Webcam Imaging Advice

[gfletcher]

Hi Guys

Having imaged Jupiter using the SPC900 webcam, using AstroCapture, i have been using a shutter speed of 1/100 sec.

At this setting i seem to be able to use about 30% gain setting.

If i use either 1/50th or 1/25 sec shutter speed i find that the gain has to be virtually at minimum otherwise the image seems over exposed.

Looking at tutorials it seems to recommend 1/25 th sec shutter speed.

I thought that if the gain was too low then you are not achieving sufficient detail to produce a decent image.

Anyone any thoughts on this?

By the way , ihave been imaging through my 250PX with a 2X barlow.

thanks

Graham

[red dwalf]

from what i`ve read, when the shutter speed is increased the object gets darker so you will have to up the gain to get the object bright enough to see detail, also need 5 or 10 frames per second so you don`t lose any detail as above this you start compressing the imformation.

[bus_ter]

The gain control just amplifies the sensor signal (including noise) to create a brighter image. Increasing the exposure allows more light to collect on the sensor creating a brighter image.

The gain control works like the ISO on a digital camera allowing you to increase 'exposure' without actually increasing the exposure time. This is essential to create sharp images in low light conditions. The downside is that increasing the ISO (gain) increases noise.

Because we stack the images the noise is less of a problem, and it seems having some gain improves images on the SPC900NC. The best thing to do is experiment. Try two runs of video of the same exposure, but achieved with different combinations of the exposure and gain controls and then compare.

[brianb]

If i use either 1/50th or 1/25 sec shutter speed i find that the gain has to be virtually at minimum otherwise the image seems over exposed.

Looking at tutorials it seems to recommend 1/25 th sec shutter speed.

I thought that if the gain was too low then you are not achieving sufficient detail to produce a decent image.

Minimum gain is actually ideal because you get less noise in the individual frames - meaning you need to stack less frames, so you can pick & choose the very best.

1/25 sec is based on a normal sort of image size, around f/25 ... your scope will be much shorter than this, unless you use a Barlow of Powermate to increase the image size. If you're getting goodexposure at 1/100 sec then adding a 2x barlow will mean the exposure has to be increased to 1/25 sec (4x, i.e. 2 squared) at the same gain as the same light collected by the objective is spread over 4x the area.

Larger image scale needs better seeing, & is more difficult to focus. Go at it steadily & don't be put off by your many mistakes - we all make lots of them! You should however learn from them.

[skybadger]

Brian

While I understand the rationale in selecting lowest gain for least noise and maximum signal based on exposure, my own experience and one Ive seen elsewhere on this forum is that if the gain isn't turned up high registax produces concentric rings on the image which I haven;t managed to get rid of. So while the images in the AVI are visually better and less noisy at low gain, longer exposure, same frame rate, the stack result is far better for higher gain.

The other approach to this is that higher gain allows shorter exposures which gives more images in the few moments of good seeing to stack.

Do you have a way to avoid this ringing at low gains ?

I attach an example of ringing and a more usual stack taken at f/30 through a 2.5x powermate on a VC200L.

regards

Mike

[brianb]

Do you have a way to avoid this ringing at low gains ?

No ... except stacking more frames ... you're not going to get minimum gain anyway but there is a compromise which is usually somewhere around whatever gain gives you a reasonable histogram with an exposure of 1/30 sec. Underexposing is a pretty sure way of getting "onion rings", overexposing washes out detail which is more of an issue ... the "onion rings" are less offensive if you don't stretch the levels too far in post processing, and can be pretty well eliminated with minimal loss of actual detail using the blur brush in PS Elements.

[skybadger]

Those are some good points

Thanks.

Mike