2013 Buyer's Guide to Eyepieces
Posted 28 September 2013 - 09:02 AM
Posted 28 September 2013 - 09:08 AM
Posted 28 September 2013 - 09:30 AM
Posted 28 September 2013 - 10:05 AM
That column automatically populates when you enter your telescope's focal length in the red-letter field at the top of column N.
What happened to the # in the calculated Field Stop column in the third version or am I missing them on my iPad copy ??
Other fields populate when you put your telescope's f/ratio in the red letter field at the top of column O.
Posted 28 September 2013 - 10:11 AM
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.
Also how we're the various Field Stops calc.? By actual physical measurement with a tape, inside callipers etc. ?
Posted 28 September 2013 - 10:18 AM
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.
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 !?
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.
Posted 29 September 2013 - 07:36 AM
Posted 29 September 2013 - 07:46 AM
Posted 29 September 2013 - 10:56 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 !
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.
Posted 29 September 2013 - 11:04 AM
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.
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
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.
Posted 29 September 2013 - 11:20 AM
Posted 29 September 2013 - 11:26 AM
Posted 29 September 2013 - 01:00 PM
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
Posted 29 September 2013 - 01:46 PM
Posted 29 September 2013 - 02:03 PM
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:-)
Posted 29 September 2013 - 02:31 PM
Posted 29 September 2013 - 02:39 PM
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.
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 !
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.
Posted 29 September 2013 - 04:10 PM
Posted 03 October 2013 - 08:03 PM
Posted 18 December 2013 - 01:34 PM
Kudos to Don who -as it turns out- is either single, or has a very accommodating (another optometrical term) wife...
Well done and great work Don,
Like I mentioned in my article.. you are truly the telescope guru!!!
Posted 18 December 2013 - 01:39 PM
Yes, ..for the discerned scientists..
I did make up the word optometrical...
just fitted the context I guess..