Canon darks...

[perfrej]

Folks!

I've been thinking...

1.

Has anyone tried to use the internal darks frame functions in astrophotography (1000D or simular). Before I started to fiddle with the settings, Nebulosity would say "Exposing...", then "Subtracting internal dark frame", then "re-constructing image".

2.

Wouldn't it be great if the software could tell the canon DSLR to fire up the CCD and expose - without opening the shutter? That would be an awesome way of catching matching darks during capture, one for each light, all with relevant temperatures.

Can APT do this?

/per

[Psychobilly]

Wouldnt this be the same as leaving the in camera long exposure noise reduction switched on...?

Whilst this approach should give the best possible long exposure noise reduction you halve the number of lights that can be taken and its only the lights that contain the useful signal...

Peter...

[adamsp123]

Personally I did a set of dark libraries, using various exposure times that I normally use and did a series at 10, 5 and zero degrees C.

I used my fridge for the 5C (main compartment) and zero I used the Ice box, after checking the temperature with a thermometer, the 10C I did one evening when the air was 10C !!

I found that nearly all runs with my Canon 1000D are close enough to one of these figures to be used in the stacking routine.

[MG1]

One advantage of APT is that it can record the chip temp into the file name so it's easy to generate a whole load of darks in the fridge and match them up.

If you don't have APT you can use this :

ExifTool by Phil Harvey

then just drag the file over to review the hidden exif data.

HTH

Michael

[Glider]

As Psychobilly says, with the shorter nights and the way the weather has been lately imaging time is short. Grab as many lights as you can and do the darks at your leisure later in the fridge as per adamsp123.

[MG1]

You will find however that even with a constant air temperature the chip temp can vary significantly. A recent session gave me files with chip temperatures between 3C and 26C when the air temperature at the scope varied by only 2-3 degrees across the session.

This will allow you to match the darks with the lights and not waste good sky time taking darks.

[Glider]

Indeed. Ivano had a very interesting thread on this temperature difference.

[johnb]

Personally I did a set of dark libraries, using various exposure times that I normally use and did a series at 10, 5 and zero degrees C.

I used my fridge for the 5C (main compartment) and zero I used the Ice box, after checking the temperature with a thermometer, the 10C I did one evening when the air was 10C !!

If i take a dark in the fridge and its say 300 secs at 5 Deg can I simply copy the file say 9 times giving me 10 darks at 300 Secs and 5 Deg - Im assuming not but not sure why ?

John B

[Demonperformer]

Darks are meant to capture the 'random' noise generated. If you take one and copy it nine times, you are not getting a random result, you are enhancing the specific noise of one sub.

[MG1]

I don't think so. There are slight differences each time and this is why you take multiple images and the software averages them.

Part of those differences will be due to the fact that after the first shot the chip is warmer and unless you wait until it has cooled each time you will find that the chip temperature (and this is the important variable, not the air temp) will change through the series you are taking.

I would recommend APT so that you can access this EXIF data.

If you are going to the trouble of creating temperature specific darks, there seems little point in not getting it as accurate as possible.

Using this chip data is the closest you're likely to get to achieving the accuracy of set point cooling with a DSLR. Even if you do the Peltier mod (which is quite destructive as far as I can tell) you still will want to know the actual temperature, not the average air temperature at the time the light, or dark was taken.

Sorry if I'm labouring the point.

Michael

[johnb]

thanks, just thought I would ask.

John B

[johnb]

So im off, will take say 50 x 300 in the main comp of the fridge then another 50 x 300 secs in the cool box and then put them all in a dark library folder and then use software callled Dark Library which from what i understand will let you load a DSS file list in and it will suggest the best matching Darks based on Temp, exposure length and ISO and it will append those files to the DSS list

Does this make sense before I press go /

John B

[George]

I know there's quite a few people who have accumulated a "darks bank" and it really makes sense todo this on those cloudy nights when there is little todo in the way of imaging. Question is what degree of temperature variation would have a effect on the quality of the darks? EG you could a start imaging session at say 10C and over a course of say 3 or 4 hours see a finishing temperature of 0C, not unheard of and probably quite common in the UK climate. If like me you take your darks unattended at the end of the session you would end up with darks at a temperature considably less than if you had taken them at the beggining.

I suppose a good test would be to get a group of lights and apply different temperature gradients of darks to them.......as anyone done this and what were the results?

Here's a link to a dark frame library software

[johnb]

Apprecited thanks. John B

[Demonperformer]

John

There is a thread on SGL somewhere going into the maths, but basically the ideal number seems to be 20, more than that and you get very little improvement for the increased number. Guess it depends how much time you are willing to devote to it;)

[johnb]

thans for that, camera has been in and out of the fridge all day, so im approaching 20 for 2 deg and 3 deg so at this rate in a couple of weeks i should have a 20 for every 300 sec ISO 800 image i can take and have set set of 20 darks that will be + or - 2 Deg the temp of the lights - seems really useful.

When I have those can I stack just darks in DSS and simply have 1 master for each occasion (if that makes sense) then after adding my lights I can simply the appropirate dark

Regards

John B

[George]

I also read somewhere that DSS prefers a odd number hence I always take 21 darks.

[Psychobilly]

Interesting stuff..

I haven't bothered with darks for ages... and have never gone to the trouble of temp matching etc...

I have downloaded the Darklibrary software to have a play with...

Peter...

[Glider]

Having just downloaded it that Darklibrary software really is pretty good.

[johnb]

It does seem good and if you can build a library which has darks for ever occasion that will be so handy in that we will have so much more imaging time.

Regards

John B

[Demonperformer]

Will darks be affected by changes to the camera over time (like some on the forum have suggested that you do new bias masters every six months or so)? If so, keeping the library updated could be quite a job.

Just a thought ...

[MG1]

yes DP they are. Depending upon usage obviously....

[Shibby]

When you have tracking errors such as mine (assuming amp glow isn't too bad) you don't really need darks

[Jannis]

This is a single 30 min unguided sub taken with the 550D/T2i with internal dark turned on. image at full res.

http://jannis.no-ip.info/astro/Polaris31min4darkflat.jpg 6MB (be paitient, i have slow upload connection)

don't know how it would be with separate dark frame, but at least this shows how well internal dark does the job with a 30 min sub at ISO 100 (i think)...

[AndyUK]

As mentioned above, it's certainly worth checking the exif data for the ACTUAL temperature of each dark after they're taken. I shoved my 40D in the fridge over a weekend and took sets of 20 for a string of exposures from 30s-600s - The fridge was 3-4C, so I was quite surprised when I looked at the exif data. As I took more and longer exposures, the chip heated up and seemed to level off at 20C...