Dithering time wasted?

[Gerry Casa Christiana]

Hello all

Ive been through this a few times but I thought I would look at what other people think on this. 

Is the time spent on dithering really worth it? Or is it better to use that time to get more subs? 

It seems to me in my experience that im spending at least 2 minutes between shots for dithering. When I look back through my pictures when I wasn't even guiding they look much better. One thing I have changed is not using darks anymore. I used to always use them. Now I use flats and bias shots but they seem noisier than my older shots.

What sort of times are others getting in dithering? Is there a way to speed it up? I'm using APT. EQMOD PHD2 eq6. 

Many thanks 

Gerry

 

[spillage]

My opinion is this for what is worth. 

Dithering: couple of minutes in between lights. No darks needed.

No Dithering: Darks needed so I then have to match the light exposure times with darks. That may be 5 mins or 10 mins. I also have to load them into dss if you really want to add up the extra time it takes. This also takes up more space on the hard drive.

Are you using a dslr?

Dithering works for me buy maybe you will find you need to take darks.

cheers

Spill.

[tooth_dr]

Ive recently discovered dithering, and am using a DSLR.  Although I've only captured one image (16 x 600s), it was much smoother than previous images of equal or more data, and there were no darks to worry about.  I allow the camera 30-60 seconds between exposures anyway so it isnt really much more wasted time.

[PeterCPC]

I use dithering with Backyard EOS and I would doubt that it's more than 40 secs between exposures.

Peter

[geordie85]

1 hour ago, Gerry Casa Christiana said:

Hello all

Ive been through this a few times but I thought I would look at what other people think on this. 

Is the time spent on dithering really worth it? Or is it better to use that time to get more subs? 

It seems to me in my experience that im spending at least 2 minutes between shots for dithering. When I look back through my pictures when I wasn't even guiding they look much better. One thing I have changed is not using darks anymore. I used to always use them. Now I use flats and bias shots but they seem noisier than my older shots.

What sort of times are others getting in dithering? Is there a way to speed it up? I'm using APT. EQMOD PHD2 eq6. 

Many thanks 

Gerry

 

The big question here since you claim your images are better without guiding is are your subs the same length in exposure time? If not, then this could very well be why you think your images were better before guiding. Are you using the same setup?

[Stub Mandrel]

Mr Penrice will be along in a bit to say 'no darks needed, but do use bias frames or a hot pixel map'.

[swag72]

I set SGP to dither every 5 exposures..... So that would cut down on your time between dithers. Also the time between my dithers is probably about 20s..... Perhaps you can change dither settings?

[brrttpaul]

being new to this how do i set sgp to dither every 5 exposures? I am pretty sure i have dithered but not for every 5 or whatever

[StuartJPP]

As mentioned in other posts above, try to get your dither time down to sub 30 seconds and it won't be much of an issue. At 2 minutes I'd say you would be wasting imaging time.

 

[swag72]

Just now, brrttpaul said:

being new to this how do i set sgp to dither every 5 exposures? I am pretty sure i have dithered but not for every 5 or whatever

It is part of SGP ...... You can change the size of the dither depending on your focal length as well as setting to dither every x subs.

[steppenwolf]

I have found dithering to be a very worthwhile exercise indeed and I dither between each exposure. It takes about 45 seconds so I am surprised at your two minute figure and when I'm taking 1800 second exposures, this is a minor concern. I like the fact that the stacking algorithm is giving me full value for money and my images are entirely hot pixel free. Perhaps your 2 minute experience includes the time it takes to re-acquire the guide star and that is slowing you down?

[spillage]

35 minutes ago, StuartJPP said:

As mentioned in other posts above, try to get your dither time down to sub 30 seconds and it won't be much of an issue. At 2 minutes I'd say you would be wasting imaging time.

 

Edit: Sorry didnt realise op also stated 2 mins. Think I'm have a moment.

Sorry this was just plucked out of the air as a comparison. I admit a poor one based on at least a 5 min exposure and as he uses apt I think the default time out is 180 seconds. But again not at home so this is also from memory.

Dither=No Darks. But please do not take my word for it just give it a go.

[ollypenrice]

32 minutes ago, Stub Mandrel said:

Mr Penrice will be along in a bit to say 'no darks needed, but do use bias frames or a hot pixel map'.

No, but he may be along to say that this is the system which works best for him! I've no idea how well it will work for others. Actually we can't dither on the dual rig because we run different exposure times in the two parallel scopes. Even so the bias-as-dark and bad pixel map system works fine.

I also know about the lost time problem because, try as we might, we could never persuade PHD/Nebulosity to dither while shooting at very high res through an off axis guider and Yves' 14 inch Dall Kirkham. It would happily have spent all night trying to re-settle for the second sub and in the end we gave up.

However, if the OP is losing only two minutes between subs and is following Tony Hallas by going for a 12 pixel-ish dither then I'd very very surprized if this didn't give the better result. I'd be inclined to look into how to get the dither to settle faster. I can't help on that, though.

Olly

[spillage]

This previous thread from here might help.

[steppenwolf]

48 minutes ago, ollypenrice said:

However, if the OP is losing only two minutes between subs and is following Tony Hallas by going for a 12 pixel-ish dither

What I love about SGL and the members on it is that they all have different ways of doing stuff!! I set a maximum dither of just three pixels, working on the basis that if the hot pixel is in a different place in each frame - even by only a small amount - it will be viewed as an 'outlier' pixel by the stacking algorithm. Any thoughts on why Tony Hallas recommends a much larger dither?

[ollypenrice]

16 minutes ago, steppenwolf said:

What I love about SGL and the members on it is that they all have different ways of doing stuff!! I set a maximum dither of just three pixels, working on the basis that if the hot pixel is in a different place in each frame - even by only a small amount - it will be viewed as an 'outlier' pixel by the stacking algorithm. Any thoughts on why Tony Hallas recommends a much larger dither?

