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Help understanding sampling / choosing barlow

Astrophotography
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#1 PcAstronomy

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Posted 26 June 2022 - 12:02 PM

I'm having a bit of trouble understanding sampling when it comes to planetary imaging. According to this calculator https://astronomy.to...ccd_suitability with my setup I'm oversampled no matter what I do. The only way I can get it right is by setting 2x2 binning which I read somewhere was a bad idea. I also heard I should be aiming for an F ratio somewhere between 5-7x the CCD pixel size which would be F/18 - F/26. Should I be using a 5x barlow to get close to this or should I stick with the 3x barlow? I'm tracking manually so it might be hard to keep the planet centered with a 5x. I can't seem to get any decent images of Saturn even in pretty good seing and with collimated optics and the rings just seem to merge together without the cassini division. My equipment is Sky-Watcher 8" Dob, Asi 224mc, 2x/3x barlow. Here is an example of a recent capture with a 3x barlow in decent seeing. 

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#2 james7ca

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Posted 26 June 2022 - 12:28 PM

Hard to tell from that image but you either had fairly poor seeing or a focus problem or an issue with your optics (collimation?) or some vibrations in your scope/mount. If I had to guess, it looks like bad focus or vibrations (since you said the seeing was "pretty good" and that the scope was collimated -- but how did you determine that the seeing was good?).

 

If you were manually tracking then it's possible that the drift of Saturn was too great to be "frozen" given your exposure time or that you were introducing extended vibrations when you manually moved the mount.

 

You can calculate an approximate lower limit in exposure time when imaging from a non-tracking mount by dividing the number 50 by your focal length. So, if your effective focal length was 2400mm (with the barlow factor) you should NOT be using any exposure time greater than 50/2400s or 20ms. Actually, to be totally save you probably want to go even lower than that, maybe 10ms. And, if you were moving the mount while actually taking your images then I'm not certain how fast of an exposure time you'd need, but it would be less than any of the above limits, perhaps MUCH less (basically NOT recommended at any speed).

 

As for sampling, I'd go at least 7X your pixel size, so an effective f/26. If below f/20 you might even want to consider doing a drizzle integration/stacking (but only after you figure out why the image looks so soft).


Edited by james7ca, 26 June 2022 - 12:31 PM.


#3 Hesiod

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Posted 26 June 2022 - 12:39 PM

Well, the capture is just the frist step, the hardest part is how to process the data.

 

Anyway, that site is for deep sky imaging, for hi-res planetary imaging you want to sample in order to cover ca 1/3rd of your telescope's potential resolution.

As an example, with a 8" aperture (which should resolve up to roughly 0.6") you'd strive for a 0.2"/pixel sampling*, meaning that the Barlow 3x should bring you closer to the ideal sampling.

Because of this manual tracking is especially bothersome, so probably the best option is to give yourself a tracking platform (several years ago I remember there were "blueprints" on the web, allowing you to craft one from plywood and the motor from a dismissed printer. Mind that you do not need a 10micron-grade tracking)

 

 

 

 

 

*these are a sort of mean value for the visual spectrum because actual resolution changes with the wavelength; from a practical standpoint you may use broader sampling if are shooting IR, tighter sampling if are shooting UV (these are obviously the two extremes) 



#4 PcAstronomy

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Posted 26 June 2022 - 03:09 PM

Hard to tell from that image but you either had fairly poor seeing or a focus problem or an issue with your optics (collimation?) or some vibrations in your scope/mount. If I had to guess, it looks like bad focus or vibrations (since you said the seeing was "pretty good" and that the scope was collimated -- but how did you determine that the seeing was good?).

 

If you were manually tracking then it's possible that the drift of Saturn was too great to be "frozen" given your exposure time or that you were introducing extended vibrations when you manually moved the mount.

 

You can calculate an approximate lower limit in exposure time when imaging from a non-tracking mount by dividing the number 50 by your focal length. So, if your effective focal length was 2400mm (with the barlow factor) you should NOT be using any exposure time greater than 50/2400s or 20ms. Actually, to be totally save you probably want to go even lower than that, maybe 10ms. And, if you were moving the mount while actually taking your images then I'm not certain how fast of an exposure time you'd need, but it would be less than any of the above limits, perhaps MUCH less (basically NOT recommended at any speed).

 

As for sampling, I'd go at least 7X your pixel size, so an effective f/26. If below f/20 you might even want to consider doing a drizzle integration/stacking (but only after you figure out why the image looks so soft).

Through the eyepiece the cassini division was nice and crisp with minimal bobbling due to the atmosphere so I just assumed it was decent seeing. As for collimation, I check it every time I'm about to start imaging using a cheshire. However, Saturn was quite low on the horizon so that might have had an impact as I've yet to get an ADC. I processed this by using PIPP to centre, autostakkert to stack 20% and then some wavelets in registax but it came out horrible. I was also pushing the scope slightly to keep the planet centred as I used a small region of capture for a higher frame rate so it was jumping around quite a bit. Should I be using a much higher gain with a much faster exposure time?



#5 Tulloch

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Posted 26 June 2022 - 06:39 PM

I'm having a bit of trouble understanding sampling when it comes to planetary imaging. According to this calculator https://astronomy.to...ccd_suitability with my setup I'm oversampled no matter what I do. 

As mentioned before, that site is designed for DSO imaging, not planetary.

 

I also heard I should be aiming for an F ratio somewhere between 5-7x the CCD pixel size which would be F/18 - F/26.  

For good seeing, a focal ratio of 5x the pixel size of your camera is around opimal. There is a link in the FAQ showing the maths and discussion about this value, see section 6.1.

https://www.cloudyni...ated-june-2022/

 

 I'm tracking manually so it might be hard to keep the planet centered with a 5x...  My equipment is Sky-Watcher 8" Dob, Asi 224mc, 2x/3x barlow.

Tracking manually makes is difficult to capture high resolution images, especially with a small sensor camera like the ASI224MC. For best results you should be imaging Saturn for 6 minutes, however at a f/num 5x the pixel size of the camera you will struggle - lots.

 

However, Saturn was quite low on the horizon so that might have had an impact as I've yet to get an ADC.

This will also reduce the quality of your images, the atmospheric effects at low elevation angles will significantly reduce the image quality. This is probably the biggest factor affecting your images at the moment.

 

So, what can you do to improve your images? IMHO, you should look at mounting your OTA on some sort of tracking mount, an EQ platform or alt/az goto/tracking mount so you can keep the planet on the sensor for at least 3 minutes, more for Saturn.

 

If you haven't already done so, reading the FAQ will help you, there are also some great tutorial videos at the end which may help.

https://www.cloudyni...ated-june-2022/

 

Andrew



#6 james7ca

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Posted 26 June 2022 - 08:49 PM

<snip>... Should I be using a much higher gain with a much faster exposure time?

What was your expousre time? If it was 10ms of less then that should have been okay, unless you were touching or moving the scope during the exposures. But, this also depends upon what your effective focal length was, shorter or longer than the 2400mm I quoted before.




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