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  #21  
Old 24-04-2015, 07:02 PM
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I think the key here is that there's some trade-offs and you need to balance those in accordance to your situation.

For me it's a matter of trying to identify "optimal exposure time" such that with an equal integration time, the SNR will be just as good as if you'd done longer exposures.

Once you can figure that out, don't go over it because you're just putting each exposure at a higher risk of being discarded due to events such as wind and tracking errors.
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  #22  
Old 24-04-2015, 07:36 PM
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Hi All,

Very interesting discussion. Some parts of Ray's hypothesis I would tend to agree with, some parts not.

I don't reject a sub unless it has an obvious fault, i.e. visibly poor star shapes. In most cases I get to use all my subs, (ROR jobs, PMX, off-axis guiding etc) even small defects are rejected by the data processing. So, depending on the brightness of the object I vary the exposure time. For NB imaging, 10 -15 minute subs bring out the detail in the DSO, shorter subs generally don't allow the signal to be collected in the dimmer regions of extended objects.

For LRGB, 5 minute subs are all I can do, LP from Melbourne kills the subs after that.

I'm also a fan of collecting lots of subs, but not so you can reject them based on a FWHM measurement, but so the data rejection algorithms are more accurate. So I tend to let the software decide which parts of the sub to retain and which parts to discard. After this a combined image can be sharpened heavily without showing artefacts, which in essence is removing the blur induced by seeing.

Cheers
Stuart
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  #23  
Old 25-04-2015, 01:39 PM
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Originally Posted by Shiraz View Post
Hi Greg
re your question on how seeing varies, I did a couple of analyses on multi-night data from a couple of other targets (3 to 4 minute subs) - the attached data is HFR (in pixels as before) and shows how much and how quickly the seeing varies at this site. Hope it is interesting - it seems that short term fluctuations of around 0.2 pixels or more (or about 0.4 arcsec FWHM) are common, which is a fair bit really. Regards ray

edit: none of this data is normalised to remove the effect of elevation on seeing.
Isn't this just the autoguiding log graphed? In which case it looks a lot like a PEC curve. So how do you extract the seeing from the PE?
I know Pempro has a calculation to do this. I am not sure what it would be other than its probably the outliers in otherwise repeating pattern.

Greg.
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  #24  
Old 25-04-2015, 05:55 PM
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Originally Posted by codemonkey View Post
I think the key here is that there's some trade-offs and you need to balance those in accordance to your situation.

For me it's a matter of trying to identify "optimal exposure time" such that with an equal integration time, the SNR will be just as good as if you'd done longer exposures.

Once you can figure that out, don't go over it because you're just putting each exposure at a higher risk of being discarded due to events such as wind and tracking errors.
agree - my philosophy as well

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Originally Posted by rat156 View Post
Hi All,

Very interesting discussion. Some parts of Ray's hypothesis I would tend to agree with, some parts not.

I don't reject a sub unless it has an obvious fault, i.e. visibly poor star shapes. In most cases I get to use all my subs, (ROR jobs, PMX, off-axis guiding etc) even small defects are rejected by the data processing. So, depending on the brightness of the object I vary the exposure time. For NB imaging, 10 -15 minute subs bring out the detail in the DSO, shorter subs generally don't allow the signal to be collected in the dimmer regions of extended objects.

For LRGB, 5 minute subs are all I can do, LP from Melbourne kills the subs after that.

I'm also a fan of collecting lots of subs, but not so you can reject them based on a FWHM measurement, but so the data rejection algorithms are more accurate. So I tend to let the software decide which parts of the sub to retain and which parts to discard. After this a combined image can be sharpened heavily without showing artefacts, which in essence is removing the blur induced by seeing.

Cheers
Stuart
Very interesting post Stuart - thanks. I guess that you and Fred favour leaving data in to keep the SNR up and then deconvolving to recover detail - so you would have no interest in leaving out data that is lower in resolution. Both approaches work - I wonder which is best?

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Originally Posted by gregbradley View Post
Isn't this just the autoguiding log graphed? In which case it looks a lot like a PEC curve. So how do you extract the seeing from the PE?
I know Pempro has a calculation to do this. I am not sure what it would be other than its probably the outliers in otherwise repeating pattern.

Greg.
Hi Greg. No, it's nothing at all to do with the autoguiding. These are plots of the how the HFR star shape parameter varies from sub to sub. I used "FITs image grader" to analyse the final subs - for example, each point in the lower graph on the second image shows the HFR value for one of the 183 individual subs in that multi-night sequence (ie these 183 points represent measurements from about 10 hours of imaging). I think that FITS image grader works somewhat similarly to CCD inspector, analysing the shapes of multiple stars across a sub - however, it reports a single average star shape parameter (HFR) for the sub. HFR is similar to FWHM/2, so each data point represents what the seeing was like over the 3-4 minutes when the sub was taken (at my image scale, an HFR of 1.2 pixels would be pretty close to a FWHM of about 2.2 arc seconds).

