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Shorten total integration time with new equipment

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#126 sn2006gy

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Posted 19 August 2020 - 05:25 PM

why does a globular cluster typically require much less total integration time than a faint galaxy?  two things I wanted to learn about:

 

1)  how long should total integration time be for a particular target?

 

2)  how long should each sub exposure be?

 

(1) and (2) are independent as far as I am concerned, but your model links them together.

 

We are just using different approaches, no worries.

 

and....

 

I am sure Frank is going to log in and show us how it is done!!

 

Experience?

 

I can see a globular with my 50.00 binoculars.  I took a photo of one with my 15 year old DSLR when i first got started and it came out really good.

 

I didn't do any of this math until i started doing more difficult targets.

 

Not sure why this question is posed as a challenge.

 

I started this entire post saying I think in integration time.  10 hours is more than i'd ever do on a single object from my home. SO i chose 10 hours.

 

You can change that number and everything should update.

 

With experience i know 5-8 is probably my number... some people say they don't do anything less than 15-20. Again, not sure why this question poses any change to anything?

 

edit, i dunno what i did with my edits that show the graphs over time... i'll update this again.

 

Basically, with 10 hours being my ceiling it would show the SNR diminishing returns over time based on the conditions and i could see at which point its not worth moving forward.


Edited by sn2006gy, 19 August 2020 - 05:29 PM.


#127 Peregrinatum

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Posted 19 August 2020 - 05:29 PM

Experience?

 

I can see a globular with my 50.00 binoculars.  I took a photo of one with my 15 year old DSLR when i first got started and it came out really good.

 

I didn't do any of this math until i started doing more difficult targets.

 

Not sure why this question is posed as a challenge.

no challenge, I was just sharing my thought process on the whole topic... the math helps me understand how the variables interact but experience is the best guide



#128 sn2006gy

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Posted 19 August 2020 - 05:30 PM

no challenge, I was just sharing my thought process on the whole topic... the math helps me understand how the variables interact but experience is the best guide

I'll say it again..

 

It's just helpful to me. It could be wrong, terrible, bad advice... but it makes sense and it works.

 

I'm sorry i actually brought this up.

 

I really have no interest in fighting, bickering and wars on forums people. None at all.


Edited by sn2006gy, 19 August 2020 - 05:31 PM.


#129 sn2006gy

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Posted 19 August 2020 - 05:34 PM

I'll capitulate.. y'all win. My 2600mc sucks, OSC is a joke, i don't know what i'm talking about and its all lies, SNR is bs, i shoulda bought a KAF

 

I need to move on to other more enjoyable things people.



#130 freestar8n

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Posted 19 August 2020 - 06:37 PM

Frank,

 

I believe that most of the time when people are talking about Entendue on here they are talking about pixel entendue not sensor entendue.  Most of the basic calculations that I have used dont have any input for sensor size, just pixel size.  In the contest of discussing overall "speed" (or whatever we want to label it) of a system with respect to integration time pixel entendue is really the only relevant part of the discussion... is this accurate?

 

Regarding QE, that has a huge impact on Entendue calculation.   If a theorized 2600mono sensor has the same QE as the 6200 (being the same sensor tech) it might have around 90% QE.  Compare that to the ASI 1600 which has a QE of 60%, that alone would make the 2600 sensor 1.5x "faster" would it not?  That seems very significant to me.  (Pixel size is almost identical between the two cameras).

 

Thoughts?

I think the use of etendue to refer to pixels is a fairly recent thing and it is unique to CN.  You can indeed refer to pixel etendue - but it is easy to have a large pixel etendue simply by binning to large effective pixels.  That places no special demand on the optical system since the pixel can still be small compared to the fully corrected field of a good instrument.  That means if you say your system has a high pixel etendue it isn't impressive by itself until you know how big the pixels are in arc-seconds.

 

In contrast, the etendue of the LSST is 319m^2degrees^2 - and that is impressive by itself.  It means two equivalent things:  You have a very fast system and a large, well corrected imaging area, or you have a very wide field system that is large aperture.  There is no way an amateur system can touch this.  But the system itself is f/1.2 and its pixels are 10um - so I know I can take my 11" f/10 with 3.75um pixels and bin them until they are about 80um wide - and they will receive just as much signal as the pixels in the lsst.  In one usage the number is immediately impressive and in the other it is lacking details to know how to interpret it.  The etendue of the system tells you how good its optics are and how well its field is corrected without even referring to the sensor you intend to use.

 

In this context the pixel etendue will tell you how quickly the pixels fill when aimed at nebulosity - but it won't tell you how much detail you will capture.  So people need to be clear on what exactly they are trying to optimize or hold fixed if they want to expose less and still get a good result.

