Flats and Dark Flats......

[Digz]

Ok here goes, I hope someone can help me

I was out last night capturing some more data of NGC7000 with a view to processing over the weekend.

At the end of my imaging session I took my Darks as usual, although the battery died meaning I couldn't get as many as I wanted after that I used my Mac screen (turned white) to capture some Flats and then Dark Flats with the lens cap on.

My question is what do I do with these? I read in various books and internet pages that say the Flats (and Darks to a certain extent) need to be calibrated into Masters. As I understand it, for example, the Flats must be Dark Flat subtracted and Bias Frame Subtracted (?) - not sure if I've got this right as my memory is bad!

Is this something Deep Sky Stacker does for you when you load in the Darks and Flats? Or is there another process I need to go through to achieve these calibrated Masters?

TIA for all and any help offered

[Demonperformer]

Load all your subs into the appropriate section in DSS and it does it all for you

[Paxo]

There's a nice little diagram of what's subtracted from where in the DSS help files.

Steve

[Digz]

Ok, cheers folks, that was easier than I thought Feels a bit of a dumb question now the answer was so simple.

[ollypenrice]

Hang on, be careful not to take your bias out twice. Isn't the bias contained within the dark anyway? I never use bias other than for flats and then I use a stacked master bias as a dark for flats.

Sorry if I am confusing the issue...

AstroArt also has boxes for bias and darks for flats but I ignore them and use only a master dark, a master flat (pre calibrated with the master bias) and nothing else. It works for me.

Olly

[Digz]

Cheers Olly, thats just confused the hell out of me now, lol

Ive loaded everything into DSS and gonna see what the outcome is. I would assume that DSS should know what to do with each file type?

Ill see what the outcome is and maybe try again at the weekend without bias to see if there is any noticeable difference.

[Digz]

OK, here is a link to an image I processed last night:

http://stargazerslounge.com/imaging-widefield-special-events-comets/115529-ngc7000-again.html

This actually didnt have any flats as when added (along with dark flats) the image had some serious problems with vignetting - it made it a lot worse than when they were not added.

[jgs001]

Something has gone wrong with the flats then... Can you post one of them...

I agree with Olly, and it's stated in the DSS help, you don't need Bias and darks... only one or the other.

[Digz]

hmmm, thats interesting as I am using both Bias and Darks, I guess I should try another interation without the bias frames and see what the final result is.

Ill post a flat when I get home tonight, thanks for the input John

[jgs001]

The problem is, and I think it's inherent in DSS as it applies all the calibration frames to all the image frames... if you apply darks to your lights, and you have flats, you should use bias. But then you subtract the bias from the lights twice, and I've always found this to produce a horrible green tint with my SLR images. So I've stopped using Bias.

[Digz]

Ok, here is an example flat, just converted from RAW to Jpeg.

Is it any good?

[jgs001]

can you post up a light frame too ? you do seem to have more of a vignette on the left than the right...

Camera Maker: NIKON CORPORATION

Camera Model: NIKON D200

Lens: 70.0-200.0 mm f/2.8

Image Date: 2010-09-29 22:53:26 +0100

Focal Length: 70.0mm (35mm equivalent: 105mm)

Aperture: f/3.2

Exposure Time: 0.020 s (1/50)

ISO equiv: 800

Exposure Bias: +0.67 EV

Metering Mode: Matrix

Exposure: shutter priority (semi-auto)

White Balance: Manual

Light Source: Flash

Flash Fired: No

Color Space: Adobe RGB (1998)

The EXIF from the flat... I notice this was shot in Shutter Priority.. that means the shutter speed was locked and the camera adjusted the lens aperture... What aperture setting were you using for the data capture ?

The Aperture should remain fixed or you will get flats that don't match.

[Digz]

Ah right that helps, so the flats need to be taken with the same aperture value as the lights? I read somewhere that it flats were easiest taken set in shutter priority but I guess if the aperture must the same as the lights then this isnt right.

Here is a single light frame:

[jgs001]

Here's the EXIF from the light

Camera Maker: NIKON CORPORATION

Camera Model: NIKON D200

Lens: 70.0-200.0 mm f/2.8

Image Date: 2010-09-29 20:37:02 +0100

Focal Length: 70.0mm (35mm equivalent: 105mm)

Aperture: f/4.5

Exposure Time: 128.000 s

ISO equiv: 800

Exposure Bias: +0.67 EV

Metering Mode: Matrix

Exposure: Manual

Exposure Mode: Manual

White Balance: Manual

Light Source: Flash

Flash Fired: No

Color Space: Adobe RGB (1998)

The Aperture was changed, it may not seem like much, but that will allow more light through and alter the way dust etc is "seen" from the sensor. Alter the aperture, you're actually altering the focal ratio, this is the number reported on the camera (the ratio, not the actual hole, which is why as the hole gets smaller the number goes up). You are also more likely to get vignetting (if I've understood this bit right) with the lens opened up wider than with it stopped down some, which means that your flats have more vignetting than the lights.

Altering the aperture, alters the light path, and you can't do that if you want good flats (that also means you can't adjust focus etc). The other thing, if you have the shutter speed to close to the refresh rate of the display you can get very odd effects in the flats as the camera will capture the refresh rate, and I think 1/50s is in that ballpark.

[Digz]

Hi John, thanks for explaining that quiet neatly. Makes perfect sense now. I do feel a bit stupid though as I should have cottoned onto this being a keen photographer as well.

Still, big lessoned learnt which I shall be implementing on my next outing.

Thank you

[ollypenrice]

Good points above. BTW, that flat would arouse my suspicions instantly since it too flat to be credible.

Olly

[jgs001]

I thought you might be a keen photographer, just looking at the kit in use... that's expensive glass ... I nearly didn't reply...

Oh something else to watch for... it looks like you've dialed in 2/3's stop exposure compensation... makes no difference in manual, but using one of the semi auto modes it will affect your flats. You want to dial that back out for the flats I reckon, you might have slightly over exposed flats.

[Digz]

Cheers guys.

@Olly, Im not too surprised at the 'flatness' of the flat as it was taken through my 70-200VR which does not suffer from vignetting, plus the sensor was cleaned beforehand. I do get what your saying though, when you take a flat you want to see something in it to ensure its a good flat.

@John - thanks for comments Im a bit dumb sometimes and should have removed the exposure compensation as your quite right it will obviously effect the exposure settings in semi automatic modes, as you say.

[Photosbykev]

.........You are also more likely to get vignetting (if I've understood this bit right) with the lens opened up wider than with it stopped down some, which means that your flats have more vignetting than the lights.....

Can you clarify this wrt to vignetting please? If the aperture is more open then the depth of field is less and the vignetting is less apparent because it is defocussed more. If the aperture is closed down then the DoF increases and the vignetting is more obvious because it is more in focus.