Strucured noise in Canon 1000D

[old_eyes]

Working on data captured with my Canon 1000D, I notice that when I am trying to dig out detail there is a structure to the noise on teh camera. Two things I see regularly. Amp glow I understand and can cope with, but there is also noise as fine diagonal lines running from top right to bottom left of the image.

Below is an example after stacking in DSS, stretched to buggery and with the black point independently set for each colour channel to bring the peaks back into line. The sample has been blown up to 400% to show the noise clearly. Lots of colour noise in there.

I think I have seen a similar effect in other images people have created with Canon DSLR's. Can anyone shed any light on where it comes from, and can anyone suggest how to minimise its effects? It is definitely a limiting factor on how far I can push my data.

old_eyes

[Psychobilly]

Im not sure and probably wrong but it could be down to mismatched darks...are you taking them and how are you matching them...?

I havent used them for ages... but then again I used to do most of my imagign in the colder winter nights...

you might find this thread in another place interesting...

http://www.astronomyforum.net/astrophotography-forum/102645-matching-darks-easily-dark-library.html#

Peter...

[old_eyes]

Im not sure and probably wrong but it could be down to mismatched darks...are you taking them and how are you matching them...?

Peter...

Peter - thanks for the link. That certainly describes my problem. I am collecting images using APT so I get the temperature recorded with the EXIF data. I have been trying to use a library I created by sticking the camera in the to get a long series (suggested to me at SGL6). I will chekc how close it is the the recorded temperatures for that run.

How close do you need to be in temperature for darks to work properly?

old_eyes

[Psychobilly]

Pass I stopped using Darks ages ago because of the streaking problem...

I used to shoot loads of subs with a hint of deliberate mis-alignment on the mount which meant there was a slight pixel shift between subs...and use statistical stacking methods...

Used to work for me... well I thought so anyway

Peter...

[roundycat]

Very close!

What you are seeing might be sensitivity variations in the bulk silicon in which case proper flats should take care of it.

I understand 'stretching to silly lengths' but if you are having to do that it points to one thing - serious underexposure.

Dennis

[old_eyes]

Very close!

What you are seeing might be sensitivity variations in the bulk silicon in which case proper flats should take care of it.

I understand 'stretching to buggery' but if you are having to do that it points to one thing - serious underexposure.

Dennis

Dennis - I didn't use flats and that may be part of the problem. Although I would be surprised if the variations in electronic properties of he silicon could create such a regular pattern. I suppose the manufacturing process could introduce a patterned variation.

I drastically over stretched the image to show the pattern noise. It was a much greater stretch than I would employ with the quality of data I am getting. However, it is always there in the background limiting what I can do with the data. Best I have been able to produce with dimmer subjects are things like the Leo Triplet here.

I think that fundamentally you are right; a better signal to noise ratio is needed. But if I can make the most of the data I do have, it can only get better with more exposure.

Thanks

old_eyes

[old_eyes]

Im not sure and probably wrong but it could be down to mismatched darks...are you taking them and how are you matching them...?

Peter...

Memo to self. Read what you write and see if it makes sense . What I meant was:

Peter - thanks for the link. That certainly describes my problem. I am collecting images using APT so I get the temperature recorded with the EXIF data. I have been trying to use a library I created by sticking the camera in the fridge to get a long series (suggested to me at SGL6). I will check how close it is the the recorded temperatures for that run.

old_eyes

[old_eyes]

Pass I stopped using Darks ages ago because of the streaking problem...

Peter...

No darks! Isn't that heresy?

old_eyes

[old_eyes]

OK. Next check.

I have taken a single original sub and stretched it in the same way as the stacked image. Lots of noise, but no strong structure to it. SO the siagonal lines come in somewhere in dark subtraction and stacking process.

old_eyes

[Psychobilly]

No darks! Isn't that heresy?

old_eyes

My friend I'll say it clear

I'll state my case of which I'm certain

I've lived a life that's full

I traveled each and every highway

And more, much more than this

I did it my way

Regrets I've had a few

But then again too few to mention

I did what I had to do

And saw it through without exemption

I planned each charted course

Each careful step along the byway

And more, much more than this

I did it my way

Yes there were times I'm sure you knew

When I bit off more than I could chew

But through it all when there was doubt

I ate it up and spit it out, I faced it all

And I stood tall and did it my way

I've loved, I've laughed and cried

I've had my fill, my share of losing

And now as tears subside

I find it all so amusing

To think I did all that

And may I say not in a shy way

Oh no, oh no, not me

I did it my way

For what is a man what has he got

If not himself then he has not

To say the things he truly feels

And not the words of one who kneels

The record shows I took the blows

And did it my way

Yes it was my way

Now where have I heard those words before....:icon_scratch:

[old_eyes]

"Regrets I've had a few" certainly sounds familiar .

