# Is a 9mm eyepiece the same as 9.7mm?

### #1

Posted 14 August 2020 - 09:14 PM

### #2

Posted 14 August 2020 - 09:36 PM

No. At 1250mm focal length, a 9mm will yield 139x power and a 9.7mm will yield 129x power.

However, that’s not likely enough of a difference for you to even notice.

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### #3

Posted 14 August 2020 - 09:37 PM

It's not the same but is close.

**Edited by kfiscus, 14 August 2020 - 09:38 PM.**

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### #4

Posted 14 August 2020 - 09:47 PM

Hi:

From what I can find, your Meade NG-90 comes with 26 mm and 9.7 mm Meade Super Plossls. I think you got the right eyepiece. Meade has had the 9.7 mm Super Plossls in their lineup for about 30 years. It's a good eyepiece.

https://www.meade.co...0-observer.html

Jon

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### #5

Posted 14 August 2020 - 09:57 PM

Hi:

From what I can find, your Meade NG-90 comes with 26 mm and 9.7 mm Meade Super Plossls. I think you got the right eyepiece. Meade has had the 9.7 mm Super Plossls in their lineup for about 30 years. It's a good eyepiece.

https://www.meade.co...0-observer.html

Jon

The eyepiece is fine, I was just wondering because it’s listed as a 9mm, not a 9.7.

https://www.meade.co...-telescope.html

**Edited by Atlantic Devil, 14 August 2020 - 09:58 PM.**

### #6

Posted 14 August 2020 - 11:41 PM

Well y'all wait till you own a Pentax XO 2.5, and someone corrects you, pointing out is 2.58.

Greg N

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### #7

Posted 15 August 2020 - 05:30 AM

Meade is not the best at editing and correcting their ad copy. In the specifications for the NG-90, I find:

* Two (2) Series 4000 Supper Plossl Eyepieces (26mm and 9mm), 1.25 *

I guess these Plossls are to be used only after you had a good meal before heading out to look at the stars.

**Edited by Ulmer Spatz, 15 August 2020 - 05:31 AM.**

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### #8

Posted 15 August 2020 - 06:06 PM

No. At 1250mm focal length, a 9mm will yield 139x power and a 9.7mm will yield 129x power.

However, that’s not likely enough of a difference for you to even notice.

this is math, not engineering. Engineering is not math.

meaning, as an engineering question, your calculations are simply not true as they don’t account for significant digits. I once explained this all in another thread and it was deemed somethingorother and I think the mods excised it. But the principle is that “9.7” does not mean the same thing as 9.70, or 9.700. 9.7 could be 9.74, or 9.66. In fact, the same is true for 9mm, which might actually be 9.4, or even 8.6. You simply don’t know, from an engineering standpoint anything more accurate than 9.7. You have the same accuracy problems with your main optics. You cannot assume digits that are not stated. If you do calculations with these presumed numbers, the errors potentially add up, and you can be significantly off from what you expect. Engineers do such calculations taking significant digits into account (which explains why important engineering tasks, like going to the moon in the 60’s was possible using slide rules, before calculators existed). The same errors potentially exist, but an engineer doesn’t presume they don’t.

so as a practical matter, I think we need to start being a lot more forgiving in these kinds of calculations, rather than approaching them as math problems. It is simply not true that “139x” or “129x” are accurate. There is no way to know that with the numbers given. I personally think we should stop even using the ones digit completely. I know rounding these two numbers to the tens gives pretty much the same result, but it is also a bit more realistic generally. I hate when I see things like 422x and 37.5x. This numbers simply cannot be accurate as an engineering fact using the numbers typically given for Astro gear, like 150mm f8. 420x and 40x is just a possible and, I think, more usable numbers generally.

try some test calculations for yourself with a 1200mm FL and 422x. You will find that very small differences in eyepiece FL make big differences in magnification. Maybe that will illustrate the importance of my point.

I'm always surprised that a forum that prides itself on Science™ fights this concept.

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### #9

Posted 15 August 2020 - 06:37 PM

back to the original question - 9+/-0.5 <>9.7+/-0.05 regardless of whether you are a machinist, engineer, scientist, or mathematician. The biggest problem I see as an instrumentation engineer is that none of us know what the manufacturing tolerance of the eyepiece is. We assume it is a tight tolerance, but would have to measure to be sure.this is math, not engineering. Engineering is not math.

