Don Pensack

Updated link with revisions and corrections:

https://www.cloudynights.com/topic/700069-2020-eyepieces-buyers-guide/?p=10103883

I have compared the 2 directly, visually, and the only things I note are:

--a (slightly) great amount of light scatter in the field of the ES FE

--additional in-focusing required with the ES FE versus essentially none with the PM.

This is a Zombie thread, but whether the differences would be noted would depend on a lot of other factors.

21 minutes ago, miguel87 said:

This is all I was saying, that it will dim with magnification.

I know it's not huge because they are such ti y objects.

Having said that, the brightness of Vega last night in my 32mm and 6mm was very noticeably different.

 

That could have been the eyepieces in question.  Assuming your scope is f/5, I would guess a 1mm exit pupil (5mm eyepiece) would be about the minimum for the Airy Disc to present a noticeable size.

At 6mm, you should not have detected any decrease in brightness of Vega.  The overall field brightness would be dimmer, but Vega?  I would suspect your 6mm eyepiece may have a significantly lower transmission than the 32mm.

On 11/05/2020 at 12:19, miguel87 said:

Even tho the star is a point source, it still reflects of every part of the primary mirror/lens, and because the exit pupil is an image of the primary, the larger the exit pupil, the brighter the star will be.

Even if the exit pupil is 10mm, you will still get 5mm+ into your pupil. Brighter than 1mm.

It might appear relatively brighter because the background sky will be much dimmer at 1mm. But if you measured the brightness of the star point it will be dimmer in a 1mm exit pupil than a 2, 3, 4 etc

Nope.  The star image's brightness represents the entire primary.  When you use a higher power, you stop down the field size of the eyepiece, but every point on the telescope's focal plane is STILL illuminated by the entire primary.  You aren't reducing the brightness of the star point.  That's why the faintest stars are always visible at high power, not low.  Yes, there is an increase in contrast, but that increase in contrast would not occur if the star images dimmed with magnification.

On 11/05/2020 at 11:35, John said:

I guess this may or may not be related to all this but I find that using high magnification (very high sometimes) and obviously very small exit pupils helps me pick dim point sources out, eg: super novae, quasars, faint planetary moons etc. These don't seem to be as apparent at lower magnifications.

Or is that something different at work ?

 

That is making the detail large enough for the eye to see.  It's a fine line: dimmer with magnification, but larger and easier to see.  There is a "eutectic" point somewhere in there that yields the best visibility.  That usually requires experimentation.

Say you use and eyepiece that yields a 7mm exit pupil and your pupil size is 7mm.  That will be the maximum brightness you can see in the scope.

Use an eyepiece yielding a 3.5mm exit pupil and the image will be 1/4 as bright due to the smaller exit pupil.

You can also look at it another way--the doubling of the magnification results in the brightness being 1/4 as great.

Now, use an eyepiece that yields a 14mm exit pupil (refractor in this illustration).  You have effective stopped down the scope by only admitting 1/4 of the light in the telescope's exit pupil.

Why is the image the same brightness as the eyepiece that yields a 7mm exit pupil?

Because the magnification is only 1/2 as much, which results in a 4X increase in brightness.  1/4 x 4 = 1.

That's why using a larger exit pupil than your eye doesn't result in a dimming of the image, and why using a smaller exit pupil than your pupil does.

22 hours ago, miguel87 said:

If an eyepiece 'outputs' parallel lines, I dont understand how the exit pupil comes about, hovering above the eyepiece?

The exit pupil is an image of the primary mirror.  It is not the image of the sky.  It is the distance we hold our eye from the eyepiece to see

the entire field of the eyepiece.  To show an eyepiece is not focusing the light, back away from the eyepiece.  The image stays in focus, but you are progressively seeing less and less field.

Exit pupil and field go hand in hand, but focus is in the eye.

On 05/05/2020 at 09:33, bingevader said:

And this is still true if the magnification remains the same but the diameter of the EP increases (so no reduction in magnification to brighten the image)?

I will quite happily stand corrected, I've been living with a misconception for some years!

There was a vogue for larger and large EPs at one point and I remember the same conversation then.

Maybe I didn't remember it and that's the problem!

I think you meant if the field size increased, not the diameter of the eyepiece (which has little relevance).

The exit pupil (brightness) and magnification go hand in hand.  A larger apparent field spreads the light farther into your peripheral vision, but does not brighten the image.

If it did, we'd all want to use 150° eyepieces.

The purpose for larger eyepieces is to get wider true fields, because a 32mm 50° eyepiece will have the same brightness as a 32mm 100° eyepiece, but the latter has 4x the field area.

