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Starter issues: not enough stars in frame & post processing workflow

Astrophotography Beginner Imaging Software
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#1 NL_Sorin

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Posted 13 October 2024 - 04:57 AM

Hi guys,

 

It's my first time actively posting here after I have been following this forum for almost two years. For the past two seasons, I've been experimenting with my gear, before finally deciding I want to continue exploring this hobby and therefore investing in some nice gear. Currently, I am using the SkyWatcher 150p F/4 600mm with coma corrector, ZWO294MM cam + LRGB Astronomik + 6nmHa/SII/OIII Astronomik Max FR filters, on a SkyWatcher EQ35 mount. Autoguiding is done by ASI290mini + ZWO 120mm mini guiderscope. For autofocus, I use the ZWO EAF. Everything is hooked up to a ASIAIR Plus. 

 

I can now finally say that I think I worked out all of the hardware related issues: I took the whole mount apart, greased it in with some high quality stuff & put it back together, so now everything turns nice and smoothly. I manage to polar align down to ~3 arcmin. 

 

In short, my gear works at intended and I've started to get the hang of it. I still face some issues that I don't yet understand and I would highly appreciate it if you could share your thoughts with me on the matter. I am very eager to learn :)

 

So, what am I facing? After taking 30s subs of the Helix Nebula with the RGB & Ha filters from my Bortle 5 location, I use DSS to calibrate my frames. I used 15 x 30s subs for light frames, 30 corresponding dark frames, 30 flat frames, 30 dark flat frames and 50 bias frames. I set the star detection threshold at 2% and DSS only recognized ~100 or so stars in each frame. With the Ha filter, there were only about ~10 stars. I have no idea why there were so little stars in the images. The sky was clear the night I took the frames. Is this amount of stars normal? And if not, what do you think the issue could be? 

 

I tried to post process the images anyway, so I compiled a master frame for each of the channels R/G/B/Ha with DSS. I then use Photoshop. The first thing I did is to assign each of the master R/G/B frames to a respective RGB channel on a new image. My image looked extremely green though, and I have no idea why. The stars overlapped fairly nicely without apparent trails. I tried stretching the channels anyway, but I noticed in the channel histogram that there was a quite broad spike in the middle, with almost no data to the left or to the right of the middle. So basically, stretching seemed like it would do no good to the image. I became really frustrated and started the workflow all over to make sure I didn't miss anything, but I had the same result.

 

Needless to say, my first experience with the new gear was no fun, and I would really like to turn that around. Therefore, any help or insights that you may have, are highly appreciated!

 

Thanks so much in advance!

Sorin

 



#2 Tapio

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Posted 13 October 2024 - 05:10 AM

30s exposure is very short.
Might be ok for stars, and maybe even broadband RGB, but for Ha you need to take longer exposures.
And your total exposure is also too short.
For fainter target like Helix you need to have hours of exposure.

Instead of DSS try Siril or Astro Pixel Processor.
Siril is fast and is content with fewer alignment stars.
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#3 Chrisofweden

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Posted 13 October 2024 - 05:56 AM

As Tapio has said, a 30s sub is very short and having only 15 of those gives you a total integration time of 7.5 minutes.
To put that in perspective, some imagers take individual subs that are longer than 7.5 minutes - EACH - and they take many of them!
Sounds as though you have your gear working well together, including guiding, which is great. I suggest that “all” you need to do is increase the length of the individual exposure to say 90 seconds or a couple of minutes and take many, many more of them…
You’ll be surprised (and impressed) with what you see 😊

/Chris

#4 NL_Sorin

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Posted 13 October 2024 - 06:33 AM

Thanks a lot for the quick replies! I only got such short exposures because Helix was the only DSS I could image from my balcony on that specific night and it was the first time I got the entire rig to work together. Therefore, I just wanted to try all the filters out. In retrospect, I should have focused on one filter only and see what its potential was. 

 

So that fixes the subs issue. Any idea though why my RGB image was way green? And why the channel level histogram showed no peak on the left (so that I could stretch it), but a broad peak around the middle? Could that have anything to do with stray light entering my newt?



#5 Tapio

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Posted 13 October 2024 - 06:55 AM

I'm sure Helix is not the only target.
For short exposure a globular cluster like M15 would be ideal. And now more than ideal because of moon interfering.

Green cast could be light pollution (although mostly it's more yellow-red).
But colour balance as well as light pollution can be fixed with proper post processing tools.

But concentrate first taking could data and learn the tricks if taking it - including calibration data.

#6 Notdarkenough

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Posted 13 October 2024 - 02:29 PM

I have the 294MC Pro, and there is quite a bit of information showing bias frames don't work with 294 sensors. Maybe that holds for the MM version as well?




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