Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Are there any disadvantages with wide-angle AFOV eyepieces?

Eyepieces Maksutov Beginner
  • Please log in to reply
35 replies to this topic

#1 Vatsumok

Vatsumok

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 21 Sep 2021

Posted 21 September 2021 - 09:13 PM

Hi all,

 

I am new to the hobby and looking to build out a 2 or 3 eyepiece collection for my Maksutov scope.  (More equipment details and intended use details are at the bottom of the post).

 

I have noticed that eyepieces come in a wide range of AFOV angles from 40' in entry-level zooms to 110' super wide.  And based on my reading, it seems like wide AFOV eyepieces show more context at a given magnification and provide an immersive space-walk like experience. 

 

It seems too good to be true, so I wanted to ask:

  1. Are there any downsides to wide AFOV eyepieces (besides cost / weight)? 
  2. Does stretching the view over a wider AFOV make the view dimmer compared to a narrow AFOV piece?
  3. How does a focal reducer / barlow affect AFOV and brightness / contrast?
  4. Wide AFOV eyepieces are expensive.  Should I get one high-end eyepiece and use it with a barlow + focal reducer to have 3 usable magnifications?  Or are there better strategies for covering a range of magnifications?

 

Scope details:

I am using an Orion Maksutov scope with 127mm aperture, 1540mm focal length (f/12.1). 1.25" Focuser.  No electronics.

 

Intended use:

  • I use my scope to see the moon, Jupiter, and Saturn from home since they are not affected much by light pollution and are visible on most nights.
  • I hope to take my scope along on camping trips and start exploring deep space objects from national parks and dark sites.

 

Thank you!

 

 

 


  • Cali likes this

#2 Echolight

Echolight

    Gemini

  • *****
  • Posts: 3,232
  • Joined: 01 May 2020
  • Loc: Texas

Posted 21 September 2021 - 09:23 PM

They cost more. They weigh more.
And over 70 degree, they get progressively more vague around the edges. What I mean is, that with a 70 the whole field will be sharp in your vision. With an 82, your eyes might not be able to focus on the whole field of view at once. And 82’s, the Naglers and clones, often have a need for more critical eye placement. The 70’s are just easier to look through.

And with 100’s, the edges are in the peripheral vision. The extra field provides context. A background if you will, for the object you are viewing in the central field. You can of course, peer towards the edge, and then see it more clearly. 100 degree eyepieces also have a little less eye relief. You’ll need to get close to see the field stop.


Edited by Echolight, 21 September 2021 - 09:33 PM.

  • JOEinCO and Vatsumok like this

#3 Mitrovarr

Mitrovarr

    Skylab

  • *****
  • Posts: 4,282
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 21 September 2021 - 09:24 PM

1. Well, cost/weight is the big one. Large eyepieces also require more elements to make work. So, there is a tiny bit less transmission even in good ones, and they may not always be the absolute sharpest eyepieces available. Really high quality eyepieces help alleviate this. There is also rectilinear distortion (if you looked at a grid it would not have straight lines) but this is pretty unimportant outside of terrestrial use and/or actual science.

 

2. No. You just get more field. It's not meaningfully fainter.

 

3. It's complicated. It doesn't cut down AFOV intrinsically but the system can vignette if not properly designed. There are almost no systems I can think of which successfully combine a 2" eyepiece and a focal reducer. Some combinations of a larger focal reducer (like the F/6.6 SCT ones) and 1.25" eyepieces work without reducing AFOV - it actually depends on the baffle tube diameter in that case. Many barlows work without reducing AFOV, particularly 2" barlows. Brightness and contrast are mostly affected through changing the scope's focal ratio, but not that much beyond that if the reducer/barlow is high quality.

 

4. That's a tricky question. What are you planning to use for a focal reducer? SCT focal reducers kinda work on Maks but they are reducer/correctors that aren't tuned to correct the Maksutov, so they probably introduce some issues (coma?) The focal reducers that screw in the filter threads of an eyepiece are generally not recommended visually.


