Why does the image move?

[Eruliaf]

OK, so first of all sorry for the title as I could not think of how to describe this correctly (there is probably a name for it), secondly - not sure if it is in the correct thread.

When I am taking images with my DSLR I note that as the images progress they are moving in a zigzag/non uniform pattern across the screen - that is as you scroll through the images you can see them moving say down and left a bit, then up and left a bit which then repeats. I have noticed this over a set of 40 images I have just taken and you can see the target moving across the screen.

I spent some time balancing and aligning my scope tonight, which I was pretty happy with (still a beginner though), but if I have not aligned it correctly would this happen or would it only scroll in one direction?

To assist with understanding my very poor description, I have created a GIF of the 40 image set.

Any assistance greatly appreciated.

 

[ollypenrice]

You're not guiding? 

The GIF won't play for me but if the stars return, more or less, to the same point after a wander around then your PA is not the main problem. More likely you'll be getting a mixture of periodic error and backlash in RA and backlash in Dec. Perfect balance is not to be desired on a mount with backlash. It is better to run slightly east heavy in RA so the drive is always pushing against a resistance rather oscillating across the backlash. Running out of balance in Dec is harder because being, say, camera end heavy is OK till you get near the zenith but then the imbalance reduces to zero at the zenith because the scope tube is vertical.

Olly

 

[michael8554]

Unintentionally (I assume) you have done a Good Thing called Dithering.

With dslr images this randomises the inherent noise pattern, because different areas of the noise pattern are stacked in each image.

As for the diagonal movement, it is due to your mount not exactly following the apparent path of the stars across the sky, due to imprecise Polar Alignment, this is not the same as the Star Alignment of the mount.

There is also an amount of Periodic Error and Dec movement.

I'm puzzled by the large jumps between the images, which have mostly round stars, were there large pauses between images?

I would google for explanations of the names mentioned if they're new to you.

Michael 

[Mered]

I've had this before, sometimes it's better, sometimes it's worse, but it's usually better when I have better polar alignment. I have a guided setup, so there's probably some flexure in there too.

Last time I really looked into it, I sorted the images by dX and dY in DSS and it clearly shows the movement. You can then have a go at working out what is due to PE and what is due to polar alignment.

As said above, at least you get some dithering from it!