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  #21  
Old 02-07-2017, 02:24 PM
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Slawomir (Suavi)
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Thank you Rick for clarifying that.

So it looks like one day I should invest in a camera with about 6micron pixels to give me just under 2"pp with my telescope at f/6, I think something like KAF16200 would be nice. I was hoping that I can get a wider FOV with my tiny ICX814 at f/4.5, but such fast f-ratio on a short refractor is a challenge to get spot on in terms of alignment and spacing. And on top of that frequent refocus is needed...I much prefer riding at f/6. Time to start slowly putting some dollars aside for a camera upgrade! LOL
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  #22  
Old 02-07-2017, 02:46 PM
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That would be a good combination, Suavi. We have a FLI ML16200 at DSW and it looks like a really nice camera. Read noise is only 5e- which is great for a KAF sensor.
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  #23  
Old 02-07-2017, 04:06 PM
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Quote:
Originally Posted by Slawomir View Post
I have read somewhere that sampling at 1/3 of your usual FWHM is optimal for getting the most detail from data, so I believe your observations are spot on Allan. Drizzle x3 requires more subs/better data than x2, but if it works then why not use it I like Troy's suggestion of directly comparing different methods visually and by measuring noise/SNR.

As for twisting Mike's arm - good luck with that!
Yes that's right Suavi. Nyquist theorem for sampling is a minimum of 2. Minimum being the main word there. So practical use is to go for 3X.

Greg.
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  #24  
Old 02-07-2017, 04:10 PM
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Originally Posted by Slawomir View Post
I have read somewhere that sampling at 1/3 of your usual FWHM is optimal for getting the most detail from data, so I believe your observations are spot on Allan. Drizzle x3 requires more subs/better data than x2, but if it works then why not use it I like Troy's suggestion of directly comparing different methods visually and by measuring noise/SNR.

As for twisting Mike's arm - good luck with that!
Quote:
Originally Posted by Slawomir View Post
Thank you Rick for clarifying that.

So it looks like one day I should invest in a camera with about 6micron pixels to give me just under 2"pp with my telescope at f/6, I think something like KAF16200 would be nice. I was hoping that I can get a wider FOV with my tiny ICX814 at f/4.5, but such fast f-ratio on a short refractor is a challenge to get spot on in terms of alignment and spacing. And on top of that frequent refocus is needed...I much prefer riding at f/6. Time to start slowly putting some dollars aside for a camera upgrade! LOL
Roland Christen of AP fame always recommends 1 arc sec/pixel for those with average seeing of around 3 arc secs so 2 arc sec is still not quite there. I have noticed this with the various scopes and camera combos I have used over time. When you are close to that 1 arc sec the images look nicer.

I would also add though that full well depth is a very important and underrated aspect of a sensor. Too low and you lose a lot of dynamic range if you overexpose. That's why I tend to keep coming back to my 16803 camera. Its still the best out there for overall performance.

Maybe these Sony CMOS full frame sensors if someone can get their hands on a mono version of the Sony A7r2 sensor it would be pretty sensational and may leave CCDs for dead. At higher ISO this thing's read noise is .58 of an electron. Its backside illuminated etc etc. QE would be very high and 42mp would give super high resolution.

Greg.
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  #25  
Old 02-07-2017, 06:57 PM
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Sounds good in theory Greg, but with my scope I would need to get even smaller pixels - ICX834 with 3.1 micron pixels would give me exactly 1"pp, but that means even shallower wells. So big fat pixels on a big scope but that also means heavy (end very pricey!) rig, and I need to set up/down for every session. That's why I need to stick with a light scope = drizzling

FLI 16200 sounds VERY good, maybe in a few years... :-)
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  #26  
Old 02-07-2017, 07:47 PM
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When it comes down to trying to get perfect arcsec/pixel and trying to be seeing limited is what you do when you're imaging with something larger than a 4" scope
Start worrying about it when you get an ASA 10" F/3.6 newt, that way you can use your QSI690 and have a perfect thing going on, albeit a narrow FOV.
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  #27  
Old 02-07-2017, 08:09 PM
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Start worrying about it when you get an ASA 10" F/3.6 newt, that way you can use your QSI690 and have a perfect thing going on, albeit a narrow FOV.
Something similar to Mike's setup, so it must be very good
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  #28  
Old 04-07-2017, 07:45 PM
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Suavi,

from the appearance of the undrizzled images you've shown in your posts, I'd guess that the subs have been adjusted by full pixels prior to stacking.
If my guess is right: Did you ever compare drizzling against sub-pixel alignment?

