How do I calculate my local seeing?

[White Rabbit]

Hi Guys.

I'm in the process of upgrading all my gear. I've just upgraded the mount and I have an SBIG STF3800m camera on the way. I'm looking at upgrading my scope as well at some point but I need to calculate my local seeing so I can match that to my imager and get the optimal resolution.

Another reason I want to know this is because my new mount has a PE is 35, 11 after PEC :sadeyes: and I'll likely need to by a new worm to get it guiding properly if I dont send it back. If my seeing is as I suspect pretty bad (7.5Km from Syd cbd) then I dont need get my mount pe down past a certain level. I'm looking at either a new worm or a TDM. I'd like to crunch the numbers before i decide what path to go down as a TDM may be over kill for my seeing.

Or do I have all this backwards.

[Merlin66]

The easiest and best way is to take some sample exposures and measure a few star images FWHM sizes.
I use AstroArt5, but I'm sure all the other "suspects" can do the same.
Knowing your focal length/ plate scale you can get a good estimate in arc sec.
Here in Melbourne Subs I usually have seeing around 3 arc sec.....

[White Rabbit]

Thanks Merlin.

Am I correct in assuming then that you would not need to guide better than 3 arcseconds? If you are guiding better than 3 you would just be chasing atmosphere refraction.

My set up is a 714mm 102 refractor and the imager will be an STF 3800m pixel size is 5.4 microns. Un binned gives me 1.56 ar/px but that will be two high res for my seeing so I'll need to bin this to 2x2 making it 3.12 arcseconds per pixel. So am I correct in thinking that my guiding doesnt need to be better than 3.12 due to my imaging rig? But then how does my local seeing play into this.

I'm just starting to get my head around all this its a bit of a can worms :screwy:.

Thanks

[Merlin66]

I no longer do "pretty picture" imaging....;)
For the spectroscope I have to hold the star image on a entrance slit 20 micron wide for as long as possible. Usually a hour or more exposure is required.
The seeing disk ie FWHM 3sec must remain centred on the slit for maximum efficiency. I use a beamsplitter guider/ Lodestar/ PHD and Al's reticule - so I'm guiding on the target star image at 2750mm focal length.
I'd don't worry too much about PHD - it can guide on the image and keep it centred - the guiding must be much better than 0.5 x FWHM (or I'd start to see a drop off in the light intensity)
For proper sampling (Nyquist) the image should cover at least 2-3 pixel, therefore your unbinned sec/pixel would be good for a seeing of 3 arc sec.

Looking at the SBIG data:
https://www.sbig.com/products/cameras/stf/stf-8300m/
That's one hellava camera!!!!
How do you intend guiding? OAG?
Also, I use the C11/ spectroscope on a NEQ6pro mount....never measured (why?) the PE etc - all I know is that with proper polar alignment and good balancing it just does it's job - PHD locks on and holds that star image on the entrance slit......what more would you want?
I'm sure some of the active imaging members may give their opinions....

[Geoff45]

To avoid chasing the seeing, take longer guide exposures--6 to 10 seconds. This averages out the seeing variation and gives a better idea of the true guide error.
Geoff