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vpcirc
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670485 - 02/09/13 10:45 AM

Frank could you provide us a link to your images. I bet you do fantastic work. I'd really like to see how you apply your knowledge.

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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neptun2]
      #5670496 - 02/09/13 10:51 AM

Yes - I think when people talk about hitting a brick wall - it's simply describing the slowness of sqrt(N) as N gets big.

With one sub, you add another and get a big gain. To get the same gain again, you add 2. To get the same again, you add 4. Once you reach 16, you add 4 and - not much change. So you add 4 again and - not much change. But if you keep going from 16 to 32, you will get a net big change. Same with 64 to 128.

But as the image improves, something else happens in that you reach a limit of SNR where the image is very clean and you just don't get any improvement in appearance. But that should be when the image shows no obvious speckly pattern anymore.

But if you really do hit a brick wall, and the image is just as noisy at 16 as it is at 128, then I would look into a problem in the calibration and combining. If you see noise in the bright areas - that points to a problem with the flats. If in the dark areas, a problem with darks or bias. Anyway - it points to a problem.

But I think for most people - it is just less rewarding to have to add a bunch of exposures to get an extra bump in quality. So many people just switch to a different target and move on. But it doesn't mean the SNR isn't still improving according to the model. The model says the relative change with each added frame should decrease - and so it does.

Frank


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korborh
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Re: A Comparison of Exposure Lengths... XMas Tree [Re: vpcirc]
      #5670637 - 02/09/13 12:23 PM

Quote:

Your shorter subs will show far less detail resolution and contrast no matter how many you take. That is the point of Jimmy's experiment. Even with far less total imaging time the amount of faint data he was able to obtain is clearly visible. Secondly, with narrowband, you are never going to reach the sky limit from what Don Goldman has told me. There are bright objects that will get blown out by longer subs such as M 42, but if you stick to short subs, you'll miss this wonderful detail that exists elsewhere. Adam Block has a great tutorial on how to combine short and long subs in CCDStack using missing value to combine. Tony Hallas demonstrates another great procedure in his newest addition to Photoshop processing in Vol 5 using the reveal all layer mask in CS5 and CS6. Jimmy used the Hallas method in his M42.




Unfortunately you are not able to benefit from the discussion as you do not seem to understand the simple concept of statistical sum. If you did appreciate this, you would know why there can be really no sharp cutoffs on number of frames and SNR. I very much doubt the experts you refer to say this, and if they do they are wrong.
But one is free to shut themselves off from basic math and science, and instead base their understanding on discrete bits of information from authority they worship.


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vpcirc
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Re: A Comparison of Exposure Lengths... XMas Tree *DELETED* new [Re: korborh]
      #5670668 - 02/09/13 12:40 PM

Post deleted by vpcirc

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dawziecat
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670710 - 02/09/13 01:06 PM

Without falling into the unfortunate polarization that has fractured this and the original thread, I am curious about the choice made by the STSCI imaging team for the ultra deep field Hubble images.
The Hubble is a native f/24 system . . . VERY slow! Who of us here would even consider an f/24 OTA for doing AP?
The individual subs comprising the UDF used a typical exposure time of 1200 seconds according to this Wikipedia article.

Now, I am totally unsure if the Hubble UDF images were actually taken at f/24. If not, just what was the EFR for the exposures?

1200 seconds at f/24 is mighty short! f/24 is about 4.5 f stops slower than a more typical imaging OTA used by folks here . . . say f/5.6 just for discussion.

A 1200 sec exposure at f/24 is approximately equal to a mere 120 seconds at f5.6. Or a 1 minute exposure at f/4.0

Seems to me the STSCI folks are using mighty short subs considering the slow FR of the instrument. Of course they are using total integration times approaching two weeks!!!
More than 800 20-minute subs! No "16 sub rule" here!
What wuzzes we are in comparison!!

Why? Perhaps . . .
1/ They are limited by the observing window of opportunity, i.e., the target is not available long enough per orbit to allow longer subs. Or . . .

2/ I am completely wrong and they are NOT imaging at f/24 at all, but at a much faster FR.

I am unsure why this thread has become so contentious. The consensus seems correct to me, i.e., go as long as your gear and sky allow but, do not deny the obvious either. At some point, short and long sub results will converge so as to make the the difference in the final result imperceptible. The point at which this happens will be a factor of equipment, sky condition and the image scale at which the final image is viewed. Also exposures must not be so long that saturation and/or blooming become overly obtrusive.

