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How to select an eyepiece ?

The true field of view (II)

As we speak about the field of view, be accurate and remind you that the apparent field of view is indirectly linked to the true field of your eyepiece. The apparent field is defined by design. A Plössl eyepiece for example yields a field of 40 to around 52° or more if its original design is modified (63° for the Paul Rini series). All "super wides" optics yield an apparent field between 50 and 70° wide. "Ultra wide" series yield an apparent field over 70° and up to 90° (BW-Optik, Zeiss/Docter, etc). But all these etiquettes are relative and do not always respect standards.

Here is an example. The 30mm Widescan from Leitz and the one from Apogee provide an apparent field of view of 90° and 84° respectively. Some dealers like Markus Ludes from APM discovered these famous eyepieces had an internal field stop of 44 mm and 46mm respectively. That means the apparent field of view drops depending your measurement : a 90 degree apparent field with a 44mm field stop is equivalent to a 84 degree apparent field with a 46mm field stop. In practice, both eyepieces yield an apparent field just a bit smaller than 84°.

Tele Vue 14 mm Radian, Zeiss Doctor 12.5 mm and Carl Zeiss 12.5 mm Diascopic. Documents from manufacturers.

Then the true field of view is relative to your aperture ratio. A "fast" scope, using a short f/ ratio will  wider your field of view and will circle a larger star field than a longer focal scope. 

Using a NexStar 5" f/10 for example, a 14 mm Radian from Tele Vue with its comfortable 20mm of eye relief yields a true field of 40' and a magnification of 89x. Using a focal reducer 0.63x the f/ratio is reduced to f/6.3 and the same eyepiece yields a true field as wide as 1°04' at 56x.

If you ignore the apparent field on your eyepiece but you want to estimate its true field, here is a simple way to calculate it. Without engaging the equatorial motion place a star at the edge of your eyepiece taking care it moves by the center of the field to the opposite edge. Evaluate the time T of transit and apply next formula : (T / 0.997271 * 15) / 60 to get the true field size in arc-minutes. Using a Celestron 8" with a 40mm Meade SWA eyepiece, a star transits the field in 290 seconds. The true field of this eyepiece is 72.6' or 1.21° which is confirmed on my Excel sheet.

Two effects of eyepieces that determine their selection : as the power increases the true field of view decreases while the number of fainter stars increase (above) or, for a same magnification they enlarge the field of view (below). The first solution is recommanded as it provides a progessive magnification. Psychologically speaking the second method is barely used because an amateur does not see the advantage to get a second eyepiece that yields a wider field if the power does not change. He will automatically select the eyepiece provinding the widest field and will never use the first one anymore.

At last as we will see below, the eye relief and your way of looking through your eyepiece has also an "virtual" influence on the true field of view. While observing an extended field with a wide eyepiece, to say over 50°, many of us move our head to see the edges of the field. This way we compensate for the parallax of our pupil as we are no more centered with respect to the eyepiece. I presume that most observers do not explore their entire field of view and prefer moving their scope to center the subject in the ocular field. But using their eyepiece this way we do not take advantage of the wide field, excepting that helps in framing the next subject attracting our attention. But consequently, we really care about the central ~40° or maybe less which sharpness must be perfect, the image contrasted, crisp, without aberrations. So in these conditions why buying super wide eyepieces ? Think about that next time you will look through your wide eyepieces. I can save you some money… But as most deep sky hunters I prefer to frame the widest field and anticipate my next subject.

Aberrations

If you haven't the slightest idea of what could be the optical aberrations of a scope, I suggest to you to read first my report about aberrations in which I summarize the common aberrations that a scope may display.

Fast scopes are much more sensitive to residual aberrations than a scope using a longer focal and they become more critical on eyepieces quality you use on them. Here are some examples. The27 mm Panoptic from Tele Vue with its 68° of apparent field of view is in its range of focals one the sharpest low power eyepiece, suited for the majority of scopes supporting a 2" barrel. It displays pinpoint stars images on all the field, it is sharp and bright to the edge at f/8 and slower scopes but coma aberration becomes visible at f/4.6 or so.

