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

#26 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 52,767
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 25 September 2021 - 01:30 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!

1. no.

2. no

3. Field size on the sky (true field) is related to magnification, so a doubling (Barlow) halves the width of the true field (not affecting the apparent field), while a focal reducer

enlarges the true field (not affecting the apparent field) by an amount related to the amount of focal reduction.

4. Your scope doesn't illuminate extremely large fields.  Hence, I do not recommend the use of a focal reducer.  The outer part of the field will be dark and you will not achieve the wider field you want.

Stick with 1.25" eyepieces, maximum field in the scope = 1.00° (sounds like it was planned. eh?)

You should have an eyepiece that yields a widest field.  That could be:

40mm Plössl, 40°

32mm Plössl, 50°

26mm 62°

24mm Wide field, 68°

Above that, you can use Barlows or more eyepieces.

All of those eyepieces have different apparent fields, but all yield the exact same true field in your scope.

 

The visual back on the scope has a 10mm threaded section for a certain type of camera adapter.  By machining it off, you can move the diagonal closer to the scope, which shorten the

focal length of the scope to around 1510mm +/-, gaining you a tiny bit of field.  Use of a prism diagonal shortens the focal length a bit more (though I haven't measured it). 

Also, using a star diagonal with a full 28mm clear aperture all the way through, will help illuminate the outer edges of the maximum field.

 

Note that the scope's real aperture is ~120-121mm per the S&T test.  I've verified this with a laser.  So the f/ratio is a bit longer than the mfr. claims.

 

Here is a list of objects all of which have been observed in that very scope:

https://www.cloudyni...list/?p=6165843


Edited by Starman1, 25 September 2021 - 01:31 PM.

  • Dave Mitsky, Jon Isaacs and Vatsumok like this

#27 MeridianStarGazer

MeridianStarGazer

    Voyager 1

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

Posted 26 September 2021 - 12:08 PM

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.



My experience was similar except that 24mm and up do not require touching the lens.


Also, 100 deg takes extra skill to see the full view at once, but once learned, I enjoy the majestic view. Naked eye must deal with hills, trees, houses, stars, and lights, making this 100 deg very nice by comparison. My complaint is the size of the eyepiece.

68 degrees is nice. Even 52 is fine. But I now notice the difference between 68 and 82 and no longer see 68 as majestic. It is just roomy.

I think 82 is majestic enough and comes in smaller sizes.
  • aeajr and Vatsumok like this

#28 Mitrovarr

Mitrovarr

    Skylab

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

Posted 26 September 2021 - 06:33 PM

One thing I will say about wide field EPs is that improvements at the low end matter more than at the high end. 55 deg is a huge improvement over 40, 68 is pretty large over 55, 82 is significant over 68, but 100 over 82 doesn't feel that crucial. At least to me.
  • Vatsumok likes this

#29 bikerdib

bikerdib

    Apollo

  • *****
  • Posts: 1,127
  • Joined: 30 Apr 2014
  • Loc: Southeast Texas

Posted 26 September 2021 - 06:41 PM

I have many Naglers and quite a few ES 82° as well as one ES 100° and they all perform nicely.  But I also have a complete set of TV P,ossels.  Sometimes such as on lunar and planets, less glass is more.


  • ButterFly and Vatsumok like this

#30 Vatsumok

Vatsumok

    Lift Off

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

Posted 27 September 2021 - 10:33 AM

Thank you all!  I have one more question about exit pupil size:

 

Take two example eyepieces of identical quality: one is 26mm 50' AFOV and second one is 13mm 100' AFOV.  In a given telescope, both of these will cover the same TFOV.  However, the exit pupil for 13mm will be half the size of 26mm EP.   Does it mean that the view will be dimmer?

 

In other words, the first 26mm eyepiece will take the same TFOV and it project a lower magnification 50' circle.  The second 13mm eyepiece will magnify the same TFOV more and project it in a 100' circle.  Will the eye perceive both circles as similar brightness?