meaning, as an engineering question, your calculations are simply not true as they don’t account for significant digits. I once explained this all in another thread and it was deemed somethingorother and I think the mods excised it. But the principle is that “9.7” does not mean the same thing as 9.70, or 9.700. 9.7 could be 9.74, or 9.66. In fact, the same is true for 9mm, which might actually be 9.4, or even 8.6. You simply don’t know, from an engineering standpoint anything more accurate than 9.7. You have the same accuracy problems with your main optics. You cannot assume digits that are not stated. If you do calculations with these presumed numbers, the errors potentially add up, and you can be significantly off from what you expect. Engineers do such calculations taking significant digits into account (which explains why important engineering tasks, like going to the moon in the 60’s was possible using slide rules, before calculators existed). The same errors potentially exist, but an engineer doesn’t presume they don’t.

so as a practical matter, I think we need to start being a lot more forgiving in these kinds of calculations, rather than approaching them as math problems. It is simply not true that “139x” or “129x” are accurate. There is no way to know that with the numbers given. I personally think we should stop even using the ones digit completely. I know rounding these two numbers to the tens gives pretty much the same result, but it is also a bit more realistic generally. I hate when I see things like 422x and 37.5x. This numbers simply cannot be accurate as an engineering fact using the numbers typically given for Astro gear, like 150mm f8. 420x and 40x is just a possible and, I think, more usable numbers generally.

try some test calculations for yourself with a 1200mm FL and 422x. You will find that very small differences in eyepiece FL make big differences in magnification. Maybe that will illustrate the importance of my point.

I'm always surprised that a forum that prides itself on Science™ fights this concept.

### #10

Posted 16 August 2020 - 10:30 AM

back to the original question - 9+/-0.5 <>9.7+/-0.05 regardless of whether you are a machinist, engineer, scientist, or mathematician. The biggest problem I see as an instrumentation engineer is that none of us know what the manufacturing tolerance of the eyepiece is. We assume it is a tight tolerance, but would have to measure to be sure.

But my point is you ALSO have to account for the tolerances of the mirror. The argument I’m making is not that one eyepiece isn’t different from another one, based on the stated numbers, but that we cannot with any certainty at all state that some combination of mirror FL and eyepiece FL can determine magnification down to a single digit. In some cases, you might even be off by more that 10x! Even moving the same eyepiece from one 6” f6 to another 6” f6 could have very different true magnifications.

I think we should accept this fact and stop using these precise numbers all the time and start speaking in more general terms. 254x is not the answer to ANY question involving optics no measured to far more precision than we ever see stated.

magnification, in the real world, is not a math problem.

### #11

Posted 16 August 2020 - 03:00 PM

this is math, not engineering. Engineering is not math.

. If you do calculations with these presumed numbers, the errors potentially add up, and you can be significantly off from what you expect. Engineers do such calculations taking significant digits into account (which explains why important engineering tasks,

like going to the moon in the 60’s was possible using slide rules, before calculators existed). The same errors potentially exist, but an engineer doesn’t presume they don’t.

? Univac and the CDC 6600 were hardly slipsticks.

Actually calculators existed on-board space craft (primitive) and I visited a suitcase sized calculator that was finding prime numbers when I was a kid (my older brother's high school class, open house, probably around 1963). And of course there were calculators on earth. Indeed there were fire control calculators (not slipsticks) on WWII battleships and submarines.

In the 1960s of the Ranger program two missed the moon entirely (!!!). Perhaps they needed more significant digits? Two of the surveyor program missions crashed. Maybe they were missing a significant digit or two as well.

Greg N

**Edited by gnowellsct, 16 August 2020 - 03:18 PM.**

### #12

Posted 16 August 2020 - 03:17 PM

I think we need to start being a lot more forgiving in these kinds of calculations, rather than approaching them as math problems. It is simply not true that “139x” or “129x” are accurate. There is no way to know that with the numbers given. I personally think we should stop even using the ones digit completely. I know rounding these two numbers to the tens gives pretty much the same result, but it is also a bit more realistic generally. I hate when I see things like 422x and 37.5x. This numbers simply cannot be accurate as an engineering fact using the numbers typically given for Astro gear, like 150mm f8. 420x and 40x is just a possible and, I think, more usable numbers generally.

I think this is by and large correct. But everyone knows this. The focal length of Newtonians often varies. An f/6 might be f/5.9 or f/6.2. Up to a certain tolerance determined by the manufacturer. I have a very pricey apo triplet marketed as f/6.9 but the paperwork with it dutifully reports f/6.85. With SCTs these statistics are always hazy, because the changing the focal length is how you change focus.

And let's not start with the oculars.

When I have timed celestial equator star transits to verify fov there are a number of systems in play (eyepiece, scope, timing techniques e.g. did you really pick a star crossing the midpoint of the eyepiece) and it has always come out to within 5% or better of what you would calculate using the usual numbers, which all things considered, is pretty good.

Nonetheless, some conventions are useful because generally accepted, no point in lambasting people for a trivial point. People in the know will drop the non-useful digits in any case. Nobody expects to see something at 322x that they can't see at 320x.

