sharkmelley

I'm pretty sure the black level on the 1100D is 2046-2048, but this was already subtracted from the values I posted

So my original analysis stands then.  Removing the microlenses (and maybe some additional effect) is reducing the pixel sensitivity by a factor of very roughly 3. 

I think the main reason for having a debayered DSLR is narrowband imaging. Now you have every pixel collecting light, say with a H alpha as oposed to only 25% of them. This is a huge improvement. For colour images, I think we would be better off with a sensor with the CFA intact as the colour interpolation algorithms are pretty good and the microfilters are still there. But for narrowband imaging... definitely, the mod is truly worthwhile.

This has always been my understanding also.  All the pixels will now be sensitive to H-alpha instead of one quarter of them.  But if the price to pay is a reduction in sensitivity by a factor of 3 then not much has been gained. 

I'd love to be proved wrong so if someone has some evidence of the improvement.  I would also happily accept that resultswith  the 1100D may not be typical of what can be achieved with debayering other DSLR sensors.

Mark

I'm not sure it works this way. A good experiment would be to compare a mono ccd luminance Flat frame with mono ccd R, G & B filtered Flat Frame. And see if the ADUs add up?

The flaw in my argument is that the flat frame needs to be bias subtracted. 

I'm assuming the CR2 file you posted was recorded using white light (since the R,G & B pixels all have values) The debayered pixels are now able to collect the R,G & B elements of the light because the filter has been removed.  So once the bias is subtracted then the (bias-subtracted) pixels on the left hand side (the debayered side) ought to show roughly the sum of the (bias-subtracted) R,G,B on the right hand side (the non-debayered side) unless losing the microlenses is having a very significant effect.  

Does anyone know the bias level for the 1100D?  For other Canon sensors it is generally a power of 2 e.g. 512, 1024, 2048.    Armed with the bias level, a very rough approximation can be made of the effect of losing the microlenses.

Mark

Has anyone noticed that the 1100D signal levels seem to drop once debayered?

I debayered half the sensor and took a flat then measured the pixel values on either side. The debayered side was on average lower.

That's a very interesting result.   Intuitively (as someone who has never tried debayering)  I would expect each debayered pixel to have a value equal to the sum of R,G & B values from the CFA pixels and then reduced by a factor due to the loss of the microlenses.  So are we saying that the loss of the microlenses reduces the sensitivity by a factor of around 3 or so?  Or is something else happening as well?

If this result is typical then it indicates to me that debayering an 1100D will result in very little overall advantage from an imaging point of view, though it's a very interesting technical exercise.

Is this result also typical of those obtained from other sensor types?

Thanks for posting the CR2 file, it was most informative

Mark

There is also the matter of bit depth, which is why decent star colour from DSLRs is rarely seen, though it is sometimes seen.

Bit depth is a valid point.  But in practice it is not too much of a problem.  I will often take a few shorter exposures with no saturation of the brighter stars.  The stack of shorter exposures is then combined with the stack of longer exposures to form an HDR stack before the main post-processing sequence begins.  Thus there is no loss of image quality but it does increase the data acquistion time.

Mark

You have an Epsilon and, at F2.8, DSLRs really can perform close to CCD levels on many targets. At more mundane F ratios I'm firmly of the CCD persuasion. But they cost.

Olly

Exactly right.  At fast F-ratios, unless the sky is pristine, the only real difference in quality between a DSLR and a cooled one-shot-colour camera is down to the quantum efficiency of the sensor.  At slower F-ratios the OSC has a clear advantage unless the DSLR sensor is cooled. So it makes me very interested to see how the JTW performs - though I think the cooling has been taken to the extreme!  For narrow band imaging the choice is obvious - it has to be a cooled mono CCD.

Mark

I am a great fan of DSLRs - a Canon 350D (with the internal filter removed )is still my main imager.  If budget is your main limiting issue then a DSLR is no-brainer.

However, from my experience I should point out the following couple of points.

The C9.25 is still a slow optical system even with a focal reducer.  This means that without cooling, you will probably find that thermal noise becomes the limiting factor in your images in the Summer and mild Spring/Autumn evenings.  I think an uncooled DSLR is ideally paired with a fast scope.

I also think you will quickly find you want to mod the camera for H-alpha sensitivity.

Mark

That should definitely make a difference to your imaging.  Banished, the blindspot at zenith! 

Mark