Remember, every point on the focal plane of both eyepieces is illuminated by the entire primary (* see below)

* In practice, we do not choose secondary sizes that illuminate the edges of the field to 100%, we choose secondaries to have about a 30% light drop off at the edge (we don't see it, though a camera can,

so photographic secondary sizes are larger).  So if a 32mm 50° eyepiece has a 30% light loss at the edge, a 32mm 100° eyepiece would have significantly more light loss at the edge.

That's why we choose the size of our secondary mirrors to illuminate the field stop of our lowest power, largest field, eyepieces to 70° at the edge.  At some point, as the magnification goes up and the field stop of the higher power eyepieces get smaller, the illumination at the edge reaches 100% because the effects of secondary edge of field light loss gradually fall to zero.

Yes, KUO does make these eyepieces.

On 05/05/2020 at 08:01, miguel87 said:

Probably getting out of my depth here but anyway...

I was  looking at exit pupil sizes and all that jazz and my largest is 6.5mm and with my eye snuggled up to the eyepiece I can only just see the entire FOV without moving my gaze. This fits with the idea that my pupil size is similar to the exit pupil image.

BUT, imagine that the exit pupil was 7.5mm and I had to 'look around' a little to see the edge of field. The exit pupil image is a resolved image, so not being able to see the edges would not dim any objects in the middle because my pupil is receiving all the light available for the object in the middle. This makes me think that a 7, 8 or 9mm exit pupil is not 'wasting light' like other people sometimes suggest. That's like suggesting that once an oil painting becomes so big that you cant see it all without moving your head, that it is somehow dimmer and the artist was wasting canvas.

Finally this got me thinking about the link between AFOV and exit pupil. If I have one of these expensive 100° eyepieces, the exit pupil must be bigger than 7mm, otherwise I could see the edges without moving my pupil around?

I know I must be wrong because AFOV is not a factor in calculating exit pupil size.

So can anybody explain where I am going wrong and how (if at all) exit pupil and AFOV are related?

Thanks!

You misunderstand the relationship between exit pupil and apparent field.

The exit pupil is the image of the primary mirror.  Essentially, every single point in the exit pupil is illuminated by the entire primary mirror.

When your iris blocks part of that exit pupil, you are blocking part of the primary mirror (an astute observer would note that so does the secondary mirror in both center and edge), but it doesn't reduce the apparent field of the eyepiece.

It reduces the field illumination in the eyepiece.

But you can simply move your eye laterally to see one edge of the field or the other since the field is being illuminated exactly as before, merely that your eye is not taking it all in.

Your supposition that a larger exit pupil is not wasted is correct.  The reduction in magnification brightens the image and the light loss exactly equals it, so for the field you see, it will be exactly as bright as the image when the exit pupil matches your pupil diameter.

That is only safe with a refractor, though.  With a reflector, as the exit pupil grows larger, so does the shadow of the secondary.  At some point, the shadow becomes a large portion of your pupil diameter and you start noticing its presence.

7 hours ago, chiltonstar said:

Just a comment. I have a 15mm SLV which I bought (partly on advice from @John) for my Mak 180, mainly for planetary viewing as it gives about x190 - ideal for me usually for Saturn and Jupiter.

I have not seen any obvious reflections with it although I've not used it for Venus (a cheaper well-known planetary EP I have does have objectionable reflections from the brighter planets, and gives visibly less detail). The SLV gives superb detail in the centre of the field and a wonderful colour rendition, showing the warm peach colours of Saturn very well. Eye relief is good and it is nearly parfocal with some orthos I use. The only optical failing I've found is slight CA in the outer part of the field - very obvious if a double star is allowed to drift across the field. A small failing though.

Chris

and it's a rare eyepiece that doesn't have some chromatic aberration in the outer field.

If you have to stick to 1.25", the APM 24mm Ultra Flat Field would be a good candidate.

It's also sold by Altair as their Ultra Flat in a fairly unique green anodizing.

As has been reported elsewhere, Meade dealers don't have the 26mm, only Meade,  and I'd be afraid they would run out permanently.

On the other hand, it could be they've just arrived.

The thread on the eyepiece:

https://www.cloudynights.com/topic/701095-meade-26mm-100afov/

Looks like it is not the 100° claimed, also that the eye relief is perhaps less than 20mm.

It looks like there is no Omegon version of it, unlike the other focal lengths.

It sounds like perhaps the issue has been addressed with current batches.  If so, when buying used, caveat emptor.

Plössls have very little eye relief when shorter than about 14mm focal length.  These go down to 3mm with about 14mm Plössl eye relief.

It's reasonable to think of them as longer eye relief Plössls.

On 29/04/2020 at 15:30, bomberbaz said:

Have you not tried the Televue DIOPTRIX Don. I use one now and I love it. Very easy to use too. 