Regards Ray

Last edited by Shiraz; 25-04-2015 at 07:53 PM.
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  #25  
Old 26-04-2015, 08:21 AM
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Ray,

Thanks for your explanation of why you chose averaging. Seems a decent experiment and conclusion*

I suppose the only definitive test would be to use two scopes and two identical cameras at the same time, one taking short subs, the other one long sub, and compare. Too hard.

Peter

* when you stacked the subs I assume you didn't do any data reject, right?

Last edited by PRejto; 26-04-2015 at 08:48 AM.
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  #26  
Old 26-04-2015, 09:55 AM
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Originally Posted by PRejto View Post
Ray,


Peter

* when you stacked the subs I assume you didn't do any data reject, right?
correct - basic average stack in Nebulosity with no smarts, although I did use hot pixel rejection in both cases, to ensure that bad pixels did not distort the HFR results. I thought about 2 scopes, but even then they would not look through the same air - and besides, as you say, too hard. However, I am reasonably satisfied that the conclusions are reliable enough for most purposes .
regards Ray

Last edited by Shiraz; 26-04-2015 at 11:40 PM.
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  #27  
Old 26-04-2015, 05:06 PM
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Hi Greg. No, it's nothing at all to do with the autoguiding. These are plots of the how the HFR star shape parameter varies from sub to sub. I used "FITs image grader" to analyse the final subs - for example, each point in the lower graph on the second image shows the HFR value for one of the 183 individual subs in that multi-night sequence (ie these 183 points represent measurements from about 10 hours of imaging). I think that FITS image grader works somewhat similarly to CCD inspector, analysing the shapes of multiple stars across a sub - however, it reports a single average star shape parameter (HFR) for the sub. HFR is similar to FWHM/2, so each data point represents what the seeing was like over the 3-4 minutes when the sub was taken (at my image scale, an HFR of 1.2 pixels would be pretty close to a FWHM of about 2.2 arc seconds).

Regards Ray[/QUOTE]

I see. But still how possible is it to remove the effects of autoguiding from an image so the balance is wind, seeing.

Add to that the fact you may get additional flexes at different angles as the night progresses to further complicate the data.

So even though an algorithim has been used to try to say this is the seeing part I doubt it can be done very accurately. Further evidence that this is the case is the need to do a huge TPoint model then a huge CPU intensive calculation to work out a supermodel, get rid of the outliers, try different possible flexes in the system.

Even the tracking rate of the mount would not be correct at lower alititudes than near the zenith due to atmospheric effects (AP mounts enable different rates of tracking at different altitudes to compensate for this).

After having spent endless hours watching the guide errors on many many nights of different conditions I think it would be very hard to differentiate between simple PEC and seeing variability.

Planetary imagers probably know what the seeing is like the best as they fight it all the time. I wonder how often the seeing is quite stable and how often it isn't.

Regardless though if its seeing, PEC, a wind gust, balance of the mount, flex, cable drag, boundary layer on the mirror, mirror cooldown times, ground effects, polar alignment I suppose the idea of shorter subs giving sharper images is still true. But I don't think its scientific to say its 100% seeing related.

I wonder even if any mount works the same all night given there could be slight shifts in voltage from the power supply, efficiencies at one angle are not as efficient at other angles, etc etc. You could go on and find other minor factors that influence the result. Hence TPoint super models.

You can't be sure of that with your data input. What percentage the data is affected by seeing would be an educated guess and probably varies by location a lot.

At my dark site I don't think seeing varies much at all. Especially 3 hours after dark onwards.

Balance would be a big one. Scopes are often top heavy and what is in balance horizontally can be badly out of balance at 75 degrees - the common imaging angle.

It would be interesting to see FWHM numbers from the really long subexposure strategy versus SNR versus stacked short subs for the same duration.

I predict it would come up with the same result as you have shown and it boils down to the read noise of the camera as the major determining factor at a dark sky site exposure length (the usual advice is to go longer on subs at a dark sky site using Kodak chips).

Greg.
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  #28  
Old 26-04-2015, 06:35 PM
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Originally Posted by gregbradley View Post
Regardless though if its seeing, PEC, a wind gust, balance of the mount, flex, cable drag, boundary layer on the mirror, mirror cooldown times, ground effects, polar alignment...I wonder even if any mount works the same all night given there could be slight shifts in voltage from the power supply, efficiencies at one angle are not as efficient at other angles, etc etc.
LOL

You have composed a rather long list of our adversaries Greg...