The exposure limit determined by "practical" matters will, for many of us, be less than the ideal as defined by the mathematics. For some of us, it will be WAY less!

Insisting on going longer than your mount can track/guide well is a sure-fire way to ruin an exposure totally. Much more so than using an equivalent total exposure duration using shorter, even MUCH shorter, subs.

Many here are pretty expert imagers but I know from my own personal experience that there are people perusing this forum that are just considering CCD imaging. They may presently be using a DLSR, and contemplating making the leap.
They could get some pretty discouraging ideas here, that you just must be able to do 20 minute or longer subs to get nice narrow band images. I once thought that. If you want "world-class, APOD contenders" that may well be so. Especially with slower OTAs.

However, for "pretty pictures" that a hobbyist can take great satisfaction in and amaze his friends with, it isn't so.


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dawziecat
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: vpcirc]
      #5670724 - 02/09/13 01:11 PM

Quote:

From John Smith

I have found that prima-donnas, attention-seekers, know-it-alls and other personality types that vex the spirit are to be avoided. Rather than try to change or convert such individuals, an impossible mission in most cases, I will choose to disengage from these vexatious types as much as possible, whether individually or the venue in which they operate. They just aren't worth any of my remaining time.




OK, I'm outt'a here. This is just name-calling and hostility.


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psandelle
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Reged: 06/18/08

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670726 - 02/09/13 01:11 PM

Nicely said Terry - that's kind of the conclusion I've come to on all this stuff (both threads).

As for the Hubble...I haven't a clue, but MY SPACE TELESCOPE will definitely have a faster focal ratio.


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psandelle
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Reged: 06/18/08

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670731 - 02/09/13 01:14 PM

Oh, and anyone who wants to use my space telescope (once it's up) is free to.

Paul


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vpcirc
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Reged: 12/09/09

Loc: Merced CA
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: psandelle]
      #5670768 - 02/09/13 01:33 PM

I don't think Hubble has an atmosphere detracting light or light pollution to deal with!

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orion69
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Reged: 05/09/10

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670824 - 02/09/13 02:06 PM

Frank, thanks for explanation, I'll try your advices in practice when my new astrograph (F/4.5) arrives. It's very possible that I would need to shorten my subs, at least for LRGB filters. I intent to replace my Baader NB filters with 3nm Astrodon's so hopefully I wouldn't need to shorten subs for NB.

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Alph
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670850 - 02/09/13 02:28 PM

Quote:

I have question that has been already addressed in this and other posts but to make things more clear (at least for me): suppose I shoot somewhat dim object and I'm using NB filters. After 10x30 min subs I've got my new stacked picture. Now I'm shooting same object, with same gear, same sky quality (completely same conditions as before) only now I'm shooting 5 min subs x times.

Question: is there some detail on first image that would not be visible on second image no matter how many 5 min subs I take?





That’s a good question although it has already been answered in this thread and many times in the other thread. If the photon count from dim parts of nebulosity does not exceed significantly the readout noise then you will need to stack hundreds (maybe even thousands) of images to expose/show faint detail in nebulosity. This might be quite impractical and in practice you will need longer exposure times to increase the photon count from nebulosity well above the readout value.

Once again, if the Poisson noise of your object does not exceed sufficiently the readout noise, then stacking will not be as effective as taking longer exposures. In other words a stack of two images will not be equivalent to a single image with exposure time twice as long. This is the key thing to understand and this is why we have minimum exposure time calculators.


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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670885 - 02/09/13 02:57 PM

Hi-

The details of the SNR calculation for the UDF are described here. The noise model is the same, but the question they are asking is - if I want to see a certain nebula with SNR 10 - how many exposures will I need? So the underlying model is the same, but they are asking it in terms of SNR. Note that they calculate the measured result in comparison with the model, and they come up with a freakin' error of 0.01 mag in table 5. They deal with the same issues as suburban imagers - read noise and sky glow. Obviously - the model works.

Whether imaging with the HST or with a backyard setup - there are concerns with SNR and noise rejection. The same applies on the internet - so I wouldn't let an occasional, or repeated, cosmic ray impact your contributions to this thread. It can be random noise or pattern noise - but either way it is something that obfuscates the signal - and the increase in informative contributions from you and others will increase the SNR that draws people to CN as a source of good information.

