Re: Question about darks


Aug 17, 2010

 


----------------------------

#42265 Aug 17, 2010

Hello

I have a Starlight H9C. It is cooled but not temperature controlled. I keep

all my darks in a single folder and let maxim organize them. However, a

10min dark taken in the summer is going to be different from a 10 minute

dark taken in the winter isn't it? Does maxim take dates into account as

well as exposure lengths when apply darks automatically? Lately I have just

taken darks at every session and kept them with the images and only used

darks from the same night for calibration.

What about master darks? Are these combinations of different exposure

lengths and temperatures?

Just trying to learn how to use darks properly.



Dale



----------------------------

#42267 Aug 17, 2010

With the H9C you shouldn't really need to subtract dark frames. Mainly you're just adding noise. Take a look at a dark frame. If it is basically flat with some warm/hot pixels here and there, I would suggest just making a hot pixel map and forget the dark frames altogether.



If you decide you do need dark frames, then it's best to average (better yet sigma-combine) a ton of them to reduce the noise in the master dark as much as possible.



On Aug 18, 2010, at 1:44 AM, Dale Ireland wrote:



> Hello

> I have a Starlight H9C. It is cooled but not temperature controlled. I keep

> all my darks in a single folder and let maxim organize them. However, a

> 10min dark taken in the summer is going to be different from a 10 minute

> dark taken in the winter isn't it? Does maxim take dates into account as

> well as exposure lengths when apply darks automatically? Lately I have just

> taken darks at every session and kept them with the images and only used

> darks from the same night for calibration.

> What about master darks? Are these combinations of different exposure

> lengths and temperatures?

> Just trying to learn how to use darks properly.

>

> Dale

>

>

>

>

---------------

>

> Yahoo! Groups Links

>

>

>



Robert J. Vanderbei

Princeton University

www.princeton.edu/~rvdb

Co-author of forthcoming book Sizing Up The Universe















[Non-text portions of this message have been removed]



----------------------------

#42268 Aug 17, 2010

Why doesn't it need a dark? When do you need one?

Dale

> -----Original Message-----

> From: MaxImDL@yahoogroups.com

> [mailto:MaxImDL@yahoogroups.com] On Behalf Of Robert Vanderbei

> Sent: Tuesday, August 17, 2010 11:16 AM

> To: MaxImDL@yahoogroups.com

> Subject: Re: [MaxImDL] Question about darks

>

> With the H9C you shouldn't really need to subtract dark

> frames. Mainly you're just adding noise. Take a look at a

> dark frame. If it is basically flat with some warm/hot

> pixels here and there, I would suggest just making a hot

> pixel map and forget the dark frames altogether.

>

> If you decide you do need dark frames, then it's best to

> average (better yet sigma-combine) a ton of them to reduce

> the noise in the master dark as much as possible.

>

>

> On Aug 18, 2010, at 1:44 AM, Dale Ireland wrote:

>

> > Hello

> > I have a Starlight H9C. It is cooled but not temperature

> controlled. I

> > keep all my darks in a single folder and let maxim organize them.

> > However, a 10min dark taken in the summer is going to be different

> > from a 10 minute dark taken in the winter isn't it? Does

> maxim take

> > dates into account as well as exposure lengths when apply darks

> > automatically? Lately I have just taken darks at every session and

> > kept them with the images and only used darks from the same

> night for calibration.

> > What about master darks? Are these combinations of

> different exposure

> > lengths and temperatures?

> > Just trying to learn how to use darks properly.

> >

> > Dale

> >

> >



----------------------------

#42270 Aug 17, 2010

Make a master bias frame by stacking a zillion 0 second "dark'' exposures.

By "zillion", I mean a zillion. The more the better. Say 1000.

It's not so hard to do. Do it in batches of 50. Sigma-reject combine each batch.

Do 20 batches. Then average the 20 individual stacks. Of course, you can delete the individual images after each batch is stacked so that you don't have to have 1000 images on your computer at one time.



Make a master dark the same way. Since the individual exposure time is not 0 seconds but something like 5 minutes, it would take a looong time to do a stack of 1000 dark frames. Do as many as you can. Let it run all day. At 11 per hour you can get 110 in 10 hours. That should be enough. Sigma combine them.



