OCULAR COMPARISONS

What follows are subjective visual comparisons of various 1.25 inch diameter oculars, most of which were conducted in my own yard. I used my personal telescope for all of these comparisons, primarily my Celestron C8. At the time most of these comparisons were made, I was the manager of France Photo, a stocking Celestron dealer, so the full line of Celestron oculars was available to me.

My purpose for making these comparisons was to determine which oculars I should purchase for my own use. (If I remember correctly, my final set of oculars for my last C8 included the 9mm Ortho, 12mm Ortho, 15mm Plossl, 22mm Plossl, 30mm Plossl, and 36m Plossl, plus the 2" diameter 50mm Plossl or the RFA w/22mm Plossl) Since I kept records of my impressions of these comparisons (hand written in pencil), I decided to transcribe them into my word processor, and make them available to others on my web page.

Oculars: Celestron 7.5mm Plossl, Celestron 10mm Plossl
Test telescope: Celestron C6 (clear aperture=6"; f=750mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: average to good; moderately dark sky
Test subjects: Jupiter, Ring Nebula, Andromeda galaxy, Double Double in Lyra
Results: This test location is about 500' above sea level, with moderate light pollution. The nearest metro area is Eugene/Springfield (population about 160,000) approximately 14 miles to the north, and behind a range of hills. Skyglow from the Eugene/Springfield metro area is visible in the northern sky. There is also some slight sky glow from nearby Creswell, Oregon (population about 2800). Jupiter: The view through the 10mm Plossl is good. The two main cloud belts look crisp. The image through the 7.5mm Plossl is larger, and hints of a 3rd cloud belt, but overall the image appears fuzzier and slightly degraded. Ring Nebula: With the 10mm Plossl the image is smaller, but somewhat sharper appearing; with the 7.5mm Plossl the reverse is true. No evident superiority either way. Andromeda galaxy: With this telescope, the 10mm Plossl provides a useful view of the central part of the galaxy. The 7.5mm Plossl is useless on this test subject. Double Double in Lyra: The binary pairs could not be split with either ocular, although the 10mm seems to hold more promise at splitting double stars in general. The 10mm is easier to focus because it is brighter. Conclusion: the 10mm is probably a better all-around short focal length (high power) ocular for this telescope than the 7.5mm. The 10, 15, and 26mm Plossls make excellent ocular choices for a C6.

Oculars: Celestron 7.5mm Plossl, Celestron 9mm Ortho
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, and Hilltop east of Saginaw, OR
Seeing conditions: Fair; moderately dark sky (near Creswell)/Good; dark sky (near Saginaw)
Test subject: Moon
Result: The site near Creswell is as stated above. The hilltop site is on a ridge about 1500' above sea level, a few miles east of Saginaw, Oregon (a very small town of a couple hundred residents). It offers generally dark sky conditions and a depressed horizon through about 270 degrees centered on the south, with a small rise to the north. The nearest town with any appreciable sky glow is Cottage Grove (population about 8800), about 10 miles to the south-west. The nearest metro area is Eugene/Springfield, about 20 miles to the north, and behind a couple ranges of hills. Both oculars are too powerful to give optimum views under "fair" seeing conditions near sea level. The 9mm Ortho is judged closer to satisfactory from this location. From the higher, dark sky location, under good seeing conditions, the 9mm Ortho is capable of awesome views of the moon and planets. Both of these oculars have inadequate eye relief (especially the 7.5mm Plossl), and are difficult to view through. However, the 9mm Ortho is judged superior. Also, the Orthoscopic ocular is not bothered by the internal reflections that sometimes plague the Plossl ocular.

Oculars: Celestron 9mm Ortho, Celestron 10mm Plossl, Celestron 12mm Ortho
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Average to good; moderately dark sky
Test subjects: Moon, Mars, Jupiter.
Results: 12mm Ortho is judged superior from this near sea level location with only moderate light pollution. Compared on several successive evenings, the 9mm Ortho was judged to be too powerful for optimum views on all the evenings tested. The 10mm Plossl was judged best on very clear evenings with steady seeing.

Oculars: Celestron 10mm Plossl, Celestron 15mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: fair; moderately dark sky
Test subjects: Ring Nebula, Jupiter, Double Double in Lyra
Results: It was more difficult to focus the 10mm Plossl than the 15mm Plossl, due to the 10's higher power. All four component stars of the Double Double in Lyra could be resolved, with difficulty, using the 10mm Plossl. The 15mm Plossl, although lower is power, also resolved all four stars. The ring nebula was faint through the 10mm, but actually looked pretty good. It looked better through the 15mm, however, because it was brighter. The view of Jupiter through the 10mm Plossl was plagued by annoying internal reflections. This is a common problem with high power (short focal length) Plossl oculars when used on bright subjects like Jupiter and Venus. I believe the symmetrical design of Plossl oculars is what makes them so succeptable to internal reflections. It was easier to see Jupiter's two main cloud bands through the 15mm Plossl, which was not bothered as much by internal reflections.

