Averaging Dark Frames.

[Vesmaobs261]

Hi Group,
I am just starting out in astro imaging with my new ASI 224 camera and am experimenting with dark frames in Sharpcap Pro.
I have heard it said that if you take your dark frames before imaging, and then take another set of dark frames
after your imaging session, then average the two sets to get a master dark that this action goes a long way to
clean up fixed pattern noise ( raining noise etc. ). Has anyone tried this to control fixed pattern noise?. I am not
a tech head so any answers please make them simple so as not to confuse my 70 year old brain, it's been through
enough. Thanks guys and gals.
George

[Menno555]

Hi George and welcome.

The fixed pattern noise has 2 main causes:
1) Darks en temperature. The methode of taking darks before and after is okay. But makes sure that it's all at the same sensor temperature and that you have enough darks. So 10 before and 10 after would do. Or more, depending on your exposure time.
2) Fixed pattern noise, also known as walking noise, can be reduced by dithering. If you take exposures without moving the camera, a pattern can be captured, Dithering can reduce this. This can be done automatically if you use guiding or you can do it manually by moving you scope in a random direction by just a few pixels in between exposures. I do that and it works just fine.
When you do long exposures, that can be done after every 5 captures for example.
Shorter exposures (like 60 seconds) need it after each 2 or 3 exposures.

Menno

[Vesmaobs261]

Hi Menno,
Thanks for the clear and concise reply which was easy to digest.
So now I'm set to take images by starting of with dark frames as you suggested
and nudge the scope a few pixels between exposures. Can't wait to try it out.
One other question though is how does one average two different sets of dark
frames ( before and after imaging ) to get a master dark when the first set of
dark frames ( before imaging ) have already been averaged by Sharpcap?.
Sorry if this is a stupid question, but I suppose the only way to learn is to ask.
Thank you
George

[Menno555]

Hi George

There are no stupid questions, especially with this hobby
With your uncooled camera it all depends on the outside temperature.

If the temperature during your capture session drops just a few degrees, then you can safely make the darks and masterdark in SharpCap (SC) at the beginning of your session and keep on using that during your session.

But if the temperature during your session for example goes from 10 degrees C to 0 degrees C (or in the summer from 16 degrees C to 10 degrees C), then averaging is useful.
It's not really averaging though in this case when you let SC do it, it can't make 1 average masterdark out of 2 masterdarks.
In this case you take your darks at the beginning and let SC average it and you apply that. Then halfway your session you do it again and then you apply that average to the captures.

Having said that: someone correct me if I'm wrong or half wrong. It's been a long time ago that I let SC do the calibration and masterfiles

Menno

[Vesmaobs261]

Hi once again Menno,
Thanks again for your reply.
That sounds straight forward enough.
I am going to put that into practice after the moon goes
down and have a go at imaging the Horsehead Nebula ( a favourite )
to see how it all comes together.
I'll be using an 80mm refractor ( 600mm focal length ) to practice with,
and if all goes well maybe even try imaging through my 2700mm Mak.
This space stuff really gets one hooked.
All the best and thanks for the info.
George

[Vesmaobs261]

Hello group,
Attached see a pic of the Horsehead Nebula that I was able to image
with some dark frame information that Menno was kind enough to help
me out with.
I'm thrilled as to how it came out considering that I am using only basic
equipment,such as 80mm refractor and Zwo 224 imaging camera.
It's not the the bees knees but it's my pride and joy.
I need to practice more on my dithering though.
Thanks Menno,Zwo and Dr. Robin of Sharpcap.

Horsehead.jpg (41.52 KiB) Viewed 2275 times

[mixon]

menno,
I'm newbie ! Just learning and beginner. Question, shooting Flat frames in sharpCap Pro 4.0
After choosing 10 flat frames in the drop down menu, I set the exposure so the histogram was 20-80%. Captured the frames, but it only averaged one flat frame in the flat frame folder. is that right? shouldn't i see at least 10 separate frames? or ? This is a huge learning curve !
Thanks

[Menno555]

This is correct. The 10 flat calibration files are added together (stacked) and that makes 1 averaged flat file. This file is called the Master Flat.
This goes for all calibration files. So also with darks which produces a master dark calibration file.

Menno