  • wrnchhead and Vatsumok like this

#4 esd726

esd726

    Surveyor 1

  • *****
  • Posts: 1,551
  • Joined: 30 Sep 2004
  • Loc: Kewanna, IN

Posted 21 September 2021 - 09:31 PM

Nope smile.gif


  • Vatsumok likes this

#5 ButterFly

ButterFly

    Mercury-Atlas

  • *****
  • Posts: 2,691
  • Joined: 07 Sep 2018

Posted 21 September 2021 - 09:48 PM

1- It's harder to keep jerk stars out of the field of view, like Alnitak and Propus.  The bright star can make it harder to see the dim nebulae nearby. See 2 below.

 

2- The magnification determines the surface brightness of the sky.  In a city, the sky is bright.  At a given magnification, the sky is some brightness.  When the AFOV gets bigger, at that magnification, there is more total light getting into the eye from the background.  That can make the pupil smaller as well as provide glare to bleach the rhodopsin that you need for dark adapted viewing.  An unfiltered 31 Nagler was useless in NYC for me in my 120mm f/7.5, the 21 Ethos less so.  The 13 Ethos was just fine.  On the flip side, for a given true field, a wider AFOV is at a higher magnification, and thus has a dimmer background.  A mak has a huge focal length, so this is not a real concern.  A 13 Ethos, for example, is already ~120x.

 

3- Reducers are fairly bad for eyepieces.  AFOV remains the same unless and until there is vignetting and clipping, which is very likely, and it's hard to predict without knowing everything about the eyepiece and reducer and their relative spacing.  Barlows need to be very, very bad for the view to get noticably affected.  A better barlow is advisable, just to help eliminate unwanted reflections away from the eyepiece.  I add barlows to my Ethos fairly often.  A two inch barlow is BIG and heavy and long, so I stick to 1.25" barlows on 1.25" eyepieces.

 

4- Good visual reducers are hard to find.  Just barlow.  At lower powers, steps in true field of view (for framing) are more important.  At higher powers, steps in magnificaiton (for seeing) are more important.  Always consider your eyepiece collection as a set that is intended to work together.  There shouldn't be much overlap in either true field or magnification unless there is a good reason for it.

 

Widefield eyepieces (anything over 80 degrees, really) are try before you buy.  Some people just don't like them.  You may  be one of them!  More generally, a mak is NOT a widefield instrument.  An 80mm refractor around f/6 is a much cheaper way to get more true field.  For your mak, a widefield eyepiece is better for keeping planets and such in the field longer, which is in line with goal #1.  For goal #2, a 13 Ethos is ~120x with a true field of around 0.8 degrees.  A 17 Nagler is ~90x with a true field of 0.9 degrees.  That's not bad at all because there aren't that many things that are much much bigger.  Just pan around to see the other parts of the North America nebula.  APM's 70 degree zoom should be coming out soon, so it may be worth the wait to see how they do, and how they would fit into your set of eyepieces.


  • JOEinCO and Vatsumok like this

#6 vtornado

vtornado

    Aurora

  • *****
  • Posts: 4,545
  • Joined: 22 Jan 2016
  • Loc: 42N 88W

Posted 21 September 2021 - 09:49 PM

How does a focal reducer / barlow affect AFOV and brightness / contrast?

 

The barlow does not affect AFOV, it effects TFOV or total field of view.

AFOV is the angular size of the image presented to your eye.

TFOV is the amount of sky visible in the eyepiece.

 

I use a focal reducer on my C5 SCT with a 32mm eyepiece to expand the TFOV that I can see.

I believe the view is vignetted, meaning that the outter portion of the field is not as brightly illuminated

than the center.  The human eye is somewhat insensitive to this (a camera less so) and I don't

care.  Others are pickier.  I only use this when I have one scope available (like on vacation),

and I want medium power and high power viewing.

 

Using wider field eyepieces will increase the drift time of your target compared to narrow field eyepieces.

You can try a 65 degree eyepiece (expanse clone ) for $30.00.

It will perform well in your mak.


Edited by vtornado, 21 September 2021 - 09:55 PM.

