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Are the entry line of Meade Super Plossls parfocal?

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#1 zanxion72

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Posted 10 April 2020 - 01:35 AM

I have a mix of Meade Super Plossls, some of them marked as Series 4000 and others not. I have noticed that they aren't really parfocal, having to focus by quite a bit especially if switching from the 26mm or 15mm to the Series 4000 9.7mm. I have also noticed that the 26mm LP focuses at a very different point than the non LP and on one of my telescopes (Astromaster 130) one must draw the focusing tube all the way out and still have doubt that it has been focused. Is it normal?

Also, I have disassembled one of my 26mm Super Plossl. Is this the way the lenses should go into the barrel?

 

IMG-20200409-095633.jpg

 

Many thanks,

Nikos


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#2 sg6

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Posted 10 April 2020 - 02:58 AM

Most "parfocal" eyepieces are not exactly parfocal. and the Meade's are very likely not.

I tend to find it a little irrelevant, I have never myself or met anyone that swaps an eyepiece without then adjusting the focuser.

 

I suppose the feature would be most appreciated on an SCT/Mak where focusing can be painful.

 

I usually say that "parfocal" means "approximately parfocal".

My Paradigms/BST's are or were said to be parfocal, most are within reason, except one that is way different.

 

Concerning the lens, I think that is the arrangement.


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#3 zanxion72

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Posted 10 April 2020 - 03:21 AM

Many thanks SG6 for the reply. The frequent re-focusing when changing eyepieces made me thing if there was a problem with my telescopes and/or my eyepieces. It seems that parfocal eyepieces just go like that. My ES eyepieces are more close to parfocal than the Meade's super plossls that in many cases are not even close to being parfocal.



#4 bbqediguana

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Posted 10 April 2020 - 08:45 AM

Yes, the lens arrangement you are showing is correct. There should be a spacer (a metal ring) that goes between them to keep the round ends separated by a few millimeters. 

 

When I bought my Tele Vue Plössls, one of the features I liked was that they are all parfocal with each other. My 6mm ortho is not close for focusing. 

 

However, as sg6 mentioned, I still refocus every time I swap eyepieces. When the 6mm goes in, it just means a few extra turns of the focuser knob - but its not something I really notice or think of. Because my primary scope is an SCT, I always refocus and finish with a counter-clockwise adjustment to ensure the primary mirror is squared up with the baffle tube it rides on (thankfully, the mirror shift on my SCT is minimal!). I have a helical focuser on my diagonal to do any final tweaks.

 

Rick


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#5 hoof

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Posted 10 April 2020 - 03:21 PM

“Parfocal” even when “perfect” changes with eye nearsightedness or farsightedness. Even people who see 20/20 vary a bit in optimal focus in a telescope. Thus there is no incentive to make eyepieces that are precisely parfocal, approximate is enough.

I’m working myself back from significant nearsightedness to normal (over a few years) and even only being a third of the way there, I notice a significant change in the focus positions of my eyepieces on my telescopes. It’s actually pretty cool seeing that principal (nearsightedness changing parfocal positions) in action!

#6 bbqediguana

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Posted 10 April 2020 - 03:51 PM

“Parfocal” even when “perfect” changes with eye nearsightedness or farsightedness. Even people who see 20/20 vary a bit in optimal focus in a telescope. Thus there is no incentive to make eyepieces that are precisely parfocal, approximate is enough.

I’m working myself back from significant nearsightedness to normal (over a few years) and even only being a third of the way there, I notice a significant change in the focus positions of my eyepieces on my telescopes. It’s actually pretty cool seeing that principal (nearsightedness changing parfocal positions) in action!

What are you doing to work yourself back from nearsightedness? Just curious!

 

Rick



#7 hoof

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Posted 11 April 2020 - 01:50 AM

What are you doing to work yourself back from nearsightedness? Just curious!

 

Rick

Well it's a long story, but in essence, I'm nearsighted because of tons of time looking closer than the neutral focus distance for my eyes. Then I got glasses to "correct" that and the cycle continued again, over and over again until I peaked at about 5.5 diopters in 2001. 

 

Well, it turns out you can push focus about 0.5 diopters further than the neutral point and that drives eyes in the opposite direction. It's why far sighted people also get worse with time.

 

I use a bazillion glasses of various powers to keep my eyes having to push focus at different subject distances. It's much harder than inducing myopia because the far sighted accommodation range is so much more limited than the eye's ability to focus close.

 

My current attempt started at 4.75 diopters of myopia (neutral focus) last Jume, and is now at about 3.6 diopters that I can actively push down to about 3.2. 

 

 My back of the napkin calculations is about 1000 hours of pushing focus like this (with minimal close focusing) per diopter. So it takes years :) But it took that long per dioper to get myopic, it just was so easy I didn't notice it.  I'm hoping to be able to see stars as points naked eye again sometime in 2023/2024.

 

So it can be done. There is a small community that does this, PM me if you want the link.


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