Similarly the 30 mm Widescan from Leitz is a wonderful eyepiece, providing highly contrasted images and very sharp. But on a f/6.8 apochromatic refractor it starts displaying some distorsions at the edge of the field. Worse, on a f/4.5 newtonian the center of the field is fine but it becomes less crisp at 40% off axis where astigmatism and coma appear. In the same way the 24.5 mm SWA Plössl 4000 from Meade is a good eyepiece. It yields a super wide field very suited to binoculars fixed on f/10 catadioptrics and slower scopes for example but it is essentially unusable on a f/5.4 dobsonian. At that short ratio the image is not as crisp or sharp as in the 22 mm Panoptic or the 21 mm Pentax XL which are color and distortion free by comparison, it has false brightness at the edge and the field stop is not quite as distinct due to fast focal ratio. 

On the contrary the 22 mm Nagler IV from Tele Vue is as good in a f/10 catadioptric than in a f/4.2 Starmaster. In both scopes images are contrasty, comfortable with the instadjust eyecup and even of better quality than in a 22 mm Panoptic.

Tele Vue 27 mm Panoptic, Pentax 21 mm SMC XL and Tele Vue 22 mm Nagler type 4. Documents from manufacturers.

This is not necessary a reason to withdraw all low-end eyepieces or long focals if you are using a fast scope. For all fast scopes some accessories can prevent aberrations to rise, specially the Powermate and the Paracorr from Tele Vue. See my report about barlow for more details. In some configurations indeed a coma corrector can be useful, mainly using ~f/5 achromat refractors and dobsonians with longer focal length eyepieces like the famous Nagler 31 mm type V super wide and other Meade SWA.

At last when panning the sky with wide fields eyepieces, the starfield may seem magnified. The effect is more prononced to the edges where the frame looks bent. Is also more noticeable in daylight. This aberration is the pincushion and can bother some. To be accurate a positive distorsion in called pincushion, a negative one barrel. This geometrical distortion may distort up to 1% of the field. "Avantage", when the distortion is present, theoretically performances of your eyepiece are corrected up to the edge of the field. This is true for Tele Vue Panoptics's.

The eye relief

This is nothing to do with the human eye but rather with the light cone going out the eyepiece. Also known as the exit pupil distance, the eye relief is defined as the distance recommended by the manufacturer between the field lens of the eyepiece and your pupil to get the optimum performance. This distance expressed in millimeters must be held from the eye to see the full field and get a comfortable view. Generally as the power increases the eye relief decreases but in some eyepieces series manufacturers have preserved a long eye relief whatever the eyepiece focal to name designs as different as Celestron Vixen Lanthanum LV, Nikon ultra wide, Tele Vue Radian or Vixen LVW Lanthanum Superwide. They all have an eye relief of 20 mm or longer.

A sample of Vixen Lanthanum's LV and LVW superwide. Documents from manufacturers.

Observers generally consider that an eye relief exceeding 15 mm is comfortable as it doesn't require you to get too close to the eyepiece to see the complete image. This is also a very important factor to consider for eyeglass wearers that need a very long eye relief due to the room necessary for eyeglasses. Hopefully many eyeglass wearers don't need their glasses when viewing at scopes, including binoculars as they can change the focusing to adapt to their sight, excepting astigmatics who must keep their glasses while observing through a telescope.

But a long eye relief is not mandatory. It depends first of your apparent field of view. It must be know that a 20° wide eyepiece having an eye relief of a few millimeters can be supported by a eyeglass wearer whereas a 60° wide eyepiece requests an eye relief of at least 20 mm for eyeglasse wearers. This is one of the reason that explains why most lowest power eyepieces yield very long eye relief, exceeding sometimes 50 mm. But generally observers wearing eyeglasses and using long focals are satisfied with an eye relief between 20-30 mm.

But even using a 20 mm eye relief some observers experience blackout in the light cone of some eyepieces as they cannot find the optimum position to circle all the ocular field, using or not an eyeguard as distance of reference. This may occur with eyepieces from 32 mm and lower, even Plössl's 45° wide.