  • Vatsumok likes this

#7 Jethro7

Jethro7

    Vanguard

  • *****
  • Posts: 2,329
  • Joined: 17 Dec 2018
  • Loc: N.W. Florida

Posted 21 September 2021 - 09:53 PM

Hello,

No, not for me, I love Space walking. My TV21Ethos and TV31mmNT5  are two of my personal all time favorite eyepieces. (Perfect for wide field star hopping) There is a caveat, some people seem to experience a form of vertigo using these ultra wides.  I do prefer Tele Vue's, but I am also impressed with the APM XWA 5mm 110° (4.77mm) and XWA13mm 100° eyepieces, ( nice for Lunar and Planetary) they are much cheaper than Tele Vue's and perform IMO about 95% as well.

 

HAPPY SKIES TO YOU AND KEEP LOOKING UP Jethro


Edited by Jethro7, 21 September 2021 - 10:04 PM.

  • esd726 and Vatsumok like this

#8 MeridianStarGazer

MeridianStarGazer

    Voyager 1

  • *****
  • Posts: 11,744
  • Joined: 01 Dec 2013
  • Loc: Southern Idaho

Posted 21 September 2021 - 10:39 PM

Your Mak can use cheaper wide angle eyepieces without edge issues. Your Mak could also use a barlow and reducer. Those won't change AFOV a lot, but the barlow might reduce it a few degrees and the reducer might increase the afov a few degrees. People who know less about optics will disagree since this is not widely known.

Larger AFOV does not dim the image. Raising the magnification with a barlow does.

A mak is not picky about eyepieces like newts and refractors often are with their faster optics.

The main reason for you to get a high end eyepiece is if you want more eye relief at high power.
  • Vatsumok likes this

#9 belliott4488

belliott4488

    Ranger 4

  • *****
  • Posts: 387
  • Joined: 05 Oct 2020
  • Loc: MD, US

Posted 22 September 2021 - 12:20 PM

I don't have anything to add about the virtues of wide-angle eyepieces, but for a counterpoint I'll just offer this:

 

There a people who prefer old-school narrow FOV eyepieces such as the venerable orthoscopic eyepiece, which almost went extinct after the introduction of the Plossl and then all the various wide-angle options. They've made a bit of a comeback, however, especially for those who like to do planetary and lunar observing and appreciate their clarity and lack of coloration (not to mention relatively lower price, when they're available).

 

Mel at AstroBaby wrote this nice article on orthos some years back: https://www.astro-ba...c Eyepieces.htm


  • Vatsumok likes this

#10 ShaulaB

ShaulaB

    Soyuz

  • *****
  • Posts: 3,556
  • Joined: 11 Oct 2012
  • Loc: Missouri

Posted 22 September 2021 - 12:41 PM

Some people absolutely love the ultra wide FOV eyepieces. Some observers hate them. The only way you will know if you benefit from such eyepieces is to actually look through them

If there is an astronomy club near you that holds star parties, see if you can participate. Some folks buy the ultra wide field EPs for big Dobsonians. Some people buy them to show off their wealth. Ask the owners lots of questions and keep an open mind when looking through an eyepiece.

With a long focal length Mak (giving a small FOV) I can understand why you want a wider field of view.
  • JOEinCO, Jethro7 and Vatsumok like this

#11 Vatsumok

Vatsumok

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 21 Sep 2021

Posted 22 September 2021 - 07:40 PM

Hi all, thank you so much for the insights and advice.  At this point, I am convinced that I need at least one wide AFOV eyepiece for my Mak, especially given its narrow f/12.1 ratio. 

 

I will visit the local astro club and try to figure out which AFOV angle looks the best to my eye. 

 

Thank you!


  • Jethro7 likes this

#12 Dave Mitsky

Dave Mitsky

    ISS

  • *****
  • Moderators
  • Posts: 100,656
  • Joined: 08 Apr 2002
  • Loc: PA, USA, Planet Earth

Posted 22 September 2021 - 08:09 PM

With a slow telescope like a Maksutov-Cassegrain, highly corrected eyepieces are not required.  Since your telescope has a 1.25" focuser and a long focal length, your maximum true field of view will be restricted.  A focal reducer may help along with a 32mm Plössl or a 24mm wide-field eyepiece such as a 24mm APM UFF.

 

https://www.eyepiece..._p/17102024.htm

 

If you can find some of the shorter focal length 70-degree Bresser or Explore Scientific eyepieces on the used market, they'll work just fine.

