Why are my darks dark?

[carl20171]

Hello, I just got an Atik 490 CCD and set up in the driveway and took some darks with my Edge 11 HD. I am running the camera at -20 and the air temp is already 28 deg F or -2 C for all my friends on the other side of the ocean. I was told this was a vey low noise CCD and darks my not be needed. The darks look completely black. Could this be that low of a noise at that low of a temp? Would that mean I don't have to subtract darks? Or could there be things in the dark frame I cant see with my eye? I dont mind taking darks and subtracting them anyway but was just looking for other opinions. 

[Stargazer33]

Have you tried stretching one of your darks to see if anything is there?

[Davey-T]

If you have some software that can analyze the image you can run your cursor over it get pixel values.

Dave

[lensman57]

Hello, I just got an Atik 490 CCD and set up in the driveway and took some darks with my Edge 11 HD. I am running the camera at -20 and the air temp is already 28 deg F or -2 C for all my friends on the other side of the ocean. I was told this was a vey low noise CCD and darks my not be needed. The darks look completely black. Could this be that low of a noise at that low of a temp? Would that mean I don't have to subtract darks? Or could there be things in the dark frame I cant see with my eye? I dont mind taking darks and subtracting them anyway but was just looking for other opinions. 

How long was the sub length for the Darks? I have not used Darks for my Atik 428 below 1200s sub length. If you stretch the master dark you may see some hot pixels or cosmic ray hits on the sensor but besides that the semsors are so quite that you do not need Darks unless you go to 1200~1800s . Just take about 100~ 200 Bias frames rand as many Flats as possible and the end result will be good.

A.G

[ollypenrice]

Just give them a stretch to confirm the situation. 

It is always best to take darks with Atik's metal chip cover on the camera. Light leakage is remarkably sneaky when it comes to creeping in on long exposures!

You might find that just a master bias will serve perfectly well as a dark.

Olly

[carl20171]

This is great info. Thanks to all. I will try the stretch to see what I get. 
I use hyperstar and was only taking 60 second exposures of M42 and that was over exposed I think , because when I slide the stretch all the way to the right in Maxum the trapezium is just a big burnt out white blob.  So I was taking the darks to match the lights. 

For the metal chip cover , Does that mean pulling the CCD off the scope and covering it for darks every time I change exposure time?

[Davey-T]

You only need a very short exposure for the Trapezium then mask together in Photoshop.

If you have Maxim, open a dark frame and use the "info" tab and run cursor over the image to see pixel values.

You can make a library of master darks in Maxim and use them for all your images matching temp/exposure.

Dave

[carl20171]

Yep the stretch was the ticket. There is a lot of noise there. Evenly covers the whole picture with white speckles. 

Davey when you say mask together in photoshop , is that the same as stacking in Maxum?

What should I see as a pixel value? My dark average is 350?

If I am going to use darks, how many on average should I use to make the master dark?

[Davey-T]

Not the same as stacking, you need stacked exposures for the nebula and stacked short exposures for the Trapesium then save as TIFFs in Maxim and import into an image processing program, you can then blend them together to reveal the whole nebula, what they call a high dynamic range image where it's impossible to capture it all at one exposure setting.

Completely black is zero, not sure what mine read as the imaging laptop is in the obs'y and it's chucking it down outside.

Lots of difference of opinion on how many darks make a master dark after 16 or so the law of diminishing returns means the extra images contribute very little, however some people claim they get better results stacking 30 or 40 darks to make a master, I only do 16 so suck it and see.

Dave

Another thought I never found any advantage running my Atiks at more than minus 15c