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Going deeper

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#1 meeus

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Posted 11 September 2019 - 02:17 PM

Here is my attempt at a two-panel mosaic of Cepheus area.

 

I did an experiment trying to go deeper in strongly light polluted sky conditions by using long exposure (over 15 hours per panel), narrowband filters (down to 3nm) and very fast optics (f/2).

 

The results are interesting. I can certainly pick up quite faint Ha/SII signal.

However, shooting with a small aperture (70mm) I could not get enough signal-to-noise on a really dim OIII target (OU4) so I had to turn to my big reflector to get extra photons for additional contrast.

Part of the problem may be that a 3nm filter may not wok very well at f/2 but I still need to figure out how much I lose.

 

The palette is a modified SHO where I kept some green to showcase the weaker Ha regions. This may not be the most eye-pleasing color combination but possibly the most informative.

 

The Samyang 135mm lens was coupled to ZWO ASI1600 mono camera with a filter wheel. More acquisition details on my Astrobin page.

 

HD-KZOTleJdZ_1824x0_wmhqkGbg.jpg


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#2 kingjamez

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Posted 11 September 2019 - 02:25 PM

Very nice! I'd love to give OU4 a try soon.

 

I did a bunch of testing on my RASA at F/2 when I first got it (and still am really). I did comparisons between very narrowband filters (3nm) and wider narrowband filters (7-10-12nm-20nm) and found that 3nm filters were essentially useless. They virtually reduced the aperture in dramatic fashion. I've settled on 12nm filters as the best option for F/2, it's a tight balance between gradients and allowing the blue-shifted filter band to let the emission line through.

 

-Jim



#3 stargazer60

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Posted 11 September 2019 - 02:39 PM

Beautiful image, I have never viewed this object before. Great detail and fantastic color.  Look forward to seeing more of your deeper images in the future.  bow.gif waytogo.gif 



#4 Joe G

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Posted 11 September 2019 - 03:00 PM

Real nice mosaic!  Yves (CN member vdb) recently posted the same FOV but in color/OIII.  The title of the thread "Squid VS Bat VS Elephant Trunk."

 

I'm trying this same framing right now in Ha from my light polluted backyard.

 

Great job.



#5 meeus

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Posted 11 September 2019 - 04:28 PM

Thank you all for the comments.

 

I saw the other image of this area that was posted recently. Really nice. I wish I had dark skies closer to home smile.gif

 

Also next time I will do a real shootout between my 3nm and 8.5nm oxygen filters at f2.


Edited by meeus, 11 September 2019 - 04:29 PM.

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#6 MARA25

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Posted 11 September 2019 - 06:58 PM

Baader has filters dedicated to fast optics: https://www.baader-p...-iii-s-ii).html

#7 meeus

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Posted 11 September 2019 - 08:17 PM

Baader has filters dedicated to fast optics: https://www.baader-p...-iii-s-ii).html

Interesting, I have not seen these before!

Apparently, they do not want to disclose the actual bandwidth. At the same time time, the price is relatively low for the sizes (at least compared to Astrodon). 

I guess one would have to get them and do a comparison. Still, having different filters for each optical tube sounds expensive. 



#8 kingjamez

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Posted 11 September 2019 - 08:25 PM

They don’t disclose the bandwidth because it’s wide.. probably wider than 15nm. However, physics dictate how wide they must be to sufficiently account for the wavelength shift due to the steep light cone.

#9 meeus

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Posted 11 September 2019 - 10:14 PM

I did not have time to do enough research but I found a few web pages that discuss the actual amount of wavelength shift. They provide somewhat conflicting information.

This one seems to be able to estimate both effects of the f-number and variation across the sensor.

 

Overall it looks like 8.5nm Baader would work better than 3nm Astrodon at f/2. What was I thinking?

 

On the other hand, 3x more light signal means that I would need to drop exposures to one minute or less, even on moonless nights. Hello hundreds of subs lol.gif

 

The good thing is with my Newtonian at f/4 the signal loss is not as bad and the gain from filtering our background should bring the expected benefit.

I will still try to do an experimental comparison.


Edited by meeus, 12 September 2019 - 12:46 AM.

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