 

https://agenaastro.c...nd/bresser.html

 


  • Jon Isaacs, Jethro7 and Vatsumok like this

#13 sevenofnine

sevenofnine

    Surveyor 1

  • -----
  • Posts: 1,788
  • Joined: 16 Apr 2016
  • Loc: Santa Rosa, California

Posted 22 September 2021 - 08:16 PM

Welcome to C/N! flowerred.gif  Turning a Mak with a narrow field of view into a wider one is tricky because the end result is so debatable. I have a 5" Mak and decided to go the safe route first. An Explore Scientific 1.25" ES 30/52 works very well with this scope. No modifications necessary and it's a visual improvement over a standard 32mm plossl. 

 

As others have said, a 5" Mak is not a wide field instrument. I agree that you will be better off buying a wide field scope to begin with like an 80-102mm refractor with a 2" star diagonal. Add eyepieces until you are broke grin.gif  


  • vtornado and Vatsumok like this

#14 rhetfield

rhetfield

    Vanguard

  • *****
  • Posts: 2,026
  • Joined: 14 Aug 2019
  • Loc: Suburban Chicago, IL, USA

Posted 23 September 2021 - 10:59 AM

When I tried a couple of the ES 82's at a star party, my experience was that if I wanted to see the full field, I had to pretty much touch my eye to the lens.  Once that was accomplished, I could see that the 82 was pretty much the largest I could see without moving my eye around to look at the edges.  I probably wouldn't appreciate anything wider and would probably be happiest with around 70 degrees.

 

If one has a fast scope, one has to be careful about wide angle eyepieces.  Some get distorted in the outer 1/3 of the field.  Especially in the shorter focal lengths.  With that situation, it is best to go to the eyepiece forum and ask what works best in the scope in question.  Only thing heavier and more expensive than a wide angle eyepiece is a wide angle eyepiece designed for a fast scope.


  • Vatsumok likes this

#15 Paul Sweeney

Paul Sweeney

    Viking 1

  • -----
  • Posts: 696
  • Joined: 19 Feb 2018
  • Loc: Heidelberg, Germany

Posted 23 September 2021 - 02:23 PM

The human eye can take in about a 50° FOV. That is why most eyepieces used to have a 50° FOV. Wider fields require you to look around, giving the "window in space" effect.

You should try wide field eyepieces out before you buy them. As mentioned above, some love them, some don't. I'm on the fence. I like having a wider FOV, but I think it is much easier to pick out faint fuzzies in a 50° Plossl.
  • Vatsumok likes this

#16 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 96,049
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 23 September 2021 - 09:57 PM

The human eye can take in about a 50° FOV. That is why most eyepieces used to have a 50° FOV. Wider fields require you to look around, giving the "window in space" effect.

You should try wide field eyepieces out before you buy them. As mentioned above, some love them, some don't. I'm on the fence. I like having a wider FOV, but I think it is much easier to pick out faint fuzzies in a 50° Plossl.

 

The human eye can take in a much wider field than 50 degrees.  You can test this yourself just by using your fingers or by closing one eye and seeing how close you can get to your computer screen and still see the edges. 

 

My screen is 20 inches wide, I can see easily see both sides of the screen from about 6 inches = 120 degrees.  According to Wikipedia, both eyes horizontal without moving the eyes, is 210 degrees. One eye vertical is 150 degrees. 

 

https://en.wikipedia...i/Field_of_view

 

I find it much easier to find those small faint galaxies with a wider field of view.. Narrow fields limit the star field and the context.  Wider fields allow greater magnification without unnecessarily narrow fields of view.  With my largest scope, very often my finder eyepiece is 280x, sometimes 350x.  I am using 100 degree eyepieces.  As it is, 280x is a 0.36 degree field, about 1/3rd of a degree. If I were using 50 degree eyepieces, that would be about 1/6th a degree, very narrow.  

 

I can take in the entire field of view of a 100 degree eyepiece without moving my eye but I will move my eye to point my averted vision.  

