How do you know how much data to collect?

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ram United States of America
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How do you know how much data to collect?

#1

Post by ram »


Sometimes I get really busy and so it's easier for me to keep collecting data on a target rather than try to find something new to image (as you can see my recent images keep going longer and longer). Because of work I've also embarked on a long term project that will collect lots of data to be processed later. I'm not sure when to stop: if I was being honest with myself, half my frames are somehow partially compromised: clouds, wind, moon, smoke, etc. But that "partial" can go from anywhere from 10-70% or so. My view on it as long as a frame can add data, might as well bung it in. Of course this means making sure there are enough high quality frames - that the 50% of good frames is achieved. I'm not confident I'm doing so so I keep going rather than switch or even just pause.

Today the transparency is supposed to be bad and the wind is bad too but I'm seemingly capturing good frames - everything "appears" okay but unless analysis is done (which I don't have time to do until I get done with deadlines and such) I won't know for sure.

So my question is how do people make these judgement calls? When do you know you have enough to start processing (assuming you wait until you collect all your data to start)? And what kind of tools do you use to determine "this is enough" or "I need more" especially for those of you who wait to process later (I saw a suggestion here to collect data in the summer and process in the winter which I think would work well for me)?

--Ram
Tubes: Celestron 9.25" 235mm f/10 XLT EdgeHD SCT; Meade ETX 80mm f/5 achromat; Coronado SolarMax II 60mm f/6.6 Hα <0.7Å BF10 solar; Stellarvue 70mm f/6 triplet apochromat; Obsession UC18 457mm f/4.2 with Argo Navis & ServoCAT; Takahashi FS128 5" f/8.1 and FC100DF 4" f/7.4 fluorite doublet apochromats. Mounts: AVX; LXD75; Paramount MyT. Eyepieces: 2" Tele Vue Ethos 4.7/13/21mm, Paracorr, 2,4x Powermate; Stellarvue 0.8x, Takahashi 0.7x, 0.66x reducer/corrector. Cameras: ZWO ASI120MC-S; Lodestar X2c; X2m; Canon T7i; QHY163M; QHY247C; QHY294M-Pro. Filters: 1.25" Astrodon 5nm Ha, 3nm O3 and S2; Chroma LRGB.
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KathyNS Canada
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Re: How do you know how much data to collect?

#2

Post by KathyNS »


Simple: MORE!! :lol:

I have a "standard" sequence that collects about 2.8 hours of total data. With most targets, that gives me a pretty fair image. But then I consider how many frames I had to eliminate, and what the signal-to-noise ratio looks like, and I might re-shoot the target on the next clear night, to get more data.

If I know ahead of time that it is a challenging target, I will schedule more frames right from the start. The Cat's Eye Nebula is one that I have been trying to get for years. I scheduled it for a period when we had several clear nights forecast in a row, and shot 12.9 hours on it. It was my best result ever on that target, but I can see that I could still use more data.

So I judge it by first taking a guess, based on my own experience and other people's results. (Which is why the technical data published with an image are so valuable.) Then, I adjust it upwards based on feedback from looking at my results.
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Re: How do you know how much data to collect?

#3

Post by Greenman »


Hmm, I think the analyst in me would do a small sequence of these runs and the project out from that. Sort of standard curve...

In realty it is the same as for any artist, when is the picture done? When you put down the brush.
Cheers,

Tony.

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Re: How do you know how much data to collect?

#4

Post by STEVE333 »


For some of us it all depends upon circumstances. At my age I am very happy if I can get 4 hours worth of data in an evening. Not having an observatory I have to stay up and bring in the equipment when data taking is done, so, each night of data taking leaves me tired the next day. Thus, my endurance enters into the equation. Now using NB I only capture data with one filter in an evening, so, an SHO image would require three evenings. If I can capture 4 hours data with each of the S, H, and O filters (12 hours total exposure) I am very happy and very tired! I may want more data, but, my body may not agree!!!

