• ASTAP versions released between 2025.04.13 and 2025.04.28 suffer a from bug which could hamper solving. Replace by version 2025.05.21 or later.
• 2025.05.29  ASTAP solver improved for star poor images.
• 2025.06.15  For stacking the full implementation of reverse mapping with bilinear interpolation for less background noise and better star shape.
• 2025.11.28  Image stitching/mosaic routine much faster.

You will need only one database. Is your field of view 0.6 degree or larger you can download either the D05 or D20 or D50 or D80. The D05 is the smallest. The D80 is the largest. Using the D80 has no drawback accept it is larger, about 1.25 gbyte.  If you have one of the older H17,  H18, V17 G17, G18 star databases, they can be uninstalled/deleted. 

Star databases usability:


Instead of a magnitude limit the new databases have a density limit.  These databases have been sorted on star density up 500, 2000, 5000 or 8000 stars per square degree. This should guarantee that in star-poor-areas there will be sufficient faint stars in the database for navigation (solving). In star-rich areas only a limited amount of bright stars is included keeping the star database size moderate. If required these databases will go as deep as magnitude 21.

This will be beneficial for setups with a small field-of view. There should be always enough database stars available for navigation. 

The V50 photometry database has like the D50 a 5000 stars per square degree density except the magnitude is the calculated Johnson-V and it also contains the Gaia Bp-Rp magnitude difference. The V05 photometry database is like the G05 except the magnitude is the calculated Johnson-V and it also contains the Gaia Bp-Rp magnitude difference.


For comment feedback and questions there is the ASTAP Forum. The ASTAP Manual is below

For photometry you could download and install the V50 star database. It contains the calculated Johnson-V magnitude and colour information (GBp-GRp) for star annotations. This one also works best for solving an image with a FOV of more then ten degrees

Hyperleda, a very large galaxy database for deep sky annotation. 2.190.000 objects. Based on extract from leda.univ-lyon1.fr/  Will be placed in the program directory.


Alternative links & development version:

• Questions, feedback: ASTAP Forum
• Older versions are available at Sourceforge
• Version history
  
• Source code (Object Pascal, Lazarus/FPC)
•  

• Introduction
• Program installation
• Windows
• Linux
• MacOS
  
• Program operation, stacking astronomical images
•  file formats
• Stack menu 
• Lights tab
  
• Classify by options
• How to exclude poor images
• Satellite tracks
• Popup menu
• Copy selected list to clipboard
• Batch menu unknown stars (Novae search)
  
• Darks tab
• Flats tab
  
• Flat darks tab
• Results tab
• Stack method tab
• Stacking grayscale of colour stacking
• Stacking raw one shot colour images (OSC)
• RAW conversion of OSC images (one shot colour images)
• Stacking LRGB
• Image stiching
  
• Alignment menu tab
• Internal_alignment
• Astrometric_alignment
• Manual alignment
• Ephemeris alignment
• Blink tab
  • Track and Stack function
• Photometry tab
  
• Photometry
• Testing your photometric measurements
  
• Transformation
• Transformation test
  
• Measure all visible variables in one step
• Measuring asteroid magnitudes
• Inspector tab
• Mount analyse tab
• Live stacking
• Monitoring
  
• Pixel math tab 1 and 2
• Background equalization tool
• Popup menu
• 
  

• Viewer:
• Reported angle
• File menu
• file formats
  
• Opening, preview files
• FITS thumbnail viewer
• Tools menu:
  
• Batch processing
• Image inspection
• F5 menu
• Curvature and tilt indication
  
• HFD_2d_contour
• HFD_diagram
• Median_background_values
• HFD values
• Unroundness of the imaged stars.
  
• Show distortion
  
• Aberration Inspector
  
• Photometry calibration
• SQM measurement
• Star annotation and photometry
• Asteroid annotation
• Deep sky annotation
• View menu
• Pop-up menu viewer
• MPC1992 report line.
• Measure total magnitude.
• Star info to clipboard.
• Online query.
• Gradient removal tool
• Dark spot removal tool
• Copy paste tool
• Remove deep sky object 
• Remove borders
• Crop fits image
  
• FITS tables 
• Other pop-up menus
  


• Solver, usage as astrometric solver and command line options
• Conditions required for solving
• Test if an image is solvable (full blind solving)
• Command line usage ASTAP
• Usage as a PlateSolve2 substitute
• Performance blind solving
• Pop-up notifier
• Installation of the external astrometry.net solver 

• Appendix 1 stack process
• Appendix 2 Why using flat-darks or bias
• Appendix 3 the star database 1476 and 001 format.
• Appendix 4 FITS keywords read for solving.
• Developer information: Using SIP Coefficients for optical distortion correction
• Background info, ASTAP star pattern recognition and astrometric (plate) solving algorithm.
• Background info, Reverse mapping in image stacking.

