I recently acquired a nice small 80mm F/7 refractor. Viewing through it for the first time last night Jupiter, Saturn, Mars and the moon looked very good and pretty sharp.
When viewing stars and for example the Pleaides using a 40mm Pentax XL and Meade 24.5mm wide angle I noticed that I could not quite get the stars in to pin points, pretty close but not quite. Just a little in or out of focus would produce some seagulling effect. If I put in my 12mm Brandon or 9mm UO ortho it seemed to clean up some.
I am suspecting it is seeing condition related. It has been clear but pretty cold here.
Anyway I'm hoping that it is related to seeing as one of the main reasons I got this scope was to use it for low power sweeping.
Any thoughts on this would be appreciated.
For most people when the iris is opened much greater than 2 mm the aberrations of the lens are quite severe. With the 40 mm your scope's exit pupil is 5.7 mm. So between FC and the eyes aberrations, it is difficult to get pinpoint stars over the entire field or even in the center. With the 9 mm ortho, the actual field is very small (just look at the diameter of the field stop in that eyepiece) so that FC is practically non-existent. Furthermore, the scopes exit pupil is only 1.3 mm, which is equivalent to stopping down the eye lens to that aperture with a corresponding reduction in SA, CA and astigmatism.
Poor seeing is more of a problem at higher powers than low.
Edit: One more point: When looking at the moon or bright daytime objects your iris is contracted to its minimum diameter so that even at low powers the view is sharp. In addition, the reduction of the iris diameter reduces the effective diameter of the objective.
Edited by daquad, 23 November 2020 - 11:51 AM.