Now that Mars is coming to opposition, it's a lot of fun to
compare what you see through your backyard telescope to a
simulated image of the planet with features labeled. I'm using
XEphem's Mars viewer to do this, but I find that the (beautiful)
Global Surveyor shaded relief map that comes with XEphem isn't
much good for this, since the albedo features that one sees
visually are poorly correlated with altitude on the relief map.
There's an easy workaround that I've found works quite well,
which is to replace the stock Mars image that comes with XEphem
by an albedo-based image. In my distribution, the Mars map image
that I have replaced is here:
/usr/local/xephem/auxil/marsmap.jpg
In fact, what I find works best of all is slightly goofy, and
that is to replace the shaded relief map with one compiled
visually by backyard observers. That way it's particularly easy
to compare what one sees at the telescope with what one expects
to see at the telescope. (This also means that the seeing
convolution dial in the Mars viewer becomes fairly pointless, but
what the heck.)
Anyway, I found that starting from this image works pretty well:
I made the following hacks to this file to get it to work with
XEphem:
1. Increase image size to 2880x1440 (not sure if this is
necessary, but since that's the size of the original
marsmap.jpg file, I figured I'd better do it)
2. Rotate by 180 degrees
3. Convert to grayscale
4. Invert the color map
I then saved the file as marsmap.jpg, and then substituted the
original marsmap.jpg with the hacked marsmap.jpg file (after
saving the original, of course). Using the new map I find that a
comparison between what I see through the eyepiece and what
XEphem tells me I'm seeing through the eyepiece becomes rather
good.
Hope this is useful to other people...
Bob Abraham
PS. In case anyone wants to try this, here is a link to the
hacked version of the image (300K) to save you the bother of
making one yourself.
PPS. I seem to recall that XEphem used to come with a different
map... perhaps that would have worked better for this specific
purpose? It might be nice to have a choice of maps included...
may I suggest this be considered as a feature request?
