can a a CCD be too sensitive?

[Daniel-K]

i ask this because last night i took this 4 min exposure of the crab and blew it out. any ideas how to stop this? would it do the same with RGB or narrowband?

thanks

Dan

[Gina]

Use a shorter exposure.

[Daniel-K]

Use a shorter exposure.

kinda defeats the object of imaging DSO dont you think?

[Cath]

The trouble is even 16-bit camera's don't have the dynamic range to capture the darker areas whilst not blowing out the brighter areas, we really need 24-bit or better (which don't yet exist).

You have to take images that are blown out to get the darker areas, and also take non blown out images (short exposures) to get the brighter area detail, then combine them in a HDR way (normally manually) to get the dynamic range you want.

[IanL]

kinda defeats the object of imaging DSO dont you think?

Kind of reminds me of a guy I saw once talking about computer backups. He said "The objective is not to have they system report it has successfully taken the backup, it is to be able to restore the data when you need it."

Same principle applies here. The objective of DSO imaging is not to expose for as long as you possibly can, it is to maximise the signal to noise ratio in your image. In other words, what is the point in exposing for four minutes (or more) if your combination of scope and camera saturate pixels before this?

Assuming you want to improve the SNR in your image further with the current configuration, reduce your exposure time until you are almost, but not quite, saturating the brightest part of the object. Then take lots of exposures (subs) and stack them using your preferred software. That way you will increase the SNR without blowing out the highlights.

Other things you can try:

1) If your camera has a gain setting (for CCD) or an ISO setting (DSLR), same thing, different name, then set it as close as possible to unity gain, i.e. so that one photon = 1 ADU unit in the image. That will give you the maximum dynamic range and allow you to expose for the longest time possible. You will still saturate at some point, but if your gain is current more than unity then you can expose for longer.

2) Use filters (colour or narrowband). These obviously block a range of wavelengths depending on the filter, and thus reduce the number of photons per minute on to the sensor. Thus you can expose for longer without blowing the highlights. A narrowband filter such as Ha will block nearly everything, allowing (requiring in fact) much longer exposures than unfiltered or a simple R, G or B filter.

3) Do some HDR processing. Basically take a set of subs at a length that doesn't blow out the highlights, then take another set that does, but hopefully increases the visibility of any fainter nebulosity in the outer regions. Not sure if this will work with the crab which is not particularly extended, but for M42 it is an essential technique as you cannot expose for the outer regions without blowing out the core. Once you have the two sets of data you combine them. This could be by means of layer blending in PhotoShop (ergh) or by using HDR stacking in something like PIxinsight (yay) or another astro processing package. This will join the two sets of data and allow you to see all the detail despite some of the exposures having the brighter regions blown out.

[Daniel-K]

i tried the gain settings diddnt do much i will invest in some filters which im going to need anyway, thanks for the advice ian

[riklaunim]

What camera did you used? It's rather hard to overexpose a Crab Nebula on 4min exposures with typical 16-bit low gain camera.

[Daniel-K]

What camera did you used? It's rather hard to overexpose a Crab Nebula on 4min exposures with typical 16-bit low gain camera.

exactly what i thought, its the mammut bright star using Sony ICX429 Mono Exview CCD chip

[riklaunim]

I did played a bit with Mammut and didn't got such problems, but I was using Nebulosity mostly. The image looks like if the histogram was clipped (like when doing a preview).

[Daniel-K]

i was using NEB3 to capture too, no editing just saved as jpeg to upload here

[andrewluck]

Looks like the image got clipped when it got saved as an 8 bit jpeg.

Andrew

[RikM]

Does Nebulosity apply a screen stretch to preview the image?

[riklaunim]

i was using NEB3 to capture too, no editing just saved as jpeg to upload here

The preview is by default clipped so the real FITS would be not overexposed.