The reported magnification 37.5 means, "I was using this ocular, I just cranked the number, it says 37.5x". Which really, is good enough, isn't it? Why should someone round just to please someone who enjoys being sarcastic? People in the know will know that the person writing probably has not measured the ocular field stop, probably not found out the scope's true focal length. Nor do they need to.

Greg N

### #13

Posted 16 August 2020 - 03:23 PM

I think we should accept this fact and stop using these precise numbers all the time and start speaking in more general terms. 254x is not the answer to ANY question involving optics no measured to far more precision than we ever see stated.

And I would add, notwithstanding what you write (which has its merit) Al Nagler chooses to put 17.3 mm on his Delos and Nikon chooses to put 17.5 mm on their competitive ocular. Zeiss puts 16.8 on their diascope piece. These are some of the most prestigious optics companies in the world. I think most people are going to stick with what's burned into their oculars more than they are going to care about a significant digit rant from some guy on the Internet. So we need to get over it and move on. --GN

**Edited by gnowellsct, 16 August 2020 - 03:24 PM.**

### #14

Posted 16 August 2020 - 04:06 PM

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### #15

Posted 16 August 2020 - 04:12 PM

I have the Olivon two inch 22mm eyepiece and it is a great eyepiece with pinpoint stars to the edge in my F6 scope. Howewer, stenciled on the eyepiece its says its a 1.25" eepiece, so go figure!

### #16

Posted 16 August 2020 - 07:23 PM

You guys TOTALLY lost me.

"Meade is not the best at editing and correcting their ad copy" in post #7 is probably the best explanation. If the eyepiece is a Super Plossl, then what they sent you is what they meant to say in the ad; they're unlikely to make one type of eyepiece in both 9 and 9.7mm focal lengths. There would be very little functional difference; *assuming* the indicated FLs are accurate, you would get magnifications of about 139x from a 9mm, and 129x from a 9.7mm. When manufacturers start specifying their mass-produced FLs in fractions of millimeters, I figure I can round the numbers either way.

- gnowellsct likes this

### #17

Posted 17 August 2020 - 01:35 PM

"Meade is not the best at editing and correcting their ad copy" in post #7 is probably the best explanation. If the eyepiece is a Super Plossl, then what they sent you is what they meant to say in the ad; they're unlikely to make one type of eyepiece in both 9 and 9.7mm focal lengths. There would be very little functional difference;

assumingthe indicated FLs are accurate, you would get magnifications of about 139x from a 9mm, and 129x from a 9.7mm. When manufacturers start specifying their mass-produced FLs in fractions of millimeters, I figure I can round the numbers either way.

It can get a little tricky because some eyepieces get sort of known for the decimals. The XL 10.5 was such a one, as I recall. And when I was at the Televue booth at NEAF (remember NEAF?) I kept getting corrected every time I said "Delos 17."

If you said you had a Pentax XL 11 you would get blank looks from those who know the line.

Greg N

**Edited by gnowellsct, 17 August 2020 - 11:24 PM.**

### #18

Posted 17 August 2020 - 09:27 PM

You guys TOTALLY lost me.

This is what happens when somebody tries to give a beginner a simple answer to what "should" be a simple question...

An engineer shows up and all bets are off.

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### #19

Posted 18 August 2020 - 12:16 AM

Thanks all, this has been an interesting thread.

Yes, I'm aware of significant digits.

gnowellsct tossed around a bunch of really interesting info. "slidestick", I like it. I'd never heard that term before. Sliderules were a bit before my time, but I have a slidestick and can use it a bit.

### #20

Posted 18 August 2020 - 10:16 AM

You guys TOTALLY lost me.

A good portion of the thread revolves around the fact that there are many variables that affect the actual magnification any scope/eyepiece combo gives and that it is highly unlikely that the 9mm eyepiece will really give 139x vs 129x from the 9.7mm.

Some are arguing that since we know that the magnification is not likely to be 129x or 139x that we should not use those numbers - but instead should round to some other unspecified number. Unfortunately, we do not have enough data in the form of manufacturing specs or other data on either the eyepiece or scope to even stand a chance of guessing at what number we should round to. Only if we were able to test the system as a whole would we be able to real data to know the real number. Without that, there is no right answer and no point in arguing which "wrong" answer is the correct one.

If we were trying to build a bridge, plane, nuke plant or spaceship it would matter enough to do the necessary testing - and then you get to figure out how to test the test instrument (and find yourself in Alice's rabbit hole real quick). But in a 9-10mm amateur astronomy eyepiece, it really doesn't matter. It would matter more if one is comparing a 4mm to a 4.5mm to a 5mm eyepiece, let alone a 2.5mm to 3mm to 3.5mm eyepiece. A little change in a short eyepiece adds up to a much bigger change in magnification - though one may not notice the difference between 450x and 500x as much as the change in 50x to 100x.

