Mars with dispersion correction

[ErwinvdVelden]

Hi everyone,

Here's my second Mars observation for this apparition, I did the first one with the usual 3x apochromatic Barlow but the camera couldn't cope with the atmospheric dispersion that goes with the current altitude of around 45 degrees, resulting in double edges and lineair artefacts.
Since I had good experiences with using a 30 mm Kellner eyepiece off-axis on Mercury, I was trying to find a simular solution on Mars, but needed a bit more magnification without using a longer extension. In other words, I needed a good quality 25 mm Kellner with a 24 mm barrel, which nowadays is very hard to find. Luckily this was exactly one of the last remaining 24 mm eyepieces in the shop, a Tasco brand made in Taiwan with good coatings and baffling!
This gives me a focal ratio of f/34.5.

There is a solid NPH visible plus a lot of haze near the terminator.

See

http://www.erwinvandervelden.id.au/2005-09-09-erv.htm

Cheers,

Erwin van der Velden

[h0ughy (David)]

very nice Erwin! you almost got the landers in that shot. Your patience and dedication is admireable! Fantastic!

[davidpretorius]

to the noobie, what is off axis?

great images!!!

[iceman (Mike)]

Very nice Erwin, it's almost the exact same CM as my shot from the other day.

I love the image scale, great shot!

[[1ponders] (Paul)]

Good to see you getting out and taking a few more images Erwin. Always a pleasure to see your work

[asimov (John)]

Nice image scale & details there. Great shot!

[ErwinvdVelden]

Originally Posted by iceman

Very nice Erwin, it's almost the exact same CM as my shot from the other day.

I love the image scale, great shot!

Hi Mike,

What amazes me about your first shot is the used focal ratio of f/15 and still get a great amount of detail. Most top imagers use anything between f/30 and f/55, although I've found that anything between f/30 and f/35 works best, depending on the seeing.

Off-axis projection means putting the camera not straight behind the eyepiece but a bit off-centre. With a simple eyepiece design like a Kellner this method introduces colour abberation, which can be used to compensate for atmospheric dispersion (the lower the altitude of an object the more different the blue light will be bent by the atmosphere than the red light, causing colour fringing). Used correctly it compensates almost entirely the dispersion, resulting in higher contrast images with less artefacts.

Cheers,

Erwin

[davidpretorius]

thanks erwin for explaining that for me, cheers!

[asimov (John)]

Thank you Erwin.

[asimov (John)]

This thread was bumped in honour, & a personal tribute from me. RIP Erwin.