Singlet refractors and Astronomical Observation Report
Optical Evaluation and Astronomical Observation Report
Singlet Refractors
I. Comparative Optical Assessment of the 1300 mm Objectives
Regarding the two 1300 mm focal length singlet objectives:
The 25 mm aperture objective polished on felt produces diffraction-limited images when stopped to its nominal 25 mm clear aperture. The Airy disc is clean and well formed under good seeing conditions.
However, when the aperture is increased to 28 mm, the image quality deteriorates. At 28 mm, three distinct diffraction spikes or rays emerge from the Airy disc, indicating residual zonal error or edge defects. At this aperture, the objective can no longer be considered diffraction limited, and the stellar image appears compromised.
In contrast, the paper-polished objective performs noticeably better. It produces diffraction-limited star images at both 28 mm and 29 mm aperture. The Airy pattern remains symmetrical, without parasitic rays or significant deformation.
At present, the paper-polished lens is optically superior to the felt-polished one.
A controlled comparison will be conducted using both objectives stopped to the same effective aperture (25 mm) in order to evaluate intrinsic figure quality independently of aperture-dependent aberrations.
All observations were carried out using the same eyepieces across both instruments to maintain consistency, although earlier reports lacked sufficient detail.
II. Observational Report
2.1 m Focal Length Singlet Objective
Focal length: 2100 mm
Full diameter: 47 mm
Effective aperture used: 37 mm
Polishing method: Felt
This is likely my best-performing felt-polished objective to date. The recently completed 3.5 m and 2.6 m focal length lenses do not yet reach the same optical quality i will try to correct them other time.
Atmospheric conditions during observation were stable, allowing meaningful planetary evaluation.
Jupiter Observations
1. Keplerian Eyepiece 25 mm Focal Length
Magnification: 84×
Design: Plano-convex singlet eyepiece
At 84×, Jupiter appears very sharp and well defined. The planetary limb is clean, and the image exhibits best performance/aperture.
The North and South Equatorial Belts are clearly visible, with a distinct separation from the brighter equatorial zone. A faint suggestion of color is detectable within the belts, especially during moments of steady seeing. The polar regions appear as darker shadings near both poles.
Chromatic aberration is present but modest at this magnification. The image remains high in contrast relative to the aperture employed.
This magnification provides the best balance between resolution, contrast, and chromatic control.
2. Keplerian Eyepiece – 11 mm Focal Length
Magnification: 190×
At 190x, Jupiter appears very large in apparent diameter. However, a noticeable blue halo surrounds the planetary disc, characteristic of longitudinal chromatic aberration in long-focus singlet objectives.
The equatorial cloud bands remain visible, but contrast is reduced compared to the 84× view. Importantly, the level of resolved detail does not increase proportionally with magnification. The image scale enlarges, but fine structural information remains essentially the same.
Thus, 190x exceeds the optimal magnification range for a 37 mm effective aperture under these conditions.
3. Galilean Eyepiece – 20 mm Focal Length
Magnification: 107x
When using a 20 mm Galilean eyepiece, the image quality is better to that obtained with the 25 mm Keplerian eyepiece. I did not observe a significant difference in clarity or contrast in the overall performance. The same level of detail is visible in both.
At the center of the field, the image is quite sharp, and chromatic aberration appears slightly reduced, likely due to the narrower effective field and different exit pupil geometry inherent to the Galilean configuration.
4. Galilean Eyepiece – 43 mm Focal Length
Magnification: 48x
With the −43 mm Galilean eyepiece, the image is very sharp and aesthetically pleasing. Jupiter appears small, bright, and crisp.
The equatorial cloud bands, especially the main belts, are clearly visible with surprisingly good contrast. The reduced image scale enhances perceived sharpness, and chromatic aberration is less intrusive at this low magnification.
Although detail is limited by magnification, the overall visual impression is excellent.
Lunar Observation
Plossl 20mm eyepiece
Magnification 107x
The Moon was observed and recorded at the eyepiece using a 20 mm Plössl eyepiece at 107× magnification.
At this power, the lunar surface shows strong contrast. Crater rims are sharply defined, and shadow relief is pronounced. The long focal ratio of the objective helps maintain reasonable spherical correction, though chromatic fringing is visible along high-contrast limb regions but very low, its a very pleasing image with very good contrast and brightness using the 20mm Plössl eyepiece, 107x.
The 20mm Kepler Plano convex eyepiece (107x)
The field of view its narrow but the overall brightness its a little higher then with the Plössl, better contrast, because its just one lens the light its passing through, but lateral aberrations are visible, not in the Plössl but there are downs and pro's.
The 2.1 m singlet performs particularly well on lunar detail at moderate magnifications between 80x and 110x.
III. General Optical Conclusions
The 2.1 m felt-polished singlet performs optimally at moderate magnifications (approximately 80–110×).
High magnifications reveal chromatic limitations without adding meaningful detail.
The paper-polished 1300 mm objective demonstrates superior correction at larger apertures compared to the felt-polished equivalent.
Singlet refractors of long focal ratio can achieve impressive planetary performance when well figured, even at small apertures. Galilean eyepieces provide high central sharpness and contrast but at the cost of extremely narrow fields of view. Keplerian eyepieces allow greater magnification and field size but make chromatic aberration more apparent.
Here is the moon with the 2.1m felt polished lens filmed at the eyepiece through a 20mm plossl eyepiece at 107x.
Observations conducted with both Keplerian and Galilean eyepieces suggest that their overall level of optical performance is nearly equivalent. Chromatic aberration appears to be slightly reduced in the Galilean configuration compared to the Keplerian; however, lateral field aberrations are comparable in both systems. At the edge of the field of view, the Galilean eyepiece exhibits substantial distortion, and when examined carefully, both designs display similar off-axis limitations. I compared a −20 mm focal length Galilean eyepiece with a 20 mm focal length Keplerian eyepiece. Both delivered essentially the same level of detail and image structure. The narrow apparent field of view of the Galilean eyepiece creates a subjective impression of higher image quality, likely because the observer’s attention is confined to the central region of the optical axis. Upon closer examination, however, both eyepieces demonstrate comparable optical performance. Personally, I prefer the Keplerian design due to its wider field of view, which greatly facilitates centering and tracking objects.
For double star observations with singlet objectives, the Galilean eyepiece provides a particularly interesting visual experience. In this specific application, the Galilean configuration performs very well, especially when the target is maintained near the optical axis.
A notable limitation arises with short focal length Keplerian eyepieces. At high magnifications, they exhibit significant edge distortion, making precise centering of a planet difficult. At 190× magnification, for example, I must often wait for the planet to drift into the central portion of the field before conducting detailed observation.
The reason for employing such high magnifications is deliberate: I aim to test the objective near its practical resolution limits. Additionally, I systematically explore a wide range of magnifications—from low to very high—in order to evaluate the full performance envelope of the instrument and to determine how image quality evolves as magnification increases.
For double star observations with singlet objectives, the Galilean eyepiece provides a particularly interesting visual experience. In this specific application, the Galilean configuration performs very well, especially when the target is maintained near the optical axis.
A notable limitation arises with short focal length Keplerian eyepieces. At high magnifications, they exhibit significant edge distortion, making precise centering of a planet difficult. At 190× magnification, for example, I must often wait for the planet to drift into the central portion of the field before conducting detailed observation.
The reason for employing such high magnifications is deliberate: I aim to test the objective near its practical resolution limits. Additionally, I systematically explore a wide range of magnifications—from low to very high—in order to evaluate the full performance envelope of the instrument and to determine how image quality evolves as magnification increases.
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