dither scale

I don't know why the descriptions of dither make it so complicated when it's a 2 click option in APT. Enter your focal length in the tools dialogue, connect your mount and set a 15 pixel dither. Unless you've the HST, that's it!

For those with HSTs who really need to complicate it, why not add PHD2 to the mix?

The default dither setting in APT doesn't produce a big enough movement, so together with the sgl and phd2 guys I worked out this for my combination of zwo120 and 700d guide and imaging cameras respectively.

m13 in hercules full frame

pixel scale summary
courtesy of the pixel scale calculator with the Canon 700D which has 4.3 micron pixels

focal length pixel scale
 1200           0.74
  760           1.17
  675           1.31
  600           1.48
  400           2.22
  200           4.43
  135           6.57
   50          17.7
   28          31.6
   18          49.2

SETTINGS for APT with PHD2
full example for 760mm telescope and 400mm guide

APT set a maximum dither of 5 could also set 1 to 4
APT dither: 5
PHD2 dither scale set at 1.0 we need to adjust this to a 15-20 pixel dither for the canon 700d

ZWO pixels: 1.93
Canon pixels: 1.17
with 240mm guide
 ZWO pixels = 3.22

therefore with a 760mm fl telescope dither on the Canon is 1.93/1.17 x 5 = 8.24 pixels, so say set PHD2 dither scale at 1.9 to give around 15 pixel dither

with the 240mm guide
therefore with a 760mm fl telescope dither on the Canon is 3.22/1.17 x 5 = 13.7 pixels, so say set PHD2 dither scale at 1.1 to give around 15 pixel dither

Here are a few combinations to get a 15 pixel dither on the current APS Canon chips

400 guide

240 guide

1200mm 1.93/0.74 x 5 = 13.1 phd2 1.2

3.22/0.74 x 5 = 21.7 phd2 0.8

675mm 1.93/1.31 x 5 = 7.3 PHD2 2.0

3.22/1.31 x 5 = 12.2 phd2 1.3

600mm 1.93/1.48 x 5 = 6.25 PHD2 2.4

3.22/1.48 x 5 = 8.9 phd2 1.9

400mm  1.93/2.22 x 5 = 4.34 phd2 3.45

3.22/2.22 x 5 = 7.25 phd2 2.1

200mm 1.93/4.43 x 5 = 2.17 phd2 5.55

3.22/4.43x 5 = 3.6 phd2 4.1

135mm 1.93/6.57 x 5 = 1.47 phd2 10.2

3.22/6.75 x 5 = 2.38 phd2 6.3

50mm 1.93/17.74 x 5 = 0.54 phd2 27.7

3.22/17.74 x 5 = 0.90 phd2 16.7

28mm lens 1.93/31.6 x 5 = 0.31 phd2 48.38

3.22/31.6 x 5 = 0.51 phd2 29.4

18mm lens
1.93/49.2 x 5 = 0.2
phd2 75.00

with 240 guide
3.22/49.2 x 5 = 0.32
phd2 46.9

crop of m13
canon 700D on bresser 152s 36 x 120s @ ISO 800 stacked with kappa-sigma in DSS
and goodbye dark frames