Oculars: Celestron 12mm Ortho, Celestron 15mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Fair to poor; moderately dark sky
Test subjects: Jupiter, Double Double in Lyra, Ring Nebula, double star Albiero in Cygnus
Results: Very difficult to fully resolve the Double Double with either ocular. The 15mm Plossl is easier to focus, primarily because it is brighter, and was the marginal winner. Jupiter is not impressive on this night, but looks a little better through the 15mm Plossl, although this ocular has noticeable internal reflections which are absent in the 12mm Ortho. The seeing conditions are such that a 26mm Plossl gives a better view of Jupiter than either of the test oculars, which is not usually the case. Albiero shows no appreciable difference in either ocular, although again the 15mm Plossl suffers somewhat from internal reflections. The 15mm Plossl is definitely superior when both are used to look at the Ring Nebula. Here, the additional brightness and ease of focus really pays off. All in all, the 15mm Plossl was the winner of this comparison under relatively difficult seeing conditions.

Oculars: Celestron 15mm Plossl, Celestron 17mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Average; moderately dark sky
Test subjects: Jupiter, Moon, Mars, Ring Nebula, Double Double in Lyra
Results: The 17mm is brighter, and has superior eye relief; it is a bit easier to look through. It proved superior for viewing the Ring Nebula. The 15mm is superior for viewing the Moon and Jupiter, and slightly superior on Mars, due to it's greater magnification. Both can split the Double Double--very close here with maybe the slightest advantage to the 17. Both are good oculars, bright and sharp. In a C8, the 17 seems slightly superior for deep sky objects, and the 15 slightly superior for the Moon and Planets.

Oculars: Celestron 17mm Plossl, Celestron 18mm Ortho
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Average; moderately dark sky
Test subjects: Jupiter, Moon, Mars, Ring Nebula, Double Double in Lyra
Results: 17mm Plossl judged superior in all cases. The Plossl is brighter than the Ortho, and this is the main area of superiority.

Oculars: Celestron 17mm Plossl, 20mm Erfle
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: Hilltop east of Saginaw, OR
Seeing conditions: good; dark sky
Test subjects: M-13 globular cluster (Hercules), Ring Nebula, Andromeda galaxy, Jupiter, Mars, Double Double in Lyra
Results: The 20mm Erfle appears to have superior brightness and field of view. The 17mm Plossl has slightly superior sharpness. M-13 and Andromeda look better through the 20mm Erfle. Jupiter, Jars, Ring Nebula, and the Double Double look better through the 17mm Plossl. I also tested both of these oculars in a Criterion Rich Field Adapter (focal reducer), and the 20mm Erfle was clearly superior in that application. Without the RFA, I would prefer the 17mm Plossl for all-around use in the C8.

Oculars: Celestron 17mm Plossl, Celestron 22mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: fair; moderately dark sky
Test subjects: M-13 globular cluster, Ring Nebula, Andromeda galaxy, Jupiter, Double Double in Lyra
Results: The 22 Plossl proved superior on the Ring, M-13, and Andromeda (not as good as a 26 Plossl for the last two, however). The 22mm could split the Double Double, but not as easily as the 17mm. The 17mm was superior on Jupiter. The big surprise (to me) was the superiority of the 22mm on the Ring Nebula, where the 17mm has performed very well in the past with this telescope. This may have been due to the seeing, which was not as good as during previous tests of the 17mm Plossl. Conclusion: the 22mm Plossl is superior to the 17mm Plossl for most deep sky objects; the reverse is true for most planetary viewing. Both are excellent oculars for a C8. I also tested this pair using the Celestron RFA (which reduces the effective focal length of the C8 to 1000mm). Used in the RFA, the 22mm Plossl was superior on all subjects.

Oculars: Celestron 15mm Plossl, Celestron 17mm Plossl, Celestron 22mm Plossl, Celestron 26mm Plossl
Test telescope: Celestron C8 with Celestron RFA (clear aperture=8"; effective f=1000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: fair; moderately dark sky
Test subjects: M-13 globular cluster, Andromeda galaxy, Double Cluster (open clusters)
Results: The 22mm Plossl proved superior on all subjects. The 17mm was second, the 26mm third. Always use the 22mm Plossl (or the 20mm Erfle) with the RFA.

Oculars: Celestron 22mm Plossl, Celestron 26mm Plossl
Test telescope: Celestron C8 (clear aperture=8", f=2000mm)
Test location: 3.5 miles south of Creswell
Seeing conditions: Average; moderately dark sky
Test subjects: M-31, M-42, Ring Nebula, double stars in Orion
Results: The 22mm Plossl is superior for splitting double stars. It also provided superior views of the Ring Nebula. The 26mm Plossl provided superior views of the large deep sky objects M-31 and M-42 due to its superior brightness and actual field of view. The 22mm Plossl is an acceptable medium focal length ocular, but the 26mm Plossl is a superior all-around medium focal length ocular for this telescope, particularly if complemented by a 17mm Plossl.

Oculars: Celestron 24mm Erfle, Celestron 25mm Ortho, Celestron 26mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Average; moderately dark sky
Test subjects: Double Cluster, Andromeda galaxy, Orion Nebula
Results: The 24mm Erfle and 26mm Plossl are both clearly superior to the 25mm Ortho for deep sky viewing, particularly in brightness. Viewing the Orion Nebula, the 24mm Erfle seemed best. On the Double Cluster, the 26mm Plossl seemed best, perhaps because of its brightness and contrast. The 24 has less critical eye relief, in regard to image black-out, than the 26. The 26 is slightly brighter, and possibly slightly sharper, than the 24. Very close overall. Final decision is very slightly in favor of the 26mm Plossl over the 24mm Erfle, with the 25mm Ortho a distant third.

Oculars: Celestron 26mm Plossl, Celestron 36mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test locations: 3.5 miles south of Creswell, and hilltop east of Saginaw, OR
Seeing conditions: Average; moderate to dark skies
Test subjects: Andromeda galaxy M-31, globular cluster M-13, various terrestrial subjects
Results: Both oculars are sharp and contrasty, with similar actual fields of view. The 36mm Plossl is definitely brighter, but the 26mm Plossl gives more magnification. Viewing M-13, the 26mm gives a good view, but the 36mm is spectacular. The greater brightness really pays off here. The Andromeda galaxy looks good in the 26, but is brighter and shows more nebulosity through the 36. M-32, the companion galaxy, looks better in the 26 because it is larger (magnified more). Conclusion: the 36mm Plossl is definitely superior for deep sky objects. It is an excellent deep sky ocular in all respects, except for its relatively narrow (for its focal length) actual field of view.

Oculars: Celestron 32mm Erfle, Celestron 36mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Average, moderately dark sky
Test subjects: Orion Nebula, globular cluster M-3, open cluster M-35, also terrestrial daylight test viewing trees on a ridge approximately 1/2 mile away.
Results: The 32 Erfle has a considerably larger field of view. This makes aiming the telescope easier. It is also superior for viewing very large deep sky objects. Subjects are slightly larger as viewed through the 32mm Erfle, but not much. The 36mm Plossl appears brighter, and slightly sharper, than the 32mm Erfle--it is a very nice ocular to look through. In addition, the eye relief of the 36 is just about ideal; the 32 suffers from excessive eye relief, which makes positioning the head for the optimum view more difficult (but it's not as bad as the 45mm Plossl in this respect, see below). The 32 gave the best view of the Orion Nebula, due to its greater actual field of view. On the globular cluster, there was little to choose. Open clusters look prettier through the 36, due to its superior sharpness and contrast. On the terrestrial subject, the 32 performed better due to its greater magnification. Conclusion: because of its excellent views and ease of use, I generally prefer the 36mm Plossl.

Oculars: Celestron 32mm Erfle, Celestron 45mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Average; moderately dark sky
Test subjects: Orion Nebula, globular cluster M-3, open cluster M-35.
Results: The 32mm Erfle has a slightly larger actual field of view, despite its shorter focal length. This is due to its great apparent field. The 32mm Erfle also makes things appear larger than the 45mm Plossl. The 45mm Plossl is noticeably brighter than the 32mm Erfle. The Plossl is also slightly sharper than the Erfle, and appears to have more contrast. The main problem in viewing with the 45mm Plossl is maintaining your eye in the correct relationship to the ocular--the eye relief is very excessive. (This problem can be eliminated by replacing the 45mm Plossl with an RFA using a 22mm Plossl, but that is another story.) The 32mm Erfle also suffers from excessive eye relief, but it is not quite as bad in this regard as the 45mm Plossl. However, both require exact eye placement. On the Orion Nebula, and open cluster M-35, the 45mm Plossl gave the best views. On globular cluster M-3, the 32 was superior. Both are good oculars for wide sky views. The 45mm Plossl was judged superior due to its brightness and sharpness.

Oculars: Celestron 40mm Kellner, Celestron 45mm Plossl
Test telescope: Celestron C8 (clear aperture=8"; f=2000mm)
Test location: 3.5 miles south of Creswell, OR
Seeing conditions: Average; moderately dark sky
Test subjects: Various deep sky objects
Results: The 45mm Plossl is much brighter than the 40mm Kellner. It is also sharper and has a wider actual field of view. There is no comparison, the 45mm Plossl is the superior ocular.