Using the best Dark Frames

[Eris]

Hi Guys,
I have heard that the best dark frames are those taken on the night the observation is made. Is this so or can I more conveniently draw on a series of darks in a Dark frame library provided that exposure time and temperature is the same.
I would appreciate any advice you can offer.

Thanks,
Eris

[Geoff45]

Hi Eris
If your camera has not changed over the time between darks and lights, then it doesn't matter when you take your darks, as long as they are at the same temp and exposure. However, cameras may change with time (different hot and cold pixels for example), so you need reasonably up to date darks. The best thing is to make a library of darks and update them on cloudy nights. If you find your dark frame calibration is not working properly, it's probably time to update.

[leon]

Although this doubles your imaging run, it is best to use ICNR, as you take your lights, but it is time consuming.

Leon

[rogerg]

I presume this would be using your ST2000 you mentioned in another thread. If so, a library is fine because you can accurately control exposure time and temperature.

Often people build up libraries of reduction groups (flat, dark, bias) with a new set for each combination of temperature, exposure time and binning that they use.

I do this, have a directory of reduction groups, and it works well. I create a new (replacement) reduction group if the one I have for the required temperature, exposure time and binning is more than a couple of months old. I keep the old ones in case I need to reduce old files in the future.

Roger.

[Eris]

Thanks for the good advice guys. Personally I always favoured the library option but was concerned about compromising the end product - image quality.
You can blame that on inexperience.

Regards,

Eris :)

[wysiwyg]

I am looking into building a library myself and have stumbled across this product:
http://www.southern-astro.com.au/php/software/dfm.php
If you are using CCDSoft or MaximDL, this little program integrates nicely and can automate the entire library building process for you.

The only thing that strikes me on this site, at the bottom it mentions that he has a dark Library for Ha, and RGB???
Why the hell would you need a dark library for those. There is no light passing through, so the filters make no difference, that's why its called a DARK.

I certainly dont have an explanation for this or do I find it logical.

Cheers
Mark

[Eris]

Thanks Mark

[KISA]

Hi,

I'm also looking to produce a dark and bias library for my 350D but am unsure of the best way to produce the darks and biases. Could someone please confirm if it's best to take multiple of each (same ISO, temp and exp time), average them and use the final single images on the actual astro image or just use a single bias and dark of the same ISO, temp and exp time on the astro image? If averaged multiple is best could you please throw a rough number of a number to average?

Thanks,

Markus

CS

[Bassnut]

A rough number of darks would be 10 if your fussy, 5 would do. Average or mean combine is fine, youd be hard pressed to tell the difference.

I have used the same dark library on my SBIG for over a year. Again, it depends how fussy you are.

The Southern Astro site note means the longer exposure bin 1 darks are typically used for Lum-Ha, and shorter bin 2 darks for RGB. For the act of taking darks, as you say, filters are irrelavent.

Brads app is great for totally automated capture and processing on a large number of darks on an automated rig. DL itself can provide one click automated processing with a whole folder of bias/dark/flat subs, and spits out all the masters in one go.

It will even scale different temps (if the cam inserts temp in the FITs header, NOT DSLRs tho).

For a DSLR, from the results ive seen, ICNR is the most effective for most users, given the lack of temp control.

[KISA]

Thank you very much for that. Is it also beneficial to take bias and flat shots in a similar way (e.g. averaging 10)?

I was previously using ICNR which worked well but unfortunately it used up a lot of the evening time just taking darks. I recently converted my 350D to be temperature controlled so I'm hoping it will allow me to produce and use a dark, bias and flat library to give me more capture time (and hopefully more sleep :zzz:). Looks like time to put it to the test :prey:.


Thanks again,

Markus

[Bassnut]

yes, same with flats, 10 or 5. For bias 2 is enough.

Well, if your 350D is temp controlled, then a library is the go, much more convienient and more imaging time to boot.

[KISA]

Thanks again, I better get into it :).

[Tilt]

If a dark frame library is created, do you have your darks grouped into temp categories? For instance with a 350D you would have ISO800/10mins/15degrees? then have other groups (of say 10 dark frames each) at 14, 13, 12 degrees ect.

Michael

[rogerg]

Yes.. To put it in perhaps a techincal way: The unique identifiers of a dark frame record woud ideally be:
- exposure time
- chip temperature
- image dimensions (in pixels)
- date
... and perhaps camera.

I only started labelling mine by date about 6 months ago, but attached is an example listing from my "reduction" directory.

You can see I'm a bt slack in accuracy of naming sometimes :)

Roger.

[Tilt]

So how many dark frames per group would be acceptable?

Michael

[rogerg]

Well even 1 is acceptable. I usually aim for 10. But sometimes only have 3 (if it's 20 minute exposures and I am doing them manually that's a long time to sit around waiting for dark frames!), 5 is common, I have some groups with about 30.

10 is a nicenumber. 5 is defintely good. 3 or less is less than ideal.

Roger.

[AlexN]

I have recently built up my reduction library...

My directory structure is as follows..
Darks
-> -10c
-> -15c
-> -20c
|-> 300sec
-> 240sec
-> 180sec
-> 120sec

Etc etc... I have 15x 1, 5, 10, 30, 60, 120, 180, 240, 300, 360, 420, 480, 540 and 600 sec at 5° increments from -5°c through to -35°c.. I have the same directory structure for bias frames, however I only have 5 bias frames at each temperature, and obviously, the exposure duration is always the minimum exposure duration of the camera..

Flats, I dont do a flat library, because dust and imperfections in the optical system can change frequently... as frequently as every use some times... so I find its always better to take fresh flats either at dusk before imaging, or dawn after imaging....

[Bassnut]

Yes, as Alex points out, every time you move the camera (or anything else in the image train) new flats are required, and dust builds up over time, so the longer the time between sessions, the more likely you need new flats. If the rig is mobile, you need new flats after every session.

Photometrists on GRAS like to see new flats every week, even though the rigs are permanent.

I renew flats on my permanent home rig about every 5th session (the cap is on between times)