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

gradual accommodation to fc?

  • Please log in to reply
19 replies to this topic

#1 CollinofAlabama

CollinofAlabama

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2,995
  • Joined: 24 Nov 2003
  • Loc: Lubbock, Texas, USA

Posted 09 December 2019 - 01:36 AM

I was wondering if anyone else has had this experience.  I was looking through my new SkyWatcher 72mm ED scope, basically an F/6 ED optic.  It was very windy on the ground and the stars were twinkling like mad, so I avoided all high power and cruised around the Albireo way with my new APM UFF 30mm.  When I first put the eyepiece into the scope, the field curvature seemed horrible to me.  The outside of the field could be brought to focus, but only at the expense of the center of field.  I began to feel a little depressed, because the SW 72 ED is an F/6 optic, and those guys are gonna have some field curvature, but this seemed ridiculous.  Still, I focused for the center and began cruising around the Albireo/Vulpecula stellar neighborhood.  The more I did, the nicer the field seemed to be.  I could still see some field curvature, but it wasn't bad at all.  I had to refocus for the edge to get completely clean, but if I focused between the center and edge, everything seemed to look quite nice.  I could get the center or edge a bit tighter with some fine focus adjustment, but basically I found the view acceptable.  Can a person accommodate over time?  Can one start out stuck, then get okay after a while?  It was a strange, though oddly pleasant experience, but I was wondering if accommodation for FC can be acquired in a few minutes of viewing?  Strange question, but that was my experience.  Anyone else have this happen to them, or hear of it?


Edited by CollinofAlabama, 09 December 2019 - 01:11 PM.


#2 clearwaterdave

clearwaterdave

    Fly Me to the Moon

  • *****
  • Posts: 5,115
  • Joined: 27 May 2014
  • Loc: Western Maine

Posted 09 December 2019 - 02:20 AM

This is what I have experienced.,I think the eye/brain connection tries to bring the image to focus.,please don't quote me on this.,lol.,I notice it most with my cheap bino.,cheers.,


  • CollinofAlabama likes this

#3 ButterFly

ButterFly

    Apollo

  • *****
  • Posts: 1,240
  • Joined: 07 Sep 2018

Posted 09 December 2019 - 04:27 AM

Your eyes are accomodating to the lack of focus, not the field curvature per se.  Whether one's eyes can do that depends on their own eye muscles, and can lead to strain.  The brain also has a way of glossing over things it just doesn't like.  Motion helps the brain correct the image (peripheral vision has "color" only becuase the brain remembers them, actual color vision is in a very small angle).  Finding the good average point based on your viewing habits helps a lot.

 

Eyepieces introduce some level of distortion (pincushion or barrel) to compensate for the field curvature while maintatining sharp focus.  That is the art behind the science.  It is much more apparent at large real fields of view and long straight things.  From the ballfields of Inwood Hill Park to the stantions of the George Washington Bridge is about a mile and a half.  They fill the vertical field of view of a 21 Ethos in an ES 80mm f/6.  It is sharp to the edge, but the stantions get noodly as you bring them to the far right or left of the field of view.  In binoculars, where panning around fairly quickly is common, the distortions are also noticable in sharp to the edge optics, like the Nikon Monarchs.  The Oberwerk 25x100s are not sharp to the edge, so I focus about 1/3 of the way out and I accomodate just fine.  Unless you spend a lot of time looking right at the edge of the field stop, focusing somewhere near the middle seems to work fine.  The other alternative is to stop down the aperture a bit to increase the effective f ratio, which makes the light cone gentler for the eyepiece.


  • CollinofAlabama and payner like this

#4 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 86,601
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 December 2019 - 04:30 AM

Collin:

 

Some thoughts:

 

- Probably a left brain-right brain thing. When you first looked, you left brain is telling you, "OMG, this is bad, I screwed up, I shouldn't have bought this scope, I shouldn't have bought this eyepiece."  Once that settled down and you decided to just enjoy the view, as horrible as it was, your right brain took over and you discovered it wasn't bad at all.  

 

- Did you go back and look at the region around Albireo after it seemed better?  Maybe the seeing settled in, the combination of bad seeing and curvature will make the curvature seem worse. 

 

- The eye can accommodate for curvature in the sense if you stare at a region, it can bring that to focus but it cannot bring the entire field to focus simultaneously. 

