Exposure time for Narrowband

[mike1485]

I am taking my first tentative steps into narrowband imaging using ASI2600MM and Baader narrowband filters.  The exposure calculators that are based on swamping read noise give very long exposure times for Narrowband imaging - understandable given the very small bandwidth being captured.  However, even at 300 seconds I am finding that the centre of bright stars are completely filling the pixel well leading to clipping.  This would only get worse as I increase exposure.  It seems I have various options;

1. Keep exposure time to the maximum that does not lead to clipping in the centre of stars - this leaves the bulk of the histogram well to the left (albeit separated from zero so not clipping at the low end) but this would seem to me to be leading to a very compressed dynamic range in the area of principle interest (nebula) and while multiple exposures making up equivalent time should compensate on SNR it feels like the dynamic range issue would remain and I am not really utilising the strengths of the ASI2600MM.
2. Expose long, eg based on swamping read noise, and aim for histogram perhaps 25%/35% from the left - this is likely to give better SNR for the nebula but the stars will be clipped badly leaving all bright stars as clipped pure white and probably bloated.
3. Expose long and then replace the stars from a separate RGB capture - replacing bloated stars with unbloated may be a processing challenge and furthermore it seems a little odd to be replacing with natural star colours when the rest of the image is probably combined based on a false colour eg SHO combination.
4. Something else I haven't thought of that maybe someone here can enlighten me on!

 

Edited August 16, 2021 by mike1485

[vlaiv]

What do you hope to gain from NB stars? They usually don't look nice and most people try to correct their color in some way during processing.

If you really want to have proper star profile - there is another option - take set of short exposures at the end of imaging session for that filter and use those short exposures to replace clipped parts of the image.

This is nice way to handle clipped parts of the image in general like in RGB imaging where you want to preserve color / definition in both star cores and clipping regions of bright targets.

As for NB - I'd say to get RGB color for stars and then create starless version of NB image / nebula (see starnet++ for example) and then put back in colorful stars from RGB data.

[mike1485]

2 minutes ago, vlaiv said:

What do you hope to gain from NB stars? They usually don't look nice and most people try to correct their color in some way during processing.

If you really want to have proper star profile - there is another option - take set of short exposures at the end of imaging session for that filter and use those short exposures to replace clipped parts of the image.

This is nice way to handle clipped parts of the image in general like in RGB imaging where you want to preserve color / definition in both star cores and clipping regions of bright targets.

As for NB - I'd say to get RGB color for stars and then create starless version of NB image / nebula (see starnet++ for example) and then put back in colorful stars from RGB data.

Thanks, this sounds like sensible advice.

[symmetal]

Star bloating with narrowband isn't really an issue compared to LRGB even with long exposures. Brighter stars will always be clipped unless you only expose for a few seconds.

With my ASI6200MM I expose for 10 mins for narrowband at gain 100, where the HGC kicks in. At this gain setting of 4 x unity I would need around an hour of Ha to swamp the read noise and significantly longer for SII and OIII. At this exposure the brighter stars are clipped on all the narrowband channels so always appear white in processing which doesn't look out of place. Adding real colour stars afterward tends to imply that the colours assigned to narrowband are also 'true colours' which they aren't if you apply the common NB colour palettes to give pleasing colourful images. 🙂

10 mins is a good compromise between allocating enough dynamic range to the NB data, and avoiding a lengthy ruined image due to guiding problems or a plane flying through. 😦

NB filters tend to give halos unless you pay huge amounts of money, my OIII gives smallish halos. Having strange colour halos around real colour stars looks more odd than strange colour halos around white stars. To try and hide the halos takes a lot more effort in processing. Starnet++ doesn't remove halos satisfactorily. 🙂

Alan 

Edited August 16, 2021 by symmetal

[mike1485]

39 minutes ago, symmetal said:

Star bloating with narrowband isn't really an issue compared to LRGB even with long exposures. Brighter stars will always be clipped unless you only expose for a few seconds.

With my ASI6200MM I expose for 10 mins for narrowband at gain 100, where the HGC kicks in. At this gain setting of 4 x unity I would need around an hour of Ha to swamp the read noise and significantly longer for SII and OIII. At this exposure the brighter stars are clipped on all the narrowband channels so always appear white in processing which doesn't look out of place. Adding real colour stars afterward tends to imply that the colours assigned to narrowband are also 'true colours' which they aren't if you apply the common NB colour palettes to give pleasing colourful images. 🙂

10 mins is a good compromise between allocating enough dynamic range to the NB data, and avoiding a lengthy ruined image due to guiding problems or a plane flying through. 😦

NB filters tend to give halos unless you pay huge amounts of money, my OIII gives smallish halos. Having strange colour halos around real colour stars looks more odd than strange colour halos around white stars. To try and hide the halos takes a lot more effort in processing. Starnet++ doesn't remove halos satisfactorily. 🙂

Alan 

Thanks Alan.  All good points and advice.  I had noticed Starnet++ has a hard time with halos!