At night the pupil of your eyes opens and you can consider a shorter eye relief.. However when looking at a bright subject like a gibbous Venus, Jupiter or the Moon your pupil will close down a bit, reducing the advantage your have just won. So the solution is in-between, in a compromise between a long eye relief and potential blackout or keeping the eye relief but reducing a bit the field of view. Some manufacturers think to get the solution achieving a long eye relief with super wide eyepieces but this is most of the time to the detriment of some definition loss at the edge of the field. This happens quite often with manufacturers marketing cheap eyepieces. In their serie the eye relief is comfortable, the field extends over 65° but yields astigmatism at edge or display a poor quality image. We will never repeat enough that in a quality eyepiece the design cannot sacrify a parameter for the benefit of marketing concerns.

Relation field of view - eye relief. Adapted from ATM.

To be fully objective we have to clarify a last thing. Some experienced observers will claim that altough some recent lines of eyepieces (Radian or Nagler) have an ample eye relief, there are hard to use by eyeglasses wearers, what seems in contradiction with what we just explained. We can rightly wonder how such a difference can arise between individuals about a factor that we all consider as sufficient and comfortable. Can we explain this fact by invoquing the personal equation or something related to the observer like the type of eyeglasses or something else ?

Indeed some amateurs wearing glasses can see the whole field of certain eyepieces where others amateurs cannot circle the whole field even with their glasses off. The problem occurs with high-ends eyepieces among which the 27 mm Panoptic that has an ample eye relief of 19 mm but also Radian's or Nagler's. Several other brands are also subject to this problem. It occured for example that an amateur could easily see the entire field of view without glasses and could not see it entirely wearing his glasses, no matter where he put his eye to catch the light cone.

Answering to a post, Albert Nagler pointed out that any eyepiece is designed such a way that we see the entire field of view assuming that we look straight ahead and do not move our eyeball. So when our head or our pupil moves laterally quite a bit as we scan to the edges of the field it is no longer centered with respect to the eyepiece which explain that we lost partially the light cone. Such a move can exceed 20 mm what is quite often larger than the field lens of most eyepieces and often sufficient to move the pupil clear out of the area where the image is entierely visible.

But there is another factor more subtle. By saying we can see "the entire field" if we don't move Albert Nagler passes over the fact that our eye cannot see with all necessary acuity the entire field it observes.  In fact the eye can only resolve a few degrees around its central axis, to say ~10°; the illusion of an extended field comes from the fact that we scan over the field with our fovea by moving our eye and/or our head. Inconsciously we tend to be closer to the eyepiece in order to see the entire field of view. This phenomenon occurs probably quite often with all wide fields eyepieces, whatever their eye relief. 

University Optics Konig MK-70 of 40 and 25 mm, Tele Vue 31 mm Nagler type V and Leica 30 mm widescan type II with its movable rubber eyeguard. These four eyepieces provide AFOV from 68 to 88° wide. Documents from manufacturers.

This behaviour explains also why certain observers consider that some eyepieces yield crisp images and pinpoints stars at the edges of the field while others don't appreciate their image. That depends of the way they look and the field, with their peripheral vision or using their fovea. Using the averted vision the seeing accuracy decreases at the advantage of faint lights. On the opposite,  looking at the edges of the field with the fovea you will get very accurate pinpoints stars but your sensitivity to faint lights will be strongly reduced.

Therefore if you are subject to myopia you will probably not fully explore the entire field of the widest eyepieces as you compensate and go closer to field lens. You will probably not appreciate the blackout you experience when looking at the edges of the field "in once" were astigmatism can also appear. Such feelings, half physiological and half voluntary are not easy to recognize and their measurement requests a high level of concentration and numbers of tests both in situ and in front of a mirror to really achieve a complete study.

So don't blame or disapprove too fast amateurs having a bad appreciation of some quality eyepieces as their opinion could reveal the true limits of these models.

Next chapter

The exit pupil

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