 

- You can become accustomed to field curvature. One can accept field curvature.  At first, you were probably looking for it and it obviously bothered you. 

 

- My own experience is that some nights field curvature seems more bothersome than others.  It also depends on the region of the sky, the brightness of the stars in the star fields.  In a scope like the Skywatcher ED-72, it will always be there with an eyepiece like the 30mm APM and the stars will not be sharp-sharp across the field but sometimes it quite apparent, sometimes not so much.   

 

Jon


  • CollinofAlabama, GlenM and 25585 like this

#5 Ernest_SPB

Ernest_SPB

    Apollo

  • *****
  • Posts: 1,451
  • Joined: 13 Nov 2010
  • Loc: St.-Petersburg, Russia

Posted 09 December 2019 - 04:34 AM

> I was wondering if accommodation for FC can be acquired in a few minutes of viewing...

 

(1) yes, eye is very adaptive (especially it is true for young observers) and after some time of observing brain learned how to receive more correct information from our visual system. Curve of learning more or less high, but exists. It is a reason why we need some time to start sense dim nebulae

(2) way to focusing in conditions of strong FC is not symmetric: focusing on edge, we will see strong defocus in center FOV, and opposite - focusing on central zone FOV we can get quite a tolerable image at edge. It is coming from difference in sign of de-focusing - positive one could be worked out by normal accommodation, negative - not.


  • CollinofAlabama and 25585 like this

#6 Astro-Master

Astro-Master

    Apollo

  • *****
  • Posts: 1,075
  • Joined: 09 May 2016
  • Loc: San Diego County,Ca.

Posted 09 December 2019 - 04:35 AM

I have experienced this using a Baader 31mm Hyperion on my 18" F/4.5 DOB.  If I focus for the center and then look cross axis at the edge, the edge looks pretty bad.  If I look at the center, relax and just let my vision flow naturally out to the edge without looking off axis, it looks much better.


Edited by Astro-Master, 09 December 2019 - 04:37 AM.

  • CollinofAlabama likes this

#7 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 86,601
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 December 2019 - 04:47 AM

Eyepieces introduce some level of distortion (pincushion or barrel) to compensate for the field curvature while maintatining sharp focus.  That is the art behind the science.  It is much more apparent at large real fields of view and long straight things.  From the ballfields of Inwood Hill Park to the stantions of the George Washington Bridge is about a mile and a half.  They fill the vertical field of view of a 21 Ethos in an ES 80mm f/6.  It is sharp to the edge, but the stantions get noodly as you bring them to the far right or left of the field of view.  In binoculars, where panning around fairly quickly is common, the distortions are also noticable in sharp to the edge optics, like the Nikon Monarchs.  The Oberwerk 25x100s are not sharp to the edge, so I focus about 1/3 of the way out and I accomodate just fine.  Unless you spend a lot of time looking right at the edge of the field stop, focusing somewhere near the middle seems to work fine.  The other alternative is to stop down the aperture a bit to increase the effective f ratio, which makes the light cone gentler for the eyepiece.

 

I don't think it works quite that way.  Distortion of the field does not correct for a curved focal plane.  A curved focal plane means the focus at the center is a different distance than the focus at the edge.  In an 80mm F/6, the edge of the field of the 21mm Ethos focuses about 1.0 mm closer in than the center.  

 

The 21mm Ethos, in a scope that is corrected for field curvature, a long focal length Newtonian with a Paracorr or a modified Petzal, the stars at the edge of the field are sharp-sharp, meaning if one focuses near the edge, the center will also be in sharp focus..

 

Field curvature is more difficult to see during the day because there are not all those pinpoint stars out there to sharply focus on.  And the field, what you are looking at, is rarely flat. 

 

Jon

 

Edit:  Collin, rest assured that the field curvature in the 31mm Nagler in a 72mm F/6 is very apparent.  The 31mm Baader, has a lot of astigmatism as well so it presents additional issues.


