Astronomy, Photography and Weather

General Category => Weather & Atmospherics => Topic started by: martinastro on October 07, 2008, 07:59:55 pm



Title: Mammatus From Frontal Clearance
Post by: martinastro on October 07, 2008, 07:59:55 pm
The warm front finally broke at sunset this evening and by 19.45 a long duration display of mammatus pouches formed over my area. At first they were small in size, grew larger, then ended up covering much of the sky as they moved in from the west at moderate speed. The low Sun illuminated the first set an intense orange colour then as the Sun dropped they developed a purple hue. At dusk they became dark as the Sun vanished for good. The display was visible for over 30 min's. Unfortunately my images are very poor because I took them hand held in low light. As a result the shutter was too slow which caused some blur.

(http://i35.tinypic.com/2jcsrwy.jpg)

(http://i36.tinypic.com/hss500.jpg)

(http://i38.tinypic.com/2dl2mq1.jpg)

(http://i37.tinypic.com/ekkei8.jpg)

(http://i38.tinypic.com/sen711.jpg)

(http://i35.tinypic.com/2wem8e9.jpg)

http://i37.tinypic.com/keuezk.jpg

http://i36.tinypic.com/4qmp9u.jpg

http://i37.tinypic.com/213rky.jpg


Title: Re: Mammatus From Frontal Clearance
Post by: Paul on October 07, 2008, 08:05:27 pm
Excellent Martin - I shall expect to see one of these on the weather tomorrow evening :)

P.


Title: Re: Mammatus From Frontal Clearance
Post by: martinastro on October 08, 2008, 12:01:34 am
Thanks very much Paul. I might just send in one or two of those. Pity I messed up the shutter speed - but lesson learned - next time use a tripod and appropriate shutter.

Paul, any idea why the images are so grainy?. These are all ISO100 yet they look noisy and poor. I have noticed this trend for some time but ONLY when near dusk when the light level is low. At this time I seem to get this result and I have problem trying to get a focus. Yet during daytime I have no problems. My camera has a manual focus ring on the barel like a DSLR but it's not a truemanual, the manual movement triggers a digital focus which gives a slight delay before I see the result on the LCD screen. I wonder why it's so tempermental in low light. I find I can get focus accurately and rapidly with the aperture closed down but with a wide open shutter the focus is so fine line that it can be frustrating. I suppose practice makes perfect.



Title: Re: Mammatus From Frontal Clearance
Post by: brianb on October 08, 2008, 12:39:17 am
Quote
any idea why the images are so grainy?. These are all ISO100 yet they look noisy and poor. I have noticed this trend for some time but ONLY when near dusk when the light level is low. At this time I seem to get this result and I have problem trying to get a focus. Yet during daytime I have no problems. My camera has a manual focus ring on the barel like a DSLR but it's not a truemanual, the manual movement triggers a digital focus which gives a slight delay before I see the result on the LCD screen. I wonder why it's so tempermental in low light.
I'm sure the noise is an effect caused by the sensor being so small - it's just quantum noise. AF mechanisms all struggle in poor light to some extent, too, but the bigger the sensor elements, the better.


Title: Re: Mammatus From Frontal Clearance
Post by: brianb on October 08, 2008, 09:33:12 am
Quote
The warm front finally broke at sunset this evening
Funny how the weather differs so much over a relatively short distance. Here there was little sign of clearance during the early evening, the rain changed from light & continuous to heavy & intermittent, that's all. Solid thickish high cloud until after 12:30 am. (BST)

By 3:30, sky clearing, some patches of high(?) cloud but the sky was very transparent between them. Very strange incident happened around 4:00, there was a thin patch of the high(?) cloud which blew over, no thick cloud at all, yet it rained for 5 mins!!! Still, managed to get a bit of observing done - looking at the weather chart it could be the last chance for a few days. Shower blew in around 6:00, sky didn't clear properly before it got too light to be useful.


Title: Re: Mammatus From Frontal Clearance
Post by: Paul on October 08, 2008, 12:27:01 pm
Martin, the graininess you're experiencing arises from two factors - firstly as Brian says, a small CCD. The pixels on a compact/bridge camera are very small indeed so not much light falls on them giving rise to the need for quite a lot of amplification. The second factor is low contrast - when you have very flat lighting you're only using a small range of the brightness values that your CCD could accommodate, and the "noise" part of the signal is very significant in relation to this. It's also the reason why autofocus struggles - it works by maximising contrast and ideally would like nice sharp edges to lock on to.

Solutions range from the expensive - a bigger sensor - if you buy me a new Sony A900 with full frame 24MP sensor I'll let you have my A700 for a good price :) Or alternatively set the ISO to the very minimum and use a tripod and manual focus set to approximately the right distance (can be tricky).

With static subjects of course this is where stacking comes in - take loads of images, add them up, the signal is always the same so adds up, the noise is random so averages out!


Title: Re: Mammatus From Frontal Clearance
Post by: brianb on October 08, 2008, 01:43:46 pm
Quote
take loads of images, add them up, the signal is always the same so adds up, the noise is random so averages out!
Not quite as simple as that - the noise comes from three sources:

1) quantum noise from the signal - Poisson distribution

2) thermal noise from the sensor - approximately normal distribution

3) electronic noise from the amplifiers - approximately normal distribution

The way the composite adds up is not simple but the basic truth is that the signal to noise ratio increases with the square root of the number of frames added.

It's still best to take subs as long as possible because this minimises the contribution from amp noise, which is independent of the exposure length.


Title: Re: Mammatus From Frontal Clearance
Post by: martinastro on October 08, 2008, 05:27:28 pm
Brian, Paul, that's very useful information. I will have to re-read these posts several times to let it all sink in and do a bit of research on those tech points. It's amazing to think that some people out there, who know nothing about photography, think it's just a matter of pointing the camera and pressing a button lol....the learning curve is phenomenal and I'm at the bottom of it  :). I've learned something new from this so thanks guys.

I was using manual focus for those.

I will just have to put up with it until I get myself a DSLR. Anybody got any spare cash?  :)

Brian, that's amazing about how the weather can vary from location to location. It was clear here all last night from just after sunset until now as I write this. Looks like big frontal systems will be sweeping over us for the next few days so tonight will be my last observing session.


Title: Re: Mammatus From Frontal Clearance
Post by: Roman White on October 10, 2008, 05:58:02 pm
I had similar situation as Martin with photographing at dusk. It was interesting to read this thread.

It's amazing to think that some people out there, who know nothing about photography, think it's just a matter of pointing the camera and pressing a button lol....the learning curve is phenomenal and I'm at the bottom of it  :). I've learned something new from this so thanks guys.
I completely agree with you! But I think that the bottom isn't single, there are a few 'bottoms', and I'm also at the one of them  ;)