Darks with different length exposures

[Andrew*]

You can tell when I go out imaging, because the next mornign there's a flurry of "HELP" posts coming up on SGL!

I have some data with different exposure lengths and a dark for each one. How can I get DSS to subtract the correct dark from the pic in question?

I've had problems doing darks in DSS before, so I just used photoshop. A bit longer but it worked! Shall I just do this or is there a quicker way?

Cheers

Andrew

[Psychobilly]

DSS should just sort it out matching darks to lights...

Billy...

[Sam]

You have to make sure that DSS knows which darks to match up so just check in the file list that DSS recognises the exp lengths - if it doesn't then you'll have to manually put them into the properties - just right click on the file (you can also do to multiple selections). By doing this it will match the right darks to the light frames.

hope that helps.

Sam

[narrowbandpaul]

you'll need to include a set of offsets/bias frames, for proper scaling of the darks...the dark current scales linearly with time, but the bias does not

example...

say you took a 5 minute bias frame, and the average intensity was 500...

now suppose a series of bias frames were taken, and the average is 100

the actual thermal component of the first 5 minute dark is 500-100=400

lets say you want a 10 minute dark, but you didnt actually take one....

double the cark current =400*2=800 add the bias back in...=900 for a 10min dark

if you double the first 5 minute dark, thinking that this was fine, the the intensity would be 500*2=1000

that was reasonable 900 isnt a mile from 1000, but with the SXV H9 the offset is 969 and the average dark current around 1100 DN

so true dark = 1100-969 =131 DN

double this, add offset = 262+969 =1231...compare doubling the original dark mistakenly 2*1100=2200>>1100DN

so its important to have a series of bias frames to work out the offset, for correct scaling of darks

Paul

[Andrew*]

Sorry - you lost me on the first line! Straight over my head. I didn't take offset/bias frames. Maybe this is for a different thread, eh?

I've now got dark corners on my pics. I assume this is because DSS made a master dark and applied it to every photo. Can I get it to do is a different way?

Andrew

[Sam]

It will make a Master Dark. Did you take flats? I wouldn't worry too much about the bias frames at the moment - the ones that make the biggest difference are flats and dark - it would seem if you are getting dark corners then a flat might fix that?, maybe.

Sam

[Andrew*]

I think the dark corners are because the master dark was too light in the corners for the exposure length of the individual frames, so when it subtracts it from them, those areas end up too dark, if you get my meaning.

So with a 4 minute dark (lots of amp glow) combined with a 2 minute one, subtracted from a 2 minute light, too much amp glow is subtracted and the result is dark corners.

It's very unlikely to be vignetting, because only the top corners are affected.

I think I'll just do it manually...

Cheers for your help.

Andrew

[narrowbandpaul]

for my SLR, the bias is quite low, and the noise HIGH, for the H9 CCD the bias is high and the noise low. If yours is the first case, darks are more important..if yours is the second then bias will have a greater effect....why not use both?

bias is just the shortest exposure possible...then average them together...about 30 works OK, depending on the read noise (mine is high...poor electronics), but more recent SLR will have lower read noise.

bias are the easiest to take...just keep a cap on the front of the lens...

Paul

[narrowbandpaul]

basically you have subtracted a 3 minute dark from a 2 minute light...so the amp glow is too blame, as you said.

however subtract the bias off the 4 minute dark...half this value to simulate a 2 minute dark...then add the bias back on, the average with your true 2 minute dark. This should produce a far better result. I think