M101 HA

[pyrasanth]

The moon was moderately bright last night so it was either a HA capture night or a play with telescope calibration. I did a bit of both. I remodeled the T-point, did a few more focusmax runs & ran a new calibration for focuslock.

I then captured 4 hours of HA under the moon using M101 as a target. I need another couple of sessions to finish the HA capture. These were 30 minute subs 2x2 bin Atik 460 on the Celestron C11. The Galaxy nearly fills the whole field at F7- its a nice object. I will begin the L capture on 1x1 bin when the moon has gone -weather permitting.

I've posted the bit of HA data I have to kick things off. There are a few hot pixels- I think I might have left dithering off accidentally so that will make my life harder on these first eight subs.

As always clear skies & let me know what you think of the data so far but its early days. This is still 2x2 bin but it will be scaled to 1x1 when I have the LRGB data. The only frame I will shoot at 1x1 will be the L

 

[gorann]

Looks promising Mark!

Still, I have not understood why imaging in Ha (compared to all other wavelengths) would help against moonlight. Moonlight is reflected sunlight and my impression is that the sun is sending out a lot of Ha (sun-telescopes are Ha-telescopes). Maybe someone can set me straight on this?

[pyrasanth]

25 minutes ago, gorann said:

Looks promising Mark!

Still, I have not understood why imaging in Ha (compared to all other wavelengths) would help against moonlight. Moonlight is reflected sunlight and my impression is that the sun is sending out a lot of Ha (sun-telescopes are Ha-telescopes). Maybe someone can set me straight on this?

Moonlight is reflected broadband which would also contain a percentage of HA light. I don't think the issue is really the moon, which just throws more undesired glare, more the selected filter.  I think it is  down to what wavelength of light is being registered on the CCD vs the wavelength of the light pollution which we hope is not being recorded. I think HA does better because it does not allow most of  the light pollution through to the CCD.  OIII filters might be closer to the wave lengths of light pollution hence a greater chance of catching more pollution depending on filter wave length selection.  It would be interesting to learn where most light pollution sits for a better answer to this question.

This would be my inspired guess but alas it might be completely wrong!

[andyo]

That's looking like a good start plenty of detail there, look forward to seeing this one when finished

[Scott]

I can't offer anything theoretical but I can state catagorically from personal experience that imaging even in Ha on a full moon is not ideal sure its better than broadband imaging but nothing makes up for dark skies regardless of whether the lp is natural or man made 

[gorann]

I can see that Ha imaging is a good alternative if the sky suffers from human light pollution, but I do not think it will reduce the effect of the sunlight reflecting from the moon.

[pyrasanth]

I will try and process more in PixInsight as sometimes the Photoshop tools can make images look a bit harsh if not used with care & I'm very clumsy! This is a raw version from PixInsight with no cosmetic retouching in Photoshop. The image is different & does not look so aggressive although the lack of data shows as its very noisy. The noise was removed mostly with the ACDNR, Deconvolution after an unsharp mask improved things somewhat. Notice how PixInsight has stacked the image the other way than CCDstack!

 

 

 

[pyrasanth]

The image below has the black nearly to the point of clipping and shows the massive amount of noise in the image. This might be a result of the moon last night and very thin haze in the sky. I will soon find out when the moon has gone & I shoot the next set of subs.

 

[wimvb]

2 hours ago, gorann said:

I can see that Ha imaging is a good alternative if the sky suffers from human light pollution, but I do not think it will reduce the effect of the sunlight reflecting from the moon.

Unless the moon doesn't reflect the Ha.

[gorann]

Unfortunately, the moon is just a pile of sand and rock, so it reflects all visual wavelengths

[laudropb]

Very promising start.

[pyrasanth]

I looked at the noise again & generated a binarized defect map which was applied to the 8 subs. This has produced a far better removal of the hot pixels. I'm puzzled as to why a few were left- might be the clipping level of the defect map.

 

 

[pyrasanth]

This is the latest file which has been cleaned using the new noise reduction binarised file. It's getting better!

 

[ollypenrice]

12 hours ago, gorann said:

I can see that Ha imaging is a good alternative if the sky suffers from human light pollution, but I do not think it will reduce the effect of the sunlight reflecting from the moon.

The fact is that it does, though. I don't subscribe to the view that Ha is 'moon-proof' but it is, without doubt, partially  moon-proof and the narrower the bandpass the better. Why this should be so is an interesting question since the moon does reflect Ha and can be photographed in this wavelength. Indeed it can give very good results in Ha because the long wavelength light is less affacted by the seeing than the shorter wavelengths. So why is Ha filtration fairly effective against moonlight?

My guess is that the explanation lies in the earth's atmosphere. Just as the daylight sky is blue, so is the night time sky and for the same reasons. (The Rayleigh effect tells us that scattering is inversely proportional to the fourth power of the wavelength in question. Blue light experiences ten times the scattering experienced by red.) I'd have thought that this helps the Ha deep sky imager in two ways, but this is just my reasoning so don't trust it! We'll get less disturbed light from deep red targets than from any other part of the visible spectrum. More importantly, though, the Ha filter blocks the bulk of the scattered light and allows the chip to record dark parts of the field of view which would otherwise be 'filled' by the scattered spectrum. In the end what we are trying to record with our cameras is the difference in flux between one part of the field and the other. We are not doing photometry so the absolute values don't matter to us. We simply want contrast and will, in effect, calibrate our Ha layer in order to blend it with the rest of the image. (We'll adjust its intensity relative to the rest of the image.)

