Newbie Questions about Bias/Darks/Flats

[P.K.]

Hello Experts,

First the info for you that my english isn't the best. Netherless I hope you understand my bad english and I can then read your tips to implement it im my workflow with PixInsight. In worst case and for complex points, my best friend will be 'google-translater'. No matter... let's go!

Actually i work with the trail version but I'm shure to buy later the commercial. I have studied many tutorial-videos and read a lot of hints on some websides. Bad for me: Most of the best infos are in english. In addition, many steps in videos are shown very quickly and I often have to stop-rewind-play-stop-rewind-play and so on. That's really tricky for me. Therefore I hope now for your 'written' help.

The general point 'darks/bias/flats' is very controversial. My plan for the first steps with PI is to use all of them. Also I think it will be useful to create a 'database' with some of these masters to increase the time for calculating the stacks.

My plan:

BIAS:
Shooting of 100 Images at shortest shutter time (1/8000s) in every ISO that I will use (200, 400, 800). Sensor and Viewer are darkened while that shootings. Then I create from every ISO one Master-BIAS and use it in all further projects.

Short question:
- Is that OK?

DARKS:
Shooting 100 images in different ISO (see above) and also create Masters of them. Renew this masters every month or minimum every quarter of a year to compensate (a little bit) the temperature difference along the seasons also the 'aging' of the dslr-sensor.

Questions about darks:
- Is this procedure generally OK?
- I think it is enough to place for shooting the darks the camera on a tripod. To mount the camera at the telescope isn't necessary in this case. Isn't it? The fact that the sensor and viewer is darkened at creating the darks is no question.

FLATS:
Here is my idea to go the same way like for darks. But now the cam is mounted on the telescope respectively the lenses are adapted to the camera. The most difference to darks will be the sorting of the master-flats additional by the used lense. (e.g. Telescope - 300mm - 17mm - 10mm)

Questions:
- Shooting flats with different ISO is no question, but what colour of the illumination is the best? pure white with external 'lightbeam'? cloudy sky? or the light-colour doesn't matter?
- The exposure of the camera whould be set to '0' and the flats should be shoot in mode 'AV'. Is that right?
- How important is the position of the camera if it is mounted on the telescope? Must the cam exactly be at the same focus and same rotation angle as the light-frames (while shoot the object)? Or doesn't matter? If yes: I can't create a database of flats and I must produce the flats again every foto session ... Is that right?

Sorry for this long story and pure newbie-questions. I'm hopeful you can give me short answers about my questions, or tell me links where i find some more solutions.

If important, here my equipment:
Canon EOS 60D, Telescope 10"/1250mm on NEQ6-GoTo, some Lenses like 10mm, 17mm, 300mm. All lenses have f2.8

Best regards from germany!
Peter

[ChoJin]

1/ once you have your master bias, you should generate a superbias using the superbias process

2/ for your darks you should match as much as possible the temperature and the time exposure. Of course once integrating you'll ask PixInsight to "optimize" the darks, but it's still better to match temperature and exposure. You could use the BatchStatistics script on your darks to double check whether or not they seem compatible (comparing MAD, Sn, Qn, BWMV). On this matter, http://www.blackwaterskies.co.uk/2015/02/pixinsight-dslr-workflow-part-2a-dark.html was quite interesting

3/ You CAN'T take flats "in advance" and reuse them. You always should take them before or after your light without touching anything (the focus has to be the same, the orientation of the camera as well etc.). The reason is that you want the defects/dust's projection on the image to be exactly the same. It's the only way to insure they will get corrected. The AV mode etc. isn't that important. The general idea is to have the histogram's peak roughly at the middle. Manual mode would work too as long as you check your histogram.

[John_Gill]

Hi,

I think with Bias & Darks you can create the library of images.   With Flats you should keep the camera attached to the scope and with the same focus and ISO and orientation, so you cannot make a library.

Having said that, to create the Bias is very quick so I do mine after every shoot.  With Darks, the temperature is important, so you would need to create many variations and perhaps every 6 months you would need to renew your Dark images.  This becomes messy and a waste of time and effort.

To me the solution is to create the Bias and Darks right after the shoot and I leave the camera attached to the scope and not change the focus or the ISO.  The next morning, I create the Flats by covering the scope aperture with a white cloth and shine a light directly into the scope.  I set the camera to AV and run about 25-30 images.

I hope this helps.
Regards
John

[P.K.]

Great expert help from you both!
Also thanks for the excelent link with a lot of helpful technical informations.

The topic 'bias' is now clear and i know all what is importand for my first steps with PI.

My understanding all around 'darks' is now much better. I'll try to create a library of dark-masters sorted by Date/Quarter (representing the temperature), ISO and Exposure-Time. The future will show me wether this way is OK for my claim of image qualitity.

Sad to know I have to shoot flats every photo-session again. Bad not really bad. Because it takes not so a long time to shoot the required number of images.

And that's now the last open topic:

How much flats I need?

It's really controvers what others say about this. Even about darks. 

Do you think it's OK to take the half of light images?
That means: 100 Lights -> 50 Flats

Darks and bias I'm planing to take 100 images of every "type" (see above) as basis for the master(s).

Note:
I'm really a hobby-photographer without professional equipment or the possibility to spend to much 'night-time' for my hobby.
My demands on the quality of the images aren't so high like for an expert. Until now ... 

regards
Peter

[John_Gill]

Hi,

I guess you can take hundreds of Bias, Darks and Flats but there is a point of very little gain for the extra images.  Personally, I take about  30 Bias and Flats.  If I shoot 60 Lights then I do 20 Darks.  (that's the ratio I use.)

I am sure others astro-photographers would recommend more of everything but we have to sleep sometime   

Look up
John

[P.K.]

Hey, don't worry! 8) your answers musn't have to be immediate. 
I'm aware of the time shift between Germany and USA. 

btw.
for me it's lunchtime now ... 

Good appetite for you all 
Peter

[ChoJin]

for flats 20-30 should be enough.

for darks, I read statistic theory says you should shoot 5 times more darks than lights (if you don't want to add more than 10% of noise), but I don't think I've ever seen anyone actually following this rule ^^

So I'd say, as many as possible, if possible no less than lights.

With my very limited experience, I compute noise statistics before and after my dark subtraction to be sure I improve stuff instead of degrading it (in case I don't have enough darks).