Top 10 EAA Challenges: Part 2

In my July Blog I came up with a list of the 10 Challenges, one or more of which, anyone getting started in EAA is likely to face at some point.  I explained covered the first 3 of these providing a troubleshooting guide for each one.  If you haven't read that Blog yet, I suggest you start there.  In any case here again are the Top 10 Challenges:

1. Cannot Get the Camera to Connect to the Imaging SW
2.  Cannot See Anything in the Image
​3.  Can See Stars, but Not DSOs
​4.  Live Stacking Not Working
5.  Poor Focus
6.  Elongated stars
7.  Comet Shaped Stars
8.  Noisy background
9.  Significant Vignetting
10. Plate Solve Fails
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​In this Blog I will address the 4th and 5th of these challenges.

#4. Live Stacking Not Working

​Live stacking is the key ingredient to EAA.  The ability to take many short exposure sub frames and stack them in real time, averaging out the background noise so as to improve the SNR is why we can see so much detail in Deep Sky Objects (DSOs) as we allow the stacking process to proceed.  It is not uncommon for the Live Stacking software (SharpCap, ASILive, etc.) to reject some frames from the stacking process for a number of different reasons.  This is no reason for alarm and can be a good thing as it may be eliminating bad frames from messing up the stacked image.  Moments of poor seeing, a light wind gust, errant mount behavior, etc. can cause a frame here or there to be rejected.  However, sometimes one finds that after the first frame is captured, no subsequent frames are successfully stacked no matter how long we wait.  There are several reasons why this may happen.  In this Blog we will use Sharpcap as our example Live Stacking software.  Other such software will behave generally the same, but may not have all of the stacking settings available to the user in Sharpcap.  Generally they don't.  Live Stacking is one aspect of EAA that you will not be able to check out during the daytime.  However, if you run into trouble at night, follow the guidelines below to, hopefully, quickly pinpoint and solve your problem.

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   4A.  Check the "Align Frames" Box in the Live Stacking Feature

To begin stacking you must start the Live Stacking function and under "Controls" click "Align Frames".  This is likely only a novice oversight, but it doesn't hurt to check.

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   4B.  Not Enough Stars Detected

For live stacking to work, the software needs to detect at least 3 stars in each image to be able to shift and rotate frames to align to the first image frame.   That is not to say that alignment will always work or even that it will do a good job with just 3 stars as that is the bare minimum required to compute the necessary adjustments that need to be made.  Sharpcap specifies that 10 - 20 stars with good distribution across the frame are required for the best possible stacking to proceed.  The user can set the number of alignment stars under the "Alignment" tab.  Defaut is 15 stars.  You may not be able to see the stars yourself even though Sharpcap can.  But, you can check the number of stars detected in the "Alignment" tab under "Star Detection Status".  If not enough stars are being detected there are several quick things to try.

   1.  First, increase the exposure and/or gain.  If the exposure is too short the camera might not capture enough stars in parts of the sky where the number of visible stars is small, such as pointing away from the galactic center.
   2.  Check the focus.  A poor focus will make stars dimmer and harder to detect, as well as, broader and less star like.
  3.  If both of those fail you can try adjusting the Star Detection parameters.  While Sharpcap automatically adjusts the Sensitivity (up if less than 25 stars are detected; down if more than 200 stars are detected) you can still try to see if this helps.  Increasing the Noise Reduction setting helps with star detection when the overall image is noisy and when hot pixels may be causing a problem.  Lowering the Maximum Star Size measured in pixels can help prevent small deep sky objects from being misidentified as stars.  Suppress Hot Pixels is a default setting which will help to prevent hot pixels from being identified as stars.  It's default setting is to be checked.  If there are obviously faint stars not being detected you can try checking Optimize for Faint Stars.  On the other hand, this can also cause non-star objects to be falsely identified as stars so if it is turned on and you are having trouble stacking and nothing else seems to work, try turning it off.  Highlight Detected Stars is a good trouble shooting tool to see what and where stars are detected.  A yellow highlight indicates a star used for stacking while a red highlight indicates stars not used for stacking.

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   4C. Elongated Stars

Elongated or egg shaped stars may be one of the most common reasons for live stacking to not work on some or all frames.  Live stacking software is looking for stars to be point like and may not identify a star shaped like an elipse as a star. Stars become elongated when the exposure time is too long for the mount to keep the stars from moving within the frame.  If you are using an Equatorial Mount, either go back and re-do the polar alignment or shorten the exposure time.  You should also make sure that your mount is up to the task for the given exposure time.  This includes keeping within the mount's rated capacity, obtaining a good balance of the optical tube and shielding the mount from gusts of wind.  If you are using an Alt-Az mount, it is important to keep the exposure short enough to avoid star elongation due to field rotation.  A maximum of 30sec is the general rule of thumb for Alt-Az mounts, but the fact is that longer exposures are possible in some areas of the sky such as due east and due west.  For a more in depth discussion on this check my page on Field Rotation.

