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HA: what am I doing wrong?

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#1 N_DD

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Posted 19 September 2020 - 06:10 AM

Hi,

 

I am starting narrowband imaging. I am using a 7 nm H-alpha filter (this one https://www.teleskop....php/info/p9231) and I am imaging with a QHY163M and a C8+fr.

I have tried imaging the Crescent nebula, but after stacking 30x4 min. subs (gain 150, chip at 0 C plus bias, flats and offsets from a green zone), the result looks a bit underwhelming... guiding was good, and there were no other issues during the session.

Here is the final image (stretched, 50% reduction for fitting CN). I have to say, the C8 is an old Ultima8, probably 30+ years old, and the coatings may not be the best ones around, but looking through the tube I cannot see any problem... is the Crescent nebula inherently dim, and am I expecting too much for 2h integration time? Or is there anything obvious I am missing?

 

 

Attached Thumbnails

  • Crescent_HA.jpg


#2 TOMDEY

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Posted 19 September 2020 - 06:30 AM

It's a nice picture! Your filtration is pretty severe, which snuffs the stars. Is the light pollution bad? Twenty hours of data should improve things a lot. Here's what I got in 1986, unfiltered, dark skies, with two hours of data.   Tom

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  • 05 Toms Old Film Crescent 1986 35-094 jpg.jpg


#3 kathyastro

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Posted 19 September 2020 - 06:56 AM

A couple of things.  You are shooting at f/10.  I used 80 minutes at f/4 for the Ha portion of my most recent image of this target..  The equivalent at f/10 would be over 8 hours.  So get more data.

 

Shoot longer subs if you can.  The term in vogue here on CN is "swamping the read noise", basically making sure that you are recording faint details.  That requires longer subs.  I shot 8x 600s subs.

 

The other thing is that your stretch looks pretty severe: very high contrast, not much mid-range.  Use Curves to bring out the mid-range tones.  In order to do that, be sure that you use the maximum dynamic range available to you.  Use 16-bit processing, and never save as JPG until you are ready to publish.

 

80 minutes of Ha at f/4:

Ha.jpg


Edited by kathyastro, 19 September 2020 - 07:00 AM.


#4 N_DD

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Posted 19 September 2020 - 07:33 AM

Hi,

 

thanks: I am shooting at f/6.3, using the Celestron reducer. The camera I have as a 20ke well, therefore going for longer sub will quickly saturate... the camera has a native 12-bit ADC but all the processing was performed on 16-bit files. The stretch and .jpg are only for displaying purposes: the lack of middle tones is exactly what I am wondering, but it has less to do with processing. The data itself looks like it is lacking the faint bits...



#5 kathyastro

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Posted 19 September 2020 - 07:54 AM

Sounds like you are doing everything right.  If you can't shoot longer subs, shoot lots more.  Given the camera limitations, I'd guess that you'd want quite a few hours on this target.




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