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Equipment Discussions >> Eyepieces

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wags1
member
*****

Reged: 05/23/13

Loc: PA, USA
Re: 2013 Buyer's Guide to Eyepieces new [Re: Maverick199]
      #5986306 - 07/23/13 03:44 PM

Don,
Just stumbled on this..incredibly useful! Thank you!


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GregLee1
professor emeritus


Reged: 07/21/13

Loc: Waimanalo, HI
Re: 2013 Buyer's Guide to Eyepieces new [Re: wags1]
      #5990050 - 07/25/13 06:17 PM

Wanting to try a wide angle EP but being pretty tight, I ordered a "UW80[deg]" (shown on photo) from a Chinese seller on Ebay. It is called "New 1.25" Multi-coated F16mm Untra Wide Angle 80 Degree Telescope Eyepiece". It matches pretty well entry #482 in the spreadsheet: KnightOwl, $44.95 (I paid $68.68, free shipping). I wonder if it is this exact EP.

If it turns out to be really wonderful, I'll post. Otherwise, I might not get back to you.


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JustaBoy
Post Laureate


Reged: 06/19/12

Re: 2013 Buyer's Guide to Eyepieces new [Re: GregLee1]
      #5990258 - 07/25/13 08:46 PM

Please tell us that you are just kidding...?

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AllanStaib
journeyman


Reged: 10/15/12

What's the difference between planetary and plossl new [Re: Starman1]
      #5999906 - 07/31/13 11:38 AM

Thanks Starman1 for the excellent spreadsheet. I'm curious about the listing of the Meade HD 60 series as "planetary" type eyepieces and the series 4000 Meade eyepieces as plossls. What defines an eyepiece as a planetary? I the the HD series are also plossls so there must be another feature which earns the planetary designation. Thanks for all the information and posts!

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: What's the difference between planetary and plossl new [Re: AllanStaib]
      #6000150 - 07/31/13 02:46 PM

The HD series was billed by both the manufacturer and distributor (Meade) as "planetary" eyepieces. The HD-60s are a 6-element design and may be identical internally to the Celestron X-Cel LX and similar or identical to the Astrotech Paradigms and its 4 or 5 clones (from Orion, Teleskop Service, etc.).
They may be considered general use eyepieces. There is no specific name for the design that I know of. They may all be copies of the TeleVue Radian, which is now discontinued. I used the "planetary" moniker out of convenience.

A Plössl has 4 elements in 2 groups (2 doublets). There are at least 4 different configurations for these 4-element eyepieces, called Plössls or Super Plössls. Unfortunately, there are also some 5-element designs, in a 2:1:2 configuration that are also called Plössls or Super Plössls. They are not Plössls, but more closely resembling Erfles, and may be designed similarly to a WWII design of Carl Zeiss, called the Astroplanokular. And the actual symmetrical design we now call Plössls are not the original design (the "original" may have come from König, it has been argued).

Confusion reigns in design names. Partly, it's that many designers live a long time and have many designs. Albert König had 28 patents for different designs. At least 6 or 7 have been sold as telescope eyepieces, and many more used in binoculars. Eyepieces of 3, 4, 5 and 6 elements have been sold that were designed by König, so what is a König eyepiece?


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AllanStaib
journeyman


Reged: 10/15/12

Re: What's the difference between planetary and plossl new [Re: Starman1]
      #6000597 - 07/31/13 09:33 PM

Thanks! You've really gotten me interested in the details of eyepiece design. Are there any publications that go into detail on the various designs? Thanks again!

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: What's the difference between planetary and plossl new [Re: AllanStaib]
      #6000946 - 08/01/13 03:00 AM

Lots!
Start here:
http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EVOLUTIONofEYEPIECE...


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GregLee1
professor emeritus


Reged: 07/21/13

Loc: Waimanalo, HI
Re: 2013 Buyer's Guide to Eyepieces new [Re: JustaBoy]
      #6009530 - 08/06/13 01:51 PM

Quote:

Please tell us that you are just kidding...?



