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Using bias frames will allow you to scale your darks. This means if you shoot 10 minute darks, you can use them to calibrate shorter exposures taken at the same nominal temperature and the same ISO. This saves a lot of work. You certainly don't need one for every 30 second increment.

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How do you calibrate your darks using bias frames? Is this something Image Plus does automatically for you?

I'm using Nebulosity2 for acquisition and stacking and it does have an autoscaling dark feature but it's documentation clearly indicates that if you are using darks you should not use bias as the dark already contains the bias error, so the bias error will actually be removed twice, therefore adding noise.

However, Nubulosity2 has a feature called Bad Pixel Mapping which allows to use the same dark for different exposure time as long as the dark duration is of longer duration as the light frames. With BPM however, bias frames should be used.

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750mm 6" Skywatcher Reflector f/5
Celestron CG-5 GT mount
SSAG autoguider mounted on Orion 80mm short tube
Canon 40D unmodified / TC-80N3 remote timer
Orion SkyGlow for Astrophotography light polution filter.
Orion AccuFocus & Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny

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I'm running 40 darks a night in my garage...It's interesting, the first few look different than the rest, so I throw them out. I believe the camera warms up and reaches equilibrium after the first few shots.

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Good to know, it never occurred to me to check the consistency. Thanks.

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750mm 6" Skywatcher Reflector f/5
Celestron CG-5 GT mount
SSAG autoguider mounted on Orion 80mm short tube
Canon 40D unmodified / TC-80N3 remote timer
Orion SkyGlow for Astrophotography light polution filter.
Orion AccuFocus & Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny

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Jerry, I know from reading other threads that you've been using the same master darks since 2005 and have been very successful doing so.

In the thread link above you say the following

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Using bias frames will allow you to scale your darks. This means if you shoot 10 minute darks, you can use them to calibrate shorter exposures taken at the same nominal temperature and the same ISO. This saves a lot of work. You certainly don't need one for every 30 second increment.

---

How do you calibrate your darks using bias frames? Is this something Image Plus does automatically for you?

I'm using Nebulosity2 for acquisition and stacking and it does have an autoscaling dark feature but it's documentation clearly indicates that if you are using darks you should not use bias as the dark already contains the bias error, so the bias error will actually be removed twice, therefore adding noise.

However, Nubulosity2 has a feature called Bad Pixel Mapping which allows to use the same dark for different exposure time as long as the dark duration is of longer duration as the light frames. With BPM however, bias frames should be used.

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Images Plus does it if you just check the box that says "automatic master dark frame matching".

Yes, there is bias contained in the darks, so you don't want to subtract the bias twice, but to you need bias frames to scale darks.

I'm sorry, I can't help you with Nebulosity.

Jerry

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A Beginner's Guide to DSLR Astrophotography
http://www.astropix.com

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A beginners question: must the darks always be the same exposure time and ISO as the images of the object? Just wondering if I can shorten the time compared to the actual exposure.

Thanks;
Steve Coe

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ISO Yes. For duration it depends on the software you use for sacking. If your software has an "autoscale dark" feature then duration is not as important as long as your dark is longer than your lights. Darks are also temperature sensitive but you can do just fine with darks within 5 degrees of your lights IF you use autoscale dark, otherwise temperature should be the same as your lights.

--------------------
750mm 6" Skywatcher Reflector f/5
Celestron CG-5 GT mount
SSAG autoguider mounted on Orion 80mm short tube
Canon 40D unmodified / TC-80N3 remote timer
Orion SkyGlow for Astrophotography light polution filter.
Orion AccuFocus & Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny

Post Extras:

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Yes, there is bias contained in the darks, so you don't want to subtract the bias twice, but to you need bias frames to scale darks.

I'm sorry, I can't help you with Nebulosity.

Jerry

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Thanks Jerry, I will follow the 'letter of the law' as per Nebulisity's documentation and not prodive BIAS frames initially. If I'm unsatisfied, I'll shoot a bias library, It will only take a few minutes anyway and compare the results with and without bias.

Much appreciated.

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750mm 6" Skywatcher Reflector f/5
Celestron CG-5 GT mount
SSAG autoguider mounted on Orion 80mm short tube
Canon 40D unmodified / TC-80N3 remote timer
Orion SkyGlow for Astrophotography light polution filter.
Orion AccuFocus & Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny

Post Extras: