Ha Narrowband Imaging with a OSC

[David_Pickles]

This is the result of my first attempt at narrowband imaging with a OSC CCD. I used the QHY8L and a Baader 35nm Ha filter (not quite narrowband - more like moderately narrowband). The image is the result of 10 x 300s standard RGB exposures with 3 x 1800s Ha exposures, binned using the 'low noise 2x2 bin' feature of Nebulosity 3, and added as a luminosity layer in Photoshop. I am a little disappointed that the outer halo didn't show up but I guess that is the limitation of using a OSC CCD with only 25% of the incoming light being captured. It may also be down to the broader bandwidth of the 35nm filter. However, adding the Ha does show up a little more detail in the nebulosity that my standard exposures, so it was a worthwhile experiment although I won't be forking out on a full set of narrowband filters just yet.

[lensman57]

That is really nice, 1800s subs in Ha are quite adequate why not try and add the Ha to the red channel of the RGB and see if things improve a little.

Regards,

A.G

PS: On my laptop there appears to be a very faint but almost colourless halo around the nebula, I am not sure if this is in the capture or a result of you trying to mask the nebula off for selctive stretching, it is worth further investigation.

[David_Pickles]

Thanks for your advice - I will have another go at processing and see if adding the Ha to the red channel brings out anymore detail. I will also try getting some more Ha exposures and drizzle them to try and reduce the effect of background noise and allow the faint Nebulosity to stand out a little more.

[lensman57]

 precisio

Thanks for your advice - I will have another go at processing and see if adding the Ha to the red channel brings out anymore detail. I will also try getting some more Ha exposures and drizzle them to try and reduce the effect of background noise and allow the faint Nebulosity to stand out a little more.

Drizzling will have an adverse effect on the noise, if you have enough signal and your subs are clean and devoid of plain trail and the likes, the best S/N ratio is achieved by stacking using the good old average routine. Drizzling is only effective if your telescope_ seeing conditions  is capable of resolving more than the CCD sensor will resolve, very long winded but that is the short of it. For this to be effective provided that the first condition is fulfilled you need a large number of decent subs and as repeatable a dithering pattern as possible, none of our hobby stuff is up to it BTW you'd need HST mechanical precision. If so then drizzle will yeild improved resolution, but to get rid of the noise the most effective tool is high dithering and a large number of subs otherwise you might just as well up sample the image in PS. Hope this helps.

Regards,

A.G

[David_Pickles]

Thank you for explaining that - everyday is a learning day in this game . I will see if I can get some more Ha exposures in an attempt to reduce the S/N ratio.

[David_Pickles]

Another 1 1/2 hours worth of Ha added (3 x 1800s) from last night and a complete reprocess but only a touch more halo visible - still not as much as I was hoping for. I guess it must be that the 35nm Ha filter is a little too wide and my OSC CCD is not sensitive enough to capture it during a full moon, meaning the faint nebulosity is washed out.  Below is a cropped version which I am quite pleased with - some nice detail in the brighter nebulosity and relatively round stars considering the exposure times.

[lensman57]

Excellent, I think that if you keep data you' get there.

A.G

[ollypenrice]

How about showing us just the Ha in mono, stretched as hard as you can? Or putting the calibrated linear Ha in Dropbox or whatever? As is, I don't think you have gained a great deal from the NB data over what might come up in natural colour, though I don't know your sky of course. You still have a very decent RIng indeed but not much outer shell.

Olly

[David_Pickles]

How about showing us just the Ha in mono, stretched as hard as you can? Or putting the calibrated linear Ha in Dropbox or whatever? As is, I don't think you have gained a great deal from the NB data over what might come up in natural colour, though I don't know your sky of course. You still have a very decent RIng indeed but not much outer shell.

Olly

Hi Olly, I have uploaded the stacked and stretched Ha data as a TIFF file https://www.dropbox.com/sh/65hto7xvt3gh4d4/AADSWzVrPqJf80uFbrLDJCqca

This is the result of 6 x 1800s exposures, captured over 2 nights using a QHY8L OSC CCD, binned 2x2, attached to a 190MN with a 2x2" Tele Vue Powermate. Conditions were stable but the full moon was well above the horizon.

As you say, apart from a little more detail in the ring I don't think the Ha has added much to the image - that said, the detail added by Ha may well be the result of the longer exposures! Processing was done entirely in Photoshop with Levels/Curves, Gradient XTerminator, Noel Carboni's Local Contrast Enhancement, Sharpening and Colour Saturation. As this is my first experiment with NB imaging I don't have anything to compare it to, but I was rather hoping for more outer halo; especially after 30 min subs in Ha. However, the 35nm Baader filter is quite wide and probably lets a lot of background light through that is drowning out the fainter detail - add to this the lack of sensitivity due to being a OSC and the fact it was a full moon last night and I probably didn't have the best conditions. Normally my skies are relatively dark, although not perfect. I think I will try again when there is no moon and see if the darker sky allows more faint stuff to be teased out. Any other advise most welcome.

Regards,

David

[David_Pickles]

Excellent, I think that if you keep data you' get there.

A.G

Thanks A.G - I will try again with less moon and see if the darker skies help.

[ChrisLX200]

That's a nice clean image of M57 with lovely colour but I do feel you've thrown away some of your hard-earned image data by clipping the background rather harshly to black.

ChrisH

[David_Pickles]

Thanks Chris - I probably do tend to make the background a little too dark in some of my images. I will be a little less aggressive when I add some more data to this.