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30 minutes subs? Seriously?

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#1 nyx

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Posted 19 September 2020 - 06:34 PM

So, I took my narrowband setup for a test drive, mainly to check what kind of exposure times I'm up against. Gear:

 

Sharpstar 61EDPHII 274mm F/4.5

ZWO ASI183

Astronomik 6nm SHO filters

ZWO UV/IR cut filter

ZWO EAF

 

I followed my usual routine: slew to the target, auto focus, plate solve, all using the Luminance filter. So far so good. Then I switched to SII and fired up SharpCap in order to measure optimal exposure time using the Smart Histogram feature. I noticed that the test exposures were taking longer than what I'm used to see with my OSC, so I let it do its thing and went to grab something to drink. Half an hour went by and SharpCap has finished measuring. Optimal exposure time: 1700 seconds!!!! (115 gain)

 

SharpCap suggested 30 minutes as optimal exposure time at unity gain. Bare in mind that this is a Bortle 6 zone we are talking about. My regular AP site is Bortle 4, which translates to even longer exposures!

 

I guess my question is....is this normal? I cannot imagine being able to calibrate the amp glow properly with 30 minutes exposures. Hell, that's not even the biggest problem! Even with decent guiding and spot on polar alignment, 30 minutes will definitely be a stretch for my CEM25P.

 

Should I "trash" my Sharpstar and go for a f/2 lens in order to reduce the exposure time on my subs? That would also mean getting other filters, since the 6nm ones won't be suitable for f/2....

 

Any suggestions are highly appreciated!


Edited by nyx, 19 September 2020 - 06:49 PM.


#2 dghent

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Posted 19 September 2020 - 06:46 PM

So uh, no where in this did you mention what gain you are driving the camera at. Aside from that, narrowband exposures are always going to be far longer than broadband because of the exceedingly little amount of light that comes through the filter. Additionally, sulfur is going to almost always be the weakest signal of the narrowband holy trinity.

Are you running at a gain of 0 or something quite low?

#3 nyx

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Posted 19 September 2020 - 06:49 PM

So uh, no where in this did you mention what gain you are driving the camera at. Aside from that, narrowband exposures are always going to be far longer than broadband because of the exceedingly little amount of light that comes through the filter. Additionally, sulfur is going to almost always be the weakest signal of the narrowband holy trinity.

Are you running at a gain of 0 or something quite low?

 

My bad. I ran the analysis aiming for unity gain. SharpCap came back with 115 gain and 1700s subs.


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#4 bobzeq25

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Posted 19 September 2020 - 06:58 PM

So, I took my narrowband setup for a test drive, mainly to check what kind of exposure times I'm up against. Gear:

 

Sharpstar 61EDPHII 274mm F/4.5

ZWO ASI183

Astronomik 6nm SHO filters

ZWO UV/IR cut filter

ZWO EAF

 

I followed my usual routine: slew to the target, auto focus, plate solve, all using the Luminance filter. So far so good. Then I switched to SII and fired up SharpCap in order to measure optimal exposure time using the Smart Histogram feature. I noticed that the test exposures were taking longer than what I'm used to see with my OSC, so I let it do its thing and went to grab something to drink. Half an hour went by and SharpCap has finished measuring. Optimal exposure time: 1700 seconds!!!! (115 gain)

 

SharpCap suggested 30 minutes as optimal exposure time at unity gain. Bare in mind that this is a Bortle 6 zone we are talking about. My regular AP site is Bortle 4, which translates to even longer exposures!

 

I guess my question is....is this normal? I cannot imagine being able to calibrate the amp glow properly with 30 minutes exposures. Hell, that's not even the biggest problem! Even with decent guiding and spot on polar alignment, 30 minutes will definitely be a stretch for my CEM25P.

 

Should I "trash" my Sharpstar and go for a f/2 lens in order to reduce the exposure time on my subs? That would also mean getting other filters, since the 6nm ones won't be suitable for f/2....

 

Any suggestions are highly appreciated!

Yep, depending on your skies and setup, can be normal.   The tiny pixels on the 183 are part of the issue.

 

I use 10 minutes with my F7 and my 4.5 micron pixeled CCD, even though the added subs gives me more read noise.  Tradeoffs.  I have done 20, which is what my particular setup calls for.  30 is not silly.

 

I also sometimes use an F2 RASA, which feeds the tiny pixels on the 183 well.  Many people turn up the gain on the CMOS camera. I use gain 0 for broadband, 100 for narrowband.

 

Lots of options.  Note that this is one reason why you cannot have "too good" a mount.