Frank


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jaddbd
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670943 - 02/09/13 03:52 PM

Quote:

1200 seconds at f/24 is mighty short! f/24 is about 4.5 f stops slower than a more typical imaging OTA used by folks here . . . say f/5.6 just for discussion.
--edit--
Why? Perhaps . . .
1/ They are limited by the observing window of opportunity, i.e., the target is not available long enough per orbit to allow longer subs. Or . . .






In the paper Frank just referenced (see page 7), seems that they used 1200sec and sometimes 850sec subs in order to get in 2 exposures per orbit with a "micro-dither" in between...

John D


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Mike Wiles
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Reged: 02/04/09

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: jaddbd]
      #5671273 - 02/09/13 07:27 PM

Just sayin....



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jaddbd
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Mike Wiles]
      #5671394 - 02/09/13 08:39 PM

I Agree... "paralysis by analysis" already for me . I found the Hubble paper Frank referenced pretty neat - it offers some cool insights into what they do and it is not completely unreadable.

JD


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Alph
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5671579 - 02/09/13 11:27 PM

Quote:

The Hubble is a native f/24 system . . . VERY slow! Who of us here would even consider an f/24 OTA for doing AP?
The individual subs comprising the UDF used a typical exposure time of 1200 seconds according to this Wikipedia article.




Let me put this in the right perspective.
The WFC3 camera has 0.13 arcsec image scale and Hubble's primary mirror has a dimater of 94.5 inches (2.4m). This results in the speed factor of 12.3 (94.5*0.13)
The Celestron 14" EdgeHD has a focal length of 3905mm. A camera with 9 micron pixels has 0.48 arcsec image scale. This results in the speed factor of 6.7 (14*0.48)
So as you can see, the HST is twice as fast as the 14" EdgeHD if you are willing to spend $10,000 on a camera.
If you are not willing to spend that much money on a camera then consider the KAF-8300 which has 5.4 micron pixel. This results in the speed factor of 4 (14*0.29) and it makes the HST configuration 3 times as fast.


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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Alph]
      #5671675 - 02/10/13 12:34 AM

As described in the paper I referenced, the ACS pixels used for the HUDF are 0.05" and provide an fwhm of around 0.08". So it is indeed slower than a 14" f/11 with 9um pixels - which itself is well matched to an fwhm in the 1.5-2" range. In both cases you want to realize the resolution capabilities of the instrument - so you can capture detail in lots of tiny background galaxies.

Frank


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bilgebayModerator
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5672594 - 02/10/13 03:57 PM

I am following this thread with great interest and learning a lot.

By pure chance, I have discovered today that I had shot this same area 2 years ago with my C11 Edge + Hyperstar setup with a Canon T2i camera. At the time I shot this frame, I believed I had saturated all the stars. Today, I reprocessed the stacked file again and saw that the stars were not saturated at all. My post processing skills are still far from being on the expert level but I was glad to see color in the stars. I will share the stacked file with you guys just to add to Jimmy's comparison.



Hires image.

There is more data than I was able show here.

The stack is 9x 600 second lights, shot through a Lumicon DeepSky filter with Canon T2i at ISO400.

I guess I was experimenting with the camera and Hyperstar at that time as an explanation for using ISO 400.

Had I used ISO 800, 5 minutes would suffice I guess.

File Name CONE NEBULA_LIGHT_600s_400iso_+17c_LUMICON_DEEPSKYfilter_20110131-01h54m32s.CR2
Camera Model Canon EOS REBEL T2i
Firmware Firmware Version 1.0.8
Shooting Date/Time 31.01.2011 01:45:07
Author BILGEBAY
Owner's Name
Shooting Mode Manual Exposure
Tv( Shutter Speed ) 600
Av( Aperture Value ) 0.0
Metering Mode Evaluative Metering
ISO Speed 400
Auto ISO Speed OFF
Image Size 5184x3456
Image Quality RAW
Flash Off
FE lock OFF
White Balance Mode Daylight

And here is the FIT file for those who want to try their ...

Clear skies


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Peter in Reno
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: bilgebay]
      #5672606 - 02/10/13 04:05 PM

Hi Sedat,

Your link does not appear to work. Also, it's TIF, not FIT. It's funny that TIF is an anagram of FIT.

Peter


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bilgebayModerator
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Peter in Reno]
      #5672615 - 02/10/13 04:10 PM

Peter dropbox is still uploading. Should finish in an hour...net is slow today.

Oh, and yes...it's a tif sorry for the confusion.


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