1. Native astrometric solver, command line compatible with PlateSolve2.
2. Stacking astronomical images including dark frame and flat field correction. 
• Filtering of deep sky images based on HFD value and average value.
• Alignment using an internal star match routine,  internal astrometric solver.
• Mosaic building covering large areas using the astrometric linear solution WCS or WCS+SIP polynomial. 
• Background equalizing.
3. FITS viewer with swipe functionality, deep sky and star annotation, photometry and CCD inspector.
• FITS thumbnail viewer.
  
• Results can be saved to 16 bit or float (-32) FITS files.
• Export to  JPEG, PNG, TIFF( ASTRO-TIFF), PFM, PPM, PGM  files.
• FITS header edit.
• FITS crop function.
• Automatic photometry calibration against Gaia database, Johnson -V or Gaia Bm
• CCD inspector
• Deepsky and Hyperleda annotation
• Solar object annotation using MPC ephemerides
• Read/writes FITS binary and reads ASCII tables.
  
4. Some pixel math functions and digital development process
5. Can display images and tables from a multi-extension FITS.
6. Blink tab.
7. Track and Stack function
8. Photometry tab
9. Inspector tab for measuring curvature.
10. Mount analyse tab.
11. Live stacking tab.
    
12. Available for MS-Windows 32 & 64 bit,  Linux 32, 64 bit, MacOS 64 bit, Raspberry-Pi Linux 32 and 64 bit.

This is a screen short of the stack menu. It contains several tabs for the file list and settings. File can be sorted on quality and values.The image can be visually inspected in the viewer by a double click on the file or using the pop-up menu.



Program requires FITS images or RAW files as input for stacking, but it can also view 16 bit PGM /PPM files, XISF files or  in 8 bit  PNG, TIFF or BMP files. For importing DSLR raw images  the program DCRAW from David Coffin or LibRaw is used.

• Stacking methods: average and sigma-clipping-average.. Internal calculation using floating point numbers. Latest program versions are using reverse mapping with bilinear interpolation.
• Simple and intuitive user interface.
• Automatic saving of selected options and files.
• Can create master files for dark and flat & flat-darks to reduce processing time.
• Limited memory use, independent of the number of images stacked.
• Bayer algorithm for DSLR/OSC cameras

The installer and separate star database will be installed by default at c:\Program Files\astap.  But they can be placed anywhere as long as all files are in the same directory.

Note that for Windows ASTAP is seen as an unknown publisher.  For Win10 you have to click trough the following screens.

Click on astap_setup.exe to install.



Click op "More info"




Click op "Run anyway"

Click op Yes.


Do the same for the database installer.

• 1a) Several light frames. Images of deep sky object unprocessed.
• 1b) Several dark frames of the same temperature and exposure as the light frames. A dark frame is a frame that represents an exposure duration done in total darkness. This signal includes the bias signal, but also includes any dark current charge accumulation, and thus any dark current noise that exists within the dark current signal. For a dark suburban area (SQM=20.4) you should take about the same amount or more darks as lights. For a light polluted area you could take less darks then lights since the noise in the lights generated by the sky background will be abundant.
• 2a) Several flat frames. A flat frame is a frame that represents the field flatness and taken from an uniform light source. Ideally with a significant signal level. This to compensate for vignetting and dust particles. Vignetting can greatly darken the corners of your image and have to be compensated.
• 2b) Flat dark frames or bias frames ideally taken of the same temperature and exposure duration as the flats. Since flats are taken with very short exposure times, either flat dark or bias images of almost zero seconds will do. See also why flat-darks

1. The flats will be combined to an average and the combined average flat-darks will be subtracted to  have a near ideal presentation of the vignetting called the master flat frame.
2. The darks will be combined to an average master dark.
3. From each light frame, the master dark will be subtracted to extract the pure deep sky signal.
4. Each light frame will be flattened by dividing it by the master-flat resulting in corrected light frames.
5. The corrected light frames are combined to the final image using the average or sigma clip mean (to remove outliers as satellite tracks) method. 

  


All the program settings and file selections will be save d by leaving the program or click on the "Stack button".

Back to index


The stack menu:

For a generic description how to stack see Program operation, stacking astronomical images

Lights tab

Browse button: Images, darks, flats can be added using the Browse button or can be drag dropped on the form. You can also drag drop multiple directories on the stack tabs. Note it will included the files in any subdirectory or subsubdirectory. (v2025.01.16)

Analyse and organise button: Images placed in the first tab will be organised based on the FITS header keyword IMAGETYP. As soon as you click on the image  Analyse and organise button, dark and flats and flat-darks/bias images will be moved to the corresponding tab. If the button is pressed are also the images are analysed for HFD, background and other details.

