DSS dx dy angle values

[alacant]

Hi

What do the values mean? I'll be going great on unguided 60s exposures, then the next few will have trailing stars. As if by magic, the accuracy comes back a few snaps later. I'm guessing angle will show polar align error (?). Do the dx and dy have anything to do with polar alignment?

TIA

 

 

[wimvb]

dx and dy are the offset between the reference image (the sub with the highest score, unless you specify which sub should be the reference). Angle is how much each frame should be rotated to cover the reference.

In an ideal setup (perfect balance, perfect polar alignment, perfect guiding, etc, etc) all values should be 0, unless you use dithering (= intentional offset between frames to minimize noise).

These values don't have any relationship to polar alignment, just differences between subframes.

 

Cheers

[hjw]

Hi Alacant, in an unguided setup you will lose frames to what's called periodic error. On every revolution of the worm gear is a point where the scope slows down for a short period to be right again for the rest of the turn. In your case you see trailing stars for one or maybe two frames and when guiding you see a short period of time where the guiding system has to work harder.

Clear skies!

HJ

[wxsatuser]

I gather any flexure can also be seen with the dx dy numbers.

 

[alacant]

12 hours ago, wimvb said:

 

In an ideal setup (perfect balance, perfect polar alignment, perfect guiding, etc, etc) all values should be 0, 

These values don't have any relationship to polar alignment, just differences between subframes.

So it does relate to polar alignment?

[wimvb]

Ok, I should express myself more carefully, so here's another go.

Dx and dy relate only very indirectly do polar misalgnment. The values dx and dy are the differences in x and y coordinates between subframes. Your reference frame is the origin. If DSS needs to move another frame to match stars in the reference, it calculates how many pixels (or parts thereof) to move in x- and y-direction. This becomes the offset dx and dy, and rotation angle.

The cause of the offset may partly be due to polar misalignment, i.e. due to misalignment stars will slightly drift between exposures. This is one cause for dx and dy. But there are many other causes. If you see a periodic change of dx and dy, this may very well be caused by periodic error in the mount, as mentioned by HJ.

Because there are many possible causes for the offset, it is not possible to directly relate it to polar alignment.

Hope this clears it up somewhat.