Back on the image, and as I said on another thread, I don't worry about trying to make galaxy Ha look like a good image. I only concern myself with that part of it which will appear in the final L Ha-R G B image. In my case that's the brightest parts of the Ha image and really nothing else. Noise in the background sky, for instance, will be below the threshold which will be applied to the final image. But this depends on the method chosen for incorporating Ha. If you're going to use it as luminance then it has to be good across the board, but in galaxies I never would use it this way.

Olly

[gorann]

6 hours ago, ollypenrice said:

The fact is that it does, though. I don't subscribe to the view that Ha is 'moon-proof' but it is, without doubt, partially  moon-proof and the narrower the bandpass the better. Why this should be so is an interesting question since the moon does reflect Ha and can be photographed in this wavelength. Indeed it can give very good results in Ha because the long wavelength light is less affacted by the seeing than the shorter wavelengths. So why is Ha filtration fairly effective against moonlight?

My guess is that the explanation lies in the earth's atmosphere. Just as the daylight sky is blue, so is the night time sky and for the same reasons. (The Rayleigh effect tells us that scattering is inversely proportional to the fourth power of the wavelength in question. Blue light experiences ten times the scattering experienced by red.) I'd have thought that this helps the Ha deep sky imager in two ways, but this is just my reasoning so don't trust it! We'll get less disturbed light from deep red targets than from any other part of the visible spectrum. More importantly, though, the Ha filter blocks the bulk of the scattered light and allows the chip to record dark parts of the field of view which would otherwise be 'filled' by the scattered spectrum. In the end what we are trying to record with our cameras is the difference in flux between one part of the field and the other. We are not doing photometry so the absolute values don't matter to us. We simply want contrast and will, in effect, calibrate our Ha layer in order to blend it with the rest of the image. (We'll adjust its intensity relative to the rest of the image.)

Back on the image, and as I said on another thread, I don't worry about trying to make galaxy Ha look like a good image. I only concern myself with that part of it which will appear in the final L Ha-R G B image. In my case that's the brightest parts of the Ha image and really nothing else. Noise in the background sky, for instance, will be below the threshold which will be applied to the final image. But this depends on the method chosen for incorporating Ha. If you're going to use it as luminance then it has to be good across the board, but in galaxies I never would use it this way.

Olly

Thanks Olly,

your reasoning is both interesting and makes a lot of sense!

[wimvb]

13 hours ago, ollypenrice said:

The fact is that it does, though. I don't subscribe to the view that Ha is 'moon-proof' but it is, without doubt, partially  moon-proof and the narrower the bandpass the better. Why this should be so is an interesting question since the moon does reflect Ha and can be photographed in this wavelength. Indeed it can give very good results in Ha because the long wavelength light is less affacted by the seeing than the shorter wavelengths. So why is Ha filtration fairly effective against moonlight?

My guess is that the explanation lies in the earth's atmosphere. Just as the daylight sky is blue, so is the night time sky and for the same reasons. (The Rayleigh effect tells us that scattering is inversely proportional to the fourth power of the wavelength in question. Blue light experiences ten times the scattering experienced by red.) I'd have thought that this helps the Ha deep sky imager in two ways, but this is just my reasoning so don't trust it! We'll get less disturbed light from deep red targets than from any other part of the visible spectrum. More importantly, though, the Ha filter blocks the bulk of the scattered light and allows the chip to record dark parts of the field of view which would otherwise be 'filled' by the scattered spectrum. In the end what we are trying to record with our cameras is the difference in flux between one part of the field and the other. We are not doing photometry so the absolute values don't matter to us. We simply want contrast and will, in effect, calibrate our Ha layer in order to blend it with the rest of the image. (We'll adjust its intensity relative to the rest of the image.)

Back on the image, and as I said on another thread, I don't worry about trying to make galaxy Ha look like a good image. I only concern myself with that part of it which will appear in the final L Ha-R G B image. In my case that's the brightest parts of the Ha image and really nothing else. Noise in the background sky, for instance, will be below the threshold which will be applied to the final image. But this depends on the method chosen for incorporating Ha. If you're going to use it as luminance then it has to be good across the board, but in galaxies I never would use it this way.

Olly

So you're saying that in the same way that a daytime B/W photographer uses a red filter to dramatically darken the sky, so does an Ha filter for a (B/W, or narrow band) astrophotographer?

[ollypenrice]

15 hours ago, wimvb said:

So you're saying that in the same way that a daytime B/W photographer uses a red filter to dramatically darken the sky, so does an Ha filter for a (B/W, or narrow band) astrophotographer?

I didn't know that daytime B/W photographers do this but, yes, I guess we have the same effect in mind. In my earlier post I should have said that the moonlit night sky is blue for the same reasons as the sunlit daytime sky is blue. The thing is that there is no doubt that Ha does make some DS imaging possible in moonlight so the thing is to find an explanation.

Olly