Ok so I guess I’m going to show just how stupid I am but I thought maybe others might want to know how this is done. I’d like to know what formula is used to calculate your seeing on a given night in arc second seeing. I’m sure that FWHM, image scale as well as PSF is used somewhere in the calulation, and I’m sure there are many other factors that play into this. A friend of mine had mentioned that it was your FWHM x image scale. When I do this with my 12” f5 newt (1500mm FL) and the FWHM number from SharpCap, my numbers are generally 2.7”-3.5” which based off my experience seems to correlate. But if I used this same formula on my wide field setup Esprit 100 it just doesn’t match what I get with my 12”. I’m sure there is some variation based on a 4” scope verses a 12” scope but my 100 shows much higher “arc second seeing” number. Full disclosure here, I’m far from understanding high math which I’m positive it gets into so if you decide to reply in this thread keep in mind your talking to someone that is STUPID! Thanks in advance! Dale BTW I had googled this subject and found this article. Seems like it’s pretty high math oriented. https://www.innovationsforesight.com/education/astronomical-seeing-tutorial/ |
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In terms of actual instrumental seeing, your formula of seeing = FWHM in pixel x image scale in "/pixel is correct and applies across the board. This is not the "seeing out there" which refers to the maximum possible value that can be obtained a priori of the actual material instrumentation used but to the sum of all possible factors, including the size of the telescope (i.e., the convolution of tracking, pixels scale, optical aberrations, thermal issues within your local environments/telescope and so on), that affect the effective resolution of your images.
While there might be an ephemeral "seeing" out there what matters is what is being recorded there and then by your camera. And yes, smaller telescopes record larger "seeing" values than bigger scopes in the main.
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Image scale is “/px, so px x image scale is just “, but yes fwhm in arcseconds is *your* seeing assuming good focus etc.
Cheers,
Scot
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Regarding seeing, IMHO for a permanent site, a seeing monitor is a great idea. They are purpose built and keep track of trends graphically as well as being able (in most cases) to report to the ASCOM observing conditions hub so they tend to be more useful than using your primary instrument(s). There are a number of commercial units available from the spendy Alcor Cyclope to the more reasonable SBS SM3 and there may even be a few used old SBIG units out there. I still have an old SBIG unit at home that has been working for 15 + years! Of course we would all like this sort of seeing (note location at upper right).   |
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andrea tasselli:
In terms of actual instrumental seeing, your formula of seeing = FWHM in pixel x image scale in "/pixel is correct and applies across the board. This is not the "seeing out there" which refers to the maximum possible value that can be obtained a priori of the actual material instrumentation used but to the sum of all possible factors, including the size of the telescope (i.e., the convolution of tracking, pixels scale, optical aberrations, thermal issues within your local environments/telescope and so on), that affect the effective resolution of your images.
While there might be an ephemeral "seeing" out there what matters is what is being recorded there and then by your camera. And yes, smaller telescopes record larger "seeing" values than bigger scopes in the main. Ok thanks for confirming this then Andrea, I was really confused that my smaller rig does give me larger numbers. At least I know that I’m calculating things correctly. Can you explain to me why this is the case for a smaller “aperture” setup? Only because I would like to understand why and “NOT” questioning your explanation. In fact you are telling me that what my numbers are telling me are “in general” correct. Dale
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Dale,
no worries, we are all stupid most of the time or in most of the fields and usually both. or in other words: there are no stupid questions only stupid answers ;-)
the formula is correct, but your FWHM is, as Andrea pointed out, the (quadratic) sum of several effects: resolution, seeing, guiding errors etc, ie
FWHM^2=resolution^2+seeing^2+ guiding error^2 + …
in the ideal world you have only seeing and resolution. Further complication: you need sufficiently small pixels to properly sample the PSF. If you have 10" pixels, you get a 15-20" FWHM (if not more) basically irrespective what your telescope resolution and seeing is.
For your Newton you probably have a resolution 0.4" and a similar sampling, these numbers are all small compared to the FWHM, so what you measure is dominated by seeing.
For your esprit, the resolution is more like 1.2", the sampling probably more like 1.5-2", so you would expect assuming identical seeing a ~2-3" larger number for your FWHM
Matthias
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Scott Badger:
Image scale is “/px, so px x image scale is just “, but yes fwhm in arcseconds is *your* seeing assuming good focus etc.
Cheers,
Scot Thanks Scot, Yes agreed, focus, tracking/guiding, thermals etc… all taken into consideration on our/my final number for seeing. Dale
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Bill McLaughlin:
Regarding seeing, IMHO for a permanent site, a seeing monitor is a great idea. They are purpose built and keep track of trends graphically as well as being able (in most cases) to report to the ASCOM observing conditions hub so they tend to be more useful than using your primary instrument(s). There are a number of commercial units available from the spendy Alcor Cyclope to the more reasonable SBS SM3 and there may even be a few used old SBIG units out there. I still have an old SBIG unit at home that has been working for 15 + years!
