A couple of questions on acquisition/processing

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BGazing
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A couple of questions on acquisition/processing

Post by BGazing »

Just thought I might tap into the collective wisdom of old hands here:
- Do you always use gain 0 unless you are dealing with DSs? Is there any purpose to use gain if expositions are under3.5ms?
- Is there a point in recording more than 1000 images in one go?
- Where do you cut off your stack? Is anything above 40-50 acceptable, or is it better to go for 100 (out of 1000) unless there is a dramatic drop-off in order to have less noise to work with? I am just trying to avoid double work...tried both and did not come to a particular conclusion so far but my sample is small.
- When shooting proms only (and planing to mask the surface during processing) is it better to up the histogram into the 90s? Or keep it at 80-90?


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Re: A couple of questions on acquisition/processing

Post by pupak »

Exposure is dependent on the data throughput of the system. It doesn't make sense to use 3ms and have 60 FPS. It is set as much as corresponds to the max FPS, but the gain is as low as possible. A compromise is chosen with regard to seeing. Bad seeing = higher FPS and vice versa.
Those who want details and have SW for it, choose 16 bit mode, otherwise 8 bit mode is more advantageous for more FPS. Smaller gain = higher picture dynamics. It's always a compromise. There is no universal setting. Different telescop, different camera = different setting.


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Re: A couple of questions on acquisition/processing

Post by christian viladrich »

Hello Vladimir,

Here are some hints :

- Exposure time comes first. In order to freeze turbulence, you need an exposure time shorter than about 5 to 7 ms. Some prefer to go even shorter than 5 ms. If you can have this with gain 0, this is great. Otherwise you need to adjust the gain value accordingly.

- In order to maximize the number of good images, the acquisition duration should be as long as possible (and fps as high as possible). The limit depends on the resolution of the telescope. In other words, the duration of the acquisition is proportional to the diameter of the telescope. Basically, you can go to about 40 s with a 150 mm refractor. In case this is a fast evolving feature (prominence), you might need to reduce this duration.
So, the duration of acqusition comes first. The number of images is just the result duration x fps.

- Be pragmatic about the number of frames to stack. Just try and see what is the best result. Forget about the quality graph of AS3. With AS3 you can make four different stacks (and even 8), so try and see what is best. This will educate your eye and your hability to see the difference between images of different quality.
After a while, you will find that the number of frames to stack is more or less the same for a given filter (continuum, Ca K, Ha, etc.).


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Re: A couple of questions on acquisition/processing

Post by BGazing »

Thank you both very much. As with 80MT my expositions are below 3.1ms under most conditions and even critical sampling, I guess it does not make much sense to add gain, exposure times are fairly short. I will try with some longer exposures, especially on smaller ROIs, feel like 1000 is too short on closeups.


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Re: A couple of questions on acquisition/processing

Post by Carbon60 »

Hi Vladimir,

I usually capture around 4000 frames for close views and stack 1, 5 and 10%, in AS!3 to select the best for further processing depending on conditions, and 500-1000 frames for FD mosaic segments, stacking the same percentages.

Stu.


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Re: A couple of questions on acquisition/processing

Post by marktownley »

I would echo exactly what Christian says, this is what I do too.

Foor my fukll disks I normally take 3 x 1000 frames, and then pick the sharpest disk.

For normal closeups I take 3000 frame file sizes in normal conditions, if conditions are worse I may capture 4 or 5000 frame files, I normally take between 5 and 8 acquisitions with these to find the best stack.


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