First Horsehead: Should there be less noise than this...

[Gerry Casa Christiana]

On 27/02/2018 at 22:15, ollypenrice said:

Yes, what I would try would be the master bias, renamed, as Master Dark and the original master bias as Flat Dark. This will not double-subtract the bias from the lights.

I think the image is sound for an unmodded camera and a short data set. The images of this target which the OP may have seen and liked quite possibly have ten hours or more of data - including strong Ha signal captured by a modded DSLR or through an Ha filter in a monochrome CCD. No free lunches in this game!

Olly

Olly

Just to clarify as I'm trying your method now. So I have just lights bias and flats. Can I use the master bias as a master dark and then use the master bias as a dark flat but then do not use master bias as master bias. 

Is that right? 

Cheers 

Gerry

[ollypenrice]

1 hour ago, Gerry Casa Christiana said:

Olly

Just to clarify as I'm trying your method now. So I have just lights bias and flats. Can I use the master bias as a master dark and then use the master bias as a dark flat but then do not use master bias as master bias. 

Is that right? 

Cheers 

Gerry

Yes, that's the idea. It works well for me but let's see how it does for you.

Olly

[tooth_dr]

7 minutes ago, ollypenrice said:

Yes, that's the idea. It works well for me but let's see how it does for you.

Olly

I tried master bias as a master dark and used same master bias as a master bias and the image came out weird!  What does using the master bias as a dark flat do?

[ollypenrice]

9 minutes ago, tooth_dr said:

I tried master bias as a master dark and used same master bias as a master bias and the image came out weird!  What does using the master bias as a dark flat do?

Primarily it removes the bias pedestal, without which your flats risk over-correcting. It also stops the flat adding noise to your picture. You can use a master bias as dark flat because the overwhelming majority of the noise in a short exposure flat comes from the bias. Thermal noise doesn't have a chance to build up. While trying to solve a different problem I did extensive tests on dedicated darks-for-flats versus master bias as dark-for-flats. I found no difference at all, but the time saved is prodigious. The use of master bias as flat dark is widely endorsed in the imaging community. It isn't something I dreamed up, it's commonplace.

Olly

[tooth_dr]

16 minutes ago, ollypenrice said:

Primarily it removes the bias pedestal, without which your flats risk over-correcting. It also stops the flat adding noise to your picture. You can use a master bias as dark flat because the overwhelming majority of the noise in a short exposure flat comes from the bias. Thermal noise doesn't have a chance to build up. While trying to solve a different problem I did extensive tests on dedicated darks-for-flats versus master bias as dark-for-flats. I found no difference at all, but the time saved is prodigious. The use of master bias as flat dark is widely endorsed in the imaging community. It isn't something I dreamed up, it's commonplace.

Olly

Thanks Olly, I wasnt suggesting you made it up!  I've keen to try out new ways as I still find noise in my images using Bias as bias and Flats as flats with no darks.  Is it ok if the timings dont match then, cos they wont I guess!

[ollypenrice]

1 minute ago, tooth_dr said:

Thanks Olly, I wasnt suggesting you made it up!  I've keen to try out new ways as I still find noise in my images using Bias as bias and Flats as flats with no darks.  Is it ok if the timings dont match then, cos they wont I guess!

The bias exposure time will be different from the flats exposure time but, assuming the flats to be of short exposure, this just won't matter. I was very glad to discover this a good few years ago since shooting dedicated flat darks and reducing them takes forever.

Olly

[tooth_dr]

4 minutes ago, ollypenrice said:

The bias exposure time will be different from the flats exposure time but, assuming the flats to be of short exposure, this just won't matter. I was very glad to discover this a good few years ago since shooting dedicated flat darks and reducing them takes forever.

Olly

Cheers Olly, off to do more stacking this evening!

[Gerry Casa Christiana]

19 hours ago, ollypenrice said:

Yes, that's the idea. It works well for me but let's see how it does for you.

Olly

Yes does work and I can't see any difference on the final result so thanks for that Olly. It's good to have another option that works  now I'm wondering how long I can use the same master bias and even flats or if I need to take them each time. What do you do Olly new set each time? 

[tooth_dr]

Tried this last night, seems to be the same or possibly slightly better, certainly no worse.  Handy tip.