1. Light filter classification: If classify by light filter is check marked then the stack routine will combine the available filters to a RGB image. If only Red + Green +Blue image are available they will be combined in a RGB image. If Luminance images are available it will first stack the RGB colors and then apply a most-common-filter and Gaussian blur on the RGB result. Finally the luminance image is coloured with the RGB result. The filter factor should be set typically near 20. The filter names can be set in tab alignment
2. Dark classification: The dark tab can contains multiple master darks. To select automatically the best compatible master set the check-mark for exposure and or temperature.  If set the master dark with compatible exposure duration and or temperature is automatic selected. If more then one compatible dark is found then the dark with nearest date is selected. Master darks with incompatible dimensions are ignored. 
3. Flat filter classification: The flat tab can contains multiple master flat made with different filters. To select automatically the best compatible flat set the check-mark for classify flat on filter. Master flats with incompatible dimensions are ignored. 
4. Object classification: Several image series of different objects can be stacked in one run. If the classify-by-object is check-marked, the program will stacked in groups based on the OBJECT header keyword value.
   

• HFD is the image median HFD. The lower the value, the better. The HFD value is depended on focus quality and guiding precision. Note that low values could be the result of streaks as a result of loss of tracking. 
• Quality of the image. The higher, the better. Based on the number of star detections divided by HFD. Depending on sky transparency of the sky and focus. Note that loss of tracking could results in a low HFD and low star count so a low quality factor..
• Star level. The higher the better. Depending on transparency of the sky and focus. Note that a high values indicate satellite tracks.
• Background. Depending on sky darkness and transparency. The lower the better. A higher value means in most cases a cloud was blocking the sky.
• Sharpness. The lower the better.Measures the change between dark and bright pixels. The measurement is very sensitive to satellite tracks. Could be used to detect satellite track when compared with HFD value. Could also be used to sort images of the Moon and Sun on sharpness.(but they can't be stacked) For correct sharpness measurement of OSC images either the FITS header should contain the keyword BAYERPAT or check-mark "Convert OSC to colour" in tab "Stack method".

Lights tab, Copy selected list to clipboard

This menu allows the export the listed FITS data to a spreadsheet. Sselect all relevant files and copy the data with right mouse button. Then copy the data into a spreadsheet for analysis.





Here and example of the data analysed in a spreadheet:


Back to index

Below an example of a nova and non-star detection in M101

Browse button: use this button to add  frames to the list. Frames can also be drag dropped on the form.

Replace check-marked by one or more master darks: This will replace the individual check-marked frames with a single master dark. The existing master darks will not be effected.

Classify during creation
If the list contains frames with different exposure durations and the option "classify by" exposure time is check-marked then for each exposure duration a different master dark will be created. Same applies for gain and temperature. If not classify check- marks are set then all loaded individual dark frames will be combined in one master dark and values are averaged.

Browse button: use this button to add  frames to the list. Frames can also be drag dropped on the form.

Replace check-marked by master flat: This button will combine the individual check-marked frames into a single master flat using the flat-darks loaded in the flat darks tab. So the flat-dark tab should be filled with frames prior to pressing this button. The existing master flats will not be effected. All individual frames will be combined in a master flat with flat-darks included. The flat-dark tab will be cleared after the operation.

Browse button:  use this button to add  frames to the list. Frames can also be drag dropped on the form.

After master flat(s) are created the flat-darks tab will be cleared.

The stack results are reported in the results tab. By a double click they can be viewed the viewer. The number of files and exposure times are given. With the pop-up menu it is possible to copy the image file path to the clipboard for use in a file explorer.
 

Calibration column  

This column will indicate if the stack was calibrated with a dark (D), Flat (F) and flat-dark (B).  The S stand for stacked. So ideally it indicates DFBS. Calibration stauts is stored behind keyword CALSTAT.

Issues column

Possible dark frame issues are D_temperature, D_exposure, D_gain. This indicates that the dark sensor temperature, exposure duration and sensor gain are different then the light frame values. Ideally light and dark frames should have been taken with the same sensor temperature, exposure duration and sensor gain settings.  

Possible minor flat frame issues are FD_temperature, FD_exposure, FD_gain.  This indicates that the flat-dark sensor temperature, exposure duration and sensor gain are different then the light frame values. Ideally flat and flat-dark frames should have been taken with the same sensor temperature, exposure duration and sensor gain settings.  

Issues between the darks and lights are more important due to the low light conditions for lights.

These issues are stored in the FITS header behind keyword ISSUES.

The best stack option is "Sigma clip average". For only 2 or 3 images or when you are in a hurry or for testing "average"will do.
    

• 1) Stacking of image of a grayscale camera or raw images of a DSLR camera.  Stacking in either grayscale of colour goes fully automatic. The program will detect the type of images. Stacking in colour can be forced if the BAYERPAT keyword is not found in the FITS header.
• 2) Stacking of (L)RGB images made with seperate filters. This mode is activated if option Classify by "Light filter" is checked.

• AstroC, colour for saturated stars, similar to the bilinear method but for saturated stars the program tries reconstruct the star colour. Select the range which matches with the value of brightest stars.
• AstroM, white stars, similar to the bilinear method but if there is an inbalance between the 4 red, 4 blue or 2 green pixels it uses luminance only. Effective for unsampled images and stacks of a few images only. Star colour is lost if undersampled but stars will become white.
• AstroSimple ©, each R,G, G, B pixel colour information is used in a 2x2 pixel range. Simple but very effective for astro images. Works best for a little oversampled images. Stars have very few artifact if any. 
• Bilinear, a basic demosaic method using the colour information from a 3x3 pixel range.

