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Some advice needed for first narrowband imaging session - M106

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#1 Professor2112

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Posted 10 April 2021 - 06:49 PM

Hello all!  I’m sitting at home at my bortle 8(ouch) location which at the moment only has a good view to the north.  I recently acquired a 2” astronomik 12nm H-alpha filter to use with the artesky adapter I have which is currently holding a 2” L filter for my 294mc pro for rgb, and for luminance with the 2600mm on the rasa 8.  What I’d like to do is image in Ha with the 2600mm since I’m in pretty heavy LP, but there isn’t much in the way of heavy Ha targets out that way right now, so I figured I could image m106 since it has very nice Ha structures.  Then I can get an rgb image to combine with later, or share the data with someone that wants to combine some Ha to their image and make a collaborative effort.  
 

The thing I need advice with, is how long should these exposures be?(approx.)  With the 2600mm/rasa 8 and the L filter, I was still blowing out some pixels(around 400 per sub) even at only 30” at 0 gain.  But I know narrowband is a totally different animal and I’ve never shot NB before.  Is there a “rule of thumb” with how much longer than your L subs your Ha subs should be? Like 2x, 3x, 4x, or something along those lines?  I’d prefer to keep using 0 gain as the dynamic range of this camera is incredible, and I plan to always use 0 gain with the rasa.  
 

So with the info I’ve provided, where would you start experimenting as far as sub lengths for Ha with a 2600mm on an f/2 system?  I know it won’t be a clear cut answer, just looking for some kind of starting point as having never done nb before, I don’t wanna start somewhere “way out of the ballpark” and waste a night of imaging.  Thanks everyone and clear skies!


Edited by Professor2112, 10 April 2021 - 06:49 PM.

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#2 bobzeq25

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Posted 10 April 2021 - 07:46 PM

Hello all!  I’m sitting at home at my bortle 8(ouch) location which at the moment only has a good view to the north.  I recently acquired a 2” astronomik 12nm H-alpha filter to use with the artesky adapter I have which is currently holding a 2” L filter for my 294mc pro for rgb, and for luminance with the 2600mm on the rasa 8.  What I’d like to do is image in Ha with the 2600mm since I’m in pretty heavy LP, but there isn’t much in the way of heavy Ha targets out that way right now, so I figured I could image m106 since it has very nice Ha structures.  Then I can get an rgb image to combine with later, or share the data with someone that wants to combine some Ha to their image and make a collaborative effort.  
 

The thing I need advice with, is how long should these exposures be?(approx.)  With the 2600mm/rasa 8 and the L filter, I was still blowing out some pixels(around 400 per sub) even at only 30” at 0 gain.  But I know narrowband is a totally different animal and I’ve never shot NB before.  Is there a “rule of thumb” with how much longer than your L subs your Ha subs should be? Like 2x, 3x, 4x, or something along those lines?  I’d prefer to keep using 0 gain as the dynamic range of this camera is incredible, and I plan to always use 0 gain with the rasa.  
 

So with the info I’ve provided, where would you start experimenting as far as sub lengths for Ha with a 2600mm on an f/2 system?  I know it won’t be a clear cut answer, just looking for some kind of starting point as having never done nb before, I don’t wanna start somewhere “way out of the ballpark” and waste a night of imaging.  Thanks everyone and clear skies!

If you like your broadband exposures, just set the average level of the narrowband to the same point.

 

Robin Glover suggests about 25X when going from luminance to narrowband.

 

https://www.youtube....h?v=3RH93UvP358

 

I went from 10 seconds at gain zero, one shot color, to 30 seconds at gain 100 with a duoband.


Edited by bobzeq25, 10 April 2021 - 07:47 PM.

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#3 elmiko

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Posted 10 April 2021 - 08:17 PM

I use 300 second subs with my Zwo 071 mcpro at unity gain and my Hyperstar f1.9 and L-Extreme filter which is a duo narrowband filter at 7 nm. This is from my Bortle 8 backyard. No problems with blown out stars.


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#4 Professor2112

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Posted 10 April 2021 - 08:20 PM

If you like your broadband exposures, just set the average level of the narrowband to the same point.

 

Robin Glover suggests about 25X when going from luminance to narrowband.

 

https://www.youtube....h?v=3RH93UvP358

 

I went from 10 seconds at gain zero, one shot color, to 30 seconds at gain 100 with a duoband.

Ahh that makes sense, so if my mean ADU is around 650-700 for the L subs, aim for somewhere in that range in general, regardless of filter, because it’ll expose the bright and dim areas similarly?  Is that the right way to understand it?   That would be fantastic if I could do longer exposures.  These file sizes have no mercy haha!  I figure my 30” L was too long and could have been 10”-15” be properly exposed.  25x that would be roughly 4-6 mins which I’m pretty sure my mount could handle. 


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#5 Professor2112

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Posted 10 April 2021 - 09:30 PM

I use 300 second subs with my Zwo 071 mcpro at unity gain and my Hyperstar f1.9 and L-Extreme filter which is a duo narrowband filter at 7 nm. This is from my Bortle 8 backyard. No problems with blown out stars.

Well it seems I was typing my last post figuring out how long my exposures would be when you replied and right around 5 mins is what I came up with.  Great to have real world experience back it up.  Looks like I’ll start at 5 and go from there.  Thanks for the feedback. 
 

Bob, thanks for that video link, I’ve had a chance to take a look and it’s a very good watch. 


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#6 elmiko

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Posted 11 April 2021 - 01:06 AM

Your welcome. Good luck with your image. Post your results.



#7 bobzeq25

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Posted 11 April 2021 - 12:33 PM

Ahh that makes sense, so if my mean ADU is around 650-700 for the L subs, aim for somewhere in that range in general, regardless of filter, because it’ll expose the bright and dim areas similarly?  Is that the right way to understand it?   That would be fantastic if I could do longer exposures.  These file sizes have no mercy haha!  I figure my 30” L was too long and could have been 10”-15” be properly exposed.  25x that would be roughly 4-6 mins which I’m pretty sure my mount could handle. 

Close.  It sets your subexposure so that read noise is not a significant factor, while preserving dynamic range.

 

Most people would set the gain up a bit to shorten the 4-6 minute subs, that's just a personal choice.

 

 



#8 Professor2112

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Posted 11 April 2021 - 02:19 PM

Close.  It sets your subexposure so that read noise is not a significant factor, while preserving dynamic range.

 

Most people would set the gain up a bit to shorten the 4-6 minute subs, that's just a personal choice.

Okay, that makes sense, and I can imagine many people have no choice than to increase gain with slower systems.  
 

Unfortunately, I wasn’t able to get any data last night.  Before I was about to setup I noticed I couldn’t see Polaris, which is usually visible even in my location, so there must have been more smog and/or haze in the air than usual.  Plus the satellite showed the clouds were close by so I knew any data I did get would be sub par at best.  The good news is that being able to setup right out front for my Ha subs means I can snag data any clear night so it shouldn’t be too long until I can give it a try.  I’ll definitely keep you all posted. 


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