Kokusai Kohki 30mm Widescan III

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Kokusai Kohki 30mm Widescan III
I also looked at this one and compared it rather extensively with the cheap BW Optik 30mm. The BW Optik is a clone of the Type II widescan, which is supposed to have the same optical design as Type III widescane, so the difference is only mechanical.

Frankly, their performance is rather close. So, it affects how I rate its value. It worths a solid 9 without the BW Optik in the market.

It does provide a wider field of view than the BW Optik, I believe the BW Optik has a small field stop, or better say, a thicker barrel so that it reduces the field and also put some more weight in it.

Overall Rating: 8
Optics:9 Value:8
Weight: 1 (Unreliable Vote)
Link to this vote: http://excelsis.com/1.0/displayvote.php?voteid=347813

Kokusai Kohki 30mm Widescan III
The Kokusai Kohki WideScan III 30mm was manufactured and assembled completely in Japan and was first released in the beginning of 2003 as a follow-up of the model type II, which was partially made in Taiwan and did show some fabrication issues. In the US, these WideScans are provided e.g. through KK/Magellan Helix Observing Accessories, in Germany e.g. through APM, Markus Ludes, Saarbruecken. The German price was about 190 (in 2006) - 295 (in 2003) Euros, the US price was 225$ for the 2 inch 30mm ocular. There are some Chinese clones of the type II in the market, known as BW Optics, J+T, 1RPD, Astrolan or the Moonfish 30mm 80 degree. The series III has been ended up recently in the spring of 2006.
It is an impressive, beautiful designed but relative handily and light 2 inches, 84 degree AFOV ocular; it is constructed with 5 multicoated optical elements in 3 groups, but it is not an Erfle design offspring (2-1-2).
It weighs about comfortable 472grs (16.65 oz.) without protection caps, the overall height is 112mm (4.4 inches) from my own measurements. The 2 inch chrome barrel has a safety undercut and measures 39mm (1.535 inches) in height. The main diameter is about 57mm (2.24 inches).

The filter thread fits perfectly with 2 inch Lumicon, TeleVue and Astronomik UHC and OIII filter with 48x0.75mm thread.
The multicoatings are excellent made to the state of art, shimmering decently in slight purple and green, without tending for ghosting and internal reflections. The interior of the eyepiece and the lens edges are perfectly blackened.

The eye positioning is very comfortable and not affected by any kidney-beaning or blackening-out effects. Eye relief is stated being about 18.4 mm on the Kokusai Kohki page. The light transmission or throughput is very high in a direct comparison with an older University Optics Konig II 32mm 2 inch and a Meade 4000 SWA 32mm, it becomes clearly obvious.
The back focus or in-travel is moderate 3.5mm inwards, quite uncomplicated for all 2inch focusers.
What about the optical performance with a telescope?
In an f/10 telescope only the outer 10% of the FOV or so are getting soft and astigmatic and some false color can be seen at the very edge. Pincushion I could detect none at all, you won’t get seasick by a quick panning around through the Milky Way.
In an f/8 Newton it is also behaving quite well, about 80 to 85% of the field is quite sharp, with 60% being very good; 15-20% of the field is unusable and suffers from astigmatism.
At f/6 or f/5, the view will degrade more, 50% of the field staying perfectly sharp while 67-75% are quite useable with slightly degraded stars at the edge, the rest is pure astigmatic!
Remember: 75% of 84degrees are at least about 62 useable degrees or approximately 35mm effective of the field stop diameter. Calculating the stated field stop diameter of 47mm to the focal length of 30mm reveals an AFOV of 90degrees or more (other sources giving 44mm Fieldstop diameter that will result 84 degrees).

I use this ep with an air spaced doublet achromatic refractor of 4 inches and 660mm focal length (f/6.5) and with my 250/1252mm travel truss dobson. Just scanning through the Virgo cluster with the 2.1 degree TFOV with the Dobson or panning in the Milky Way or at the Veil Nebula is most impressive and pure fun! You can easily get accustomed to ignore the worse edge performance knowing that a perfect ocular would cost you about the threefold of the regular price for the WideScan. It’s just fun for the money. Overall value 9 to 10, great!
Clear skies to you all,
Randolf K., Germany

Overall Rating: 9
Optics:9 Value:10
Weight: 5 (Veritable Vote)
Link to this vote: http://excelsis.com/1.0/displayvote.php?voteid=508584

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