Image of the day

Captured by
Peter Bresler

Trifid Nebula, m20

My Account

New to Astromart?

Register an account...

Need Help?

Review of TeleVue Parfocal 1.25" Eyepieces

Posted by Wing Eng   09/20/2005 12:00AM

Review of TeleVue Parfocal 1.25" Eyepieces
By Wing Eng

My favorite refractor is an "accidental" Tele Vue 102mm f/8.6 apochromatic scope. I acquire this lovely white-tube refractor "accidentally" after being broadsided by another vehicle, which resulted in an insurance payout which not only paid for a used replacement sedan, but left me with over $2,000 which was enough to buy the Tele Vue 102.

My 1 ¼ inch Tele Vue eyepiece collection was recently upgraded as I went on an "eyepiece hunt" for another Tele Vue eyepiece at a local telescope dealer in the San Francisco Bay Area. Since I already owned Tele Vue's 19mm Panoptic Eyepiece and Tele Vue's 12mm Radian Eyepiece, which are parfocal to each other requiring virtually no refocusing when switching from one eyepiece to the other, my attention turned to several shorter focal length 1 ¼ inch Tele Vue eyepieces. I was intrigued by the many Radian and Nagler eyepieces that were between 3mm and 8mm focal lengths.

I enthusiastically tried several of them using a Tele Vue Pronto Scope on a Gibraltar Mount. The "viewing object" used to evaluate the eyepieces was a foreign currency-dollar bill mounted on a far wall, located about 35 feet away from the scope. I focused on the finely detailed facial features of the persons featured on the dollar bill in evaluating the Tele Vue 1 ¼ inch eyepieces. Even after I completed my one-hour-long evaluation of various Tele Vue eyepieces, the proprietor cheerfully proclaimed "Come back again, anytime, to do more testing!"

With respect to eyepieces, the meaning of "parfocal" generally refers to the fact that in a particular series of eyepieces such as the Tele Vue Plossls, every eyepiece will focus at approximately the same point, which in practice means the maximum amount of focus travel in going from one eyepiece with a particular focal length to another eyepiece with a different focal length will be no more, than say, 1/8th to 1/16th of an inch. Practically speaking, when owning non-tracking scopes such as a Tele Vue 102 on a standard Gibraltar Mount, or a Coulter Odyssey I on a Dobsonian Mount, parfocal eyepieces greatly assist in identifying celestial objects, especially under "high power" situations where the field of view is quite small. By this, I mean that it is very easy to switch from a parfocal low or medium power eyepiece to a parfocal high power eyepiece and still identify the same object being viewed as, say, a double star as opposed to a galaxy or globular cluster. This is impossible to do if the switched eyepieces are not parfocal since all objects, such as an in-focus-star, or an in-focus-galaxy, at low or medium power will look greatly expanded, unfocused and "mushy" at high power if the eyepiece series is not parfocal.

Finally, by the time one "refocuses" an object being viewed in the non-parfocal high power eyepiece, the star or galaxy, will have drifted close to edge of the field of view or perhaps even completely out of the field of view.

For Tele Vue's current line of 1.25 inch Plossl eyepieces, which span the following focal lengths of 40mm, 32mm, 25mm, 20mm, 15mm, 11mm and 8mm, every eyepiece, except for the 40mm is parfocal. The 40mm eyepiece is not parfocal to the other plossls in Tele Vue's series since it is optimized for the widest possible field in a 1.25 inch barrel. It is interesting to find that within the complete line of Tele Vue eyepieces, the set of 1.25 inch Plossl eyepieces (excluding the 40mm) is also parfocal with the 1.25 inch Panoptic eyepieces of 19mm and 15mm focal lengths, parfocal with the Nagler series of 1.25 inch eyepieces in the following focal lengths of 16mm (type 5), 13mm(type 6), 9mm (type 6), 7mm (old style), 7mm (type 6), 5mm(type 6), 4.8mm(old style), 3.5mm (type 6), 2.5mm (type 6), and also is parfocal with the complete 1.25 inch Radian series of eyepieces in 18mm, 14mm, 12mm, 10mm, 8mm, 6mm, 5mm, 4mm and 3mm focal lengths, and finally parfocal with the 1.25 inch Nagler Zoom eyepiece (3mm to 6mm focal length). It is also important to understand that sometimes a 1.25 inch, 90 degree, star diagonal will not allow every eyepiece within some series of eyepieces to work as "parfocal." For example, some of the shorter focal length, higher power, Radian eyepieces actually have much longer barrel lengths than the longer focal length, lower power, Radians, resulting in the fact that the Radians with longest barrels will "bottom out" within many 1.25 inch focusers and will not be parfocal with their corresponding shorter barrel length Radians. The way I found to "overcome" this issue was to purchase a 2 inch star diagonal that allowed all Radian eyepieces to work as "parfocal" since not even the longest barrel Radians "bottomed out" within the bigger diagonal. One interesting thing I found was that the 7mm Nagler and the 5mm Nagler eyepieces are both virtually parfocal to the 19mm Panoptic, 12mm Radian, and 5mm Radian eyepieces. This finding confirmed the data I saw on Tele Vue Optics' Webpage - Eyepiece Specification Listings for current and discontinued eyepieces, which details a plethora of valuable data such as Tele Vue Product Code, Focal Length(mm), Eyepiece Type (Plossl, Panoptic, Nagler, Radian, Nagler Zoom), #of Lens Elements(4 to 8), Apparent Field (43 to 82 degrees),Eye Relief(6 to 38 mm), Field Stop Diameter(3.3 to 46 mm), Barrel Size (2", 1 ¼" or hybrid), Weight(0.1 to 2.3 lbs.), Parfocal Series(A, B or C), and Dimensions(A- Height of black barrel, B-Height of 1-1/4" barrel, C-Height of 2" barrel, D- Diameter of black barrel, E-Depth of 1-1/4" barrel, and F-Approx. location of field stop, from 1-1/4" or 2" dia. Flange).

I was pleasantly surprised that the four(4) Tele Vue 1 ¼ inch eyepieces(Panoptic, Radian and Nagler), I currently own and that are listed in the table shown, tested out as being even more "parfocal" than the Radian 4, 5, 6 and 8 mm eyepieces I tried out. Now, don't get me wrong, the four (4) different focal lengths of Radian eyepieces I evaluated only required a maximum of about 1/64 to 1/16 inch of focus travel, within the series of the tested focal lengths, to achieve optimal focus.

When testing my Tele Vue eyepieces for "parfocalness," I found that it was fruitless to test them out on celestial objects located near the heat-wave-plagued horizon. For example, I tried to test out my eyepieces on the 2 ½ day old crescent moon about 1 hour after sunset. I noticed that the best focus point, even for a single eyepiece, kept on changing as the lunar limb appeared very unsteady and looked as if it were "boiling." Patiently, I waited until the sky steadied, and swung my Tele Vue 102 scope to view the 2 plus arc second separated pair of white component stars of the Double-Double in Lyra-using the 12mm Radian, the 1.9 arc second separated orange and blue component stars of Izar in Bootes-using the 7mm Nagler, and the brilliant first magnitude orange star, Arcturus in Bootes. In switching between all four(4) eyepieces, I did not need to change focus at all in my TV 102; in fact, Arcturus displayed a neatly framed Airy Disc in using both my 7mm Nagler and 5mm Nagler eyepieces. I hope you may have found this article interesting as I had fun writing it. May all your focal lengths be Parfocal Lengths!

Best Wishes and Clearest Skies,
Wing(1st name) Eng(last name)
San Ramon, CA

Click here for more about this subject. -Ed.