A Dark Question Please

leon · #1 12-12-2007, 05:56 PM

Hi Guys,

I realise that taking ICNR images is the way to go and i have been doing this always, but lets face it, it doubles your imaging time which could be better spent taking real data.

So my question is this, how critical is it to take in camera darks, is the temp factor so important, or can one get away with it, and take darks seperately at close the temp, with satisfactory results.

If i spend 5 hours out side 2.5 of this time is taken up with the camera doing its dark stuff.

How do others do this, and is using at least some darks of different temp better than not using any at all.

Thank You

Leon

rogerg (Roger) · #2 12-12-2007, 06:15 PM

I don't have the patients for ICNR unless I'm doing exposures of 30sec on a fixed tripod or something. Sure, they might be better, but I prefer to be getting light data.

I take dark frames every couple of hours or so. It depends how much the temp is changing. If it's a hot summer's day cooling down to 10 degrees at night then I'll take lots (10 throughout the night perhaps). If the temp is more stable I take only a couple.

I do some manual editing, manually choose how much to apply the dark reductions.. .etc.. It works out ok in the wash, but probably isn't as ideal as ICNR, but a fair compromise for my lazyness

dugnsuz (Doug) · #3 12-12-2007, 06:17 PM

I think I may start to take an equal number of darks AFTER I've taken my light frames for a different reason. After a number of 3-5min exps I note my guidestar has drifted a bit in the PHD window, so by running my lights one after another with no ICNR I'll minimise the time it takes for this drift to occur in my mount between exps. Darks taken later can drift all they like!!!
I can go in and have a coffee and browse IIS when the darks are firing off too.
G11 users shouldn't have this problem though!
Doug

Questions...If you take 10 lights do you take 10 darks for calibration in ImagesPlus. If you took 15 darks after 10 lights would this improve the noise reduction after calibration? Or have no significant effect?

leon · #4 12-12-2007, 07:31 PM

Doug about your question, I think, now heed this, I think, you are suppose to take more dark frames than light ones, so if you took 10 lights than at least fifteen to twenty darks would be the go.

It is also the rule by many on IIS that a batch of 16 flats be recorded before the session, and the lot calibrated and stuff in Image Plus, or similar program.

I was just wondering about the effectiveness of a dark if it wasn't the same, or nearly the same temp as the light.

because i need that time for imaging real data

Leon

ballaratdragons (Ken) · #5 12-12-2007, 07:37 PM

Leon, if temp changes are making it awkward, It makes sense to me that you could take a Dark in between every light.

Then the darks would match the lights.

leon · #6 12-12-2007, 08:14 PM

Yea, I know Ken, that would be the same as letting the Camera do it, in time wise.

I understand all the reasons for the flats and darks, i just want to know the effect of using darks that do no necessarily match the temp of the light.

If there was no real significant difference in the end results, my time could be better spent at imaging the light data of objects, rather than waiting the same amount of time for each dark to be had at the length of the light exposure.

For example, if I took 10 images of m42 at 3 mins each, I also have to wait for the camera to take 10 darks at 3 mins each, which in turn doubles the time, for which i could be imaging lights.

So at the end of the night I could let the camera fire off as many darks as it wants and go to bed, so to speak.

Leon

pixelsaurus (Mike) · #7 13-12-2007, 12:06 AM

I'm not sure if this piece of freeware is of any use. It is a EXIF reader and I have noticed that in the data is included the sensor temperature. I have some pix of McNaught that were shot at ISO 3200 but never shot any darks. My intention is to reshoot darks at the same temp as the originals to see if I can reduce noise. http://www.photome.de/

ballaratdragons (Ken) · #8 13-12-2007, 12:15 AM

Ahhh, I understand, Leon.

What the DSI tutorials suggest to do (and you may want to try it) is to have a Library of Darks.

