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Author Topic: Lights and Darks  (Read 1984 times)

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Offline graham.b

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Lights and Darks
« on: 17:12:00, 30 September, 2009 »
Ok i am new to this and only in the last couple of days got into a bit more serious side of imaging. I got to understand the dark side, but this light subtraction, i have tried to a little background work.

 It goes as this, it is the light just before dark fallls, but what time do you know when this is. just before street lights. Then there are some that build a box to fit over the end of the tub, with a small wattage light in it.
 So could some explain in lay mans terms how and when, or what to do.
 
 Regards Graham

Offline JohnH

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Re: Lights and Darks
« Reply #1 on: 21:44:46, 30 September, 2009 »
Hello Graham, most CCD images including DSLR's require calibration to remove image sizzle/hot pixels/variation in brightness across image and dust marks.  The lights are the actual images you take, every now and again take a dark with exactly the same exposure with cap on camera this will show noisy pixels - minimum of 3-5 darks, take when cloud passes over to save imaging time.  Bias frames are the fastest shutter speed to isolate readout noise which show as faint lines across image - take 15 of these.  Finally Flats can be taken in twilight or using a diffuser at an evenly illuminated surface - 1/4 - 2sec exposure take 15 of these.  Some make a light box to go over refractor which is easier and will give a better quality flat.  Put your lights/darks/flats/bias into automated software and lights will be calibrated removing all the rubbish from the individual flats and your clean lights can be stacked and then processed to give a smoother image with better detail.

We need to know what scope/camera your using?

John.
SPX350 F4.53 Ultra Grade (BIG WHITE MONSTER) - EQ6mod - ATK-16HR -ATK-EFW - TS 9mmOAG - Mintron12V6 guidecam. 
DMK21AF - ASTRODON Series I LRGB + Ha/S2 5nm + 3nm OIII + UVenus filter.  Astronomik IR742nm/UHC.  Wratten 47.

Offline graham.b

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Re: Lights and Darks
« Reply #2 on: 22:04:24, 30 September, 2009 »
Hello John, in a word, wow. I did not think there was so much to this. But to think of what you have said, it know more than what i would do in dark room with negs. I got the darks, you could take them at any time as long as they are the same size as what i taken with the subs.

 Bias ? you say this is at the fastest shutter, i have a 5D which i will in turn use, at the moment i use the 716, so no shutter on them. This only for the digi crowd.
The flats, so this is, correct me if i am wrong, the Sun is not in view, no stars but a clear sky but not bright. These then would have to be taken on the same day as the main image, for consistence with the sky to be observed that same night.
 For your advice the other day on filters, i got hold of Bern and Q 5 camera and LRGB, with a filter wheel.

 Scopes, LX90 10", with a 3" Sky watcher on top, will be imaging with a both, dippens on the subject.
 
Thank you.
 Graham

 

Offline JohnH

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Re: Lights and Darks
« Reply #3 on: 22:37:43, 30 September, 2009 »
Hello Graham, glad you are sorted with filters and guidecam QHY5.  Digital is totally different from film and does require fiddling but results are way better.  MX716 CCD is cooled so will not need bias frames.  These calibration frames darks/flats/bias are needed for DSLR's because they get warm.

The 716 may need darks but not necessarily each night as hot pixels are in the same place usually, but I would take some each night anyway.  Flats using twilight sky (ccd must be in focus) move each time in case stars are seen just to average out and pixels in CCD only 1/3 full no more this will remove dust shadows if any and even out brightness on CCD removing central bright area that sometimes shows especially with DSLR's.  You don't have to take flats each night unless you get new dust on CCD

Hope this helps, John.
SPX350 F4.53 Ultra Grade (BIG WHITE MONSTER) - EQ6mod - ATK-16HR -ATK-EFW - TS 9mmOAG - Mintron12V6 guidecam. 
DMK21AF - ASTRODON Series I LRGB + Ha/S2 5nm + 3nm OIII + UVenus filter.  Astronomik IR742nm/UHC.  Wratten 47.

Offline graham.b

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Re: Lights and Darks
« Reply #4 on: 23:55:03, 30 September, 2009 »
I think i got this now. All we need is a clear sky, and some days of work. Again thank you it all helps. Just practice.
 Graham

 

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