Back to First Principles - 16 Dec.

[GeorgeIonas]

After a discussion with Ralph (Solar) I decide to go back to square one today and perform a checkerboard type test where I varied the Gain, Gamma and the Exposure time. After capturing about 50 images it became evident that my DMK51 camera produces a fairly flat image with little contrast at its lowest Gain value (Gain=142). However, on tweaking the gain up to Gain=162 I was able to get a dramatic increase in the image's dynamic range which also allowed for better processing. This slight increase in gain did not seem to introduce any extra noise to the image at least in my hands. Although the seeing was not the best I used the 1.6x and 2.5x Barlow.

I have attached some of the better images from this trail both in mono and colorized.

1. Four-frame mosaic using the Zeiss 63/840 OTA with the SM40 Etalon.

2. Single stack SM90+Barlow 1.6x give a nice image scale FL=1142mm

3. Single stack SM90+Barlow 2.5x this image is a little soft as the seeing conditions weren't the best at FL=1785mm

George

[solar]

Well done George.
These are a Quantum leap from the previous. : :
For your mosaic try using I Merge it it much better.
http://jaggedplanet.com/iMerge.html

[mattwastell]

Great results - these are your best to date - congratulations!

[Derek Klepp]

Impressive

[Montana]

Wonderful results :

Alexandra

[swisswalter]

Hi George


wonderful results, congratulations. Maybe that hint helps me too :thanx: :thanx:

[eroel]

George:
Very nice images, the 40mm etalon gives a beautiful full disc mosaic.
Best wishes,
Eric.

[GeorgeIonas]

Thank you all for your kind comments.

Walter: As I mentioned in my post, I found that with my DMK51 camera that it just needed a tiny amount of Gain which resulted in increasing the dynamic range of the image. This may not be the case with other cameras or even with a similar brand of camera. It would be advisable that one does a similar checkerboard type of test with their own imaging cameras to find which values provide optimum results (best dynamic range with little or no noise).

In my test I ran about 40 to 50 images with varying Gain, Gamma and Exposure times. It was evident that in all images that were taken with the Gain set to it lowest value, that the images just looked "flat" and exhibited poor contrast. The small amount of Gain that I introduced had a dramatic effect on the image but not on the amount of noise. I would think that the video cameras would be similar to a dedicated astro camera where one has to find the optimum camera performance by changing the Gain and Offset values. Some manufacturers do this for you, other don't and it is left to the individual to work out the optimum values.

George

[swisswalter]

Hi George

thank you very much for the additionell information

[GeorgeIonas]

You're welcome Walter.

I would be interesting to see what others get? It could be just that my camera is the odd one out here?

George

[swisswalter]

Thank you george

we will see

[Montana]

What is a checkerboard test?

Alexandra

[GeorgeIonas]

Alexandra: A checkerboard type test is perhaps not the best term to use. I keep two values constant and change one. For example, I kept the Gain and Exposure times constant at on value and then I varied the Gamma. I then changed the Gain and the Exposure time to another value and again varied the Gamma setting and so on.... One then sees a trend occurring where the image is dark flat and has no contrast through to an image that is over exposed. Somewhere in the middle there is an optimum value.

George

[marktownley]

Top shots George and an interesting thread. Something for me to try next year when the sun returns...

[brianb11213]

Nice work George. I guess we all are going to be slightly variant in the way we capture, & consequent post processing. In my case, I want it as FLAT AS A TACK at capture time. I'll add my 'gain' in after the fact because I can vary it. You can't, once it's in the capture. As for dynamic range, it's already there in the capture despite a flat looking capture due to mainly, the characteristics of the Sony ICX274AL chip plus the exposure time has a LOT to do with it...
Yeah ... so long as you remember what dynamic range is. You lose dynamic range (and signal to noise ratio) by underexposing and you lose dynamic range (and get a horrendous looking result) by overexposing & clipping the highlights. As sharpening uses up some of headroom you want the high end of the histogram at around 75% rather than fully right on the graph.

Tradeoff between exposure time & gain is dependent on light intensity & seeing. But the dynamic range isn't affected by that tradeoff ... noise is, so is capture & processing time.

The results you've obtained are pretty darned good.

[marktownley]

Some great points here Fellas!

[pstew]

Excellent work George :

[GeorgeIonas]

Thanks Paul.

[Derek Klepp]

Yep gotta agree with John in the end we all have different expectations of what we deem satisfactory with our equipment whether its visual or imaging it really is a shame that we all live so far apart in global terms as sometimes its easier to understand if you're there witnessing what's going on.

[swisswalter]

Hi Derek

that is a very good Point. I second that