  • aeajr likes this

#31 ButterFly

ButterFly

    Mercury-Atlas

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

Posted 27 September 2021 - 11:04 AM

Thank you all!  I have one more question about exit pupil size:

 

Take two example eyepieces of identical quality: one is 26mm 50' AFOV and second one is 13mm 100' AFOV.  In a given telescope, both of these will cover the same TFOV.  However, the exit pupil for 13mm will be half the size of 26mm EP.   Does it mean that the view will be dimmer?

 

In other words, the first 26mm eyepiece will take the same TFOV and it project a lower magnification 50' circle.  The second 13mm eyepiece will magnify the same TFOV more and project it in a 100' circle.  Will the eye perceive both circles as similar brightness?

Great question.  The issue is one's own contrast sensitivity when it comes to which eyepiece permits that observer to better discern the object against the background.  One discerns a region of object plus background from a region of just background.  Each person can do that differently, and it can change throughout the night.

 

With the same TFOV, all the light from the background plus object is the same.  The total light in each eyepiece, minus optical losses, is thus the same.  In the lower focal length eyepiece, with a larger AFOV, both the object and background are correspondingly dimmed, but the sky background has a lower surface brightness, and the object is larger.  Seeing a larger, albeit dimmer, object against a dimmer background, can be easier for that observer.  One's contrast sensitivity changes throughout the night, and based on ambient conditions.  There is an excellent discussion of this in the intro chapters of Pellier, Planetary Astronomy.  Planetary astronomy has the added issue of different colors contrasting differently with different colored backgrounds (red spot on beige Jupiter), rather than the mere "easy" problem of blue green nebula against gray light polluted sky background.

 

For your eyes and your skies, switch eyepieces until you have the best view for the particular target you are after.  When looking for spiral arms, the light from the disk of the galaxy can be added background, on top of whatever light pollution there is.  One of the benefits of zoom eyepieces is to be able to dial in the best exit pupil for some particular target against some particular background, for your eyes and your skies.  Glare riddled eyepieces just add to background.


  • Jon Isaacs, dave253 and Vatsumok like this

#32 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 52,767
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 September 2021 - 12:21 PM

Thank you all!  I have one more question about exit pupil size:

 

Take two example eyepieces of identical quality: one is 26mm 50' AFOV and second one is 13mm 100' AFOV.  In a given telescope, both of these will cover the same TFOV.  However, the exit pupil for 13mm will be half the size of 26mm EP.   Does it mean that the view will be dimmer?

 

In other words, the first 26mm eyepiece will take the same TFOV and it project a lower magnification 50' circle.  The second 13mm eyepiece will magnify the same TFOV more and project it in a 100' circle.  Will the eye perceive both circles as similar brightness?

Surface brightness, as seen or measured, is the brightness per unit area.  That will diminish with magnification.

The object's surface brightness doesn't change, of course, because you are not altering the distance to the object, but the surface brightness you see will change.

Total integrated magnitude is the overall total brightness of the object as if it were a star.

That is the same at all magnifications.

 

So, taking the eyepieces in question, the object covers 4x the area in the higher power eyepiece so the apparent surface brightness is now 1/4 as high, a 75% dimming.

That's merely another way of saying the exit pupil is 1/4 the size at 1/2 the diameter, so the amount of light entering the eye is less.

As was pointed out in post #31, the object is also now 4X larger.  Multiply the two to get 1 and you see why the overall brightness of the object doesn't change.

[at least not if all of the object is contained in the exit pupil]

Merely our perception of its surface brightness changes.

The contrast between object and background has not changed either, but a larger, dimmer, object against a darker background will likely be more visible.

 

Up to a point.  Once the apparent surface brightness of the object gets too dim, it will become less visible, despite being larger.

So there is a balance point between making it larger to make it easier to see, and making it too dim.

As "Butterfly" mentioned, switch eyepieces until you get the best view.

If all you want to do is to detect the object, a low power will usually suffice.

