A bad attack of the pox

[MarkS]

Here is Wednesday night's attempt at the veil with the newly modded Canon EOS 300D (scaled down by factor of 3)  21 x 5 min exposures:





I'm not at all happy with it - the stars look absolutely terrible.  They have a bad attack of the pox - a white pimple surrounded by angry red.  Here is a region of the image at full size:





The problem is chromatic aberration on my ancient Nikon 300mm lens - the one I normally use for guiding.  It will not focus the H-alpha in the same plane as the blue.  I guess it was never designed to do so.

Looking back at my test focus image the problem becomes quite apparent:





The telescope focus knob is adjusted as the star slews from left to right.  At the left hand end (2nd segment in) the blue is clearly focused but is surrounded by a red haze. At the right hand end the red is clearly focused but is surrounded by a blue haze.  The veil was imaged with the focus knob near the left hand end of the scale.  Hence every star is surrounded by red haze.

In retrospect, the best compromise is around the 3rd segment where nothing is completely in focus but at least all the colours are more or less registered.  The other alternative is to stop the lens down further (say F8 instead of the F5.6 which I used) but this would reduce the light too much.

[Carole]

Your talk was very interesting last night Mark, and I can now understand where you are coming from when you talk about your imaging on the forum as I tend to forget you are using a DSLR.  The only subject you did not get to explain was modding.

Any chance of any explanation here, or is it too long/complicated to put in a post. 

I thought you method of focusing was rather clever. 

Carole

[MarkS]

Carole,

It's easy to explain.  A CCD is sensitive to a much greater range of wavelengths than the human eye is e.g. infra-red.  So the manufactureres of consumer digital cameras put a filter in front of the CCD that allows only the visible wavelengths to reach the CCD.  If they had no filter then your digital camera photographs would look very strange - they would include features that are invisible to the eye!

The problem is that the IR filter put in by the manufacturer also cuts out most of the Hydrogen-alpha wavelength.  The H-alpha wavelength is very useful (and pretty!) in astrophotography even though it is invisible to us.  Modding the camera means replacing the manufacturer's filter with an alternative that lets the H-alpha reach the CCD.  In the case of the Canon, this improves the brightness of H-alpha by a factor of 4.  Depending on the alternative filter you use, the camera can also be made sensitive to infra-red.

A modded camera is great for astrophotography but it means everyday photos now come out extremely pink and show features invisible to the human eye.  A special additional filter can be put on the lens to return the camera back to its "normal" sensitivity range (i.e. only the visible wavelengths) to make everyday photographs appear with the correct colour balance.

Mark

[Rick]

I'm not at all happy with it - the stars look absolutely terrible.  They have a bad attack of the pox - a white pimple surrounded by angry red.
...
The problem is chromatic aberration on my ancient Nikon 300mm lens - the one I normally use for guiding.  It will not focus the H-alpha in the same plane as the blue.  I guess it was never designed to do so.

Is any of that red the result of infra-red appearing red in the modded camera? That would definitely focus a way off from the main visible components...

The H-alpha wavelength is very useful (and pretty!) in astrophotography even though it is invisible to us.

Ummm... I think the point is that it is visible (or at least in the visible part of the spectrum)...

[Ian]

but only just, it's right at the edge of the eye's red response. If it wasn't visible, a Ha filter would appear opaque, rather than a very deep red. I've never seen a reasonable explanation why DLSRs have such a poor response to Ha by default...

[Rick]

Yeah, but it's visible enough for instruments like the Coronado solar telescopes to show a nice image, admittedly from a high intensity source...

[MarkS]

Rick,

Your suggestion about the infra-red focusing to a different plane is very plausible.  However I was using an Astronomik CLS filter and I believe it cuts the IR but I'm not absolutely certain.

On the other point, yes, the eye is not completely insenitive to H-alpha.

Mark

[Ian]

the other thing to bear in mind when modding a camera in this way is the bayer pattern filters on the CCD are not completely opaque to IR, so you need some sort of IR filtering for anything approaching a reasonable colour rendition.

[Rick]

the bayer pattern filters

Yeah, that could make things complicated...

