CFWHM Plug-In for CCDSoft, version 1.1.1     


StarSieve and CFWHM have been superceded by a new product: CCDInspector 1.0.  CFWHM is no longer available as a stand-alone product.

CCDInspector contains both original products, and provides significant enhancements and innovations. The new package is being distributed by CCDWare, and is available for a trial download.

Read all about CCDInspector 1.0

Measure and Plot In Real-Time Any Star, Any Sub-Frame, Any Image:


Focus like a Pro... Evaluate Seeing Conditions... Compare Images for Sharpness... Monitor Focus while Guiding  +  MUCH MORE.

A common complaint from CCDSoft users is that this software does not provide an instant FWHM display for critical focusing and image quality measurement.

CFWHM is designed to work seamlessly with CCDSoft to provide this function, along with a number of other useful tools to estimate seeing, image quality, and tracking, and guiding performance. A real-time profile of the star in X and Y directions is also available.


Disclaimers and Copyright

Copyright 2005 by Paul Kanevsky. All Rights Reserved.

CFWHM is NOT freeware. The author accepts no responsibility for direct or consequential damage caused by the use of this software. CFWHM is a supported software.

Table of Contents

  1. Disclaimers and Copyright
  2. What is it good for?
  3. Installing CFWHM
  4. Using CFWHM
  5. Setting Image Scale
  6. Display Settings
  7. Chart Settings
  8. Definitions
  9. Requirements
  10. Download CFWHM
  11. What's new in this version?
  12. Problems, Support

What's it good for?

CFWHM was designed to perform the following functions:

1. Real-time focusing using Full-Width-at-Half-Maximum (FWHM), Peak Value, Half-Flux Diameter (HFD), and other display statistics
2. Seeing conditions estimation by measuring FWHM or HFD of a star
3. Focus quality monitoring during a long exposure by measuring the quality of the star image on the autoguider chip
4. Fast and easy way to evaluate the quality of a long exposure containing multiple stars and extended objects
5. Measure FWHM or a number of other statistics of a specific star in the image by selecting it in CCDSoft
6. An easy and objective way to sort a number of exposures by sharpness.

There are many other uses. For example, the eccentricity measurement allows one to evaluate the coma distortion of a star near the edge of the chip compared to the one in the center. The star profile plot allows one to see at a glance whether the star is saturated, or has reached the non-linear region of the CCD chip, or has been distorted in some way by poor guiding or poorly adjusted optics.

CFWHM contains some very sophisticated algorithms to provide the most accurate measurement possible, while ignoring spurious data, noise, and artifacts.

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Installing CFWHM

To install CFWHM,

1. Download the installation file

2. Exit CCDSoft if it's running

3. Locate CCDSoft Version 5.0 folder. It's usually located in C:\Program Files\Software Bisque\CCDSoft Version 5.0

4. Double-click on the CFWHMSetup.msi file to start the installation.

5. Follow prompts to select folders for CCDSoft and CFWHM application. In most cases, accepting the default values will be sufficient.

6. On completing the install, start CCDSoft, and locate and double-click the CFWHM Panel icon on your desktop.

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CFWHM Panel application can be started or closed as needed. You don't have to shut down and restart CCDSoft -- CFWHM will automatically connect to the running instance of that application. The panel position, size and selected options are remembered between sessions. If two instances of the Panel are started, only one will receive real-time updates from CCDSoft.

Start CCDSoft. Double-click on the CFWHM Panel icon on the desktop:


The panel will float on top of CCDSoft. If you will be focusing and standing far away from the monitor, simply drag the bottom right corner of the panel window to increase its size. The text size will increase in proportion to the size of the window. If desired, the panel can be stretched to occupy the size of the full screen.


The display panel is designed to be as simple to use as possible, and as easy to see in the dark and from a distance, as possible. If you would like to see the profile plot of the star or a running real-time chart, click the expand button ">>" at the right :

There are some additional statistics shown below the profile plot: FWHM value, Aspect ratio, Peak value, and centroid coordinates. Aspect ratio indicates how far out-of-round the star image is. For example, a star that's elongated due to tracking error, or coma, will exhibit much higher aspect ratio. The ratio is expressed in %. A 9% out of round means that the longer axis of the star is 1.09 times longer than the short axis. Usually a value below 30% or so is nearly invisible, but the elongation becomes more obvious as the ratio increases.

