seeing and guiding: which telescope?

[alacant]

Hi everyone

I'm trying to find a way to relate seeing to being able to use longer focus telescopes. It seems a pity to use short focus when the seeing would allow a longer focal length. 

I'm seeing (sorry) some sort of relationship between the steadiness of a star when I first go to focus before imaging begins and the guiding thereafter. It seems the steadier and easier it is to focus, the better the seeing; on those occasions I could use the longer telescope.

IOW, an example I sometimes have, guiding an 812 f3.9 is always OK. Guiding a 150 f8 is only OK on steady nights.

Do we have any way of deciding which nights those are? With galaxy season upon us, it would be useful to know.

Cheers and clear skies. 

[vlaiv]

Not really sure what you are asking, but if it is how to judge the seeing on a particular night (and hence feasibility of using high resolution setup), here are some guidelines that might help:

- first check out seeing forecast to be able to tell in advance if it is going to be good night for high resolution work:

https://www.meteoblue.com/en/weather/forecast/seeing/alicante_spain_2521978

Also useful is jet stream forecast (jet stream overhead - crappy seeing):

https://www.netweather.tv/charts-and-data/jetstream

Not related to seeing, but you might want to check out this as well if going after galaxies:

https://atmosphere.copernicus.eu/maps/aerosol-forecasts#1._aerosol_optical_depth_at_550_nm_(provided_by_cams,_the_copernicus_atmosphere_monitoring_service)/4/46.19/9.34

So if forecast for the night is sub 1.5" seeing, then I would say it is worth going for high res stuff.

- Next thing to do is take 2 seconds exposure on fairly bright star and check FWHM (using best focus). PHD2 will give you this stats, but it is better to do it with main scope, rather than guide scope / OAG as those tend to present less than perfect star profile and Gaussian fitting might be a bit problematic in that case.

Actual star FWHM will differ from seeing FWHM depending on aperture used. In 2s exposure tracking precision error is minimized so you don't need to guide for that - provided you have decent mount that is able to track 2s OK (almost any mount). To be sure take several measurements. Over time using this technique you will get the feel for "good" values for high res work (for that particular setup).

Star FWHM will be larger in actual sub because of tracking/guide error of long exposure.

See spreadsheet in attachment (libreoffice/openoffice spreadsheet) for calculation of FWHM in final sub based on 1. Aperture, 2. Seeing FWHM (not your measurement, but aperture independent one - like from meteoblue forecast), 3. PHD2 RMS (sigma of gaussian for guide error).

Now, final point is somewhat undefined at the moment:

Optimum sampling resolution for image with star FWHM. General rule should be 2-3 times FWHM value, so if for example your star FWHM is 3" - you should be good for resolutions in range 1"/pixel - 1.5"/pixel.

HTH

FWHMCalc.ods

[ollypenrice]

The FWHM in a sub of about three seconds gives a good measure of the seeing. It's long enough to record the 'wobble' but short enough not to be much affected by the guiding. You need to know what your best FWHM values are, of course, since the values are scope-and-camera-specific. If the values are far above the best you can ever record then it would be better to use a shorter FL setup that night. One of the advantages of mono, of course, is that you can still shoot the RGB when the seeing is poor. It's the L that needs good seeing.

Olly

[alacant]

3 hours ago, vlaiv said:

first check out seeing forecast

Thanks vlaiv. Those links are wonderful:)

3 hours ago, vlaiv said:

if forecast for the night is sub 1.5" seeing

Yep. That's what I was looking for...