Photometric analysis

Thist of the exercise uses a slightly different way of using Midas, as a background process. After "startmidas", a display window is started. Any regular Midas command can then be given with the "domidas" Unix command.

A set of small scripts are available to simplify the usage of DAOPHOT (even further). Study the excerpt from the DAOPHOT manual made available. The full DAOPHOT manual is available on the local Web pages.

Note that in the object detection and PSF definition phase, we use the total average image, in order to have the best signal to noise ratio. Also, the PSF is produced from this image and used in all the subsequent analysis.

• The first step is to determine the point spread function. "preppsf" starts by detecting and measuring all objects in the frame. It then selects 30 PSF candidate stars. These are marked on the displayed image. "dao_sort" is then used to sort the output from the previous step.

• "mpsf" is used to extract and analyse the candidate stars. A set of fitting deviations are presented on screen and the data is written to (e.g) yaverwf2.lst. The programme marks with an asterisk stars that are unsuitable and also notifies bad stars for other reasons. You must then (in a separate Unix window) edit the .lst file and remove lines containing such stars. "mpsf" will also output a file with the detected stars close to each candidate star. Finally, it will create the first version of a PSF (.psf) file.

• "allst_nei" measures all candidate star members.
• "subnei" subtracts the neighbour stars in order to clean the PSF candidate stars.
• "mpsfs" is similar to "mpsf", but operates on the cleaned image. Again, residual values will be given for each candidate. Edit the ".lst" as required. "mpsfs" creates the final version of the PSF.

• "startdaomeas" starts the (sequential) measurements of the three averaged images. For each image, the analysis is done in a special subdirectory ("a", "b", etc). The result file has the suffix ".als". Study this file when available.

• The final ".als" files are uploaded to "ser" in the directory "linde/Lab1/Results". Please use a unique and descriptive filename.

Copy the files allstar.opt, daophot.opt and photo.opt from the directory $HOME/ser/linde/Lab1/bin to your work directory (PC/y, etc). Take a look at them and study what they do.

32	startmidas	U	Start Midas in the background, get window
33	domidas load yaverwf2 scale=-4,-4	U	Load the total average image
34	preppsf yaverwf2	U	Detect all objects, choose 30 PSF candidates
35	dao_sort yawerwf2	U	Sort the output file from the previous step
36	mpsf yaverwf2	U	Compute first iteration of PSF. Remove unsuitable candidates
37	allst_nei yaverwf2	U	Measure all PSF candidate neighbours
38	subnei yaverwf2	U	Subtract all PSF candidate neighbours
39	mpsfs yaverwf2	U	Compute final PSF. Remove unsuitable candidates
40	stopmidas	U	Stop the background Midas.
41	startdaomeas wf2 y a b c	U	Start the measurements.
42	Upload result files