Do we really need offset (bias) frame??

Post by **weixing** » Tue Jan 31, 2006 6:42 pm

Hi,
This is a question on Astrophotography processing and hope those expert here can help answer this question: Do Offset or Bias frame really required??

We know that theoretically a image taken is:
Image Taken = (Real Image * Flat) + Dark + Offset

So to get the "Real Image", we do the following:
Real Image = (Image Taken - Dark - Offset) / Flat
Where Flat = Flat Image - Flat Dark - Flat Offset

But in practical, the Dark Frame actually also contain the Offset Frame, right?? So in practical, it should be:
Real Image = (Image Taken - Dark) / Flat
Where Flat = Flat Image - Flat Dark, Flat Dark = Flat Dark Frame + Flat Offset and Dark = Dark Frame + Offset Frame

Anyway, I've read this on some article and it look logical enough... just want to check with the image processing expert here and confirm this before I applied on my images.

Have a nice day.

tanguan2001 · Post by **tanguan2001** » Sun Feb 05, 2006 2:01 am

Weixing,

Hi - in theory the offset is only required if you want to scale the dark, ie if the dark is taken at a slightly different temp to the lights. This is because once the offset is subtracted, the dark signal depends on temp and can be multiplied as needed. IRIS is able to do dark scaling automatically - the opt command.

So offsets not needed if the darks are taken at the same temp? What I have found in practice with DSLR is that it is still necessary to take offsets. This is because temperature fluctuates with time during a session and it is very useful to be able to scale the dark during processing.

I have now moved on to using a home-built Artemis 285 camera (same as Atik 16HR). This has such low dark current that darks are not needed at all. However the offset is very temp dependent, and I take offsets as a matter of course.

Hope this helps.

TG

ariefm71 · Post by **ariefm71** » Sun Feb 05, 2006 2:10 am

Hi TG, welcome back!

How's things down under?

Arief

Post by **weixing** » Sun Feb 05, 2006 2:44 am

Hi,
Hello TG!!!

Welcome back to SingAstro!!! Now got one more astrophotography mentor to ask when got more questions... hee hee hee

Thanks and have a nice day.

jermng · Post by **jermng** » Sun Feb 05, 2006 2:57 am

Looking at the smiles ... Weixing is one happy man ....

Post by **Airconvent** » Sun Feb 05, 2006 3:39 am

hi tan guan
glad to see you here again...you must have been extremely pampered by clear austrialian skies
do let us know the review you were performing on the ED80 before you went down under...
cheers

tanguan2001 · Post by **tanguan2001** » Sun Feb 05, 2006 7:34 am

Should have known that you guys don't sleep! A post at 2 am and four replies by 4 am!

I'm pretty well - Perth skies are pretty good, outside of Perth even better - went camping last week in Margaret River about 500 kms south of here and there the starlight was bright enough to see by on a moonless night. LMC and SMC + eta carinae clearly visible. Pleiades' dipper shape and nebulosity visible.

Have been fine tuning the ED80 and ART285 camera - last night got guiding down to +- 2 arcsec accuracy over 7 minute exposures. Will post results later today (on eta carinae) once processed.

TG

rlow · Post by **rlow** » Sun Feb 05, 2006 12:36 pm

Hi Tan Guan,

Glad to see you back on Singastro! We do want to see your photos once you have finish processing it!

Richard

tanguan2001 · Post by **tanguan2001** » Sun Feb 05, 2006 1:26 pm

All,

Here's the Eta Carinae in H-alpha. Very good guiding and seeing. Apologies for the file size - 220KB, but a smaller size didn't seem to do justice to the detail in the image.

Eta Carinae

TG

tanguan2001 · Post by **tanguan2001** » Sun Feb 05, 2006 1:29 pm

Oh - and forgot to say that the little 80ED is brilliant - look at the star sizes!