Don Pensack

But that is simply not relevant to the use of telescopes. If a star occupies 1/10 of one pixel and is magnified 10x, it still occupies 1 pixel. Hence, all its light falls on one pixel. In both cases, the star appears to be equally bright. It is not until magnification increases the size of the spurious disc to a visible size that from that point on the object dims with increasing magnification. Hence, contrast will improve right up to that point and stay the same from that point on. It's rational to argue that brighter stars appear to have larger spurious discs, because more of the light in the center of the Airy disc is above the visibility threshold. The Airy disc is actually the same size as a faint star, but a lot less of the spurious disc is visible. Therefore, the point at which the star image begins to dim with increasing magnification because it is visibly an extended object will differ for bright stars than it does for dim stars. There are stars that stay points to the eye all the way to the maximum magnification because of that. So the books that discuss a 1mm exit pupil as the point where the Airy disc becomes an extended object to the eye are not taking the apparent size of the spurious disc into account. Here is a cross section of the Airy disc pattern. The Airy disc is from the first minimum on one side to the first minimum on the other. The horizontal line is the threshold of visibility. It slides upward when stars get fainter, reducing the width of the spurious disc in the center and increasing the apparent gap to the first ring.

You are arguing that the contrast between two extended objects changes with magnification. This is contrary to everything I've ever read or studied in school about optics.. In the case of your two extended objects, multiplying by 10 lowers the surface brightness of each equally to 1/100 of its former surface brightness. If the ratio were originally 100,000:1 (your first example) the final example is still 100,000:1(your second example). Contrast is a ratio, not the amount of difference. Yes, taking 2:1 in size, a difference of 1, and doubling, yields 4:2, a difference of 2, but this is not how contrast is measured. And all the discussions of the eye I have seen or run across describe the eye as a contrast-sensing device, and they are describing contrast as a ratio of dark to light, not a simple arithmetical difference. And that the eye's contrast threshold lowers with decreasing brightness, from about 1.02 in daylight, to 1.20 or more with fully scotopic vision. In your example, the difference in surface brightness is lessening with magnification, which implies that we would require larger differences between brightnesses at high powers than we do at lower powers. Yet, the opposite appears to be true. Yes, contrast will change between a star and the background with magnification, but it is because one acts like an extended object and the other one doesn't.

The ONLY bummer about the Nikon NAV-HW 17mm is that there is no optimum position in a TeleVue Paracorr for it. Whereas the 17mm Ethos is optimized at setting A, the Nikon should have a setting lower than A to get it close enough to the lens. It is because the lower barrel extends farther beyond the lens than it does in the Ethos. A machinist could take a few millimeters off, but I doubt anyone would bother. Setting A on the Paracorr won't be optimum, but the coma correction will still be 95%, so it may be a moot point. Obviously, this doesn't apply to other types of scopes than newtonians.

I've compared the two of them along with the 24mm APM Ultra Flat Field (also available from Altair). The only one compatible with glasses is the APM. The best in a 12.5" f/5 scope was the Panoptic because the star images were the best at/near the field stop. The ES at that f/ratio had too much astigmatism in the outer field. The APM came close to the Panoptic, but did deteriorate right near the field stop. In my f/7 Apo, though, the ES cleans up and only has a bit of astigmatism right near the field stop and the other 2 are essentially equal. [Bill Paolini's scopes, it should be noted, are all f/8 or longer, so weight his reviews accordingly] I didn't have a longer scope on hand, but given the difference between f/5 and f/7, I could pretty much guess that at f/8 or longer, all three would be essentially perfect. One odd thing I would note: the APM seems to have a dead-flat field of view, like looking at a map. The Tele Vue has a sharp field to the edge, but seems to be a bit "bowl-shaped" as if the field is a tad curved--this is noted as the scope is panned while looking through the eyepiece. I attribute this to about a 5.5% field distortion on the Panoptic compared to 0.27% in the APM.. The ES was ~4.7% All had noticeable pincushion distortion when used in daylight on straight line targets like a telephone pole. This was assuming the manufacturer's apparent field claims, which may or may not be accurate. I did not measure the apparent fields exactly.

