Introduction  Look (2 x Extender)  Look (3 x Extender)  Look (both Extenders)  Eyepiece Data without and with Focal Extenders  First Experiences  First Conclusions  References
On this page, I present my Explore Scientific Focal Extenders (Tele Extenders) 2 x and 3 x (1,25")  as well as first experiences that I have collected so far using them.
Many inexpensive telescopescome equipped with a 2 x or 3 x Barlow lens, which makes it possible to virtually "double" the number of eyepieces. Using a 2 x Barlow lens, the SkyWatcher kit eyepieces, for example, change lens into a 12.5 mm (25 mm) and a 5 mm (10 mm) eyepiece. Admittedly, I do not use the Barlow lenses that I own because the achievable quality is too poor  I do not even use the slightly better lens from my Revelation eyepiece bag. But, of course, I find the idea of changing the focal length of telescopes interesting...
In May 2017, I "stumbled" by coincidence across a focal extender (TeleExtender) on the Explore Scientific Website and just ordered one, namely the 2x extender because I thought the lower the multiplication factor, the better the quality (which does not necessarily have to be true ...). This extender is also available in a 3 x and a 5 x version. In addition to multiplying the focal length of a telescope, Explore Scientific highlights a further advantage of the extenders, namely that you can benefit from the better eye relief of eyepieces with a long focal length when using the extenders.
In contrast to Barlow lenses, which use a socalled "telenegative" design, the Explore Scientific Focal Extender has a "telecentric" design with 4 lenses, one of which is a dispersing lens. A similar design was sold by Meade, and Explore Scientific probably took it over (both versions are supposedly manufactured by JOS), and another one is being sold by TeleVue (obviously at a much higher price ...).
When the 2 x focal extender arrived at my home, the cardboard box was unfortunately "crumpled" at one corner, but fortunately the extender was left undamaged:
Photos: The "crumpled" box of the focal extender
I notified Explore Scientific of this issue, but did not ask for an exchange. I got a voucher of 4.95 EUR, which I used when ordering a 3 x focal extender from them  so to speak without shipping cost thanks to the voucher.
The following table shows for my current eyepieces, which magnifications and true visual angles can be achieved with my current telescopes without and with the two focal extenders:
Telescope  Telescope Focal Length (mm) 
Extender  Eyepiece Focal Length (mm) 
Max. useable Magification (1.5/2) 

4 
7 
10 
16 
24 
32 

Magnification 

Heritage 100P  400 
  100.00 
57.14 
40.00 
25.00 
16.67 
12.50 
150/200 
Explorer 150PDS  750 
187.50 
107.14 
75.00 
46.88 
31.25 
23.44 
225/300 

Skymax102  1300 
325.00 
185.71 
130.00 
81.25 
54.17 
40.63 
204 

Heritage 100P  400 
2 x  200.00 
114.29 
80.00 
50.00 
33.33 
25.00 
150/200 
Explorer 150PDS  750 
375.00 
214.29 
150.00 
93.75 
62.50 
46.88 
225/300 

Skymax102  1300 
650.00 
371.43 
260.00 
162.50 
108.33 
81.25 
204 

Heritage 100P  400 
3 x  300.00 
171.43 
120.00 
75.00 
50.00 
37.50 
150/200 
Explorer 150PDS  750 
562.50 
321.43 
225.00 
140.63 
93.75 
70.31 
225/300 