If the master dark made as above looks basically the same as the master bias, then you don't need darks.



I think most people who have done experiments like this have decided that the SXV*-H9* family of cameras do not need dark frames. They also don't need biases (because the master bias just looks basically like a flat gray image with some hot pixels) but if you go to the trouble of making a master bias, you might as well use it.



--Bob

On Aug 18, 2010, at 3:34 AM, Dale Ireland wrote:



> Why doesn't it need a dark? When do you need one?

> Dale

>

>> -----Original Message-----

>> From: MaxImDL@yahoogroups.com

>> [mailto:MaxImDL@yahoogroups.com] On Behalf Of Robert Vanderbei

>> Sent: Tuesday, August 17, 2010 11:16 AM

>> To: MaxImDL@yahoogroups.com

>> Subject: Re: [MaxImDL] Question about darks

>>

>> With the H9C you shouldn't really need to subtract dark

>> frames. Mainly you're just adding noise. Take a look at a

>> dark frame. If it is basically flat with some warm/hot

>> pixels here and there, I would suggest just making a hot

>> pixel map and forget the dark frames altogether.

>>

>> If you decide you do need dark frames, then it's best to

>> average (better yet sigma-combine) a ton of them to reduce

>> the noise in the master dark as much as possible.

>>

>>

>> On Aug 18, 2010, at 1:44 AM, Dale Ireland wrote:

>>

>>> Hello

>>> I have a Starlight H9C. It is cooled but not temperature

>> controlled. I

>>> keep all my darks in a single folder and let maxim organize them.

>>> However, a 10min dark taken in the summer is going to be different

>>> from a 10 minute dark taken in the winter isn't it? Does

>> maxim take

>>> dates into account as well as exposure lengths when apply darks

>>> automatically? Lately I have just taken darks at every session and

>>> kept them with the images and only used darks from the same

>> night for calibration.

>>> What about master darks? Are these combinations of

>> different exposure

>>> lengths and temperatures?

>>> Just trying to learn how to use darks properly.

>>>

>>> Dale

>>>

>>>

>

>

>

>

---------------

>

> Yahoo! Groups Links

>

>

>



Robert J. Vanderbei

Princeton University

www.princeton.edu/~rvdb

"Just what the truth is, I can't say anymore." -- Moody Blues









[Non-text portions of this message have been removed]







----------------------------

#42271 Aug 17, 2010

Hi Dale,



Dark frame subtraction is only worthwhile if doing so improves the signal to noise ratio of your images. Dark 'noise' is an unwanted signal which builds up as time passes, due to thermally released electrons being collected by the image pixels and adding to your wanted signal. Subtracting a 'master dark' will reduce this unwanted signal, but will also add some noise of its own - the amount being dependent on the number of darks that were used to create the master. If you use a large number of averaged dark frames, the noise added will be pretty small, but the H9C camera has an exceptionally low inherent dark noise and so the gain in signal to noise ratio may be vanishingly small. I personally never use dark subtraction - I just kill the hot pixels with a defect map in Maxim and leave it at that.



Regards,

Terry

----- Original Message -----

From: Dale Ireland

To: MaxImDL@yahoogroups.com

Sent: Tuesday, August 17, 2010 7:34 PM

Subject: RE: [MaxImDL] Question about darks







Why doesn't it need a dark? When do you need one?

Dale



> -----Original Message-----

> From: MaxImDL@yahoogroups.com

> [mailto:MaxImDL@yahoogroups.com] On Behalf Of Robert Vanderbei

> Sent: Tuesday, August 17, 2010 11:16 AM

> To: MaxImDL@yahoogroups.com

> Subject: Re: [MaxImDL] Question about darks

>

> With the H9C you shouldn't really need to subtract dark

> frames. Mainly you're just adding noise. Take a look at a

> dark frame. If it is basically flat with some warm/hot

> pixels here and there, I would suggest just making a hot

> pixel map and forget the dark frames altogether.

