Resolving Mars's Disk

[snabald]

So, I am using a 4.5" 1000mm reflector, last night I took it out for my fist test run with it. At 40x Mars still looked like a very bright point, at 111x, it still seemed like a bright point! This scope should be large enough to resolve mars as a disk, could my mirrors be misaligned?

I'm sure it was mars, I checked my astro software just before going out and I even let the scope cool down for an hour (while checking my astro software).

I expected to see a tiny orange disk, what have I done wrong here?

[cjl]

Mars is (surprisingly enough) actually pretty hard to resolve to a disk compared to, say, jupiter. A couple of nights ago, me and my dad were out observing and could barely resolve mars as a disk in a 10" scope at 175x. This is primarily because of mars's small size (smaller than earth), as well as that it is not that close to earth anymore (although it is still close enough to resolve as a disk theoretically). If you can resolve it to what appears to be a point, then I'm betting your scope is not the problem, as I can't think of a problem that would make a disk appear to be a point. If the seeing is good enough, I would try a little more magnification.

[snabald]

Mars is (surprisingly enough) actually pretty hard to resolve to a disk compared to, say, jupiter. A couple of nights ago, me and my dad were out observing and could barely resolve mars as a disk in a 10" scope at 175x. This is primarily because of mars's small size (smaller than earth), as well as that it is not that close to earth anymore (although it is still close enough to resolve as a disk theoretically). If you can resolve it to what appears to be a point, then I'm betting your scope is not the problem, as I can't think of a problem that would make a disk appear to be a point. If the seeing is good enough, I would try a little more magnification.
I gave it some thought, it was more of a very small blob that a point, it has been very cloudy lately and mars was only 50 degrees or so from the horizon, it may have just been bad seeing, I guess I'll have to wait until next September and hopefully I can get a disk, until then there is always Saturn! :)

[Kaptain K]

Mars won't really be worth looking at again until late 2007. Remember that by the time Earth gets back to where Mars was "good", Mars has gone over halfway around the Sun. It will take us more than another year after that to catch up again. Thus, Mars oppositions occur about 26 months apart. Also, because of the eccentricity of the two orbits, the 01/ 2008 opposition will not be as close as the 11/05 opposition, which was not as close as 08/03.

[Tog_]

I think you'll have a harder time next September. Mars reaches it's best spot to be seen every 25 months (or so). This is called opposition. It's when Mars is directly opposite the the Sun from the Earth. (Like a full Moon). Since this is the closet approach the two planets make to one another, Mars will have the largest agular size on these dates. This happenes about every 25 months. The most recent was October 2005. It should still be a good target for a few more weeks, but by next September, it will be on the other side of the Sun, and the smallest it will appear.

The first night I had my Meade ETX 90 (one of the first models with much that was usless) I went to a star party. It was in March of '97 I think. Mars was at opposition, Hale-Bopp was a naked eye object, and the Moon rose about 10 minutes ito a total eclipse. All in all, a good night for a star party. The ETX is a 90mm (3.5") by 1250mm Maksutov, and we were able to resolve Mars to a disk, as well as make out the darker areas.

The first thought that comes to mind is the mirror alignment. This will have a major impact on the sharpness of a planetary disk. Get Mars as focuses as possible in the center of the field, then slowly bring it out of focus. It should be round like a donut. If it gets teardrop shaped, or of it's brighter on one side, you make need tp adjust the mirrors.

Another may be where you're looking. What lies on the ground between your observing site and the Horizon in line with Mars? Some ground features, asphalt parking lots, apartment complexes, and so on will radiate heat and cause turbulence in the air above them. Air you need to see through.

Try making an aperture mask. This is a flat bit of paper or carboard with a round (as round as possible) hole cut in it that is the same diameter (or a litle smaller) than the distace from the edge of the secondary mirror to the inside of the tube. On a 4.5" this may end up being about 1" to 1.5" across. It will reduce the brightness of the objects, but it will also make the images a bit sharper, and with better contrast. In a small scope like this, it may not help all that much, but it's cheap and easy to do.

