SGP Dithering parameters with PHD2

Hello All,

I’ve just started to use SGP with a Canon T3i camera so I’d appreciate if somebody here could help me with the following doubt about dithering setup:

Below you will see calculations I used to setup dithering values between PHD2 and BYEOS for one of my OTAs (SW Esprit 100ED):

Main scope focal length 550 mm
Main camera pixel size 4.3 pixel (Canon T3i)
Arc-sec/pixel 1.61

Guidescope focal length 207 mm
Guide camera pixel size 2.4 pixel (QHY5III-178)
Arc-sec/pixel 2.39

Imaging scale ratio: 1.48 arc-sec/pix

If I want to dither 13 pix on the main camera, then use the following factors:

PHD2 dither scale should be: 8.77
BYEOS dither aggressiveness should be: 1

If I keep my PHD2 dither scale in 8.77 then, which dither scale should I select in SGP to again ensure the same dithering level in the main camera (13 pix)?

What are the multiplying factors for each of the dithering options used in SGP?

Thanks in advance,

The dither amounts are:

Small      +/- 0.5 px
Medium     +/- 1.0
High       +/- 2.0
Very High  +/- 3.0
Extreme    +/- 5.0

The units are guide camera pixels, and PHD2 will multiply the value by the dither scale factor setting (on the Global tab in the brain).

Hi Andy,

Thank you for getting back to me!

As I mentioned above, the imaging scale of the combo main scope-main camera is 1.61 arcsec/pixel and the imaging scale of the combo guiding scope-guiding camera is 2.39 arcsec/pixel. This means that a 1 pixel movement on the guiding camera is equivalent to 1.48 pixels in the main camera.

Now if I want to have a total dithering of 13 pixels on the main camera, the guiding camera would have to move 8.77 pixel (13 / 1.48).

In this case and just to be clear, I should choose “medium” dithering in SGP while keeping 8.77 in PHD2 to get the desired movement in the main camera (13 pixels), correct?

Thanks again,

That is correct.

Thanks Andy!! Appreciated!


Just to chime in a little here with a related tip. For settling, since you are using PHD2, I recommend figuring out what your guide RMS is in guider pixels, and set your settling in SGP to no less than that.

I’ve found in my own testing, as well as when assisting others with settling issues in SGP, that if you try to settle below your guide RMS, if and when it happens, it is mostly by chance. This can greatly increase the amount of time it takes to settle, without any clear benefit.

If you settle at your guide RMS in guider pixels, then you can settle pretty quickly (couple/few seconds usually, vs. say 30, 60, 90 seconds.) So if you have a guide RMS of say 0.55", in guider pixels with a scale of 1.61", that would be 0.89 pixels. You would want the settle factor in SGP to be say 0.9. On the other hand, if you had a guide RMS of 0.8", then you would want the settle factor to be 1.3 in SGP.

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Hi Jon,

Thanks for the reminder! I had this in mind after I read a similar comment you posted while ago in CN.