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Old 13-02-2017, 12:13 PM
Barnacle (Bill)
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Meade MA eyepiece – plastic lens element…DIY conversion

Hi All,

I have a current version of the Meade MA (modified achromat) 1.25 inch eyepiece which came with the scope, and it looks exactly the same as the one sold per link below (note, my eyepiece came with the scope, so did not buy from this seller):

https://www.telescopehouse.com/meade...iece-1-25.html

When I disassembled this Meade MA 25mm to clean the dust and blacken the lens edge (which does make a difference), I learnt the following:

1. The eye lens is an achromat doublet (glued together) made of glass.
2. The field lens is a bi-convex lens made of plastic (easily scratched)

The design is almost the same as a Kellner, barring the plastic bi-convex field lens instead of one made of glass which is somewhat more plano-convex.

I was keen to see why the MA 25mm eyepiece spacing between the eye lens and field lens elements being much closer than the other 25mm Kellner eyepieces I have, the answer unfortunately lies in the curvature of the plastic field lens, plus its refractive index being plastic and not glass.

What disappointed me the most is to learn that a plastic lens being used as a Meade telescope eyepiece element (albeit a low end eyepiece for Meade) when other brands’ 25mm eyepieces use glass lens elements for their field lens.

Cost cutting in Chinese suppliers have crept into Meade’s eyepiece inventory (by design or by stealth)… so buyers beware…

PS:
Given I have other 25mm eyepieces already with glass lens elements, to save this dud MA eyepiece to having more practical use for me, I have increased the spacing between the field lens by moving the plastic field lens further away from the eye lens (which is easily achievable in this MA design).

That is, I now place this plastic field lens just before the field stop for the field lens, and changed this MA 25mm to around 37.5mm instead, a 1.25 inch eyepiece which I don’t have at this focal length.

The field of view is narrow, probably 35 degrees (but still better than the cheap Huygens that is marked with a silver lettering H sticker coming out of China, which comprised of two identical plano convex lens). It does now have a lower power than 25mm, which is something I like to have in my collection. And low power won't magnify the plastic scratches and optically flaw as readily inherent in this plastic field lens.

Thanks.

Kind regards,

Bill
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Old 13-02-2017, 08:19 PM
Wavytone
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Reminds me of my DIY experiments long ago when I had a box of assorted surplus lenses I acquired for next to nought.

A couple of MgF coated 30mm binocular objectives (plano-convex doublets) arranged convex-to-convex with their flat sides facing out made a startlingly good classic 40mm Plossl. Inserting a third coasted plano-convex doublet in the middle with the convex side facing the sky also made a pretty darn good eyepiece though the eye relief was very short. Note these doublets were all coated, although rejects from binocular assemblies.

Likewise you could buy some smaller doublets from SurplusShed.com and make your own Plossls which would easily outdo the Meade MA eyepieces... but the downside is those won't be anti-reflection coated so the light losses will be horrendous (about 60% transmission).

But these days the old Meade MA eyepieces really are rubbish ... take a close look at the plastic field lens and I'd bet it wasn't antireflection coated, which is why it scratches so easily. In comparison $50 buys a very nice (fully coated) product from China. My last buy was a Prostar 38mm 70 degree affair in a 2" barrel, which cost a whopping $49. Awesome eyepiece for the money.

IMHO there is no reason to pay the TeleVue Tax if you do some research.

Last edited by Wavytone; 13-02-2017 at 08:35 PM.
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Old 14-02-2017, 09:57 PM
Barnacle (Bill)
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Anti-reflection

Yep, you are right.

The plastic field lens is not anti-reflection coated for the Meade MA 25mm eyepiece.

But then again, the same applies for the glass field lens for the Skywatcher Super 25mm Kellner eyepiece or other unbranded Kellner eyepieces supplied with mass manufactured telescopes, at least the ones that I come across and disassembled to blacken the lens edges.

Kind regards,

Bill
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Old 14-02-2017, 10:08 PM
Wavytone
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you... actually use kellners ? hmmm.
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Old 16-02-2017, 12:25 PM
Barnacle (Bill)
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Eyepieces

No, I generally use my labelled as "Plossls", but when I took them apart, and blacken the lens edges, are really symmetrical (also called dialsight)eyepieces instead.

http://www.quadibloc.com/science/images/eyevar34.gif

Keen to take eyepieces apart and experiment.

The Kellners come in real handy when you have kids around wanting to see thru your scope and they will touch the lens despite you tell them not to.

Bill
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Old 16-02-2017, 04:04 PM
Wavytone
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Ah. True I have a keen little one also who takes everything apart. Keeping a pair of old eyepieces for the same purposes...
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Old 16-02-2017, 06:35 PM
miker
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Bill, what do you use to blacken the edges?

Michael
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Old 17-02-2017, 03:33 PM
Barnacle (Bill)
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Lens edge blackening - it is not a myth

Hi Michael,

To blacken the lens edge, I use black permanent marker, and fine Black Sharpie for the small groove between cemented doublet lens.

Just make sure I don’t clean them later with acetone or other solvent cleaners, or else I will create a mess.

I can vouch that lens edge blackening is not a myth.

I have a pair of 10x50 Tasco Zip binoculars which suffered a loss of contrast when I got it second hand. I blacken the cemented doublet objective lens first, it did improve contrast a little.

I then blacken the two eyepieces as well (each eyepiece comprised of a doublet lens and a plano convex lens) and the improvement in contrast is definitely noticeable with the naked eye.

Nothing else was done to the binocs other than blacken the lens edge for the objectives and eyepieces. It is not perfect but I am much happier now with it than when I first got it.

Kind regards,

Bill
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Old 22-02-2017, 03:11 PM
miker
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Thanks Bill I appreciate your detailed reply.
I have a couple of Plossls that may benefit from edge blackening.

Michael
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Old 23-02-2017, 06:33 AM
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dannat (Daniel)
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Bill very few plossl's left in existence, just all ep's labelled as plossl are in fact symmetrical..i have a couple of the vite eyepieces, cheap on ebay, low lens count, also have plastic lenses; not a problem if they are figured well, plastic can be just about as good as glass

there are still some clave plossl at Melb Obs in botanic gardens, true French plossl from yesteryear..they are good eyepices but nothing spectacular
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Old 24-02-2017, 05:04 PM
Barnacle (Bill)
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Plastic lens elements

Hi Daniel,

The issue I have with plastic lens elements is that they are much more prone and easier to scratching than glass.

They also tend to yellow more readily over a quicker time.

Lastly, I think plastic lens have a different refractive index, which actually may help reduce violet fringing and spherical aberration in short tube refractors that I experimented with.

I agree, Plossls are really Dialsight/symmetrical but mass eyepiece manufacturers (from China) should label them correctly as Dialsight/Symmetrical and not as Plossls when they are not.

Kind regards,

Bill
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