I've extensively used the GCE filter (and about 51 others) in the last few years. I find the GCE is a very gentle filter and makes virtually no difference at all in light polluted skies. In fact, I find the light scatter inside the filter to make the image yield lower contrast with the filter than without. But in a very dark sky(mag.21.5mpsas), the slight improvement in contrast can be seen, though it's certainly not a great improvement. It's interesting you mention the Astronomik CLS, as this was another filter I thought was very similar to the GCE. However, I gave the nod to the GCE for a very subtle increase in contrast over the Astronomik. In neither case did the filter do as much for the view of nebulae as the Baader UHC-S, which has a narrower bandwidth than the DGM or Astronomik (yet still qualifies as a broadband filter) or the Lumicon DeepSky. I didn't really find that any filter helped the appearance of galaxies, though any nebula filter did help the visibility of H-II regions in M33. Steve is generally right about galaxies--if you want to see them better, up the magnification, to at least 15x/inch in my recommendation.

Sorry, the logic is poor. If stars behave like extended objects and dim with magnification, it is the case that they must have a surface brightness across the spurious disc of X amount. The background sky surface brightness is, let's say, Y. So the contrast is X/Y (we'll ignore the fact that X is really X+Y). As we increase the magnification, the ratio between X and Y cannot change. So contrast between the star and the sky cannot change with increased magnification. If you have a faint star sitting by itself, you should be able to see it at low power just as you can see it at high power. But you can't. No one can. There is not enough contrast to do so. So you have to argue that, somehow, the contrast between star and sky must change with increased magnification. That is illogical, since if the star acts like an extended object, the contrast cannot change. Yet, we have the experience of thousands of observers over hundreds of years that faint stars do become more visible with magnification. This can ONLY be true, logically, if stars do not behave like extended objects in a telescope. Here is Schaefer's paper on limiting magnitudes in a telescope. http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1990PASP..102..212S&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF And a comment about the article by Nils Olof Carlin: http://web.telia.com/~u41105032/visual/Schaefer.htm and a limiting magnitude calculator based on Schaefer's work: http://www.cruxis.com/scope/limitingmagnitude.htm Note how the limit goes deeper with increasing magnification. The point of the links is to note that there is ample work to show that contrast increases with magnification. THIS CAN ONLY BE TRUE IF THE SKY ACTS LIKE AN EXTENDED OBJECT AND STARS DO NOT. My earlier point that stars do not behave like extended objects until the eye resolves the spurious disc is a possible compromise position.

But to the eye, it is a point source up to about a size of 1'. Another way to say that is that the star image may expand, but if it still occupies only 1 pixel, the camera isn't going to see it dim. The eye has to begin to resolve the Airy disc with a visible size before magnification treats it as an extended object. Were that not true, we would see the faintest stars at low power, which is not the case. We see the faintest stars at high powers. And contrast with the sky will continue to improve even above that magnification because the star's image will be ever brighter to the eye in comparison to the sky. So, obviously, the dimming of the sky does not match the dimming of the star. Ergo, stars do not behave as extended objects in a telescope.

OK. We know the size of the Airy disc in a scope isn't relevant because then we'd all use only short f/ratio scopes, where the Airy disc is smaller. But, the scale of the focal plane is smaller also. At the same magnification, the Airy disc is aperture-dependent, not f/ratio dependent. In general, the Airy Disc becomes resolved as an extended object in scopes at around 25x/inch (200x in an 8" scope), or 1x/mm or an eyepiece focal length that equals the f/ratio. This is assuming 20/20 vision normally, but very high contrasts might allow us to see the spurious disc at lower magnifications. Viewing Vega might be an example of high contrast, so I'll grant the Airy disc might be visible in the 6mm eyepiece. At whatever magnification the Airy Disc becomes an extended object, you have to exceed that before the surface brightness dims with increasing magnification. With stars, though, this occurs are really high magnifications. Slightly exceeding the magnification where the Airy Disc becomes visible (or the spurious disc in the center of the Airy Disc more accurately) will not noticeably dim the star. You would have to greatly exceed the 1x/mm magnification to appreciably dim a bright star like Vega. To make it noticeably dimmer would require MUCH higher magnifications than a 6mm eyepiece would yield, as Vlaiv said in his post. So I don't buy that the reason you saw it dimmer had anything to do with the size of the Airy disc. There is likely something else at work, here.

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.

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.

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.

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.

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.

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.

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.

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.

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.