Quote:
Originally Posted by Filippo
... then all of a sudden it would all go black...
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OK... this is a "known issue".
Practice makes perfect.
The telescope has what is known as an "exit pupil". You must align the pupil (iris) of your eye and hold it there to allow all the light from the telescope to enter your eye. See
https://www.google.com/imgres?imgurl...9QEwAHoECAQQBg
There are two snags.
Telescopes with have a secondary mirror (like your 4SE) are very prone to this at low power, because the black spot is actually the image of the secondary mirror as seen by the eyepiece. The problem is significantly worse in daytime because the iris of your eye will have closed to 1-2mm aperture - which makes accurate eye placement absolutely critical, whereas at night your iris may open up to 5...7mm, making it a lot easier. Alternatively - in daytime - use a higher power eyepiece (shorter focal length).
If your eye is off to one side of the exit pupil, the black-out can occur, very suddenly. Secondly this is also dependent on the specific eyepiece - some are worse, some are very tolerant of poor eye placement. The black-out effect is also known as "kidney bean" (it may resemble a big black kidney-bean in the field) and is caused by spherical aberration in the eyepiece. Some have it, many do not. A couple of Televue Naglers were notorious for this.