Eye Relief Thoughts

[Rick Towns]

It was cloudy last night so I set up my scope in the dining room and aimed at a street light down the road. I then got out my new Fujiyama 6mm Ortho and my old Tanzutsu 6mm Ortho that it replaced. I was surprised by how much closer I had to get my eye into the Tanzutsu in order to see the entire field of view. The Fujiyama doesn't have fantastic eye relief (4.9mm) but it must still be within my threshold of "comfortable" and the Tanzutsu must have near 0mm! Perhaps 2? Regardless, the difference was quite noticeable.

For fun, I also tried an Orion Edge-On Planetary 5mm, which has 20mm of eye relief. The views through all 3 eyepieces were very similar (Fuji being the best/sharpest, Tanzutsu second, Orion a little softer, but it is 1mm shorter and a more complex design), but the comfort on all of them was different. With the Orion, I was barely touching the eye cup with my... um... face (?) and I had to move my head around to avoid black outs. So I had to hold my head perfectly still over the sweet spot to get a good view. With the Fuijiyama, I can press my eye socket (yeah, that's better!) gently against the eye cup and easily find the exit pupil. After just a few minutes of observing a spider egg sack on the street light, I overwhelmingly preferred the view through the Fuji over the other two. I also have a BST UWA Planetary (9mm) that I see a kidney bean effect unless I hold my head just in the right spot.

So, I think I may have figured out why I prefer Orthos and Plössls to other designs - at least at shorter focal lengths. Or I need to maybe try some higher quality long eye relief glass? Perhaps a TV DeLite or an ES 82 would not have this same issue with locating and holding the exit pupil? Thoughts?

[Knighty2112]

I think eye relief can also be affected by different focul  lengths on any telescope. I have both a f5 and a f11 'frac that has slightly different positions for eye relief in the same EPs when used on both the scopes. 

[John]

The ergonomics of the eyepiece top, the design and flexibility of the eye cup (if fitted), the extent to which the eye lens is recessed and (with long eye relief / wide field eyepieces) the figure of the eye lens top surface can all make a noticable difference to the amount of actual eye relief available. This is often not quite what the specifications of the eyepiece promise !

Although I don't wear glasses to observe and have enjoyed using orthos and plossls for high power viewing, I tend to prefer longer eye relief designs these days. Must be old age !

[Mr Spock]

There has to be a lot said for comfort. Some of those ortho and Plössls, while sharp, aren't all that confortable to use.

[Moonshane]

3 hours ago, Knighty2112 said:

I think eye relief can also be affected by different focul  lengths on any telescope. I have both a f5 and a f11 'frac that has slightly different positions for eye relief in the same EPs when used on both the scopes. 

This is more likely to be a change in exit pupil size.

[Moonshane]

I always seemed to prefer the plossl and ortho view but possibly based on age, now prefer the eyepieces with a more easily accessible field. Other things do come into it though such as weight/balance, apparent field of view and focal position (i.e. how far out an eyepiece focuses). I do still like plossl/ortho types with longer focal length / eye relief despite their narrower fields.

[Rick Towns]

33 minutes ago, Mr Spock said:

There has to be a lot said for comfort. Some of those ortho and Plössls, while sharp, aren't all that confortable to use.

That is the funny thing - so far, I find Orthos and Plössls more comfortable to the longer eye relief ep's I have. Maybe I'm a freak? LOL!

[bomberbaz]

I bought my Nikon EP which claimed to have 16mm of eye relief which I believe will be right. However the lens is quite concave, markedly. So you are losing 3, maybe 4 mm of this eye relief in this. When I used this wearing my glasses I had to do an awful lot of head moving to get the full fov out of the eyepiece and this somewhat spoiled the experience.

Not an issue now as I use a dioptrx but thats another story. 

[Louis D]

4 hours ago, Rick Towns said:

With the Orion, I was barely touching the eye cup with my... um... face (?) and I had to move my head around to avoid black outs. So I had to hold my head perfectly still over the sweet spot to get a good view.

The Orion likely has spherical aberration of the exit pupil.  It's not confined to just inexpensive positive/negative eyepiece designs like the Orion.  The Type 1 Naglers had it as well as the Televue Radians.  Sensitivity to it is increased in daytime viewing because the restricted entrance pupil of your eye (2mm or less) can't possibly accommodate all the different focus locations inherent with this aberration.  Some parts of the exit pupil will strike your iris no matter where you place your eye, thus causing the kidney-beaning.

Higher end positive-negative designs like the Ethos, Delos, XW, Morpheus, newer Naglers, etc. don't have it and thus don't have such sensitivity to eye placement.  I don't know about the ES lines if any of them suffer from it.

None of the simple positive-only eyepiece types like plossls and orthos suffer from SAEP, though you can induce it in some cases by putting them in extremely short barlows.

[YKSE]

The claimed ER for an eyepiece is measured from the center of eye lens, all the factors mentioned by John (recessed eye lens, hard/soft eye guide, concave/conve eye lens), and our physiological structure (how recessed are our eyes, bone structure around the eyes) will all affect the actual usable ER.

Are you sure you see kidney bean? or it's just blackout when you move your eye too close (since you're quite accustomed to shorter ER eyepieces).

