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# For Atmospheric seeing, what is better 1.6 arcsecs or 2.5 arcsecs?

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### #1 Procyon

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Posted 17 May 2018 - 11:42 AM

Lower arcseconds are better correct? 1.6 would be better than 2.5 (example)?

Thanks

### #2 photoracer18

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Posted 17 May 2018 - 11:52 AM

Yes.

### #3 Bob4BVM

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Posted 17 May 2018 - 12:39 PM

Yes.its the size of the resolvable image.  Smaller is better.

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### #4 REC

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Posted 17 May 2018 - 01:05 PM

How does one know what the number is for their area? Also what is a simple explanation of "arcseconds"? Do they use this term when explaining the separation of double stars ect? I read about the sizes of planets like Jupiter which the current size is listed at 44". What does that mean exactly?

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### #5 ismosi

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Posted 17 May 2018 - 01:14 PM

An arc second is an angular measurement. One degree contains 60 arc minutes. And one minute of arc has 60 arc seconds. Thus an arcsecond is 1/3600 of a degree.

Double star separations are expressed in arc seconds, yes.

When Jupiter's current (apparent) size is denoted as 44" that means is apparent diameter is 44 arc seconds (about 3/4 of a minute of arc). The Moon and the Sun are about 30 arc minutes (1/2 degree) in apparent diameter.

For astronomical seeing, try this site:

https://www.meteoblu...america_4562148

(enter your own location, naturally )

Edited by ismosi, 17 May 2018 - 01:15 PM.

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### #6 REC

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Posted 17 May 2018 - 02:41 PM

An arc second is an angular measurement. One degree contains 60 arc minutes. And one minute of arc has 60 arc seconds. Thus an arcsecond is 1/3600 of a degree.

Double star separations are expressed in arc seconds, yes.

When Jupiter's current (apparent) size is denoted as 44" that means is apparent diameter is 44 arc seconds (about 3/4 of a minute of arc). The Moon and the Sun are about 30 arc minutes (1/2 degree) in apparent diameter.

For astronomical seeing, try this site:

https://www.meteoblu...america_4562148

(enter your own location, naturally )

Thanks, I'll try and visualize this in my brain. So compare the space between a double like Alberio or Mizar, they have a pretty wide separation. I'm thinking Mizar's two stars or Alberio could fit inside the width of Jupiter ect?

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### #7 Procyon

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Posted 17 May 2018 - 02:41 PM

How does one know what the number is for their area? Also what is a simple explanation of "arcseconds"? Do they use this term when explaining the separation of double stars ect? I read about the sizes of planets like Jupiter which the current size is listed at 44". What does that mean exactly?

Hey REC, this is my preferred site for seeing and transparency conditions: https://www.goodtostargaze.com/#/ (just enter your postal or zip code).

By the way, anyone know what 0.3 means for transparency? Does the same apply here, as in lower is better?

Edited by Procyon, 17 May 2018 - 02:42 PM.

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### #8 gavinm

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Posted 17 May 2018 - 05:52 PM

Yes, lower transparency is better. Normally we talk about extinction, not transparency because it takes into account more things (including transparency)

The easiest way to measure you seeing is by taking an image and measuring the size of a star (assuming all else equal, like focus, collimation etc)

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### #9 Eddgie

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Posted 17 May 2018 - 10:19 PM

As mentioned, seeing in arc seconds describes the diameter of the blur circle.

What this does not really explain though is how that relates to the way that seeing will look in a telescope, and for telescopes of different apertures, the seeing can appear quite different.

Since your C11 can resolve sub arc second detail, 2.5 arc seconds of seeing would make a star look very bad.

In an 80mm refractor though, the Airy disk is so much larger, that this amount of seeing would mean that much of the scattered light would fall inside of the diameter of the Airy Disk.

Here I have modeled two different telescopes showing the effects on a star with the about the same amount of seeing.

The scope on the left is an 80mm f/10 unobstructed scope, and the scope on the right is a 280mm 33% obstructed scope (C11).

As you can see, the "Damage" looks to be much less on the smaller scope because the Airy Disk is so much larger that it "captures" much of the scatter within its diameter.  In the C11, the scatter falls outside of the diameter of the Airy Disk at that aperture, so this makes the seeing seem worse, but in essence it is the same amount of seeing and if you look, the blur diameter in the 11" is still smaller than the Airy Disk in the 80mm, so in spite of the fact that the star looks messy, the larger scope could still resolve smaller detail though the nature of seeing means that this may be a moment to moment thing.

What this means though is that seeing will limit the smallest size detail that can be imaged to the blur circle size.   When the seeing is pretty bad (5 arc seconds) the C11 won't be able to resolve any more detail than the 80mm scope.

