ICNR or Darks

[leon]

Hi Guys, I have read and have heard and have noticed that there is some small controversy about haw to obtain darks.

Some have suggested that it is better to take the darks seperatly, and some have said that ICNR is the better way to go.

What I'm probably asking is, dose it really matter, if the time and temp range is the same for both methods, or is there a preferred way.

I would really like to hear views on this subject.

Thank You.

Leon

[Terry B]

I prefer to take darks separately but there are arguments both ways.
If you take a series of darks and median combine then the you will reduce the readout noise by a formula that I can't remember, with more being better up to a point.
This leaves you with what is hopefully the consistent dark current for your particular sensor with the random readout noise reduced. This is assuming that the temperature when you take the darks is the same as when you take your lights. There are ways around this however using scaling of darks etc.

On a practical note it is also probably more time efficient to take separate darks. I tend to use 300s exposures and take about 7 darks. This will take 35mins and can be done after I have finished imaging and packing up or gone to bed. If I used ICNR and took 24 exposures in the session then my 2 hours of imaging would have expanded to 4 hours with 24 darks being taken along the way but not median combined. I also don't have access to these dark files as they don't exist as the camera internals have used and then deleted them.

[Dennis]

Hi Leon,

The answer in my case is both!

I have only had a few brief outings with my Canon 40D, but as it is not temperature regulated I have chosen to use ICNR to ensure that each light image is reduced by a dark immediately after the light exposure.

Also, I have noticed that processing the much larger .CR2 raw files in ImagesPlus and Photoshop take so much longer, so I am content at this stage in my DSLR career to do ICNR rather than suffer the burden of more post capture processing. It came as quite a shock processing images 3888 x 2592 at 10.1Mp!

With my trusty old SBIG ST7E it is quite a different story. With a chip size of 765 x 510 pixels the files are less that 1M each. Also, the camera is thermally regulated so it makes sense to capture lights and darks separately. The very tight integration between the ST7 CCD camera and the software CCDSoft also make it very, very easy to take separate darks and then perform the reduction later.

Cheers

Dennis

[leon]

Thanks guys, your responses, you have been very helpful, and Terry to what you explained as so much time lost with using ICNR. I too have lost so much good imaging for the lights because of the doubled time required to take each image.

I suppose everyone has different views on this, and i suppose it is 50/50 each way, and personal preference.

Thank You.

Leon

[iceman (Mike)]

I *want* to take separate darks, simply because of the time involved with ICNR (doubling the time out at the telescope, as already mentioned).

But I just haven't had good success with separate darks yet - they always end up spotty, and when subtracted from the lights, end up with black spots in my nebula.

It was possibly due to not being at the same temperature, but I haven't gone back to do more experimentation to eliminate or at least find the cause, yet. But I will.

For now, I'm using ICNR because the results are fantastic. But the plan is to go back to taking darks.

[EzyStyles (Eric)]

hi Leon, ICNR is much better as the hot pixels will get subtracted evenly after one expo shot and will give a more accurate subtraction. With taking darks, you are averaging from eg: 10 x 6 minutes (1 hour) shots therefore tempreture will fluctuate during that 1 hour. I have to admit, i do darks rather than ICNR and have a collection of darks varying from different tempretures and exposures. Reason being is so i can spend more time taking other objects at night lol i know you are guilty of this also

[DJDD]

Originally Posted by EzyStyles

i do darks rather than ICNR and have a collection of darks varying from different tempretures and exposures. Reason being is so i can spend more time taking other objects at night

this is a very scientific approach.
and as long as the standards you prepare are updated every now and then you cannot go wrong.

thanks for the tip.

[EzyStyles (Eric)]

no worrries, just the way how i do things heheh.

[leon]

Yes Eric i am guilty as well, I reckon one should be able to access a cool room, set it to zero, take twenty darks with the scope and camera in there, than turn it up to one degree, and so on, up to about 10 degrees, and build the dark libarary.

You could then have 20 dark at each temp, and time

Leon

[EzyStyles (Eric)]

actually you just gave me an idea Leon. I have this portable air con in my computer room which displays the current room temp. Couldn't i take my darks then decrease the aircon temp by 1, then take some more then decrease etc etc ?? might work...hmmmm

[dugnsuz (Doug)]

very timely thread!
Cloud last night forced me into starting a darks library.
One question though - Outside temp was around 17°, will these darks be useable over a range eg for lights taken between 15-20° or will they only be useable for lights taken at 17°??
Doug

[EzyStyles (Eric)]

Hi Doug, i wouldn't use darks with a 5 degree range eg:15 - 20 degrees too big of a gap. I use it per each degree. even with 1 degree difference, you'll notice it.

i normally do something like this:

19-20 degrees (any lights ranging from 20-19 eg: 19.5, 19.2, 19.8 degrees will be using these)
19-18 degrees (" " " ")
18-17 degrees (" " " ")

[jjjnettie (Jeanette)]

I know I'm not an expert like you guys and shouldn't probably be posting an answer here.