The vertical line in the middle of the plot points to the location of the centroid. The coordinates of the center of the star, expressed in CCD pixels, are shown in parenthesis at the right as (x,y).

You have a choice of display unit in CFWHM Panel: the values can be displayed in arcseconds or pixels. Simply place a check mark next to In Arcsecs option if you'd like to see the display in arcseconds, and uncheck it to see it in pixels:

CFWHM will always show the units next to the value. The following abbreviations are used: 

  " (double quotation mark) stands for arcseconds
  pix stands for pixels
  a stands for ADU or analog to digital units
  a/p stands for ADU/pixel and is used for the average flux display
  % is used for the aspect ratio display
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Setting Image Scale

In order to display information in arcseconds, you will also need to ensure that CCDSoft knows the focal length of your imaging setup. For use with CCFWHM, this should be expressed in units of mm. This can be entered in CCDSoft in the Camera Setup screen by pressing the File Defaults button:

Please make sure that the focal length is set in mm, and not inches, otherwise the arcseconds calculation will be incorrect.

Note: If you select display in Arcseconds, but the focal length or pixel size parameters are not available from CCDSoft or the image, CFWHM will display measurements in pixels by default.

If you request arcseconds display and focal length or pixel scale is not available from CCDSoft, CFWHM will ask if you'd like to enter a default pixel scale to be used for images where it's not known. If you say yes, you will be prompted with the following dialog:


Enter the image scale value in arcseconds per pixel for the main imager and the autoguider in your setup. These values will be remembered and used in the future, whenever image scale is not available from CCDSoft. You can always change the default settings by activating this dialog from the CFWHM control menu:

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Display Settings

CFWHM has an automatic display mode that will measure the star immediately from an image captured by the camera. You can choose to monitor images captured by either the main camera chip, or by the autoguider chip. To monitor the autoguider chip, place a check mark next to the Autoguider option. To monitor the main imager, uncheck this option.

CFWHM will automatically refresh its display and recalculate the values when a new image is captured. If you'd like to measure a star from a specific image after it's already been captured (or loaded from disk), simply make this image active in CCDSoft, and click on the Get FWHM button. This will measure the currently active image in CCDSoft.

If the image contains only one star, then CFWHM will display its measurements. This works great for the autoguider window, or in rapid focus mode with the sub-frame set to display one star. CFWHM is smart, and will make every attempt to ignore hot pixels, noise, airplane tracks, and other artifacts that are not stellar in nature.

In case of an image that is too noisy, or contains no valid star data, CFWHM panel may display N/A or Not Available. This can also happen if the star is very unfocused or extremely saturated, causing the FWHM value to exceed 30 pixels.

When an image contains many stars, and possibly other objects, such as nebulae, galaxies, etc., CFWHM will pick a single star to measure, based on a number of attributes. It will first isolate up to 1000 stars from the image.   From these stars, it will discard stars that are saturated, or very close to the background. It will then sort the remaining stars by their FWHM value and will pick the median (middle) star from the list. This star will be most representative of the quality of the image. In this way, CFWHM allows a very easy and automatic measurement of the quality of any image. You can judge the guiding performance of your mount, the changing seeing or focus conditions, and also sort your exposures based on sharpness.

In addition, you can select an area of the image, or even a single star that you'd like to measure using CCDSoft's rectangular selection tool:   

Once selected (the selected area can include one or many stars, or even extended objects), press Get FWHM button on the display panel, and the best representative star from the selected rectangle will be measured and displayed.


Note that you can load a saved image at any time after it's been captured, and measure its quality using CFWHM by clicking on the Get FWHM button.

The CFWHM display panel shows two values: FWHM and HFD by default. The top value will always show FWHM, but you have a choice of what metric to display in the bottom value. Click your mouse directly on the label next to the lower number display that contains the word HFD. You will see a pop-up menu with the following choices:


Click on the value you'd like to see displayed below the FWHM. See the Definition section for explanation of what each one of these values measure. For example, here is a display with peak value option selected:

Note that the Peak Value display in CFWHM is not exactly the same as the image maximum value: the peak value is the maximum value of the selected star, not the whole image.