That's why, in an attempt to not go entirely mad while trying to solve and understand all issues, I expose for as long as stars in my subs are nice and round(ish)...
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  #29  
Old 26-04-2015, 08:25 PM
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Hi Greg. No, it's nothing at all to do with the autoguiding. These are plots of the how the HFR star shape parameter varies from sub to sub. I used "FITs image grader" to analyse the final subs - for example, each point in the lower graph on the second image shows the HFR value for one of the 183 individual subs in that multi-night sequence (ie these 183 points represent measurements from about 10 hours of imaging). I think that FITS image grader works somewhat similarly to CCD inspector, analysing the shapes of multiple stars across a sub - however, it reports a single average star shape parameter (HFR) for the sub. HFR is similar to FWHM/2, so each data point represents what the seeing was like over the 3-4 minutes when the sub was taken (at my image scale, an HFR of 1.2 pixels would be pretty close to a FWHM of about 2.2 arc seconds).

Regards Ray
I see. But still how possible is it to remove the effects of autoguiding from an image so the balance is wind, seeing.

Add to that the fact you may get additional flexes at different angles as the night progresses to further complicate the data.

So even though an algorithim has been used to try to say this is the seeing part I doubt it can be done very accurately. Further evidence that this is the case is the need to do a huge TPoint model then a huge CPU intensive calculation to work out a supermodel, get rid of the outliers, try different possible flexes in the system.

Even the tracking rate of the mount would not be correct at lower alititudes than near the zenith due to atmospheric effects (AP mounts enable different rates of tracking at different altitudes to compensate for this).

After having spent endless hours watching the guide errors on many many nights of different conditions I think it would be very hard to differentiate between simple PEC and seeing variability.

Planetary imagers probably know what the seeing is like the best as they fight it all the time. I wonder how often the seeing is quite stable and how often it isn't.

Regardless though if its seeing, PEC, a wind gust, balance of the mount, flex, cable drag, boundary layer on the mirror, mirror cooldown times, ground effects, polar alignment I suppose the idea of shorter subs giving sharper images is still true. But I don't think its scientific to say its 100% seeing related.

I wonder even if any mount works the same all night given there could be slight shifts in voltage from the power supply, efficiencies at one angle are not as efficient at other angles, etc etc. You could go on and find other minor factors that influence the result. Hence TPoint super models.

You can't be sure of that with your data input. What percentage the data is affected by seeing would be an educated guess and probably varies by location a lot.

At my dark site I don't think seeing varies much at all. Especially 3 hours after dark onwards.

Balance would be a big one. Scopes are often top heavy and what is in balance horizontally can be badly out of balance at 75 degrees - the common imaging angle.

It would be interesting to see FWHM numbers from the really long subexposure strategy versus SNR versus stacked short subs for the same duration.

I predict it would come up with the same result as you have shown and it boils down to the read noise of the camera as the major determining factor at a dark sky site exposure length (the usual advice is to go longer on subs at a dark sky site using Kodak chips).

Greg.[/QUOTE]

Hi Greg. What I did was use real measured data to show the consequences of throwing out subs in which the stars were fatter in shape than some chosen threshold. It doesn't matter at all how they got to be fat - seeing, guiding, flexure etc. all have basically the same effect, as you point out - and I have not tried to separate them out. I have used the term "seeing" to describe the combined result, but it was noted in an earlier post that the measured HFR includes more than just the atmospheric effects. What I found was that, if you decide to try for better resolution by throwing out bad subs, you can do so more efficiently using short subs rather than long ones. I was particularly surprised to find that the SNR can be better if short subs are used cf long ones - didn't anticipate that.

Re seeing stability in planetary imaging, I find that it varies over quite short timescales - it is rare to have seeing that stays stable over more than 10 seconds - it occasionally does, but not often. At my site, it is far more common to face waves of ordinary to bad seeing flowing through, with occasional bursts of a few seconds of better seeing every now and again. And of course at 50Hz framerate and 0.2 arcsec sampling, the primary things affecting the imaging are atmospheric seeing and scope PSF - guiding, flex etc are unimportant.

The measurements have nothing to do with read noise. The RN component of this data is well below 10% of the total noise - ie it is not significant.


Quote:
Originally Posted by Slawomir View Post
LOL

You have composed a rather long list of our adversaries Greg...

That's why, in an attempt to not go entirely mad while trying to solve and understand all issues, I expose for as long as stars in my subs are nice and round(ish)...
Do you also take star size into account Slawomir, or just shape?

Regards Ray

Last edited by Shiraz; 27-04-2015 at 12:02 AM.
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  #30  
Old 27-04-2015, 05:24 AM
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Do you also take star size into account Slawomir, or just shape?
3nm filters tend to help big time in keeping star sizes in check, in fact, I feel that quality of filters used is often heavily underestimated in their impact on the overall sharpness and quality of astro images.

An example of an image resulting from collecting subs with a 3nm filter, low above the horizon (30-50 degrees) and humidity was relatively high (January in Brisbane), no subs were rejected: http://www.astrobin.com/full/148715/D/
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Old 27-04-2015, 12:00 PM
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Thanks for posting Ray. It does shed some light on what may be the best imaging strategy on the night with a particular setup.

Greg.
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