FITSalize: a software-tool for deformation, SQM and focuser temperature gradient analysis

[inFINNity Deck]

Especially to those among us that do unguided imaging a new and free software-tool I recently created may be of interest: FITSalize.

When doing unguided imaging stability of the set-up is of the essence. Question is how one can properly assess the stability of a tripod or an observatory and distinguish flexure in the imaging train from deformation of the basis of the set-up. From the images themselves it is impossible to assess whether elongated stars are the result of tracking errors or due to stability issues.

FITSalize is a command-line tool that runs under JAVA and uses ASTAP to plate-solve the images taken and to determine their SQM values. It is built to analyse FITS-files from a stationary scope (i.e. a scope firmly attached to the pier or base of the mount and pointing in a fixed direction) and to convert the images to accurate alt/azi coordinates. Being taken with a stationary scope the alt/azi coordinates should remain unaltered during the imaging period. Any deviation may indicate issues with the stability of the set-up. As these plate-solves result in J2000 RA/DEC coordinates they cannot be directly used to analyse stability. FITSalize corrects for precession to produce mean JNOW RA/DEC coordinates from the J2000 RA/DEC, then corrects for the effect of nutation in longitude and obliquity of the ecliptic and for the effect of annual aberration to find the apparent JNOW RA/DEC. These are then converted to alt/azi and stored (with all intermediate results) in a .csv-file.

The algorithms in FITSalize are based on J. Meeus, Astronomical Algorithms, (Richmond (VA), 2005) and have been compared to results from the SOFA-library (many thanks to Massimiliano Chersich for testing this, he also initiated this type of measurements last year at the 10Micron forum) and with the algorithms in Han Kleijn's ASTAP and Stellarium (only Stellarium does not apply annual aberration to the calculations).

Using your favourite spreadsheet (examples are provided) graphs can be produced to visualise the stability:

Above example shows the stability of InFINNity Deck (my observatory) over a period of approximately six hours. Deformation in altitude was less than 4 arc-seconds, in azimuth less than 2 arc-seconds.

In addition to deformation measurements FITSalize can also be used to plot SQM-values against alt/azi. For this regular FITS that were acquired to image a target are analysed:

In this example light pollution affects the SQM-values in the north-east of the observatory.

From those same FITS the focuser gradient and intercept can be calculated:

For more info and download of the software see the FITSalize-page on my web-site.

Nicolàs