Eye relief vs F/ratio

[vlaiv]

We know that barlow extends eye relief of eyepieces and some time ago I asked why would that be and was given an answer that I did not really understand well.

Yesterday I had a brief session from my balcony - just a bit of planetary / lunar with Mak102 - very rewarding session indeed although seeing was not perfect (I have not been observing in ages). I had my BCO 6mm on my desk as I was meaning to sell it for quite some time - and for some reason I decided to try it in Mak.

I was surprised by how much eye relief there is in this eyepiece. I remember it being very tight on my F/6 8" dob. Then I realized that it must be the same effect as with barlow. I'm now using it with F/13 scope and it feels quite comfortable.

In any case, I do have one plausible explanation that I'll try to sum up in a diagram, but I'm not 100% sure it is solely down to that.

Ok, I'm pretty sure I made a mess with above diagram, but let's try to decipher what I draw.

It is about exit pupil size. Large exit pupil will have tighter eye relief than smaller exit pupil. In red we have rays of larger exit pupil. If we mark intersection of all of them - we get eye relief position - that is dark red vertical line.

If we now observe smaller exit pupil - marked in blue - we can see that full intersection of those lines actually moved further away from eye lens.

Here in diagram I'm making one assumption that stands to reason, but I'm not completely sure it is true - edge of the field rays that are still at full illumination will emerge at extreme ends on eye lens - like it has been drawn and not closer to center. Or to put it in another way - edge of the field "pencil" will touch edge of eye lens.

We know that exit pupil is determined by F/ratio of scope and focal length of eyepiece (when we divide the two). 6mm BCO in F/6 scope will give 1mm exit pupil while same eyepiece in F/13 scope will give 0.46mm exit pupil.

This effect is more noticeable with narrower field eyepieces because it depends on max angle exit pencil will make. With simple designs that angle is often 50 degrees or less (or rather half of that - because angle is observed with respect to optical axis).

Does this make sense and could it be explanation for why barlows make eye relief longer and why eye relief varies with telescope (f/ratio) used? Maybe this is the reason why people used Orthos in the first place - they had decent eye relief on F/15 scopes

 

[jetstream]

Maybe the effect is related to how a barlow works?

[vlaiv]

15 minutes ago, jetstream said:

Maybe the effect is related to how a barlow works?

Barlow indeed produces higher F/ratio for any given scope and if above is correct - that would be explanation, but I don't remember that being offered as an explanation in previous discussion.

Maybe it would be beneficial if I find actual discussion I was referring to and explanation given there.

Ok, here is original discussion:

And here is interesting point:

Quote

Long focal length telescopes produce near parallel light cones. They will give a short eye relief.

Short focal length telescopes produce slightly divergent light cones. They will give a slightly longer eye relief with the same eyepiece.

The light cones after passing a Barlow diverge strongly. The Barlow will noticeably increase the eye relief of the same eyepiece.

A Focal extender may produce an even more parallel set of light cones than the original objective does. In that case, the extender will give the shortest eye relief, again with the same eyepiece.

 

[Louis D]

I don't know about longer eye relief on longer f-ratio scopes, but I do notice much stronger SAEP and CAEP in longer f-ratio scopes as I showed in this thread.

[vlaiv]

I just realized that Ruud's explanation given in the thread that I linked is much more likely than mine above. Mak has secondary mirror that magnifies - that makes rays look like they come from much shorter focal length - so I guess that is the reason behind longer eye relief in Mak?

In any case, I made one wrong assumption - smaller exit pupil will not necessarily touch edge of eye lens and whole thing with exit pupil can easily be tested - one just needs aperture mask to create smaller exit pupil and leave everything else the same (focal length and eyepiece). Adding such aperture mask should change eye relief if it is related to this.

On completely different note, @Louis D, very interesting thread you linked and your images seem to show something that has been troubling me. I get this dark spot in the center of the view sometimes and I don't understand why it is there. It is not round and it causes blur / scatter and some light fall off.

It resembles the most to images of GSO and Orion plossls 32mm. When I move my eye - this dark spot "counter moves" - or rather it's position relative to eyepiece stays the same. I can often "look behind it" (sort of). If I place it directly over bright source than everything turns nasty (like placing Jupiter behind it) - so much scatter appears in FOV of eyepiece.

This is not scope related nor barlow related nor eyepiece related since I experienced this with different eyepieces / scopes / barlow or no barlow situations - but it does not happen always.

For example in ES82 11mm and Mak102 - it was there but very subtle, more like just a bit of "misting" in that place rather than shadow. With ES82 6.7mm it was very obvious and distracting. With BCO 6mm it was evident but somewhere between other two in intensity.

What could that be and why does it happen?

[jetstream]

35 minutes ago, vlaiv said:

What could that be and why does it happen?

Re :scatter -have you checked your eyes for scatter on a street lamp at night? If I remember I looked through eyepieces from the bottom, our eye scatter is shocking showing under certain conditions.

The dark spot would imply a mismatch between your eye and exit pupil of the scope/eyepiece I think?

[andrew s]

I may well be recalling this incorrectly but my understanding is the the the exit pupil is the image the eyepiece makes of the entrance pupil or aperture stop i.e. the objective lens or mirror. (I can't  recall which but they are the same on a  normal telescope.)