When the seeing falls to less than the diameter of the 80mm scope's Airy Disk, the C11 will pull ahead.   This though takes about 3 arc seconds of seeing (for this comparison). For the C11 to work near full resolving power, seeing will have to be around 1 arc sec.  At 1 arc second, the first diffraction ring as it appeared in the C11 would look about the same as the way the first diffraction ring is looking in the model for the 80mm above.  The first ring would not be formed but would have segments that waver around and flare.  To really work near the scopes's full resolution then then at high power, the image has to look like on the left.  If the ring is mostly formed and wavering so that areas of the ring dim out, that is quite excellent sub arc second seeing (as seen in the C11), and there is little new detail to be seen visually with seeing better than this in the C11.

I know that this is not really what you asked, but I hope it helps understanding of how seeing has different effects on different size apertures and how it looks different in different size apertures even though it is the same amount of seeing.  Bad seeing in a small scope does little to handicap it, but even so-so seeing can be a major limit on the resolution of larger apertures.

Edited by Eddgie, 17 May 2018 - 10:24 PM.

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### #10 gnowellsct

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Posted 17 May 2018 - 11:11 PM

Well you know what a 360 degree view is.  And you know what a 180 degree view is.  And you probably can see that it is logical that if you look straight ahead you have about 160 degrees total field of view, from one side to the other.  A little less than half of a circle.

This is angular measurement and it is true regardless of whether you are sitting about five feet from a car (so all you can see is the car from one end to the other) or looking at a distant mountain range.  You get about 160 degrees.  It's an angular measurement using your eyeballs as the center of the theoretical circle.

If you use a telescope to zoom in on a distant mountain range you no longer can see the the entire mountain range.  You no longer see 160 degrees from where you are.  You might see just one degree.  You might even see a fraction of one degree, which you would measure in arc minutes.  So you might be looking at a road going up one mountain in the range, maybe 40 arc minutes.

You drop in an even more powerful eyepiece and you can see people waving at you.  Their hands might be a fraction of an arc minute, say 10 arc seconds out of the full 160 degrees that are available to you.

Now look at the sky.  On a perfect plain, 180 degrees to go from your left to your right, horizon to horizon.  Halfway from the left to straight up, is 90 deglrees.  Half way from the right to straight up is 90 degrees.

No look at the moon.  That's about half a degree: 30 arc minutes.  Zoom in on a crater on the moon: maybe 20 arc seconds.  (I guess the very largest might be a full arc minute.)  Zoom in on a tiny crack on the side of the crater on the moon:  a few arc seconds out of the total 180 degrees available to you in the sky as a whole.

Now if you were sitting five feet from the car, and were to say front bumber to the rear bumper is 160 degrees, different features of the car could be measured in degrees.  The two doors might be 40 degrees each.  The door handles 5 or 10 degrees.  The keyhole might be one or two arc minutes out of your total view.  If you pulled out a magnifying glass and examined you might be able to get "arc second" levels of detail in terms of dust and specks on the door.

The point of angular measurement is that you can express the angle of separation from where you are observing regardless of whether you are looking at something close or far away.

For space it means you can say that two stars appear close together (say 2 seconds of arc) even though in reality one is light years behind the other.  Alcor and Mizar are separate by about 12' 36" or 12 mins 36 seconds.  36 seconds divided by 60 = 0.6 so you could express the distance as 12.6 arc minutes and get a decimal value.

It's a sexagesimal system meaning you got a lot of sixes going on, 60 arc seconds make one minute, 60 minutes make one degree, 360 degrees make a full circle, and one hour = 15 degrees.

### #11 Procyon

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Posted 18 May 2018 - 12:27 AM

Thanks. I've never had bad seeing like the pictures Eddgie put up but starting to understand what you mean. For some reason, when theres no clouds the seeing is always in the 1.3-1.6 area here. Can go up to 0.7. Further off towards forested areas it's always in the 1.9-2.5 area. There was horrible seeing tonight but transparency was good so there was some decent Galaxy sights.

### #12 gavinm

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Posted 18 May 2018 - 01:09 AM

I'd be happy there

We're always between 1 and 2 - never below

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### #13 Alex McConahay

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Posted 18 May 2018 - 09:27 PM

There are 1,296,000 arc seconds in a circle.

A soccer ball at thirty miles is one arc second wide.

Alex

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### #14 noisejammer

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Posted 19 May 2018 - 06:54 AM

There are 1,296,000 arc seconds in a circle.

A soccer ball at thirty miles is one arc second wide.

Alex

That's a surprisingly accurate heuristic. I get 0.94(0) arcsec.

A standard soccer ball is 22cm in diameter. Also, 30 miles = 30*1609 metres.

The subtended angle = 0.22/(30*1609.344) radians = 206265*0.22 / (30*1609.344) = 0.940 arcsec.