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Chart Settings

CFWHM provides a number of chart displays that can be useful in both, monitoring focus and image quality in real-time, and in measuring an existing image. To view the chart, expand CFWHM Panel by clicking on the >> button.  The Chart display will appear, showing the default selected chart.

Chart settings can be changed by clicking the left mouse button anywhere on top of the chart area:

The following chart options are available:

1. Star X-Y Profile chart  (default) shows currently measured star's profile in horizontal and vertical directions. Note that Moving Average setting is not applicable to this chart type.

2. FWHM  - a real-time running display showing a number of latest FWHM value measurements in pixels or arcseconds. Real-time, or Moving Average or both can be displayed at the same time.

3. HFD - a real-time running display of a number of latest HFD measurements. Shown in pixels or arcseconds.

4. Peak - running display of a number of latest peak values of a star. Shown in ADU.

5. Aspect Ratio - a running display of a number of latest aspect ratio measurements, in %.

6. Centroid - a running display of the distance, in pixels or arcseconds, from the initial star position centroid. Can be used to evaluate seeing, and tracking problems.

All charts except for the X-Y Profile can be used to display Real-time measurement, a Moving Average, or both by placing a checkmark next to the appropriate menu selections.

For moving average selection, you have a choice of 2 points, 3, 4, 5, 6, 7, 8, 9, 10, or 20 points moving average.

The chart can also be set to display more or less points, as desired. To pick the number of points to keep on the chart, select Points on the Chart menu, and pick the desired number (50,100,200,500, or 1000).

To restart the chart, and to reset the chart scale choose Clear Chart option. This will erase all historical data and will start measurements from scratch.

If you'd like to pause the automatic chart refresh, click on the Pause Chart Updates option. The chart will not be updated until the check next to the Pause Chart Updates option is removed.

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A point source of light, such a star illuminates a number of pixels surrounding the center of the star. The center pixel is usually the brightest, with the intensity of the pixels falling off rather rapidly the further away one looks from the center pixel. The faster the light falls, the better the quality and sharpness of the star. The slower it falls, the more pixels the star occupies, the more bloated it is, and therefore less focused. To come up with a standardized way to measure star quality, independent of the brightness and magnitude of the star, the FWHM measure is used. FWHM stands for Full-Width at Half-Maximum and is simply the width (or the diameter) of the circle surrounding the star where the intensity has fallen off by 50% from the peak value (half-maximum).

FWHM can be measured in pixels, or in arcseconds. Pixels is the default in CFWHM if the focal length and the pixel size of the camera cannot be determined.  If these are known and specified in CCDSoft, CFWHM can be used to automatically convert pixels to arcseconds. The value in pixels is dependent on both, focal length and pixel size, and so cannot be used to directly compare two images from different cameras, or from different telescopes. On the other hand, measurement in arcseconds is independent from telescope focal length or the camera pixel size, and can be used for direct comparison between different instruments.

Lower FWHM values are better -- means the star brightness falls off faster.   There are many factors that limit just how low an FWHM value one can get. Some of these are:

1. Focus: telescope must be well focused to produced lowest possible FWHM. This is where CFWHM plugin can help a lot.
2..Collimation: poor collimation will lead to more bloated stars  with higher FWHM
3. Seeing conditions:  atmosphere is one of the major limiting factors even for professional astronomers.  On a good day, an average amateur with excellent optics and collimation can hope for FWHM of 2"-2.5". On rare occasions or at really great locations, the seeing can dip to as low as 1.6" but not much below. More often than not, you're likely to see FWHM in the 3" to 4" range. Anything much above 4" is usually a signal to go inside and watch some TV -- the seeing is really poor.
4. Tracking and guiding: FWHM of a star in a long exposure can be much greater than FWHM of the same star in a really short exposure. This happens because the air masses keep moving causing seeing fluctuations during long exposures, and guiders and mounts usually have small errors that add up over a long period to smear the star a bit more.