 

With regard to QE - sure it is important.  But if you double the aperture and keep everything else fixed you will have x4 light into each pixel.  The f/ratio and binning can have huge impact on signal received, whereas differences in QE will only amount to a fractional change.

 

Frank


Edited by freestar8n, 19 August 2020 - 06:39 PM.

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#131 freestar8n

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Posted 19 August 2020 - 06:52 PM

I was not convinced by Bill's example because of several facts. First of all I don't know the optical system being used. Second of all, that's an old noisy CCD camera. Third, it appeared to me that he was shooting narrow band images. Fourth, he was apparently shooting in a location with good seeing but lots of skynoise. When I put them all together, I wonder if the example actually would come out the same if I used a modern CMOS low noise camera with the ability to adjust gain (and offset).

 

I guess I'll just have to try it! All of those differences made me wonder if the hour long exposure was actually long enough. And,  had it gone on longer would the skynoise etc made the situation worse.  Now if the skies will just clear!

 

Rgrds-Ross

One side thing I like about the example is that if this was taken near the Astro-Physics site in northern Illinois, then it is an example where you can get 1.5" results without anything special about your location.  He describes it as a brightly lit industrial park - and I am assuming it is northern Illinois but I could be wrong.  But I have imaged in many places around the world and find good seeing everywhere for deep sky imaging when the jet stream isn't around.  So I think people tend to underestimate how good the seeing can be at given locations - and if they focus on "optimal" pixel sizes of 1" or so - they will never know what they are missing.  So be careful focusing on pixel etendue without losing sight of potential detail lost - if you care about detail.

 

He is also indirectly touting the quality of his optics, guiding, and mount - but fwhm's in that regime don't require high end equipment.

 

It's unfortunate when people do take the time for a comparison such as this - since to me any examples are good to have.  But at the least there needs to be more details on how it was all done.  The stacked image has a mottled appearance consistent with a well guided stack that has pattern noise.  If they had drifted at all they would show ugly streaks - but if they had been dithered they would be very clean.  And unless there is additional info to convey there was no change in sky transparency - that is another unknown in the comparison.  They were presumably calibrated with different masters - and how good are they - etc.

 

Frank


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#132 555aaa

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Posted 19 August 2020 - 07:17 PM

Frank, I think Roland has a place in Hawaii now. Did you notice that the FWHM of the star in the stacked image is worse than the single long exposure? That's why the snr measured on the star is better in the long exposure. I do think the method he is using does produce a meaningful SNR number, because the object in the image is known a priori. It's a point. Which is why I keep harping on having the best guiding and collimation you can.
- Bruce
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#133 freestar8n

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Posted 19 August 2020 - 09:36 PM

Frank, I think Roland has a place in Hawaii now. Did you notice that the FWHM of the star in the stacked image is worse than the single long exposure? That's why the snr measured on the star is better in the long exposure. I do think the method he is using does produce a meaningful SNR number, because the object in the image is known a priori. It's a point. Which is why I keep harping on having the best guiding and collimation you can.
- Bruce

Ah - Hawaii would make more sense for small fwhm - but then it is odd he chose to image from an industrial park.

 

I looked in more detail at the numbers and now I don't think the frames were calibrated and they may just be a raw, average stack.

 

If it were an average stack of calibrated frames compared to a single calibrated frame then the background should be 1/6 what it is in the 1 hour exposure - since it would represent a calibrated 10m frame.  The stellar flux ("intensity" in maxim) matches that pretty well since it is about 6x bigger in the 1hr frame - but the background avg is less than 1/3 in the stack.  That suggests the stack has the full pedestal plus 1/6 the background in it (after averaging the sum) - compared to the full pedestal plus the full background in the single exposure that was not averaged.  As a result the background isn't 6x bigger, as it should be if calibrated frames were used.

 

This doesn't affect the fwhm or the star snr directly - but it suggests these are all raw and we are looking at a stack of raw, non-dithered frames and seeing noise in it.  Yes - that's why we calibrate with good masters, dither, and align/stack well with rejection.

 

I'm happy to be corrected but I shouldn't have to deduce this stuff myself if the intent is for people to draw a direct conclusion from the comparison.

 

There is no reason at all that the fwhm in the stack should be larger - so that is strange right there.  It may depend on exactly which stars were measured.  And the aperture he chose is pretty large for measuring those stars.

 

Frank

 

addendum:  I get the pedestal as 89 adu and the background flux (sky plus dark current) in 10m as 41 adu.  So the bkgnd in the stack is 130 and in the single exposure it is 336.  Assuming all raw frames and the stack is summed and divided by 6.


Edited by freestar8n, 19 August 2020 - 09:56 PM.


#134 rockstarbill

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Posted 19 August 2020 - 11:04 PM

It was him imaging by the A-P facility, not in Hawaii. :) Strip malls, mini marts, etc nearby. 




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