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Note on ES 100* parfocalarity

Bart

By Bart
in Discussions - Eyepieces

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Bart

Bart

Posted
Posted

Yes, I did make that word up!

Just thought I would give my findings comparing the 14 and 20mm 100* ES range. I have the 20mm some time and love it. I got to use my new 14mm for the first time last night, it having been sitting in a box for 2 weeks due to cloud!

The 14mm is equally as nice as the 20mm. Lighter though, noticeably. The thing that surprised me is that they are miles apart in terms of parfocal(arity)(ness)??? I mean, I thought something was wrong. Mind you, I found the 14mm 100* almost exactly parfocal with the 30mm 82*, which made me feel better :)

I know it bothers some a lot, so thought I'd stick it up here for anyone interested.

Barry

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Doug

Doug

Posted
Posted

Hello Barry

Well I'm still waiting to use my E/S14mm and it look like tomorrow may be my fist chance but by then I may be up to my ankles in snow :embarassed:

it's nice to see someone getting a chance :grin: which scope did you use with the 14mm ?

congrats

Doug

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John

John

Posted
Posted

Parfocal is when there is little or no need to re-focus when an eyepiece is changed. By little I'd say just a tiny tweak. It's a "nice to have" rather than essential feature I reckon.

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russ.will

russ.will

Posted
Posted

For the record, my ES82s are all over the shop. Mind you, I find the mere act of changing an EP is enough to require a focus tweak, so it matters little whether it's 1/10 or 1/2 a turn. It seems to be one of those features that's less useful than the literature would have you believe.

Russell

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alan potts

alan potts

Posted
Posted

At the moment I have with me the E 8mm, 13mm, and 17mm and they are not parfocal with each other, it will be interesting when I get the 21mm and the 10mm to see if they are. I have to say the eyepieces being parfocal is not a big issue with me. All my Nagler T6's that I have had were.

Alan

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John

John

Posted
Posted

I reckon parfocality is a feature that has largely had to be sacrificed in order to achieve hyper-wide fields of view and longer eye relief that us fussy astronomers are demanding. Tele Vue have parfocal groups with their eyepieces, either A, B or C where you can be sure they are "...approximately parfocal -- require little (if any) refocusing when used together." I notice that that don't do this with the Ethos range or the 14mm and 17.3mm Delos so presumably they are "off the scale" !

What TV do for all eyepieces is give you data that allows you to work out the approx position of the field stop within the eyepieces. This is column F in the table below with explanatory notes and diagrams at the bottom of the page. This should enable you to get an idea of how they will compare:

http://www.televue.c...=214&plain=TRUE

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Dunkster

Dunkster

Posted
Posted

That's an interesting read, thanks John :cool: does that mean that all the EPs with the same value of F (at least, within a form factor of either 1.25" or 2") are parfocal?

Given our weather and variable seeing, I always find myself having a little tweak of the focuser knob anyhow just to check I'm as close to the focal point as possible (but maybe that's just my OCD :eek: )

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Dunkster

Dunkster

Posted
Posted

Thats what microfocusers are for in my opinion, to give you all the little tweaking you want without throwing your focus a mile out :)

I use a motorised focuser for the C8... helps reduce disturbance to the mount (I usually observe from the worst hard surface in history!) while focussing too :cool: still can't help fiddling :D

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