      AstroSimple is © Han Kleijn, www.hnsky.org, 2020. and licensed under a Creative Commons Attribution 4.0 International License.
 which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.                       

What to select:

• In general de-mosaiced OSC astro images are suffering from colour artifacts due to the small size of stars and pixel saturation. If the pixels iluminated by a star are saturated, the red, green and blue values will have the same maximum value and the star centre will appear white. In most case this can be avoid by taking short exposures of 60 seconds or shorter.
• Best results are achieved with de-mosaic methods AstroC and Simple. 
• In most cases the option "Auto level and colour smooth" is required for the correct colour balance and colour smooth. First the three colour channels are adjusted to make the background colour neutral and the stars average white. Secondly the bright stars are smoothed. Both actions can be done manual in the tab pixel math, option "colour correction" and "smart colur smoothing"
  
• If the images are under-sampled and the star colour is random after stacking, use AstroM, white stars. Stars will be whiter. Star colour will be lost. 

• LibRaw (full active area)
• LibRaw  (Cropped active area)

• A + C:   The Active Area, which is the largest area from which a useful image can be formed.
• C:   The Crop Area, which is the subset of the Active Area which many raw converters convert into a useful image. The main reason why C is smaller than A is to provide some extra pixels all around for a raw converter's demosaicing algorithm to use.
• M :  Masked Area, used by some cameras, especially Canons used a dark.

    ||      , button to stop blinking cycle.

   ⯈       or      ⯇    , to start a continuous un-aligned blinking cycle. This is intended to find visually outlier images where guiding has briefly failed.

Activating (L)RGB mode:


The filter names should match the filter names in the FITS headers. This is the case if in tab Lights the filter column is displaying red, green, blue or gray icon. If there is no match the icon will be a question  mark.



Stack method tab, Image stiching method (Mosaic)

Astrometric image stiching is possible with the internal astrometric solver. The reference of each pixel is the astronomical position. So stacking is not done against a reference image but against an position array set by the first image. If if image stitching is selected the SIP option in tab alignment will be activated. This will allow for correction of optical distortions.
 .

Here a suggested work method:

1. Stack the tiles separately using method "SIGMA-CLIP-average" and use for the alignment the internal STAR alignment method.  Inspect the resulting tiles and crop them if required. You can also crop them later automatically with "Mosaic skip outside pixels" Do this for each color separately if you have separate files.
2. In tab  "stack method" select option "IMAGE STICHING METHOD" and select astrometic alignment using either the internal solver.
3. In tab "stack method" check-mark the option "equalise background".  If the input images have poor borders, set option crop images larger then 0%.
4. Select the files. Most likely the files names contain "_stacked, so you have the check-mark the files after selection.
   
5. Click on the button   Stack check marked images|  
6. Crop the stacked result using the image crop option in the viewer mouse pop-up menu.
7. Adjusted the stretch range and save as JPEG, 90% quality.

Possible error message:  Abort!! Too many missing tiles. Field is 11.7x1.3°. Coverage only 14.5%.
This could happens if you have by accident an image from a different series in the collection. The program tries to adapt the canvas to include the outlier but it will be too empthy.  Check the positions of the images to stitch and remove any outlier.
Here an example mosaic x 4 of M31 made with ASTAP:





Here an example of a mosaic build of DSS images:

The size can be reduced by a crop function (right mouse button) later. Making the oversize too large could result in memory overload.



If you have  DSLR/OSC sensor and using a monochrome filter like H-alpha, you can split the raw the images in seperate R, G, G, B  image using the viewer Tools, Batch processing, Raw colour seperation menu. In case of H-alpha use only the R=red image for future processing.



The alignment menu tab:

For alignment there are four options, internal star alignment,  native astrometric solver, manual alignment or ephemeris alignment. For mosaic building you have to use the internal astrometric solver.



 Internal star alignment



This internal star matching alignment is the best and fastest option to stack images. It is not suitable for mosaics. No settings required; fully automatics alignment for shift in x, y, flipped or any rotation using the stars in the image. It will work for images of different sizes/camera's with some limitations.

The program combines four close star detections into an irregular 2D tetrahedron or kite like figure (and compares the six irregular tetrahedron dimensions with irregular tetrahedrons of the first/reference image. It selects at least the three best matches and uses the centre position of the irregular tetrahedrons in a least square fitting routine for alignment.  The four star detections are called a quad. The six geometric distances are used to construct a hash code.

There are only three relevant settings, but normally you don't have to change them.

