Why do we wait for opposition to observe Mars? Answer it is at its largest
in the sky for us. Well ask any serious lunar observer to look at the full
Moon and you will get a funny look from them. They only usually look for ray
features or other bright features at this time. For observing difficult to
observe features they look for them when they are close to the terminator.
Mars at the moment is showing a terminator with a phase of about 90% at
this moment in time. Valles Marineris stretches about 60 degrees in length
around the surface and about 7 degrees in latitude if you include the
escarpments either side of the valley. Mars has an apparent diameter of
about 12 seconds of an arc on October 23rd and at about 02:30 UT the widest
part of the valley is close to the terminator.. At its widest 7 degrees
equals 1/25 size of mars. Mars is 12" in size the so Valles Marineris is
about 0.5" in size then. Using Dawes limit this means a 10 inch telescope
and above should theoretically image it. The feature will appear at the
terminator 37 minutes later each day after this so do make allowances for
this. I have chosen this date to start observations but you can start
earlier but deduct 37 minutes for each day from my time for the 23rd.
May I suggest that as you are trying to observe at your telescopes optimum
performance that usual precautions are taken, i.a. let the temperature of
the scope reach the outside temperature, the telescope is properly
collimated and you use the fastest speed that your camera is capable of to
try to catch moments of good viewing. I do not expect any naked eye viewing
as rarely does the viewing get below 1 second of an arc, but who knows with
mist at this time of the year you may get lucky. Try also using a neutral
density filter as well as the usual ones on Mars for visual and photography
to get contrast.
The dust storms should not be a cause for concern as these usually occur at
perihelion and the occurred in late May and shut down the explorers on the
Martian surface for a few months. So hopefully the Martian atmosphere this
as it is should not be an hindrance.
Finally do not forget flat field and dark screen to get the best from your
images.
For those of you on the east coast of the USA with big telescopes may I
suggest that on October 23 rdOlympus Mons will be close to the terminator
and the shadow may be discernable with a 20 inch telescope at 07:00 UT.
If all else fails overexpose Mars and use your planetarium program to
locate Phobos and Deimos and try to pinpoint them Phobos is 12.51 mag and
Deimos is 13.60 mag at this time. All my predictions are using Starry Night
Pro ver. 4.5, so if I have got my data wrong please let me know.
The reverse logic also stands for viewing after opposition. So observe now
and then at opposition and the again after and compare results. My own setup
is a Meade LX90 200mm with 2 times Barlow ,check out what Barlow mag you
require to obtain the image at your focal point is at your theoretical
resolving power. In my opinion it is not worthwhile going to higher than
necessary as the image will wander around on the sensor a lot more.