Saturday, May 14, 2011

Learning to Swim

My son is learning to swim and consequently I am regularly spending time at the pool. I recently took a camera along.

Swimming Lesson
Swimming Lesson
Stefan Schoberth of is watching a student who is quite low in the water ...


... until he reaches solid "ground".

I like how closely Stefan pays attention to the children, creating a safe environment for them to experience water on their own terms. It seems that it's not just the children who are having a great time!

Swimming Instructor Stefan
Swimming Instructor Stefan

A word of advice: because a lot of intense sunlight passes through the giant windows the air temperature can easily reach close to 40°C (104°F) with high humidity. Cold metal or glas immediately collects a film of condensation. On the one hand this can quickly damage the electronics, on the other hand it is not really possible to take a picture through a fogged lens.

In order to avoid this, the camera and lens should have enough time to acclimatize and reach the ambient temperature. This is best done in an airtight plastic bag. I have had good results using Ziploc bags, such as Ziploc Double Zipper Gallon Storage Bags. Pick a size suitable for your camera. If you do not get the double-zipper bag, you can use two bags, one inside the other, to ensure that the content is really kept dry.

Friday, May 13, 2011

Uncurling Nascent Fern Spiral

With seemingly nothing better to do than to crawl around on my belly wrangling a tripod with a camera, I found the this little baby fern:

Fern Spiral
Fern Spiral

I find the hairy stem of the fern quite lovely and I really like the way the fern spiral repeats itself in the background.

Sunday, May 8, 2011

Tech Tip: Soving the need to Authenticate a "Print Operator" to Change Printer Settings on Mac OS X 10.6

On Mac OS X 10.6 (Snow Leopard) a non-administrator user can not change all settings of the installed printers. This is really annoying when a printer error causes the print queue to go on hold: the user can not resume printing when the problem has been solved. Instead, a dialog pops up asking for the user name and password of a member of the "Print Operators" group. Usually, that means a system administrator has to manually enter the credentials before the user can resume working.

The problem is caused by the fact that only members of the group "Print Operators" are allowed to control the printer. Normal users (i.e. members of the group "Staff") are not automatically added to the group "Print Operators". This makes sense in an environment where you don't want Joe Public messing with printer settings. It makes less sense in a lab or production environment where the staff can be trusted with the printers.

Fortunately, I found a simple solution here:

In, type the following:

dseditgroup -o edit -u ADMINUSER -p -a USER -t user _lpadmin

where ADMINUSER is the user name of an administrator and USER is the user name you want controlling the printer.

You will be asked for the password of ADMINUSER and then USER is added to the "_lpadmin" group, solving the problem permanently.

A word of advice to the wise: I do not know if adding a user to the group "_lpadmin" has any side effects. The users of the machines I administer are considered trustworthy. The only reason they do not have administrative privileges is to protect the machines from unintended damage. So giving limited administrative privileges is not a problem. This may be different in your environment.

Consider the implications carefully before making changes!

Background Information
It appears that the problems observed are not uniform: some machines always show the problem, some not at all, and some only for some users.

It looks like Apple changed the defaults some time during the life-cycle of Mac OS X 10.5 (I have been unable to ascertain when exactly). Users created before the change were all members of "_lpadmin", users created later are not members.

The best solution would be to tell CUPS (the printing system used in Mac OS X) to allow members of the group "Staff" to control the printer. To do so, the file /etc/cups/cupsd.conf section

    AuthType Default
    Require user @AUTHKEY(system.print.operator) @admin @lpadmin
    Order deny,allow

needs to be changed to something like

    AuthType Default
    Require user @AUTHKEY(system.print.operator) @admin @lpadmin @staff
    Order deny,allow

I did not try this myself. If you do, you may find cupsd.conf documentation useful. I would appreciate a comment telling about your experiences. Good luck!