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zoom eyepiece and FOV

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#1 mike in arkansas

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Posted 07 February 2013 - 06:20 PM

Ok-- One baby step at a time here. I received a Meade zoom eyepiece(8-24) for my Dob 6". I wanted to play with different fl eyepieces to see the difference. (still too cloudy and rainy to actually look at stars!). At 24 the fov is fairly wide. As I get to the 8mm it is very narrow-- which seems understandable. My question is--if i get a stand alone 8mm (or thereabouts) will the fov still be very narrow? Does a Wide fov eyepiece help correct that?

Just askin (before I spring for single eyepieces)

#2 rflinn68

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Posted 07 February 2013 - 06:47 PM

What is the apparent fov of your zoom? If you like the 8mm range the best one for the money, IMHO, is the Explore Scientific 82 degree 8.8mm

Which dob do you have? What is the focal length? If it is the Orion dob with a 1200mm focal length the 8.8mm will give you 136X magnification and about a 0.60 degree true field of view which is a little bigger than the full Moon. For a comparison, a 14mm plossl eyepiece with a 52 degree afov will give you about the same fov as the ES82 8.8mm but the magnification will only be about 85X. Hope this helps.

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#3 lamplight

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Posted 07 February 2013 - 06:56 PM

Hi Mike and welcome

I keep this formula handy myself as its not committed to memory myself yet. First determine the power (or magnification), then your actual field of view is determined:

POWER = scope Focal length / Eyepiece focal length

FOV = AFOV / POWER (field of view and "apparent" field of view)
ex: with "31mm ES 68º" in 1000mm scope = 68 / (1000 / 31) = 2.11º TFOV
ex: with "12mm AT ED 60º" in 1200mm scope = 60 / (1200 / 12) = .6º TFOV   

Generally higher magnification is going to be narrower field of view, its only natural with the same scope. But some eyepiece designs can offer a wider field.. Hopefully this explains how to figure out what you'll,actually see when buying, say, a 100° AFOV EP.

#4 Jon Isaacs

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Posted 07 February 2013 - 08:23 PM

Ok-- One baby step at a time here. I received a Meade zoom eyepiece(8-24) for my Dob 6". I wanted to play with different fl eyepieces to see the difference. (still too cloudy and rainy to actually look at stars!). At 24 the fov is fairly wide. As I get to the 8mm it is very narrow-- which seems understandable. My question is--if i get a stand alone 8mm (or thereabouts) will the fov still be very narrow? Does a Wide fov eyepiece help correct that?

Just askin (before I spring for single eyepieces)


Mike:

It is important to distinguish between the True Field of View (TFoV) i.e the size of the patch of sky you see, and the Apparent Field of View (AFoV), the apparent angle you see when looking through an eyepiece.

The Meade 8mm-24mm Zoom is a standard Japanese zoom so the AFoV is about 40 degrees at 24mm and increases to about 60 degrees at 8mm. This means while you see a larger part of the sky at the 24mm focal length and you see a smaller part of the sky at the 8mm focal length, the field appears larger at 8mm.

One can estimate the TFoV (True Field) if one knows that magnification. If you 6 inch Dob has a focal length of 1200mm, then the magnifications are:

1200/24mm = 50x

1200/8mm = 150x.

To estimate the True Field of view, one can use the following equation:

TFoV = AFoV/Magnification

This makes intuitive sense, if you do not magnify the image, then the True field of View is the same size as the Apparent Field. If increase the magnification by a factor of two, then the True field is half as large.

So the 24mm with it's 40 degree AFoV at 50 x:

TFoV = 40 deg/50x = 0.80 degrees

At 8mm with it's 60 degree AFoV at 150x:

TFoV = 60 deg/150x = 0.40 degrees

In general, a 40 degree AFoV is considered narrow, a 60 degree AFoV is probably a bit wider than average. Zooms generally have wider AFoVs at the short focal length end, narrower AFOVs at the longer focal length end.

Jon

#5 mike in arkansas

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Posted 08 February 2013 - 06:19 PM

Ok-- this has been very helpful -- more stuff to think about (and get out my calculator!). Any thots on the difference between a 1.25 and 2" eyepiece? More important at low magnification or hi?