### #15 Alex McConahay

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Posted 19 May 2018 - 08:58 AM

The error is in rounding the km (45) to "30 miles." The original stat is "soccer ball at 45 km is one arc second." But, if I say  27.9617037 miles, I am being too precise for heuristics. And besides, the size of a soccer ball varies a bit.

Some comparisons...…..

…..Parallax, the first major way astronomers tried to show how far it was to nearest stars, used a measurement of .76 arc seconds--how far a nearby star moved when observed against background stars from when the earth was on one side of its orbit to the opposite side.

…..Eddington's big idea from the 1919 eclipse that proved Einstein was a genius by showing a movement in the stars of 1.75 arc seconds.

…..Hubble Space telescope has a resolution of .05 arc seconds, and a pointing accuracy of .007 seconds.

…..The earth "wobbles" about half an arc second every 435 days, changing the direction of "north."

And scientists can measure these changes.

They do not use soccer balls.

Alex

### #16 gavinm

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Posted 19 May 2018 - 10:47 PM

Hubble Ultra Deep Field - one of the most iconic images from space and contains about 10,000 objects....... is about 2 arc-minutes across (120 arc-sec).. equivalent to a square of paper 1mm across held at arms length.

### #17 Asbytec

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Posted 19 May 2018 - 10:54 PM

Thanks, I'll try and visualize this in my brain. So compare the space between a double like Alberio or Mizar, they have a pretty wide separation. I'm thinking Mizar's two stars or Alberio could fit inside the width of Jupiter ect?

REC, I have the same question. Still do. But, if I gather what Eddgie is saying and others have explained, I think it has to do with long exposure imaging, not visual. I really don't know how to evaluate seeing in terms of arc seconds. That's not how us visual guys roll.

One thing that gets me is 2" arc seeing is supposed to be average. I think it'd be terrible.

In Eddgie's illustration, note how disturbed the 80mm is, probably Pickering 5/10 or so. But, also note how small the FWHM of the blur is on the C11 image. The C11 is still holding onto some of it's resolution despite being what looks like Pickering 3/10, give or take. So, is that two arc seconds seeing?

Edited by Asbytec, 19 May 2018 - 11:00 PM.

### #18 jwheel

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Posted 20 May 2018 - 09:54 AM

This link may also be of interest:

https://weather.gc.c...o/seeing_e.html

### #19 CrazyPanda

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Posted 20 May 2018 - 07:58 PM

An arc second is an angular measurement. One degree contains 60 arc minutes. And one minute of arc has 60 arc seconds. Thus an arcsecond is 1/3600 of a degree.

Double star separations are expressed in arc seconds, yes.

When Jupiter's current (apparent) size is denoted as 44" that means is apparent diameter is 44 arc seconds (about 3/4 of a minute of arc). The Moon and the Sun are about 30 arc minutes (1/2 degree) in apparent diameter.

For astronomical seeing, try this site:

https://www.meteoblu...america_4562148

(enter your own location, naturally )

That site's measurement for seeing has never been accurate for me. I've had nights at 0.9 arc seconds that were far worse than nights at 2 arc seconds.

Sometimes when it says it's about 1.2 arc seconds, it doesn't account for the MASSIVE geometric distortions of larger turbulence cells. Sure, maybe those cells are so large that they don't obscure tiny details, but the sky is sloshing around so aggressively that it doesn't matter.

Whatever seeing prediction models they are using do not seem to apply to New England - the Bermuda Triangle of astronomy.

Edited by CrazyPanda, 20 May 2018 - 08:00 PM.

### #20 Starman1

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Posted 21 May 2018 - 12:00 AM

Lower arcseconds are better correct? 1.6 would be better than 2.5 (example)?

Thanks

well, yes, but with a clarification:

Sites don't have a seeing of 1.6" or 1".  That is the way astrophotographers describe a sky because it limits

what can be seen in their instrumentation with long exposures.  It is a "lowest common denominator" figure, i.e. the worst

it gets.

For visual observers, the seeing is always varying.  on a night the professionals might call 1", you might see seeing vary from 0.5" to 1".

Or, the seeing could be quite steady and only vary from 0.9" to 1".

But, patiently observing and waiting for fluctuating seeing to reveal smaller details for a few seconds before the atmosphere blurs the image again

is the name of the game in planet, Moon, and double star observing.

If we had a visually-oriented way to describe seeing, it would be a range from the best we saw for a few seconds to the worst it got.

Seeing varies from second to second, minute to minute, hour to hour, and night to night.  It's our fate we have to do what we can to mitigate its effects:

cooling the optics, collimating the optics, avoiding low altitudes of pointing the scope, finding sites with more stable seeing, observing often to catch the nights when seeing is great,

etc., etc.

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