EQAlign

"Who affirms that the money does everything, ends up doing everything by the money"
Voltaire

Improving Pointing


There could be various reasons for a poor pointing accuracy even if you have done a good polar align:

- Mounting fabrication errors (i.e. axis non-perpendicularity)
- Bad collimation between scope optical axis and mount axis
- Erroneous scope synchronization.

For the first two issues there is a good work of Toshimi Taki (“The Matrix Method for coordinates transforms” pp 21 and further)

For the last issue we can use astrometric processes to determine the exact equatorial coordinates of an image centre taken on-line.

Of course there are many commercial solutions for that two problems, but our goal is to implement both solutions into GNU/GPL open source EQAlign.

EQAMatch


EQAMatch is a C# implementation of Michael Richmond’s program for matching star list http://spiff.rit.edu/match/ Many thanks to Michael for his permission to use his solution!

EQAMatch compute the exact coordinates of an image centre, matching all the image detected stars into the UCAC2 cathalog. The user must know the approximate centre coordinates, the scope focal length in mm and camera x/y pixel size in microns (the program will take into account non-squared pixels)

EQAMatch will extract a star zone from UCAC2 catalogue and will perform a spiral like search until the catalogue stars and image stars centroids match. With this matched pairs EQAMatch will compute a transformation between image coordinates and equatorial coordinates, so that it is possible to know the exact image centre equatorial coordinates, that is where the scope is exactly pointing to. User can select into linear, quadratic, or cubic fitting algorithms.

EQAMatch is just a test project before it can be fully implemented into EQAlign. A cheap wide field cam as QHY5 or even a still image from a DLSR camera should be great for performing an exact scope synchronization. This will come into a better further goto commands.

You need to download UCAC2 catalogue into your hard disk. It fully length about 2Gb should not be a problem for our current laptops.

EQAMatch is a C# implementation and it needs Microsoft Framework 2.0. A Linux implementation for EQAlign is into our tasks. But please, be patient: no body (nor Microsoft) pay us for our work and we offer it freely.

You can download EQAMatch windows installer here (Last edition date: Sep 8, 2008)
You can download EQAMatch Source code here. (Last edition date: Sep 8, 2008)

Quick guide:

First of all it is recommendable, but not necesary, to download UCAC2 catalog to a local drive. You can download UCAC2 catalog from the web of our friend Juan Antonio Henríquez Santana.

1) The first folder will show a sky projection for the selected observatory site:

- Select you observatory site (you can add new observetory sites from options->Observatories)

- Select the suitable scope controller, for example ASCOM (you need to install ASCOM platform 2008)

- Select a target by typeing its equatorial coordinates, or by clicking on sky projection.

- Push “GoTo” button.



2) On second folder, you can see image capture properties and some search parameters.

- Select a camera from the detected cameras combobox.

- Select a proper exposure time; between 10 an 20 secconds should be enogth, depending on the camera sensitivility.

- Select proper focal lengt and camera pixel size in microns. Depending on selected cameras, the camera pixel size will automatically be changed when selected; if you select a webcam you must verify that data.

- Push “Single” button for a image adquire.

- You can leave the rest of parameters at default values. “Star detect/Search” group: if “Spiral search” is checked, program will search in an spiral-like into the cathalog around initial target coordinates. The limit of search in degrees will allow a limit for “Search limit (degrees)”. EQAMatch will use “Max star radious (píxeles)” value for exclude wide illuminated areas as star. You can vary “Star light/shadow threshold” for detecting more or less stars.

Alternatively you can open a FIT, bmp o jpg format image already stored in hard-drive. If it is a FIT format image, EQAMatch will try to extract obs.date, camera pixel size, scope focal lenght and image centre coordinates from image info-heather.


Now you can push “Solve” button at main tool bar (the calculator icon).


3) If EQAMatch could find the exact solution, the equatorial coordinates contol at first folder will be actualized with the computed image center coordinates. You can push “Sync” button for scope syncronization, following slew-commands will be improved.


EQAMatch computes also camera angle, exact focal lenght for the given camera pixel size and the possible horizontal and vertical image flips.

You can leave the third folder parameters as default. Only at first time you should select the location of UCAC2 catalog. You can also select to ask for on-line catalogs like UCAC-B1, but the solution process will be slower. Remember that UCAC2 catalog is limited to less than 40ºN declination.

Next figure shows solved position and camera angle, into catalog model at third folder for the example:


You can locate source code at SourceForge.net.

You can learn more about matching stars lists at Michaels web or in the publication of Valdes et al., Publications of the Astronomical Society of the Pacific, vol 107, page 1119 (1995).

Thanks for reading and best regards!
Antonio Fraga May 2008