But it seems to me that the temperature fluctuates during the course of the night anyway, so if you take your darks at the end of the night, it may in fact be 7 or 8 degrees cooler than in the beginning of the session.
So to get accurate darks, shouldn't they be taken at the end of each individual image. Whether it is taken manually or using the in camera dark subtraction.
But if there is cloud coming up over the horizon and you know that time will be limited, I guess the only thing to do is grab what images you can while you can and take your darks later.
With the Gstar, I just image for x number of minutes, whack the end cap on and continue to image for another x number of minutes. You never mix up which darks go with which set of video images that way.

[dugnsuz (Doug)]

Originally Posted by EzyStyles

Hi Doug, i wouldn't use darks with a 5 degree range eg:15 - 20 degrees too big of a gap. I use it per each degree. even with 1 degree difference, you'll notice it.

i normally do something like this:

19-20 degrees (any lights ranging from 20-19 eg: 19.5, 19.2, 19.8 degrees will be using these)
19-18 degrees (" " " ")
18-17 degrees (" " " ")

Makes sense Eric.
Do you keep 10-20 specific darks to use to calibrate images, or do you make a master dark and use that?
Doug

ps...2008, time for a new avatar!!!

[avandonk]

I have found with bitter long experience that for very dim and faint objects that are just above noise levels NOTHING beats ICNR!

There are a few reasons for this.

1. The camera sensor will heat up during a long exposure by a few degrees from its idle temperature. Even the idle temperature is higher than the 'off ' temperature and the 'off' temperature most closely approximates ambient.
If you take a series of exposures using ICNR the first image will have darker 'holes' due to the sensor heating up during the light exposure. The situation gets worse if the ambient temperature is dropping.

2. Neither amp glow nor noise is completely removed by dark subtraction as it is impossible to know what temperature your sensor was actually at. The only dark that comes close is the dark taken immediately after the light and even then it is the second exposure where all temperatures have settled to a quasi equilibrium. If the ambient temperature is dropping the rate of temperature change will control the residual noise even with ICNR.

The only thing that really works is ICNR.

The only way to beat this is to have a constant ambient temperature. The fridge I am working on will be thermostatically controlled and then I will start using darks as I can "set" the ambient temperature.

Bert

[EzyStyles (Eric)]

Originally Posted by jjjnettie

I know I'm not an expert like you guys and shouldn't probably be posting an answer here.

But it seems to me that the temperature fluctuates during the course of the night anyway, so if you take your darks at the end of the night, it may in fact be 7 or 8 degrees cooler than in the beginning of the session.
So to get accurate darks, shouldn't they be taken at the end of each individual image. Whether it is taken manually or using the in camera dark subtraction.
But if there is cloud coming up over the horizon and you know that time will be limited, I guess the only thing to do is grab what images you can while you can and take your darks later.
With the Gstar, I just image for x number of minutes, whack the end cap on and continue to image for another x number of minutes. You never mix up which darks go with which set of video images that way.

Totally agree with you JJJ. yes tempreture will fluctuates throughout the night. To do this, i normally jog down the tempreture using a $12 dicksmith thermometer measuring ambient temp of the first frame. eg: 18 degrees. Then, jog down the last frame eg 17.4 degrees. I'll then use the 17-18 degrees set of darks

[EzyStyles (Eric)]

Originally Posted by dugnsuz

Makes sense Eric.
Do you keep 10-20 specific darks to use to calibrate images, or do you make a master dark and use that?
Doug

ps...2008, time for a new avatar!!!

I use my master dark frames. LOL noooo i don;t want to change avatar

[dugnsuz (Doug)]

Originally Posted by EzyStyles

LOL noooo i don;t want to change avatar

Not you Eric, Me!!!!!!!

[Terry B]

The other war around the problem you have all noted about temperature fluctuations is to scale the darks. It is explained quite well in this tutorial for use with Iris.
It is usable with other programs also.

[leon]

Terry that is certainly different, must have a good read of that,

Leon