The FITS Image Grader is a handy utility designed to perform a focus quality analysis and assign relative grading metrics to your sub-frames. Simply select the FITS files you want to grade, click "Grade" and FITS Image Grader will calculate "whole-image" Half Flux Radius (HFR) in order to determine the image's relative focus (lower values are better), Number of Stars found and finally a proprietary "Score" that takes both HFR and Number of Stars into account to provide a single metric (higher is better).

What FITS Image Grader is not...

The image grader tool is not an absolute measure of the quality of your images... it grades images relative to one another. This means that if all of your images are consistently and badly out of focus then your images will all likely have around the same HFR and bad images will have good scores (which is less than helpful). You will need at least one image that is of decent quality in order for images of lesser quality to stand out.

Why use star count? 

HFR is a fairly resilient metric and as a result, it is normally capable of providing accurate measurement in poor seeing and even through a thin film of clouds. Using star count gives you another point of reference in terms of image quality. While an image with thin cloud cover might have a very similar HFR to a "good" image, the star count is likely to be reduced.

How the final score is calculated

The final score is nothing complex. It is a number between 0-100 (higher is better) that reflects the average of an image's HFR and Star Count scores. The score is RELATIVE and will often give a value of "0" to PERFECTLY GOOD IMAGES. A very low score indicates that an image was of lesser quality than similar images in the set... and that's all. You should always visually inspect low score images.

How images are grouped

When you add images to the grader (use either "Add FIles" or "Add Folders"), it will automatically attempt to categorize the images by filter type and binning mode. It will first inspect the file name for certain patterns and then if that fails, it will open the FITS file and inspect the headers for this information. Only images of similar type are scored against one another (e.g. HA images at 1x1 binning).

Note:  Checking include sub-folders will add all files in the selected directory and all files found in its sub-directories.

Renaming images

FITS Image Grader provides several options to rename images as they are graded (see options below). You can choose from several patterns and select from using score, HFR or both as part of the new file names. Additionally, you can copy the file before renaming (see options below).

Reporting and Marking

After a grading session is complete, you can choose to mark images as good or bad (renames files). Either right click on individual images or select them using the check boxes on the left and click either "Mark Checked Good" or "Mark Checked Bad". Additionally, you can create a CSV (for Excel or Google docs) report of the grading run for later viewing.


FITS Image grader will currently operate on all known variants of the 16-bit FITS format (mono and color). It is optimized to run analysis on well integrated sub-frames from about 2 minutes to 30 minutes in length.


These options control how images are renamed during the grading process.

  • Rename images:  Check this to rename images (in-place) during the grading process.  See the rename using section below for more details.
  • Copy and then Rename Images:  Check this to copy the image and then rename images during the grading process.  Images will be copied to the selected directory (browse button).  See the rename using section below for more details.
  • Rename using:  Rename the files using the calculated HFR, final score or both.
  • Rename location:  Add the data selected in "rename using" to the beginning or end of the file name.
  • Overwrite existing:  Check this to allow overwrite of existing files during the grading process.