Any Barlow or Cassigrain mirror will change where this is. I would need to do the sums to see if this has the effect of making the objective seem closer to the eyepiece and so increase the eye relief. 

Regards Andrew 

Edited July 28, 2020 by andrew s

[vlaiv]

33 minutes ago, jetstream said:

Re :scatter -have you checked your eyes for scatter on a street lamp at night? If I remember I looked through eyepieces from the bottom, our eye scatter is shocking showing under certain conditions.

The dark spot would imply a mismatch between your eye and exit pupil of the scope/eyepiece I think?

Yep, I've got some very strange astigmatism in my right eye so I amuse myself by trying to figure out PSF of street lamp (I see three moons / street lamps with my right eye).

This is more like grease smudge - that sort of ghosting / scatter - it does not  move with my eye but rather stays put with respect to eyepiece.

My first reaction was that it is grease from eyelashes / eye - since it is in center of the eyepiece and it looks like that, but cotton swab with cleaning fluid made no difference - I expect at least to spread it around if not clean it completely. It is also present in various eyepieces (although that means nothing since I could have contaminated eye lens in all of them - but effect changes with eyepiece / scope combination).

 

[Don Pensack]

The exit pupil is the image of the primary mirror.

As you approach the eyepiece (we'll assume it is in focus), you see the truncated field until you are at the exit pupil, where you finally see the field stop.

Essentially, when you were farther out, the edges of the lens vignetted the field with a hard stop because the rays from the outer parts of the field hit your eye outside the pupil and you could not see the entire field.

If you move your head back and forth when outside the exit pupil, you can see different parts of the field.

At the exit pupil, all the light of the field goes into your pupil and you see the entire field.

When you move inside the exit pupil, your pupil is now smaller than the light cone from the lens to the exit pupil and once again your pupil sees only part of the field.

But now there is no hard edge to the field created by the aluminum surrounding the lens.  You do not see the entire field, but you see blacked-out portions of the outer field floating around because

your head and eye are always wandering.  Unlike the top of the eyepiece, there is nothing outside the visible field to focus on, so the blacked out areas appear vague and amorphous as they float around.

 

Now, if the exit pupil is smaller than the pupil of your eye, you do have some freedom to drift in and out relative to the eyepiece and still see the entire field, though you may need to refocus as you do so (depends on your age and your ability to accommodate).  Likewise, you can drift back and forth laterally slightly and still see the entire field, though we instinctively try to hold out eye in the center so our foveal vision can examine the image.

 

If the light rays enter the eyepiece parallel, they are bent by the eyepiece and the image at the exit pupil is a certain distance from the lens.  If the light rays enter the bottom of the eyepiece diverging, as, for example, after a Barlow lens, 

then the bending of the rays that occurs in the lenses will still occur to the same degree but because the incident rays are diverging, the image formed at the exit pupil will be extended outward from the eye lens.

[andrew s]

1 hour ago, andrew s said:

I may well be recalling this incorrectly but my understanding is the the the exit pupil is the image the eyepiece makes of the entrance pupil or aperture stop i.e. the objective lens or mirror. (I can't  recall which but they are the same on a  normal telescope.)

Any Barlow or Cassigrain mirror will change where this is. I would need to do the sums to see if this has the effect of making the objective seem closer to the eyepiece and so increase the eye relief. 

Regards Andrew 

A quick ray trace with pen and paper shows a Barlow produces a virtual image of the objective closer to the eyepiece. The eyepiece will then reimagine this further out from its back focus and hence increase the eye relief. 

Regards Andrew 

Edited July 28, 2020 by andrew s

[vlaiv]

2 minutes ago, andrew s said:

A quick ray trace with pen and paper shows a Barlow produces a virtual image of the objective closer to the eyepiece. The eyepiece will then reimagine this further out from its back focus and hence increase the eye relief. 

Regards Andrew 

In the end, I guess that is proper explanation, as for Maksutovs (and other Cats) - it is magnifying secondary that acts the same, right?

[andrew s]

7 minutes ago, vlaiv said:

In the end, I guess that is proper explanation, as for Maksutovs (and other Cats) - it is magnifying secondary that acts the same, right?

Yes it's exactly the same. Both the Barlow and Cassigrain secondary have negative focal length and so, I belive,  in the paraxial approximation they are equivalent.

Regards Andrew 

Edited July 28, 2020 by andrew s

[John]

The extent to which eye relief is altered depends on the focal length of the barlow lens I believe - the shorter the barlow FL, the more eye relief is pushed out. It's not easy to find that specification for barlows though. The actual positional change also depends on the focal length of the eyepiece as well I think.

 

 

 

[andrew s]

Just now, John said:

The extent to which eye relief is altered depends on the focal length of the barlow lens I believe - the shorter the barlow FL, the more eye relief is pushed out. It's not easy to find that specification for barlows though. The actual positional change also depends on the focal length of the eyepiece as well I think.

 

 

 

Yes quite right. You won't find it specified as it depends on the focal length of the objective, the Barlow and the eyepiece and exactly where they are position with respect to each other.

For an objective at infinity the eye relief would be at the eyepiece back focus. As the objective gets nearer eye relief moves out away from the eyepiece. A stronger Barlow reimagines the objective close to itself (and the eyepiece)  than a weaker one. But the detailed position will change to achieve focus.

Regards Andrew