 

Jon


  • Dave Mitsky, esd726, DHEB and 3 others like this

#17 Keith Rivich

Keith Rivich

    Gemini

  • *****
  • Posts: 3,233
  • Joined: 17 Jun 2011
  • Loc: Cypress, Tx

Posted 23 September 2021 - 10:19 PM

Lots of good answers above. The only real downside is all the extra glass in the eyepiece. Each lens has a bit of transmission loss. If you really push your scope to it limits you may want to keep a few ortho's on hand for those "maybe I see it, maybe I don't" objects. I have a couple of old Brandon's just for that purpose. 


  • ButterFly, ColdestCrow and Vatsumok like this

#18 Piero DP

Piero DP

    Viking 1

  • -----
  • Posts: 773
  • Joined: 28 Jan 2015
  • Loc: Cambridgeshire, UK

Posted 23 September 2021 - 10:27 PM

Disadvantages:

1) they can hurt the wallet;

2) they can be addictive

 

bugeyes.gif  <-- typical reaction to these disadvantages


  • vrodriguez2324 and Vatsumok like this

#19 Dave Mitsky

Dave Mitsky

    ISS

  • *****
  • Moderators
  • Posts: 100,656
  • Joined: 08 Apr 2002
  • Loc: PA, USA, Planet Earth

Posted 24 September 2021 - 12:59 AM

I can take in the entire field of view of a 100 degree eyepiece without moving my eye but I will move my eye to point my averted vision.

As can I.  If I wasn't able to make full use of an eyepiece with a 100-degree AFOV, I would have never bought 3 of them.


  • Jon Isaacs, esd726 and Vatsumok like this

#20 Tony Flanders

Tony Flanders

    ISS

  • *****
  • Posts: 20,155
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 24 September 2021 - 05:39 AM

The human eye can take in a much wider field than 50 degrees ...
 
I can take in the entire field of view of a 100 degree eyepiece without moving my eye but I will move my eye to point my averted vision.


Not really. People can't see anything at all without moving their eyes; if your eye stops moving you go blind. Human eyes are edge detectors, and they work by scanning across those edges and noting the change as the edge sweeps across the retina.
 
However, these crucial but involuntary movements, called microsaccades, are very small, spanning no more than 2 degrees. At a larger scale, moving your eye is also essential if you want to see more than a few percent of all possible detail within a scene. That's because your fovea, the part of the eye responsible for detailed vision, spans an angle of about 5 degrees, and the sharpest part of it spans an angle of about 1 degree. To actually see all the details in a scene -- as when viewing the brushwork on a painting in a museum -- you need to intentionally scan that 1-degree circle all the way across the scene, over and over again, horizontally and vertically. That's one of the major reasons that so many people fail to see so much of what's in front of them.

 

You may think that your eye is motionless while scanning a huge scene, but you're wrong. You are necessarily making all those microsaccades, and almost certainly also glancing around at the scene semi-voluntarily. If you notice something interesting with peripheral vision you will almost certainly steal more direct glances at it, even if you're trying to stare at the center of the scene.

 

Mind you, astronomy is a bit different from looking at a painting because most deep-sky observing is done with averted vision, intentionally keeping your target out of your fovea. That's because while the central fovea is packed full of detail-specialized cone cells, it utterly lacks the rod cells that do best at low light levels. Even so, the part of your retina where averted vision works best is still a very small fraction of the retina's total area.

 

For what it's worth, my own subjective impression is that I cannot even begin to take in all of a 50-degree field of view at once. That subjective impression is confirmed by the fact that it's quite easy for a faint deep-sky object to be hiding out inside that 50-degree field of view, quite invisible until I make a conscious effort to look at precisely the right spot within that field of view.

 

As for a 100-degree field of view, I'm happy to look through a 100-degree eyepiece if that's all that's available. After all, most of them also have superb optical quality. But given my choice, I would almost always pick an eyepiece with a 70-degree field of view in preference to one with identical magnification and optical quality and a 100-degree field of view.

 

I'm quite sure that the difference between Jon and me has little or nothing to do with physiology and a great deal to do with personal preference. I will also grant that there are some situations -- fairly rare -- where I do indeed find fields of view bigger than 70 degrees to be useful.