Yeah, I have tried them on the Ethos, but noticed 3 things:

1) the eye relief shrinks a lot--to about my eyelashes length

2) the images at the edge seemed more distorted when looking at the edge directly because of the oblique angle through the DioptRx lens

3) it was harder to look at the edge of the field with the DioptRx in place.

In contrast, I have no similar issues with the DioptRx on eyepieces with longer eye relief or narrower fields.  Fortunately, I can still use Ethos eyepieces below 11mm without glasses.

I didn't want to pre-prejudice your review with my comments.  No posted comparison from me.  I'll wait for your comments.

On 29/04/2020 at 18:08, Louis D said:

Actually, I want a 25mm ES-92 for under $600.

Looks like Meade did it for you: the 26mm MWA is around 90° and very inexpensive.

1 minute ago, John said:

What does astigmatism from the eye look like in the eyepiece Don ?

 

That will vary, John, with the nature of the eye's astigmatism.  For me, stars became small stick men instead of points.  Others see the stars turn into comets, even in the center of the field.  Others see a few spikes that don't correspond to the spider vanes.  But, it is visible in the very center of the field unlike coma.  With my current prescription, I can look at the sky with my glasses on and all the stars as points.  If I take my glasses off, all the stars look WORSE than Venus when Venus is on the horizon.  And faint stars disappear.

9 hours ago, jetstream said:

Me too, with the exception of the 42mm LVW for large nebula observing- the TSA120 is a super, super nebula telescope which will show IFN easily.Inserting a Delos into the focuser gives as close to a zero scatter combination I think is possible, using the prism diag. Actually I must say the Vixen HR's are the least scatter EP's I've seen.Getting down to splitting hairs here.

I wish Televue would make a 25mm Delos.

 

Would you want a 25mm Delos if it was 2" and weighed close to twice what a 17.3mm weighed and cost 50% more?

15 hours ago, Stu said:

I really enjoyed my 22mm Nag and still miss it to a degree. Not small, but compared with others like the 21 Ethos it is much more compact. The 21 Ethos is better though if you can stretch to it or the bulk/weight is acceptable. Dropping another 1mm gets you to the APM 20mm 100 degree which is lighter, 677 vs 1020g

Some discussion here:

I used the 20mm APM XWA for a night to compare it with the 20mm ES 100° and the 21mm Ethos.  To my eye, the ES and APM were nearly identical in performance, but neither had the contrast or sharpness of the 21 Ethos.

Was it a 2.7:1 difference in performance, given the respective prices?  Maybe not.  It just depends on what you can afford, I think, or require.

19 hours ago, jetstream said:

If I was going to get a 22mm eyepiece I would drop 1mm and get the 21mm Ethos. The other one that might tick the boxes is the 26mm T5 Nagler. They are heavy eyepieces however.

The TSA120 can make almost any eyepiece look good IMHO

Mine seems to like the Baader/Zeiss 2" prism diagonal- what a low scatter combination.

Assuming you don't wear glasses.  I just went back to a 22mm Nagler after 10 years with a 21mm Ethos because my astigmatism has gotten bad enough I needed the eye relief.

21 hours ago, tonyowens_uk said:

I have an APM 30mm UFF on backorder with APM. I plan to assess the Pentax XW30 v the APM UFF 30 when I get my hands on one. I'll do this with and without the field flattener fitted to the 80mm APO.

I'll be interested in reading your comparison.  I did just that with a 31mm Nagler, 30mm UFF and 30mm XW just 2 months ago.

5 hours ago, JeremyS said:

I'm looking for one of the above to use in my Takahashi TSA 120. I want wide field, ~82deg. Oh, and perfect optically to take in those wide fields.

Also I want as light weight as possible and not a bulky profile (i.e. not one of those EP's with a shroud/carapace).

The Nagler 22, type 4 looks good. But it's quite dear. Also why is it only type 4, whereas other FL's are on T5 or 6? 

The ES 24mm 82deg is a lot cheaper, actually half the price!, but is it as good optically?

Thoughts?

The Nagler Type 4s were long eye relief.  Type 5s were to create longer focal lengths.  Type 6s were to create shorter focal lengths and binoviewer compatibility.

No real difference in performance.

The ES eyepiece has more scattered light and unresolved outer field astigmatism, but is pretty good above f/6.

I personally use a 22mm T4 Nagler and I'd recommend it.  It's perhaps THE eyepiece that defines "immersive".  Seriously, it's a special eyepiece.

within your requirements, what Louis recommended is a great one.

I have an f/7 triplet as well and found the TeleVue Delites gave superb performance.

I hesitated about a longer focal length, but if you do not wear glasses, the obvious eyepiece is the TeleVue 24mm Panoptic.

If you do wear glasses, the 24mm APM Ultra Flat Field.