What ratio of LIGHTS : DARKS : FLATS : BIAS ?

[Craney]

Evening everybody,

Is there some magical formula for working out the ratio of   LIGHTS :  DARKS :  FLATS : BIAS  for DSO's  ??       Is there a 'Bible of AP' that people refer to ?

So if I took 20 lights or 'subs' ( I think they are called as well),  how many of the others should I take to make the stacking and processing bring out the finest quality image ?

At the moment (due to weather and trying to get a first attempt at any object going) I tend to take about 20 LiGHTS, then about 2 DARKS, no FLATS ( I don't have the light panel) and a couple of BIAS.

So my workflow ratio would be      20 : 2 : 0 : 2

What is optimal ?        Or is this the type of question that keeps forums awake at night as well as the imaging??

 

Thanks in advance.

 

Sean.

 

 

[frugal]

Flats, Darks, and Bias are calibration frames, the Lights hold the image you are trying to capture. So as many lights as you can manage.

Darks and Bias can be generated at any time (and you only need to generate them once every six months or so, so you can do it on a cloudy night). I have an stock DSLR, so I let it run overnight taking 300s darks outside to match the 300s lights I normally use. The following morning there were 112 darks. Bias frames I take by putting the exposure at it's fastest speed and the camera drive into high speed, and the lens cap on, and just held the shutter button down whilst I was reading something else ( I ended up with 490, probably overkill, but I only needed to integrate it once).

Flats I take at the end of each imaging session. I do not have a light box, so I hold my tablet showing a white screen with the brightness turned down over the end of the scope and put the camera into AV and shoot about 50 frames (at about 1/250s it only takes a minute or so).

My camera is so noisy that I generally stack my final image once with the flats/darks/bias, and once with just the flats to see which has come out better on any given day.

[glowingturnip]

edit: frugal beat me to it...

 

 

well bias definitely don't have to be taken in the same session, so I tend to shoot mine with the camera tucked away in a dark drawer, x100, then use those for a couple of months or so.

Darks don't really need to be the same session either so long as they're roughly at the same temperature and at the same ISO and exposure time - I tend to leave my camera outside on a night when I'm not imaging, in a secure and covered location of course, and leave it to click away all night, x100.

Flats you have to do at the same time as your session, I do mine when I'm packing up and just use a white screen on my laptop as a panel, about 30 or so since they're quick to do

Edited March 22, 2017 by glowingturnip

[johnrt]

21 minutes ago, Craney said:

Is there some magical formula for working out the ratio of   LIGHTS :  DARKS :  FLATS : BIAS  for DSO's  ??

No.

You need to take as many as possible to average out the random noise that you camera will generate (shot and read noise), to isolate the thermal signal in darks and bias signal in bias. I would aim for 100+ bias (because they are quick and easy) and in the region of 50 darks and flats. There is no ratio of flats to lights, or lights to darks etc. You need to take many to generate a quality master frame you can use to calibrate your light images with.

 

24 minutes ago, Craney said:

Is there a 'Bible of AP' that people refer to ?

Our very own Steve Richards book is the most often recommended if your just starting out - https://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html

Hope that helps

[frugal]

IanL has also done a very detailed series of articles on Bias and Dark frames specifically for DSLR astrophotography: http://www.blackwaterskies.co.uk/category/image-processing/

 

[Craney]

Thanks everybody.   This is good stuff.   I like the Laptop monitor solution to 'flats'.  

The book mentioned often gets quoted by the cognoscenti as well.   Must acquire one. 

Thats for the other URL  Frugal.    Will chase that one up.

 

Cheers, Sean.

Edited March 23, 2017 by Craney

[ollypenrice]

Be aware that flats can be tricky. What works for one may not work for the other. (I run three imaging rigs and know this to my cost! One rig refuses to play like the other two.) Be prepared to be flexible.

Olly

[happy-kat]

Be flexible as you can't get a light panel be creative. You can take flats using a dusk or dawn sky or use the app lightbox and a tablet if your objective lens is small enough or make a flower pot lightbox. Flats are not optional really they make a big contribution to your image in my experience in this challenging area of imaging.

[JohnSadlerAstro]

I use Laptop and Sail Cloth (!) an Aunt (!) gave me!!  And it works great!!!  

John