Reported FWHM

Hi Anil @anilketkar,

Thank you for your question.

The FWHm is reported in pixels.

• The abs(olute) value after star analysis reports in pixels of each frame separately.
• The rel(ative) value is reported after registration. In this case the absolute FWHM in pixels is adjusted (and reflected in the relative value) to be stated in pixels of the reference frame for all images. So if you combine images of different image scales, the relative FWHM value in pixels tells you how the stars are sized in the image scale of the reference frame to which the data is registered and will be integrated. So you have better quality calculation and comparison for integration.

Min, max indicate the roundness af the average starprofile in the image. If Min and Max are the same, you have perfect round stars. If Max is twice (2x) Min, then you clearly have elongated stars.

So if you have a FWHM of 4 pixels with 3,9 micron pixels, the FWHM is 4x3,9 micron = 15,6 micron. You will need to know the focal length of your setup to actually calculate the image scale in arc seconds.

There are a lot of online articles that explain the calculation, perhaps this is nice article form Sky & Telescope:

https://www.skyandtelescope.com/astronomy-blogs/imaging-foundations-richard-wright/understanding-field-of-view-pixel-scale/

Let me know if this is clear 😉

Mabula

Thanks, Mabula. A couple more questions, if I may  - 

1. For the some indiv subs, APP reports a tight min/max FWHMs of 3 to 3.5 (pixels), the lowest I have ever seen with my system. But, on the same night with the same set-up without changing focus etc, some other subs show min/max FWHMs of 4/6 - i.e. much poorer (squarish) stars? So, is that because of seeing conditions changing over the course of the night? 

2. I use a Stellarvue 102ED scope with a native f ratio of 7.0 without a reducer (Focal length=714mm). So for this system with my DSLR having 3.9 micron pixels, the maximum resolution is 1.13 arc seconds per pixel. So with the best FWHM of (avg) 3.2 pixels, how would I calculate the actual resolution I am achieveing in my images?

Thanks in advance ..... Anil

Hi Anil @anilketkar,

You're most welcome 😉

1. yes, larger FWHM can be due to a number of reasons,
   1. out of focus due to not changing focus with different outside temperature, or
   2. worse atmospheric conditions.
   3. Using filters or not will have it's influence as well 😉
   4. worse guiding performance due to wind sheer perhaps
2. If you are shooting with 1.13 " / pixel, then a FWHM of 3.2 pixels will be 1.13 * 3.2 = 3.6 " (arcseconds) or on the focal plane on the sensor : 3.2 pixels times 3.9 micron = stars with FHWM of 12,48 micron.

Kind regards,

Mabula