Oh what a to-dither

[rubecula]

Just before I went on holiday I had a go at dithering.  After looking at various references on the web I did the following calculation:

Calculation of Image Scale

   Image scale = (camera pixel size in micron / scope focal length in mm) x 206.3

EdgeHD

   focal length = 2000mm

   with 0.7 focal reducer = 1400mm

Canon 550D

   Pixel size = 4.2 micron

   Pixels = 3516x5344

   Image scale = (4.2 / 1400) x 206.3 = 0.6189

GPCAM

   Pixel size = 3.75 micron

   Pixels = 960x1280

   Image scale = (3.75 / 1400) x 206.3 = 0.5526

Dithering

   Guider scale / Imager scale = 0.5526 / 0.6189 = 0.8929

   For 12 pixels move on Imager requires 12 x 0.8929 = 10.7 pixels on guide camera

   So; for Dither aggressiveness of 4 in BYE if I use dither scale of 3 in PHD the guide camera will move 4 x 3 -= 12 pixels

When it came to dithering the scope set off across the sky at a great rate of knots, with the following result:

 

 

Quite amusing at the time.  Clearly I have done something wrong, I'd be very grateful if someone could put me right.

 

[PeterCPC]

How long did you specify the calm down time in BYE to be?

Peter

[rubecula]

10 seconds Peter

[PeterCPC]

I would give it much longer - try 20 and it may need more.

Peter

[rubecula]

Thanks Peter, it's looking good here for tomorrow night so I'll give that a try.

[rubecula]

Just in case I've got anything else wrong the rest of the BYE settings are:

Dither aggressiveness = 4

Settle Dither at           = 0.25

Dither every x images  = 2

Thanks

[PeterCPC]

I would dither every image.  I would also set aggressiveness to 2. It's all a bit of an experiment really.

Peter

[StuartJPP]

I have only played around with dithering on one occasion so I am quite interested in this as I haven't had much opportunity to test it fully.

Does the mount actually slew to the dithered position or is it just affecting (i.e. adjusting) the guide rates to get there?

Is the settling time the time for the mount to actually stop moving (which APT should  know via ASCOMM) and for the vibrations to have fully settled?

I can't quite fathom why for some people it takes so long to perform a dither. At 2 meter focal length I can understand it takes longer than say 500mm for vibrations to settle but it should move ~12 pixels on the imaging sensor faster. I have read people saying that the time should be set to 120 seconds??? Personally I'd rather not dither if that is the case, eating too much into valuable imaging time. I want the dither to be over as soon as possible.

 

[rubecula]

Thanks again Peter, I'll try an aggressiveness of 2 and jack PHD2 dither scale up to 6 to give me the 12 pixel dither.  Forecast still looks clear for tonight, so I'm resigned to spending the night trying to master the dither.

Stuart, I'm a real beginner with this, so don't know how to answer your question.

[wxsatuser]

On 01/10/2016 at 16:40, StuartJPP said:

I have only played around with dithering on one occasion so I am quite interested in this as I haven't had much opportunity to test it fully.

Does the mount actually slew to the dithered position or is it just affecting (i.e. adjusting) the guide rates to get there?

Is the settling time the time for the mount to actually stop moving (which APT should  know via ASCOMM) and for the vibrations to have fully settled?

I can't quite fathom why for some people it takes so long to perform a dither. At 2 meter focal length I can understand it takes longer than say 500mm for vibrations to settle but it should move ~12 pixels on the imaging sensor faster. I have read people saying that the time should be set to 120 seconds??? Personally I'd rather not dither if that is the case, eating too much into valuable imaging time. I want the dither to be over as soon as possible.

 

My Lacerta guider dithers and the 20sec gaps between each sub seems enough at the shortish focal lengths I use, no more than 300mm.

As soon as the sub finish is triggered you can see the DEC and RA lights come on and if you listen careful the mount makes obvious slew noises.
This is normally well finished before the next sub start and the count down on the camera timer starts, it's pretty consistant.
I don't see any vibrations and the stars look pretty good as a rule, btw I run as east heavy as possible without causing mount problems.