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Using the SkyTools 3 Exposure Calculator

The purpose of the exposure calculator is to plan your imaging session interactively, one target at a time.  Don't think its simply about computing exposure times; this tool is about much more than that.  The Exposure Calculator is a sort of experimental laboratory that you can use to get the most out of your images without having to use your telescope time to do the testing.

When planning an imaging session many questions crop up:

• How far into twilight should I expose?  Which filter is least affected by twilight?

• What will be the difference in my final image if I stack 5 minute sub-exposures rather than 30 minute subs?

• Should I continue to observe after the moon rises?  If so, which filter is least affected by moonlight?

• What order should I use for my filters?

• How much would I gain by moving to a dark site?

• What effect will poor seeing have on my final image?

• How long can I image Jupiter tonight before the final stacked image will appear smeared from rotation?

• What is the penalty, if any, for imaging Jupiter in daylight?

These questions are often answered approximately using "rules of thumb" handed down by experienced observers, either in person or via guide books.  But there are many variables; what works best for one imaging setup under one set of observing conditions is often not best for another.  The Exposure Calculator allows you to explore these "what if..." type questions for your setup and your conditions.  By giving you answers directly it takes the guess work out of imaging.

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Deep Sky and Stellar Imaging

Planetary and Lunar Imaging

Double Star Imaging

Deep Sky and Stellar Imaging

The Signal-To-Noise Ratio (SNR)

In your final image the signal comes from the stars and deep sky objects.  The longer you expose the greater the signal. But imaging also involves noise and the noise generally gets worse with longer exposures.  The ratio between the signal and the noise, or the SNR, is a sort of contrast.  An image with a higher SNR will look cleaner and sharper--less "grainy."  Higher SNR images contain more detail and are more pleasing to the eye.  But how much SNR is necessary?

Here is a table of SNR values and their corresponding general result:

Use the Calculate Required Exposure Time function to estimate the exposure time required to obtain a target SNR.

Tip: use your favorite astronomical image processing program to determine the SNR for some of your best images; this will give you a good idea of the SNR you want to obtain in the future.

Stacking Images and Sub-Exposure Times

One way to beat the noise down and increase SNR is to stack multiple images together.  Rather than one long exposure the final image consists of multiple exposures stacked together in a way that will reduce the noise, increasing the SNR.  

The sub-exposure time is the exposure used for each individual image in the stack.  The optimum sub exposure time will depend on many variables, including your telescope/camera system, filters, and your local observing conditions.  Some cameras are limited by "dark" noise in the detector itself, but for most the primary factor is the sky brightness at the target location, including your light pollution, moonlight, twilight, etc.  

SkyTools will recommend an optimum sub-exposure time.  But computers can sometimes be rather dumb.  SkyTools will tell you the sub-exposure time that optimizes the final SNR (after stacking), but it doesn't tell you how sensitive the final SNR is to the sub-exposure time.  Depending on your equipment and conditions, SkyTools might suggest a 1 minute sub-exposure.  But in practice there may be very little difference in the final result if you use 10 or even 30-minute sub-exposures.  So how can you be sure?  Use the calculator!  Under the Calculate SNR for Exposure property click the Auto button to disable the automatic sub-exposure time.  Enter any sub-exposure time you want to the right and wait for the program to update the final SNR with the new sub-exposure taken into account.  Does the SNR change quite a bit with sub-exposure time?  If not, then you are free to use any sub-exposure time is convenient.  

Also note that you can set limits on the sub-exposure time recommended by the program.  See Allowable Sub-exposure Times on the left side of the dialog.  This can be useful, if for instance SkyTools keeps suggesting very short sub-exposures that are impractical.  In this case set the minimum sub-exposure to something you can live with.

For more about sub-exposure times see the Sub-Exposure Times guide.

How to Use the Calculator to Best Advantage

When you open the Exposure Calculator it will have automatically selected the best exposure opportunity for the night.  The exposure opportunity will appear on the NightBar as two vertical yellow lines.

The Exposure Opportunities table breaks the night into blocks of time, one per line.  Each block represents a period of the night with a similar value of Q.  When you open the calculator one or more blocks will automatically be selected as the best exposure opportunity for the night.  This is the time period indicated on the NightBar with the vertical yellow lines.

You can also experiment with the exposure opportunity time "blocks" themselves.   Select another block in the table and watch what happens on the NightBar and in the Calculate SNR for Exposure area.  To select more than one block at a time hold the Ctrl key down as you select them. To select a group click on the top line of the group, depress and hold the shift key and select the bottom line.  All the lines in between will be selected.

You can skip the pre-computed blocks entirely by dragging the vertical yellow lines on the NightBar or by entering specific Exposure Start and Total Exposure times in the Calculate SNR for Exposure area.

Sometimes you can group several grade "B" blocks together to give a larger SNR than a single grade "A" block.  Other times exposing during the grade B period is not worth the time.  As you experiment keep an eye on the Total SNR in the Calculate SNR for Exposure area.  If want an SNR of 100, it doesn't really matter how you reach it, whether it be short period of time when Q is high, or a longer period when the Q is lower.  If you use different filters don't forget to change them and see the results.  Some filters will be much more sensitive to moonlight than others.  In this way you can see which filters to go for in darkness and which ones can be left for after the moon rises.  

Above all, Experiment!

Some final thoughts: 

• Use the Night (ahead/back) buttons. As you click you can quickly preview the coming nights.  This can help you decide to go ahead and observe this target now or wait a few days/weeks.

• Don't forget the Current Weather Conditions.  The weather data you enter affects the results, which in turn can affect your decisions. Experimenting with different seeing conditions tells you how important seeing is to the final result.  At the very least be sure to enter typical values for your site.

• Keep an eye on the effective resolution.  This value will depend on the seeing conditions and the altitude of your target object.  If you want good resolution or want to combine images with similar resolution you should pay attention to this value.

• Remember the option to enter your own target data.  You may want to override the default values derived from the catalog data for your object.  By entering your own values you can expose for a general stellar magnitude limit or choose a generic surface brightness value to shoot for.

• Help the calculator out by setting reasonable limits on sub-exposure time.  The calculator has a tendency toward short sub-exposures.  Use your experience with your camera here.  For instance, if readout times are long a 1-minute sub exposure is probably too short.  And in general, a 1-minute sub-exposure may technically lead to the highest overall SNR, but it may be only very marginally so.  It's ok to limit sub exposures to 5 or 10 minutes, or even more.  Enter your limits in the Allowable Sub-exposure Times area.

Calibrating SkyTools for use with your Camera

The results of any computer model depend on the quality of the inputs and there are a lot of them that go into this model.  Not everything can be fully quantified, which ultimately leads to some error (or uncertainty) in the SNR values computed.  Fortunately the calculator uses a trick: the most useful aspect of the program doesn't lie in predicting the exact SNR but in comparing SNR values under differing conditions.  Even if the calculator doesn't give highly accurate values of SNR for your camera, the difference in SNR when you change a variable, such as adding moonlight, will remain relevant.

Regardless, fine tuning SkyTools to better predict your SNR values can't hurt.  Use one or more raw images taken with your system.  For best results use a single star as your target object.  A major source of uncertainty is the surface brightness of a deep sky object, which isn't always accurately known, so a star is better. It is important to know the observing location, date, exact time, duration, filter, binning, light pollution (sky brightness overhead on a dark night), temperature, relative humidity, and astronomical seeing at the time the exposure was made.

Use a program to determine the maximum signal of the target star and the background sky brightness (both in adu) on your raw image.  Carefully enter all of the date/time/conditions data into the exposure calculator.  Under Calculate SNR for Exposure set the start time and enter the exposure time where it says "total exposure time."  Click the Auto button to disable the use of the computed optimum sub-exposure time and enter the same value for the sub-exposure time as for the total exposure time.

There will be a line like this in the results below: Max. signal: 36020 ADU Sky: 5080 ADU/pix

The values on this line are always for a single sub-exposure rather than the final stacked image.  But in this example case there is only the one image anyhow.  You should be able to directly compare these numbers to what you measured on the image.  If they are very different, check all your inputs.  If they are still very different verify the parameters of your camera on the Add/Modify Cameras dialog (accessed via the Add/Modify Telescopes dialog).  To do this highlight the camera in the listing on the left and click the Edit button. 

Narrow Band Filters

It is important to understand that it is not currently possible to create a general exposure calculator for most narrow band filters. The reason for this is that most narrow band filters are designed to isolate an emission line. The problem is that a comprehensive database of emission line strengths for various astronomical objects does not yet exist. Without knowing the emission line strength it is impossible to estimate the signal. Without an estimate of the signal the calculator lacks a critical piece of information. For this reason SkyTools only works fully for filters that do not isolate an emission line.

The (B-V) color index gives us the shape of the continuum spectrum only. Any emission line is going to "spike" above the continuum producing a much greater signal at that specific wavelength.  We may be able to one day estimate the emission line strengths for use in the calculator but that is a frontier that will take further development and testing.

For narrow band imaging the SNR and sub-exposure times are not reliable. BUT, the relative effect of altitude, moonlight, and twilight will still provide useful information for general planning purposes. In other words, go ahead and use the narrow band filters for planning when to observe your object, either from the information displayed in the planner or the exposure opportunity table in the Exposure Calculator. Just remember that the actual SNR and exposure times won't be accurate.

