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How can you tell the exact magnification while using a zoom eyepiece ?

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#1 Voyager 3

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Posted 10 April 2021 - 08:55 AM

Most 8-24mm zooms have their focal lengths marked in 4mm interval like 8mm , 12mm , 16mm , 20mm and 24 mm . But how can you say at what focal length it's operating when you are somewhere between 8mm and 12mm ?

 

PS : I've not seen a zoom EP in flesh , so this maybe silly but I've broken walls the length of Great wall of China by hitting my head thinking this bangbang.gif .


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#2 clearwaterdave

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Posted 10 April 2021 - 09:22 AM

You may not know exactly but you can have some idea if your quick with math or you have a calculator.,You can certainly guesstimate anyway.,To me the view matters more than the magnification.,so rest at ease.. it ain't all that important.,.Cheers


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#3 Taosmath

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Posted 10 April 2021 - 09:56 AM

I have several zooms and I'm with clearwaterdave: the view matters more than the magnification.  So I adjust the focal length until I get the view I want (best framing or sharpest view the seeing will allow etc) and never worry about what the actual setting is.

 

The only time I ever actually need to know is when I  am logging things for my Astroleague observing projects and then I look at the numbers on the scale and guesstimate if I need to.

 

I don't know any reason why you would ever need to know the exact focal length.  However if for some reason you ever did, you could try the method TOMDEY  outlined in this thread:

 

https://www.cloudyni...-zoom-eyepiece/

 

It would require that you have previously calibrated the EP of course, as the OP in that thread was looking to do.

 

Again, like clearwaterdave said, use it, enjoy it and don't stress out over something that is not important.


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#4 MisterDan

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Posted 10 April 2021 - 09:58 AM

You can always spend some time adjusting your zoom between the 8mm and 12mm indicators - perhaps even comparing views with a fixed-focal-length 10mm eyepiece - then adding your own 10mm indicator/mark.

 

If you find the resulting 10mm indicator to sit equidistant between your 8mm and 12mm indicators (or "fairly" equidistant), then that the 9mm and 11mm "positions" are likely ("fairly") equidistant between the 8mm & 10mm indicators (for 9mm) and the 10mm and 12mm indicators (for 11mm).  -Or you could, of course, just repeat the adjusting/comparing process as you did for the 10mm indicator.  You would not necessarily need 9mm and 11mm fixed-focal-length eyepieces for comparison -- simply compare image scales at 8mm and 10mm and then estimate where the 9mm scale "sits" (ditto for 11mm) in your zoom.

 

Repeat for 14mm, 22mm, and the other intervals, if you wish.

 

Best wishes.

Dan


Edited by MisterDan, 10 April 2021 - 09:59 AM.

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#5 Voyager 3

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Posted 10 April 2021 - 11:26 AM

The only time I ever actually need to know is when I  am logging things .

Exactly this . Without the magnification/exit pupil it's a waste of time . Also while sketching . 



#6 Jeff Morgan

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Posted 10 April 2021 - 11:36 AM

I used to interpolate between markings. For about two observing sessions.

 

Then I realized it really did not matter at all and haven't thought about much in the last seven years.

 

No scientist is going to unearth my observing logs 1000 years from now and find the the key observation to solve the mystery of Eta Carina. And all because he knew the observation was made at 183x and not 179x.

 

Have you ever wondered if your fixed length 8mm eyepiece is really 8mm?

 

Maybe it is 7.8mm? Or 8.08mm?

 

It just doesn't matter.


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#7 DavidSt

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Posted 10 April 2021 - 11:47 AM

I’m curious. I guess I don’t understand why it’s a waste of time. Why is’t an approximation good enough? Few fixed focal length lenses are accurate either, they are usually rounded off to the nearest integer. Some manufacturers tell you to the nearest tenth but many if not most don’t. Or does that make your head sore too?

Edited by DavidSt, 10 April 2021 - 11:50 AM.

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#8 LDW47

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Posted 10 April 2021 - 12:12 PM

Most 8-24mm zooms have their focal lengths marked in 4mm interval like 8mm , 12mm , 16mm , 20mm and 24 mm . But how can you say at what focal length it's operating when you are somewhere between 8mm and 12mm ?

 

PS : I've not seen a zoom EP in flesh , so this maybe silly but I've broken walls the length of Great wall of China by hitting my head thinking this bangbang.gif .

Calculate it in 2-4mm increments, write it down on a paper and carry it with you when you are viewing with the zoom. A close estimate is as good as anything, unless you are building rockets, lol !


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#9 areyoukiddingme

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Posted 10 April 2021 - 01:43 PM

Measuring the actual focal length of an eyepiece is difficult, but checking the increments on the zoom is easy.

 

Maybe the scale on the housing is correct, but if you want to know you can just set up a tape measure and measure the ratio of the true fields at different settings, and check the correspondence with the increment on the housing.

 

But just seems like something to satisfy curiosity. I've never been concerned with the exact magnification of my zooms. I just want to know what the range of magnifications are, and I go from there.


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#10 eyespy

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Posted 10 April 2021 - 02:02 PM

Hi Voyager 3

 

All of my zooms are marked at roughly 4mm intervals and these are equally spaced around the eyepiece so I presume that if I am halfway between 12mm and 16mm, then I’m at 14mm or close enough.  As others have suggested, just use the zoom and adjust for the conditions and/or objects being viewed.  I do not recall ever actually looking at and setting my zooms to a specific mm setting.  Pretty hard to do in the dark anyway. Depending upon the object being viewed, I either begin at the high or low setting and then adjust visually for the appropriate view.  I believe that only if you are using a pair of zooms in a binoviewer that you would ever need to actually set the zooms to a specific mm setting.

 

Doug.....


Edited by eyespy, 10 April 2021 - 02:06 PM.