Of course we would all like this sort of seeing (note location at upper right).
Ohhh wow this is interesting Bill! While I never thought there was actually something like this for amateurs, I figured there was something for the big guys! So I’m intrigued now! I’ll have to look into this a bit as it would be kinda cool to have something like this in my observatory. Or maybe outside??? Dale
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Dale,
no worries, we are all stupid most of the time or in most of the fields and usually both. or in other words: there are no stupid questions only stupid answers ;-)
the formula is correct, but your FWHM is, as Andrea pointed out, the (quadratic) sum of several effects: resolution, seeing, guiding errors etc, ie
FWHM^2=resolution^2+seeing^2+ guiding error^2 + ...
in the ideal world you have only seeing and resolution. Further complication: you need sufficiently small pixels to properly sample the PSF. If you have 10" pixels, you get a 15-20" FWHM (if not more) basically irrespective what your telescope resolution and seeing is.
For your Newton you probably have a resolution 0.4" and a similar sampling, these numbers are all small compared to the FWHM, so what you measure is dominated by seeing.
For your esprit, the resolution is more like 1.2", the sampling probably more like 1.5-2", so you would expect assuming identical seeing a ~2-3" larger number for your FWHM
Matthias Thank you Matthias for making me feel better about my “stupidity” 😌 This makes total sense as this is about what I get when I run the numbers for the Esprit! Its right about what your saying 2-3 times what my 12” figures out to be. This helps me to understand whats going on with these numbers a bit better. Dale
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Dale Penkala:
Bill McLaughlin:
Regarding seeing, IMHO for a permanent site, a seeing monitor is a great idea. They are purpose built and keep track of trends graphically as well as being able (in most cases) to report to the ASCOM observing conditions hub so they tend to be more useful than using your primary instrument(s). There are a number of commercial units available from the spendy Alcor Cyclope to the more reasonable SBS SM3 and there may even be a few used old SBIG units out there. I still have an old SBIG unit at home that has been working for 15 + years!
Of course we would all like this sort of seeing (note location at upper right).
Ohhh wow this is interesting Bill! While I never thought there was actually something like this for amateurs, I figured there was something for the big guys!
So I’m intrigued now! I’ll have to look into this a bit as it would be kinda cool to have something like this in my observatory. Or maybe outside???
Dale Of course the image shown above is not from an amateur system. It was a snapshot I took of a monitor at the VLT in Chile during a walk thru the control room in 2018.  Amateur systems are typically mounted outside on a separate and very solid pier so as not to get any vibrations from other activities. Mine sits about 6 feet from the roll-off on a separate steel pier. Here is a snapshot from it from a few nights ago.  This is the from the old, no longer made, SBIG unit. The most commonly used these days is from Santa Barbara Scientific, owned by Alan Holmes, one of the owners/founders of SBIG before it was sold to Cyanogen. Link to his unit below. SBS and SM3These things are typically a tiny refractor tied to what is basically a guide camera and equipped with special software. They monitor the movement of Polaris so just need to have enough FOV that it stays on the chip all night and all year. The software adjusts for latitude so supposedly gives you the theoretical seeing at zenith. They are especially useful if one has a long and a short focal length system since they inform you as to which might be the better system for a given night and time. Here is A link to the monitor output from a SBS unit at my remote site, Skies Away Observatory in Ca. As you can see, the seeing was pretty decent there last night... SBS Monitor at SAROS |
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Dale Penkala:
Ok thanks for confirming this then Andrea, I was really confused that my smaller rig does give me larger numbers. At least I know that I’m calculating things correctly.
Can you explain to me why this is the case for a smaller “aperture” setup? Only because I would like to understand why and “NOT” questioning your explanation. In fact you are telling me that what my numbers are telling me are “in general” correct.
Dale Hi Dale, The smaller aperture isn't most likely sampled the same as the 12" (there's a factor of 2 or so I expect) so it won't give you the same reading. Where it sampled to take advantage of its own "best" seeing you'd probably have had better "seeing" numbers on the smaller rig except in the better moments of seeing, all other things being equal (i.e., mounted side by side).
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andrea tasselli:
Dale Penkala:
Ok thanks for confirming this then Andrea, I was really confused that my smaller rig does give me larger numbers. At least I know that I’m calculating things correctly.
Can you explain to me why this is the case for a smaller “aperture” setup? Only because I would like to understand why and “NOT” questioning your explanation. In fact you are telling me that what my numbers are telling me are “in general” correct.
Dale
Hi Dale,
The smaller aperture isn't most likely sampled the same as the 12" (there's a factor of 2 or so I expect) so it won't give you the same reading. Where it sampled to take advantage of its own "best" seeing you'd probably have had better "seeing" numbers on the smaller rig except in the better moments of seeing, all other things being equal (i.e., mounted side by side). Ok thanks Andrea! Appreciate the info and explanation! Dale
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