Good Morning all,
I hope you didn't run out of popcorn Suzy, or fall asleep in the front stalls
Let me say there is always a lot of hype about Televue eyepieces and justifiably so; they are very good. Being a US grown product they have a proliferation in the US astronomy market and community, which is easily the biggest in the world. Televue also spend an enormous amount of money advertising in the astronomy magazines and always have a personal presence at the major US star parties. This up front marketing ensures their market domination in the USA. That doesn’t mean they are the best and the only top quality eyepieces available, although many posters on US dominated forums, would have you believe that and in fact think that themselves. In the widefield eyepiece market some of the other top brands include Pentax, Nikon, Vixen and Docter; and a few others. Not all of these companies specialise in Astronomy equipment and in fact astronomy equipment manufacture is only a very small part of their business. Televue only makes astronomy equipment and a fairly limited range of astronomy equipment at that, essentially they only make refractors, eyepieces and optical accessories. Pick up any Sky and Telescope magazine from the past 10 years and you will find a Televue advertisement for their eyepieces and equipment. Pick up those same magazines and see how many advertisements you will find for Pentax or Nikon eyepieces. Not too many, if any. It doesn’t mean the product isn’t as good, or in some cases better.
At the end of the day Pentax XW’s and Televue eyepieces (most breeds including Naglers), are excellent eyepieces. It comes down to which eyepiece performance criteria are most important to you. You need to determine this before making your purchase decision. In many cases a mix of both might be the best long term option. They each excel in some performance criteria and have some faults in others. In some cases they have been designed to optimise different performance criteria. Further, they are not “scaled designs” which means different focal lengths have different lens configerations and do not necessarily perform the same, although in some cases they do. In the attached diagram you can see all of the different lens configurations and design differences for the entire Pentax XW range.
http://pentaxplus.jp/archives/tech/xo-xw/61.html
This is borne out when comparing the short focal length XW's with the longer focal length XW's. If you look at the attached graph you can see that the Pentax XW’s with a focal length of 40mm to 14mm have different Field Curvature characteristics to the eyepieces with a focal length of 10mm to 3.5mm.
http://pentaxplus.jp/archives/tech/xo-xw/64.html
The longer focal length eyepieces have positive field curvature, the shorter focal length eyepieces have negative field curvature. This gets further complicated when the field curvature characteristics of the eyepieces need to be “matched” to the field curvature characteristics of the telescope. For instance a Newtonian reflector which has positive field curvature when used with the short focal length Pentax XW’s, which have negative field curvature, work very well because the two field curvatures cancel each other out. When the positive field curvature of the longer focal length Pentax XW’s is combined with the positive field curvature of a fast Newtonian reflector, the observed field curvature is compounded. When used with a paracorr which has inherent negative field curvature, the longer focal length Pentax XW’s work very well.
Just about all of the major eyepiece lines and types have these different performace characteristics across types and focal lengths. Pentax however is one of the few companies that makes these graphs and diagrams available
There are many different types of Naglers and also ETHOS and DELOS. Those currently available include the Type 4’s (12mm,17mm and 22mm), Type 5’s (16mm,20mm,26mm and 31mm) and the type 6’s (2.5mm to 13mm). They differ greatly in many respects and have different design objectives. The type 4’s have long eye relief in all focal lengths but have inferior edge of field (EOF) performance to the T5’s and T6’s, which have shorter eye relief. The 17mm and 22mm T4’s IMO need a paracorr to deliver top quality images in a Newtonian. When combined with a paracorr they work exceptionally well and are very comfortable to use. The eye relief on the T6’s and most of the T5’s is too short for eyeglass wearers (The 31mm T5 being the exception). The design goal with most of the Nagler’s (except the T4’s) was to have sharp stars at the EOF. They do this very well.
For a good comparison you need to consider the strong points and weaknesses of each type and design and weigh up what are the most important criteria to you. To do the comparison properly one needs to look at the Pentax XW’s 3.5mm to 10mm and then over 14mm and over. Similarly one needs to consider the different design criteria from the T4 Nagler's to the others.