• Hash code tolerance  Quads matching tolerance. Default setting 0.007. Leave this at 0.007 or 0.005 unless you have severe optical distortion. If you have too many false detections then set this lower.
• Maximum number of stars. Number of star detections used to build quads. Default setting 500. In some cases like for images with a lot of hot pixels you could set it lower to avoid false detections.
• Ignore stars less then [HFD]. Setting for stack alignment to filter out hot pixels out of the star detection. Default value 0.8. Single hot pixels have a HFD of  less then 0.8. { 2*(0.5/PI())^0.5=0.798 }
  

The following image shows the selected quads. The six geometric distances between the four star detections form an irregular tetrahedron and will be used as a hash code:


The matching process is described here Background info, how does the ASTAP astrometric solving works internally


Alignment tab, astrometric alignment.

• Field height:  This is the square size of the star field in degrees used for detection and will be set automatically for most FITS files. It should equal to the image height in degrees. If unknown, you could try initaly the option "auto".
• Radius search: Search radius in degrees. If there is no match, the program will move the search field around in a square spiral and increasing the distance form the initial position up to the radius specified.  A radius of 30 degrees could be searched in a minute.
• Ignore star less then ["]  Any star with a HFD below this value will be ignored. This will filter out hot pixels. Default setting 1.5".
• Star database used:  If you select "Auto" the following logic will be used:
  
  FOV>20°  ==> if exist W08
  ELSE
  FOV>6°   ==> if exist G05
  ELSE
  FOV<0.5° ==> if exist D80
  ELSE
  ==> if exist V50, D50, D20, D05, D80, G05
• Downsample:  For large images, the downsampling will speed up the solving and increase the signal noise ration of the stars. Default position is 0 equals auto. Any image with a height dimension above 2500 pixels will be binned 2x2. Also colours are combined to monochromatic so this option is beneficial for colour images. Avoid too much binning. Resulting height should be 960 pixels or higher. In mode downsampling auto=0 any pixel scale less then 1"/pixel is binned to get a larger pixel scale. This is beneficial for the typical astro images but for images taken by a telescope high on mountain with superb seeing you might choose a fixed downsample factor.
• Maximum number of stars to use:  Number of star detections used to build quads. Default setting 500.  If the database density is not sufficient the maximum number of stars will be reduced automatically.
• Hash code tolerance:  Quads matching tolerance. Default setting 0.007. Leave this at 0.007 or 0.005 unless you have severe optical distortion. If you have too many false detections then set this lower.
• SIP coefficients:  Using this option the solver will add 3th order SIP polynomial coefficients to the header to cope with image distortion. This option is not relevant for stacking since the distortion for each frame will be the same. It is only important for positional astrometry.
• Use triples. Experimental option for images with a low star count only. Default unchecked.
  
• Slow. This will force a 50% overlap between search fields. Default unchecked. Use this in rare cases where solving fails while still many star are detected. If applied this will slow down blind solving.
• Solar drift compensation  This is a special option to stack aligned on minor planets or faint moons. The stack process will align on the stars but with a angular movement correction you have to enter theα and δ rate in arcseconds/hour. Using the date of observation, the solar object will be sharp in the stack while the stars will form streaks.  This allows imaging faint objects especially faint moons where no ephemerides are available. For common solar objects you better use the option ephemeris alignment.  Angular movement rate can be extracted from JPL Horizons using the custom output option "Rates; RA & DEC". The rates from JPL Horizons have to be entered without any modification.

• Select an alignment star in the first image.
• Select all images in the Lights tab.
• Select from the pop-up menu of the  Lights tab "Auto alignment star for selected images"
• If it stops halfway, then it doesn't lock on that image star halfway. Select manually in that image the same star and try again or continue with the next images.

• Configure and test the viewer Asteroid & Comet annotation menu shortcut CTRL+R

• Select ephemeris alignment
• Browse for all the images in the image tab and add if available the dark, flats.
• Press the button  Analyse the selected file in the tab images.    
• Select from the combo box the asteroid or comet to align on. See screenshot below. If no objects are listed then check if there is a comet and/or an asteroid database available. See the Asteroid & Comet annotation menu, shortcut CTRL+R') If still none, then increase the limiting count and/or limiting magnitude in the same menu.
• Hit the  Stack button. 
• After the stacking is finished it is possible to annotate the result.  

1. Get images of a standard field made with B, V filters or DSLR camera.
2. Load them in the ASTAP Photometry tab.
3. Select in tab photometry for AAVSO annotation either the local database or the online standard field like "Online DB mag 13 std field".
4. Select in the Photometry tab for the Gaia comp stars "Online Gaia transformed". This is required for the blue and red filter images. The local database contains only the Johnson-V magnitude.
5. In case of a DSLR images use the photometry tab popup menu to split them in TR (red),  TG (green) and TB (blue) For DSLR cameras it is important to keep the stars a little out of focus since red and blue sensitive pixels are only one-quarter of the pixels.
6. Calibrate the images with dark and flat & flat-dark if possible.
7. Press the transformation button. If required the image will be solved. Check the found coefficients.