  • CollinofAlabama likes this

#8 BillP

BillP

    ISS

  • *****
  • Posts: 20,343
  • Joined: 26 Nov 2006
  • Loc: Spotsylvania, VA

Posted 09 December 2019 - 09:39 AM

There are a host of factors that "may" be playing into why you experienced what you did.  Your posit about whether you may be accommodating over time is also a distinct possibility.  Last week came across the 2011 research on the human eye where they showed that the brain takes the aberrations out of each individual's eye optics.  The interesting point is that it is not "fixed" processing but dynamic based on aberrations present in the particular eye's optics.  So similar to the research on how if one wears glasses that invert the view the brain will eventually flip the view for you, your brain does lots of post processing on the image from your eye's optics.  So who knows, perhaps what you are suggesting may indeed be happening!

 

https://journals.plo...al.pone.0027031


  • CollinofAlabama likes this

#9 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 47,300
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 09 December 2019 - 02:29 PM

Plus, focusing on a point half way from center to edge allowed what accommodation your eye has to more of less focus the entire field.

It is a common way to deal with mild field curvature.

Accommodation improves with relaxation, too, and this could have occurred.


  • jjack's likes this

#10 SteveG

SteveG

    Cosmos

  • *****
  • Posts: 8,826
  • Joined: 27 Sep 2006
  • Loc: Seattle, WA

Posted 09 December 2019 - 03:50 PM

I was wondering if anyone else has had this experience.  I was looking through my new SkyWatcher 72mm ED scope, basically an F/6 ED optic.  It was very windy on the ground and the stars were twinkling like mad, so I avoided all high power and cruised around the Albireo way with my new APM UFF 30mm.  When I first put the eyepiece into the scope, the field curvature seemed horrible to me.  The outside of the field could be brought to focus, but only at the expense of the center of field.  I began to feel a little depressed, because the SW 72 ED is an F/6 optic, and those guys are gonna have some field curvature, but this seemed ridiculous.  Still, I focused for the center and began cruising around the Albireo/Vulpecula stellar neighborhood.  The more I did, the nicer the field seemed to be.  I could still see some field curvature, but it wasn't bad at all.  I had to refocus for the edge to get completely clean, but if I focused between the center and edge, everything seemed to look quite nice.  I could get the center or edge a bit tighter with some fine focus adjustment, but basically I found the view acceptable.  Can a person accommodate over time?  Can one start out stuck, then get okay after a while?  It was a strange, though oddly pleasant experience, but I was wondering if accommodation for FC can be acquired in a few minutes of viewing?  Strange question, but that was my experience.  Anyone else have this happen to them, or hear of it?

Just a note, the FC is seen because of the scopes very fast 420 mm focal length, not the f6 ratio. You are lucky that you can accommodate. I can’t, even at 600 mm. For me it disappears somewhere between 700-800 mm. 
 

You rarely hear about visual observers complaining that their AP 130 f6 is showing FC. Again, that’s due to the 820 mm focal length.


  • Starman1, jeffmac, Peter Besenbruch and 1 other like this

#11 Peter Besenbruch

Peter Besenbruch

    Fly Me to the Moon

  • *****
  • Posts: 7,328
  • Joined: 21 Aug 2014
  • Loc: Oahu

Posted 10 December 2019 - 03:44 PM

Plus, focusing on a point half way from center to edge allowed what accommodation your eye has to more of less focus the entire field.

It is a common way to deal with mild field curvature.

Accommodation improves with relaxation, too, and this could have occurred.

For me at my age, that makes the center and the edge of field off just enough to be really annoying. :(


  • SteveG and Sarkikos like this

#12 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 47,300
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 10 December 2019 - 05:11 PM

For me at my age, that makes the center and the edge of field off just enough to be really annoying. frown.gif

lol.gif

The MOST annoying part of losing accommodation as you age is that your vision doesn't "freeze" on infinity focus, either.

I'm presbyopic, but I still need glasses to drive.  Everything is perfect at 20' without glasses, but nowhere else.

Annoying.


  • Sarkikos, Peter Besenbruch and 25585 like this

#13 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 86,601
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 10 December 2019 - 05:16 PM

lol.gif

The MOST annoying part of losing accommodation as you age is that your vision doesn't "freeze" on infinity focus, either.

I'm presbyopic, but I still need glasses to drive.  Everything is perfect at 20' without glasses, but nowhere else.

Annoying.

 

I must have been living right because as presbyopia set in, my focus gradually shifted towards infinity.. 

 

Everything is pretty good from about 6 feet out..