You're aiming to remove hot pixels, Steve. Tony Hallas is also trying to remove what he calls 'colour mottle,' the medium-scale colour irregularity seen in so many DSLR background skies. They tend to be very 'colour busy' rather than neutral grey. So the large scale dither benefit doesn't apply to CCD cameras, but to DSLRs.

Olly

[steppenwolf]

19 minutes ago, ollypenrice said:

Tony Hallas is also trying to remove what he calls 'colour mottle,' the medium-scale colour irregularity seen in so many DSLR background skies. They tend to be very 'colour busy' rather than neutral grey. So the large scale dither benefit doesn't apply to CCD cameras, but to DSLRs.

Aha, perfect sense - don't have this problem, of course, with the image type that I capture.

[Gerry Casa Christiana]

Just going through all your messages. Thanks for all the advice just to answer a couple of things. I'm using a autoguider now and so I'm doing longer exposures. My setup is the same except for that. 

Yes my images before was when I was using ISO 3200 for 120 seconds without guiding a no dithering but always darks.

Ok let's ask it like this for dummies. 

On APT my dithering is set to 15. I am guessing this is 15 pixels? I seem to remember that's wrong.  Is there somewhere in PHD2 where I can define dithering there? I have a field of view of 102 x 67.8 arc minutes. That's with a Canon 550d and a Skywatcher 150pds. 

In a word. HELP!

Thanks for all your input. Brilliant!

Gerry 

 

[Gerry Casa Christiana]

6 hours ago, spillage said:

My opinion is this for what is worth. 

Dithering: couple of minutes in between lights. No darks needed.

No Dithering: Darks needed so I then have to match the light exposure times with darks. That may be 5 mins or 10 mins. I also have to load them into dss if you really want to add up the extra time it takes. This also takes up more space on the hard drive.

Are you using a dslr?

Dithering works for me buy maybe you will find you need to take darks.

cheers

Spill.

Yes I'm using a Canon 550d on a Skywatcher 150pds and a eq-6. 

[Gerry Casa Christiana]

2 hours ago, steppenwolf said:

Aha, perfect sense - don't have this problem, of course, with the image type that I capture.

Yes I've seen that video too in fact it was that that started me dithering. I think though I have more colour mottle than before that's the thing!

before no guiding and 2 minute subs. 

Here is Lagoon taken last month

Gerry

[spillage]

39 minutes ago, Gerry Casa Christiana said:

Yes my images before was when I was using ISO 3200 for 120 seconds without guiding a no dithering but always darks.

On APT my dithering is set to 15. I am guessing this is 15 pixels? I seem to remember that's wrong.  Is there somewhere in PHD2 where I can define dithering there? I have a field of view of 102 x 67.8 arc minutes. That's with a Canon 550d and a Skywatcher 150pds.

With your camera I would start at 800 iso and then go up or down one to see if this helps.

I am sure if you are running phd to guide then you should choose phd in the dithering settings as the guiding program and set dithering to 3 first of all and may up it if needed.

 

[michael8554]

If you want to dither 12 pixels on your imaging camera, you need to work out how many pixels PHD needs  to move the guidecam.

Imaging pixel scale = say 2 arcsecs/pixel, so 12 pixels is 12 x 2 = 24 arcsecs

Guide pixel scale = say 8 arcsecs/pixel

So to move the guidecam 24 arcsecs, the guidecam needs to move 24/8 = 3 pixels

The PHD dither range is:

Dither Level     |       Pixels

---------------           ---------

1       -                +/-  0.5  x  Dither Scale   

2       -                +/-  1.0  x  Dither Scale

3       -                +/-  2.0  x  Dither Scale  

4       -                +/-  3.0  x  Dither Scale

5       -                +/-  5.0  x  Dither Scale

So to dither the guidecam 3 pixels you could select 2 in Dither Level, and 3 in the Scale box.

Or 4 in Dither Level, and 1 in the Scale box.

I think I've got that right........

Michael

Edit:  If your dithering is taking 2 minutes to recover it's possible you're dithering too much.

Mine usually takes 45 secs max to recover.

[wimvb]

I'm probably missing something, but as far as I can tell you're trying to compare the following:

1. No guiding, no dithering, 2 minutes exposures, ?? number of subs, good astro-darkness & cold (Orion image, so probably winter)

2. Guiding, dithering, ?? minutes exposures, ?? number of subs, no proper astro-darkness & not cold (Lagoon, "last month" = July)

Others have already commented on time for dithering to settle down. But if you're comparing noise, I think you've changed so many parameters, that a fair comparison is impossible. The increase in noise can be explained by the absence of astro darkness and the higher temperature.

[alacant]

Hi. Here are the APT settings for my 700d on an 818mm focal length telescope guided by a 60mm f4 guidescope and an asi120mm via PHD2 dither scale=1.0, giving random dither up to + or - 15px. The next frame starts in under a minute.

I find dark frames introduce more noise and so use light, bias and flat frames with dark frames for the latter only. To see the benefit of the dither, You need at least 20 (?) light frames. HTH.

[Gerry Casa Christiana]

1 hour ago, alacant said:

Hi. Here are the APT settings for my 700d on an 818mm focal length telescope guided by a 60mm f4 guidescope and an asi120mm via PHD2 dither scale=1.0, giving random dither up to + or - 15px. The next frame starts in under a minute.

I find dark frames introduce more noise and so use light, bias and flat frames with dark frames for the latter only. To see the benefit of the dither, You need at least 20 (?) light frames. HTH.

Ok here is mine as it is now. What do you think?