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Why do you image at different colours?


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I dont do any imaging at the moment (see my equipment list :)) but i will get into it at some point, so this might be a n00b question...but why do you image at different colours of the spectrum?

My best guess is that, say the crab nebula, has all the RGB colours, but as they are really faint you need to image just those and then increase the intensity of that colour in the final image stack...anywhere close?

cheers

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A hot topic, single shot cameras or mono cameras with filters.

The simple answer is that mono cameras use one pixel on the chip for the light whereas the single shot cameras including dslr's use a matrix to grab the different colours.

In practice and tests prove it, that mono cameras are much more sensitive and hence give you better results than single shot matrix camera's for the same exposure times.

When you work it out, single shot may be slightly quicker overall including processing time, but not by much when you are used to filters. Remember filters are LRGB (luminance, red, green, blue) so you are only doing 1/4 the time per channel.

Also with mono camera's it is much easier to upgrade to narrowband imaging with a simple filter replacement. One shot colour camara's will struggle.

See this link for the BAYER MATRIX on single shot camera's

Bayer filter - Wikipedia, the free encyclopedia

Edited by Catanonia
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so its nothing to do with the lack of intensity at that channel?

i thought that nebulae usually look grey from earth, and that you needed to up the colour intensity individually inorder to get a nice looking image?

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so its nothing to do with the lack of intensity at that channel?

i thought that nebulae usually look grey from earth, and that you needed to up the colour intensity individually inorder to get a nice looking image?

There might be somewhere technically down the line, but when we do separate channels we either adjust the exposure settings for each channel or calibrate them afterwards.

Basically LRGB separate channels gives best colour and sensitivity combination over single shot colour.

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All the comparisons have been between a mono and colour camera of the same type. Unfortunately mono cameras are often 50% more expensive than the colour equivalent, as although the cameras are often the same price, you still have to buy a filterwheel and all the filters for a mono setup. You could instead, of course, buy a 50% more expensive one shot colour camera.

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i thought that nebulae usually look grey from earth, and that you needed to up the colour intensity individually inorder to get a nice looking image?

The reason things look grey to the human eye is not because they lack colour. it's because at the very low light levels our eyes can only see in black (or grey) and white. Our eyes need a fair bit of light to distinguish colours.

Having said that, the sensors in astro cameras (and all other cameras, come to that) don't respond to colours in the same way that our eyes do - they are more sensitive to some colours than eyes, and less sensitive to other colours.

What that means is that we usually have to "balance" the colours, so they look normal to the viewer. if you use a DSLR, then it has a filter built in to balance the volours, so photos taken in daylight look the same as if we'd seen the scene. However, that filter also reduces the DSLR's sensitivity, so some people take it out - or replace it with a different filter (e.g. one that passes Hydrogen-alpha, dark red, light)

There is another technique, that some people use. There are certain wavelengths of light (colours) that are particularly prevalent in astronomical objects. By using some narrow filters, these colours - associated with very specific atomic wavelengths - are let through and all the "noise": the light pollution, the ordinary light from stars and galaxies is removed. When people take photos at these wavelengths, they can combine several photos and assign "false colours" - one colour to each specific wavelength. this produces very striking images, but with entirely artificial colours. A lot of NASA's showcase images are produced this way.

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i thought that nebulae usually look grey from earth, and that you needed to up the colour intensity individually inorder to get a nice looking image?

They do if you're observing. Thing is, our eyes work differently to cameras so they're able to pick up the colour our eyes don't see. Imagers doing 'normal' colour imaging should keep the colour balance as normal as possible so the colours you see on the screen should the the colours of the actual object. You're also able to do 'false' colour imaging with narrowband filters where you're imaging very narrow parts of the spectrum and assigning that part of the spectrum to a Red, Green or Blue part of the colour image. What most people do is image the Hydrogen alpha (Ha) wavelength which is Red, Oxygen III (OIII) which is Blue/Green and Sulphur II (SII) which is also Red, during processing what you can do for example, is assign Ha to Red, OIII to Green and SII to Blue (or any combination thereof) to create a colour image but it's not the actual colours of the object, hence it's called false colour.

HTH

Tony..

Edited by Whippy
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All the comparisons have been between a mono and colour camera of the same type. Unfortunately mono cameras are often 50% more expensive than the colour equivalent, as although the cameras are often the same price, you still have to buy a filterwheel and all the filters for a mono setup. You could instead, of course, buy a 50% more expensive one shot colour camera.

In my case £1300 for a QHY9 colour compared to £1750 for a QHY9 mono + filter wheel + 4 filters.

They can be in total more expensive, but not as much as you may expect.

But then the argument can go why not step up to the next colour and gain more back and so on.

My experience is that Mono is far more flexible allowing all filters including NB and I had a budget to spend.

The simple fact that the majority of CCD imagers use mono sort of says something.

All that said, I would love a one shot colour ccd for the pure simplity to go alongside the mono.

Edited by Catanonia
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The simple fact that the majority of CCD imagers use mono sort of says something.

Though the author of the book everyone suggests all new imagers should buy (Making Every Photon Count) appears to use OSC - and I believe that OSC is very popular amongst US imagers.

But in the end I bought a mono camera - because of the reason you mention.

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Though the author of the book everyone suggests all new imagers should buy (Making Every Photon Count) appears to use OSC - and I believe that OSC is very popular amongst US imagers.

But in the end I bought a mono camera - because of the reason you mention.

Well the other side of the pond is a different world eh :):eek:

Everyone wants best quality images, hence it makes sense to go mono for that reason. The price hike to the next camera is so high, it makes sense to splash a little more and have the flexibility. I know that is the reason I went mono.

Also is OSC probably OSC as he hasn't sold enough books :) LOL Sorry mate :eek: I have actually recommended your book tonight to a member :o

Edited by Catanonia
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