Tutorial:
Can dithering replace dark calibration?

Content:

• To page I, What is dithering, My hardware and software
• To page II, Setup APT, Setup PHD2 * (You are here!)
• To page III, Sigma clipping, Calculation of calibration noise

Page II

1. Setup APT
2. Setup PHD2

2.1: Setup APT

There isn't very much to setup in APT to do dithering. Maybe not necessary to put in the telescope focal length and camera pixel size. But I think it's wise and you need it for other purpose also. Here how it looks in the APT, down right:

What's more important is to find out the dithering distance. You have to do a test to see how much you have to separate your hot pixels or other patterns you want to get rid off.

In my case I found that dithering distance =3 is optimal. It works like a scaling factor to the settings in PHD2. It correspond that the mount moves about 10 to 20 pixels every dithering. A mono chrome camera maybe needs less. But if there is static column pattern then it could need very big movements. If you use the software DSS to stack the sub images there is information about how much the align process have moved the images in x and y axis.

When I increased dithering distance from 2 to 3 I got problem with the auto guiding, I had to increase the timeout to 25 seconds.

If you working with short focal lenses you maybe don't have auto guiding. Then you can change setting from PHD guiding to APT guiding in the APT guide menu. I haven't tried this but then APT communicate with the mount to force it to move (dithering) between each sub image. Don't forget to connect scope.

2.2: Setup PHD2

Push the brain symbol in PHD to come to the advanced setup menu.

Here you set the focal length of your guide telescope. For me it's the same as my main telescope because I have an off-axis guider (no guide telescope).

Here you set the pixel size. Note! The guide camera's pixel size!

With my equipment I have found out that Dither Scale =2 is the optimal setting in PHD2.

Don't forget to enable server under tools!