Erwin
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  #29  
Old 04-07-2017, 08:14 PM
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Hi Erwin,

To be honest I do not know how exactly PixInsight creates a drizzled image. I just followed a regular path of Star Alignment that also creates drizzle data while aligning all subs, and then Image Integration allows for refining pixel rejection algorithms (it also updates individual drizzle files for each sub), and finally I used Drizzle Integration tool with standard settings to create drizzled masters.
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  #30  
Old 05-07-2017, 05:52 AM
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Quote:
Originally Posted by ErwinL View Post
from the appearance of the undrizzled images you've shown in your posts, I'd guess that the subs have been adjusted by full pixels prior to stacking.
If my guess is right: Did you ever compare drizzling against sub-pixel alignment?
Quote:
Originally Posted by Slawomir View Post
To be honest I do not know how exactly PixInsight creates a drizzled image. I just followed a regular path of Star Alignment that also creates drizzle data while aligning all subs, and then Image Integration allows for refining pixel rejection algorithms (it also updates individual drizzle files for each sub), and finally I used Drizzle Integration tool with standard settings to create drizzled masters.
Hi Suavi,

I think that Erwin is asking if you created the original (non-drizzled) images using nearest neighbour interpolation as opposed to one of the other modes that does sub-pixel alignment. Nearest neighbour can give a blocky appearance.

Erwin: I'll let Suavi answer about his data but my experience with undersampled data is that Drizzle still does a much better job of rounding up stars than sub-pixel interpolation without Drizzle.

Cheers,
Rick.
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  #31  
Old 05-07-2017, 07:24 AM
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Got it, thank you for clarifying that Rick

The non-drizzled image is a stack of original subs without upscaling, at about 1.6"pp. My system is a bit under-sampling when I use a reducer, thus there are blocky stars in the non-drizzled image. I rarely upscale final images as I am confident that drizzle will yield a superior result to upscaling a final image, but perhaps one day I could actually try to measure if there are any differences.

Suavi
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  #32  
Old 05-07-2017, 06:24 PM
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Originally Posted by Slawomir View Post
I rarely upscale final images as I am confident that drizzle will yield a superior result to upscaling a final image, but perhaps one day I could actually try to measure if there are any differences.
Ah, ok, I see things are a bit different in PixInsight (which I never used). What I'm doing regularly and meant in my question is sub-pixel alignment and interpolation without upscaling. The basic calculations are comparable to drizzling but the image size remains unchanged.

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..my experience with undersampled data is that Drizzle still does a much better job of rounding up stars than sub-pixel interpolation without Drizzle.
Rick: Do you think this is also valid for this 'in-place' drizzling-like interpolation? (Unfortunately, my equipment and the seeing at my location do not allow undersampling, so I can't do real tests.)

Erwin
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  #33  
Old 05-07-2017, 07:42 PM
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Ah, ok, I see things are a bit different in PixInsight (which I never used). What I'm doing regularly and meant in my question is sub-pixel alignment and interpolation without upscaling. The basic calculations are comparable to drizzling but the image size remains unchanged.
Hi Erwin, the default interpolation done by PixInsight image registration is sub-pixel interpolation without upscaling. You can also set the interpolation to be nearest neighbour but I doubt that many people do.

Quote:
Originally Posted by ErwinL View Post
Rick: Do you think this is also valid for this 'in-place' drizzling-like interpolation? (Unfortunately, my equipment and the seeing at my location do not allow undersampling, so I can't do real tests.)
With a FSQ-106ED and KAF-16803 sensor I have an image scale of 3.5 arcsec/pixel. Even with sub-pixel registration smaller stars don't cover many pixels and they look blocky. Drizzle x2 is a big improvement in this case.

Cheers,
Rick.
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