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Equipment Discussions >> Eyepieces

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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]
      #6107373 - 09/29/13 12:20 PM

I would have thought that the amount of distortion is so minute that it would have virtually been unmeasurable or insignificant !? My thoughts were that it was, somehow, just a straight line measurement between the narrowest part of the EP at the lenses but I now see there is a lot more to it once you add in how the lense(s) function ! It's just amounts to getting all these little goodies straight in your mind !

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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]
      #6107385 - 09/29/13 12:26 PM

That is exactly what my problem is so I will access it on my Microsoft desktop ! Measuring it would be strictly eyeballing as you are correct in saying you can't actually get to it ! You would probably be within a mm or so if it actually means anything based on your other answer !?

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JustaBoy
Post Laureate
*****

Reged: 06/19/12

Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6107583 - 09/29/13 02:00 PM

It actually is a bit more complicated than just what we would think.

In an eyepiece that has an internal field stop, it is always placed after the negative lower group, and before the upper positive group. – Now just for the sake of simplicity let’s call this lower negative group a ‘barlow’, as it does produce diverging rays like a Barlow, but because it may be designed just to complement the specific upper group in in use , then I suppose that it is more correctly called a Smyth lens. – But please; for our purpose here, ‘barlow’ it is, with a small ‘b’ out of respect for Peter Barlow, capital ‘B’.

Now a ‘barlow’ is usually considered to be part of the telescope’s optical train rather than the eyepiece’s, but in this case we are making it part of the eyepiece, as is certainly the case with the negative lens being before a field stop in all eyepieces that use them.

Now for an example, let’s do something easy, a 32mm 50° Plossl, which has a easily measured field stop of very close to 27mm (please, lets not nit-pick here).

Now let’s attach a 2x ‘barlow’ to the bottom of it so that we can no longer get to the field stop to measure it... Now we have what appears to be a 16mm Plossl, still with a 50° AFOV, so the field stop ‘must be’ 13.5mm, to get the 16mm and 50° field.

But, when we open this hybrid to gain access to the internal field stop we see it is indeed 27mm.

So, the moral of this story is that measuring the internal field stop by disassembling the eyepiece tells us nothing if the focal length of the upper group (used as a stand alone unit), is not known. How much amplification is our ‘barlow’ providing to arrive at the resultant focal length as a whole eyepiece?

So Uncle Al uses what is really a derived figure for the size of the field stop for the eyepieces that can’t be measured directly.

If you don’t believe any of this, just take your $500 Nagler apart and measure the field stop directly (NOT recommended), and you will find your measurement to be quite a bit different than what AL is trying to tell you. – He is only telling you the derived figure to help you in your field calculations, so trust him on this.

Now, like Don has mentioned the TFOV and AFOV will not compute because of the eyepiece’s distortion, but will typically vary only about 5% in a Nagler design, Perhaps more in a T1, T5, and approaching nil in a T4.

I think I need another cup of coffee, and then I will wake up and discover once again, that everything that I thought I knew was all wrong

Good day,

-Chuck


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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: JustaBoy]
      #6107665 - 09/29/13 02:46 PM

With so many different EP 's out there with differing specs, are all these numbers ever going to be truly indicative of anything in real life or are they just a good, average guide of what you have in your particular EP set ? It seems that with the number of manufacturers / designs / performance characteristics for each that to arrive at anything truly accurate would be difficult because of this complicated nature !? If that is how the results is to be taken ie.a really good set of numbers to be used, then I have no problems with that as long as we all know ! I have just been wondering how accurate / meaningfully this issue is !! Very interesting and kind of what I thought but thanx for all of your help and considerable knowledge !

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JustaBoy
Post Laureate
*****

Reged: 06/19/12

Re: 2013v.3 Buyer's Guide to Eyepieces new [Re: LDW47]
      #6107698 - 09/29/13 03:03 PM

Try a eyepiece and see how it works for you, in your scope. - See if it's what you want.

Here's a little trick... Multiply the focal length of the eyepiece by it's quoted AFOV. - If we are talking 1.25" eyepieces, then the number you are looking for to max out the TFOV is around 1600 - For 2" you want 2700.

As you can see there are many ways you can end up with the number you want with all of the different AFOV's you will find.

Bottom line is; see the top line of this post:-)

Have fun
-Chuck


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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: JustaBoy]
      #6107746 - 09/29/13 03:31 PM

The top line is exactly correct ! Like with everything else it's just nice to have a number for reference / comparison and to take it for what it is worth !

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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]
      #6107755 - 09/29/13 03:39 PM

Quote:

The top line is exactly correct ! Like with everything else it's just nice to have a number for reference / comparison and to take it for what it is worth !



Which is one of the reasons why, though it is a little less accurate than the field stop calculation, the standard TF=AF/M works just fine [true field = apparent field/magnification]. Just keep in mind your actual true field will be slightly smaller than the calculated one from that formula, and you're fine.
Because what we don't really know is the EXACT focal length of our scopes, or the EXACT focal lengths of the eyepieces (they're usually rounded off).
And, really, how important is the EXACT magnification? I almost always round off the magnification of my eyepieces when people ask me what power I'm using, and that's close enough.
And you can always time a star to yield the correct true field for the eyepiece.
With TF and M and approximate AF, you're set.


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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]
      #6107841 - 09/29/13 04:44 PM

I agree ! Thanx again for all of your expertise !!