 

Jon



#14 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 47,300
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 10 December 2019 - 05:55 PM

Jon,

Must be all those hours observing.waytogo.gif



#15 Miranda2525

Miranda2525

    Vanguard

  • *****
  • Posts: 2,407
  • Joined: 12 Jul 2016

Posted 10 December 2019 - 09:53 PM

I was wondering if anyone else has had this experience.  I was looking through my new SkyWatcher 72mm ED scope, basically an F/6 ED optic.  It was very windy on the ground and the stars were twinkling like mad, so I avoided all high power and cruised around the Albireo way with my new APM UFF 30mm.  When I first put the eyepiece into the scope, the field curvature seemed horrible to me.  The outside of the field could be brought to focus, but only at the expense of the center of field.  I began to feel a little depressed, because the SW 72 ED is an F/6 optic, and those guys are gonna have some field curvature, but this seemed ridiculous.  Still, I focused for the center and began cruising around the Albireo/Vulpecula stellar neighborhood.  The more I did, the nicer the field seemed to be.  I could still see some field curvature, but it wasn't bad at all.  I had to refocus for the edge to get completely clean, but if I focused between the center and edge, everything seemed to look quite nice.  I could get the center or edge a bit tighter with some fine focus adjustment, but basically I found the view acceptable.  Can a person accommodate over time?  Can one start out stuck, then get okay after a while?  It was a strange, though oddly pleasant experience, but I was wondering if accommodation for FC can be acquired in a few minutes of viewing?  Strange question, but that was my experience.  Anyone else have this happen to them, or hear of it?

Over time seeing fc doesn't get any better. It gets worse, lol. IMO, it is also best to avoid any observing in windy conditions. 



#16 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,322
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 11 December 2019 - 07:37 AM

lol.gif

The MOST annoying part of losing accommodation as you age is that your vision doesn't "freeze" on infinity focus, either.

I'm presbyopic, but I still need glasses to drive.  Everything is perfect at 20' without glasses, but nowhere else.

Annoying.

I have myopia and presbyopia.  My focus gets sharpest at about 20" - yeah, inches.  But even then it's still pretty fuzzy.  So at no distance - from eye to infinity - do I achieve sharp focus.  This is why I leave my eyeglasses on whenever possible.

 

Mike



#17 Peter Besenbruch

Peter Besenbruch

    Fly Me to the Moon

  • *****
  • Posts: 7,328
  • Joined: 21 Aug 2014
  • Loc: Oahu

Posted 11 December 2019 - 05:10 PM

I have myopia and presbyopia.  My focus gets sharpest at about 20" - yeah, inches.

In my late 30s, I noticed I couldn't focus close as well as before. In my mid 40s I lost the ability to focus to infinity. Yes, I have hyperopia. You've heard of the phrase, "To infinity and beyond?" My eyes are only good for "beyond."


  • Sarkikos likes this

#18 Heitman

Heitman

    Ranger 4

  • *****
  • Posts: 374
  • Joined: 26 Jan 2007
  • Loc: Reeds, NC

Posted 11 December 2019 - 05:19 PM

Had a Pentax XW20 that I had to sell because I was never able to compensate for the FC.



#19 Migwan

Migwan

    Vanguard

  • *****
  • Posts: 2,060
  • Joined: 24 Jul 2017
  • Loc: Meeechigan

Posted 11 December 2019 - 06:05 PM

When I got that same eyepiece and slapped it into the f5 ST120, my initial impression was worry.  I wasn't able to accommodate the field curvature.  Then I put it in the f10 C11 and realized the eyepiece was fine.   I ordered a field flattener for the ST120 and it's all good there.

 

Regretfully, now the the view thru the ST120 is so flat (90% flat with an APM 30, 80% with an Ethos 13) I can't seem to help but notice the field curvature in the C11.  

 

jd


  • Sarkikos likes this

#20 Sarkikos

Sarkikos

    ISS

  • *****
  • Posts: 31,322
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 12 December 2019 - 07:47 AM

Had a Pentax XW20 that I had to sell because I was never able to compensate for the FC.

The XW20 had some of the worst FC I've ever seen in an eyepiece.  Then is when I owned one, maybe seven years ago.  Back then, I needed to put the XW20 in a Paracorr and focus the focuser about midway between center of field and edge of field to get a barely acceptable view.  I imagine now it would be even worse to my eyes.  

 

Mike




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






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics