Star Eccentricity and Guiding Accuracy

The Pix Insight documentation specifies that most people would consider a star to be round when the Star Eccentricity is less than 0.42. The equation for Star Eccentricity is a bit convoluted but a star eccentricity 0f 0.42 is also equivalent to a Star Aspect Ratio of 0.91 or a Star Flatness of 0.10.  I have chosen to have CaLIGHTs calculate what I call Star Roundness(%) which equals the Star Aspect Ratio multiplied by 100. So a CaLIGHTs Roundness value of 91% equals a Pix Insight Star Eccentricity of 0.42. Lower Star Eccentricity values correspond to higher CaLIGHTs Roundness values.

Guiding can influence Star Eccentricity only when the RA and DEC guiding rms are significantly different. If they are equal then guiding will typically result in round stars and as total guiding rms guiding increases the HFD of the stars will increase. We all strive for low HFD stars but it would be a shame if the stars looked oval because the RA and DEC guiding rms were different.

So I am going to develop some relationships that link HFD values to guiding rms so that you can judge when your guiding can result in oval stars. Both the guiding rms and the HFD values need to be in units of arc-seconds. PHD2 Guiding can display guiding rms for RA and DEC in arc-seconds. You will need to know the pixel scale for your LIGHT frames to be able to convert HFD values from pixels into arc-seconds.

After scratching my head a lot, I finally came up with this chart. I plotted my results for various Total Guiding rms values ranging from 0.2 arc-seconds(a-s) up to 1.5a-s. The legend along the right edge of this chart indicates which pen color is associated with each total guiding rms. I chose to make the lines for 0.5a-s, 1.0a-s and 1.5a-s thicker. The y-axis is the Guiding Ratio of the RA and DEC guiding rms. It equals the higher of the two values divided by the lower of the two values. As an example: if the RA guiding rms was 0.48a-s and the DEC guiding rms was 0.3 then the Guiding Ratio was 0.48 / 0.3 = 1.6.

So let’s draw some conclusions…

If I have a Guiding Ratio of 1.6 and my total guiding rms is 0.5a-s this corresponds to a HFD value of 2.7a-s. This means that if the HFD of my stars is less than 2.7a-s my stars will be noticeable oval. This can be interpret as follows:

For any give Total Guiding rms staying to the right side of the curve ensures round stars. If your situation puts you on the left side of the curve you will have oval stars attributable to your guiding ratio. More examples:

If my Guiding Ratio is 1.6 and my total guiding rms is 1.0a-s the graph indicates that if my stars have a HFD less than 6.1a-s they will be noticeable oval. Ouch! This is a big reason why we all want better guiding.

Yet another way to use the graph… let’s assume that my LIGHT frames have HFD values of 2.5a-s and my guiding rms is 0.5a-s. Using the graph I need to keep my Guiding Ratio less than 1.55 to avoid oval stars.

On a typical night I can have my guiding rms at 0.5a-s. On an excellent night it can get down to 0.3a-s. When I am using my 8”EdgeHD @F10 I typically get stars with an HFD of 6 pixels. The pixel scale is 0.47a-s/px so this HFD equals 2.82a-s. According to this chart, with a total guiding rms of 0.3a-s, it would take a guiding ratio higher than 3.2 before I would start seeing oval stars. This makes sense because tighter guiding means tighter RA and DEC guiding. Higher guiding ratios are possible but they will be based upon two very small values which will have a smaller affect on the star profile.

Please note that there are lots of other issues that can cause oval stars. Differential flexure being a big culprit here. The goal here is to give you some perspective as to whether issues with your guiding are responsible for oval stars.


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