1° Forum di Astronomia Amatoriale Italiano

Da come si presentano sembrano anelli di Newton e potrebbe essere che l'effetto sia dovuto al doppietto con lenti molto vicine. È un "gioco di luce" dato da interferenza ottica, ma non significa che sia certamente un problema. Fammi chiedere a proprietari dello stesso strumento e ti faccio sapere.

these are Newtonian rings:

https://en.wikipedia.org/wiki/Newton%27s_rings

For the optical design of the FC-76 it's completely normal. Every FC-76
shows these rings. They arise because of the small distance of the both
lenses in the lens cell. May be, it sounds crazy for you, but this is a
sign of a perfect centered optic. The rings are round and symmetric. If
somebody has the will, he could measure the rings and gets impression of
the quality of the optic.

The rings does not affect the image in the focal plane in any way. The
explanation is quite simple: If you have two lenses with an air gap, you
get reflections between the two lenses. This is always the same in every
lens cell, it doesn_t matter which construction or manufacturer, it's
physics. The only question is: Does the reflected light interfer with
itself? It can interfer with itself, if the optic is quite good and if
the distance is fitting (see Wikipedia). If the light does not interfer,
nevertheless the reflected light between the lenses is present. So the
phenomena do not add something to the light beam reaching the focus or
is loosing something. This the the amount of light getting lost between
the two lenses, regardless if it interfers or not. In the case it
interfers, you can see it in the Newtonian rings, otherwise the light is
scattered without you notice.

So all I can say, you have a perfect lens and you can see it in the
Newtonian rings.

Sì, infatti, non è indice di difetto. Quello che mi lascia perplesso è che non abbia mai ricevuto feedback in tal senso, pertanto preferisco chiedere ad attuali proprietari della medesima ottica. Se è tutto a posto come presumo l'effetto che vedi frontalmente non ha nessun riscontro poi al piano focale. In caso di lenti erroneamente spaziate te ne accorgi poi al piano focale... eccome se te ne accorgi!

Mauro