Edited by bobzeq25, 19 September 2020 - 07:01 PM.


#5 wrnchhead

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Posted 19 September 2020 - 07:02 PM

I just recently switched to mono and narrowband and even at home in the city or out in the country, it’s about 10 minutes to break away the histogram.

#6 nyx

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Posted 19 September 2020 - 07:09 PM

Yep, depending on your skies and setup, can be normal.   The tiny pixels on the 183 are part of the issue.

 

I use 10 minutes with my F7 and my 4.5 micron pixeled CCD, even though the added subs gives me more read noise.  Tradeoffs.  I have done 20, which is what my particular setup calls for.  30 is not silly.

 

I also sometimes use an F2 RASA, which feeds the tiny pixels on the 183 well.  Many people turn up the gain on the CMOS camera. I use gain 0 for broadband, 100 for narrowband.

 

Lots of options.  Note that this is one reason why you cannot have "too good" a mount.

Well, the mount is just one thing. For what it's worth, it does track very good with guiding. I just feel like 30 minutes is just asking for trouble. Throwing away a 2-minute sub is one thing. Having to trash a 30 minute one just because a small cloud went by is going to hurt. And judging from the skies I'm up against, I'd say it's going to hurt too much, too often.



#7 ChrisWhite

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Posted 19 September 2020 - 07:38 PM

I've never used sharpcap to estimate exposure length.  Do you know what kind of read noise swamp factor it is trying to achieve?  10x RN^2?  20x RN? 

 

In any event, with narrowband you may never really be able to get the kind of swamping you want to get.  With broadband it is easy to get optimal read noise swamping, but with narrowband you might only be able to get 3x or 5x read noise.  This is not generally a problem, especially with a low RN camera like the 183.  Honesty, you will probably be totally fine with 10min subs even at Unity. 

 

I would however recommend bumping the gain to 178 and trying a ten minute sub. 

 

You shoudnt NEED to do 30min with this camera for any scope... I was doing 10min subs at f10 with NO problems at all, using 3nm NB filters. 


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#8 nyx

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Posted 19 September 2020 - 07:48 PM

I've never used sharpcap to estimate exposure length.  Do you know what kind of read noise swamp factor it is trying to achieve?  10x RN^2?  20x RN? 

 

In any event, with narrowband you may never really be able to get the kind of swamping you want to get.  With broadband it is easy to get optimal read noise swamping, but with narrowband you might only be able to get 3x or 5x read noise.  This is not generally a problem, especially with a low RN camera like the 183.  Honesty, you will probably be totally fine with 10min subs even at Unity. 

 

I would however recommend bumping the gain to 178 and trying a ten minute sub. 

 

You shoudnt NEED to do 30min with this camera for any scope... I was doing 10min subs at f10 with NO problems at all, using 3nm NB filters. 

I cannot really answer that question right now. I'd have to check on that and come back to you another day.

 

So you're saying, I should "ignore" SharpCap's suggestions and go for an "artitrary" sub length time? SharpCap will always try to swap read noise when measuring optimal exposure time, which according to your sayings isn't what I should be worrying about.

 

Why 178 gain specifically? Isn't increasing the gain higher than unity going to reduce dynamic range?

 

What would be the benefits of the 30 minute subs vs 10 minute subs in this particular setup?

 

By the way, one can set a limit on the acceptable amount of read noise. By default this is 10%, which is what I used in my analysis. More info on that from the SharpCap manual:

 

You can also set the time you intend to image for (setting this value is not critical, changing this will *not* change the suggested values) and the contribution you are prepared to tolerate from the sensor read noise in the final image noise level. If you select a ‘Read Noise Limit’ of 10%, that means that the calculations will allow the total noise level in the final stacked image to increase by 10% above the minimum achievable noise level (i.e. to go from 10 to 11 on some arbitrary scale).


Edited by nyx, 19 September 2020 - 07:54 PM.


#9 ChrisWhite

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Posted 19 September 2020 - 08:01 PM

Most of us use arbitrary exposure lengths for pretty much everything.  60s, 5min, 10min, 3min.... Most people don't do something like 127 seconds because something told them it was optimal. 

 

This is a little bit of a tangent, but I generally expose as long as I can without clipping too many stars.  For RGB that might be 3 min or 5min.  For narrowband clipping stars isnt as big of a risk, and I'm more concerned with getting enough signal to swamp read noise... within reason.

 

You've mentioned some pros for doing shorter exposures.... less time lost if you toss a sub, maybe your mount isnt up to the task, etc...

