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EAA Exposures: How short is short? How short is too short?

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#1 Cobalt5120

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Posted 22 May 2019 - 09:04 AM

So the big draw of EAA is short exposures allowing near realtime viewing of deepsky objects.  When reading, there is alot of talk about short exposures and the use of Sharpcap and watching the photo object build on the screen.

 

So, assuming an F4 to F6 scope, in moderately light polluted areas (not dark sky and not metro)

 

1) how short is short?

2how short is too short?

3) how many exposures are usually taken based on the above answers?

 

Of course there is no DEFINITIVE answer and many things need to be taken into consideration such as aperture, focal ratio, light pollution, etc.  However is there a general "ballpark" answer for EAA that would be the equivilent of shooting an 85 in golf?  Not a pro at par or less, but not a total duffer shooting 100, but just a fun day out on the course for the average joe, or in this case, a fun night at the scope for average joe looking at some neat stuff with no post processing but using live stacking.


Edited by Cobalt5120, 22 May 2019 - 09:04 AM.


#2 davidparks

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Posted 22 May 2019 - 09:50 AM

I'm tempted to answer 5 -  10 seconds...  and as you indicate, no definitive answer.  1 and 2 seconds exposures are commonly used as well... depending on target, light pollution and all the things you mentioned.  These are all generally at HIGH gain settings, assuming modern CMOS cameras with good SNR.  Some, like the 294 even have HCG (High Conversion Gain) which reduces noise at higher gains.  Stacking increases SNR, thus reducing noise in the image, again making high gain useful for short exposures.  So basically it's a dance between gain and exposure, with many folks landing in the 5 - 10 second area.  However, 15, 30, and even 60 second exposures can be used to take you deeper, there really isn't any rule to how you optimize your experience.  The only 'rule' here on Cloudy Nights I believe is a 5 minute total integration time if you want to post a picture.  EAA is considered a visual discipline, not an imaging one, so posting pictures are intended only to demonstrate results, capabilities, provide guidance, and otherwise help communicate visual expectations.

 

You can use Sharpcap's Smart Histogram Brain Window to automatically calculate an optimal exposure/gain for a chosen area of sky and integration time.


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#3 alphatripleplus

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Posted 22 May 2019 - 09:51 AM

If you are asking the question what is the optimal sub-exposure length you should use for a given camera and sky conditions, Robin Glover (the developer of Sharp Cap) has an excellent discussion of how to determine that here for Deep Sky targets. His idea is to choose a sub exposure long enough so that read noise is a very small component of the total noise  in a stack of sub exposures - making the sub exposure longer would have a minimal impact on the overall noise once sub exposures exceed this value. If you look at his calculations, this sub exposure length depends on several factors - camera read noise, Quantum efficiency (QE) of the camera and the sky background per pixel.

 

If you are not interested in the details, he gives some examples at the end of the last post. For example, with a 2e read noise camera, 50% QE, and moderate light pollution, he comes up with about 7s. The optimal sub-exposure length is proportional to the square of the read noise in his analysis, so if you have an even lower read noise camera, the optimal sub exposures will be shorter. 

 

Edit: As David noted, the Smart Histogram Brain in SharpCap has implemented the calculation described above.


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#4 Astrojedi

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Posted 22 May 2019 - 10:05 AM

Minimum workable sub exposure length depends on:

 

1. Brightness of the object

2. F ratio & aperture of the optical system

3. QE, read noise and gain of the image sensor

4. Sky conditions 

5. The observer’s criteria for an acceptable image

 

With my C8 and ASI290 mono from my heavy LP backyard I have observed faint galaxy clusters with 1-4s sub exposures. From a dark site usually 0.5s exposure is sufficient for similar objects.

 

With the C8 Hyperstar I have found I can just browse the brighter objects using 1-2s exposures with no need for stacking.

 

If you click the link to YouTube in my signature you can find examples of this.


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#5 Stargazer3236

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Posted 26 May 2019 - 02:25 PM

As pointed above, it all depends on these factors:

 

Light pollution (Bortle Sky number)

Sky conditions: Transparency and Seeing (dictated by the Jet Stream and any warm/cold fronts passing by)

Moon presence: Where it the Moon located? East and just rising, no problems. West and setting, much better.

Definitely Focal Ratio and aperture: F10...Good Luck! F/6.3 or F/5...much better.

Camera used: OSC (One Shot Color), a little more difficult than Mono. Mono, more sensitivity, but lacks color.

Imaging capture software: SharpCap highly recommended, for EAA.

 

All put together employs a made up formula to judge what is best. Also, magnitude of the object you are wanting to EAA capture, will also factor in to the equation. A dim object may require high gain, long exposure, high digital gain to get alignment stars. Brighter objects do not require long exposures, but still may require more gain. 300 Gain is almost certainly the best to start off with. Higher gain on dimmer objects may need to be used. Noise drops off with higher gain. Unity Gain at 120 with a 294MC will give more noise, so a boost to 300 is always welcomed.

 

M42 does not require a lot of gain nor exposures. Probably 200 gain or so and 2-4 seconds of exposure will give optimal results. However, if you want Trapezium detail, low exposures and low gain are required. If you want wispy detail in the wings, then higher gain and longer exposures may be required. It all depends on the surface brightness, hence magnitude of the the object in question. I typically do not EAA capture the same way every night. It all depends on the formula above. Nights of average transparency are so-so, nights of above average transparency are much better and excellent transparency are very rare and the best for EAA capturing! I mostly get average to above average transparency, however, if I am antsy about getting out to stargaze, I may go out on a below average night, just to soothe my aching mind.



#6 Cobalt5120

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Posted 26 May 2019 - 04:36 PM

So based on reading here and other threads, I could be anywhere from a few seconds to several dozen depending on the many dependent variables.  

 

Best thing for me to do is to start trying some of this for myself and see what happens.  Unfortunately, I only have a 450D DSLR right now and will probably get a CMOS camera.  224, 385, 294?????  My brain hurts!

 

I'll keep reading and figure it out.  smile.gif



#7 Astrojedi

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Posted 27 May 2019 - 11:51 AM

So based on reading here and other threads, I could be anywhere from a few seconds to several dozen depending on the many dependent variables.  

 

Best thing for me to do is to start trying some of this for myself and see what happens.  Unfortunately, I only have a 450D DSLR right now and will probably get a CMOS camera.  224, 385, 294?????  My brain hurts!

 

I'll keep reading and figure it out.  smile.gif

It is easy to do in this hobby but don’t overthink it : )

 

The range will be more like 1-15s. Since I switched to dedicated CMOS cameras I cannot remember many instances when I exceed 10-15s. You just stack more sub exposures. 


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