SharpCap Forums

I think I may have found the issue. Under options there’s the option for how many pixels to dither by. Default is 20 pixels. Thing is, it says something about using ascot drivers and if so it’s arc seconds. So it was moving like 400 pixels each time…. No wonder it kept moving !

I have shingled it to 3a/s and will see how that goes
Thank you for your advice so far

Hi,

actually the wording on the settings is a bit misleading - it should always be pixels if used via PHD, it's arc seconds if you are using the 'dither only' guiding option. I will try to make that clearer.

cheers,

Robin

Ok thanks. So what value should I be using?

Hi,

that's a difficult one to answer - it depends on the pixel size and focal length of your guiding setup and the relative values of the same things for your main imaging scope. For instance if both cameras have 3.75 micron pixels and your guide scope is 200mm vs 800mm for imaging scope, the number of pixels the main image will seem to move will be four times as much as the guiding image (due to the longer FL).

In general I'd say avoid setting this to less than about 3 - I'd be worried that very small dither movements may not really work as well as expected. 5 to 10 is probably sufficient for most cases. You may want to go higher if you are using off axis guiding (same FL for guiding as imaging). There is always going to be an element of trial and error here.

cheers,

Robin

Ok thank you Robin. I have used 5 to start with.

I have related question. If I have dithering set in both PHD2 and SharpCap, will this be a conflict? If so, should dithering be turned off in PHD2?

Hi,

the normal setup is that you configure in PHD2 the details of how each dither should work, but SharpCap sends the message to PHD2 to actually perform each dither operation - that makes sure that the dithering happens between the capture of frames rather than during them.

I wasn't even aware that you could manually trigger a dither operation in PHD2, but the manual seems to suggest that you can...

However, you can do manual dithering or experiment with dither settings using the controls at the bottom of the dialog.

Whatever that refers to, I have been unable to find within PHD2 - anyway, don't do that, let SharpCap trigger the dithering movements.

cheers,

Robin

Hi Robin/All, Thanks for the discussion. I have now used dither, and find the image moves too much, so the frame edged are shifted, and clearly different stack lengths. I think i should be able to adjust this in the settings, but any tips welcomed. (see example attached)

In the manual, it indicates the live frame stack is halted before the dither is performed but it looks like the camera continues exposing and that frame is then lost. So each dither results in a lost frame. Why cant the the next frame start when the dither and settling period is completed ? Or have i miss understood? - cheers

Hi Tim,

in the guiding settings (main settings window), you can adjust how far the dither movement is - change the 'Max dither step' to a smaller value. Note that this measurement is in pixels for the guide camera, so the number of pixels moved by the main camera will very likely be larger than this value.

SharpCap has to discard any frame that is in progress while the dither movement happens, since it will be spoilt. There are two ways to minimize the impact of this on your imaging

1) Tick the 'Reduce exposure while dithering' box in the Live Stack Guiding configuration - this drops the exposure to 1s during the dither, then goes back to full length when the dither is complete

2) Put the camera into still mode before starting stacking - in still mode, the live stacking can ask for exposures exactly when it needs them, and therefore leave a gap between frames to allow the dither to happen.

cheers,

Robin