Old MOS News and Tips

Look here for old news items and interesting tips. These items were once on the MOS home page, but are now just slightly stale. We are keeping them here in case people still find the information useful.

Information on the spectral calibration lamps on the web !... Aug 14, 1998

We are in the process of installing on the web an atlas of ghe calibration lamps for various grisms with MOS (and OSIS). You are encouraged to go  here  for more on the subject...

A new version of the  MOS/OSIS Autofocus Procedure (CAF) - July 5, 1998

Implemented for the May/June run of MOS, a new CAF procedure has been installed for MOS and OSIS. While its basicoperation mode is not very different from the previous one, the observers should read carefully the text written in the various CAF windows which will pop-up after clicking on CAF in the menu bar.

Here are the main changes:


An autofocus on LAMA - Feb 13, 1998

Thanks to the efforts of CFHT staff members led by Jon Seerveld, the mask cutting machine, known as LAMA, is now equipped with an autofocus which will make the mask cutting much easier for the observers.

This new feature has been made available to the MOS observers of the February run. You won't need to stand up close to LAMA with one hand onthe focusing screw and your eyes on the TV screen trying to find the right focus at the right time. A videoscreen in the control room will allow you to control what LAMA is doing by itself while you are observing. Those with 100 slits per mask and plenty of fields will certainly appreciate that !

TOs certified as OAs on MOS - Jan 5, 1998

In last December (1997!), all the telescope operators went successfully through a check-out on MOS for their certification as Observing Assistants.

The main consequence for the observers is that they will receive more help and advice from the person operating the telescope while they are observing. The more you will share with them (the images you get, the questions you have, ...), the more they will be able to help you identifying possible problems which could impair the success of your run.

If you have a hard time preparing and cutting masks while observing, the OAs will be able to help you efficiently in the mask preparation and cutting, if the telescope operations allow it.

Some improvements for the MOS operations - Nov 26, 1997

Asking for a CAF sequence (automatic focussing using a bi-prism) will automatically

It will then avoid the risk of some time being lost if the observer is not careful enough.

The new Gumball is arriving ! - Nov 12, 1997

Many items have been changed in the Gumball (the calibration unit for the F/8 instruments), mainly for improving its efficiency when used with OASIS. Some of the planned features (etalon calibrations), not available yet, could even be useful for the MOS observers (especially in the blue).

As a consequence, all the exposure times for calibrations are different from what they used to be in the past, and the information given in the manual is no longer correct.

New exposure times are given in the quick reference sheets.

A new filter for MOS - Nov 6, 1997

For allowing the observation over a wide spectral range, and avoiding second order pollution, a GG455 Schott filter with an AR coating has been ordered. By using this filter, it will then be possible to cover a [450nm - 900nm] spectral domain with a minimal impact of the second order on the observed spectra.This filter arrived on Nov 5, 1997 at CFHT, well in advance for the 1997 November/December MOS run, and will be made available after the necessary checks to be performed before the next MOS setup.

An example of second order pollution - Nov 6, 1997

This figure shows a calibration spectrum made with MOS-STIS2 and O300, and points some of the second order blue He lines embedded in the first order lines of He and Ar...

MOS and second order - Oct 20, 1997

These last weeks, various information and comments circulated concerning the second order being observed on MOS (or not ?), with some confusion arising from the fact that recent OSIS observations with low dispersion grism gave undesirable second order images. As long as the spectral coverage of the whole instrument (telescope/filter/grism/detector) is larger than [l,2l] (l is a wavelength), one will observe the second order of the spectrum coming into its main first order image. STIS2 and a low or mid-resolution grism used withoutany filter will allow to cover more than just a [l,2l] interval, and then the second order will be here, even with a much lower intensity than the first one.

If you intend to observe an object on a spectral range larger than [l,2l] and want to avoid any second order parasitic image, you have to observe this object twice, splitting the spectral range in two, and using appropriate filters. If your domain is near [l,2l], a single exposure is possible but you need to get the right high pass filter which will cut near l... Check the CFHT filter list@. If you don't find anything, and if the domain you require seems to be of general interest, CFHT can purchase the filter. You can also bring a filter with you for your observations. It is unlikely to happen with a high dispersion grism absorber

Checking the object/slit alignments - Oct 20, 1997

In the Recipe 2: Summary of Steps for Long Slit or Multi-slit Spectroscopy (Chapter 5 of the manual), the order of the various steps to be performed for aligning the field and the mask could lead to a bad alignment if the observer doesn't check carefully the telescope status (guiding or not...). Why ? Because you leave the guiding mode while taking an image of the mask with the "halogen image lamp". And you have to be back guiding on the same star before performing the offset.

Two alternate sequences can be made in order to avoid this problem :