Onion Rings

[Matty P (Matt)]

My recent Juptier images all show onion rings around the edge of the planet. No matter what I do, I just can't seem to get rid of them. I've tried to adjust the gain, gamma and exposure but have had no luck. My first batch of Jupiter images did not show any onion rings and I am not sure why is that.

I have heard that onion rings appear because of under exposure or compression but I am certain that is not the case. I have a huntch that onion rings occur because of the seeing conditions and/or too much gamma but I could be very wrong.

Can anyone shed some light on why onion rings appear on most of my Jupiter images and how to get rid of them.

Thanks in advance.

[davidpretorius]

bird gave the best explanation within the last month.........may have to search for that reply

he was saying thay underexpsoure and bad seeing around the edge of the planet are the main causes.


here we go... http://www.iceinspace.com.au/forum/s...ht=onion+rings

[Matty P (Matt)]

Thanks for the link Dave, Bird's explanation really helped.

I still get onion rings even when the seeing conditions are good and I always make sure that the histogram is properly exposed.

So I can rule out underexposure and bad seeing.

Any ideas?

[davidpretorius]

just to bring me to date on your setup

- 8" SCT?
- DMK in 8 capture
- no filters as yet

Not being versed with the imaging capture software for the DMK, what is the histogram showing (hopefully values between 0-255...8 bit)?

Also are you estimating the exposure by eye? The dark adapted eye can think something is bright, when it is not really. I always let my histogram read out in my capturing software to tell the truth (0-4096...12 bit)

I also do not touch the setting of brightness / gamma / contrast. I want as "natural a caputure" as possible.........ie I do not want to try and capture at 60fps and at first get a dim image, then try and "artificially" boost the signal by upping the brightness / gamma etc.

Do you have a single frame from one of the runs before registax played with it?

[Matty P (Matt)]

Originally Posted by davidpretorius

just to bring me to date on your setup

- 8" SCT?
- DMK in 8 capture
- no filters as yet

Not being versed with the imaging capture software for the DMK, what is the histogram showing (hopefully values between 0-255...8 bit)?

Also are you estimating the exposure by eye? The dark adapted eye can think something is bright, when it is not really. I always let my histogram read out in my capturing software to tell the truth (0-4096...12 bit)

I also do not touch the setting of brightness / gamma / contrast. I want as "natural a caputure" as possible.........ie I do not want to try and capture at 60fps and at first get a dim image, then try and "artificially" boost the signal by upping the brightness / gamma etc.

Do you have a single frame from one of the runs before registax played with it?

My setup is:

8" SCT
DMK 8bit camera
No filters

The histogram on the capture software shows between 0 and 255.

I estimate the expsoure by eye but always keep a watch on the histogram. I've been capturing all my recent Joop images with a 1/60s expsoure shooting at 60fps. I then usually adjust the Brightness, Gain, Gamma so the histogram is properly exposed.

Here is a single frame and the final image from an imaging run on the 30th. As you can see in the final processed image there is severe onion rings.

[davidpretorius]

i would try and cut back to 30 fps, leave brightness and gamma at default values and then set your exposure to maximum.....and then drop your gain back from 100% to adjust the histo.

i would avoid changing the gringhtness and gamma

[Lester]

I use a colour camera and am convinced its gama that causes onion rings.

I did a test on 3rd April on Jupiter in steady 6/10 conditions, on one avi I used 12 gama (10 gama is minimum with my DFK) which isn't very much and the image was ruined with onion rings.

All the other avis with minimum gama had no sign of the onions. Exposure in all cases was with the histogram almost at maximum.http://southcelestialpole.org.au/for...ic,7504.0.html

[Matty P (Matt)]

Thanks for the help Dave,

What are the default values for the brightness and gamma?

[Matty P (Matt)]

Hi lester,

I too believe that gamma causes onion rings. Do you know how much gamma I should be using?

Thanks

[Lester]

Hi Matty,

I use no added gama at all. The minimum setting is 10 with the slider all the way to the left.

So no gama and just alter the brightness of the image with exposure time and gain.

[Matty P (Matt)]

Thanks Lester,

I'll give it a go when the clouds clear. I have been using too much gamma around the 60-70 mark. That's probably why I am getting onion rings.

Thanks for the help.

[Lester]

I will look forward to seeing your results.

All the best and clear skies.

[davidpretorius]

if memory serves me correctly, bird said that gamma multipies the value, brightness adds to it?