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What is FOM, and what difference do higher numbers make?

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#1 25585

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Posted 07 May 2024 - 03:25 PM

Is there a scale for FOM where a signifigant difference can be seen visually? What are those differences?

 

As prices seem higher for larger FOM, what extra is a buyer paying for? Even browsing NVDs is confusing. 



#2 PEterW

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Posted 07 May 2024 - 03:41 PM

FOM is a combination of signal to noise and resolution, so a single value can mean high resolution and poor signal to noise or the opposite or something in between. Also for astronomy EBI is important for nebulae, the lower the better. Erring on the higher side for signal to noise would be better for a given FOM, I’ll let others comment on the visibility of differences.
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#3 chemisted

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Posted 07 May 2024 - 04:10 PM

Is there a scale for FOM where a signifigant difference can be seen visually? What are those differences?

 

As prices seem higher for larger FOM, what extra is a buyer paying for? Even browsing NVDs is confusing. 

In the US, very few (if any?) potential buyers of NVDs pay any attention to FOM.  For example, I just helped another CNer buy a killer device today and FOM was never brought up.


Edited by chemisted, 07 May 2024 - 04:15 PM.

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#4 25585

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Posted 08 May 2024 - 06:51 PM

Are there any scale comparisons, say like with TV definition and signal? 



#5 chemisted

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Posted 08 May 2024 - 07:07 PM

I don't know about TV definition and signal so I'm not sure what you are asking. There are seven characteristics of image intensifier tubes that need to be taken into account when choosing a tube.


Edited by chemisted, 08 May 2024 - 07:07 PM.


#6 vicuna

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Posted 09 May 2024 - 12:59 PM

as you browse NVD devices, remember what they are made for...hint, not astronomy or bird watching.

 

FOM is a shortcut so they can quickly tell approx minimum levels of a tube. i dunno ins and outs but my understanding is the military uses FOM to make big purchases.

 

they are used to navigate treacherous terrain under combat. 

 

so just keep in mind the specs are for that.

 

there happen to be some specs that are more important for astronomy than others. browse around and there are some really good threads on here. check the threads for when people post the specs of their device and ask for advice...or threads on when people have two choices.

 

and call a few vendors and have them give their opinion on what to look for in night vision and what specs to look for.

 

ETA: also before you buy one, you should get a cell phone image of what the tube looks like pointed at a blank wall in the dark. so you get a feel for it.


Edited by vicuna, 09 May 2024 - 01:02 PM.

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#7 Deadlake

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Posted 09 May 2024 - 03:05 PM

Are there any scale comparisons, say like with TV definition and signal? 

You might be thinking of line pair density, NVD’s are not measured with pixel resolution.


Edited by Deadlake, 09 May 2024 - 03:05 PM.


#8 25585

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Posted 09 May 2024 - 04:39 PM

All I want is a good image/picture. So if buying a TV, as an allegory, I might choose 4K/UHD, rather than, say, max 1080P. Signal being the same of course. 
 

I have top class refractors and eyepieces, so want an equivalent class best-image NVD. 
 

What is "noise"? That seems like something to have at a minimum.


Edited by 25585, 09 May 2024 - 04:58 PM.


#9 vicuna

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Posted 10 May 2024 - 05:09 PM

here's my cliffs on what I look for from what I've learned (far from an expert but just bought 2 tubes and looking for another). happy to be corrected on this by anyone more experienced.

 

- higher SNR - most important. as close to 40 (or over) as possible

- high PS

- lower EBI (note higher SNR sorta pairs with higher EBI. so 39 SNR with EBI 1 >>> 33 SNR with EBI 1

- resolution should be 72. your eyes can't see more than that...and you don't want less if possible.

- higher gain

- **prioritize PS for - galaxies, globs, clusters
- prioritize EBI for - nebula

 

no chicken wire, honeycomb, no dark spots >0.003" - especially in zone 1&2, low halo

 

and look at the images. if you're experienced looking through the eyepiece and know what you like, you can see what the tube looks like. as chemisted said the other day, they are snowflakes. one might look better to your eyes.

 

if you're in the USA L3 filmless white phosphor is sorta a no brainer if budget isn't an issue (meaning you don't care if you spend $4,000 rather than $3,400 or something like that)

 

ETA: tbh even though everyone says FOM is irrelevant, it is a decent enough shortcut like your Television analogy. if you see a 2500+ FOM tube, that's a good starting point to look deeper...and if I see a 2200 FOM tube, I won't even bother looking at anything else.


Edited by vicuna, 10 May 2024 - 05:12 PM.

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