In your case, know that there will be a little difference between a 9mm and a 9.7mm, but especially in a cheap eyepiece you will not know if it is actually exactly what it says. It might actually be a 9.4mm if it was actually tested

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### #21

Posted 18 August 2020 - 10:31 AM

A good portion of the thread revolves around the fact that there are many variables that affect the actual magnification any scope/eyepiece combo gives and that it is highly unlikely that the 9mm eyepiece will really give 139x vs 129x from the 9.7mm.

Some are arguing that since we know that the magnification is not likely to be 129x or 139x that we should not use those numbers - but instead should round to some other unspecified number. Unfortunately, we do not have enough data in the form of manufacturing specs or other data on either the eyepiece or scope to even stand a chance of guessing at what number we should round to. Only if we were able to test the system as a whole would we be able to real data to know the real number. Without that, there is no right answer and no point in arguing which "wrong" answer is the correct one.

If we were trying to build a bridge, plane, nuke plant or spaceship it would matter enough to do the necessary testing - and then you get to figure out how to test the test instrument (and find yourself in Alice's rabbit hole real quick). But in a 9-10mm amateur astronomy eyepiece, it really doesn't matter. It would matter more if one is comparing a 4mm to a 4.5mm to a 5mm eyepiece, let alone a 2.5mm to 3mm to 3.5mm eyepiece. A little change in a short eyepiece adds up to a much bigger change in magnification - though one may not notice the difference between 450x and 500x as much as the change in 50x to 100x.

In your case, know that there will be a little difference between a 9mm and a 9.7mm, but especially in a cheap eyepiece you will not know if it is actually exactly what it says. It might actually be a 9.4mm if it was actually tested

Yep, and on top of that, if a manufacturer says the scope focal length is 1200, but maybe that's plus or minus a little, then maybe it's actually 1210 or 1193. Maybe the 9 isn't exactly 9.0, maybe it's really 9.35 or 8.82, etc...

When you start looking at those numbers and maybe the manufacturer rounds them to a nice number, then taking them (for instance 1200mm and 9mm) and doing the math to come up with 133x, is wrong because you don't know that the numbers that you started with were accurate enough to come up with a number as precise as 133. Maybe the real numbers are 1215 or 1185 instead of exactly 1200, and maybe the EP is really 8.77 or 9.35 instead of 9.00. If you take those numbers, then your mag could be anywhere from 127x up to 139x.

### #22

Posted 18 August 2020 - 12:26 PM

Yep, and on top of that, if a manufacturer says the scope focal length is 1200, but maybe that's plus or minus a little, then maybe it's actually 1210 or 1193. Maybe the 9 isn't exactly 9.0, maybe it's really 9.35 or 8.82, etc...

When you start looking at those numbers and maybe the manufacturer rounds them to a nice number, then taking them (for instance 1200mm and 9mm) and doing the math to come up with 133x, is wrong because you don't know that the numbers that you started with were accurate enough to come up with a number as precise as 133. Maybe the real numbers are 1215 or 1185 instead of exactly 1200, and maybe the EP is really 8.77 or 9.35 instead of 9.00. If you take those numbers, then your mag could be anywhere from 127x up to 139x.

my 6” F6 mirror, is marked “35.5”. I’ve never measured the actual FL. If it were exactly 6” and 36” FL, a 4mm eyepiece, if it were exact, would yield 228.6 (pretending this is a math problem for the moment). If it’s actually, and accurately, 35.5”, then it’s 225.4x. But even if 35.5 is generally true, given that the aim was f6, it is unlikely that 35.5 is more than a generalized number, that may only be accurate to an eighth or so. So using that, we get a range of about 224.5 - 226.2x. Not too great, I give you, but again, keep in mind that we ASSUMED it was 228x, so it might be 5x off, just with what we are stating here. Now consider that the mirror may well have been 36.5”, given the stated accuracy of the manufacturer. And the fact is, I wouldn’t even know about the half inch, but for the marking. Is YOUR mirror marked? Add a bit of fudge factor into the eyepiece measurements, and who knows?

so, as I said, we assume a LOT to be able to say something is 228x. For a forum proud of Science™, that’s not very scientific. At best, we should be rounding to tens, while always remembering that THAT is a rough number. We certainly shouldn't be drilling it into anybody’s head that you can come up with accurate numbers to a ones digit, and that such precision matters.

as I’ve stated a few times on this forum, I have no idea what the supposed magnification is on any of my eyepieces on any of my scopes (I guess I do now because of the above). It’s just not what matters beyond knowing the limiting ends and greater or lessor.

**Edited by Andrekp, 18 August 2020 - 12:27 PM.**