Quote:
Originally Posted by astro744
Yes within a series of eyepieces having them parfocal would of course be possible but making all eyepieces parfocal from a given manufacturer across different series types, e.g. Plossl, wide field, ultra wide field would be difficult if not impossible. How does one allow for the 2"-1.25" adapter thickness in ones calculations? e.g. If the 42mm LVW is parfocal with the LVW 3.5mm then what thickness 2"-1.25' adaptor must one use or must one have a zero thickness adaptor?
Zero thickness adaptors come in two types; the one in which the 1.25" lock screw slots into the 2" focusser and then there is the one that has an allen screw that locks onto the barrel of the 1.25" eyepiece effectively making it a 2" eyepiece.
You can add Clave Plossl to the parfocal list as this was a major selling point for me when I bought some in the '80s plus the fact that they claimed 0.5mag extra gain due to their highly polished surface and fine coatings and of course 4 elements only in a true Plossl (asymmetrical) design. (They have certainly NOT disappointed!) This of course is the subject of a different debate but suffice to say having parfocal eyepieces or at least within a few mm of each other is very handy when observing but I don't expect say a 31mm Nagler to be parfocal with other eyepieces and as long as I have the inward travel I'm OK with that.
It would be good if all suppliers post their detailed eyepiece specifications but they don't so most of the time one has to either find out by buying the given eyepiece or asking here on the forum if anyone experiences any drastic inward or outward focus issues with a given eyepiece.
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0.5 MAGNITUDE GAIN ...Where can I get some!!!
bigjoe.