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Old 17-01-2015, 02:54 PM
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Best exposure time for narrowband imaging?

I'm planning my first foray into narrowband imaging taking the Rosette Nebula as my target. But I'm having difficulty in deciding what exposure time to use. I understand that it is inevitable that narrowband exposures will generally require more exposure time than the LRGB filters that I have been using up to now. With 3 narrowband filters for the Hubble palette and, say, 6 subframes for each filter the total integration time increases by a factor of 18 for each additional minute of exposure time. Therefore the total integration time for a 5 minute exposure would be 1.5 hours. For a 10 minute exposure it would be 3 hours. For a 15 minute exposure, 4.5 hours and for a 20 minute exposure, 6 hours. So, like everyone I guess, I want to balance the quality of the end result with the need to avoid unnecessary exposure.


Of course one can always simply “suck it and see” and make a full exposure run using a guessed-at exposure time but I would prefer to take a more considered approach. I therefore did a test run of just 4 exposures with the Ha filter – 1 each for 5, 10, 15 and 20 minutes to see what resulted after stretching using the levels tool in Photoshop. I already had dark frames for 5 and 10 minutes but as yet I have no flat frames for Ha. But I reckoned that that didn't really matter as hot pixels and any vignetting, etc. would not be much, if any, of an issue in comparing the results and coming to a decision as to which exposure time to use. So I simply subtracted the 5 minute dark from the 5 minute light and the 10 minute dark from the other 3 exposures.


Attached is a screenshot of each stretched image showing the histogram settings used in the levels tool. As to be expected, the light distribution curve widens with increasing exposure. For simple comparison purposes I have only used the white slider button and set it at the right hand end of the distribution curve. It is fairly obvious that the 5 minute exposure is not as good as the others and can be discarded immediately. But there seems to be very little, if any, difference in quality between the other 3 exposures. Do I play it safe and use 20 minute exposures? Take a chance and use 10 minute exposures? Or maybe have a bet each way and use 15 minute exposures?



Any comment that those experienced in this field would care to make would be most welcome.
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Old 17-01-2015, 03:16 PM
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Amaranthus (Barry)
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If you are guiding well at 20 min, then use that. Longer is better for NB (within reason), if all else is holding up (tracking, wind, clouds etc.)
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Old 17-01-2015, 03:48 PM
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Ideally, you want subs long enough that camera read noise is negligible compared to shot noise in the background of your image. There's a description of a simple way to evaluate this here: http://www.iceinspace.com.au/forum/s...d.php?t=117010

Unfortunately, unless you have a very fast optical system or a camera with very low read noise this will require impractically long exposure times with narrowband filters. So, expose for as long as you comfortably can. If you have good polar alignment and guiding then the limit will be how much time you are willing to lose if something goes wrong.

I do narrowband subs of 30 minutes and have done 60 minutes on some very dim targets.

The reason you can't see much difference between your 10, 15 and 20 minute subs is probably because you're not looking at the dimmer areas closely or stretching them hard enough. Bright parts of the image will reach high SNR quickly. It's the dim areas where noise will lurk.

Cheers,
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Old 17-01-2015, 08:58 PM
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Many thanks for your replies Barry and Rick. I've tried to follow that topic thread you mention Rick but at this stage I find it to be more than I can readily absorb. Will keep trying however. In the meantime I note the general advice that for NB longer is better. With my SW AZ-EQ6 GT and Orion Mini guide 'scope and Starshoot autoguider camera mostly sheltered from wind in the observatory I don't seem to have any trouble getting PHD2 to guide the 'scope for long periods so in practical terms the only exposure limit is my patience. I'll try 20 minute subs for starters and maybe experiment with cutting back to 15 minutes at a later date.

Again thanks for your help. It is much appreciated.

David
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Old 19-01-2015, 07:06 AM
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Hi David,

I have a fast scope and OK sensitivity camera, so I find that after 10 to 15 minutes (target dependant) the SNR is enough. I also find the longer you go the more time you lose to unexpected things happening, like planes, satellites etc.

As you go longer in your subs, you will inevitably take less of them, so the statistically based rejection algorithms used in data processing are less effective, bright satellite trails aren't completely removed for example. So this is also a consideration I have when deciding sub length. As a compromise I'll tend to shoot 10 minute subs as I find the SNR is usually pretty good and the greater number allows the sigma rejection to work much better (which also increases the SNR). Really dim targets require greater exposure length mainly to get the S part of the SNR up.

So, for the Rosette nebula, I can easily get away with 5 minute subs for all three filters, though, from memory, there isn't much SII anyway.

Cheers
Stuart
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Old 19-01-2015, 08:58 AM
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That point from Stuart reminds me of another thing to check. SII! For some objects, this channel is really useful. But many others, don't bother burning your integration time on Sulphur. Best think to do is take a 10 min exposure of each of Ha, OIII and SII on your object, and then evaluate the image. From there you can work out final framing, whether to do SII, and also get a guide as to whether you need more time for the channels you are going to use. It's 30 min well spent at the start of a project.
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Old 19-01-2015, 12:11 PM
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Quote:
Originally Posted by Amaranthus View Post
That point from Stuart reminds me of another thing to check. SII! For some objects, this channel is really useful. But many others, don't bother burning your integration time on Sulphur. Best think to do is take a 10 min exposure of each of Ha, OIII and SII on your object, and then evaluate the image. From there you can work out final framing, whether to do SII, and also get a guide as to whether you need more time for the channels you are going to use. It's 30 min well spent at the start of a project.
That's good advice Barry, some objects are low in O3 as well - best to check like you suggested.

Personally, I've pushed my EQ6 to 25 min subs, and they're still sharp, but it's gut wrenching when a satellite stuffs up the sub. - I'm more of a 15-20 min guy now.
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Old 20-01-2015, 09:01 AM
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Stuart / Barry / Andy. Thanks for the further input. Much appreciated

David
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Old 20-01-2015, 11:40 AM
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Quote:
Originally Posted by Andy01 View Post
Personally, I've pushed my EQ6 to 25 min subs, and they're still sharp, but it's gut wrenching when a satellite stuffs up the sub. - I'm more of a 15-20 min guy now.
I rarely get a satellite trail that doesn't reject out cleanly if I have a reasonable number of subs, so maybe you need to adjust rejection parameters?

I also have some PI techniques for removing the trail from the original sub if it won't reject out but that's a bit more work.

Cheers,
Rick.
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Old 20-01-2015, 12:16 PM
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If you bin2 OIII and SII for colour, use half the exposure time you use on Ha (at bin 1). Even if OIII and SII are still noisier, you can smooth them right out without effecting the final image much.
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