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Question about making master darks and other calibration frames


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Hi!

Yesterday I was reading about dark frames vs in camera long exposure noise reduction, and something caught my attention. As far as my (so far little but growing) knowledge goes, the best you can do is to take the calibration frames right after the imaging session. This can be a pain in the A, and as I read yesterday, many takes these frames separately, when there is nothing better to do, like on a cloudy afternoon. This is allright, it's a good idea, you can create different master darks and other master calibration frames on different temperatures (room temp, cold, hot etc), and use these when stacking images from your light sessions according to the temperatures the lights frames were capured at.

But. As far as I know, my darks should have the exact same settings and focus that my lights have. If I know I use for an example a prime wide angle lens at F2.8 all the time, with ISO 1600 to capture the milky way, that's okay. But what if something changes? What if I use ISO 3200 for some reason? What about the focus (okay, inifinity, but not exactly the same all the time when manual focusing)? What if I use a zoom lens on different focal lenghts? What about the other calibration frames?

It's definitely not impossible to be prepared for every scenario, but when you use lenses instead of telescopes, there are more variations.

Extra info, if that matters: I'm using a Nikon D5500, which is "ISO invariant".

I'm really curious about your replies, as this could greatly improve my image's quality, if It's possible to take calibration frames this way.

Thanks in advance!

Árpád

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Dark frames are not governed by focus as they are simply imaging, well darkness so you should not have any issues there, ISO and exposures lengths are relevant.
Flat frames do rely on the focus as this is largely the point of taking them.

When I imaged with a DSLR I would try and take them outside on a cold day. Aside from placing the camera in the fridge (which some people have done) there is not a huge amount you can do about this.

This is where regulated cooling on a CCD or CMOS helps.

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So with darks ISO and exposure time should match, and it's ok if it's taken on different temperatures (best in cold I assume). It's still pretty specifc.

Maybe if I take the lights from the same location, with the same lens, and same ISO, and I can achieve the same exposure times with tracking, it can work with multiple targets. With the same lens, location, and ISO aren't a problem because if I shoot deep-sky objects in general, the ISO can be the same, but tracking time depends on polar alignment, and that's not always the same (still figuring the perfect one out with my Star Adventurer).

I'm pretty sad seeing that this could possibly only work with a well experienced routine, which I don't have yet.

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31 minutes ago, sza85 said:

So with darks ISO and exposure time should match, and it's ok if it's taken on different temperatures (best in cold I assume). It's still pretty specifc.

Maybe if I take the lights from the same location, with the same lens, and same ISO, and I can achieve the same exposure times with tracking, it can work with multiple targets. With the same lens, location, and ISO aren't a problem because if I shoot deep-sky objects in general, the ISO can be the same, but tracking time depends on polar alignment, and that's not always the same (still figuring the perfect one out with my Star Adventurer).

I'm pretty sad seeing that this could possibly only work with a well experienced routine, which I don't have yet.

Yes Darks ISO and exposure times should match. i.e. You may want to take x25 120 second expoures at ISO 800. The colder the better but as mentioned unless you retro fit cooling to your camera (probably a bad idea) then that is you limiting factor.
The stacking software in which you feed the Darks into wont care about what DSO or tracking conditions you had. Just be sure you don't mix ISO or exposure times.

This is why alot of people use cloudy evenings or day time to create their Dark Libraries. Once you have taken say a bunch of darks at ISO 800 at different exposure times and then a bunch of Darks at ISO1600 at different exposures times then you just reuse them for each DSO. People tend to refresh their darks every 6-12 months to help combat any changes within the camera sensor. Frankly I stuck with mine for a year before redoing any darks.

Just make sure you camera is in a darken room or shielded from light if you decide to take them during the day, this will prevent sun light leaking in during longer exposures.

 

Edited by Droogie 2001
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8 hours ago, sza85 said:

and it's ok if it's taken on different temperatures (best in cold I assume).

No. Temperature should match the light frames. This is actually one of the main reasons why darks don't always work for dslr imaging. Dark current and hot pixels are very temperature dependent.