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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]
      #6107886 - 09/29/13 05:10 PM

Your 01/09/06 article Eyepiece Qualities & Aberrations explains a lot !

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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]
      #6107909 - 09/29/13 05:26 PM

Quote:

Your 01/09/06 article Eyepiece Qualities & Aberrations explains a lot !



Thanks. Here are some more essays I've written in recent years:
Informational Articles


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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]
      #6112664 - 10/02/13 09:14 AM

I have them bookmarked !

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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]
      #6116060 - 10/03/13 09:03 PM Attachment (138 downloads)

I attach the 2013.5 Buyer's guide to eyepieces.

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MatthewG
newbie


Reged: 05/09/13

Loc: Dallas Texas
Re: 2013 Buyer's Guide to Eyepieces new [Re: Starman1]
      #6259038 - 12/17/13 09:47 PM

Post deleted by Scott in NC

Edited by Scott in NC (12/24/13 12:45 PM)


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michaeldurban
member


Reged: 03/06/12

Loc: Durban, South Africa
Re: 2013 Buyer's Guide to Eyepieces new [Re: MatthewG]
      #6260119 - 12/18/13 01:34 PM

Sometimes (amateur) astronomy -or should I say optics- can become a leeeeettllle bit overwhelming.

Kudos to Don who -as it turns out- is either single, or has a very accommodating (another optometrical term) wife...

Just kidding.

Well done and great work Don,
Like I mentioned in my article.. you are truly the telescope guru!!!

Edited by Scott in NC (12/24/13 12:45 PM)


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michaeldurban
member


Reged: 03/06/12

Loc: Durban, South Africa
Re: 2013 Buyer's Guide to Eyepieces new [Re: michaeldurban]
      #6260125 - 12/18/13 01:39 PM


Yes, ..for the discerned scientists..
I did make up the word optometrical...

just fitted the context I guess..



Edited by Scott in NC (12/24/13 12:46 PM)


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

Reged: 06/24/03

Loc: Los Angeles
Re: 2013 Buyer's Guide to Eyepieces new [Re: michaeldurban]
      #6260154 - 12/18/13 01:54 PM Attachment (131 downloads)

Quote:


Sometimes (amateur) astronomy -or should I say optics- can become a leeeeettllle bit overwhelming.

Kudos to Don who -as it turns out- is either single, or has a very accommodating (another optometrical term) wife...

Just kidding.

Well done and great work Don,
Like I mentioned in my article.. you are truly the telescope guru!!!



No, I have a great wife, who, while professing little interest in astronomy, can find all the Messier objects in Sagittarius without star charts. She, like many women, has somewhat of a shoe-collecting hobby. Periodically she feels guilty about spending so much money on shoes and says to Me, "Don't you need an eyepiece or something?"
It's how I collected a suitcase full of Ethos eyepieces over the last 5 years.


As for the 2013 Eyepiece Buyer's Guide, I hereby attach the latest version with all recent updates.

Edited by Scott in NC (12/24/13 12:46 PM)


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MPD
member


Reged: 11/14/13

Re: 2013 Buyer's Guide to Eyepieces new [Re: Starman1]
      #6424994 - 03/23/14 04:32 PM

This got lost in the brief 2.0 version, but I had some emails with Don, and once he finished up the 2014 updates/additions, I am going to take the data and make it into a searchable database. I have a preliminary version working, but it isn't ready for primetime.

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rodnocjolly
member


Reged: 10/13/12

Re: 2013 Buyer's Guide to Eyepieces new [Re: MPD]
      #6470166 - 04/16/14 05:38 PM

Thanks, very kind of you to share.

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Moses
newbie


Reged: 04/16/14

Re: 2013 Buyer's Guide to Eyepieces new [Re: Mark9473]
      #6506027 - 05/04/14 02:32 PM

this may sound like a terrible newbie question, but why can't I use my 1.25 Barlow with a 10 mm to get a closer look at Jupiter or Mars? They simply don't appear and I'm sure I've got them in my finder.

Thanks. From a newbie. By the way, I use an Orion 90mm refractor and a 6 inch Newtonian Star Blast, also from Orion.

What I really want from Santa is a 12 inch Dobsonian.


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

Reged: 06/24/03

Loc: Los Angeles
Re: 2013 Buyer's Guide to Eyepieces new [Re: Moses]
      #6506461 - 05/04/14 06:24 PM

Quote:

this may sound like a terrible newbie question, but why can't I use my 1.25 Barlow with a 10 mm to get a closer look at Jupiter or Mars? They simply don't appear and I'm sure I've got them in my finder.

Thanks. From a newbie. By the way, I use an Orion 90mm refractor and a 6 inch Newtonian Star Blast, also from Orion.

What I really want from Santa is a 12 inch Dobsonian.



Your finder is not lined up with the main scope. This won't matter much at low power, when the field in the eyepiece is large, but it will matter at high power, when the field of view in the eyepiece is small.
So, use your 10mm eyepiece on Polaris and line up your finder exactly on Polaris.
Add your barlow and repeat, lining up Polaris perfectly with your finder (center Polaris in the eyepiece first, then adjust your finder).
Then, you'll find your target will still be in the field when you add the barlow on any other target. just remember to be quick, because the turning of the Earth will take the object out of the field in only a short time.


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