Sunday, December 3, 2017


This post is a quick picture of the moon since it is always a simple target, when it is up and a discussion about focus performance.


Autofocus was run and resulted in 1/2 Flux diameter (HFD) of 5.96pixels.
  • The half-flux diameter is the diameter in pixels that contains half the energy in a star image.
  • The question is, is a 1/2 Flux diameter of 5.9 something to write home about or be improved upon?
  • It should be possible to calculate what the theoretical HFD of the optical and CCD system is.
  • It should be possible to combine this with the seeing and estimate expected HFD. 
  • Then we would know if we are impressed by a HFD of 6.
  • A sufficient reference to begin with is Link. I have calculated the Airy pattern for the EDGE 11 SCT. It is presented below. Compared to the Celestron "glossy spec sheet" this calculation is in agreement with the 0.5arc-sec specification.

    Airy Disk for 11" EDGE SCT
  • The above result was converted to total flux in a diameter by integrating and scaled to CCD pixels.
  • HFD for 11" EDGE w/ STF8300 CCD
  • From this result the HFD of the optical system should be 0.7arc-sec.
  • Seeing is a measure used to quantify stability of star light in the eyepiece or camera. The most excellent Canadian link has a good and relevant description of seeing.
Seeing w/ Scope and CCD
5/5 or V
less than 0.4"
Less than 0.9”
1.4 pixels
4/5 or IV
~ 0.4-0.9"
Up to 1.2”
2.1 pixels
3/5 or III
~ 1.0-2.0"
Up to 2.2”
3.7 pixels
2/5 or II
~ 3.0-4.0"
Up to 4.1”
7.1 pixels
1/5 or I
more than 4"
More than 5”
8.8 pixels

  • The last column in the table above is a combination of seeing and theoretical optical performance. It was calculated by taking square root of the sum of the squares of seeing and optical performance. Expressed in arc-second units and pixels.

To return to the original question, best focus was 5.9pixels HFD and this corresponds to poor seeing. I do not recall the seeing forecast for the evening of the image. It is possible that seeing forecast was Poor and so the 5.9pixel HFD may be reasonable.

I will pay more attention next time.
    Since the moon is so bright a Ha filter was used to cut brightness and a 1mS exposure produced 3/4 full wells.

    • Focus MaximDL 1/2 Flux Dia. = 5.93
    • Exposure Control MaximDL Ha 1x1 1x0.001sec
    • Guiding Unguided
    Image Processing
    • None

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