SDM Telescopes
16-40 inch custom built classic style Dobsonians
 

Eyepiece Notes

The Europeans have a very romantic name for eyepieces. They call them oculars, ockulars and even okulares. Ocular has to do with the eye, and these names hark back to a former era when the centre of world optical manufacturing was in Germany. As I am a romantic at heart, I prefer to use ocular instead of eyepiece. According to some, it's also more technically correct.

The mirror or lens and its support structure (and that's all a telescope is) are, as one vendor has put it, only half the optical train. The rest is in the ocular. You depend on the ocular to give you the very best image possible. This is probably why amateur astronomers spend so much time fussing about the subject of oculars. It's an area of much discussion, subjective judgements and personal preferences.

I am hoping that the information provided below will assist you in making some informed choices on your ocular selection.

Most amateurs, after acquiring a telescope, often then turn to make a choice on oculars. Usually, what is sold with most telescopes are decent ordinary oculars. They do the job, but you know there are more expensive (and according to legend) better oculars out there. They have to be better, they are more expensive, right?

First, a warning about "gear acquisition syndrome". If you look into the ocular case (or cases) of many amateurs you will be surprised to find up to two dozen items. Oculars, barlows and filters, all carefully designed to bring out the best in any celestial object within their sights. This can be a real trap for those starting out in this hobby. You simply don't need a dozen or more oculars. (Pardon me whilst I put my accessory cases away). You are simply not going to play chess with them. For a reasonable magnification range, you really only need 2 oculars, plus a good quality barlow. Some people recommend that you make your ocular selection based on exit pupil, but this is essentially the same as magnification. A low magnification at x50-x100, giving about a degree field of view, one about x180-x250, and one about x300. It is unlikely you will ever need anything outside these 3 magnification ranges. On rare night of good seeing, you might need something with more magnification at a higher power. This is where your barlow comes in useful. Barlow lenses effectively double (or more) the magnification of the ocular. This also allows good eye relief at higher magnifications, as many short focal length oculars have very short eye relief.

Oculars are a series of polished and coated lenses in a metal barrel. The lens facing the observer is called the eye lens and the lens facing the telescope is called the field lens. The field stop is outside the field lens, and appears as a dark ring framing the view through the ocular.

Oculars have the following attributes, in order of importance:

focal length, in millimetres
apparent field of view (AFOV)
exit pupil
eye relief
field stop diameter
optical design

The magnification of an ocular is determined by dividing its focal length (in mm) into the telescope focal length (in mm). So that an 8in (200mm) f10 Meade LX90 has a focal length of 2,000mm, and a 26mm ocular will provide a magnification of x77 times. The AFOV depends on the ocular design. A Plossl design will give a AFOV of about 50 degrees. There are several formulas for deriving the true field of view (what you actually see in the telescope). The simplest (and most inaccurate) involves dividing the AFOV by the magnification to get the true field. So that 50 degrees AFOV divided by x77 will give a true field of 0.65 degrees of sky that can be seen in that ocular/telescope combination.

The AFOV of an ocular depends on its optical design, and varies from about 45 degrees to 84 degrees.

The exit pupil determined by dividing the magnification into the mirror diameter. So x77 with a 200mm mirror gives an exit pupil of 2.6mm. This is the cone of light that emerges from the eye lens and needs to be captured by your eye pupil. If an exit pupil is more than 7mm  it is often said that you are throwing away aperture, as your eye pupil cannot dilate wide enough to accommodate it. However exit pupils this wide with correspondingly wide fields of view allow you to move you head around and take in a much wider field than you otherwise would. See also Jeff Medkeff's optical definitions entry for "Exit Pupil Upper Limit".

Some basic information on oculars can be found here

Assuming that you now have your SDM telescope, you are probably wondering which oculars are best. This is essentially a personal decision based on what is comfortable for you. But there are a few basic facts that you need to take into account. As you may be aware, the shorter the f/ratio a telescope is, the more demanding it is of the ocular. Short f/ratio telescopes, and your SDM telescope fits into this category, with their stubby short light cones, bring out the worst in any ocular. I have an old 32mm Erfle ocular with 65 degree AFOV. In a f4.5 telescope the pincushion distortion effect is so great it feels like I am looking into a gold fish bowl. This is called the fishbowl effect, and is well known. Also, the stars at the edge of the field of view are no longer points of light, rather appear to look like seagulls. This is due to optical aberations in the ocular. To get a nice wide flat field of view through my SDM telescope, you need a modern premium ocular. These are expensive. Unfortunately, it gets worse. With short f/ratio telescopes, the larger the aperture, the longer (in mm) the focal length, then the higher the magnification provided by any particular ocular. The higher the magnification, the more apparent the aberrations become. To get the most out of your SDM telescope, with its relatively fast f/ratio and long focal length (which increases the magnification of your oculars), only the best will do.  After you have used a premium ocular, you will be disappointed with anything less. Sorry, but you will need to leave your wallet at the door.

A short story will illustrate my point. Some time ago I purchased a 30mm WideScan ocular from Sirius Optics for $360. Now this ocular is good. It has excellent correction and represents very good value for money. Using it in a 10 inch f5 produced (to me) very acceptable images. The edges were not prefect, but it was a wonderful wide field of view that took in almost 2.5 degrees of sky. Using it in a 25 inch f5 produced images that, to me, were totally unacceptable. The aberrations were horrible at the higher magnification.

So what premium oculars are recommended for your SDM?

The very wide well corrected fields of view and high price of the Televue Nagler ocular has resulted in it attaining icon status amongst amateurs. There are various types of Nagler oculars, each with a slightly different design. They all share a wonderful 82 degree Apparent Field of View (AFOV). However, not everyone finds these oculars to their taste. The longer focal lengths tend to be heavy, which can affect the balance of a Dobsonian telescope. Because the AFOV is so wide, many people cannot see the full field of view. Some users like this "spacewalk" experience, whilst others consider that they are not getting all that they paid for. Also, the on-axis sharpness and contrast of these oculars is sometimes less than other, more inexpensive types. Nevertheless, the only other ocular to even come close to the Nagler for sharpness, correction and field of view is the Speers-Walerr.

An excellent substitute is the Televue Panoptic ocular. This 68 degree AFOV ocular has a reputation of being sharp and contrasty all the way to the edge of the field of view. The only negative with this ocular is a slight "pincushion" effect near the edge of the field of view. Not everyone finds this objectionable, or even notices. Comparable oculars are Pentax XW, which are also highly regarded.

You will notice that both of the above oculars are from Televue optics, which has a reputation for producing the highest quality oculars currently available. Other ocular types from Televue are the Radian and the Plossl. The Televue Radians were designed to give a generous 20mm in eye relief and 60 degree AFOV. Having lots of eye relief makes using an ocular very comfortable to use. Comparable oculars are the Burgess-TMB. These are also very sharp with high transmission qualities.

Finally the Televue Plossl is often regarded as one of the best "planetary" oculars available. There is a good argument that any ocular that is an excellent performer on the planets is equally good for any other object, such as planetary nebula and galaxies, and emission nebula. I do not find the 50 degree AFOV any problem, even with undriven telescopes at relatively high power. For those who cannot afford the more expensive ocular types, the Televue Plossls and the Burgess TMB planetary oculars represent excellent value for money. Those who choose these less expensive types should not feel that they are missing out on anything.

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