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11 hours ago, wimvb said:

No. Temperature should match the light frames. This is actually one of the main reasons why darks don't always work for dslr imaging. Dark current and hot pixels are very temperature dependent.

Thanks for the info! I feel likeI can't be lazy then...

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I feel that callibration frames need to be taken on the same night temp..

Darks..dependant on what the temp is that you're imaging in will depend how much dark noise will be in your lights,especially on a dslr..a batch of say iso 800 will work but will it work to the same degree as it taken on the same eve straight after a imaging  run..

Flats.. as long as the orientation of the camera and focus stays the same then that's all that needs to be taken care of..

Bias..bias noise is caused but the circuits in the camera heating up so again I'd say temp related..

Saying that I know of 2 imagers that don't use any callibration frames and produce pretty good results..far better than I can atm

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1 hour ago, newbie alert said:

I feel that callibration frames need to be taken on the same night temp..

Darks..dependant on what the temp is that you're imaging in will depend how much dark noise will be in your lights,especially on a dslr..a batch of say iso 800 will work but will it work to the same degree as it taken on the same eve straight after a imaging  run..

Flats.. as long as the orientation of the camera and focus stays the same then that's all that needs to be taken care of..

Bias..bias noise is caused but the circuits in the camera heating up so again I'd say temp related..

Saying that I know of 2 imagers that don't use any callibration frames and produce pretty good results..far better than I can atm

I see no need to waste precious imaging time on taking Darks in an imaging evening. These can be taken entirely seperately, yes there maybe a temperature difference between what happens from one evening to another but overall these are not likely to make any difference. I know I would rather spend more time on light frames than darks. Newbie Alert: I believe this is what you are stating?

Flat frames are important, probably being the highest priority as these will make a significant difference with vignetting etc.
Again no need to waste time taking BIAS in an imaging evening.

You can get away with not using Darks on a CCD (I dont use darks) but the noise from the warm DSLR sensors will probably add noise if not taken.
Worth experimenting with and without them.

 

Edited by Droogie 2001
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 I see no need to waste precious imaging time on taking Darks in an imaging evening. These can be taken an entirely seperately, yes there maybe a temperature difference between what happens from one evening to another but overall these are not likely to make any difference. I know I would rather spend more time on light frames than darks. Newbie Alert: I believe this is what you are stating?

I see there's a difference between ccd and DSLR.. with some ccd you can set the sensor temp to whatever you want -20,-40 i can see that taking a dark libary  here would be beneficial and no need to take them after imaging..and at -40 there's going to be very little dark noise anyway..

with a dslr I see it is beneficial to take them on the same night @the same ish temp..yes a dark libary will work but I feel that the different temps of the UK on different nights surely on the same night would work slightly better in my opinion..

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The other thing caught my attention in the video above is this dithering thing. I heard about similar with landscape photography called as "super resolution". As far as I can see this thing works with guiding, but unfortunately I don't have guiding yet. Is it possible to do it somehow without guiding (I read that backyardeos can do it - don't know how though, but if backyardnikon can do it aswell I may invest in it)?

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7 hours ago, sza85 said:

The other thing caught my attention in the video above is this dithering thing. I heard about similar with landscape photography called as "super resolution". As far as I can see this thing works with guiding, but unfortunately I don't have guiding yet. Is it possible to do it somehow without guiding (I read that backyardeos can do it - don't know how though, but if backyardnikon can do it aswell I may invest in it)?

You can dither manually if you have a goto mount, but it's tedious. (I know from experience.) Just move the mount randomly in ra and dec. Or in a spiraling pattern. Some camera control software can dither without guiding. But it will also need to control the mount, obviously.

6 hours ago, newbie alert said:

If you're not guiding a slight pause between frames will give you a natural dither..

Actually, this can give rise to an effect named walking noise, which looks like streaks across the background, and which is a pain to get rid of ... unless you use dithering.

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On 2018. 01. 27. at 11:09, sza85 said:

As I saw backyardNikon can dither without guiding, if the mount is ASCOM compatible(? - don’t know anything about guiding yet). I will dig into finding out as soon as I will have some free time. 

Turns out that the star Adventurer doesen't have ASCOM drivers :/

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