Excessive dithering?

[edarter]

Ok, so I bough an Altair Astro 26C a few weeks ago at the PAS but for a few reasons (I'm looking at you British weather!!) I've not yet used it. However, tonight is looking clear, its a 1% moon and so a shakedown test looks to be in order. However, I'm toying with the idea of changing some of my historical practices for this new camera and one of those is how often I dither. I dither after every frame. Is there such a thing as too much dithering? I guess some imaging time is lost with settle time after dither etc but do the benefits outweigh the drawbacks?

Also - I have always used 4 minute subs with my modded DSLR, what are the thoughts on reducing that to 2 or 3 minutes to reduce possibility of guiding errors etc?

Thanks
Ed

[OK Apricot]

I've read somewhere (CN I think) to aim around the one dither every 5-6 minutes of exposure. So if I'm doing 5min exoposures I'll dither every frame. It costs some time on the mount settling but is what it is. If 2 mins I dither every 3.

[ONIKKINEN]

You definitely dont need to dither every frame with an IMX571 sensor camera. Perhaps that was the case with DSLRs but not here.

I have been decreasing my dithering frequency for a while now, i think im now at one dither per 8 subs at 120s each. If you have polar alignment error or cone error, which will result in a slight drift in field rotation then you might need to dither more frequently, otherwise dont bother since you lose a considerable amount of imaging time to mount settling.

[david_taurus83]

Every 3 or 4 subs us fine with that sensor. Say 1 or 2 dithering per 10 minutes is what I work with. I never reject frames because of satellite streaks and they always get removed along with hot pixels. I had a 2600MC and have a 533MM. No amp glow but plenty of hot pixels.

[Vroobel]

17 minutes ago, ONIKKINEN said:

If you have polar alignment error or cone error, which will result in a slight drift in field rotation then you might need to dither more frequently, otherwise dont bother since you lose a considerable amount of imaging time to mount settling.

Could you please explain why? IMHO, any drift brings exactly what the dithering does. 

BTW, I do it only when my setup is undersampling, but 1 per every 3 x 300s (300s Ha, 300s SII, 300s OIII) or so. 

Edited May 7 by Vroobel

[Ags]

Drift unfortunately is not random, so it results in walking noise. Dither is random, so it reduces noise overall.

[ONIKKINEN]

1 hour ago, Vroobel said:

Could you please explain why? IMHO, any drift brings exactly what the dithering does. 

BTW, I do it only when my setup is undersampling, but 1 per every 3 x 300s (300s Ha, 300s SII, 300s OIII) or so. 

It is like @Agssaid above, that constant small drift will bring walking noise with it which will be almost impossible to process away. Dithering will break that pattern.

[TiffsAndAstro]

1 hour ago, OK Apricot said:

I've read somewhere (CN I think) to aim around the one dither every 5-6 minutes of exposure. So if I'm doing 5min exoposures I'll dither every frame. It costs some time on the mount settling but is what it is. If 2 mins I dither every 3.

This is also what I've gleaned from various YouTubers/interwebs. How accurate or scientific it is seems hard to know.

[Vroobel]

1 hour ago, Ags said:

Drift unfortunately is not random, so it results in walking noise. Dither is random, so it reduces noise overall.

 

41 minutes ago, ONIKKINEN said:

It is like @Agssaid above, that constant small drift will bring walking noise with it which will be almost impossible to process away. Dithering will break that pattern.

 

That makes sense, thanks. I've never met this problem because I always do the PA as best as I can. 🙂

[900SL]

I vaguely recall that you should aim for at least ten dithers during an imaging session, preferably more. And if drizzling, you need to dither every frame

If I am shooting 30s to 1m exposures, I dither every ten frames, and aim for 20+ dithers. If shooting Ha narrowband 5 - 6 minute exposures, every 1 - 2 frames.

Dithering is invaluable for dealing with walking noise.

 

[900SL]

7 hours ago, Vroobel said:

Could you please explain why? IMHO, any drift brings exactly what the dithering does. 

BTW, I do it only when my setup is undersampling, but 1 per every 3 x 300s (300s Ha, 300s SII, 300s OIII) or so. 

Drift due to PA error will be in one direction, so hot pixels, colour mottling etc will appear to drift across successive frames (like streaks on a dirty window) and this is difficult to address by stacking. Dithering about both RA and Dec axis moves these at random and allows stacking to remove the outliers

[edarter]

Thanks all, much appreciated!