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SkyTools 3 Sub-Exposure Times

     

It's All About Better Images

Some images simply look better than others. "Better" also means being able to detect fainter objects. A better image starts with maximizing the signal from the astronomical object and at the same time minimizing the noise. We can assign a number to an image that is the ratio of the signal to the noise, or SNR. A high SNR means a better image. To maximize the signal from the object we simply expose longer. But longer exposures maximize the noise as well. So we use a trick: by stacking many "sub exposures" together using a median filter combine we can remove some of the noise, increasing the SNR to make a better final image. 

 

Why Not Just Use What The Masters Recommend?

Maximizing your SNR used to be much simpler. There was a time when nearly everyone used traditional CCD cameras attached to long focal length scopes at dark sites. Many books were written assuming these three truths and many rules of thumb resulted. But we have seen a revolution in astronomical imaging. Today wide fields of view and light polluted locations are common even with the more traditional CCD cameras.  The introduction of DSLRs has taken these things to an even greater extreme. The three most important factors that determine the noise reduction that can be achieved for a given sub-exposure time are the readout noise, dark signal and sky background. Today readout noise and dark signal vary greatly from one camera to another. And the sky brightness is not only a function of light pollution but field of view as well; wide fields of view magnify the sky background. The astrophotographer has a lot to gain by tailoring the sub-exposure to the telescope, camera, and sky conditions. A rule of thumb that says to use, (as an example) 5 minute sub exposures, created for a traditional CCD camera at a dark site with a small field of view simply doesn't cut it. By taking the various factors into account better images can be made with less time at the scope.

 

SkyTools uses  a scientific model that includes all these factors in its recommended sub exposure. This gives the astrophotographer a big advantage. Once you have SkyTools dialed-in (see below) you can use it to help you make decisions under different conditions. What if you go on exposing after the moon rises? Do you need to go to shorter sub exposures? And if so, how short? What if you take your scope to a dark site? What if you use a certain filter? 

 

How It Works

The optimum sub-exposure time is computed by comparing the noise that comes about when reading out each sub exposure to the noise generated during the exposure. On the one side longer exposures are better and on the other shorter exposures rule. A sweet spot is found in between that minimizes the total noise.


For most astronomical CCD cameras the dominant noise during the exposure comes about from photon noise in the sky signal. The random component of this noise can be removed by stacking the images. For very dark sites the noise is low, for brighter skies the noise is high. Wide fields of view will also increase the sky signal. So if this were the only consideration short exposures would always be better than long ones in the presence of a significant sky background.

Some cameras (mostly DSLRs) are limited by their high dark signal rather than the sky brightness. This works the same way as the sky brightness--arguing for shorter exposures--except that it doesn't matter what the conditions are or what the field of view is.

The read noise associated with reading-out the exposure is contributed each time the chip is read out. This noise compounds with each sub exposure.  If this were the only source of noise, longer exposures would always be superior to shorter ones.

So the typical game is to estimate the noise generated by the sky + dark noise on the one hand and the read noise on the other, looking for the sweet spot in between that minimizes the total noise. The sub-exposure time that coincides with this sweet spot is what SkyTools reports.

 

Dialing-In Your SkyTools for Best Results
The age old adage of garbage-in = garbage-out applies here. It is important that your camera be set up appropriately. In particular the readout noise and dark signal are important to the result. It can be useful to measure your actual values from test images rather than use the manufacturers numbers. How this is done is outside of our scope here but there is software available and/or instructions that can be found on the web to do this.   

 

Often the most critical input is your sky brightness, particularly if you are combining a light polluted sky with a wide field of view. A brighter sky will tend to drive the algorithm to short sub-exposures. It is very important to make sure you have an accurate value entered for your light pollution at your location. If the sky brightness is overestimated very short sub exposure time recommendations often result. Short of a Sky Quality Meter the best way to measure the sky brightness is to find the faintest naked eye star you can see overhead on the darkest night. For best results you need to be as dark adapted as possible. It is very easy to overestimate your light pollution particularly when there are nearby lights; a casual glance at a constellation often won't suffice.

For the most accurate results measure the sky signal on one of your images and adjust the naked eye limit for your observing site such that SkyTools will compute the same sky background. To do this make sure to set up the SkyTools Exposure Calculator to exactly match the test exposure in every way (date/time, location, filter, seeing, temperature, humidity, etc). Set the sub-exposure manually to the length of the actual exposure. The predicted sky signal will be displayed. When you compare this to the actual image make sure the image is dark subtracted first, and beware that most software adds an arbitrary offset (usually 100 ADU) that you need to subtract when you measure the sky signal. If you measure a typical sky signal on your image of 220 ADU, subtract the offset (often 100) to get 120 ADU. Experiment with changing the sky brightness for your location until SkyTools reports the same sky brightness in ADU. One final thing to be aware of in this process is that the gain value for your camera is used to compute the ADU so be sure it is as accurate as possible.

 

Once SkyTools is dialed-in it will automatically account for changing conditions such as twilight and moonlight. For even more accurate results enter the current (or expected) values for temperature, seeing and humidity each time you observe.

 

Once again the critical inputs are:

  1. Sky Brightness (set for your observing site)

  2. Readout noise (set in your camera specs)

  3. Dark signal (set in your camera specs)

How Critical Is the Sub Exposure Time for Me?

SkyTools will always compute an optimum sub exposure time but it doesn't tell you how much difference the sub exposure time actually makes.  For some the improvement in following the SkyTools recommendation can be significant. For others the sub-exposure time makes little difference in practice. Use the Exposure Calculator to answer this question. Set up a test exposure on a typical target for a typical night. Set the sub exposure time to manual. Enter various values for the sub exposure and look at how this affects the final SNR. If the final SNR varies very little then you can go ahead and use any sub exposure time that is convenient--at least for these observing conditions. If it does vary then it will be beneficial to use the SkyTools recommendation.

 

There have been a few users for whom SkyTools always recommended very short sub exposure times. In these cases there was typically light pollution combined with a wide field of view. When we tried the test above we found that although SkyTools technically claimed very short sub exposures were always better, in fact they made little or no difference. So they were free to use any sub exposure time that was practical for them regardless of what SkyTools displays. But it is important to make sure that SkyTools is dialed-in--that all the inputs are accurate--before coming to this conclusion!
  

Related Topics:

Using the SkyTools Exposure Calculator

Planning Imaging Sessions

Adding Cameras Manually to SkyTools