#21 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 96,049
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 24 September 2021 - 06:30 AM

That's because your fovea, the part of the eye responsible for detailed vision, spans an angle of about 5 degrees, and the sharpest part of it spans an angle of about 1 degree.

 

 

You did quote me on this:

 

"I can take in the entire field of view of a 100 degree eyepiece without moving my eye but I will move my eye to point my averted vision."

 

Fovea = direct vision, not averted vision.

 

I'm quite sure that the difference between Jon and me has little or nothing to do with physiology and a great deal to do with personal preference. I will also grant that there are some situations -- fairly rare -- where I do indeed find fields of view bigger than 70 degrees to be useful.

 

 

Do you wear your glasses at the eyepiece?  I don't, that is a potential physiological difference.  

 

Here is the thing:  The eye can see a much larger field than 50 degrees.  That was what my post was addressing.  Yes, the eye can see a much greater field than 50 degrees.

 

When I am viewing the planets, splitting double stars, I do use my direct vision.  The advantage of an 82-100 degree field of view is longer drift times, particularly helpful if one is using well corrected eyepieces, coma correctors and such.  

 

And as I said, I do move my eye to direct my averted vision..

 

When I am hunting down galaxies and tiny planetary nebulae, the wider field is an advantage, For me, the 100 degree eyepieces really come into their own in the 22 inch. Having the wide field makes it easier to keep the star field in context, it's easier to identify stars in relation to one another... I am using Sky Safari Pro with the GAIA extension that goes down to around magnitude 18 and I often am star hopping in the main eyepiece at 280x or even 350x, that added field is really a big help.  And as I said, I do point my averted vision so the combination of the wider field and the larger star field is useful to me. 

 

The difference between a 70 degree eyepiece and a 100 degree eyepiece is a factor of 2 in area.. I could use lower magnifications and get the same TFoV but I would be sacrificing visible stars and magnification.  For the same TFoV, the magnification is 30 percent less, the sky is twice as bright. 

 

Lots of factors.

 

Jon



#22 ButterFly

ButterFly

    Mercury-Atlas

  • *****
  • Posts: 2,691
  • Joined: 07 Sep 2018

Posted 24 September 2021 - 05:31 PM

As mentioned above in different contexts, the eyeball doesn't need to be firmly planted directly in the middle.  One can wander around the field and take it all in at leisure.

 

As an aside, wiggling fingers at the extreme periphery helps to see them the farthest out.  Have a lab assistant place a cup at the extrememost angle of your wiggling fingers.  Try to guess the color of the cup (without looking at it directly).  All color we see outside of the tiny angular field the fovea provides is filled in and remembered from previous looking!  Magicians heavily rely on our filling in.  For some reason, a hard field stop of a small AFOV does not get washed away, even after panning around the whole Orion Nebula at high power.



#23 Tony Flanders

Tony Flanders

    ISS

  • *****
  • Posts: 20,155
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 25 September 2021 - 06:37 AM

You did quote me on this:
 
"I can take in the entire field of view of a 100 degree eyepiece without moving my eye but I will move my eye to point my averted vision."


OK, I will directly challenge that statement. In some sense of the word "see," you can most certainly see the entire field of a 100-degree eyepiece all at once without moving your eye. In fact, the field of view of peripheral vision is much bigger than that. If you stare straight ahead (easier said than done!), hold your finger out directly past one ear, and wiggle it, you cannot see the finger at all. But when you start to bring that finger around toward the front, you will probably be able to detect its wiggling when it's 80 degrees away from straight ahead. That means that your field of view is perhaps 160 degrees wide as far as detecting motion is concerned -- that being the single thing that peripheral vision does best.

However, if you repeat this experiment wiggling two fingers at once, you will see exactly the same thing that you saw when wiggling one finger until the fingers get perhaps 65 degrees away from straight ahead. In other words, the part of your field of view where acuity is good enough to discern any detail whatsoever is closer to 130 degrees wide.
 
And now if you hold your finger steady and continue to bring it toward the center of the field, you will see increasing detail as it gets closer to center -- but still far outside the fovea. If you pay close attention to what you're seeing, I think you will find that the field of view within which you can see half the detail potentially visible with averted vision is much smaller than 100 degrees. In fact, I think it's much smaller than 50 degrees.
 