No, not kidding. The UW80 eyepiece arrived from China on schedule -- seems fine to me for terrestrial viewing. Stars have to wait for clouds to dissipate (perhaps left over from tropical storm Flossie). Anyhow, relevance to the Guide is that it tells me I probably overpaid by $25 or so. Useful to know.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: GregLee1]
      #6019434 - 08/11/13 06:03 PM Attachment (128 downloads)

I attach the 3rd version of the spreadsheet, with a couple hundred new eyepieces and oddballs I missed the first time around.
So many resellers are selling BST (Barsta) eyepieces that I added those as well.


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Helveticus
member


Reged: 07/10/13

Loc: Switzerland
Re: 2013v.3 Buyer's Guide to Eyepieces [Re: Starman1]
      #6020568 - 08/12/13 11:21 AM

Thanks for sharing, great job!

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LDW47
scholastic sledgehammer


Reged: 03/04/12

Loc: North Bay,Ontario,Canada
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: Starman1]
      #6105603 - 09/28/13 10:02 AM

What happened to the # in the calculated Field Stop column in the third version or am I missing them on my iPad copy ??

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LDW47
scholastic sledgehammer


Reged: 03/04/12

Loc: North Bay,Ontario,Canada
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6105612 - 09/28/13 10:08 AM

Also how we're the various Field Stops calc.? By actual physical measurement with a tape, inside callipers etc. ?

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LDW47
scholastic sledgehammer


Reged: 03/04/12

Loc: North Bay,Ontario,Canada
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6105651 - 09/28/13 10:30 AM

If not physically measured the calc. Field Stop could be significantly different ( probably much less ) based on the actual design of, shall we call it the Field Stop Ring, in the barrel of each individual manufacturers EP !?

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6105719 - 09/28/13 11:05 AM

Quote:

What happened to the # in the calculated Field Stop column in the third version or am I missing them on my iPad copy ??



That column automatically populates when you enter your telescope's focal length in the red-letter field at the top of column N.
Other fields populate when you put your telescope's f/ratio in the red letter field at the top of column O.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6105731 - 09/28/13 11:11 AM

Quote:

Also how we're the various Field Stops calc.? By actual physical measurement with a tape, inside callipers etc. ?



The manufacturer's field stop field is the one to look at first. But I wanted to figure out a way to calculate a field stop that would yield the apparent field in your scope if the manufacturer didn't quote a field stop. So, a formula in the field takes into account the focal length of the eyepiece, the manufacturer's claim for apparent field, and the focal length of your scope to calculate an "effective" field stop. It could be off by a few tenths of a millimeter.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6105743 - 09/28/13 11:18 AM

Quote:

If not physically measured the calc. Field Stop could be significantly different ( probably much less ) based on the actual design of, shall we call it the Field Stop Ring, in the barrel of each individual manufacturers EP !?



If you use the calculated field stop for the eyepiece, you can derive the manufacturer's apparent field figure and a true field for the eyepiece. So the calculated figure does work back to the apparent field that is quoted by the manufacturer. As you know, when such things are actually measured, the apparent fields are rarely exact, so in those examples where the calculated field stop differs from the manufacturer's field stop quote, it is likely the manufacturer's claim for the apparent field is off by a couple degrees.
In such a case, you can adjust the apparent field figure until the calculated field stop and the actual field stop are the same. That would work for apparent fields of view smaller than about 50 degrees. It won't work for widefield eyepieces, though, since distortion at the edge of the field changes the amount of apparent field that will fit within a particular field stop diameter.


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LDW47
scholastic sledgehammer


Reged: 03/04/12

Loc: North Bay,Ontario,Canada
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: Starman1]
      #6107056 - 09/29/13 08:36 AM

I don't understand what distortion has to do with the amount of apparent field that will fit within the field stop for a given widefield and/or 2" EP ? As well, other than in an odd case, why don't the EP manufacturers clearly identify the Field Stop spec.for their product, why make the buyer do the leg work as I think it is a fairly important # to have when buying an EP or am I wrong in my thoughts ? This is a learning experience for me as I gain knowledge in this hobby !

