A perfect example of severe Field curveture Coma and vignetting

Striker (Tony) · #1 19-07-2006, 08:25 PM

Hi Guys,

Gary Beal was kind enough to lend me a .5x focal reducer to test on the LX200R....I may be able to get an improvememnt over this with more spacing between FR and camera but I thought I would show an example for the purpose of those who dont know what Coma and vignetting is.

Coma/field curveture is all the outer stars pointing towards the center..means the field is not flat.
Vignetting is the dark outer background that surrounds the bright center image also surrounded by a circle..notice the real distinct bright center compared to the very dark outer background....these are both severe and actualy look a 100% better then the original.

On the other hand I was stocked at the image as my exposure where only 2 minutes and only 10 exposures in which case thats half the time exposures I normaly do with half the quantity....cant wait to find a suitable FR between .8x and .6x.

RB (Andrew) · #2 19-07-2006, 08:29 PM

Thanks for posting that Tony.
Actually if you just crop the middle you'll still have an awesome image.

Well done once again and can't wait till you find the right f.r.

ballaratdragons (Ken) · #3 19-07-2006, 10:05 PM

Hey Tony,

That's not Coma! That's what the stars are suppose to look like at Warp Speed travel!!

Also, sorry but I can't see the vignetting.

Great pic apart from the warp speed effect.

gbeal · #4 20-07-2006, 06:16 AM

See Striker, that's what you get for nothing. Now if you had wanted the anti-coma and field flattening parts as well, all you had to do was ask.

iceman (Mike) · #5 20-07-2006, 06:20 AM

Awesome shot.. your setup will produce some amazing images soon.

Lester · #6 20-07-2006, 07:50 AM

Thanks Tony,

the colour of the nebula is very nice.

Striker (Tony) · #7 20-07-2006, 08:13 AM

Hey guys the image wasn't about the eagle neb....this image has had nil calibration...no darks..no flats..no bias...just stacked..its all about showing the coma affect...if I new you were going to like the image then I would have done calibrations on it...lol

Quote:

Also, sorry but I can't see the vignetting

True Ken...depends on you monitor settings it may stand out more because of this...... it is a bit harder to see as I have darkened the image to reduce it.....here is a more obvious image Ken.....it's all about a bright center and darker edges which is what you dont want but taking flat frames will improve this.

Lester · #8 20-07-2006, 08:21 AM

One word=Yuk

OneOfOne (Trevor) · #9 20-07-2006, 08:34 AM

Quote:

Originally Posted by ballaratdragons

That's not Coma! That's what the stars are suppose to look like at Warp Speed travel!!

I thought the same thing!

Maybe you should buy a Parracor and if you don't like it, I could take it off your hands!

Astroman (Andrew Wall) · #10 20-07-2006, 09:16 AM

Thats a beaut image Tony, one of your best for sure..

Keep up the great work

Dujon · #11 20-07-2006, 11:45 AM

Thanks for bunging that up, Tony. The coma is obvious but I originally thought that the vignetting was merely the nebula itself; your second image highlights that it is not.

ballaratdragons (Ken) · #12 20-07-2006, 01:33 PM

Yep, very obvious in the 2nd shot. Thanks Tony.

So that is what this is in my Trap shot - Vignetting. What is the solution?

iceman (Mike) · #13 20-07-2006, 01:55 PM

There's no vignetting in your shot Ken - which part of your shot are you referring to?

ballaratdragons (Ken) · #14 20-07-2006, 01:57 PM

The dark area around the Bright Stars, Mike.

gbeal · #15 20-07-2006, 02:02 PM

Looks like you over-cooked it in the processing.

Striker (Tony) · #16 20-07-2006, 02:03 PM

No Ken....the vignetting is the tunnel affect you see on the whole image....not individual stars....notice the image is much brighter in the center then on the sides...thats vignetting.

ballaratdragons (Ken) · #17 20-07-2006, 02:05 PM

ahhhhhhhhhhh

iceman (Mike) · #18 20-07-2006, 06:42 PM

Ken the dark area around the stars is usually due to oversharpening using unsharp mask or other image processing techniques.

When sharpening, the gradual colour gradients turn into sharp short colour gradients. Dark greys can turn into black, and lighter greys can turn into white.

At the extremes, you'll end up with halos around stars as the sharpening creates a sharp gradient from black to white.

TidaLpHasE · #19 20-07-2006, 07:48 PM

Wow, that's an awsome image Tony, even with the distortion.

Here is a couple of images i took recentlyshowing the same problem big time.

The first image was taken with the 5dh 50mm f/1.4 @ f/3.5 8 seconds ISO 3200, seems o.k.

The second image, taken with the next shot, was 5dh 50mm f/1.4 @ f/1.4

the coma and vignetting have completely ruined the image.

It is amazing what a couple of f stops will do as far as lens capability and image quality is concerned.

Striker (Tony) · #20 20-07-2006, 08:05 PM

Nice comparison Trevor...sure makes a huge difference.