You're not aware of that fact when viewing a 100-degree field of view in real life, because in real life people never -- but never -- stare straight ahead. The urge to take side glances is almost irresistible. Eye doctors have to keep reminding you to stare straight ahead when they want to examine different parts of your retina. Of course your eye wanders over the scene whenever you're viewing through a telescope's eyepiece. Why ever not?
 

The difference between a 70 degree eyepiece and a 100 degree eyepiece is a factor of 2 in area.. I could use lower magnifications and get the same TFoV but I would be sacrificing visible stars and magnification.  For the same TFoV, the magnification is 30 percent less, the sky is twice as bright.


Here's how I experience that. When I'm using an eyepiece with a 30-degree apparent field of view -- and yes, I really have used some even narrower than that -- I truly do find the entire field of view equally easy to look at. So a 30-degree eyepiece has a fully useful 30-degree field of view for me.

When I'm using an eyepiece with a 50-degree field of view, if I see something interesting with 20 degrees of the center I will probably just swivel my eye over to view it. But if it's really near the edge, I will move the telescope so that the point of interest is centered in the eyepiece. I would say that with a 50-degree eyepiece I find about 40 degrees to be fully useful.

When use an eyepiece with a 70-degree field of view, I find that the fully useful field of view is about 50 degrees wide. And when I use an eyepiece with a 100-degree field of view I find that the fully useful field is about 60 degrees wide. So for me the useful field of view of a 100-degree eyepiece is actually just (60/50)^2 ~= 1.5 times the area of a 70-degree eyepiece. As the apparent field of view grows, the outer part becomes decreasingly useful -- for me, of course. I cannot speak for other people.

 

Another way of looking at this is, how do you feel about the field stop? That's where the sky stops and the total blackness all around the framed part of the sky starts. The whole idea of the "spacewalk view," to use Al Nagler's very apt phrase, is that the field stop is a bad thing. With the ideal eyepiece, there would be no field stop at all.

 

I understand that point of view, but it's not how I feel at all; the field stop is my friend, not my enemy. I like to frame my target in a telescope just as I do in photography. The essence of a good photograph is cropping in around your subject just right, with neither too much nor too little context around it.


  • JOEinCO likes this

#24 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 96,049
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 25 September 2021 - 07:01 AM

OK, I will directly challenge that statement. In some sense of the word "see," you can most certainly see the entire field of a 100-degree eyepiece all at once without moving your eye. In fact, the field of view of peripheral vision is much bigger than that.

 

I don't know why you are challenging my statement.  That was exactly my point.  You basically reiterated what I wrote in my first post. 

 

When use an eyepiece with a 70-degree field of view, I find that the fully useful field of view is about 50 degrees wide. And when I use an eyepiece with a 100-degree field of view I find that the fully useful field is about 60 degrees wide. So for me the useful field of view of a 100-degree eyepiece is actually just (60/50)^2 ~= 1.5 times the area of a 70-degree eyepiece. As the apparent field of view grows, the outer part becomes decreasingly useful -- for me, of course. I cannot speak for other people.

 

 

Do you wear your glasses when observing?  I don't... This obviously can have a significant effect on how much of the field is usable.  

 

Not all 100 degree eyepieces have the same useful field.  My first two 100 degree eyepieces were the 20mm and 14mm Explore Scientific's, I could not "take in the entire field" and to see the edge of the field, I had to tilt my head and peak.. 

 

I replaced those with the 21mm and 13mm Ethos eyepieces and the difference was immediately obvious, I could take in the entire field of view. For me, the useful field is significantly greater than the 82 degree of the Naglers.  For me, the entire field of the Naglers is useful. I can point my gaze to the edge.. 

 

Also, occasionally using a borrowed eyepiece is quite different than owning it and using an eyepiece.  Using any eyepiece is a learning process.  

 

This is not to say that narrower field eyepieces cannot be effective and certainly they are more affordable.  But for me, there is no doubt that I am a more effective observer star hopping and observing with the Ethos eyepieces.  