Planetary and Lunar Imaging

The focus of planetary and lunar imaging is on resolution (fine detail) rather than SNR, although SNR can still be important.  When you open the Exposure Calculator it will have automatically selected the best exposure opportunity for the night.  The exposure opportunity will appear on the NightBar as two vertical yellow lines.

The Exposure Opportunities table breaks the night into blocks of time, one per line.  Each block represents a period of the night with a similar value of Q.  When you open the calculator one or more blocks will automatically be selected as the best exposure opportunity for the night.  This is the time period indicated on the NightBar with the vertical yellow lines.

You can experiment with the exposure opportunity time "blocks."  Select another block in the table and watch what happens on the NightBar and in the Calculate area.  To select more than one block at a time hold the Ctrl key down as you select them. To select a group click on the top line of the group, depress and hold the shift key and select the bottom line.  All the lines in between will be selected.  Remember, some objects can be imaged in broad daylight just fine, so don't ignore the daytime.

You can skip the pre-computed blocks entirely by dragging the vertical yellow lines on the NightBar or by entering specific Exposure Start and Total Exposure times in the Calculate area.

Above all, Experiment!

Some final thoughts: 

• Use the Night (ahead/back) buttons. As you click you can quickly preview the coming nights.  This can help you decide to go ahead and observe this target now or wait a few days/weeks.

• Don't forget the Current Weather Conditions.  The weather data you enter affects the results, which in turn can affect your decisions. Experimenting with different seeing conditions tells you how important seeing is to the final result.  At the very least be sure to enter typical values for your site.

• Keep an eye on the effective resolution.  This is the actual resolution that can be expected in the final image, including any blurring from atmospheric effects.  The effective resolution for some camera/telescope combinations is more or less fixed.  For others it is highly dependent on the seeing conditions (as selected) and the blurring effect of the atmosphere itself is highly dependent on the altitude above the horizon.  This is an example of how useful the calculator can be.  Two imaging systems can be set up right next to one another imaging the same target.  For one the resolution may be as good as it gets, but for the other you may see a large improvement if you wait for a night of better seeing or when the object is higher in the sky.

Double Star Imaging

The focus of double-star imaging is on resolution (fine detail), although SNR is still very important.  When you open the Exposure Calculator it will have automatically selected the best exposure opportunity for the night.  The exposure opportunity will appear on the NightBar as two vertical yellow lines.

The Exposure Opportunities table breaks the night into blocks of time, one per line.  Each block represents a period of the night of similar quality.  When you open the calculator one or more blocks will automatically be selected as the best exposure opportunity for the night.  This is the time period indicated on the NightBar with the vertical yellow lines.

You can also experiment with the exposure opportunity time "blocks" themselves.   Select another block in the table and watch what happens on the NightBar and in the Calculate area.  To select more than one block at a time hold the Ctrl key down as you select them. To select a group click on the top line of the group, depress and hold the shift key and select the bottom line.  All the lines in between will be selected.

You can skip the pre-computed blocks entirely by dragging the vertical yellow lines on the NightBar or by entering specific Exposure Start and Total Exposure times in the Calculate SNR for Exposure area.

Above all, Experiment!

Some final thoughts: 

• Use the Night (ahead/back) buttons. As you click you can quickly preview the coming nights.  This can help you decide to go ahead and observe this target now or wait a few days/weeks.

• Don't forget the Current Weather Conditions.  The weather data you enter affects the results, which in turn can affect your decisions. Experimenting with different seeing conditions tells you how important seeing is to the final result.  At the very least be sure to enter typical values for your site.

• Keep an eye on the effective resolution.  Resolution is what determines whether you can split the pair and/or how accurately you may be able to measure their separation.  The effective resolution is the actual resolution that can be expected in the final image, including any blurring from atmospheric effects.  The effective resolution for some camera/telescope combinations is more or less fixed.  For others it is highly dependent on the seeing conditions (as selected) and the blurring effect of the atmosphere itself is highly dependent on the altitude above the horizon.  This is an example of how useful the calculator can be.  Two imaging systems can be set up right next to one another imaging the same target.  For one the resolution may be as good as it gets, but for the other you may see a large improvement if you wait for a night of better seeing or when the object is higher in the sky.  

• One other thing to notice is under Target Object Data.  The separation between the component stars on your image will be indicated here.  Example: The pair is separated by 14 pixels.  This can give you a good idea of how well separated the two stars will be on the final image.

Related Topics:

Sub-Exposure Times

Planning Imaging Sessions

Adding Cameras Manually to SkyTools

Measuring your Sky Brightness from an Image