HFD or Half-Flux diameter is yet another way to measure star quality. A star  illuminates a number of pixels on the CCD to a level above the background (sky) brightness level. Total star flux is a total number of units of light above the background that the CCD has captured. HFD is then a diameter of a circle around the center of the star such that it contains exactly 50% of the total flux of the star. Again, the diameter of the circle can be measured in pixels, or in arcseconds.

Just like FWHM, lower HFD values are better: the smaller the diameter of the 50% circle, the more concentrated the star light is, the sharper the star image. As it happens, HFD measurement at low values is very close to the FWHM value. The benefit of HFD is it is more immune to noise.  CFWHM allows you to display FWHM and HFD values side by side for a quick comparison.

Peak Value:  is the brightest pixel at the center of the star. The better the telescope is focused, the better the seeing conditions and transparency, the higher this value is going to be. This is not as good a measure of star quality as FWHM or HFD because atmospheric transparency variations can lead to quick brightness variations making   it hard to judge the best focus point. The Peak Value is also very much dependent on the magnitude of the star. This means there's no way to judge seeing conditions between two nights by comparing peak values.  A maximum Peak Value in combination with lowest FWHM value is a very good indication of critical focus.

FWHM Moving Average: because seeing causes random fluctuations in FWHM values, it is sometimes hard to judge when critical focus has been reached. To help mitigate this somewhat, you can choose to display the average of the last 3 FWHM measurements. On a turbulent night, this helps smooth out the fluctuations, and gives a more reasonable approximation of the seeing conditions and focus.

Aspect Ratio: Aspect ratio represents how much out-of-round the star image is. It is the ratio of the longest axis to the shortest axis of the star profile, expressed in percent. A number below 20-30% represents a pretty round star. A number of 0% represents perfectly round star, but you will most likely not see this in real images due to noise and measurement uncertainty.

Centroid Coordinates: Displays X, Y coordinates of the exact center of the star being measured. This is always expressed in pixels. Note that CFWHM measures a star along multiple axis, so the long and short axis may not correspond to the X and Y axis of the image.

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CFWHM requires CCDSoft Version 5.0, release of 5.00.156 or later is preferred.  Operating systems currently supported are: Windows XP, Windows 2000. 

Other version of Windows may be compatible but have not been tested. if you'd like to test it on another operating system, please e-mail me.

CFWHM contains two components: display panel executable and event-plug-in for CCDSoft. Unfortunately, the event plug-in must be in Visual Basic 6.0 to work with current versions of CCDSoft. If your computer doesn't already have VB 6.0 run-time environment, please download it from  here: Click here to download VB Run-time support files directly from Microsoft.

Software Bisque promised to support C++ based event plug ins in an  upcoming release, so hopefully then the need for this outdated VB runtime will go away.

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Download CFWHM

Newsflash 10/16/2005: CFWHM is no longer available for purchase. It will be replaced by a new product in the very near future. Existing CFWHM customers will continue to receive support, and will be offered an upgrade at a significant discount. Please, watch this space for further announcements.


Click here to download just the upgrade to Version 1.1.1   (869 KB download)

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Release History

Release Date Version Number


10/14/2005 1.1.1   Patch released for CCDMonitor.dll, version 1.1.0. This patch resolves an issue reported with slower computers that resulted in occasional images being skipped and not measured when acquired from a camera.

To apply the patch, please download a new version of the CCDMonitor.dll event plugin:

Once downloaded, extract the two files to a temporary folder, and follow the instructions in Readme.txt to copy the new CCDMonitor.dll to the appropriate folder.

09/12/2005 1.1.1   New selection for chart moving averages
Configurable number of data points to keep in a running chart
Improved main window position handling to
08/18/2005 1.1.0   Real-time running charts added, better scaling of the charts
Default pixel scale dialog added
Detect and ignore box dimensions that are for the wrong image, as provided by CCDSoft
Better noise rejection and background level estimation: better faint star detection in noisy and undersampled images
08/10/2005 1.0.7   Bug fix for focus and autoguider mode that would cause noise to be treated as a star. Please download this version to replace 1.0.6!
08/09/2005 1.0.6    First public release
08/05/2005 1.0.0    Initial beta release
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Problems, Support

There is a Yahoo Discussion group where you can submit your questions, requests for features, or discuss this and other products I make:

PKAstro Support Yahoo Group

Here are some possible problems, questions, and solutions:

1. "I selected display In Arcseconds option, but the display still shows values in pixels?"

