FIES Observing guidelines for using skew readout mode

Introduction

This is a manual for using skew readout mode for using high time resolution or observing faint objects with FIES. Skew readout mode uses pre-defined echelle order masks to sum over orders and along dispersion direction. This mask needs to be created only once (per night?), since the spectrograph is in a stable environment.

Creating a readout mask

The mask can be created at any time, with any pointing of the telescope. One has to use the object fiber for the creation of the mask. Currently (Oct 2006) the recommended fiber is the #4 med-res fiber.

Start up FIES BIAS, fserver and fib8server as normally.

log in to verushka
cd fies
startbf manual
- make sure that detector name reads: "Roy 1.5s". If not, the PROM is wrong and the SKEW mode is not available!
/home/software/fies/src/programs/fserver
/home/software/fies/src/programs/socket_fiber/fib8server [-noinit]
in fib8server:
- arm 7
- fiber 3
- lamp 1 on
in FIES BIAS:
- object halogen trace
- exp 0.5
in fib8server:
- lamp 1 off
analyse the halogen (passwords from the staff)
- ssh -X postprocess@elena
- cp /raid1/data/fies/FIpj090004.fits . (make a copy of the file to be safe!)
- make_maskin.py FIpj090004.fits 5k (5k mask, for other values try make_maskin.py without parameters.)
- scp maskin*run maskin*dat obs@verushka:fies/.
- exit
Done!

Observing procedure

Prepare the instrument, point to your target and position the target onto the fiber following normal observation instructions.

Observing with skewmode

Images obtained with skew readout mode currently still have the full width (2148 pixels), but they are truncated in height by the amount required to achieve desired resolution. For example, default resolution 10 000 images have size 2148 x 342 pixels. When done with preparations, go back to verushka.

In order to set up the controller variables and to load the mask into the controller, run the script you just transferred in FIES BIAS (example for resolution 5000):

run maskin5k.run

A lot of commands fill the command window, you can just ignore them.

An exposure script is used to get 3 bias frames before and after the exposure in order to get good bias subtraction. Edit exposure script skewobs to have correct observer, imagetype, object and exposure time. Original script template is in file skewobs.org and looks like this:
autosave+ obs Smith obj Skew predark imt calib mdark 0 3 obj Skew B Cep 5k imt sci exp 12.5 obj Skew postdark imt calib mdark 0 3 autosave-

Take exposure of the star with appropriate exposure time:

run skewobs

When you are finished with skew mode observing, turn off skew readout mode:

run skewoff

Which looks like this:
@IMTX1 @MSEE reset

and will set the controller parameters to their default values and resets the CCD to full frame.

Data reduction

Since the spectra are already extracted on-chip, normal data reduction packages do not really understand the images. For this there is a IDL task skew_extract. Available here, REMOTE SITE! Transfer your images and the maskin.YYMMDD.HHMMSS.BB.dat file to your favorite machine. Run the IDL task:

idl -vm="do_skew.sav"

Program first asks for the name of the image to be processed, second it asks for the name of the mask file.

This program creates a FITS file which contains the extracted spectrum. Now the width of the image is the same as the number of orders in the spectrum (on FIES CCD one can fit 79 orders), and the height is the same as in original image (171 for 5k). While running, the program will also do the bias subtraction, scattered light subtraction, wavelength calibration and plots all of the orders.

Known bugs

Probably many since this is still in progress.

Last modified: April 19 2023