>

> If you decide you do need dark frames, then it's best to

> average (better yet sigma-combine) a ton of them to reduce

> the noise in the master dark as much as possible.

>

>

> On Aug 18, 2010, at 1:44 AM, Dale Ireland wrote:

>

> > Hello

> > I have a Starlight H9C. It is cooled but not temperature

> controlled. I

> > keep all my darks in a single folder and let maxim organize them.

> > However, a 10min dark taken in the summer is going to be different

> > from a 10 minute dark taken in the winter isn't it? Does

> maxim take

> > dates into account as well as exposure lengths when apply darks

> > automatically? Lately I have just taken darks at every session and

> > kept them with the images and only used darks from the same

> night for calibration.

> > What about master darks? Are these combinations of

> different exposure

> > lengths and temperatures?

> > Just trying to learn how to use darks properly.

> >

> > Dale

> >

> >











[Non-text portions of this message have been removed]



----------------------------

#42280 Aug 17, 2010

Hi Dale.



I agree with Bob and Terry.



I never use dark frames with either the SXVF-H9, -H9C or -M25C cameras. You probably don't even need bias frames unless you calibrate your light frames with flat frames (as I do).



Also since your using MaxIm, use the dither function when capturing your images and a outlier rejection method (like SD Mask) when combining images. That way you won't even need to apply a bad pixel map.



Gordon

--- In MaxImDL@yahoogroups.com, "Terry Platt" tplatt@...> wrote:

>

> Hi Dale,

>

> Dark frame subtraction is only worthwhile if doing so improves the signal to noise ratio of your images. Dark 'noise' is an unwanted signal which builds up as time passes, due to thermally released electrons being collected by the image pixels and adding to your wanted signal. Subtracting a 'master dark' will reduce this unwanted signal, but will also add some noise of its own - the amount being dependent on the number of darks that were used to create the master. If you use a large number of averaged dark frames, the noise added will be pretty small, but the H9C camera has an exceptionally low inherent dark noise and so the gain in signal to noise ratio may be vanishingly small. I personally never use dark subtraction - I just kill the hot pixels with a defect map in Maxim and leave it at that.

>

> Regards,

> Terry

>

> ----- Original Message -----

> From: Dale Ireland

> To: MaxImDL@yahoogroups.com

> Sent: Tuesday, August 17, 2010 7:34 PM

> Subject: RE: [MaxImDL] Question about darks

>

>

>

> Why doesn't it need a dark? When do you need one?

> Dale

>

> > -----Original Message-----

> > From: MaxImDL@yahoogroups.com

> > [mailto:MaxImDL@yahoogroups.com] On Behalf Of Robert Vanderbei

> > Sent: Tuesday, August 17, 2010 11:16 AM

> > To: MaxImDL@yahoogroups.com

> > Subject: Re: [MaxImDL] Question about darks

> >

> > With the H9C you shouldn't really need to subtract dark

> > frames. Mainly you're just adding noise. Take a look at a

> > dark frame. If it is basically flat with some warm/hot

> > pixels here and there, I would suggest just making a hot

> > pixel map and forget the dark frames altogether.

> >

> > If you decide you do need dark frames, then it's best to

> > average (better yet sigma-combine) a ton of them to reduce

> > the noise in the master dark as much as possible.

> >

> >

> > On Aug 18, 2010, at 1:44 AM, Dale Ireland wrote:

> >

> > > Hello

> > > I have a Starlight H9C. It is cooled but not temperature

> > controlled. I

> > > keep all my darks in a single folder and let maxim organize them.

> > > However, a 10min dark taken in the summer is going to be different

> > > from a 10 minute dark taken in the winter isn't it? Does

> > maxim take

> > > dates into account as well as exposure lengths when apply darks

> > > automatically? Lately I have just taken darks at every session and

> > > kept them with the images and only used darks from the same

> > night for calibration.

> > > What about master darks? Are these combinations of

> > different exposure

> > > lengths and temperatures?

> > > Just trying to learn how to use darks properly.

> > >

> > > Dale

> > >

> > >

>

>

>

>

>

> [Non-text portions of this message have been removed]

>







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