2023-02-06, 06:25 PM
Hi Greg,
During my observing days with the 13" Coulter Odyssey newtonian, Coulter recommended using a 5" off-axis mask for planetary & lunar observing. In my experience, the image with the mask was much clearer & exhibited smaller star images while the full aperture stars were rather bloated. This was especially true earlier in the night as the atmosphere was cooling. With the mask, the stars shifted their positions in the eyepiece, but were sharp, while at full aperture the stars didn't move as much, but appeared larger.
The effect was attributed to the seeing cells being ~6" in diameter such that the 5" aperture's light path passed through a single set of cells & would shift with the cells, but give sharp images. At 13" the light passed through several cells & the image got fuzzier, but was more stable in the eyepiece.
Several experienced observers in the astro club said that the view of Saturn with the masked aperture was among the best that they'd seen.
The mask may have also reduced the tube currents.
Phil S.
During my observing days with the 13" Coulter Odyssey newtonian, Coulter recommended using a 5" off-axis mask for planetary & lunar observing. In my experience, the image with the mask was much clearer & exhibited smaller star images while the full aperture stars were rather bloated. This was especially true earlier in the night as the atmosphere was cooling. With the mask, the stars shifted their positions in the eyepiece, but were sharp, while at full aperture the stars didn't move as much, but appeared larger.
The effect was attributed to the seeing cells being ~6" in diameter such that the 5" aperture's light path passed through a single set of cells & would shift with the cells, but give sharp images. At 13" the light passed through several cells & the image got fuzzier, but was more stable in the eyepiece.
Several experienced observers in the astro club said that the view of Saturn with the masked aperture was among the best that they'd seen.
The mask may have also reduced the tube currents.
Phil S.