Edit to add a few letters that did seem to matter after all ;)

[Wolverine]

... until then there is always Saturn! :)

Don't forget Jupiter's back up in the predawn sky. ;)

[snabald]

Try making an aperture mask. This is a flat bit of paper or carboard with a round (as round as possible) hole cut in it that is the same diameter (or a litle smaller) than the distace from the edge of the secondary mirror to the inside of the tube. On a 4.5" this may end up being about 1" to 1.5" across. It will reduce the brightness of the objects, but it will also make the images a bit sharper, and with better contrast. In a small scope like this, it may not help all that much, but it's cheap and easy to do.


My scope came with a dust cover that has a 1.5" hole in it (and a small cover to go over the hole as well) now I know what this piece is for! :)

[aurora]

At 111x you should have seen a disk.

Are you able to get stars to focus to a point? If so, then Mars should look unstarlike, even though it is small.

Is your finder scope aligned accurately so that you are certain that what you were looking at was Mars?

[mickal555]

Probobly just bad seeing...

[Launch window]

From what I've seen with other people's scopes - Jupiter is much easier than Mars

for the red planet, you'll need very good skies for Mars, perhaps some Red/Blue filters, you need the planet to be closest to Earth ( opposition ) and you need patience

[Tog_]

My scope came with a dust cover that has a 1.5" hole in it (and a small cover to go over the hole as well) now I know what this piece is for! :)


Is the hole off center or right in the middle? I saw a Newt in a store once that had this built into the cover. It would be worse that utterly useless since the secondary would take up a huge fraction of the light path. If it's off to the side, give it a shot, just make sure that the spokes that hild the secondary are out of the path as well.

[Dave Mitsky]

By January 19, Mars will have have shrunk to less than 10", which is generally considered to be the minimum apparent size for worthwhile visual observation by amateur astronomers.

Dave Mitsky

[Dave Mitsky]

For the apparent size of each of the planets see the table at http://www.enchantedlearning.com/subjects/astronomy/planets/

Dave Mitsky

[Dave Mitsky]

Mars is (surprisingly enough) actually pretty hard to resolve to a disk compared to, say, jupiter. A couple of nights ago, me and my dad were out observing and could barely resolve mars as a disk in a 10" scope at 175x. This is primarily because of mars's small size (smaller than earth), as well as that it is not that close to earth anymore (although it is still close enough to resolve as a disk theoretically). If you can resolve it to what appears to be a point, then I'm betting your scope is not the problem, as I can't think of a problem that would make a disk appear to be a point. If the seeing is good enough, I would try a little more magnification.

The historical maximum apparent size of Mars is just a bit more than half of Jupiter's maximum size of about 49 arc seconds. Venus can grow to more than 1 arc minute (a maximum of approximately 66 arc seconds), or about 1/30 the apparent size of the Moon, when it's at inferior conjunction as it will be on January 13-14 (62 arc seconds). The planets are really pretty tiny as seen from Earth.

http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html

http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html

http://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html

Dave Mitsky

[snabald]

Is the hole off center or right in the middle? I saw a Newt in a store once that had this built into the cover. It would be worse that utterly useless since the secondary would take up a huge fraction of the light path. If it's off to the side, give it a shot, just make sure that the spokes that hild the secondary are out of the path as well.
It's off to the side.

and dave, thanks for all the info!!!

[snabald]

Just wanted to give you guys an update, on 1/24/06 after collimating my mirrors I was able to resolve mars as a disk, it was just a tiny red dot (did not look "point like") with no surface details, but a disk was clearly visible!

[aurora]

Congrats!

That's about all you are going to see now that Mars is getting smaller.

Mars is now a little more than 1 AU away from Earth, and is only about 9" angular size. Back in October - November it was around 20".

By comparison, Jupiter right now is about 35" and Saturn around 20" and the ring width is about 46".