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LDW47
scholastic sledgehammer


Reged: 03/04/12

Loc: North Bay,Ontario,Canada
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: Starman1]
      #6107068 - 09/29/13 08:46 AM

I am having a hard time accessing the columns to enter my data as I get no response from my iPad when I hit the data line !? What's the secret as I have tried it several times ! Also I am going to physically measure the Field Stop as a comparison on my 2" and 1.25" EP's

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6107332 - 09/29/13 11:56 AM

Quote:

I don't understand what distortion has to do with the amount of apparent field that will fit within the field stop for a given widefield and/or 2" EP ? As well, other than in an odd case, why don't the EP manufacturers clearly identify the Field Stop spec.for their product, why make the buyer do the leg work as I think it is a fairly important # to have when buying an EP or am I wrong in my thoughts ? This is a learning experience for me as I gain knowledge in this hobby !




I'll try to come up with a decent analogy.

You're in space, looking at a globe. As a city comes around the edge of the planet, it seems to move sideways, relative to you, very slowly (because a lot of its motion is toward you, not laterally). When it crosses underneath you, it seems to move fast. When it nears the other edge, it appears to slow down.

What we've just described is similar to how angular magnification distortion works in an eyepiece. The star may enter the field moving very slowly, speed up as it crosses the center, and slow down again as it nears the edge. This is because, in my example, magnification is lower at the edge of the field than it is in the center.

So let's say we time the passage of a star to see how large the true field of view is. We would get a larger true field than the size of the apparent field would predict, i.e. the apparent field would be smaller than it should be for that large a true field. If we knew the apparent field, could we predict the true field? No. If we knew the true field, could we predict the apparent field? No.

So, let's say we know the field stop of the eyepiece. Can we predict what the true field or apparent field will be from that figure? Only if there is zero distortion.

To get around that, some manufacturers quote a "derived" field stop. It's not an actual field stop like an iris, but it is a field stop size that can be plugged into formulae to calculate the true field of the eyepiece. In the example I gave of the eyepiece with significant AMD, the field stop size we'd quote would actually be larger than a true field stop inside the eyepiece because the true field stop size would lead you to predict a smaller true field than you would actually measure.

As to manufacturers quoting a field stop size, some do. But many manufacturers don't because they are not aware of the spec (their engineers certainly are), or don't care, or think it would hurt the sales, or be confusing to the customer. As I discovered when doing the buyers guides over the years, some manufacturers quote nothing more than the diameter of the eyepiece and the focal length--they tell you nothing about elements inside, or apparent field, or weight, or any other characteristic. How do they expect to make sales? I haven't a clue.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 2013v.3 Buyer's Guide to Eyepieces [Re: LDW47]
      #6107351 - 09/29/13 12:04 PM

Quote:

I am having a hard time accessing the columns to enter my data as I get no response from my iPad when I hit the data line !? What's the secret as I have tried it several times ! Also I am going to physically measure the Field Stop as a comparison on my 2" and 1.25" EP's



Your iPad is an Apple computer and the spreadsheet format is Microsoft Excel (xlsx). Can you normally read Microsoft programs? Do you have Excel on the iPad? Perhaps you only have a "Reader" for Excel and don't actually have the Microsoft Office program on the iPad.
If that's the case, you can download OpenOffice from openoffice.org and have a fully-functioning program to read and modify Word, Excel, PowerPoint, pdf, etc.

As to field stops, a lot of today's eyepieces have negative lenses in the bottom and a positive group in the top. On these eyepieces, the field stop is between the lenses, and not directly measurable. Besides, as I implied in my previous post, even if you measured it, it would only lead to the actual true field if the eyepiece had zero distortion. You're better off deriving a "effective field stop" size for the eyepiece by timing the passage of a star on the Celestial Equator and back-calculating the field stop that would yield that true field.


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