Value of FWHM during autofocus

Sequence Generator Auto Focus
1

Hi,

I have a small question about the value of FWHM that is indicated when I autofocus with SGP and Pinpoint.

During autofocus (5s bin 2 for each photo), I get a FWHM mini at the focus point of 1 to 1.3.

If I take a picture under the same conditions (5s bin 2), I get a much larger FWHM measured with CCD Inspector, between 3 and 5.

Where does this significant gap come from and what does the FWHM actually mean by SGP during autofocus?

Thank you

Jean Pierre

2

Jean:

SGP uses a metric called Half Flux Radius (HFR) for autofocus, not FWHM. It is calculated differently than FWHM and it also represents a radius, not a diameter. As such, it can’t be compared to FWHM as calculated by CCD Inspector.

Tim

3

@Discret68

If you are using Pinpoint and FWHM as the metrics engine for focus (not recommended), we are not able to account for specifics. We merely ask the Pinpoint SDK to analyze the image and it gives us a FWHM value. We have no insight as to how it is calculated.

You will need to redirect your question to the Pinpoint boards if you would like more information.

4

@spokeshave :
SGP allows autofocus with Pinpoint. In this case, a value of FWHM is indicated.

@Ken :
For the value of FWHM, I will contact DC3-Dreams for an explanation on the calculation method. All the software I use (MaximDL, Pixinsight, DSS, MaxSelector, …) indicate a similar value but very different from Pinpoint.
On the other hand, if I understand your answer, you do not recommend using Pinpoint to autofocus. For what reasons ?
Is it possible to convert the value of HFR to FWHM?

5

Because:

  • HFR is considered a more resilient measurement and, in general, is more tolerant of atmosphere.
  • The SGPro star finding algorithms will work well with “donut stars” found at the edges of AF runs.
  • We have complete control over this process, including generation of metrics, and don’t have to respond to questions with “please ask your question in a different forum”