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Mars map for backyard viewing
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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? |
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Hello Bob,
what a greatful, just-in-time hack! Thank you very much! Now i'm waiting for clear sky's for my next Mars Observings... My suggestion: Put this tip and the image on the XEphem Contrib-Page Georg Graf Am Die, den 27.09.2005 schrieb Bob Abraham um 23:13: I made the following hacks to this file to get it to work with________________________________________________________________________ -- Georg Graf <georg.graf@...> |
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Thanks Georg, it works very well for me. I did find a snag that might
bite some people though: if the image is flipped top/bottom or left/right using the View menu, XEphem inverts the colormap, so the features look funny. I'm not sure if this is a bug in XEphem or not... in any case it doesn't affect me as the default orientation is what I use anyway, but it will bite people using telescopes with an odd number of reflections unless they use a version of the file with the colormap inverted. If I have time next week I'll try to check out why this is happening in the source code and if it's a bug I'll try to send along a patch. (Odds of me finding time to do this are low, unfortuntately, because between travel for work and a wonderful new baby at home, leisure time seems almost as rare as favorable oppositions of Mars). I've had email from a couple of happy people besides yourself who are using this for their own Mars observing, so I'm glad it's proving useful. It's surprising that there wasn't a tool to do this already --- at least I couldn't find one for Mac OS X or Linux. Cheers, Bob --- In xephem@..., Georg Graf <georg.graf@g...> wrote: Hello Bob,________________________ -- |
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I've uploaded a copy to my website at as the link above appears to have succumbed to digital rot. I performed the same routine on the original to generate the new marsmap.jpg file.
I can confirm that the "T/B Flip" does in fact invert the brightness, but the "L/R Flip" does not change the image. Regards, Cathal |
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Hi Cathal,
Thanks for the update. This blurred version is perfect for visual observers. Your URL has a bug however, just change .files to /files. I tried the image and it works perfectly for me in XEphem 3.7.7: T/B flips vertically, L/R flips horizontally as expected, the colors do not change. Thanks for using XEphem. Elwood |
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Oops - that's what I get for typing it out from memory and not verifying before reply..
Corrected URL: http://www.swibble.com/files/marsmap.jpg
There is still an issue with the view when flipping though. I'm also using 3.7.7, compiled from source and using the catalogues from my paid version, ?The colours don't change, but the bits that are dark in the normal view, such as Syrtis Major, become light when flipped top/bottom. Flipping left/right behaves as normal. Both axes flipped gives inverted brightness. -Cathal |
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Hi Cathal,
Ok, now I can reproduce this. In fact, it is a "feature" that applies to the original image also. Here is a snippet of source: /* swap dark-bright color map.
?* this is used to keep mountains looking like mountains when flipping t/b ?*/ static void swap_colors() ? For the original image, to my eye at least, when the image is flipped top/bottom the shadows make the craters look like mountains and vv. I've always had this inversion of relief, it's especially bad with lunar images. To remove this feature, the simplest thing to do is just add a return as the first statement in this function so it does nothing. Elwood |
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