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#11 sevenofnine

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Posted 10 April 2021 - 07:37 PM

What I like best about my zooms is that I stop thinking and just enjoy. love.gif


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#12 MortonH

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Posted 10 April 2021 - 08:15 PM

The focal length of the telescope isn't exact, and neither is the focal length of any eyepiece (including a fixed focal length eyepiece) so you can never really know the magnification precisely.


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#13 Miranda2525

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Posted 11 April 2021 - 12:49 AM

Most 8-24mm zooms have their focal lengths marked in 4mm interval like 8mm , 12mm , 16mm , 20mm and 24 mm . But how can you say at what focal length it's operating when you are somewhere between 8mm and 12mm ?

 

PS : I've not seen a zoom EP in flesh , so this maybe silly but I've broken walls the length of Great wall of China by hitting my head thinking this bangbang.gif .

By drift timing and knowing the AFOV.



#14 Jon Isaacs

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Posted 11 April 2021 - 01:00 AM

Measuring the actual focal length of an eyepiece is difficult, but checking the increments on the zoom is easy.

 

Maybe the scale on the housing is correct, but if you want to know you can just set up a tape measure and measure the ratio of the true fields at different settings, and check the correspondence with the increment on the housing.

 

But just seems like something to satisfy curiosity. I've never been concerned with the exact magnification of my zooms. I just want to know what the range of magnifications are, and I go from there.

 

I don't think measuring the true field works..  To do that, you have to know something about the relationship between the TFoV and the focal length of the eyepiece, I know of no such relationship.. 

 

It's all a guess, an 8mm-24mm may not be truly an 8mm to 24mm zoom, the markings may be wrong.. 

 

As others have said, it is what it is, it's the view that matters.  

 

I have pretty much given up on my Baader Mk IV zoom because it is the view that matters.  It's a pretty good view but it doesn't have the magic of a type 6 Nagler etc.. 

 

Jon


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#15 Taosmath

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Posted 11 April 2021 - 01:25 AM

By drift timing and knowing the AFOV.

Yes but for a zoom the AFOV varies with focal length (except for the Nagler 3-6mm I think) , so you can't use that method.


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#16 areyoukiddingme

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Posted 11 April 2021 - 01:52 AM

I don't think measuring the true field works..  To do that, you have to know something about the relationship between the TFoV and the focal length of the eyepiece, I know of no such relationship.. 

 

It's all a guess, an 8mm-24mm may not be truly an 8mm to 24mm zoom, the markings may be wrong.. 

 

As others have said, it is what it is, it's the view that matters.  

 

I have pretty much given up on my Baader Mk IV zoom because it is the view that matters.  It's a pretty good view but it doesn't have the magic of a type 6 Nagler etc.. 

 

Jon

Ah, yes, I didn't think of the changing apparent field. My suggestion would only work for fixed apparent field zooms. Otherwise there would need to be a frame of reference--like some kind of reticle with a scale, perhaps.


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#17 Jon Isaacs

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Posted 11 April 2021 - 02:45 AM

Ah, yes, I didn't think of the changing apparent field. My suggestion would only work for fixed apparent field zooms. Otherwise there would need to be a frame of reference--like some kind of reticle with a scale, perhaps.

 

I think the bottom line, is that if you really want to know the exact magnification, don't use a zoom, don't use an SCT or MAK and only use eyepieces and telescopes from manufacturers you trust to provide accurate information.  Either that or take the time to accurately measure the magnification, a challenge in itself.

 

The other bottom line is:  Why would you really want to know the exact magnification?

 

Jon


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#18 SteveG

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Posted 12 April 2021 - 12:13 AM

As others have stated, you can’t really know.

 

I took pictures of a railing through my scope to compare fixed focal length eyepieces with an 8-24 zoom I recently purchased. I found that the markings between the 8 and 24 settings were wildly off, as much as 2 mm at the 10 mm setting. I just ignore it all and enjoy the view.


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#19 Jim1804

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Posted 12 April 2021 - 08:08 PM

My Meade Zoom (AFAIK identical to the Celestron Zoom) only has written focal lengths every four, but there are tick marks in between them so you can see exactly where you are. 

 

As others have said above, unless I'm logging for an AL program, I don't worry about it - but in those cases (I'm working on the double star list currently), I just mark where I am on the Zoom (e.g. split cleanly at 13mm), and figure out the magnification after the fact. 


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#20 RedzoneMN

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Posted 13 April 2021 - 04:55 AM

As a thought experiment, for a given scope/eyepiece combo, figure out the magnification at one end of the zoom, then calculate the same for the other end of the zoom. The difference in magnification is then a function of the degrees turned from one end to the other. 



#21 Taosmath

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Posted 13 April 2021 - 10:45 AM

As a thought experiment, for a given scope/eyepiece combo, figure out the magnification at one end of the zoom, then calculate the same for the other end of the zoom. The difference in magnification is then a function of the degrees turned from one end to the other. 

Indeed it will be a function of the angle turned.  Unfortunately you don't know what that function is; there's absolutely no guarantee it will be a linear function.



#22 Miranda2525

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Posted 13 April 2021 - 11:55 AM

Yes but for a zoom the AFOV varies with focal length (except for the Nagler 3-6mm I think) , so you can't use that method.

Yes you can. All you need to do is look here right on CN. The variations in FL for the Baader Zoom is here if you Google it. Other zooms have the variations in AFOV as well on here. JUST LOOK.

 

I have a few charts with magnifications and AFOV I made for all FL's of the Baader Hyperion Zoom.

 

The magnifications and AFOV are not exact, but close to what I need to know, which is good enough. Sold the zoom eventually anyways. The low power end wasn't even worth using in the end.


Edited by Miranda2525, 13 April 2021 - 12:00 PM.



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