Pentax XW 3.5mm to 10mm (inclusive) strong points
• Very sharp on axis
• Very sharp at EOF
• Very high light transmission
• Excellent Contrast
• Excellent eye relief and suitable for eyeglass wearers
• Very comfortable to use for long viewing sessions
• Very resistant to dewing and fogging and getting eyelash oil on the lenses
• Excellent cool neutral (white) colour reproduction
• Waterproof and a have a lifetime warranty
• Excellent construction quality and durability
• Excellent for daytime use in a refractor
Pentax XW 3.5mm to 10mm (inclusive) weaknesses
• Field of view (FOV) is limited to 70 degrees. Many people however find this to be ideal, me included
• Expensive
Pentax XW 14mm to 40mm (inclusive) strong points
• Very sharp on axis.
• Very high light transmission
• Excellent Contrast
• Excellent eye relief and suitable for eyeglass wearers
• Very comfortable to use for long viewing sessions
• Very resistant to dewing and fogging and getting eyelash oil on the lenses
• Excellent cool neutral (white) colour reproduction
• Waterproof and a have a lifetime warranty
• Excellent construction quality and durability
• Excellent for daytime use in a refractor
Pentax XW 14mm to 40mm (inclusive) weaknesses
• Field of view (FOV) is limited to 70 degrees. Many people however find this to be ideal, me included
• EOF can be a little soft on some telescopes, particularly fast newtonians. This is corrected by using a paracorr with a newtonian
• Expensive
Nagler T4’s strong points
• Sharp on axis
• Good light transmission
• Good Contrast
• 82 deg FOV
• Excellent eye relief and suitable for eyeglass wearers
• Very comfortable to use for long viewing sessions
• Very resistant to dewing and fogging and getting eyelash oil on the lenses
• Excellent construction quality and durability
• Excellent for daytime use in a refractor
Nagler T4’s weaknesses
• EOF can be a little soft on some telescopes, particularly fast newtonians. This is corrected by using a paracorr with a newtonian
• Warmer colour tones
• Expensive
Nagler T5 strong points
• Very sharp on axis
• Very sharp at EOF
• Good light transmission
• Good Contrast
• Excellent construction quality and durability
Nagler T5 weaknesses
• Eye relief except the 31mm is inadequate for eyeglass wearers. Some people can use the 26mm with their eyeglasses on, some cannot.
• Eye lenses prone to fogging and dewing and contamination from eyelash oil
• Warmer colour tones
• Not ideal for daytime use in a refractor
• Expensive
Nagler T6 strong points
• Very sharp on axis
• Very sharp at EOF
• Good light transmission
• Good Contrast
• Excellent construction quality and durability
Nagler T6 weaknesses
• Eye relief is inadequate for eyeglass wearers. Eye lenses prone to fogging and dewing and contamination from eyelash oil
• Warmer colour tones
• Not ideal for daytime use in a refractor
Now, let’s see where all the above gets us. My current telescopes are a 10”/F5 newtonian, a 14”/F4.5 newtonian and an 18”/F4.5 newtonian. The eyepieces which I currently have to use in these scopes are:-
5mm Pentax XW
5mm UO HD orthoscopic
6mm UO HD orthoscopic
7mm Pentax XW
7mm UO HD orthoscopic
8mm Televue Radian
8.5mm Pentax XF
9mm UO HD orthoscopic
10mm Pentax XW
12mm Televue Nagler T4
12mm UO HD orthoscopic
13mm Televue ETHOS
14mm Pentax XW
17mm Televue Nagler T4
18mm UO HD orthoscopic
20mm Pentax XW
22mm Televue Nagler T4
27mm Televue Panoptic
31mm Televue Nagler T5
I also have a Televue Paracorr
I am an eyeglass wearer and my preference is to observe with my eyeglasses on using long eye relief eyepieces. If I was to start over again I would build the following eyepiece collection for use in these scopes
5mm Pentax XW
7mm Pentax XW
10mm Pentax XW,
12.5mm Nikon NAV HW (or 12.5mm Docter)
17mm Nikon NAV HW
22mm Nagler T4
31mm Nagler T5
Televue Paracorr
If I was a young bloke and didn’t need eye glasses I would build the following eyepiece collection
5mm Pentax XW
7mm Pentax XW
10mm Pentax XW,
12.5mm Nikon NAV HW (or 12.5mm Docter)
17mm Nikon NAV HW
20mm Nagler T5
26mm Nagler T5
31mm Nagler T5
The above of course all assumes cost isn’t a factor.
Base your decision on what things are the most important to you in terms of performance criteria, there is no right or wrong answer and neither rulz
Cheers,
John B