Again-- incredible help-- thanks and blessings
Mike

#6 dpwoos

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Posted 08 February 2013 - 08:38 PM

The only time you NEED a 2" eyepiece is when you want a very wide-field and low-power view, as a 1.25" eyepiece doesn't have a barrel large enough to accommodate that. I suggest you google "eyepiece field stop", which will provide info that builds on the afov/tfov/fl concepts already provided.

#7 Jon Isaacs

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Posted 08 February 2013 - 09:12 PM

Any thots on the difference between a 1.25 and 2" eyepiece? More important at low magnification or hi?



It's only important at low magnifications.

When you look through an eyepiece, you see the edge of the field of view. What you are seeing is the field stop, a ring located at the focal plane of the eyepiece that actual determines the field of view. If you look backwards down the barrel of a simple eyepiece, a Plossl or something similar, you can actually see the field stop, it's a ring usual close to the bar end of the barrel.

The bigger the ring, the wider the true field of view. The ring needs to fit inside the eyepiece barrel so if the eyepiece needs a larger field stop than is possible with a 1.25 inch barrel, a 2 inch barrel is required. Generally any field stop greater than about 27mm needs a 2 inch barrel. Such eyepieces are longer focal lengths and therefore low magnification.

Does your scope have a 2 inch focuser?

Jon

#8 dpwoos

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Posted 08 February 2013 - 09:41 PM

Kind of like asking if a bigger spoon is better than a smaller spoon. Depends on how big your mouth is, and what you want to put in it! Interesting how some beginners (not necessarily the o.p.) think that somehow 2" eyepieces are "better" than 1.25" eyepieces - bigger is better?

#9 mman22

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Posted 09 February 2013 - 12:00 AM

Okay, please help me understand this. The only purpose for a 2" eyepiece is to allow for a larger AFoV? If you had a 32mm 1.25" and a 32mm 2" with the same AFov (say 52*) there would be no difference in the TFov or the quality of the image (provided the same optics), is that correct?

#10 dpwoos

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Posted 09 February 2013 - 02:12 AM

Correct, no difference besides size. However, if you wanted 31mm with a 82degree afov then you would have to go with 2" - e.g. 31mm Nagler with a 42mm field stop.

#11 Dave Mitsky

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Posted 09 February 2013 - 04:07 AM

Okay, please help me understand this. The only purpose for a 2" eyepiece is to allow for a larger AFoV? If you had a 32mm 1.25" and a 32mm 2" with the same AFov (say 52*) there would be no difference in the TFov or the quality of the image (provided the same optics), is that correct?


An apparent field of view of 52 degrees, in this case, won't work as an example, since a 1.25" 32mm Plössl eyepiece can't produce more than about a 49 to 50 degree AFOV. That's all its 27mm field stop will allow. The AFOV would continue to shrink with increasing focal length, if excessively long focal length 1.25" eyepieces (i.e., longer than a 40mm Plössl, which has an AFOV of 43 degrees) were available.

AFOV is determined by the eyepiece design and ultimately by focal length, which is related to the size of the field stop. (AFOV is actually the angular diameter of the field stop projected into space.) True field of view (TFOV) is determined by telescope focal length and the diameter of the field stop and is approximated by dividing AFOV by the magnification produced by that eyepiece. True field of view is actually equal to the eyepiece field stop diameter divided by telescope focal length times 57.3.

A 2" eyepiece produces a larger TFOV, since it has a bigger barrel and hence a larger potential field stop, up to 46mm, than an eyepiece having the same AFOV but with a 1.25" diameter barrel and a smaller field stop. The larger field stop of a 2" eyepiece allows for longer focal lengths, which mean lower magnifications, and, therefore, larger true fields of view.

link

Dave Mitsky

#12 dpwoos

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Posted 09 February 2013 - 09:50 AM

There are many, many threads and posts about measuring afov, tfov, field stops etc. Given that this is the "Beginners" forum, I wonder if a discussion of the fact that things get questionable around the maximum field stop might be better left for later.

The gist of the o.p.'s post is correct, which is that if two eyepieces have the same specs except that one is 2" and the other is 1.25", then the views (tfov and magnification) will be the same.

#13 Jon Isaacs

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Posted 09 February 2013 - 10:08 AM

There are many, many threads and posts about measuring afov, tfov, field stops etc. Given that this is the "Beginners" forum, I wonder if a discussion of the fact that things get questionable around the maximum field stop might be better left for later.

The gist of the o.p.'s post is correct, which is that if two eyepieces have the same specs except that one is 2" and the other is 1.25", then the views (tfov and magnification) will be the same.


