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More imaging time not helping much

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#1 rkayakr

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Posted 22 October 2019 - 11:26 AM

I'm trying to image the Bat and Squid with my ASI1600 on a TS 80/480 with a .8x reducer that brings the focal ratio down to F4.8. I'm using 120s exposures at gain 200. After 85 subs the HA looks good. After 120 subs the OIII showed very little. I kept collecting OIII and now I'm up to 896 subs (~30 hours) and stacking 806 (~27 hours). I'm still seeing very little. Is it hopeless to try to image this from my suburban location (19.09 mag, class 6 Bortle)?

Much of the OIII was captured during bright moon phases. Is that the problem?

 

Suggestions appreciated.

 

Image with STF applied.

 

get.jpg?insecure



#2 kingjamez

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Posted 22 October 2019 - 11:34 AM

120s seems low to me for F4.8. What is the median of your subs?

 

OIII is more affected by the moon than Ha or SII. I do SII when the moon is full.

 

-Jim


Edited by kingjamez, 22 October 2019 - 11:34 AM.


#3 2ghouls

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Posted 22 October 2019 - 11:41 AM

That doesn't look too bad. It's a very faint object. How do you plan to approach post-processing? If you remove the stars and use tone mapping I think you could get a nice result with what you have.

 

In terms of acquisition advise:

Yes, shooting OIII with the moon out hurts contrast. While I still do it with bright OIII emitters, for something like the squid I would avoid it. When I shot the flying bat and squid, I only got 6.6 hours of OIII and 3 hours of Ha, but all from very dark sites on moonless nights at f/2.8. I had an additional 8 hours from an urban setting, and found the photo was stronger by not including that data. Also, I was shooting 10 min subs at f/2.8 for the OIII. Yes, this meant I was clipping stars, but since I was planning to tonemap, and not use the OIII stars, I didn't care, but that kind of thing is really personal preference. Take all of this with a grain of salt, just sharing my experience since I've shot this object.


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#4 rkayakr

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Posted 22 October 2019 - 03:37 PM

Jim

   Typical median is .0212 with .0109 standard deviation.

 

Thanks Nico

  I've got your image bookmarked. Looking at your imaging time gave me alarm about how poor my OIII stack is after much more time.

I've never tone mapped. I guess it's time to learn something new.

I tried stacking using a subframe selector combination weight and SNR weight and didn't see any difference. I'll continue to collect images now that the moon is less of an issue. Any suggestions on a good measure to use for contrast to thin the existing herd of subs?

      

          Bob


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#5 kingjamez

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Posted 22 October 2019 - 04:18 PM

Hi Bob,

Sorry, what was your Median ADU?



#6 View2

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Posted 22 October 2019 - 05:15 PM

O3 is usually much weaker than H1 and require much more time. That is Imaging time not exposure time. Like 2 hours of hydrogen Alpha in 4 hours of oxygen 3

Edited by View2, 22 October 2019 - 05:17 PM.


#7 rkayakr

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Posted 23 October 2019 - 04:35 AM

Jim

  The medians range from 1400 to 2800. From many posts on CN it looks like 1450 is a good target for the ASI1600mm-cool.

 

View2

  I have about 3 hours of imaging Ha, which looks good, and 30 hours of OIII, which I think is faint - so I'm at 10 to 1 in imaging time.



#8 2ghouls

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Posted 23 October 2019 - 06:54 AM

Looking at your imaging time gave me alarm about how poor my OIII stack is after much more time.
I've never tone mapped. I guess it's time to learn something new.
I tried stacking using a subframe selector combination weight and SNR weight and didn't see any difference. I'll continue to collect images now that the moon is less of an issue. Any suggestions on a good measure to use for contrast to thin the existing herd of subs?


I would probably just use blink and do it visually, or I would think it would line up with the median count. A higher median would mean a brighter sky/ less contrast.

I believe my Squid was just as faint, but my background sky was a bit darker from shooting in Bortle 1/2 sites. In any case, there is not much other OIII signal in the frame so it should be easy to apply some extreme contrast adjustments to the image in the tone mapping process and get a nice final product from what you have. I suggest using Starnet++ for the star removal.
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#9 lucam

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Posted 23 October 2019 - 10:22 AM

Bob,

 

I shot the Squid this season from Bortle 5 skies using the ASI1600 at unity gain (gain=139, offset=50) with a Takahashi FSQ106 with 0.73x reducer (effective f=3.6) and 5nm Astrodon filters. I only shot OIII data on moonless hours and the total exposure time on the squid was 97x8min~13 hours. I usually target 1200 to 1500 DN for subs on the ASI1600 at unity gain. You can find my image on astrobin here. I shot a two panel mosaic to cover the whole flying bat, but you can see the exposure times for each panel in the description. I tone mapped my image but I did not have to stretch the OIII excessively. You can see the inverted OIII signal of the squid in revision G. I think there is a bit more SNR in the squid in my image but certainly not 10x. 

 

The ratio of f-ratios between your system and mine is 1.33, so that amounts to ~1.8x imaging time to achieve the same pixel SNR (same camera in both setups). I don't think gain changes SNR calculation, that's only affected by the amount of object vs background light captured (provided read noise is negligible). It just affects how quickly you fill up the wells, which sets the sub duration. At twice the gain, you could shoot 4 minute subs instead of my 8 with similar results but more star clipping. 

 

2 minute subs seem a bit on the short side and may affect how you build up SNR as a function of total integration time. However, if your subexposure ADU counts are in the range you reported, that's not likely the case. You seem to be well in the shot noise dominated regime. which then points to either significant light pollution, or wider band filters (what filters did you use? signal will be affected by transmission of the filters and noise by rejection band of the filters), or shooting with the moon in the sky. 

 

finally, given the wide range of ADU counts you reported, I agree with Nico that you may want to make sure that a few poor subs are not wrecking the whole integration. Blink is a good way to go, Subframe Selector process in PI is useful as well (you can look at star count are exclude images with star count < 2 sigma from the mean or so). 


Edited by lucam, 23 October 2019 - 10:24 AM.

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#10 rkayakr

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Posted 24 October 2019 - 02:39 PM

Thanks Nico and Luca

   Now that the moon is down most of the night I have collected more images under darker but still light polluted skies. I'm up to 38 hours of OIII subs. I'm using subframe selector to eliminate low star count and low snr values as well as outliers in FWHM and eccentricity. The next stack trial is about 22 hours or 58% of captures, down from the 90% I was stacking.

  I've been reading about tone mapping. Looks like it will be a long adventure.


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#11 View2

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Posted 24 October 2019 - 03:11 PM

Good grief! 38hrs. I think I agree that using blink would be much faster to weed out the obviously bad ones and then maybe do your subframe selector but that sounds like a lot of Subs to go through

Edited by View2, 24 October 2019 - 03:11 PM.



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