John Russell
Well-known member
I have a 65mm Diascope with the 23x (16,7mm) eyepiece and it has served me well for several years.
The comparatively low magnification offers advantages in fov, brightness, depth of field and speed on the bird and I have seldom wished for more.
However, I was recently in a situation, where I could not get any closer and was not able to distinguish between Ringed Plovers and Little Ringed Plovers, and could not see any colour in the legs of a Greenshank (?).
Of course. the Zeiss zoom would help out here but I am not a fan of zooms and was not willing to pay €450 for the rare occasions I would need it.
Both Zeiss and Baader Planetarium offer adapters to enable 1 1/4" astronomical eyepieces to be used on the Diascopes (more on these later). I experimented some time ago with an 11mm TV Plössl for 35x mag. but the expectation that the simple construction would offer contrast benefits for terrestrial use was an illusion, and the 8mm eye relief was very uncomfortable.
Many astronomical eyepieces will not achieve infinity focus on the Diascope but the Vixen LVs have a short barrel and 20mm eye relief. I decided on the 9mm version, which is the shortest focal length with a quoted afov of 50°. This would give me 43x mag. and a 1,5mm exit pupil. I have read tests indicating that there is little to be gained with exit pupils under 1,3mm for terrestrial use.
The Vixen eyepiece is claimed to be fully multicoated and to have blackened lens edges. It has a fold-down rubber eyecup, is well finished and weighs about 170g complete with Baader bayonet adapter. I set about comparing it to the 23x Zeiss and the Zeiss zoom (15x-45x), which was kindly lent to me by a friend.
Eye lens diameter seems to be related to fov and eye relief. It measured 23mm on the Vixen, 24,5mm on the zoom and 28mm on the 23x WA. By contrast, it was only 10mm on the Plössl.
Holding the zoom eyepiece to one eye at minimum magnification and the Vixen to the other eye, I could see no difference in fov. The 9mm Vixen NLV, the latest version with twist-up eyecup, is quoted with 45° afov and I assume it is optically the same as the LV, so this figure would appear more realistic.
However the view is comfortable with the long eye relief and could not be described as tunnel vision. Close focus with the 23x was 3,55m and 3,60m with the Vixen, so little change of focus was necessary.
The zoom was not parfocal across its range. It wasn't just the need for more critical focus at 45x; returning to 15x required a small correction.
Optical errors are most apparent at the field edge and the greater the fov, the greater the errors. Nevertheles, I was surprised that the inexpensive Vixen had the best overall performance. Astigmatism was not discernable, field flatness was excellent, distortion and lateral CA were low. There was some diffuse ghosting from a street lamp outside the fov but in the same situation the 23x Zeiss and the zoom at 15x showed striations.
Both Zeiss eyepieces showed moderate lateral CA and pincussion distortion.
The 23x and zoom at 15x also had a lot of field curvature with the zoom also showing astigmatism at this magnification. The Plössl, surprisingly, also showed severe astigmatism.
The Baader adapter is to be recommended. It duplicates the Zeiss bayonet and is secured to the astro eyepiece with a compression ring and 3 grub screws. The Zeiss adapter is well made but slow to change as the eyepiece has to be secured to the scope with a locking ring with a very fine thread.
All Vixen LVs (and NLVs) have 20mm eye relief, so the different focal lengths are individual designs and may differ in performance but the 9mm can be warmly recommended.
John
The comparatively low magnification offers advantages in fov, brightness, depth of field and speed on the bird and I have seldom wished for more.
However, I was recently in a situation, where I could not get any closer and was not able to distinguish between Ringed Plovers and Little Ringed Plovers, and could not see any colour in the legs of a Greenshank (?).
Of course. the Zeiss zoom would help out here but I am not a fan of zooms and was not willing to pay €450 for the rare occasions I would need it.
Both Zeiss and Baader Planetarium offer adapters to enable 1 1/4" astronomical eyepieces to be used on the Diascopes (more on these later). I experimented some time ago with an 11mm TV Plössl for 35x mag. but the expectation that the simple construction would offer contrast benefits for terrestrial use was an illusion, and the 8mm eye relief was very uncomfortable.
Many astronomical eyepieces will not achieve infinity focus on the Diascope but the Vixen LVs have a short barrel and 20mm eye relief. I decided on the 9mm version, which is the shortest focal length with a quoted afov of 50°. This would give me 43x mag. and a 1,5mm exit pupil. I have read tests indicating that there is little to be gained with exit pupils under 1,3mm for terrestrial use.
The Vixen eyepiece is claimed to be fully multicoated and to have blackened lens edges. It has a fold-down rubber eyecup, is well finished and weighs about 170g complete with Baader bayonet adapter. I set about comparing it to the 23x Zeiss and the Zeiss zoom (15x-45x), which was kindly lent to me by a friend.
Eye lens diameter seems to be related to fov and eye relief. It measured 23mm on the Vixen, 24,5mm on the zoom and 28mm on the 23x WA. By contrast, it was only 10mm on the Plössl.
Holding the zoom eyepiece to one eye at minimum magnification and the Vixen to the other eye, I could see no difference in fov. The 9mm Vixen NLV, the latest version with twist-up eyecup, is quoted with 45° afov and I assume it is optically the same as the LV, so this figure would appear more realistic.
However the view is comfortable with the long eye relief and could not be described as tunnel vision. Close focus with the 23x was 3,55m and 3,60m with the Vixen, so little change of focus was necessary.
The zoom was not parfocal across its range. It wasn't just the need for more critical focus at 45x; returning to 15x required a small correction.
Optical errors are most apparent at the field edge and the greater the fov, the greater the errors. Nevertheles, I was surprised that the inexpensive Vixen had the best overall performance. Astigmatism was not discernable, field flatness was excellent, distortion and lateral CA were low. There was some diffuse ghosting from a street lamp outside the fov but in the same situation the 23x Zeiss and the zoom at 15x showed striations.
Both Zeiss eyepieces showed moderate lateral CA and pincussion distortion.
The 23x and zoom at 15x also had a lot of field curvature with the zoom also showing astigmatism at this magnification. The Plössl, surprisingly, also showed severe astigmatism.
The Baader adapter is to be recommended. It duplicates the Zeiss bayonet and is secured to the astro eyepiece with a compression ring and 3 grub screws. The Zeiss adapter is well made but slow to change as the eyepiece has to be secured to the scope with a locking ring with a very fine thread.
All Vixen LVs (and NLVs) have 20mm eye relief, so the different focal lengths are individual designs and may differ in performance but the 9mm can be warmly recommended.
John
