Russell 65mm Super Plossl XL used with NV
I most often use NV for “Phonetography” and wanted to use my TEC 140 more for that purpose, but needed to reduce the focal ratio to cover wider fields of nebula, so I purchased a Gary Russell 65mm Super Plossl to see if it would work with my scopes and NV device afocally. I tested the eyepiece in 3 scopes, a TEC 140, ST-120 and an ES 208 Newt… and then sent the eyepiece to Moshen to test in his C-9.25 and his 92mm AP Stowaway. All together, we had scopes with focal ratios of f:4, 5, 6.6, 7, and 10.
Last year, I tested a TV-55 afocally, using its long focal length to reduce the effective focal ratio of the 140, but it showed too much astigmatism with field curvature, and I saw reflections from bright point light sources… this was before Chemisted and then Gavster showed very good results using the 55mm eyepiece in conjunction with the GSO R-C .75x reducer in certain refractors. I was interested enough to see if the 5 element Russell Super Plossl was better corrected than the 4 element TV 55.
I normally observe with NV (Mod 3C) in prime configuration, which is more simple than afocal, with a shorter stack and less concern about reflections showing up from multiple glass surfaces. So, I’ll try not to let my preference interfere with this evaluation.
I called Gary Russell and spoke with him concerning my intentions and he recommended the 65mm for my trial. His 50 and 56 Super Plossls cannot be configured with threads (either 2” or TT threads) at the top of the eyepiece. I wanted some threads for a firm, centered connection with the Envis lens on my Mod 3C. Gary made the eyepiece using Delrin for the body, with 2” female threads at the top of the eyepiece ($95 incl. shipping) and it arrived in about 10 days. It is nicely made, looks good and is very robust. It does have an odd smell from the Delrin. I had already ordered a 2” to TT adapter from Agena for an additional $25. I used a RAF Camera TT to Envis adapter, already in my possession, for the connection between the eyepiece and the Mod 3C. It looked like this when assembled… 11” long:
Before testing the eyepiece with NV, I placed it in my ST-120 which is f:5, fully expecting to see edge aberrations, but the image was quite good! I quickly found that the eye relief was very long. In advertising, the 65 is included with the 72mm and 85mm Super Plossls; eye relief is listed as at least 35mm for these three eyepieces. I knew that I would have to experiment with the eyepiece to get the distance between the eye lens and the NVD Envis lens correct. None of these long Super Plossl eyepieces (65-85mm FL) are recommended for optics with a central obstruction having a magnification with the eyepiece of less than 35x. However, this limitation is offset by using an NVD.
The 65mm eyepiece does not have a typical field stop; it relies on the bottom of the barrel to act as the field stop. When I placed my eye too close, vignetting occurred because the bottom of the barrel intrudes on the FoV. Pulling my eye too far away caused the upper housing restriction to intrude, also causing vignetting. Spacing with the NVD would be a critical issue. Here’s a photo through the eyepiece with the camera lens too close to the eye lens:
In the second daytime image, the spacing is about right, although this image was taken while hand holding the phone, so not perfectly centered. It shows that the bottom of the barrel, acting as the field stop, is not infringing on the FoV. This image was taken using the ST-120, which produced a good FoV with no noticeable astigmatism, spherical aberrations, or curvature, even at f:5. Very encouraging.
For the first attempt using this eyepiece with NV at night, I used no additional spacers and found that moderate vignetting did occur on the perimeter of the FoV, probably because the bottom of the barrel was intruding… too close. I used TT (T2 or M42) 5mm incremental spacers to reduce vignetting, but never found the “perfect” distance to completely eliminate it. I did, however, get it down to about the outer 5% of FoV. My thought is that the cone of light might have been clipped by the TT spacers I was using… I did not have 2” spacers to test this theory.
Results using the eyepiece with NV varied, probably because of the focal ratio of the scopes that were used to test the eyepiece in combination with NV. Gary Russell said that this eyepiece has been used successfully in scopes down to f:4, and I believe him. Here’s why… when using this eyepiece visually, without NV, the FoV was quite flat (see the daytime images above, using the ST-120 at f:5). Daytime tests using the 65mm eyepiece in the 140 (f:7) refractor were even more impressive. But at night, when I attached the NVD Envis (1x lens) to the top of the eyepiece, I saw astigmatism and field curvature. And I learned that the speed (focal ratio) of the scope was only partially responsible for the observed aberrations. Since a picture is worth a thousand words, I’ll show you what I saw using different telescopes.
The first image was using the 65mm eyepiece with NV in the ST-120 (achromatic doublet) at f:5. Looking at this image of a very large FoV which includes M81/82 near the center, field curvature and astigmatism are visible and obvious across 50+% of the outer field.
Using the eyepiece in the TEC 140, resulted in significantly decreased edge of field aberrations, showing in the outer 25+% of FoV, providing a much more pleasing image. Not only were aberrations visible in a smaller part of the FoV, but they were less obvious with less effect on star shapes. That’s M51 in the center FoV. At about 1 o’clock, you can see a brighter star outside the field of view, revealing minor vignetting (the star was right at the very edge of field, within the vignetting). Placing a nebulae in this large FoV (2.4°) for visual use could be very pleasing.
Shortly after the nighttime tests with the 140, I set it up for a daytime test and photo with the Russell 65mm eyepiece. Again, the field was very flat with no noticeable aberrations. The palm tree in this photo was about 200m away. The full size version of this photo shows the leaves sharp all the way to the edge of field; this photo is very compressed, so it doesn't look clear.
There were no surprises when I put the eyepiece in the f:4 Newt with NV attached. Stars in the outer 60+% were misshapen into little, pointy pollywogs:
Adding an MPCC (Baader Multi-Purpose Coma Corrector) in front of the 65mm eyepiece removed the pointy ends on stars, but instead, left them striated.