Mercury 13 5 08

[thing]

I was out at dusk this evening just before the clouds rolled in. I though I would have a go at Mercury being as it's quite high at the moment and I'd never seen it before.

I started of on the Moon to get good focus then tried Saturn but it was still too light. I knew if I couldn't get Saturn then Mercury was out of the question. I hung around for 15 minutes then finally caught Saturn in my 40 mil before changing to an 8 mil Hyp. The seeing was fantastic, even though it was still light. It was noticeable that the wind was from the North so it was blowing colder air in. I pushed up to x400 and had steady views for 10 minutes or so, no moons seen at that time, still to light.

I then had a go at Mercury. It took me a while to find it, but it was just visible in the finder scope low in the NW. I centered up and immediately went to x400. I was rewarded with a slightly less than half phase Mercury. There was some CA around it which I thought was odd as I've never seen CA on this scope or eyepiece (8" Newt, 5 mil ortho and Ultima Barlow) but it wasn't making any difference to the view. It appeared as a white disc with no detail, probably around the same size as Mars is at the moment. I've never managed to catch Mercury before and it was quite a fascinating sight. I don't think it's an easy target to catch, especially given houses, trees etc around me so I was most satisfied that I'd actually clobbered the beast. It was still light at this time, Saturn was still not visible to the naked eye. I never saw Mercury visually, which I expected to do so that was a bit of a mystery.

Feeling like a dog with two tails I then went for Mars just to take in three planets that night. I found it in the finder again and went to x400 straight away. I was amazed to see the two 'eyes' even at this distance. The seeing was as good as I've ever seen it this evening. The clouds were starting to roll in by now so I had another 5 minutes on Saturn. It was starting to turn dusk now and Saturn was clearly visible to the naked eye. Through the 8 mil Hyp (x125) I managed to count 7 moons. A quick 5 minutes cloud dodging with the Moon ended the night. A shame because it's one of the best nights I've had for a long time. By the time I'd packed away it still wasn't fully dark.......quite an evening's 'daylight' viewing really.

And I've just realised this is in the wrong section, could a kindly mod move it to 'Planetary' please............

[Talitha]

(Sorry it took so long to move this for you.

Had to shut down and unplug yesterday.. T-storms moved in.)

Could the "CA" you saw be caused by the atmosphere? Bright stars put on a pretty colorful light show when they're close to the horizon.

If my western sky is clear tonight I'll try catching Merc's phase with the 90mm Mak. I've only seen the little guy once so far, and that was a number of years ago with the 11x70 binos.

[thing]

Could be but Sirius is always pretty low on my horizon during the Spring and it never shows CA.

[Astronut]

So it's not just me then. I too saw some chromatic aberration when I looked at Mercury at the recent Wiltshire star bash (12" newt + 13mm Ethos).

http://stargazerslounge.com/index.php/topic,24251.0.html

"Mercury - I swept the horizon after sunset with the binos and eventually found it as it slowly got darker about 10 degrees above the horizon, just above the treetops. I swung the dob around and slapped in the Ethos. It was very small and somewhat lost in the FOV but the half phase was unmistakable. I was a little concerned that chromatic aberration was apparent (red on one side, blue on the other - Darren noted green too) but the small disc was a pinkish colour overall. Adding the 2x Powermate didn't improve the view. I can't imagine the eyepiece is at fault so in the end we put it down to the low elevation but it had me worried for a while."

I'd really like to know what causes this as I'm not entirely convinced it's solely due to low elevation.

[thing]

Me neither, it seems odd that you saw it like that too, especially with a Nagler. I was using a Hyperion which aren't exactly cheapo. I can catch objects in the SE pretty low down, and as the seasons advance I'm always looking for the 'next' interesting object to be seen. I never see any CA on those objects. I can honestly say that Mercury is the only object I've ever seen through my scope with CA. Perhaps it really is red yellow and blue and all the NASA shots are in B/W.........

I've just had a thought, it might be something to do with the fact it's so close to the setting sun. I don't normally look for objects in the semi dusk in the NW.

[barkis]

The colour flashing is due to light coming through earths atmosphere when the object is low down.

Venus and Sirius are usually the prime examples. The effect is called Atmospheric Prismatic Dispersion

The two examples are very bright, and scintilate quite rapidly due to the atmosphere.

Reflectors do not exhibit Chromatic Aberration. You will only get this from Achromatic Refractors.

Well manufactured Apochromatic refractors are also free from CA.

Ron.

[thing]

So this is a sort of twilight effect Ron?

[barkis]

It could be labeled such Thing. As the bright object gets lower in the West, the light has to traverse more atmosphere. Sirius is unfortunately in a permanent low declination, and therefore is rarely free from this effect.

[thing]

I've honestly never sen it on Sirius. What made it stand out so much on Mercury was the fact that it was the first time I'd seen CA of any sort with this scope. Interesting explanation though, thanks.

Dave

[Astronut]

Thanks for explanation of atmospheric prismatic dispersion. I understand the principle ie. that colours would flash and change on a stellar object as the light passes through the atmosphere's layers (which act as a lens or prism) but as Mercury isn't a stellar object and shows a distinct disc in the eyepiece it doesn't explain why it appeared permanently red on one side of the disc and permanently blue on the other ie. the colours weren't flashing and changing from red through to blue - still not convinced :?

[barkis]

Perhaps because Venus and Mercury do have appreciable size, and therefore not a point source as is the case with Sirius, that the prism effect remains static or polarised. Polarised not indicating anything to do with poles of the planets themselves.

The explanation I gave for the phenomena, could only came from a book I surely must have read.

so I will attempt to resurrect that source of the information.

I can only reiterate that the effect has nothing to do with telescope induced Chromatic Aberration.

Ron.