I'm totally in awe at the amount of data you are able to collect on some of these targets. Mathematically (and practically) it is clear that more data produces better images. Your results certainly show that is true.

Cheers,

Steve
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Re: How do you know how much data to collect?

#5

Post by XCalRocketMan »


KathyNS wrote: Fri Sep 18, 2020 11:45 am Simple: MORE!! :lol:

So I judge it by first taking a guess, based on my own experience and other people's results. (Which is why the technical data published with an image are so valuable.) Then, I adjust it upwards based on feedback from looking at my results.
I agree. More is always better! I usually check previous work from other APers as a guide (I very much echo Kathy's remark about publishing the technical data) and I generally try for at least 4-5 hrs (20x300sec each filter NB) slightly different for LRGB. I frequently add in additional data once I process the initial set to see how it looks. After seeing some of the absolutely wonderful images that are the result of 20-30hrs+, I'm starting to lean toward more subs myself.

However, that said, you have to consider what you want to get out of this hobby. I also like it when I capture lots of different objects, and spending the 3-4 nights of quality seeing on a single object is sometimes not the thing I want to do.

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Re: How do you know how much data to collect?

#6

Post by Star Dad »


Well, I started out doing 60 minutes of wideband on a target, then moved to 90 minutes. Clusters - that's all I do for them. But now I'm starting narrowband for nebulae. I am finding it extremely difficult to align images taking with the filters... it's not anywhere close to getting the same results with each filter. I just spent about 6 hours total trying to get an Ha and OIII image aligned that I took on the same night (IC1805). And then I aligned it with a wideband from 2 weeks ago. I resorted to using the GIMP and put each image in a layer and then tried to get alignment of at least some of the stars by playing with rotation. I am still processing it as I type this, but it looks like I got the layers aligned. So I am thinking now that I'm going to do maybe 90 minutes broadband and two hours each of OIII and Ha on the same night on the same target. That is going to curtail my rapid accumulation of images, but I've probably gotten close to 200. The Veil Nebula I took last week turned out very, very nice so I am hoping this strategy will work. I envy those guys out in Texas or Arizona that can collect 20 hours in two night on a target - that would take me a month based on my cloudy skies.
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ram United States of America
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Re: How do you know how much data to collect?

#7

Post by ram »


Thank you all for your responses---that helps a lot! I agree with many of you who note that we need to enough good signal to at least swamp all bad signal out and for me the rule has been 50/50 and for the most part they tend to go in blocks

I agree it is about what you get out of this hobby but I started doing EAA where I would get several images (lofi) per night but I went into AP looking to do a bit more investment on a single target than that. I do find "projects" appealing where I spend lots of time on a single target (mainly because it conveniently fits in with my deadlines and such).

--Ram
Tubes: Celestron 9.25" 235mm f/10 XLT EdgeHD SCT; Meade ETX 80mm f/5 achromat; Coronado SolarMax II 60mm f/6.6 Hα <0.7Å BF10 solar; Stellarvue 70mm f/6 triplet apochromat; Obsession UC18 457mm f/4.2 with Argo Navis & ServoCAT; Takahashi FS128 5" f/8.1 and FC100DF 4" f/7.4 fluorite doublet apochromats. Mounts: AVX; LXD75; Paramount MyT. Eyepieces: 2" Tele Vue Ethos 4.7/13/21mm, Paracorr, 2,4x Powermate; Stellarvue 0.8x, Takahashi 0.7x, 0.66x reducer/corrector. Cameras: ZWO ASI120MC-S; Lodestar X2c; X2m; Canon T7i; QHY163M; QHY247C; QHY294M-Pro. Filters: 1.25" Astrodon 5nm Ha, 3nm O3 and S2; Chroma LRGB.
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Re: How do you know how much data to collect?

#8

Post by John999R »


collecting data under dark skies is a bonus.
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Re: How do you know how much data to collect?

#9

Post by Stuart »


I usually go for 20-30 subs of whatever filter, no matter the length of the subs, to make sure I have adequate data rejection.
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