focus

vindictive666 (John) · #1 20-03-2005, 10:59 AM

gday all

just a question really

ive got 3 ep's

2x 1.25 inch plossle's (15mm,30mm)
and one 2 inch 32mm gso

is it normal to have to change the focus every time i change the ep's ?

iceman (Mike) · #2 20-03-2005, 11:16 AM

Yes, they will all have different focal points.

Some eyepieces have a claim of being "parfocal" which means the eyepiece has been designed so that they will focus at the same point, and you won't need to adjust focus when you change eyepieces.

I'm not sure which designs are parfocal, I imagine they cost more and unless there was some other reason you wanted those EP's, I wouldn't buy a set of eyepieces for parfocal reasons alone.

vindictive666 (John) · #3 20-03-2005, 11:26 AM

thanks mike

i was just wondering thats all

ps

what do you think of my avatar not too wild this time ?

trufflehunter (Wayne Robinson) · #4 21-03-2005, 10:49 PM

All the Meade Series 4000 (up to and including the 32mm) are supposed to be 'parfocal'... but not in my experience. They all require some re-adjustment of the focus.

In fact, I find even adding a filter to an eyepiece requires a slight re-adjustment. Which sounds completely wrong, I know...

Vermin (Tom) · #5 22-03-2005, 01:42 AM

Even the Televue EPs that are parafocal require some fine adjustment.

I've noticed the filter refocus requirement too (on a polarizing moon filter), it does not change the height of the EP so it must mean the glass refracts the light in some way (supposed to be optically flat).

ving (David) · #6 22-03-2005, 07:22 AM

Quote:

Originally posted by vindictive666
thanks mike

i was just wondering thats all

ps

what do you think of my avatar not too wild this time ?

you know what? I actualy liked the other one

both my moon and light pollution filters need refocusing.

MiG · #7 22-03-2005, 08:01 PM

Quote:

Originally posted by Vermin
Even the Televue EPs that are parafocal require some fine adjustment.

I've noticed the filter refocus requirement too (on a polarizing moon filter), it does not change the height of the EP so it must mean the glass refracts the light in some way (supposed to be optically flat).

The glass is optically flat but the incoming rays aren't perpendicular to it. If my thinking isn't faulty, the focal point would be slightly further.

Your filters must be different to mine. Mine do change the height of the EP.