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Originally Posted by gregbradley
I think you meant the other way round Ray. The OSC is way less QE than the mono which is the case with every OSC sensor I am aware of.
Typical QE for a OSC (not sure what a 314 is, is it the Sony ICX694 chip?) is around 25% or less but mono's are usually 50% -77%.
The bottom line, OSC is good for bright nebula but not for galaxies which are dim. Dim objects in OSC will show a ton of noise which is hard to get rid of especially in the dim areas of the image.
Straight luminance images of galaxies are quite nice even without colour. They are like Ha images.
No doubt at all - mono all the way and add colour later otherwise you will be disappointed for sure.
Greg.
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Quote:
Originally Posted by codemonkey
I found the data sheets for both of the sensors in question, which give some seemingly interesting numbers. I say seemingly because I don't even know the metric, let alone what the numbers mean, but I can only assume bigger is better:
The OSC has a more rounded spectral sensitivity curve as well. Assuming I'm correct in that bigger is better, it looks like the OSC is waaaay more sensitive.
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Quote:
Originally Posted by LightningNZ
The ICX285AL (colour) is a newer, better sensor. That's why it's more sensitive. That said, it doesn't change the fact that each pixel will lose ~2/3rds of the signal hitting it because those photons will be of the wrong colour and will be absorbed by the filter sitting above the pixel.
I'm not sure what Ray's saying about galaxies looking bigger with more pixels - I guess you can digitally "zoom" in more with more pixels, but the resolution is set by the diameter of your scope and the image size by the focal length...
One thing not otherwise mentioned here is that the 314L has a physically larger chip, and that means a larger field of view. That will likely make it easier to frame your target and give you more options for things to image without having to take mosaics.
Hope this helps,
Cam
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The cameras have quite different chips Greg. The 205 chip in the 314e has a little over half the QE of the 285 in the 314L. The 205 pixels are just over half the area of those in the 285. Putting these factors together, a mono 314L would be about 3.5 times as sensitive (per pixel) as a mono 314e. 2 out of every 3 photons will be lost through absorption in the Bayer matrix of an OSC 314L, reducing the sensitivity to 1/3 that of a mono version, but it will still detect more photons per pixel than the mono 314e - and it will provide full colour into the bargain. When you add in the time required to get colour data with the mono 314e, you should expect to spend about twice as long as you would with the 314L OSC for similar signal to noise.
Cam, agreed that the scale of the physical image will be determined by the fl of the scope, but the size of an object in an electronic image will depend on the sampling. For example, if a galaxy has an extent of 10 arc seconds and you are sampling it with pixels that are 1 arc second across, the resulting image will be 10 pixels across. If the pixels are larger (eg 2 arc seconds), there will only be 5 pixels across the galaxy (ie it will be smaller in the electronic image). objects taken with the 314e (smaller pixels) will have about twice the area (in pixels) as those taken through an identical scope with the 314L. As an illustration, the attached composite image shows an identical scene
taken through the same scope but using three different cameras. The leftmost image is taken using a camera with 5.6 micron pixels and the central one using a camera with 3.75 micron pixels - the fields of view vary with the chip size, but the central insulator is also clearly of different size due to the different pixel sizes in the cameras.
For imaging galaxies, the 314e with its smaller pixels makes sense.