Quote:
Originally Posted by Tom Hancock
crisp and sharp
|
But... At high power - say around 200x on a bright star:
- can you clearly see the Airy disk and a nice set of diffraction rings ?
- when you defocus a little, is the appearance of the rings the same inside and outside focus ?
- is there a violet halo ?
- are the rings coloured ?
Secondly, does it cleanly resolve a close double star say 2-3 arc secs apart ?
Many of the short achromatic doublet refractors fail the first test - the star looks like a hard "knot" but no diffraction rings, or have a distinct violet halo.