1. Load into the photometry tab  images made with B, V (or R) filter.
2. Select in the photometry tab for the AAVSO annotation online or local. The local DB is only suitable for  B & V  or TB & TB measurement. For R or TR use the online version.
3. Select in the photometry tab for the Gaia comp "Online Gaia " or local V50. The database selection is not essential unless you choose later Gaia ensemble as comparison. 
4. In case of a DSLR images use the photometry tab popup menu to split them into TR (red),  TG (Green) and TB (blue) For DSLR it is important to keep the stars a little out of focus since red and blue are only captured by a 1/4 of the pixels..
5. Select "measure all in the photometry tab"
6. Press the play button. The imaged stars are now measured. The listview will now contains the instrumental magnitudes based on the star database. The values are reasonably correct using the star database for a first correction.
   
7. Press AAVSO report.
8. Uncheck "Gaia ensemble". This will allow in the final report a final correction of the instrumental magnitudes using the selected comparison stars.
   
9. Check "transformation"
10. Select variable, comp stars and check star.
11. Press either "Report to file" or "Report to clipboard". 

It is important to test your setup. In the AAVSO report window there is a button  "Test mode" . Use this to test the transformation process and achieved accuracies.

In the report windows the difference between the measured and documented magnitude of the check star are reported. Also the error after transformation of the check star. This transformed check star magnitudes are not reported in the report and only reported in the window below. You can see that in the example below using not so good interference filter that the check star error reduces from about 0.1 magnitude to 0.002/0.004. For the test an image series of a standard field was used.  If the check star transformation works, then the transformation of the variable will work.

Version 2025.10.18:

Below is an example of the available variables in one image:

For a standard deviation so noise calculation  either subtraction or addition you have to combine the uncertainties quadratically. The reason that a star ensemble has a lower standard deviation than a single comparison star is that the signal is averaged. So the total noise of an ensemble of four equal stars would be  σ_total= ( (0.25σ)² + (0.25σ)²+ (0.25σ)²+ (0.25σ)² )^0.5  equals  σ/2.   This doesn't happen when you use the ensemble m₀ value for measuring the check star.  The standard deviation of the check measurement result is higher and equal to σ_{tota l}= (σ²_check + σ²_ensemble)^0.5

The tolerances shown in the graph are 2.5σ_check

Annotations could contain the following extras:

• "[At Max]"  Indicates the variable is near its brightest epoch (within 5% of the calculated cycle period).
      Note: This feature only works with the online database, as it relies on epoch data unavailable offline.
      Testing status: Preliminary—if the calculation aligns with observations, please report it.
• "Bad!"  Indicates saturation—the star’s brightness exceeds the detector’s limit, causing flux loss and invalid measurements.
• "#"  Indicates the variable star lacks an AUID (AAVSOs Unique Identifier). To report observations, request an AUID from the AAVSO first.

• Date/time (start) in YYYY-MM-DDTHH:MM:SS. This is date & time from the header keyword DATE-OBS. This is normally the universal time at the start of the exposure.
• JD (mid exposure) This is the Julian Day of the exposure midpoint. This time is calculated from the keyword DATE-OBS and half of the exposure time is added.
• HJD (helio) Heliocentric Julian Day at the exposure midpoint expressed in UTC. This is the event time as seen from the Sun centre, compensating for the maximum ± 500 seconds time difference depending on the positions of the Earth, the object outside the Solar system and Sun. For plotting purposes only the fraction is displayed.

• CV   (clear view
• V or G or TG    (Johnson-V or Bayer filter green),
• B  (Johnson-B)
• RC (Cousins-red)
• SI (Sloan i')
• SR (Sloan r')
• SG (Sloan g')

The usage is as follows:

• Prepare a series of short exposure images with different focuser positions and a lot of stars. Set the exposure time to a few seconds. Move for each image the focuser a small fixed step but only in one direction to prevent backlash problems. Images with stars having an HFD above 20 will not be analysed correctly.
• Browse with ASTAP inspector tab to the images.
• Press curve fitting.
• The difference between the focus point of each image area will be reported in focuser steps.

:

Viewer,  file menu, thumbnail viewer for FITS files

ASTAP has a FITS thumbnail viewer (ctrl-T). This could be useful to browse your FITS files. By clicking on the thumbnail it will be opened in the viewer. With a right mouse button click some options are available as changing directory, copy, move, rename or rename to *.bak.

The thumbnail size is dependent on the form size. Make it larger, the thumbnails will follow. Thumbnails are organized in 3*X. So the thumbnails are pretty big by purpose. The images are fully loaded in memory so it will consume some memory and time. So don't try to get thumbnails of 400 images.




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position in viewer and enter the object name. The position will be retrieved from the database. This position will be used as a start point for the solver.

Viewer, Tools, Batch processing:

With the batch routine several FITS images can be "astrometric solved" or converted. This conversion is not required for ASTAP. Automatic conversion is integrated in the menus.