Mike did ask about the difference between 1.25 inch and 2 inch eyepieces and whether they were more important at low magnifications or high.

In the beginners forum, my goal is to help someone understand the fundamentals, to not only answer their individual questions but also to provide an explanation on an appropriate level of the fundamentals behind the answer.

There are certainly many threads about TFoV, field stops, the errors and drift timing etc, etc, but few are written with the beginning astronomer in mind.

Jon Isaacs

#14 dpwoos

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Posted 09 February 2013 - 10:57 AM

I agree that it is a fine line between good information and too much (still good) information. In this forum, I tend to err on the side of less (especially as the op writes "one baby step at a time"). Anyone can do a google search and come up with as much detail as they want, including previous CN threads, and in fact I find myself (at work, at school) increasingly focused on providing sufficient info to do a web search and so educate oneself. Of course, you might like to strike a different balance.

#15 Dave Mitsky

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Posted 09 February 2013 - 12:58 PM

Put simply, for a given magnification, a larger AFOV (produced by a more complex design) means a larger TFOV. Using a longer focal length eyepiece also means a larger TFOV, except at the point where the maximum field stop becomes an issue.

Dave Mitsky

#16 Jon Isaacs

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Posted 09 February 2013 - 05:58 PM

Of course, you might like to strike a different balance.



Exactly.

I figure that most do a web search first before getting up the courage to post a forum like this and need someone to help them figure things out. There are many excellent webpages but Cloudy Nights takes it a step further and provides us with the opporunity for "face to face", personalized contact.

Ideally, everyone would have an knowledgeable, experienced friend to help them out, show them the ropes. Unfortunately, this is a small hobby so most don't have that special friend. My goal is to try to fill that void.

Jon Isaacs

#17 mman22

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Posted 09 February 2013 - 06:29 PM

Of course, you might like to strike a different balance.



Exactly.

I figure that most do a web search first before getting up the courage to post a forum like this and need someone to help them figure things out. There are many excellent webpages but Cloudy Nights takes it a step further and provides us with the opporunity for "face to face", personalized contact.

Ideally, everyone would have an knowledgeable, experienced friend to help them out, show them the ropes. Unfortunately, this is a small hobby so most don't have that special friend. My goal is to try to fill that void.

Jon Isaacs


Jon, that is exactly why us newbs come here, for the personalized and friendly advice. In case we forget to say it sometimes, it is greatly appreciated. My post was to both verify I was understanding correctly and to boil it down to its simpliest to help others understand (an annoying trait I have had since junior high). Ultimately I hope to post a thread which will summarize all I have (and will have) learned about the core principles of the hobby. My goal will be to both confirm my understanding and translate it from the crowd of "when determining the suitable ep one must take into account its AFov and how it translates to TFov as well as its eye relief, f/l when compared the telescope's f/l..." to more of the "I like stars, can I look at stars too" crowd :p

#18 Scott in NC

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Posted 09 February 2013 - 10:39 PM

My goal is to try to fill that void.

Jon Isaacs

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#19 dpwoos

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Posted 09 February 2013 - 11:33 PM

Ideally, everyone would have an knowledgeable, experienced friend to help them out, show them the ropes. Unfortunately, this is a small hobby so most don't have that special friend. My goal is to try to fill that void.


I'm a math helper at a local K-6 school, and increasingly I find myself motivated not to teach math facts, but rather to give kids the tools and encouragement to teach themselves. Last week I helped a girl who couldn't express .614 as a percent. I asked if she knew what percent 1 was, and she said that it was 100%. I told her that if she knew that, then she could figure out the answer for herself if she thought about it. Getting the kids to think about stuff is the challenge, and I find that often that is the challenge here. In my mind, too much of what I read here is on the order of "tell me what I should buy" and "tell me what is better" and "tell me what I need". I am all for leading folks in the right direction, but finally it is not doing anyone any favours to spoon feed information, especially when (by its nature - not a criticism per se) almost all of it is opinion.

I once assisted in a wood shop class for kids, and the teacher told me to make sure that before I helped anyone I made sure that they asked me a question, and that "help me" is not a question. I have never forgotten that.

Finally, the most bestest advice anyone posting in the Beginners forum can receive is to seek out their local astro club, where (it is likely that) they will find real live astro friends!

#20 Jon Isaacs

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Posted 10 February 2013 - 06:17 AM

I'm a math helper at a local K-6 school, and increasingly I find myself motivated not to teach math facts, but rather to give kids the tools and encouragement to teach themselves....

"help me" is not a question.


My job also involves teaching. I am a researcher at a major university and I work guiding/assisting PhD students, professors, other researchers in their work. These people are motivated, highly intelligent, they know how to learn. I makes the same assumptions about the members of Cloudy Nights, they know how to learn, they just need help.

"Help me" is not a question, it's a request. Anyone who has come to Cloudy Nights in search of answers, who is confused, if I can help them, I will. Sometimes a hooking up with a local astro club is the best thing but quite often, Cloudy Nights, as the biggest astronomy club in the world, can provide the help needed.

Jon Isaacs

#21 lamplight

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Posted 10 February 2013 - 10:35 AM

As a noob, I'm seeing that almost every question I've ever asked has already been answered here 10,000 times. I don't know how you guys have the patience for it but I am very grateful. It's a very involved and multifaceted pursuit. So many things seem to keep coming up that you ok, I) had no idea was even an issue. S anyhow...

I'm trying to use the search button more often ;)

#22 dpwoos

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Posted 10 February 2013 - 10:35 AM

If a PhD student doesn't know how to think and learn on their own then something is terribly wrong. On the other hand, most of the little kids I work with (and a lot of adults) don't know how, and actually seem to be uncomfortable with the activity (at least in math/logic). How does one get from the little kid to the PhD student? I don't think anyone really knows, but for sure somewhere in the trajectory to adulthood some folks learn how to think well, and others don't, and it seems to happen less often in the US then it does in many other countries.

Anybody can say "help me" without any thinking at all, whereas asking a question requires that some consideration/effort has gone into it. To me, there is a big difference and I don't believe anybody is being served by answering stuff like "Do I need an xxx". I would ask why they thought they might need an xxx, and if they don't have an answer then they don't need one. On the other hand, maybe a forum such as this serves a purpose other than acquiring/sharing info, and "help me", "do I need", etc. is an invitation to socialize as opposed to an actual request for assistance? Hmm - I will have to think about that.

BTW, I think the the o.p.'s question about zoom eyepieces and fov is a great one, which is why I answered in the first place.

#23 howard929

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Posted 10 February 2013 - 11:10 AM

This isn't a vocation, it's a hobby. Most if not all who come to it are busy elsewhere in their lives with full time attention. Stealing away some that attention to learn the in's and out's of all of this from square one is a daunting task.

Leaning on others to answer the most basic of questions may not be your way for anyone to learn but when answered, it sure is appreciated. What's missing from your take on this is one never knows just when someone else reaches that Ah-Ha moment. When then say to themselves "How stupid I was. I get it." And I'd hate to think that it wouldn't happen to someone around here sooner rather then later because "help me" isn't allowed.

#24 dpwoos

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Posted 10 February 2013 - 02:33 PM

What's missing from your take on this is one never knows just when someone else reaches that Ah-Ha moment. When then say to themselves "How stupid I was. I get it." And I'd hate to think that it wouldn't happen to someone around here sooner rather then later because "help me" isn't allowed.


Maybe, or maybe rewarding "help me" makes it happen later, if at all? Just to be clear, I'm not saying that anything is or isn't "allowed", but rather am suggesting that instead of everybody responding to these kinds of "help me" questions with their own opinions about what somebody else needs/wants/oughts, that folks instruct and prompt the questioner in ways that produce better, more knowledgable, and more directed questions. It takes some knowledge to ask a good question, and sometimes the best answer is not an answer at all but rather a suggestion on how to get to that more knowledgable place.

#25 Jon Isaacs

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Posted 10 February 2013 - 07:22 PM

Maybe, or maybe rewarding "help me" makes it happen later, if at all? Just to be clear, I'm not saying that anything is or isn't "allowed", but rather am suggesting that instead of everybody responding to these kinds of "help me" questions with their own opinions about what somebody else needs/wants/oughts, that folks instruct and prompt the questioner in ways that produce better, more knowledgable, and more directed questions.



You do it your way, I'll do it mine. I find the questions asked in this forum to be intelligent questions and deserving of answers. As I said, ideally, everyone has a knowledgeable friend to help them understand the concepts and show them the ropes. Unfortunately, most don't. My goal, and I think other more experienced astronomers who post here share this goal, is to be that friend.

Jon Isaacs






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