Just to keep up the story, I have just gone back and reprocessed my Leo Triplet image without darks. The diagonal stripes were certainly less of an issue. I'm not sure the image is much better, but it certainly ain't any worse!

Hmmm! Interesting!

Thanks for the tip .

old_eyes

[Psychobilly]

In my experience with my camera...

My 1000D doesnt suffer from ampglow... well not unless you use liveview to achieve mirror lock up which is soemthing i never bohter with when using the obs setup...

Primary Colour Hot pixels or Hot Pixel snakes asre easy to spot and clean up... I usually use the tools in DSS but others haev problems with this removing star centers leaviing dark holes...

For me The £no Darks" Heresy works if you have lots of exposure and dither (or in my case allow a tiny amount of drift) between subs an use a Statistical (outlier) stacking method rather than average or median...

And by not using them I dont have to worry if they are tmeperature matched...

I am probably looking to "Box Cool" the 1000D... or maybe get a 1100D...

Peter...

[roundycat]

take some bias frames, stack and then stretch them. See if the noise is mainly in the bias. I would suggest 1/10s for the bias, no need to go ultra short with the exposure.

Dennis

[old_eyes]

take some bias frames, stack and then stretch them. See if the noise is mainly in the bias. I would suggest 1/10s for the bias, no need to go ultra short with the exposure.

Dennis

Will do. We'll see what comes out.

old_eyes

[Psychobilly]

take some bias frames, stack and then stretch them. See if the noise is mainly in the bias. I would suggest 1/10s for the bias, no need to go ultra short with the exposure.

Dennis

Just wondering ...

What's the thinking behind the longer bias?

Traditionaly DSLR Bias frames have been at the shortest possible exposure time so 1/4000s for the 1000D....

Billy....

[AndyUK]

Old_Eyes, I had / have the same problem with horizontal banding with my 40D.

I took stock darks in the fridge expecting them all to have a temp of about 3 or 4C... I used them religiously (as everyone said you had to use darks), but then noticed some very nasty effects creeping in as the temp rose. I suspected that I needed some better temp-matched darks, but then came across the "no-darks" team using bias and flats only, which actually does seem to improve the results... in many cases. However, I haven't joined the "no-darks" team entirely - I now produce two versions out of DSS, one with and one without darks and then see which is best. For me I find that it's sometimes best with, and sometimes best without...

However, as part of the exercise, I looked at the exif temps of the fridge darks and found that they weren't shot at 4C after all - Over the first few frames the chip heats up, and despite leaving a 30-60s gap between exposures, the darks from 90s+ levelled out at 20C... which is also what seems to happen when imaging - The first frame temp may start at ambient (say 6C), but then rises to 20C after the 2nd or 3rd 10 minute frame.

Although I've now also bought a setpoint CCD cam, I've started on a dry cloudy-night project of trying to put together sets of temp-matched DSLR darks so that eventually I can get a range of sets of 20 between 4-20C and then by checking the temp of the subs, I can either choose which single dark set to use or, if the temps of the subs change significantly (as I expect them to), I can use different tabs in DSS to subtract the appropriate temp darks from a subset of the frames (I haven't found out if there's a maximum number of tabs in DSS though )

I figure that it's worth a go anyway... and besides, as it would appear that our fellow-members who live in much warmer climes don't seem to complain about this issue, maybe it's just that we're taking short cuts by using "stock" darks(?). Olly has also mentioned that one of his visitors has used reusable flexible gel cool pads to good effect, so I thought I'd give that a try as well...

[swag72]

So let me get this right in my head - if I take flats and bias frames then I can ditch the darks? That sounds like a wonderful idea, as long exposure darks at the same temperature are a royal pain in the butt!!

[Psychobilly]

Nope ..Just find out what works for you... especially as your in warmer climes...

Sometimes mismatched darks may be worse than none at all...

Peter...

[roundycat]

I read about the not-so-short bias exposures on a 'sensible' web site, CCD New Astro or SBIG, something like that but I cannot for the life of me remember what was given as the reason. It did sound entirely sensible or I would not have remembered the idea.

On the face of it shooting sub millisec exposures is maybe going a bit too far. When the shortest 'normal' exposure is, maybe, 60s for a cluster or PN then 0.1s for Bias is hardly going to be corrupted. No thermal pixels are likely to be generated either.

Dennis

[Psychobilly]

Cheers Dennis

Billy on the San Fran

.....