When to dither?

[rnobleeddy]

I've been using a DSLR so followed the advice to dither, but looking for advice with new cameras.

 

I tend to take 300s subs and dithering adds about 30s to the time between subs (yes, this is a long time, but large DEC dithers upset guiding and it can take this long to settle).

I've got a couple of new (to me) CCD cameras - one has setpoint cooling, the other doesn't, so just cools to around 30 degrees below ambient.

The question is would you dither for either or both of these cameras? Both appear to have pretty low dark current compared to my DSLR. 

My initial guess is:

- don't dither with the set point cooling, but create a dark library and use that

- dither with the camera that lacks set point cooling, and don't take darks

 

Anyone suggest something else? The random dark noise appears to be pretty low for both, but each had a small number of hot pixels on test darks. Obviously dithering can never be a bad idea, but  it's costing me 10% of my exposure time.

Edited December 5, 2020 by rnobleeddy

[wimvb]

Between exposures. 😋 Dither in a random direction or spiraling pattern, about 12 pixels.

It’s easiest to do if you also guide. If not and you’ve got patience, you can dither with your hand control. At a slew rate of 1x sidereal, press RA+/- or DEC+/- for as many seconds as you have arcseconds/pixel. Usually 1-2 seconds.

[rnobleeddy]

Apologies - I wrote this on my phone, accidentally pressed submit then the battery died. I've actually written the post now I'm home!

[vlaiv]

I would say dither always if you can.

Not going to go into math - but dithering reduces noise regardless of other things (dark calibration, etc ...), so it's worth doing always.

I know it adds time and if it's that much time - maybe dither every other or every third frame.

Btw, I also advise full/proper calibration (which means set point cooling, or in case it is not available - at least dark optimization in case camera has stable bias).

[wimvb]

4 hours ago, rnobleeddy said:

I've been using a DSLR so followed the advice to dither, but looking for advice with new cameras.

 

I tend to take 300s subs and dithering adds about 30s to the time between subs (yes, this is a long time, but large DEC dithers upset guiding and it can take this long to settle).

I've got a couple of new (to me) CCD cameras - one has setpoint cooling, the other doesn't, so just cools to around 30 degrees below ambient.

The question is would you dither for either or both of these cameras? Both appear to have pretty low dark current compared to my DSLR. 

My initial guess is:

- don't dither with the set point cooling, but create a dark library and use that

- dither with the camera that lacks set point cooling, and don't take darks

 

Anyone suggest something else? The random dark noise appears to be pretty low for both, but each had a small number of hot pixels on test darks. Obviously dithering can never be a bad idea, but  it's costing me 10% of my exposure time.

Ok, this makes more sense.

I would say, with a cmos, always dither. With a classical ccd, dithering is still good practice, but not as critical. If you don't dither, and your guiding is spot on, then hot pixels will always be in the same position in your registered (aligned) subs, and the rejection algorithm in stacking, doesn't have any sigma to work with. For ccd, a bad pixel map will remove hot pixels. But as Vlaiv said, you can dither every second or third frame. Cmos cameras work best with many, short exposures, and dithering will cost a lot of imaging time. But dithering every 3 subs is doable.

[KP82]

Theoretically dithering every sub is optimal if you can. However with inexpensive mounts most of us have, doing so would waste huge amount of time. Therefore I normally set dithering to every 3 subs if my total light subs isn't likely to exceed 50. If I go for more shorter exposures, then probably dither every 5.

Edited December 6, 2020 by KP82

[rnobleeddy]

On 05/12/2020 at 22:27, vlaiv said:

I would say dither always if you can.

Not going to go into math - but dithering reduces noise regardless of other things (dark calibration, etc ...), so it's worth doing always.

I know it adds time and if it's that much time - maybe dither every other or every third frame.

Btw, I also advise full/proper calibration (which means set point cooling, or in case it is not available - at least dark optimization in case camera has stable bias).

On 06/12/2020 at 00:16, KP82 said:

Theoretically dithering every sub is optimal if you can. However with inexpensive mounts most of us have, doing so would waste huge amount of time. Therefore I normally set dithering to every 3 subs if my total light subs isn't likely to exceed 50. If I go for more shorter exposures, then probably dither every 5.

 

On 05/12/2020 at 23:36, wimvb said:

Ok, this makes more sense.

I would say, with a cmos, always dither. With a classical ccd, dithering is still good practice, but not as critical. If you don't dither, and your guiding is spot on, then hot pixels will always be in the same position in your registered (aligned) subs, and the rejection algorithm in stacking, doesn't have any sigma to work with. For ccd, a bad pixel map will remove hot pixels. But as Vlaiv said, you can dither every second or third frame. Cmos cameras work best with many, short exposures, and dithering will cost a lot of imaging time. But dithering every 3 subs is doable.

Thanks. Dithering every 3rd frame or so seems like a decent solution.