 

Gain 178 does have less DR, but it also has less RN.  Not sure if you are familiar with cameras, and ISO.  But using gain 178 instead of unity is like doubling your ISO, which kind of allows you to use half the exposure length.    Its all about finding the right balance of exposure that works for you, and if you are not excited about doing 30min exposures.... change a few settings and expose shorter... or just exposure shorter.

 

It's just an expense of time to try a few things and find what you like. 


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#10 Midnight Dan

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Posted 19 September 2020 - 08:09 PM

So you're saying, I should "ignore" SharpCap's suggestions and go for an "artitrary" sub length time? SharpCap will always try to swap read noise when measuring optimal exposure time, which according to your sayings isn't what I should be worrying about.

I'm not a narrow band imager (yet) so take my advice with a grain of salt. :-)

 

But think of it this way.  Say you're taking a picture of a red fire truck against a red brick background.  Hardly any blue in the image at all.  If you aim your DSLR at it you might find an optimum exposure time is 1/60 of a second.  That would give you a good looking image, with all 3 color channels in your DSLR getting the exact same exposure time.

 

But, if you filtered out the red and green channels and just looked at the blue data, your DSLR might now tell you you need a 1 second exposure to get a "properly exposed" image.  That's because there's so little blue in the image.  But ... the correct exposure to balance all three channels is still 1/60 of a second.

 

This can be the same thing with your SII filter.  Depending on the target, there might be very little SII compared to the Ha and OIII channels.  So judging a correct exposure time by what Sharpcap tells you about that channel might not be the best route.

 

-Dan


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#11 nyx

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Posted 19 September 2020 - 08:14 PM

Most of us use arbitrary exposure lengths for pretty much everything.  60s, 5min, 10min, 3min.... Most people don't do something like 127 seconds because something told them it was optimal. 

 

This is a little bit of a tangent, but I generally expose as long as I can without clipping too many stars.  For RGB that might be 3 min or 5min.  For narrowband clipping stars isnt as big of a risk, and I'm more concerned with getting enough signal to swamp read noise... within reason.

 

You've mentioned some pros for doing shorter exposures.... less time lost if you toss a sub, maybe your mount isnt up to the task, etc...

 

Gain 178 does have less DR, but it also has less RN.  Not sure if you are familiar with cameras, and ISO.  But using gain 178 instead of unity is like doubling your ISO, which kind of allows you to use half the exposure length.    Its all about finding the right balance of exposure that works for you, and if you are not excited about doing 30min exposures.... change a few settings and expose shorter... or just exposure shorter.

 

It's just an expense of time to try a few things and find what you like. 

I've always found SharpCap's Smart Histogram feature to be a god sent tool for clueless people like myself. The science behind astrophotography and photography in general is beyond be and I know I won't be wrapping my head around it any time soon. Everyone has his/her own field of expertise in this wolrd and I prefer to use the knowledge from people who decided to put the time into something I didn't :)

 

The experimentation part you mention is important, though a bit depressing when I think how little clear sky time I get per year. I guess I'll have to twist a few knobs and see what works best.



#12 ChrisWhite

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Posted 19 September 2020 - 08:15 PM

I've always found SharpCap's Smart Histogram feature to be a god sent tool for clueless people like myself. The science behind astrophotography and photography in general is beyond be and I know I won't be wrapping my head around it any time soon. Everyone has his/her own field of expertise in this wolrd and I prefer to use the knowledge from people who decided to put the time into something I didn't smile.gif

 

The experimentation part you mention is important, though a bit depressing when I think how little clear sky time I get per year. I guess I'll have to twist a few knobs and see what works best.

You are welcome to use my suggestion of Gain 178 and 10min exposure for narrowband as a starting point.  wink.gif



#13 nyx

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Posted 19 September 2020 - 08:28 PM

I'm not a narrow band imager (yet) so take my advice with a grain of salt. :-)

 

But think of it this way.  Say you're taking a picture of a red fire truck against a red brick background.  Hardly any blue in the image at all.  If you aim your DSLR at it you might find an optimum exposure time is 1/60 of a second.  That would give you a good looking image, with all 3 color channels in your DSLR getting the exact same exposure time.

 

But, if you filtered out the red and green channels and just looked at the blue data, your DSLR might now tell you you need a 1 second exposure to get a "properly exposed" image.  That's because there's so little blue in the image.  But ... the correct exposure to balance all three channels is still 1/60 of a second.

