Steve's Place

Photography and Astrophotography

Subs & Frames

What are subs ? lights ? dark frames ? bias frames ? flats ? dark flats ?

Here, these calibration frames are explained in laymans terms – that way, I might understand it ๐Ÿ˜‰

IMPORTANT: BackyardEOS refers to each one of these calibration terms hence the reason for this little guide.

Please note that all of these camera files are raw files – jpegs are compressed and lossy so of absolute no use whatsoever for calibration.

I have obtained two sets of each, one for ISO800 and the other for ISO1600. I would love to capture at ISO400 but the exposure times would be too long and sensor noise would be a real problem. ISO3200 would considerably reduce the exposure time but again, the image noise might be too great. So ISO800 & ISO1600 will be the ones to choose – if conditions are perfect drop to ISO800 – if the moon is quite strong then select ISO1600 for a shorter duration capture and hence less light pollution.

LIGHTS also known as SUBS – These are the actual photos taken using a DSLR or CCD imaging camera. These should be grouped together – even renamed to reflect the exposure time and ISO level as these are extremely important when it comes to processing.

DARKS or DARK FRAMES – This is the same as lights but with the scope cover on. Equally important is the focus, ISO and exposure time which must remain exactly the same as above. Darks reflect the camera induced noise.

FLATS – Place a white teeshirt in front of the scope just b4 sunset or at least later in the day, set DSLR to AV mode and aim for an exposure under 1 second mine were 1/180th sec@ISO800 and 1/250th@ISO1600. Flats records the uneven illumination of the image and can also record the dust within the camera.

DARK FLATS – Same time duration as your normal flats (remember your ISO settings) but with scope cover on. Dark Flats help to reduce the noise in the flat frames above. IMPORTANT: if you use Dark Flat frames you do not require Bias frames.

BIAS – put camera cap on body (ie no lens) and fire away at the fastest speed your DSLR can do (mine is 1/4000th – remember to set your ISO to the same you are using for lights – I have done some for ISO800 and ISO1600). IMPORTANT: if you use Bias frames you do not require Dark Flat frames.

It is worth putting a library together and grouping them in two sets – ISO800 and ISO1600.
Then creating sub-directories of Darks from say 30 second to 10minute in minutely intervals and shooting at least 15 at each exposure (ie 11 folders each containing 15 images). Remember, the flats, dark flats and bias are each ISO dependant. You may ask what is the difference between dark flats and bias frames as they both essentially do the same purpose. Apparently, some software processing works better with Bias than Dark Flats and vice versa.

IMPORTANT: yes you are creating libraries but remember, these will be calibrated from the moment you start taking these frames. ie, assuming a perfectly clean sensor and allowing time for the sensor to cool down prior to taking the next frame. Over time your camera will pick up dust and will need to be cleaned or the calibration will be out. Also, the temperature of the sensor will also change depending how cold it gets outside which again will throw the calibration out.

However, using raw files will eat up a lot of disk space so best advise is to keep the sensor clean and when doing the Dark frames allow it to cool down between captures (at least 10 seconds between shots). If disk space is of no problem then you could replicate the procedure for different lighting conditions (ie a set for bright moon and a set for no moon – perfect conditions).

I would consider refreshing and updating your library files every 4 months.

I hope you find this useful ๐Ÿ™‚

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