The Sirius Plossl line is Orion's longest-enduring family of eyepieces, which first started back around summer of 1985. Before this time, Orion did not have their own dedicated line of plossl eyepieces. Instead, they had volcano-top oculars sourced from Towa, Carton, and rarely Tani of Japan and were called the "Observer Series Eyepieces". These oculars were not plossls, they were orthoscopics and kellners, and a single erfle. Plossls sold by them before 1985 came from (and were branded by) Tele Vue, Celestron, and Meade.
When the very first Sirius Plossls hit the catalogs, the Observers were ended and a newer, multi-coated set of kellners and orthoscopics would exist alongside the famous plossls for just over 10 years; the "Explorer Series".
The 1st generations were all black with some featuring a rubber grip and looked remarkably like some of the Celestron Silvertops (Deluxe Plossl) from Vixen of Japan. Like those Silvertops, the focal lengths were virtually identical, save the 35mm, which if a Silvertop, was a 36mm. These were multi-coated and some of the very best examples of this ocular line ever known.
At this time only (4) focal lengths were available: 35mm, 26mm, 17mm, and 10mm, and, if purchased as a complete set, they came in a polyethylene case with blue foam; virtually the SAME case that a set of Edmund RKEs came in.
These Sirius models were unusual in that the Sirius logo was not a star, but a cross bullseye. And the Orion oval grid pattern logo is absent.
In either winter of 1989 or spring of 1990, the 2nd generation of the Sirius Plossl appears. These are the rarest of the line, being offered in only (3) focal lengths: 25mm, 18mm, and 12mm. Also sourced from Vixen of Japan and featuring the same multi-coatings, these were not all black, but chrome-barreled with silver satin bodies and a black top rim. You could say these were IDENTICAL to the Celestron and Vixen "Silver Blacktops", which postdate the original orange and red-lettered Silvertops. A prime example would be the 2-inch 50mm Silvertop offered as a stock ocular to the Celestar Compustar 8.
At this time, we see the classic Orion oval grid icon and the "Sirius Star" disappears.
When Orion turned to Synta Technology Corp of Taiwan right about November/December of 1993, the Sirius line was visually revamped again, and we now enter the 3rd generation; the second rarest. Now featuring a milled anodized black body with white-painted characters and a very distinctive red O-ring that separated the top rim from the rest of the main body (it also secured the inner spacers). It is this red O-ring that visually sets this era of plossls apart from the other Sirius models; it earns them the term "Redlines".
Unlike the Japanese-made examples, this generation featured only fully coated optics. Nonetheless, the difference in coatings were not wasted as the optics in these plossls are remarkable.
Their available focal range increased to include 40mm, 25mm, 17mm, 10mm, and 7.5mm.
At the same time, Celestron was making their "Halloween" plossls as well Pro-Optic was making their plossls look nearly like Orion's with the red O-ring. All these were from the same Taiwanese plant.
It is also during this time, the often-remembered "Sirius Star" logo is reintroduced.
To be noted: after this generation expires in December of 1994, the Sirius Plossls will ALL be made of a lighter gauge of milled aluminum. By example, a 1994 25mm Sirius Plossl weighs a hefty 3.8 ounces whereas a 1996 25mm Sirius Plossl has been lightened to 3.6 ounces. By 2015, they will be even lighter.
In January of 1995, the 4th generation Sirius line came about and changed very little from this point on. These were the all-black bodies with a return of the rubber grip on the larger focal models. It is the first time the built-in rubber eye guard is introduced. Still coming from the same Taiwanese manufacturer, this family sees a reappearance of the 26mm. Thus, the entire line is 40mm, 26mm, 17mm, 10mm, and 7.5mm.
Logos do not change.
You may think this 4th generation Sirius Plossl line to actually be the Chinese variant. But like the red-ringed generation, the words "Taiwan" are painted on the opposite side of the focal length...and their boxes had "Taiwan" printed as well.
In spring of 1996, the introduction of the 32mm, 20mm, and 12.5mm expand the family to a total of (8) focal lengths. The largest thus far...Keeping in mind were are still in the 4th generation.