But if you want to see details, higher powers will be necessary.


  • Dave Mitsky, dave253 and Vatsumok like this

#33 rhetfield

rhetfield

    Vanguard

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

Posted 27 September 2021 - 03:35 PM

Thank you all!  I have one more question about exit pupil size:

 

Take two example eyepieces of identical quality: one is 26mm 50' AFOV and second one is 13mm 100' AFOV.  In a given telescope, both of these will cover the same TFOV.  However, the exit pupil for 13mm will be half the size of 26mm EP.   Does it mean that the view will be dimmer?

 

In other words, the first 26mm eyepiece will take the same TFOV and it project a lower magnification 50' circle.  The second 13mm eyepiece will magnify the same TFOV more and project it in a 100' circle.  Will the eye perceive both circles as similar brightness?

The object and its background can appear dimmer, which can be a good thing or a bad thing.  A bright planet will end up with reduced glare with a smaller exit pupil.  Similarly, a light polluted sky will have a darker background - which may increase contrast.  Too small and a DSO gets too dim.

 

If the exit pupil becomes too small, then the view gets diminished.  Try not to go below 0.75mm on bright planets.  DSO's often should not go smaller than 1-2mm.  Be careful about going too large also.  The upper limit is supposed to be 7mm, but many people can't go that high and 5-6mm is probably better as an upper limit for most.


Edited by rhetfield, 27 September 2021 - 03:36 PM.

  • dave253 and Vatsumok like this

#34 aeajr

aeajr

    Hubble

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

Posted 27 September 2021 - 04:24 PM

Thank you all!  I have one more question about exit pupil size:

 

Take two example eyepieces of identical quality: one is 26mm 50' AFOV and second one is 13mm 100' AFOV.  In a given telescope, both of these will cover the same TFOV.  However, the exit pupil for 13mm will be half the size of 26mm EP.   Does it mean that the view will be dimmer?

 

In other words, the first 26mm eyepiece will take the same TFOV and it project a lower magnification 50' circle.  The second 13mm eyepiece will magnify the same TFOV more and project it in a 100' circle.  Will the eye perceive both circles as similar brightness?

Just think of it this way, as you go up in mag you will darken the background sky and the objects you are focused on will be dimmer.   This is true if all of your eyepieces are 82 degree or 100 degree or 50 degree.

 

Brightness and detail are not the same thing.  You are drawing a comparison between a 26 mm and a 13 mm.  Regardless of the AFOV, the 26 will appear brighter, but the 13 will show you more detail, up to the point where the atmosphere becomes a limiting factor.   I don't think AFOV comes into play. 


Edited by aeajr, 27 September 2021 - 04:27 PM.

  • Vatsumok likes this

#35 MeridianStarGazer

MeridianStarGazer

    Voyager 1

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

Posted 28 September 2021 - 09:08 AM

Thank you all! I have one more question about exit pupil size:

Take two example eyepieces of identical quality: one is 26mm 50' AFOV and second one is 13mm 100' AFOV. In a given telescope, both of these will cover the same TFOV. However, the exit pupil for 13mm will be half the size of 26mm EP. Does it mean that the view will be dimmer?

In other words, the first 26mm eyepiece will take the same TFOV and it project a lower magnification 50' circle. The second 13mm eyepiece will magnify the same TFOV more and project it in a 100' circle. Will the eye perceive both circles as similar brightness?


The sky and galaxies will dim as you zoom in. Stars will not. Stars in globular clusters will stay just as bright. They might be brighter at 13mm if the 26mm gives an exit pupil bigger than your pupil.
  • Vatsumok likes this

#36 saemark30

saemark30

    Surveyor 1

  • -----
  • Posts: 1,537
  • Joined: 21 Feb 2012

Posted 05 October 2021 - 10:34 AM

I see fainter magnitudes with simpler fully MC eyepieces than early Televue Naglers and Meade UWA eyepieces, i.e. orthos and plossls.


  • Vatsumok likes 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