Solution: Make sure you set the focal length in CCDSoft Camera File Defaults dialog

2. "The FWHM or HFD display in arcseconds appears to be incorrect?"

Solution: Make sure to set the correct focal length in mm in the CCDSoft Camera File Defaults dialog . If you are trying to measure an existing image, then you may need to edit image header in CCDSoft, and verify that the FOCALLEN keyword is set to the correct focal length in mm. To display image header information, select View\File Information... menu and click on the Edit Header tab.

3. "It takes too long to display FWHM for a new image"

Solution: This may happen when trying to measure a large image with a lot of stars, or a lot of hot pixels. You can either use CCDSoft to select a small portion of the image with some representative stars to measure, use a sub-frame to reduce image size, and/or use auto-dark frame subtraction to reduce noise.

4. "The display does not update when a new image is downloaded from the camera"

Solution #1:  Check that the "Autoguider" option is set appropriately. If it's checked, and the camera is exposing images using the main imager, no updates will be calculated since CFWHM is waiting for an image from the autoguider.

Solution #2:  Verify that the plugin is properly installed and enabled in CCDSoft. From the Camera Setup screen, press Events Plug Ins button.  If you don't see the "CCD Monitor Relay" event plug in, then it wasn't installed properly. Try uninstalling CFWHM from the Windows control panel Add/Remove software screen. Then, re-install CFWHM, making sure to provide the correct path to the CCDSoft folder.

If you're having problems using CFWHM or have comments or suggestions for improving it  drop me a line.

5. CCDSoft Hangs on Autoguider image update with Windows 98SE.

CFWHM has not been certified to work on Windows 98. But, I've received reports that the CFWHM Panel display works just fine.

The CCDSoft Event Plug In part of CFWHM that causes the panel display to update when a new image is acquired apparently does not work. This is probably due to some compatibility issues between Visual Basic run-time, CCDSoft, and Win98.

Solution: To work around this problem, you can disable the VB-based event plug in by activating the CCDSoft's Camera Setup screen and pressing Events Plug Ins button. In the list of Event Plug Ins, uncheck the CCDMonitor Relay plug in. Once you do this, CFWHM will not get automatic updates on a new image (autoguider or imager chip). But, you will still be able to manually click on the Get FWHM button to measure a selected star, region, or a whole image. I'll continue to work to resolve this problem with Windows 98.

6. I did a plate solve to get the exact image scale, but I don't know my effective focal length. What do I do?

CFWHM will use image scale as computed by a plate solution, if available. Focal length doesn't need to be set in this case. But, if you want to compute the focal length from a "solved" image scale, here is the formula you can use:

FocalLength =  pixelsize * 206.2648 / imagescale

where FocalLength is the effective focal length expressed in millimeters, pixelsize is the size of the CCD pixel in microns, and imagescale is the image scale as computed by the astrometric solution, in arcseconds/pixel.

7. What are the upper and lower limits on star size and image scale?

CFWHM uses advanced interpolation and curve fitting techniques to measure star FWHM, even for very tiny stars. There is a limit, though, to how small a star image can be before it becomes impossible to measure. For CFWHM that limit is 1.3 pixels FWHM. In other words, if the star size is at or below 1.3 pixels, CFWHM will be unable to measure it. This is because the star spans a radius of less than two pixels, and therefore, there is just not enough information in the image to properly measure its size.

At the other extreme, if FWHM value of an object exceeds 30 pixels, CFWHM will give up on trying to measure it. This is an arbitrary choice, made mostly to help CFWHM not get bogged down in trying to measure large objects that are not stellar in nature (such as the core of a galaxy, for example, or an extremely bloated and saturated star).

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CFWHM PlugIn for CCDSoft
Copyright 2005 by Paul Kanevsky. All rights reserved.
Revised: January 30, 2006 .