   4D. Poor Frame Quality

Besides elongated stars there are other reasons that a frame can be rejected by the stacking algorithm.  Poor focus, poor seeing, bad collimation can all cause the overall frame quality to be rejected by the stacking algorithm.  Check the Full Width Half Maximum (FWHM) tab to see if frames are being rejected (highlighted red) from the stacking process because of star size.  You should use the focus tools under Focus Assistant under the Tools Tab at the top of the screen to make sure that you have good focus first.  You can also use a Bahtinov mask to check and optimize the focus if you prefer.  You can check for poor seeing with the Seeing Monitor under the Tools Tab at the top of the screen but make sure that you have a good focus first or the Seeing Monitor can be fooled.  

   4E. Forgot to Remove the Bahtinov Mask

Sometimes we so distracted that we forget to remove the Bahtinov mask if we used one to focus.  The distorted stars will give the imaging software agita and the stacking process will have problems.

   4F. Camera Issues

It is possible that frames get dropped because of problems with the connection between the camera and the software.  These can be due to cable timing issues between the SW asking for an image frame and the camera sending it over.  Try connecting the camera directly to the computer if it is not already.  You can also try a different USB cable.

   4G.  Check the Forums

When all else fails you may want to check the appropriate software forum where you can find additional help, especially for uncommon problems.  Here is the link to the Sharpcap forum forums.sharpcap.co.uk/index.php and here is the link to the ASIStudio forum bbs.zwoastro.com/t/asistudio

If live stacking is working well, very, very few frames should be dropped.  In fact, I find that if live stacking is working, I rarely see dropped frames.  

#5.  Poor Focus

Poor focus at best will result in soft images with important detail obscured, and at worst produce undiscernible images.  It can also adversely impact live stacking and the plate solving process.   During the daytime checkout of your setup, checking focus should be foremost on your list.  Focus on a distant object like a building, tree, power pole, etc.  The further away the better but even once you have achieved focus in daylight you will still have to fine tune it at night on the stars.

At night, you should focus right after everything is connected and the optical axis is pointed at a clear region in the night sky.  Start you viewing session by focusing on a region of the sky just after dusk with lots of stars.  There are a number of different tools available for focusing depending upon the software used and whether a motorized focuser is available.   Also, if you change focal length you may be far from focus at the new focal length.  With my SCT I find that I have to move the mirror with the focus knob almost all the way to the other side of focus when I change from f/10 to f/7 on my Edge C11.

  5A.  Manual Focus

If you do not have a motorized focuser it will be necessary to focus manually.  While one might think that they can tell when the optics is focused by looking at the image of a star field by eye it is unlikely that they would achieve the best possible focus without some feedback mechanism.  One of the simplest of these is an inexpensive Bahtinov mask to focus on a single bright star.  Placed over the front of the optical tube, the cutouts in this plastic mask cause the light passing through to create a diffraction pattern with two lines forming an "X" and a third passing through the "X" vertically.  When the third vertical line bisects the "X" the optics is well focused.  If necessary, increase the exposure to help make the pattern easy to discern.  A Bahtinov mask is capable of achieving a very good focus on par with other automated methods.  Sharpcap has a focus assistance tool for the Bahtinov mask that analyzes the diffraction pattern and gives a score to help determine the best focus point.

An alternative to the Bahtinov mask is to focus directly on a single star or a field of stars.  Sharpcap measures the pixel width of the star or stars and displays a value for the Full Width Half Maximum (FWHM).  A well focused star will cover the minimum number of pixel widths, hence will have the smallest possible FWHM number.   Use the FWHM method for a single star or the Multi-Star FWHM method for a field of stars.  Adjust focus while watching the FWHM feedback number until you have minimized it.

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​  5B.  Auto Focus

If you have a motorized focuser you can use the software to focus automatically.  For this to work, the software must be able to connect to the focuser which will require that you download the appropriate ASCOM driver for the focuser from the focuser manufacturer's web site.  In addition, you will also need to download the ASCOM platform which you can find on the ASCOM web site ascom-standards.org/Downloads/Index.htm.  You can use the same focusing aids (Bahtinov Mask, FWHM, Multi-Star FWHM) in the auto focus mode as in the manual focus mode.  The difference is that the software will control the focuser based upon the focus score it calculates.

  5C.  Re-Focus Occasionally 

As the temperature drops throughout the night the optical tube will contract changing the optimal focus position.  For this reason it is a good idea to check the focus from time to time.  It will also be necessary to refocus if you remove or change a filter as the optical path length can change due to differences in filter glass thickness and/or indices of refraction.

  5D.  Elongated Stars

Elongated stars due to poor tracking, for instance, will make it difficult to achieve a good focus.  In that case, the reason for the elongated stars needs to be resolve before trying to refine the focus.

A good focus is key to a sharp image showing the best detail.  And, a good focus is important for the other ingredients of a successful EAA session including automatic stacking and plate solving.