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My darks ain't dark!!


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Hi all,

So I thought I would start building a dark library.  With the benefit of a permanent set-up inside a dark observatory I took 12x600 secs and created a master.  Under normal screen stretch it looks like you would expect it to but if you exaggerate the stretch I get the very odd effect below.  Given the obsy was effectively dark (roof on and I even draped tea-towels over the camera), what could be causing this? 

Many thanks,

Ig

Master_Dark 1_1391x1039_Bin1x1_Temp-10C_ExpTime600s.jpg

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There can be several causes

1. Light leaking in. I cover my lense/camera with aluminium foil and keep it in a dark box when shooting darks.

2. Amp glow. The camera's electronics heat part of the sensor, increasing the dark current locally. As this will also occur in light frames, oddly enough you want this also in dark frames. It will calibrate out during preprocessing. Ideally you want to avoid it in lights and in darks of course.

3. ...

Check your light frames.

Check if the effect changes if you cool your camera.

Hope this helps.

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The other , and most common leakage point is the viewfinder , light easily backtracks through the prism down into the body.

You will find ( if using a Canon at least ) a small rubber "bung" attached to the carrying strap that is supplied for this very purpose that slips into the grooves and blanks it off ... :happy8:

rubbereyepiece.jpg

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Thanks. I should have mentioned I am using a cooled Atik 314L+. If the amp glow suggestion still applies I would assume it is that given there was no source of light to leak in? Will have another play today.

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It's light leakage. Atik supply a solid, black, metal screw-on cover not only to allow you to protect the chip window physically but to generate excellent darks. You really should use this. (If ever, when taking flats, you get a jet black screen it is because the chip has been saturated. It will usually reset if you reduce the exposure but, if it doesn't, you need to put the maetal cap on and take a sub. That should bring it back to life. I saw this on my 16HR.)

When you buy a CCD you are paying for extreme sensitivity to low levels of light. Even with a metal lens cap on the refractors, sealed electric filterwheels and a dark observatory I cannot take accurate darks. I have to use the chip window cover.

Olly

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Thanks Olly - is that an "extra" cover they supply or should it have been in the box originally? I have it had it a just over a year now and can't recall seeing one.  Perhaps in the meantime I should dismount the camera from the filter wheel and scope and block the window with a well taped cap?  But also, taking into account what was said above by Wim, albeit about glow, if this leak is in my optical system, should I also leave it in the darks to calibrate out or would the flats take care of it?  And lastly, could the leak possibly be down at the other end where the primary mirror sits?  I think I have noticed daylight coming in there before.  Will go and check later but should I think about covering the primary ends with blackout material? (using a 150pds newt). (Might also explain some of the gradients I seem to get on my lights!)

Your thoughts are most appreciated!

Thanks.

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All my Atiks have come with the solid metal cap. I'm sure they'd supply one.

On a Newt you will have to be very careful with flats. I have never done DS imaging with a pure Newt but I would expect light leakage to be a big issue if trying to do flats other than in darkness because of light ingress at the bottom. I'd ask the Newt imagers on here how they do it. I'll ask a friend as well and see what he says.

By definition you'll be imaging in the dark so leakage should not affect your lights. Don't try to mix the functions of darks and flats. Flats are divided and darks are subtracted. Darks should contain only the electronic noise of the camera, not leaked light. Flats should replicate the night time light path of the setup. I would strongly advise sticking, so to speak, to 'the rules.'

Olly

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So some success here I think.  If you look at the new image the leaks seem to have been banished. I am pretty certain the circulars were coming from the primary end (which I covered with blackout material) although I was still getting some gradient so cover the whole of the observing end in a towel.  (Will need to find a better solution for that!) So that is good. But I have noticed in the image below a semi-circular smear just right of centre. What on Earth could be causing that?? 

 

Master_Dark 1_1391x1039_Bin1x1_Temp0C_ExpTime600s.jpg

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