I tried it and it does help reduce time. I took a stack of 30 second darks at temp 2 degrees, a stack of darks at 40 seconds at 2 degrees etc. Then the next night was zero degrees so I did the same stacked 30, 40, 60 second exposures at zero degrees. I now have a library of stacked darks at different exposure rates and temps. What I do then is, if it's say '20second lights on an 8 degree night' I take my lights and get the darks from my library by selecting the closest (something like '20 sec @ 7 deg). It's a lot of work, but worth it for the clear nights.

What you could do on cloudy nights is build up a library of Darks at different exposure times and temps. Good use of those cruddy nights

RB (Andrew) · #9 13-12-2007, 10:19 AM

Quote:

Originally Posted by pixelsaurus

I'm not sure if this piece of freeware is of any use. It is a EXIF reader and I have noticed that in the data is included the sensor temperature. I have some pix of McNaught that were shot at ISO 3200 but never shot any darks. My intention is to reshoot darks at the same temp as the originals to see if I can reduce noise. http://www.photome.de/

I've just D/L a copy and it looks like a nice piece of software, but where does it show you the sensor temp?

Thanks for the link BTW.

[1ponders] (Paul) · #10 13-12-2007, 10:36 AM

Leon re darks and temp.

For every 6 deg C increase in temp there is roughly a doubling of the temperature dependant dark noise, for every 6 dec C drop in temp the noise reduction is halved. So if you take a light at 30 deg and then a dark at 27 deg you could expect roughly (and this is roughly) 3/4 the amount of noise temperature dependent noise. This will show up in you subtractions.

There are other "dark" factors as well, but temperature is probably the most variable and has the greatest impact.

leon · #11 13-12-2007, 11:43 AM

Well thank you Paul, that was the answer i was hoping for. at least i have a bench mark to work too.

Last night i tried some darks,and lights seperately, the lights all 10 of them, were taken in the temp frame of 13.3 degrees and 12.4 degrees, so i expect that that is not a very big difference, (I have a temp gauge attached to the Tak) which is very accurate.

The darks also fell into this range, so therefore i expect that they too would be satisfactory for the use with the lights.

I also expect that a difference of one degree is not a major issue, would this be right.

Thanks, Leon

[1ponders] (Paul) · #12 13-12-2007, 12:43 PM

One degree shouldn't be a major issue with a DSLR, especially with the later model cameras that turn the amp off until downloading the image. The old 300Ds though are a bit if a pain in this respect as they tend to generate a fair bit of internal heat anyway.

Geoff45 (Geoff) · #13 20-12-2007, 03:03 PM

Quote:

Originally Posted by ballaratdragons

Ahhh, I understand, Leon.

What the DSI tutorials suggest to do (and you may want to try it) is to have a Library of Darks.

What you could do on cloudy nights is build up a library of Darks at different exposure times and temps. Good use of those cruddy nights

Also, whenever you get a chance, update the darks ie replace an old 2 degree dark with a new one whenever you get the chance. This is because the camera will slowly change over time.
Geoff

Aster · #14 21-12-2007, 04:16 PM

As usual I am probably way of the mark, but I have been thinking, well, trying to.

Would it be possible to make a temperature controlled light proof box so you could shoot darks at varying temperatures at anty time ?

You could then shoot lights the whole night, noting down the temperature and then the next day, at your leisure, shoot matching darks.

Someone on IIS is sure to have the technical knowhow to design, maybe even make something like that for a profit if it is feasable.

leon · #15 21-12-2007, 04:38 PM

Alaxander, that thought has actually crossed my mind as well, if we could get say something like a regulated temp controlled ice box/fridge thingy, your suggestion could have some merit.

Leon

pixelsaurus (Mike) · #16 03-01-2008, 03:37 AM

Quote:

Originally Posted by RB

I've just D/L a copy and it looks like a nice piece of software, but where does it show you the sensor temp?

Thanks for the link BTW.

Sorry for the delay in replying. The version I am using is version 2. The maunufacturers notes show the camera temperature. I'm using a Pentax *ist DS. However I updated Photome on my other laptop and the camera temperature seems to have been dropped fron the maunfacturers notes.