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INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

Excellent tutorial on minimizing noise and optimizing exposure parameters

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And Craig Stark's very similar presentation on this topic:

5 years 6 days ago #36851

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I wanted to check if my noise calculation were correct so adapted my own spreadsheet to match the data presented in the first video.
Results are very similar, the only difference being the wavelength at which SQM is measured: reading the SQM documentation is should be about 500nm but to match the data in the video 300nm is to be used (well these data are not in the video but the video references to tools.sharpcap.co.uk/). Anyone have the same difference?

Too bad that the video ended before gain was taken into account. I have many doubts how to treat it.
5 years 5 days ago #36896

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The 300 nm in the SharpCap tool do not refer to a wavelength, but to the bandwidth of the filter used. It assumes a bandwidth of 300 nm for the luminance filter, it can be increased to a maximum of 350 nm.

I would say that’s about right, because the bandwidth of the spectrum generated by city lighting is probably around 300 nm.
5 years 5 days ago #36897

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I meant bandwidth, sorry. But the point was that for 'no filter' I would not assume 300nm bandwidth but a greater value as the SQM meter collects light on a wider range as far as i know.
5 years 5 days ago #36900

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but I was wrong: arxiv.org/pdf/1701.05019.pdf (page 4, figure 4): measured spectrum range is indeed 300nm, theoretical a little more.
5 years 5 days ago #36901

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Here is another informative (and at times spirited) discussion on the same topic. That one also addresses the effect of gain.

www.cloudynights.com/topic/573886-sub-ex...00-and-maybe-qhy163/

Basically, the settings I have gravitated towards in my extremely light polluted backyard is a gain of 240 on the ASI1600MM Pro and an exposure time of 8 seconds for luminance, 20 s for RGB and 120 s for Ha, O3 and S2 at f8. That keeps the median ADUs in the low 2000s with my dark ADUs around 800. That puts me just a little more than 1200 ADUs above the darks. According to the cloudnights discussion, that is the optimal setting at which saturation of stars (i.e. preservation of their color) is minimized while preserving maximal dynamic range possible under the circumstances.

I had basically empirically come to very similar values before being alerted to the discussion board by a friend in our Astrophotography group. Bottomline: In the city use short exposures and high gain settings, at a dark site use long exposures and low gain settings, because only there can you really get the return on dynamic range that longer exposures and low gain can provide. It is not possible in the city.
Last edit: 5 years 5 days ago by Jose Corazon. Reason: forgot f ratio. Exposure times are meaningless without it.
5 years 5 days ago #36904

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That is correct, the SQM meter collects photons over a wider range, but the bandwidth at which man-made light radiates is generally more limited to the visible spectrum and falls within the 300 nm range. Anything outside that range would be deep red or glaring blue/UV. Artificial light generally minimizes those parts of the spectrum.
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5 years 5 days ago #36905

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Exactly what I thought at that point! Too bad he did not elaborate on this topic which is poorly understood by myself.
5 years 5 days ago #36910

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Thanks, I read the thread (well ,not all 188 posts...), argumentations are solid but the data presented are mainly empirical. There's another useful thread on CN that discuss the theory behind the calculations:
www.cloudynights.com/topic/536809-conver...o-photon-flux/page-2 (#38)

Do you have an excel file with simulations of suggested exposure times to share? I would like to compare to mine
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5 years 4 days ago #36929

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Hi Fenriques,

Sadly, no, don't have an Excel file with simulations. That might be useful to build.

I only have the numbers I gave in my last reply, which fit the abhorrent conditions in my backyard in a white zone and which match closely what the sharpcap site says and what the formulas in the cloudynights blog also suggest. They were obtained recently with an RC-8" at f/8 using the ASI1600MM Pro at 240 gain. The ADUs come out a little higher than in the cloudnights discussion I provided the link to, I will continue tweaking times and gain some more until my results are optimized to the conditions (that doesn't say much!).

One drawback not discussed in the video presentations or the blogs is the time factor required for downloading and dithering. The shorter the exposure times, the more those come into play. By going to shorter exposure times one will progressively end up with less integration time, since progressively more time is being wasted downloading the images and dithering. That is especially critical when using a Raspberry Pi, which is much slower than a mini-PC, or when downloading the images over WiFi. Here, it becomes essential to use a mini-PC at least, preferably the more computing power one has the better. Also, USB3 becomes absolutely critical for downloading the images. For those reasons, it seems to me to make sense not to go below 30 s exposures and rather preserve dynamic range by using a lower gain for luminance, while keeping the median ADUs in the low 2000s.

That makes the scheduler more important than ever. With that, one can then program image acquisition at different gain values, i.e. perhaps 120 gain/30s for L, 180 gain/30s for RGB and 300 gain/60s for narrowband from the city at f/8.

Jasem, if you are reading this, that would be a useful addition to the capture module: Allow different gain values to be programmed for the different filter settings (rather than only one gain value for the entire sequence), without the need to set up separate entries for the same target in the scheduler.

What do y'all think?

Best

Jo
5 years 4 days ago #36935

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hi Jo,
I'm using a color cmos so I'm not really concerned about the gain issue you mentioned.
Anyway, having a different gain setting per sequence wouldn't increase time to take calibration frames too much? that could be automated of course but storage / download time would increase.

Ferrante
5 years 3 days ago #36967

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Hi Ferrante,

Don't tell anyone (:blush: ) and don't post that fact on the internet, but I am a little lazy with my calibration frames. I am reusing my darks and flats for pretty extended periods of time and given that my area is light polluted, I have no problem with that. The noise is always much greater than the subtle difference from sensor shift over time. I have to use narrowband, one shot color is just not cutting it from the city, except for the very brightest objects like M42.
Anyway, given the dead time lost in downloads and dithering, exposure times of less than 30s seem to be counterproductive as they cut down substantially on total integration time achieved during the session. That was the main point I wanted to draw attention to.
Cheers
Jo
5 years 3 days ago #36971

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