Viewer, tools, Image inspection

The following will be reported:

• Image median HFD which is an excellent indicator of the quality of focusing. The lower the value the better the focusing, the sharper the stars are. The value is also dependent on the astronomical seeing and the quality of the optics. If the image is solved it will also be indicated in arc seconds.
• Sensor tilt as the HFD difference between the best and worst corner median values. In addition as an graphical indication it draws an trapezium in the image based on the four median values.
  There can be some variation in images of the same series. Any tilt equal or less than 10% of the range is a good value. Any tilt equal or more than 20% of the range indicates a tilt problem.  
• Off-axis aberration as the delta between the HFD value in the centre area and in the outer areas of the image. The stars in the outer area are normally a little larger, oval or comet-shaped due to the optics or curvature giving an higher HFD value. The lower this value the better the optics. This value could  be a help to adjust the optimal distance between the field flattener and the camera. This measurement is only a valid if the focus for the centre area is perfect. A more advanced measuring method is available in tab CCD Inspector.

A system suffering from severe curvature:

A system with severe tilt:

The tilt scale is defined as follows:

None (0 to 5% tilt)

Almost none (5 to 10% tilt)

Mild (10 to 15% tilt)

Moderate (15 to 20% tilt)

The triangle based tilt indication is based on the following three areas:

Normally the tilt indication uses stars with a signal to noise ratio (SNR) of 30 or higher. For extra stars it will reduce the minimum SNR to 10. Then the HFD values will be a little less accurate. Extra stars is intended for short exposure only where there are not enough stars with a SNR of 30 or higher.

Data export



If the option "data to clipboard" is checked then all available data is copied to the clipboard. If the image is solved the α

fitsX    fitsY  HFD    RA[°]    DEC[°]    ADU    Magnitude

1877.70  17.97  3.843  274.093663  -14.400684  229858  13.424

 267.96  19.45  3.472  275.332120  -14.498080  141018  13.955

1172.71  18.24  3.040  274.635894  -14.444421   71522  14.692

2161.77  18.96  3.548  273.875327  -14.381897  167485  13.768

 222.94  19.92  3.075  275.366797  -14.500390  218724  13.478

 785.63  21.10  3.047  274.933882  -14.465880  169184  13.757

1392.94  22.41  3.522  274.466757  -14.427718  135043  14.002

2065.31  22.09  3.350  273.949687  -14.385719   87597  14.472

  This unroundness measuring principle is licensed under a Creative Commons Attribution 4.0 International License. 

This tool writes the median background values as numerical values in the image. Stars will be ignored but nebulae will influence the background measurement..

Viewer, Tools, SQM report based on an image

• The Unihedron is an absolute measurement of the sky glow. The ASTAP measurement is relative to star light measurement. In case of poor sky transparency ASTAP will report lower SQM values due to the higher extinction of star light.
• It is expected that the ASTAP measurement will be less sensitive to Milky Way light since it measures the background and not the total sky glow.
• The Unihedron measurement will be less sensitive to the blue part of the spectrum.

In the bottom left corner of the image an estimate for the limiting magnitude for point sources is reported using a detection limit SNR ≥ 7. Below this value detection of point sources becomes unreliable.

• Image clean up Ctrl+space
• Centre lost windows Ctrl+F12  Use this if you have multiple screens and once window is out of sight for some reason.
• Flip horizontal Ctrl+H
• Flip vertical Ctrl+H
• Zoom out  PgUp
• Zoom in     PgDn

1. Use the star database to measure the MEDIAN relation between flux of the detected stars and star magnitude from the database. (That's why solving is required and best results are achieved with the V50 Johnson-V star databases based on Gaia DR3) 
2. Measure the MEDIAN background 1 to 10 pixels wide outside the rectangle box. This median measurement will ignore stars in the field.
3. Measure the MEAN  flux inside the box.
4. Calculate the magnitude for the "inside mean flux" minus "median outside box flux" using the relation found for the stars.

 You could argue that a Johnson-V filter or green channel is required for the image but in practice the error is limited depending on the spectrum.

A MPC1992 template  for reporting asteroid/minor-planet positions to the Minor planet Center

COD XXX
CON Optional contact name. Additional contact details may then follow.
OBS Observer name(s), no programs
MEA Measurers name(s), no programs
TEL 0.2-m f/4 reflector + CCD
COM Long. 01 30 00 E, Lat. 53 00 00 N, Alt 10m, Google Earth
NET Gaia3
63343         B2018 09 10.54372 22 57 15.97 -07 38 51.9          19.76B      XXX
38826         B2018 09 10.54372 22 56 42.42 -07 50 37.1          18.19B      XXX

....

Show statistics: This menu will generate a statistics report of the image.

