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Old 18-05-2016, 01:27 PM
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Originally Posted by Shiraz View Post
I tried to set it up so that the subs of various lengths would be fairly well mixed up - eg, a the 5 minute subs were not all taken together, but were spaced out with other length subs taken in between. The intention was to minimise the effect of slow changes in seeing - if the seeing changed, it would affect the averages for all sub lengths by roughly the same amount.

sort of like shuffling a deck of cards so that all the kings are not together.

Oh I see, that's the engineer in you. Perhaps you can get a job with the PixInsight team! You interleaved different lengths of subs in the run. Got it.

Greg.
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  #22  
Old 18-05-2016, 01:43 PM
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Shiraz (Ray)
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Perhaps you can get a job with the PixInsight team!

Greg.
That's way too hard - I'm a physicist, not a masochist!

edit: although come to think of it, this hobby can generate a lot of self inflicted pain.
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  #23  
Old 18-05-2016, 01:49 PM
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That's way too hard - I'm a physicist, not a masochist!

edit: although come to think of it, this hobby can generate a lot of self inflicted pain.
Good one Ray.

Greg.
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Old 18-05-2016, 02:37 PM
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This is an interesting discussion Ray. I think you might find that longer subs still (ie 10-15-20 minutes) will remain fairly static with regard to FWHM in similar seeing. The nuances of seeing and guiding will be more obvious from 1 second to 2 minutes in my opinion and then will slowly reduce over increasing sub lengths to where there is little difference. I know from experience very short subs always have tighter star profiles than ones from even 2 minutes. I am sure this boils down to the star moving just minor amounts around the centroid. Even tight guiding in good seeing still sees this happen.

I can see uses for both short and long subs. Bright objects like the homunculus can be imaged with quite short subs. I think I used 1 second subs when I did Eta Carina last year. Other similar objects such as the core of M42 would also similarly respond well to short subs. Both from a saturation level and from a lucky seeing point of view. As you know planetary imaging has been doing this for years. Objects with high brightness levels can be imaged at very fast speeds.

However I can also see the use of longer subs just to pick up signal. I routinely use long subs for detail imaging on both systems. I think loosing a tiny bit of detail sharpness is more than compensated by increase in signal. If imaging from stable seeing locations (I know not everyone has this luxury) some slight blurring can be made up with degrees of sharpening.

Perhaps if sensors can be developed with more sensitivity (though physics will limit how much) than at present, then short subs could become the norm.
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Old 18-05-2016, 04:14 PM
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Hi Lee. No I wanted to get around all of the complications of stacking by taking just the raw subs and averaging the FWHM for the handful of interleaved subs used at each sub length (from 4 subs total at 300sec up to 200 subs at 1 sec (from memory - don't have the data on this PC) . Nothing has been stacked.
Ah, ok, thanks for the clarification, Ray. You mentioned "final stack" so I assumed they had been integrated.

There's a very clear trend in the charts that you provided, and the results are intuitive. That's what I'd hoped I was seeing in the initial charts.

From here, I think the next step is to evaluate the net gains of having more, sharper subs vs fewer less sharp subs.

Are the rejection algorithms good enough to provide a tangible benefit in the integrations? If you integrate 2x5s less sharp subs with 10x1s sharper subs, is the latter integration still sharper?

It might be interesting to do a drizzle integration as well, with a scale of 1, as this would avoid the sharpening effects of (some of?) the interpolation algorithms used in registration.
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Old 18-05-2016, 08:14 PM
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What was the frequency and range of the seeing changes that night ?

Next time try this with a DIMM (or a camera with dual apertures and appropriate software - a link exists somewhere on the net !) to measure the instantaneous FWHM and plot that.

I think you'll find there will be a correlation between the length of sub, period and range of seeing changes and the FWHM captured up to a point where Id expect it would plateau due to averaging.
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Old 18-05-2016, 09:05 PM
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Ah, ok, thanks for the clarification, Ray. You mentioned "final stack" so I assumed they had been integrated.

There's a very clear trend in the charts that you provided, and the results are intuitive. That's what I'd hoped I was seeing in the initial charts.

From here, I think the next step is to evaluate the net gains of having more, sharper subs vs fewer less sharp subs.

Are the rejection algorithms good enough to provide a tangible benefit in the integrations? If you integrate 2x5s less sharp subs with 10x1s sharper subs, is the latter integration still sharper?

It might be interesting to do a drizzle integration as well, with a scale of 1, as this would avoid the sharpening effects of (some of?) the interpolation algorithms used in registration.
Will look at stacking over the next couple of days. - should be interesting.

Quote:
Originally Posted by rally View Post
What was the frequency and range of the seeing changes that night ?

Next time try this with a DIMM (or a camera with dual apertures and appropriate software - a link exists somewhere on the net !) to measure the instantaneous FWHM and plot that.

I think you'll find there will be a correlation between the length of sub, period and range of seeing changes and the FWHM captured up to a point where Id expect it would plateau due to averaging.
Don't know the seeing statistics Rally, but I would not expect seeing to have any effect at all with periods of a minute or more (0.5 second guiding should clean up anything with a period longer than a second or so) - and yet there was no plateau up to five minute subs on both nights.

Have been toying with the idea of making a DIMM, but am not sure I want to put in that much effort and expense for now (a 10 inch+ SCT, a non-standard prism and half a grand's worth of software). Maybe when curiosity gets overwhelming).

Regards ray
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