Skymax102  1300 
975.00 
557.14 
390.00 
243.75 
162.50 
121.88 
204 

Telescope  Telescope Focal Length (mm) 
Extender  Apparent Angle of View 

82 
82 
70 
82 
65 
52 

True Angle of View 

Heritage 100P  400 
  0.82 
1.44 
1.75 
3.28 
3.90 
4.16 

Explorer 150PDS  750 
0.44 
0.77 
0.93 
1.75 
2.08 
2.22 

Skymax102  1300 
0.25 
0.44 
0.54 
1.01 
1.20 
1.28 

Heritage 100P  400 
2 x  0.41 
0.72 
0.88 
1.64 
1.95 
2.08 

Explorer 150PDS  750 
0.22 
0.38 
0.47 
0.87 
1.04 
1.11 

Skymax102  1300 
0.13 
0.22 
0.27 
0.50 
0.60 
0.64 

Heritage 100P  400 
3 x  0.27 
0.48 
0.58 
1.09 
1.30 
1.39 

Explorer 150PDS  750 
0.15 
0.26 
0.31 
0.58 
0.69 
0.74 

Skymax102  1300 
0.08 
0.15 
0.18 
0.34 
0.40 
0.43 
Red: Above maximum usable magnification; magenta: above maximum usable magnification for Newtonians
In the following, I present sample pictures taken with the Explore Scientific Focal Extender 2 x, the Revelation Barlow lens 2 x, and the SkyWatcher Barlow lens 2 x (under unfavorable conditions, camera Ricoh GR held to the eyepiece). These photos should not be taken too seriously, but they provide a certain impression of the different performances of the three focal extenders. The SkyWatcher Barlow lens 2 x is especially sensitive to stray light and shows the strongest color fringing. With the Explore Scientific Fokal Extender 2 x, I could not find any color fringing. The Revelation Barlow lens is in the midfield, but I also found it not good enough for using it in the past ...
The SkyWatcher Heritage 100P used in these tests was actually beyond the maximum usable magnification (200 x instead of 150 x) when used with the 4 mm UWA eyepiece. The following photos are unprocessed!
The following photos are ment to give an impression of the achievable magnification with the 2 x and 3 x externders (not of the achievable image quality). They were also taken with the SkyWatcher Heritage 100P and the Ricoh GR:
24 mm WA eyepiece at Heritage 100P  Explore Scientific Focal Extender 2 x, 24 mm WA eyepiece  Explore Scientific Focal Extender 3 x, 24 mm WA eyepiece 
The Explore Scientific Fokal Extender 2 x and 3 x version weigh 260 g (232 g and 254 g according to my kitchen scales) and are therefore a problem for light telescopes such as the SkyWatcher Heritage 100P. Experience will show whether I can live with this...
In progress
My first impression of the Explore Scientific Focal Extender 2 x is very good. Surely one cannot expect miracles, and "real" eyepieces with the corresponding magnifications will certainly give better results than the equivalent combinations with the extender. But as a "start," extenders are a good and not too expensive solution. Very first impressions of the 3 x extender seemed to indicate that it does not perform quite as well as the 2 x extender (which does not come as a surprise to me...), but when I checked both extenders at the moon (nearly half moon), the 2 x with an 16 mm UWA eyepiece, and the 3 x with a 24 mm WA eyepiece, I could not see any significant differences (the same accounts for the 7 mm UWA eyepiece without extender, however, this magnifies a little more in comparison to the two other configurations).
When I look at the technical data of my current eyepiece (see above), I have, however, to admit that my eyepiece set does not have clear gaps (see elsewhere) and that therefore the purchase of the focal extenders was not really necessary...
When traveling, the extenders do not seem to have any real advantages for me: Two extenders instead of two shortfocussed eyepieces actually increase the weight without really offering "must have" options. And since I mostly use the small SkyWatcher Heritage 100P telescope when traveling, heavy extender solutions are not really ideal (see above)...
The following table demonstrates possible "travel sets " from my current eyepieces and focal expanders for the SkyWatcher Heritage 100P telescope (two consisting of 5 pieces, two of 4 pieces):
3 Eyepieces (24 mm, 16 mm, 10 mm) + 2 Extenders (2 x, 3 x) = 8 Options  
Extender  2 x 
2 x 
3 x 
3 x 
2 x 
3 x 

Eyepiece  24 mm 
16 mm 
24 mm 
10 mm 
16 mm 
24 mm 
16 mm 
10 mm 
10 mm 
Magnification  16.67 
25.00 
33.33 
40.00 
50.00 
50.00 
75.00 
80.00 
120.00 
Weight: approx. 1060 g*  
3 Eyepieces (24 mm, 16 mm, 10 mm) + 1 Extender (2 x) = 6 Options  
Extender  2 x 
2 x 
2 x 

Eyepiece  24 mm 
16 mm 
24 mm 
10 mm 
16 mm 
10 mm 

Magnification  16.67 
25.00 
33.33 
40.00 
50.00 
80.00 

Weight: approx. 810 g with extender, approx. 580 g eyepieces alone*  
3 Eyepieces (24 mm, 16 mm, 10 mm) + 1 Extender (3 x) = 6 Options  
Extender  3 x 
3 x 
3 x 

Eyepiece  24 mm 
16 mm 
10 mm 
24 mm 
16 mm 
10 mm 

Magnification  16.67 
25.00 
40.00 
50.00 
75.00 
120.00 

Weight: 830 g with extender, approx. 580 g eyepieces alone*  
3 Eyepieces (24 mm, 16 mm, 7 mm) + 1 Extender (2 x) = 6 Options  
Extender  2 x 
2 x 
2x 

Eyepiece  24 mm 
16 mm 
24 mm 
16 mm 
7 mm 
7 mm 

Magnification  16.67 
25.00 
33.33 
50.00 
57.14 
114.29 

Weight: approx. 925 g with extender, approx. 690 g eyepieces alone*  
4 Eyepieces (24 mm, 16 mm, 10 mm, 7 mm) + 1 Extender (2 x) = 8 Options  
Extender  2 x 
2 x 
2 x 
2x 

Eyepiece  24 mm 
16 mm 
24 mm 
10 mm 
16 mm 
7 mm 
10 mm 
7 mm 

Magnification  16.67 
25.00 
33.33 
40.00 
50.00 
57.14 
80.00 
114.29 

Weight: approx. 1020 g with extender, approx. 790 g eyepieces alone*  
5 Eyepieces (24 mm, 16 mm, 10 mm, 7 mm, 4 mm) = 5 Options  
Eyepiece  24 mm 
16 mm 
10 mm 
7 mm 
4 mm 

Magnification  16.67 
25.00 
40.00 
57.14 
100.00 

Weight: approx. 1000 g*; *) all weights taken with kitchen scales and including caps, some bags, ... 
The "eyepieces only" solution (5 pieces) looks best to me, next come the configuration with 3 eyepieces plus 3 x focal extender (4 pieces) and 4 eyepieces plus 2 x extender (5 pieces). I decided to go for the last configuration for my June 2017 holiday in France.