Meteoblue seeing accuracy

[dan_adi]

Do you guys find meteoblue seeing forecast accurate?

For my location they seem to optimistic, between 1 and 2 arcsec. I use Prism astro software that reports seeing and star FWHM in my subs and the best FWHM I got was 1.9. Usually I have values between 2-3 arcsec. 

In Prism the difference between reported seeing and median star FWHM is 0.2 arcsec, this tells me the scope is focused ok. The autoguiding RMS with Mesu 200 is usually 0.2-0.3 arcsec.

So my guess is meteoblue is way to optimistic for my location. I doubt I should get bellow 2 arc sec seeing at sea level..

What is your experience?

Edited August 9, 2022 by dan_adi

[vlaiv]

I think that Meteoblue is actually rather accurate.

In fact - your case seems to confirm this rather than show that it is "way too optimistic".

I'll explain.

In final FWHM of the image - seeing is only one component. There are two additional components - one is mount performance which you mentioned, and another is aperture size and spot diagram of your optics.

Guide RMS figures that you have are indeed low (I'd be surprised that it was otherwise with Mesu), but those figures are in RMS not FWHM "units". There is conversion rate that is approximately 2.355 (for Gaussian profile).

0.2-0.3 RMS is in fact 0.471 - 0.7 FWHM

FWHM of diffraction limited airy disk of 8" aperture is about 0.56" - and smaller apertures only have higher figures (4" will have 1.12" FWHM from airy disk).

When you put those figures together 1-2" FWHM from seeing, 0.5-0.7" FWHM from guiding and depending on your aperture - 0.5"-1" FWHM from airy disk, and you account for any deviation from diffraction limited optics (most of the time, any coma corrector or field flattener and/or focal reducer reduces sharpness of optics from diffraction limited in order to obtain larger flat field and good correction at the edges of the field) - then you see that it is perfectly sensible and normal to have total FWHM to be 2-3".

In the end, I'd like to point out that Meteoblue can only offer values for atmospheric seeing - and not local seeing effects - like tube currents, or local thermals - which all add up to give you even higher FWHM.

In my view, if you have 2-3" FWHM - you are doing quite ok for 1-2" seeing forecast (giving all that goes into final FWHM number).

[ONIKKINEN]

If Meteoblue says that seeing is really bad, then it always is so in that way i find it to be very accurate. In the other end of the spectrum when it says that seeing is very good, this i find to be more variable and not always the case. But still its a very good guideline for general estimations of the nights seeing.

[dan_adi]

18 hours ago, vlaiv said:

I think that Meteoblue is actually rather accurate.

In fact - your case seems to confirm this rather than show that it is "way too optimistic".

I'll explain.

In final FWHM of the image - seeing is only one component. There are two additional components - one is mount performance which you mentioned, and another is aperture size and spot diagram of your optics.

Guide RMS figures that you have are indeed low (I'd be surprised that it was otherwise with Mesu), but those figures are in RMS not FWHM "units". There is conversion rate that is approximately 2.355 (for Gaussian profile).

0.2-0.3 RMS is in fact 0.471 - 0.7 FWHM

FWHM of diffraction limited airy disk of 8" aperture is about 0.56" - and smaller apertures only have higher figures (4" will have 1.12" FWHM from airy disk).

When you put those figures together 1-2" FWHM from seeing, 0.5-0.7" FWHM from guiding and depending on your aperture - 0.5"-1" FWHM from airy disk, and you account for any deviation from diffraction limited optics (most of the time, any coma corrector or field flattener and/or focal reducer reduces sharpness of optics from diffraction limited in order to obtain larger flat field and good correction at the edges of the field) - then you see that it is perfectly sensible and normal to have total FWHM to be 2-3".

In the end, I'd like to point out that Meteoblue can only offer values for atmospheric seeing - and not local seeing effects - like tube currents, or local thermals - which all add up to give you even higher FWHM.

In my view, if you have 2-3" FWHM - you are doing quite ok for 1-2" seeing forecast (giving all that goes into final FWHM number).

I wasn't paying attention on the guiding RMS conversion to FWHM.

Now it makes sense. 

I found a formula for estimating final FWHM on the ESO website, can't remember how they dealt with autoguiding RMS, but I'll check and report back.

Thank you Vlaiv 

[dan_adi]

I found the conversion factor, 2.355, comes from 2 x √(2 x ln(2)).

Seems autoguiding performance is rather important for the final FWHM.

Didn't thought seeing could be bellow 2" at sea level, live and learn 😁

 

 

[Scott Badger]

When I look at the current Meteoblue seeing chart (screenshot attached), the best seeing index value corresponds with the highest arc sec value which is opposite to what I would have expected......am I missing something basic? The info page says that the arc sec value is based on both indexes and bad layers, does that mean I should pay more attention to the arc second values than the index values?

Thanks,

Scott

[vlaiv]

6 minutes ago, Scott Badger said:

When I look at the current Meteoblue seeing chart (screenshot attached), the best seeing index value corresponds with the highest arc sec value which is opposite to what I would have expected......am I missing something basic? The info page says that the arc sec value is based on both indexes and bad layers, does that mean I should pay more attention to the arc second values than the index values?

I think that seeing index goes from 1 - worst to 5 - best, but there is two of them and seeing FWHM (arc second value) is combination of those two indices.

In above example - you have severe jet stream and I think it:

a) reflects in index 2 - being the worst all the time

b) having high FWHM values

Since there are two indices and only one FWHM value and taking into account that indices are discrete values and seeing is continuous - it is not straight 1-1 mapping

I think that in above example with very poor seeing - index 2 lacks "resolution". Both bad and very bad have been assigned the same number - 1 (jet stream speeds of 41m/s and 52m/s). In one instance index 2 having 1 and being worse - trumps other case where index 1 has value of 2 or 3 and index 2 - still having 1 but being "better" than first case - if that makes sense.

In any case - indices I think represent conditions at different heights / layers, and what you want as a final judgement of seeing is actually FWHM in arc seconds.

 

[Scott Badger]

Thanks Vlaiv! In a confusing way, that does make sense!.... :  ) And a more complete answer than I got from MB. They simply said that "the lower the arcsec, the better the seeing and the higher the index 1 should be" and "we're currently working on improving the meteoblue seeing forecast". Anyhow, I don't really need a chart, clearly my seeing is just plain bad!....

Cheers,

Scott

[creativspelerr]

I'm still skeptical about the seeing arcseconds.  I've only been keeping an eye on it for the last month or so but I almost never see a number above 1.00 and sometimes below 0.50.  The numbers are under 1.00 in the daytime when they are expected to be worse (I do solar).  Even at x2.355 that comes in around 1.00 to 1.50 FWHM.  The other day I was testing out some new white-light solar imaging equipment and the seeing was definitely crummy.  With so many new components and processes I was trying I can't pinpoint where any fault lies, but if I were to give meteoblue the benefit of the doubt, I would say it's the ground-level conditions.  I pointed the scope at a radio antenna on a hill top and the antenna looked like it was doing a hula dance from all the atmospheric shimmer.  So maybe the atmospheric air was great but the ground level air was crummy.