Barrel size: 1.25-inches
Design: 4 element plossl (2)(2) configuration
Optics: fully coated
Field of view: 50 degrees
Eye relief: 6mm
Exit aperture: 8.1mm
Threaded for filters: yes
Materials: milled aluminum and brass, rubber ring, optical glass
Weight: 2.3 ounces
Still remaining in 4th generation guise, the 6.3mm model brings the line to include (9) focal lengths. Nothing will change until very late 2000 when the Suzhou Synta becomes the manufacturing center for the Sirius Plossl. It should be noted ALL Taiwanese variants are fully coated only.
In late summer of 2000, the Sirius Plossl, enters the 5th generation; now coming from Suzhou Synta of China. The painted characters are less white and more metallic silver, but otherwise there are no changes. The optics are still fully coated.
Midway into 2003, multi-coated optics are returned and remain for the rest of the Sirius Plossl timeline.
In 2007, the 26mm is omitted and the long lost 25mm returns for good (to this day in 2024).
The final installment of the 5th and current generation occurred back in 2016 when the Sirius Plossl line went back to fully coated optics.
So in total, if you could amass the ENTIRE Sirius Plossl family, it would encompass (13) models:
40mm, 35mm, 32mm, 26mm, 25mm, 20mm, 18mm, 17mm, 12.5mm, 12mm, 10mm, 7.5mm, and 6.3mm.
IN REGARDS TO THE REDLINE ERA SIRIUS PLOSSLS...
The Orion Sirius Plossl family was exactly (5) models:
40mm, 25mm, 17mm, 10mm, and 7.5mm
By comparison, the 1993-1996 Pro-Optic "Redlines" Plossls were (9) models:
40mm, 32mm, 26mm, 20mm, 17mm, 12.5mm, 10mm, 7.5mm, and 6.3mm
And finally, the 1995-1999 Celestron Halloweens included (9) in the family:
40mm, 32mm, 26mm, 20mm, 17mm, 12.5mm, 10mm, 7.5mm, and 6.3mm
During this SPECIFIC era, Pro-Optic, Orion, and Celestron had a dedicated 2x doublet barlow from Taiwan and matched these eyepieces with is black, milled anodized aluminum body and with matching font, white-painted characters.
Collectors will know that the Sirius "Redlines" are quite rare as they were only advertised for less than 2 years. And of the smoothy-style, black anodized aluminum plossls from Taiwan, the Orion variants are the rarest as they ran for the shortest time.
The 10mm Sirius Plossl as seen here was never inclusive equipment to an Orion telescope. What is interesting is that the mpn does change. The Redline 10mm fall under 8532. When the rubber-featured variant arrives in January 1995, the mpn code is now 8736.
With a focal length of 10mm, this eyepiece is easy to sight in and will work with SOME eyeglass wearers, but not most because we are now entering the restrictive range of eye relief capability (for any plossl). It is best if you can remove them.
This ocular is perfect for viewing deep sky objects that require a good amount of power, but not too much power, and will not be nearly as sensitive to less-than-perfect seeing conditions as a 7.5mm or 5mm. Jupiter, Mars, Saturn, all of the Moon, countless double and triple stars and high surface brightness objects like M13, M22, M92, M37, and the Double Cluster are all excellent targets. It will probably be your highest power eyepiece on galaxies and dark nebulae.
Also, this is a wonderful ocular to pair up with the apochromat barlows; the Meade 140, Celestron Ultima 2x SV, or Orion's own Shorty Plus at which this 10mm becomes a 5mm with exceptionally comfortable eye relief.
If used in a high quality spotting scope such as the Orion Apex 102, Pro Optic Mak 500, or the Celestron C90, it is GREAT for birding and other wildlife observing; though at this level of magnification, you will be at the mercy of the stability limits of most photographic tripods. This is where you will want to have a Bogen, VersaGo II, or Orion's own Paragon on hand.
From an Orion/Vixen GP102 to the Celestron C9.25, to a Meade 6600 equatorial reflector; if you have an excellent quality telescope this eyepiece has a home in it.
Plossls get along with both your rich field f/4.5 reflectors and f/5 refractors just as well as with the longer f/10 reflectors and f/15 cassegrains. Consider it the golden retriever of eyepieces in your ocular case!