dithering with Nebulosity 3 software

[red dwalf]

hi all, 

can anyone help with the dithering settings in nebulosity 3, the settings mean nothing to me and not 100% certain of what it actually does but i have options for small, medium and large dithers and settle at rates.

i`m using a st80 as a guide scope and a William Optics GT 81 with Atik 314L+ ccd camera if any of that revelent.

many thanks,

Rob

[perfrej]

If you guide with PHD then Nebulosity can dither. In essence, all it does is move the image frame slightly between exposures so that artifacts you do not want end up in different places in relation to the target in each sub. Suppose you have some hot pixels. They will stay in the same place on the chip, so if you move the framing between subs you have them in different parts of the frame in each sub after alignment, which works on the target data.

Stacking dithered subs is nice because you can use standard deviation based maths to statistically remove things that don't follow the target, like airplane tracks, hot pixels, satellite trails or a passing firefly. That is called Sigma Rejection. This is the best way to handle problems like that and with PHD/Neb it is dirt simple.

The amount of dithering between each sub needs only be a few pixels. You will loose the very outer rim of your image due to the fact that it is not covered by all subs, but that is a small price to pay. Nebulosity lets PHD perform the dither by telling it to offset the guide star slightly. Small, medium and large are calculated in relation to the image scale of the guider. The settling is just how long Neb will wait for PHD to finish the dither operation, and it will be idle during that wait.

Give it a shot!

/per

[red dwalf]

thanks for the reply, certainly makes alot more sense now, many thanks

[Gib007]

I do this myself - use PHD Guiding for guiding and Nebulosity for imaging. I personally set Extreme Dither and Settle at < 0.5. Works really well on both my wide-field and narrow-field telescopes. Dithers between subframes in only a couple of seconds and the benefits when stacking the dithered exposures are fantastic. 

[red dwalf]

great thanks for that, i`ve been using a small dither rate as i didn`t know what was best and a settle rate the same as you of <0.5

[perfrej]

Just make sure you don't use to small a settle setting as it may use up a lot of time waiting for dither settle - sometimes all the time if you set it too tight.

/per

[Gib007]

Just make sure you don't use to small a settle setting as it may use up a lot of time waiting for dither settle - sometimes all the time if you set it too tight.

/per

Indeed. That's why I use Settle at < 0.5. 0.4 is also ok but I've found that below that value, it can take ages to start the next exposure and it's not worth the wait.