[Shiraz (Ray)]

Originally Posted by PRejto

Hi Ray,

I brought this up in Greg's post so I appreciate the insights posted by all in this thread. I have to admit that I had not fully considered the difference between the guide camera move (in pix) vs the imaging camera. Duh. Fortunately given that my guide camera's pix are larger than the imaging camera I think it would be safe to assume that a 1 pix move of the guide camera is a greater move on the imaging camera. But, that raises a different thought to puzzle out; if the guide camera is binned 2x2 and one moves by 1 pix is that the same as moving 4 pix (or 2?) if the camera was not binned?

Peter

Hi peter. I think that 2x2 binning gives twice the movement in the image. If the guide pix are bigger than the imaging pix, I guess you get more motion in the image.

Originally Posted by gregbradley

Doesn't this also assume perfect tracking with no drift at all between subs. Is that the case with your system? Dithering implies every sub is exactly the same without a dither command. I rarely see that even with high end mounts.

If you work your PA to perfection maybe but if you are using a higher PE mount I think it highly unlikely to the extent of 1 or 2 pixels or even more.

Another point not quite related is its common practice to bin your guider 2x2 as Peter pointed out. But if you can I think there is a gain by using 1x1 binning. Perhaps not practical if its hard to find a guide star.

SBIG STi has 7.4 micron pixels. Not sure about a Lodestar. 2x2 binned would 7.4 micron pixels would be the equivalent of 15 micron pixels. So if you are using a 9 micron pixelled camera you are already over 1.6 pixels. I suppose the guide star would land within that 15 pixel band at optimum but highly unlikely it would in practice and no doubt lands somewhere within several pixels. Even with good seeing sitting watching the guide star it changes shape all the time so the centroid would have to be moving around as well.

My point is even if you aren't deliberately dithering there would still be a significant amount of dithering occurring anyway as mounts PE, polar alignment drift, flexes and seeing are causing it to move around already.


Otherwise you wouldn't need a Protrack TPoint model if it were dead still sub after sub and within a sub.

But as an approach this is good work and it makes sense to make subs not replicate each other exactly.


Greg.

Hi Greg. The point I was trying to get at is that dither may well have to be much larger than a few pixels to be fully effective. Mount errors etc will introduce some FPN decorrelation sub-to-sub and you could dial up a couple of pixels dither as well, but it could be that 3-4 pixels is nowhere near enough.

Originally Posted by Paul Haese

Ray I am using +/- 1.3 pixels of dither via guider with a maximum guide error of +/- 1 pixel with a cycle of 10 in CCDAP. What I effectively get is a little dance around with all my subs. Added to this is that I do 3 precision slews after each focus run to 23 arc seconds only, it mainly gets to 7 arc seconds. That effectively means I don't get any real overlap from one sub to another but sometimes get the occasional sub that does correlates with an earlier one. I am not sure if I am dithering enough though the total effect of pointing error with dither should be enough. Thoughts?

Sounds like it should be good Paul, provided you do regular refocus/slew. Even occasional correlation will add measurable noise though.

Originally Posted by gregbradley

I think Peter's problem is the Sky X does dither from the camera control tab not the autoguider. I imagine that would be the same as getting a sudden spike in PE and how long does it take for the guiding to settle back down. Maxim must do it differently. It sounds like SB chose the weaker method.

Greg.

This is another good issue - it is all very well to dither by a lot, but it may be a problem if the mount takes minutes to settle down again.

Thanks for the discussion. when Peter posted his question, I realised that I did not know how much dither was enough. A rough and ready analysis suggested that I should be using larger dither and it is good to get other opinions and critique - the analysis was clearly a bit too simplistic. Has anyone done any tests by any chance - I will try to do so next time the sky clears (don't hold your breath).

regards Ray