mm or more.

Some consideration of exit pupil should be included in this. I am not sure that a 3.7mm focal length ep in an f/10 system is useful very often (0.37mm exit pupil). (This improves the strength of your argument for this case).

If you exclude the LDN catalog, then 90%+ of the objects available to view in moderate or large apertures are smaller than half a degree (30 arc min). Even then, about half the objects over 30 arcmin are Barnard objects.

Over 50% of objects are smaller than 5 arcmin. Over 75% are smaller than 10 arcmin. About 10% of those 2400 objects are 1 arcmin or less (according to SkyTools). So a case could be made, and I think has been made, that your main instrument should have a useable field of view of 30 arcsec.

It makes sense to me to buy one eyepiece that gives you the widest field that your telescope can provide and that matches well with your eye; the latter part largely determines how "wide field" that eyepiece needs to be. For an f/10 SCT, that 55mm Plossl may be ideal. That same eyepiece is not ideal for an f/5 Newtonian; a 26mm or 31mm Nagler or 35 Pan may be, depending on your eye.

I want an eyepiece that gives me the maximum magnification under the best skies I anticipate, and one that gives me the maximum magnification under my typical conditions. These need to be around the 1mm exit pupil range, probably one somewhat below and one above. Wide fields and good eye relief are very nice, here.

Other than those 3, it seems more a matter of taste and funds; that's 3 eyepieces, and either one or two more seems like enough to make a complete set.

The attraction for a lot of people for premium ($$$) eyepieces may be the very wide AFOV. For me, the fundamental appeal is that it seems that things like excellent performance at low focal ratios and availability of consistent adequate or long eye relief, independent of the focal length of the eyepiece, are only available in these lines of eyepieces, and are, for me, more important factors.

Lee


Good points. The issue with trying to select the most common or significant scopes in use is that there is a wide spread leading to lots of repetitive charts and tables to cover the permutations, and I wanted to keep the article on the shorter side. In your list you give about seven focal lengths. Pick any three specific models and the owners of the other four types cry foul.

To get around this I "invented" three telescopes: small, medium, large.

Additionally, I was taking a "clean slate" approach before building or buying. Supposing an individual had specific targets in mind, what kind of focal length (telescope and eyepiece) would frame them? With the clean slate one can truly take an object-based approach, and then work out the details like aperture, focal ratios, and exit pupils.

In many (most) instances people start with a telescope first. The scope choice is made based on constraints such as budget, weight, and logistics. For those that have already bought or built a scope of course the choices of aperture and focal ratio are set and the goal becomes the best eyepiece choice given what we already have to work with.

And certainly it is not always about total field. For example, I prefer medium to long focal ratio scopes. While I could use a 41 Panoptic with it's huge 46 mm field stop, I have always preferred the 31 Nagler with only a 42 mm field stop despite giving up about 10% true field. It is still a respectable star hopping eyepiece, and I did not feel there were many objects I was missing out on. And in looking at the sorted object list this turned out to be very true.

Having covered the role of star hopper, what to do in the medium range?

It was not until I plotted a frequency distribution of bright DSO's that I could put into words what I always felt: not only were the low-power choices not operating in the heart of that distribution curve, but none of the medium-power choices were either!

Most of us are equipment geeks, but it's really all about understanding the targets. Through analysis or experience most of us figure out that the broadest capability comes from the scopes (or binoculars), not the eyepiece.

#21 derangedhermit

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Posted 24 July 2013 - 03:31 AM

So are you saying maybe I want a high-power eyepiece to give a well-framed view(whatever I choose that to be) for the 25% smallest things, then the next eyepiece for the next 25%, the next eyepiece for the next 25%, and one more to cover whatever else I care to try for (wide TFOV), since I can't get 100%?

Say I want 50% framing - bear with me. Then the TFOVs and eyepiece focal length and magnification, using 82 AFOV eyepieces, I need for my 12" f/5 scope are:

1.4 degrees (26mm, 58x) Frames most of the largest 25% of objects
20 arcsec (6mm, 254x) Frames 75% of all objects (!)
---
10 arcsec (3mm, 508x) Frames 50% of all objects
5 arcsec (1.5mm, 1016x) Frames 25% of all objects

If objects were bright enough, and skies steady enough, I could probably go along with that. I'd love to be able to use 1000x. For one thing, it would mean I owned at least a 20" scope (at 50x per inch of aperture). But some objects aren't that bright (in surface brightness), and skies aren't that steady.

There is a lot of truth in the line of thought that you only need two eyepieces on a given night: one low-power eyepiece, to enjoy looking around in general, and maybe to find stuff, and one more, giving the highest power the atmosphere will tolerate. I do that on many, if not most objects already, and you have documented the reason why - stuff up there is very small.

I think maybe the strongest message your analysis could carry is that the atmosphere greatly limits us, much more than in the past; since 12"+ instruments are more affordable in recent years than ever before, and most can deliver more than 300x, but the atmosphere rarely allows most of us to use them to full advantage. Things do get brighter, and that can help bring out contrast at a given magnification, but we all too often can't use the magnification that would increase detail further, or get the increased resolution that the larger aperture provides.

Lee

#22 Thomas Karpf

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Posted 25 July 2013 - 02:13 PM

2) Unless you have supra-human physiology, you are only going to see one of those three galaxies better at one time. Sure, you can "see" the others separated by 80+ degrees of apparent field (remember, the Nagler didn't work!).


Frankly, that point neatly argues against buying any eyepiece currently on the market. The portion of your field of view that is in FOCUS at any time is less than three degrees.






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