Fringe killer filter in a mak?

[Ags]

I had my first look at Mars this year last night, and it was very mushy compared to the beautifully sharp views I got last time around. The biggest problem with the view was the low elevation, which leads to atmospheric refraction - there were very strong fringes of blue and red above and below the planet, to the extent that it was hard to make out even the polar cap.

Given that we are dealing with (atmospheric) chromatic abberation here, would my Mars viewing be saved this year if I bought a Baader Fringe Killer?

[alan potts]

I do not believe you will need or should need a finge killer in a Mak, I have one myself, the 180mm Sky Watcher. The affect on Mars is only because it it low in the sky, once it is higher the blue and red will go away. It is a problem that seems to vary from night to night, I have viewed Juptier when it is 10 degrees high and not seen it and also the opposite when it was as high as 20 degrees.

My advise is save your money.

Alan.

[AndyWB]

I remember reading about Atmospheric Dispersion Correctors, or ADCs, that could correct this. They sounded like something more for planetary imagers, though. http://www.skyinspector.co.uk/Atm-Dispersion-Corrector-ADC%282587060%29.htm . Not cheap though. Might be better to wait until it's at its highest, and hope for good conditions.

[Astro_noob]

I viewed mars this weekend through my 120mm f5 and although the image was poor and I could not achieve satisfactory focus I do not think there was any CA even at 120x. 600mm is far shorter than you 1800mm mak so I cannot see how CA could be apparent?

[rwilkey]

Fringe Killers are designed for achromatic refractors, I doubt if it would be useful in a mak, as others have said, save your money and wait until it gets higher.

[Ags]

You are all doubters, lacking in vision :-)

If my image is blurry because of atmospheric chromatic abberation, and I use a filter to cut out the extreme blues and reds, how can the image not be improved?

[Ags]

I tried out my theory tonight. I had a go at Mars unfiltered and with my UHC filter. The UHC is not great because it cuts too much light and it still passes a bit of far red (my UHC passes Ha too).

Without the filter mars was blurry and had blue and red color fringes. There might have been a lightening at the pole, and a lightish white spot on the equator on the eastern limb of Mars. There seemed to be hints of dark areas across the face, but sometimes they seemed to be in the north east, and other times in the south center...

With the filter Mars was much too dark (220x in a 100mm scope) and the focus point was shifted considerably. Focus was sharper and rippling less apparent - although the darker view might have made it harder to see these problems. The blue fringe was gone completely, and the red fringe barely visible. So the edge of the planet was much more well defined. I seemed to have higher contrast on some dark areas in the north east. I could not see the two white areas at all with the filter.

After a short while, I found the filtered view too dim and tiring and went back to the unfiltered view.

On the whole, I think a filter that cut the far red and far blue (as I think a minus violet filter does) but which passes far more light than a UHC might actually be useful.

[Ags]

I've put my money where my mouth is, and paid 15 euros for a Wratten #8, the same as a baader fringe killer but ¼ the price (ok, the baader does let through maybe 5% more green and red light). Hopefully I can try it out tonight :-)

I remember the first time I saw atmospheric refraction, on Venus a few years back... I thought my scope was broken!

[Ags]

Seeing was atrocious tonight for Mars (it was fine earlier on) but the atmosphere wasn't making colored fringes tonight, so I could not really carry out my experiment. I did not see any color difference in Mars when I used the filter, which is good - I didn't want the view to look unnatural. Nor was the view noticeably dimmer.

I had my sharpest moments on Mars when I was using the filter, but that was probably just fluctuations in the seeing.

[Peter Drew]

Always worth an experiment, otherwise how would we ever have known beetroot goes well with cheese.