I had a conversation with another member recently on this topic but thought it best to start a new thread so that others might fill in any gaps.
Some birding scopes are designed to accept 1.25" astronomical eyepieces (Celestron, Pentax) and others can be adapted using a bayonet or thread.
There are two categories of adapters, those attached to the eyepiece and those attached to the scope body.
Achieving infinity focus with some birding scopes and astro eyepieces can be problematic so the former type of adapter doesn't require any external clamping and can be seated more deeply in the scope body to achieve infinity focus.
The disadvantages are that one requires an adapter for each eyepiece and that a change of magnification, as with the extenders from Kowa, Leica and Swarovski, is very slow. If one were observing an object in the night sky at low magnification, it would probably have drifted out of the field of view by the time one had changed eyepieces and would be hard to find again at the higher magnification.
The on-scope adapters allow a very rapid change, so one could use something like a cheap 32 mm Plössl as a finder and then drop in a shorter focal length eyepiece for higher magnification.
Apart from infinity focus there are other compatibility issues. A €30 Plössl would probably provide excellent central sharpness but widefield and long eye relief in the shorter focal lengths is going to require complex (and expensive) designs, where distortion and edge performance may still fall short of expectations.
The OEM eyepieces on my Swarovski ATM65 HD (30xW) and on my Kowa 883 (25-60x zoom) both show minimal amounts of pincussion distortion. Both have flat fields and there is no visible astigmatism on the Swarovski and only the slightest amount on the Kowa.
All the astro eyepieces I have tried show significant pincussion distortion. This is not necessarily bad as, with the right amount, a celestial body (moon, planets) would also appear round at the field edge. With very little pincussion, as on the OEM birding eyepieces, a round object gets flattened to a tangential ellipse near the field edge. You can test this by affixing a coin to a vertical surface with some blu-tack.
However, some astro eyepieces such as the 28 mm Edmund RKE show such extreme pincussion that a round object at the field edge gets distorted to a radial ellipse.
All objective lenses and all eyepieces have field curvature and the focal planes for tangential and sagittal rays differ. Getting these to match is something of a lottery so one might end up with little field curvature but remaining astigmatisn or little astigmatism but remaining net field curvature.
It's unlikely that one would achieve similar edge performance with astro eyepieces to the above mentioned OEM eyepieces, so I think that astro eyepieces make most sense outside the normal magnification range of the birding scope, short focal lengths for high magnification and large focal lengths for use as a finder or for large exit pupils in twilight.
Lastly, as regards infinity focus, I see that all 1.25" Televue eyepieces (with the exception of the 14 and 17,3 mm Delos) have their field stops (focal planes) 1/4" below the shoulder, so they should work with any scope. On most others, e.g. Baader, Vixen, Pentax, it is at the shoulder and infinity focus is problematic, particularly on the premium Kowa scopes.
John
PS:- Kowa also offers an adapter that can be attached to eyepieces with a gain of about 6 mm of in-focus.
PPS:- I originally posted yesterday on the Optics and Astronomy subforum but will attenpt to delete that.
Some birding scopes are designed to accept 1.25" astronomical eyepieces (Celestron, Pentax) and others can be adapted using a bayonet or thread.
There are two categories of adapters, those attached to the eyepiece and those attached to the scope body.
Achieving infinity focus with some birding scopes and astro eyepieces can be problematic so the former type of adapter doesn't require any external clamping and can be seated more deeply in the scope body to achieve infinity focus.
The disadvantages are that one requires an adapter for each eyepiece and that a change of magnification, as with the extenders from Kowa, Leica and Swarovski, is very slow. If one were observing an object in the night sky at low magnification, it would probably have drifted out of the field of view by the time one had changed eyepieces and would be hard to find again at the higher magnification.
The on-scope adapters allow a very rapid change, so one could use something like a cheap 32 mm Plössl as a finder and then drop in a shorter focal length eyepiece for higher magnification.
Apart from infinity focus there are other compatibility issues. A €30 Plössl would probably provide excellent central sharpness but widefield and long eye relief in the shorter focal lengths is going to require complex (and expensive) designs, where distortion and edge performance may still fall short of expectations.
The OEM eyepieces on my Swarovski ATM65 HD (30xW) and on my Kowa 883 (25-60x zoom) both show minimal amounts of pincussion distortion. Both have flat fields and there is no visible astigmatism on the Swarovski and only the slightest amount on the Kowa.
All the astro eyepieces I have tried show significant pincussion distortion. This is not necessarily bad as, with the right amount, a celestial body (moon, planets) would also appear round at the field edge. With very little pincussion, as on the OEM birding eyepieces, a round object gets flattened to a tangential ellipse near the field edge. You can test this by affixing a coin to a vertical surface with some blu-tack.
However, some astro eyepieces such as the 28 mm Edmund RKE show such extreme pincussion that a round object at the field edge gets distorted to a radial ellipse.
All objective lenses and all eyepieces have field curvature and the focal planes for tangential and sagittal rays differ. Getting these to match is something of a lottery so one might end up with little field curvature but remaining astigmatisn or little astigmatism but remaining net field curvature.
It's unlikely that one would achieve similar edge performance with astro eyepieces to the above mentioned OEM eyepieces, so I think that astro eyepieces make most sense outside the normal magnification range of the birding scope, short focal lengths for high magnification and large focal lengths for use as a finder or for large exit pupils in twilight.
Lastly, as regards infinity focus, I see that all 1.25" Televue eyepieces (with the exception of the 14 and 17,3 mm Delos) have their field stops (focal planes) 1/4" below the shoulder, so they should work with any scope. On most others, e.g. Baader, Vixen, Pentax, it is at the shoulder and infinity focus is problematic, particularly on the premium Kowa scopes.
John
PS:- Kowa also offers an adapter that can be attached to eyepieces with a gain of about 6 mm of in-focus.
PPS:- I originally posted yesterday on the Optics and Astronomy subforum but will attenpt to delete that.
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