astronomy, equipment

Orion Optics SPX – 6 Point Mirror Cell Mod

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I’ve recently completed the first stage of modifying the mirror cell to convert it to use 6 point supports instead of 3 points.

By modelling deformations of the 350mm mirror in PLOP, I get the following figures for the current 3 points:

Support radius = 123mm
P-V: 1.65 x 10-4mm (~1/5λ at 500nm)
RMS: 3.81 x 10-5mm (~1/13λ)

For a mirror with a zygo report of 1/12λ P-V, this is less than impressive!

By using equally spaced 6 point supports, these figures are:

Support radius = 110mm
P-V: 1.07 x 10-5mm (~1/46λ)
RMS: 2.37 x 10-6mm (~1/210λ!)

So it can be shown that the deformations are potentially over an order of magnitude better using 6 points. Interestingly, allowing PLOP to run for a 9-point cell for the same mirror gives a slightly worse result than the 6 point, and though the 9 point is much better than a 3 point it’s harder to adapt this cell into.

Orion Optics Mirror Cell with 6 point mod
350mm Orion SPX Mirror Cell with mod…

My modification adds three “U” shaped supports that attach to the existing cell parts (being attached loosely via an M8 bolt to allow them to rock a little). There are two nylon mushroom head bolts that screw into these parts which the mirror will sit upon (these are the black cross heads in the image).

I think I’ll most likely need some mechanism to stop the bars rotating (but this might just be a bit of tape), and I’ll need new lateral nylon grub screws as 2 of the three current ones are well and truly stuck!

astronomy, equipment

Refurbishing the Newtonian

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"Newt" Raytrace for SPX350
“Newt” Raytrace for SPX350

I’ve been working recently towards finally completing the observatory dome in my back garden (that I bought almost 4 years ago now…) – with flooring done and power connected, I need just some steps, finish security and I can start to setup.

Attention has turned towards the Orion Optics SPX350 that’s been languishing in my garage awaiting use. I’ve known that the current secondary mirror doesn’t offer anywhere near enough completely illuminated field – and so I’ve finally measured the location of the focal plane  and using the excellent “Newt” software (now available as a web based app) and determined the size of a new secondary mirror for it. Yesterday I ordered a new 89mm secondary from Galvoptics that I plan to mount in a new secondary holder from Protostar (and hoping I won’t need a new spider :-/ ). This should give me a 100% illuminated diameter approaching 20mm – more than enough to cover the KAI2020 CCD chip I use, and good enough (with flats) should I decide to try upgrading to a KAF-8300 based camera in the future.

SPX350 - Primary Mirror
Primary Mirror – warts and all.

More importantly, the primary is going to need some work to bring it back to its full potential (the original zygo report says it is ~1/12 wave P-V) as even after washing off the dead moth remains (!) and flies, the coatings look to be past it, and there’s a load of spider poo stuck to the surface as well. Here’s hoping that they will clean off well and it’ll recoat fine – and so once I have the new secondary installed, a trip to Newcastle-under-Lyme will be needed in due course to look to get the whole thing serviced.

More to follow.

computing, sysadmin

Debian Wheezy, Apache+FCGI+PHP; changes to /etc/mime.types and php5-cgi

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Here’s a potentially useful note for anyone upgrading to Debian Wheezy on a system that uses Apache2 + FCGI + PHP. If you’ve configured it to run using one of several guides (like these: https://community.x10hosting.com/threads/debian-apache-2-2-fastcgi-php-5-suexec-the-easy-way.148894/, http://davejamesmiller.com/blog/how-to-set-up-php-fastcgi-with-suexec-on-debian) then you might well be bitten by a similar issue to that reported in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=687307.

Previously, one could define config such as the following in an apache2 conf.d file:

AddType application/x-httpd-php .php

AddHandler php-fcgi .php
Action php-fcgi /fcgi-bin/php5-fcgi

<Location /fcgi-bin/>
SetHandler fcgid-script
Options +ExecCGI
</Location>

This would instruct Apache to use the handler “php-fcgi” to process .php files – with the “Action” referencing a wrapper held at /fcgi-bin/php5-fcgi (suitably aliased in the vhost). This all looks well and good and doesn’t appear to change between squeeze and wheezy (Apache is still at 2.2).

However, if you do a straight upgrade, you may find that your server starts serving out php files in plaintext (not only is your site down, but it’s a security risk as well with potential connection details listed in config files). In Wheezy, the php MIME types have disappeared from /etc/mime.types –  php5-cgi now includes two files (in /etc/apache2/mods-available) to try and correct the missing MIME type definitions. With php5-cgi enabled in the webserver, the config as follows is included:

<FilesMatch ".+\.ph(p[345]?|t|tml)$">
SetHandler application/x-httpd-php
</FilesMatch>

This sets the handler appropriately. With this set, Apache serves out the file as text, instead of using the relevant action “php-fcgi”  – the FilesMatch directive overriding the old config. The fix is reasonably simple – comment out the AddType and AddHandler in the conf.d file and change the Action line so you have:

Action application/x-httpd-php /fcgi-bin/php5-fcgi

In the case you just want sidewide php5-cgi with no suexec, then you don’t even need the above – in php5-cgi.conf in mods-available, just uncomment the last section of the php5-cgi.conf file – this has a similar “Action” directive to that above. I keep the above as I use suExec to run the fcgi processes under individual accounts (you’re unable to call outside of the suexec root, and it’s easy to repoint the fcgi-bin location appropriately in each virtualhost).

(Note that this type config appears also to be not vulnerable to execution of files of the type evil.php.jpg thanks to the FilesMatch directive in the module .conf)

 

Featured image adapted from work by W. Rebel (Wikimedia Commons)