Odd dithering behaviour...

[Adam1234]

Does anyone know what might be causing this odd dithering behaviour I see practically every imaging session? I doesn't happen on every dither, but I do see it often.

So basically every so often (actually happened on 2 dithers in a row just now while I was outside at the scope), I get a very large dither like the one in the image below. It then doesn't settle and times out (I have timeout in APT set to 120s which is normally plenty). The result is then APT continues with the next exposure, the stars of which then sometimes appear misshapen. 

In the example below, it timed out at 120s about midway through the 'dither'. 

 

When I go back outside to check on the scope I'll get a screenshot of a (hopefully) normal looking dither for comparison.

 

Cheers 

Adam

[Adam1234]

Here's a screenshot i got just now of one of my more 'normal' looking dithers

 

[Jamgood]

What are your Dither settings in APT?

When I had mine set to small dithers (around 2) I used to get correction errors like you're seeing. Between 7 and 12 seems to be the right area for my mount. 

[david_taurus83]

Backlash in declination. It make a large movement in dec and spends the remainder of the settle time clearing backlash. The mount only then moves to the new position after APT has started the next exposure. I get this if I dither with my CEM25. RA is fine no matter how big the movement but Dec is bad if its too big.

[Adam1234]

7 hours ago, Jamgood said:

What are your Dither settings in APT?

When I had mine set to small dithers (around 2) I used to get correction errors like you're seeing. Between 7 and 12 seems to be the right area for my mount. 

Here's my dither settings ( I stand corrected on dithering timeout, 60s, not 120s). 

[Adam1234]

26 minutes ago, david_taurus83 said:

Backlash in declination. It make a large movement in dec and spends the remainder of the settle time clearing backlash. The mount only then moves to the new position after APT has started the next exposure. I get this if I dither with my CEM25. RA is fine no matter how big the movement but Dec is bad if its too big.

 

That would make sense.  It seems there is a tick box in PHD2 to dither in RA only, I guess it might be worth turning that on?

[david_taurus83]

5 minutes ago, Adam1234 said:

 

That would make sense.  It seems there is a tick box in PHD2 to dither in RA only, I guess it might be worth turning that on?

That or decrease the size of the dither. I actually asked on the PHD forum if they could implement a scale for both axis. So we can set RA at a large dither and Dec with a small dither. I didn't think this was an unreasonable suggestion given that most hobby mounts out there probably suffer from backlash in declination. The general response I got was "why would you want to do that" or "just dither in RA". I didn't ask again...😒

Now I just don't bother dithering at all as I can clean up any hot pixels on my Atik 460 with a bad pixel map.

[smashing]

9 hours ago, Adam1234 said:

Does anyone know what might be causing this odd dithering behaviour I see practically every imaging session? I doesn't happen on every dither, but I do see it often.

So basically every so often (actually happened on 2 dithers in a row just now while I was outside at the scope), I get a very large dither like the one in the image below. It then doesn't settle and times out (I have timeout in APT set to 120s which is normally plenty). The result is then APT continues with the next exposure, the stars of which then sometimes appear misshapen. 

In the example below, it timed out at 120s about midway through the 'dither'. 

 

When I go back outside to check on the scope I'll get a screenshot of a (hopefully) normal looking dither for comparison.

 

Cheers 

Adam

I started getting the exact same thing so I'm glad you've asked the question I'm going to try only in RA and see how it goes.