Question for ccd imaging

[Keithp]

Hi All,

Just a quick question regarding imaging. I can take pictures with my Fuji S5700, reasonable to middling stills and not bad avis but can someone clarify something for me so that I understand the concept correctly. The various purpose designed astro ccd imagers that people produce the stunning colour pictures with, do they record an avi or is it a single picture on the ccd that is continuously exposed as the scope tracks the object.

For instance, if it's the latter so the light over that period is recorded on a single picture, bit like a dslr, would the same amount of light recorded in an avi over the same time and then treated to registax or similer produce a similar result, maybe not as good, but again what if I recorded an hour and a half or 2, would the avi run through registax pick up the detail/colours.

Regards

Keithp

[steppenwolf]

do they record an avi or is it a single picture on the ccd that is continuously exposed as the scope tracks the object.

These astro-CCD cameras take single frames each of relatively long exposure lengths (commonly between 180 and 600 seconds) and these are then stacked to give the final image. For deep sky objects, it is necessary to take fewer but longer exposure subframes to capture detail and colour from these relatively 'dim' objects. Taking an AVI with many short exposures of a DSO over the same period of time would not result in sufficient light being captured to show structure and colour.

[Keithp]

Many thanks for the reply Steve, that's cleared it up a bit. I had somehow got the impression that the total value of the light captured in the avi, sort of analogue form could be somehow added together.

Sorry if this sounds a bit stupid, but just trying to sort it in my head for the next equipment purchase... much later in the year though!

Regards

Keithp

[steppenwolf]

Keith, the contents of an AVI can indeed be added together as you suggest and this works extremely well for Lunar and Planetary images both of which are relatively bright but the rules are different for DSOs because they are so relatively dim and the photons falling on the sensor of a webcam for example are not sufficient to register enough to produce an image in the short exposure times available.

[Milamber]

... actually, they are sufficient, but along with the few photons per frame of your image, you would have electrical noise displayed as photons also. This noise would drown out the photons you actually want to keep in short-exposure frames - which is why long exposure is the way most people go - typically, the longer the exposure the better the signal to noise ratio.

Arthur

PS - That is not to say that your thoughts are wrong, the Lucky Imaging team in Ireland do just what you are talking about, but they do have lots more money, faster/cooled cameras, (cooled cameras have lower noise since heat also shows as noise on a ccd image - hence cooled astro cameras), and much better processing systems.