dark frame problem

[alacant]

Hi everyone
When using dark frames I have black speckles:
https://drive.google.com/open?id=0B_uUt0LyuuEdZlRsQWVJX1BlZlE

Without, it's fine but then I need so many more light frames to compensate. I'm using DSS.
-only occurs when using darks frames
-canon dslr
-20 raw dark frames images
-120s exposures
-flat frames added
-bias frames added

Any ideas what's causing the speckling?
TIA,

[cjdawson]

I have a few questions that can help to indentify that.

Are you shooting in RAW or JPG?

Is your optical train clean?  (by clean I mean about 90% clear. there can be some dust on the refracting/reflecting surfices, but too much will cause problems. I don't think it is this though)

Which camera specifically?  Canon 10D, 20D, 550D, 7D etc?  There's lots of them and whilst they are generally good camera, some models can have issues.  I probably won't make a difference, but it might.

Which scope is it that you are using?

Did you have any filters, focual reducers barlows etc installed?

 

Can you post up the fully set or original images?  maybe share a zip file containing the lights, darks, flats and bias frames.  Also might help identify what's happening.

[Freddie]

Can you post up the dark?

It looks like the dark frame is subtracting data from the light frame that it shouldn't. If there are light spots on the dark where there are black spots on that final image then the dark frame is your problem.

[wxsatuser]

Darks don't match the lights?

[alacant]

Hi and thanks for the replies

Camera: RAW on a 700D filters: no imaging: a refractor method: the light, dark frames are at the same exposure.  The combination usually works great, darks or no darks.

here is the master dark tif

and this is a single CR2 from the same set.

TIA

[almcl]

Don't know if it has a bearing on this issue, but I gave up using darks with my 700D as it was impossible to get them at anywhere near the same sensor temperature as the lights.  Bias frames, yes! Flat frames, absolutely, but darks?  No, they didn't help at all.

 

[wxsatuser]

The 700D most likely has good on sensor dark current suppression, no need for darks.

[alacant]

Yeah. I've been getting the same idea. The problem has been more (only even) apparent since the summer. Nights rarely fall below 20° with the sensor commonly at 38°. Could the high temperature exacerbate the issue I wonder; I see the CCD lords cooling their sensors below freezing. Taking extra light frames and dropping the dark frames maybe the way to go. I too find adjusting the dark temperature to the light temperature all but impossible.

Thanks again for your suggestions.

[Starflyer]

Try using something like Kuso Exif Viewer to check the temperature of your darks against your lights, ideally they should be within a few degrees.

I suspect your darks will show a higher temperature than your lights and the black pixels in your calibrated image are caused by this.  You can reduce the 'strength' of your darks in DSS, try them at 70 - 80%, this may help. I can't recall the exact name of the setting, check the help or maybe someone here can help.

 

Cheers,

Ian

[ollypenrice]

A larger dither (12 pixels or so) will greatly out-perform dark subtraction on a DSLR and will reduce colour mottle in the background as well. This from a good Tony Hallas video which you might be able to Google. I saw it on here.

Olly

[alacant]

Agreed. The dither has it. Thanks to everyone here and over on the phd2 list. The dither settings are not immediately obvious so FWIW I documented them here. I really hope that dark frames are a thing of the past. Here at least. Thanks again and clear skies everyone.