[Rick Towns]

5 hours ago, YKSE said:

The claimed ER for an eyepiece is measured from the center of eye lens, all the factors mentioned by John (recessed eye lens, hard/soft eye guide, concave/conve eye lens), and our physiological structure (how recessed are our eyes, bone structure around the eyes) will all affect the actual usable ER.

Are you sure you see kidney bean? or it's just blackout when you move your eye too close (since you're quite accustomed to shorter ER eyepieces).

You are right - I am seeing a blackout.

[Rick Towns]

5 hours ago, Louis D said:

The Orion likely has spherical aberration of the exit pupil.  It's not confined to just inexpensive positive/negative eyepiece designs like the Orion.  The Type 1 Naglers had it as well as the Televue Radians.  Sensitivity to it is increased in daytime viewing because the restricted entrance pupil of your eye (2mm or less) can't possibly accommodate all the different focus locations inherent with this aberration.  Some parts of the exit pupil will strike your iris no matter where you place your eye, thus causing the kidney-beaning.

Higher end positive-negative designs like the Ethos, Delos, XW, Morpheus, newer Naglers, etc. don't have it and thus don't have such sensitivity to eye placement.  I don't know about the ES lines if any of them suffer from it.

None of the simple positive-only eyepiece types like plossls and orthos suffer from SAEP, though you can induce it in some cases by putting them in extremely short barlows.

The only thing I'll add is that my dining room (and house) were dark and I had two filters on my diagonal to cut the glare of the street light to usable levels.

[Louis D]

4 hours ago, Rick Towns said:

The only thing I'll add is that my dining room (and house) were dark and I had two filters on my diagonal to cut the glare of the street light to usable levels.

Try it out under stars after you've fully dark adapted and see if you still have blackout issues.

By the way, were the blackout issues kidney-bean shapes you chased around the periphery or simply the entire view blacking out at once.  The former is SAEP, the latter can have many causes, most of which are unknown.  My 12mm and 17mm Nagler T4s tend to be "finicky" about eye placement.  The view is steady and easy as you approach the point of being able to see the entire FOV to the fieldstop; however, the moment you reach that point, you can't go any further in by even a fraction of a millimeter or rotate your eyeball in the least or you will lose the view entirely.  The Delos, XL, XW, and Morpheus don't exhibit this behavior.  Neither does my 30mm ES-82.  As a result of this behavior, it is tiring to use either of them at full FOV, so I tend to pull back just a little.  The only thing I've noticed with them when measuring the exit pupil by shining a flashlight backward through them is that unlike the 65 to 76 degree field of view eyepieces mentioned, the T4s don't have a nice defined point of best focus where all the light reaches a sharply defined point.  It tends to extend over a couple of millimeters inward or outward with a rather fuzzy spot, so it is hard to say just exactly where the exit pupil lies.

Try shining a light backward through your Orion eyepiece onto a sheet of paper and report on what you observe.

[Rick Towns]

Both the BST Planetary and the Orion Edge-On Planetary have kidney-bean shapes that I chase around the periphery. This happens whether in my living room or out under the stars when I am fully dark adapted. That is why I keep coming back to Orthos and Plössls because I don't have any of these issues. Plus, I like to have my eye "in there" close to the lens, versus hanging back with the BST & Orion.

So I guess I was wrong earlier when I said it was black outs and not kidney beans... it is the beans! LOL!

[Louis D]

1 minute ago, Rick Towns said:

Both the BST Planetary and the Orion Edge-On Planetary have kidney-bean shapes that I chase around the periphery. This happens whether in my living room or out under the stars when I am fully dark adapted. That is why I keep coming back to Orthos and Plössls because I don't have any of these issues. Plus, I like to have my eye "in there" close to the lens, versus hanging back with the BST & Orion.

So I guess I was wrong earlier when I said it was black outs and not kidney beans... it is the beans! LOL!

Yes, a common problem with poorly executed positive/negative designs.  You gain AFOV and eye relief, but at what cost, frustration?  I refused to buy the Radians when they came out because of their strong SAEP.  Because the XW, XL, Delos, etc. all have well behaved exit pupils, they've been described as a spa for the eye on Cloudy Nights.  They're expensive, but worth it, IMHO.

[Rick Towns]

42 minutes ago, Louis D said:

Yes, a common problem with poorly executed positive/negative designs.  You gain AFOV and eye relief, but at what cost, frustration?  I refused to buy the Radians when they came out because of their strong SAEP.  Because the XW, XL, Delos, etc. all have well behaved exit pupils, they've been described as a spa for the eye on Cloudy Nights.  They're expensive, but worth it, IMHO.

Well, I am going to wander around at the next star party that I attend to see if I can take a peek. Thank you for your input on this!

[Louis D]

15 minutes ago, Rick Towns said:

Well, I am going to wander around at the next star party that I attend to see if I can take a peek. Thank you for your input on this!

A guy had a Delos in a Celestron NexStar Evolution 8" Edge HD at a local star party open to the public last summer, and everyone commented that the combination was putting up the sharpest, easiest to view images on the field.  I had to agree that it was much cleaner than the various large dobs and APOs on the field, even with Naglers and ES-100s in some of them.  Perhaps 8" was the sweet spot for viewing that night.  Perhaps this new generation of SCTs have very flat and well corrected fields.  Add to it the fact that he could control it from a tablet running sky charting software with DSO overlays was pretty impressive integration.