While we're on the matter of colour representation of IR images, how did you take your "DSC in IR" image? (Link to gallery via thumbnail below...)

[Ian]

standard Nikon D70 with a Cokin IR pass filter. In fact, that's given me an idea for IR pass photography, need to adapt the filter holder to fit my refractor...

[Carole]

Modding the camera means replacing the manufacturer's filter with an alternative that lets the H-alpha reach the CCD

Ah, is this what you were telling us about when you took the camera apart and changed the filter from a red one to a white one, in which case you DID tell us about modding?  So modding simply means modifying.

A CCD is sensitive to a much greater range of wavelengths than the human eye is e.g. infra-red.  So the manufactureres of consumer digital cameras put a filter in front of the CCD that allows only the visible wavelengths to reach the CCD.

Getting in a muddle now as one minute you are talking about a digitial camera and the next CCD.  I thought CCD was when you use a laptop instead of a DSLR and yet on CCD we put ON an IR filter.

Maybe I don't really understand the true meaning of a charged coupled device.
Can some-one elucidate all this.

Does this go part way to explaining why the naked eye does not see astronomical things in colour?   

Thanks
Carole


Carole

[Mac]

getting back to the focus problem, red blue.



Band C is the Ha line, so would be in focus when your blue was also infocus, unless you have severe chromatic aberation.

Which even on your old nikon lens would probably be nill.

The more plausable answer is the out of focus red is in fact it is the IR, as this does focus at a different plane, compared to visible light.

and now that you have the full range of your CCD which goes right in to the IR spectrum

Some of my old pentax lenses still have the IR focus point on them.

Now that you have removed your IR filter your chip will be recording the IR out of focus, whilst the rest is in focus.

You probably find that the IR filter fitted to most modern cameras probably starts from 650nm upwards.

Just as a thought have you tried photographing UV objects.

http://www.naturfotograf.com/uvstart.html.

[Ian]

Carole,

a CCD is the actual sensor chip. You can see a small one when you unscrew the lens off your webcam, or if you take the lens off a DSLR and lock the mirror up, you'll see a big one. The reason we tend to associate CCD with astro cams, is to distinguish between film and CCD, DLSRs didn't exist when that phraseology came into being. So, all digital cameras are CCD devices (actually there's a couple of types but the distinction is technical, the other is known as CMOS) and we're just a bit sloppy with names. Bit like Hoover really...

Mac, yup alot of old lenses have a IR mark on them (often in red or orange) and that can really show you how far away the focus point of IR is from the visible (bearing in mind the wavelength for IR is twice that of blue light, and refraction angle is a function of wavelength).

However, when we're talking critical focus the next question is how well corrected is the lens. In my experience, not particularly. Camera lenses rarely see the sort of exacting requirements we have on our equipment. Point parallel light sources are as hard as it gets for an optical system and camera lenses are not designed to perform perfectly with them. However, ED and APO camera lenses should do what the name suggests, in the case of APO should be corrected to three wavelengths across the visible spectrum. None of my camera lenses are APO and I don't think your Nikon one is either Mark. So actually, camera lenses can have chromatic aberration but generally get away with it. They can also suffer from significant coma and spherical aberration too. Do a google search for "Boke", there is a view that spherical aberration is good for a camera lens, as long as it's the right way...

[MarkS]

Carole,

Digital cameras, specialised astro cameras and webcams all have a sensor chip.  Most sensor chips use CCD technology (but some are CMOS - that's a different story).  I just used the word CCD as shorthand for sensor chip.

You put an IR filter on your webcam to prevent the infra-red which would otherwise appear out-of-focus on your image.  Out of focus IR is possibly the problem I am seeing with my "pox" picture.

Mark

[Carole]

Thanks Mark and Ian for your explanations.

Just one more while we are on the subject.  I tried explaining to the doctors at work why the eye does not see in colour, I know it has something to do with less sensitivity and the fact that the eye does not see in long exposure, but I felt I was not sufficiently able to explain. Obviously the eye does not see in IR and other wave lengths is there anything else I can add to this?

Carole