 

This can be the same thing with your SII filter.  Depending on the target, there might be very little SII compared to the Ha and OIII channels.  So judging a correct exposure time by what Sharpcap tells you about that channel might not be the best route.

 

-Dan

I get what you mean. Regarding SharpCap, it gets worse:

 

...you should point the telescope at an area of sky without nebulosity or many stars to get a good measurement.

 

The Smart Histogram suggests: "Point to a dark area of sky" and " Alternatively, select a dark section of the field of view".

 

The thing tries to measure sky brightness in order to calculate/suggest optimal camera values (gain, offset, exposure time). I have a feeling, if I let it run at my bortle 4 site, it will a) run "forever" and b) suggest something like 60 minutes as optimal exposure time.

 

At the end of the day, is it "safe" to expose X minutes (whether it's 5, 10 or 20) for all 3 filters and stop worrying about it? What am I going to lose compared a 30, 40 or 60 minute sub?



#14 nyx

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Posted 19 September 2020 - 08:29 PM

You are welcome to use my suggestion of Gain 178 and 10min exposure for narrowband as a starting point.  wink.gif

I cannot help wondering why 178 and not 180 or 190? :D



#15 ChrisWhite

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Posted 19 September 2020 - 08:34 PM

I cannot help wondering why 178 and not 180 or 190? laugh.gif

 

 

IIRC it was established in some analysis by Jon Rista when this camera first came out that at gain 178 it was exactly twice unity (0.5e-/ADU vs 1e-/ADU at unity).


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#16 Churmey

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Posted 19 September 2020 - 08:41 PM

I did 7nm OIII and HA NB with the 183 just last week, shooting 5 minute subs with the Lowest Read Noise setting (at F6.5 with a AT65EDQ). You get an ADU of around 800 from a red zone (my zone too). Data turned out great. Just switch gains. It might not be optimal but work with what you have and can get.

 

Result here:

 

https://www.astrobin.com/7olikw/0/


Edited by Churmey, 19 September 2020 - 08:47 PM.


#17 AtmosFearIC

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Posted 19 September 2020 - 08:54 PM

I have a QHY183M and find that if I want to hit that 10x RN mark I need 900s Ha and 1200s OIII & SII... this is at F/3 mind you with 3nm filters. The IMX183 has such small pixels that you’ll always need longer exposures compared to some of the other CMOS sensors out there.

#18 rockstarbill

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Posted 19 September 2020 - 08:58 PM

I have a QHY183M and find that if I want to hit that 10x RN mark I need 900s Ha and 1200s OIII & SII... this is at F/3 mind you with 3nm filters. The IMX183 has such small pixels that you’ll always need longer exposures compared to some of the other CMOS sensors out there.

Well f/3 with 200mm of aperture. OP is using a 61mm aperture scope.



#19 PeteM

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Posted 19 September 2020 - 10:50 PM

I am using an older CCD ATIK 383L+ (8300 chip) and with my 8" RC at f/7.8ish, my standard narrowband (7nm) is 1200s (20min) from a Bortle 4 location (20.6 sqm-l). SGP recommends around 30min and honestly I have not gone that long because I have not taken dark frames for those. But as many above state, smaller pixels and little light coming in at these wavelengths and bandpass. Usually in the past it has boiled down to if you can track that well for so long and do you plan to take enough subframes to make stacking rejection effective. Not sure such an apple to orange (ccd v cmos) is handy, but go long and many!



#20 AtmosFearIC

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Posted 19 September 2020 - 11:18 PM

Well f/3 with 200mm of aperture. OP is using a 61mm aperture scope.

That’s my point, I’m needing 900-1200s exposures with a larger & faster instrument. It makes sense that it’s suggesting 30 minute exposures. The one good thing is that if it is suggesting 30 minutes for 10x read noise then you can easily get away with 15 minute exposures as you’re still going to be 5x RN.
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#21 rockstarbill

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Posted 19 September 2020 - 11:20 PM

That’s my point, I’m needing 900-1200s exposures with a larger & faster instrument. It makes sense that it’s suggesting 30 minute exposures. The one good thing is that if it is suggesting 30 minutes for 10x read noise then you can easily get away with 15 minute exposures as you’re still going to be 5x RN.

Then we are on the same page (same point). 


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#22 nyx

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Posted 20 September 2020 - 01:46 AM

I have a QHY183M and find that if I want to hit that 10x RN mark I need 900s Ha and 1200s OIII & SII... this is at F/3 mind you with 3nm filters. The IMX183 has such small pixels that you’ll always need longer exposures compared to some of the other CMOS sensors out there.