Example of an ASTAP report:

Online query:

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Other pop-up menus:

Search for an object position:

1. An approximate α and δ celestial position is specified (for a spiral search). For FITS file this position is normally read from the FITS file header and set automatically. The position can also be passed by the command-line from your favourite imaging program. In case you view an JPEG, TIFF image the approximate  position can be entered by a double click on α input to search for a known deep sky object position from the database.
2. Correct image height in degrees specified within preferably 5% accuracy.  See  ∑  window, tab alignment, group-box astrometric settings, "Field of view (height)". For FITS images this is normally automatic calculated from info in the file header. In case of doubt you could try "Field of view (height)"=AUTO
3. At least 30 stars are visible in the image. They can be very faint, barely visible in the noise. For large field-of-view (>1°) expose 5 to 10 seconds but for small field-of-view (<0.5°) expose longer up to 60 seconds and use the D80 star database.
4. The visible stars are reasonable round and the camera is in focus. You could verify the star detection by the CCD inspector or by  the  Test button to show quads  in the  ∑  window, tab alignment). Most stars should be detected.
5. Image dimensions at least 1280x960 pixels. Set downsample at 0 (=auto) and the program will select a downsampling factor automatically. Image height in pixels after ASTAP dowsampling should be somewhere between 1000 and 3000 pixels. ASI120MC camera image are problemetic to solve.
6. Image is not stretched, very saturated or heavily photo-shopped. The total exposure time could be hours (stacked) as long it is possible to separate the brightest stars from the faint by intensity.
7. Search radius should be set large enough. See  ∑  window, tab alignment, group-box astrometric settings.  You could set this at 30° or for blind solving at 180°.
8. Use downsample factor 0 (auto). See  ∑  window, tab alignment, group-box astrometric settings. 
9. If your image is full of hot pixels you could adjust the "Ignore stars less than ["]"  in tab Alignment. This is set default at 1.5" but could be set higher for a long focal length.
10. The maximum number of stars to use should be defined. Typical set at 500. See ∑ window, tab alignment.
11. Hash code tolerance should be defined. Typical set at 0.007. See tab alignment.
12. For a field-of-view less than 40 arc minutes (long focal length) it is recommend to install the D20, D50 or even the D80 star database.  See star database usability
13. If a global cluster fills the whole field, ASTAP could struggle to solve. Forcing option "slow" could  help.
14. Best input format is FITS, RAW or 16 bit PNG, TIFF. Images in 8 bit PNG, TIFF and especially in the JPEG format are disadvantageous for solving.
15. If your image has a strong gradient, the option "Equalise background" in tab alignment (or by command-line) could help.

To test if an image is solvable follow this procedure:

A) Load an image in ASTAP

B) Check star detection by hitting button F4. Only stars should be annotated and not the hot pixels. If hot pixels are annotated see E) Hot pixels seen as stars

C) Go to ASTAP settings by CTRL+A or Σ button. Select tab Alignment. Set ASTAP to blind solving by 1) in screen shot below. So "field of view"=auto and "radius of search area[°]"=180.  (Note that blind solving will only search within FOV range 10° to 0.3°)  

D) Hit the ASTAP solve button

If too many hot pixels are detected as stars then increase "Ignore stars less than ["]" value marked by 2) in the screen shot. The star annotation values shown after hitting F4 are indicating which value is you should use. More than the hot pixels HFD values but less than the star HFD values. Save settings by hitting the solve button.

HFD_MEDIAN=3.3

or with the -debug option

astap.exe  -debug -focus1 D:\temp\FocusSample\FOCUS04689.fit -focus2 D:\temp\FocusSample\FOCUS05039.fit -focus3 D:\temp\FocusSample\FOCUS05389.fit -focus4 D:\temp\FocusSample\FOCUS05739.fit -focus5 D:\temp\FocusSample\FOCUS06089.fit -focus6 D:\temp\FocusSample\FOCUS06439.fit -focus7 D:\temp\FocusSample\FOCUS06789.fit -focus8 D:\temp\FocusSample\FOCUS07139.fit

Select then tab "inspector" and hit the "hyperbola curve fitting button" to test the functionality.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

Installation of the external Astrometry.net solver:

MS-Windows:

Install a local copy of Astrometry.net (via ANSVR  or  Astrotortilla)   as the astrometric solver. Or alternatively if you have Win10, 64 bit Creation edition you use the new Linux sub-system

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The main reason why you have to use flat-darks or bias frames is that they provide a pedestal so a fixed value to the flat routine. The same pedestal value which is present in the flat. This is typical a pixel value of something around 500 to 1000. Without this value the flat compensation will not work properly, and the darker corners of the light frame(s) will be under compensated.

This can be visualised with a simple X, Y diagram. The green part of the value is due to the light exposure. Blue is the pedestal value. If the flat-dark is included (subtracted) the intensity at the corners of the sensor drops to 40000/50000 equals 80%. If no flat-dark is included, then the measured intensity drop will be 41000/51000 equals 80.3%. This will result in an under compensation by the flat routine and the corners of the stack will be a little darker than the centre. So use flat-darks or bias frames for best flat correction.

Some cameras behave a little weird in the first second. For those it could be better to use the same exposure time for the flat-dark as for the flat.  A bias is a flat-dark with an exposure time 0 or about zero. For most cameras you will not notice the difference between using either a bias or flat-dark. 

The dark temperature should be about equal as the the light temperature. Calibration with a master dark of a lower temperature gives a poorer result than calibration with a master dark of equal temperature. See test results below:

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