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subs, flats, darks, bias........


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Could someone explain to a me the different terms sub, flats, dark, bias etc etc etc used in AP. What prompts the photographer to decide which combination to use (I bet the answer is experience), a recent post on here had something like

"60x2mins Subs, 15x Flats & 15x Bias."

What technique would typically be used to get the subs (I take it subs are individual exposures) flats etc, i.e. I presume you don't stand with the remote in your hand and click the button every two mins?

Sorry if this has been covered elsewhere, I did look but can be hard to locate in such an active forum.

Thanks in advance.

Edited by IvanT
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Hi Ivan,

Sub - this is the light frame. An timed exposure of an object.

Dark - this is the same as the light frame, and will have the same duration and settings as the light frame, but will have the lens cap on (this shows noise and hot pixels that will then be removed in the stack process).

Flat - This is a calibration frame taken with a light box (or white T shirt over the front of the scope or various other methods). The flats will be used in the stacking process to remove things like vignetting (uneven field illumination). It is VITAL that the flat frame is taken at the exact same focus as the light frame, with no change of anything in the lightpath.

Bias - These are also calibration frames use in stacking. The bias frame is a very quick exposure (1/1000s or quicker if possible) and enables read noise (I think it is) to be subtracted from the Subs.

Someone will be along soon to correct me ;)



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Thanks for the reply Ant. What's the decision thought process, i.e. why 60 subs and not 6, is it simply more is better, i.e. try 10 subs of M31 some night using the method you describe, do it again with 20 sub the next night & compare, is it just trial and error until I find what works?

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which combination to use

Ideally you would use all the kinds of calibration frames, or regret it later. None of the calibration frames need a clear sky so they can be scheduled at a different time, BUT: the "darks" need to be taken with the camera at the same temperature as the "lights" (or "subs") and the "flats" need to be taken with the optical setup exactly as it was during the "lights".

What technique would typically be used to get the subs

Depends on the camera: with a DSLR, you can use a programmable remote control gizmo or a laptop, with a CCD I guess you need the laptop. There's some advanced (and expensive) software that can organize all the operations.

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What's the decision thought process, i.e. why 60 subs and not 6,

You shoot as many as you can to improve your image. Combining the individual subs in software yields cleaner images with less noise, which in turn enables finer detail to be teased out by postprocessing on the computer.

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If you break the photography process down into parts, you can see how each calibration frame helps:

First you have your mirrors, lenses, filters, field flatteners, focal length reducers, integrated filters on the sensor and the like. All photons have to go through this. There's dust on everything affecting the chances that a photon will make it through. There's also the vignetting effect which makes the centre of the image brighter than the edges if your optical system is not up to scratch. The "flats" take care of all those variations by recording the response to a perfectly even illumination.

Then you have the photosite, the place where the photon gets converted to electrons. Each pixel comes from one photosite. They are all a bit different. Some produce electrons even when there's no light. You hunt these down with the "darks" by measuring the response to complete darkness. Some just won't produce electrons no matter what. They are taken care of by the "flats" as it's the same as if some dust had settled just in front of them.

Then you have the amplifier that turns the electrons from each photosite into something that can be measured. Again, there's variability and you measure this with the "bias" or "offset" frames.

Once you've measured all these calibration frames you can correct your "lights" (there's software like DSS and Iris to name just some gratis ones that do this) and produce what you should have seen if you had a more perfect system, a more accurate rendition of the sky.

Then it's a question of "stacking" lots of calibrated "subs" to reduce the statistical noise that is inherent in any sampling.

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