 

Jon


  • FoxIslandHiker likes this

#25 aeajr

aeajr

    Hubble

  • *****
  • Posts: 15,781
  • Joined: 26 Jun 2015
  • Loc: Long Island, New York, USA

Posted 25 September 2021 - 07:28 AM

Hi all,

 

I am new to the hobby and looking to build out a 2 or 3 eyepiece collection for my Maksutov scope.  (More equipment details and intended use details are at the bottom of the post).

 

I have noticed that eyepieces come in a wide range of AFOV angles from 40' in entry-level zooms to 110' super wide.  And based on my reading, it seems like wide AFOV eyepieces show more context at a given magnification and provide an immersive space-walk like experience. 

 

It seems too good to be true, so I wanted to ask:

  1. Are there any downsides to wide AFOV eyepieces (besides cost / weight)? 
  2. Does stretching the view over a wider AFOV make the view dimmer compared to a narrow AFOV piece?
  3. How does a focal reducer / barlow affect AFOV and brightness / contrast?
  4. Wide AFOV eyepieces are expensive.  Should I get one high-end eyepiece and use it with a barlow + focal reducer to have 3 usable magnifications?  Or are there better strategies for covering a range of magnifications?

 

Scope details:

I am using an Orion Maksutov scope with 127mm aperture, 1540mm focal length (f/12.1). 1.25" Focuser.  No electronics.

 

Intended use:

  • I use my scope to see the moon, Jupiter, and Saturn from home since they are not affected much by light pollution and are visible on most nights.
  • I hope to take my scope along on camping trips and start exploring deep space objects from national parks and dark sites.

 

Thank you!

We each have our own opinion and approach to our eyepiece set.

 

I have one eyepiece that maxes out my field of view for each scope.   For a scope with a 1.25" focuser that would be a 32 mm Plossl.  Plossls are excellent eyepieces and are quite inexpensive so you don't give up anything with a 32 mm Plossl in a 1.25" focuser.  

 

In my F15 Mak I tend to use a 32 mm Plossl and an 8-24 zoom for most observing sessions.  As you can see in my signature I have many other eyepieces to choose from, but this is my preferred observing set. 

 

 

Understanding Telescope Eyepieces- There are recommendations, based on budget,
but the meat of the article is about understanding the considerations and specifications
to know when selecting eyepieces.
https://telescopicwa...cope-eyepieces/

 

As you go up in magnification, a wider AFOV will provide more drift time when using a manual mount.  A tracking mount takes care of this so not as much of a consideration.

 

A wider AFOV will give you better context, especially for larger DSOs.  And some DSOs are quite large.   The Pleiades are about 2 degrees wide.  The Andromeda galaxy about 3 degrees and the North America nebula about 4 degrees.  This is why many of us have multiple scopes, one with a shorter focal length to enable wide field of view and, perhaps, one with longer focal length which are more optimized toward the Moon, planets and those smaller, high mag DSOs.  Your Mak would be of the latter type. 

 

In an F12 scope like yours, having a good image across the entire field of view is easier for the eyepiece as compared to using it in an F5 scope.  This has to do with the angle of the light rays as they enter the eyepiece.

 

I have several scopes ranging from F15 to F4.   An eyepiece used in the F15 scope may give me an excellent image right to the edge of the field of view.  That same eyepiece used in the F4 scope may show distortions as you get closer and closer to the outer edge of the field of view but is likely to be just fine in the center 60 to 70 percent of the field of view.  

 

The wider you go in AFOV the harder it is for the eyepiece designer to handle these outer areas in low focal ratio scopes.  This is where the more expensive eyepieces will have a performance advantage over the cheaper eyepieces.  Some people don't mind this outer edge fall off in quality, others are very sensitive to it.  

 

If you plan to have that one scope for a long time, this will be less of a concern as you buy eyepieces.   If you have plans to add a scope that is less than F8, this will be more of a consideration.  Below F6, the loss of edge quality is more apparent.

 

For my single FL eyepieces I have standardized on Explore Scientific and Meade 82 degree eyepieces.   And I am a very active user of Zoom eyepieces.  My Baader Hyperion 8-24 is my most used eyepiece. 

 

I hope that helps. 


Edited by aeajr, 25 September 2021 - 07:35 AM.

  • dave253 and Vatsumok like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Eyepieces, Maksutov, Beginner



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics