FWHM

[Howardk]

Just curious…what kind of fwhm number are you getting when doing a multi star focus assistant at best focus in average seeing?

I get around 5.0

[Gfischer]

Howard,
12” dob with coma corrector at f /4.5 533 camera . Using auto focus a good focus is 3.0

George

[Howardk]

I’m at f7 with 9.25” scope

Your fwhm is small because u are at a lower f ratio than me?

[admin]

Hi,

SharpCap usually measured FWHM in pixels, and there are lots of factors that affect that number, so it is hard to compare that value with someone else's value usefully. The very latest 4.2 beta versions will also show a value in arc seconds (where the image scale is know - for instance after plate solving) - that number is more comparable between setups.

For the value measured in pixels, the following are some of the key factors:

* Seeing and focus - poor seeing and poor focus lead to bigger values
* Focal length (including the effects of reducers/barlows) - longer focal lengths lead to higher magnifications, which lead to larger values of FWHM
* Aperture - larger apertures in theory lead to smaller stars (higher resolving power) so lower FWHM, but only if seeing and focus are both good
* Pixel size - big pixels mean smaller FWHM values, small pixels mean larger values
* Binning - binning 2x2 is effectively making pixels 2x bigger, so gives smaller values

cheers,

Robin

[Howardk]

Yes

Well i get 5.0 fwhm with f/7
Fical length 1645mm
9.25 edge scope
Average suburban seeing

Is this about good?
Equals around 2.4 arcsecs

My canera has 3.76 pixels

[Jean-Francois]

Hello,

2.4 arcsec FWHM seems OK for an average suburban seeing.
You can have a lot difference with the turbulence near the floor (hot ground or metal roof in the line of sight).
What you not mentioned is the wavelength range and the position of your measurement.
Is 2.4 arcsec an average over the full sensor and with L filter ?
You may have better FWHM with a filter and at the centre.

If you want to analyse deeper ... you can take shorter exposer (< 1 s) on a bright star (with ROI at the sensor centre and with O-filter) and then do some analysis of the FWHM during several minutes capture.

Regards,
Jean-Francois

[Howardk]

Its with a uv/ir cut filter
I use multistar focus assist but i make a roi box to exclude edge of image

[cyrille.de.brebisson]

Hello,

Using a SV705C (IMX585) with 2.9µ pixels, I get FWMH (in pixels) around 3.6 using a 250mm FL HAC125 and around 4.5 using a 72ED.
I have never managed to get anything better.

Cyrille