How can you do narrowband at f/3 with 3nm filters? I mean, of course you can but, aren't you losing signal with such narrow filters at f/3? I remember reading a thread here on CN about this. For example, Astronomic suggests that I don't go faster than f/4 with my 6nm filters and their 12nm ones can go up to f/3 "without significant tranmission loss". Isn't 3nm a bit of a stretch at f/3? Just curious smile.gif


Edited by nyx, 20 September 2020 - 01:46 AM.


#23 bobzeq25

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Posted 20 September 2020 - 01:59 AM

I get what you mean. Regarding SharpCap, it gets worse:

 

...you should point the telescope at an area of sky without nebulosity or many stars to get a good measurement.

 

The Smart Histogram suggests: "Point to a dark area of sky" and " Alternatively, select a dark section of the field of view".

 

The thing tries to measure sky brightness in order to calculate/suggest optimal camera values (gain, offset, exposure time). I have a feeling, if I let it run at my bortle 4 site, it will a) run "forever" and b) suggest something like 60 minutes as optimal exposure time.

 

At the end of the day, is it "safe" to expose X minutes (whether it's 5, 10 or 20) for all 3 filters and stop worrying about it? What am I going to lose compared a 30, 40 or 60 minute sub?

Sure you can stop worrying about it.  If you use "too short" a subexposure, you'll take "too many" subs, and have "too much" read noise, for the same total imaging time.  A fairly minor deal.  Total imaging time is the big deal, not how it's divided into subs.

 

An alternative is to ditch the "black box" of Sharpcap, and run the calculation for yourself.  I understand you think it's beyond you, but it's not that bad.  I'm not saying you have to do it, all I'm saying is that you can better understand what's going on by getting into the details yourself.  Often true in DSO AP.

 

You asked "30 minute subs, seriously?"  You continue to find this unclear.  Depending on your circumstances 30 minutes may be optimal for getting the least noise in your data.  There's no way to tell for sure without getting into the details.  I find it personally reassuring to do that, but it's not for everyone.

 

Other options are doing what Sharpcap says or ignoring it.  No one will come to your door and confiscate your equipment.  <smile>


Edited by bobzeq25, 20 September 2020 - 02:00 AM.


#24 nyx

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Posted 20 September 2020 - 02:08 AM

I see that the general consensus is too shoot long. Or better say, as long as posslble. The reservations I have are:

 

  1. Where I live we rarely get 100% clear nights. The chances of shooting a 30+ minutes exposure without having a cloud passing by are really low. And when I say cloud I mean the really thin ones that you normally can't see. You discover it only when you're in post processing and almost all stars have halos. 
  2. At 274mm, I want to drizzle. Considering what I mentioned before regarding weather and shooting very long subs, it would take years before I have enough subs to even finish a project.
  3. SharpCap's test 30-minutes-sub showed an insanely dominant amp glow. Will I be able to calibrate it out using darks of equal exposures?

On the other hand, if I replace my Sharpstar with a fast lens (think f/2), I will be exchanging longs exposures for dodgy focus, terrible stars at the edge and a host of other problems associated with lenses used in astrophotography. I'd still need 5-10 minutes subs, but drizzling is going to be an option again.

 

Frustrating :/



#25 rockstarbill

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Posted 20 September 2020 - 02:12 AM

I see that the general consensus is too shoot long. Or better say, as long as posslble. The reservations I have are:

 

  1. Where I live we rarely get 100% clear nights. The chances of shooting a 30+ minutes exposure without having a cloud passing by are really low. And when I say cloud I mean the really thin ones that you normally can't see. You discover it only when you're in post processing and almost all stars have halos. 
  2. At 274mm, I want to drizzle. Considering what I mentioned before regarding weather and shooting very long subs, it would take years before I have enough subs to even finish a project.
  3. SharpCap's test 30-minutes-sub showed an insanely dominant amp glow. Will I be able to calibrate it out using darks of equal exposures?

On the other hand, if I replace my Sharpstar with a fast lens (think f/2), I will be exchanging longs exposures for dodgy focus, terrible stars at the edge and a host of other problems associated with lenses used in astrophotography. I'd still need 5-10 minutes subs, but drizzling is going to be an option again.

 

Frustrating :/

Stop putting so much faith into what sharpcap says. You are being led by something that is doing a good job giving you information but you lack the experience to know what to do with that information. 

 

Shorten the exposure times to what you find acceptable for your environment, gear, etc... At the end of the day it is not going to matter. 

 

TL:DR; Analysis paralysis. 


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