Length of webcam 'videos'?

[TheThing]

S@N gives some conflicting information about the length of time you should video on webcam for. One page says not more than 2 mins, another says not more than 1. Any idea which is better? I'm planning on imaging Jupiter.

Also, is that total length or length of usable frames? My LPi only keeps good frames and gets rid of duff ones automatically. If seeing is bad, out of two mins I might only get 5 good frames!

[michaelmorris]

The limiting factor will be the rotation rate of the planet or the speed of the terminator travelling across the Moon's surface. If your videos are too long they will be blurred by the rotation/terminator movement.

In general, webcam imaging tends to be done at long focal lengths (1.5 metres to 8 metres), so the limitation imposed by the rotation/terminator movement can be pretty short for some subjects. The best place to source this information that I have found is Martin Mobberley's excellent book, Lunar and Planetary Webcam User's Guide.

For webcam imaging the other limitation is the size of your hard disk and the length of time you are prepared to wait for the align and stacking program to do it's magic.

I use 170 second captures for Jupiter where the fast rotation of the planet is the limiting factor. I tend to stick to 200 seconds (2000 frames) for most other shots as this usually seems to be enough frames to capture a reasonable image. I have been known to go as high as 1000 seconds (10000 frames) on a few occasions.

[Starman]

It's hard to give a definitive figure without going into the specifics of the imaging setup. Longer focal lengths will produce larger image scales and fine detail on a rapid rotator like Jupiter will be blurred quicker. So for a long focal length your capture time needs to be pretty quick to capture crisp detail. The problem then becomes one of balance because the shorter your capture time, the fewer the frames you'll collect and the noisier and potentially less well defined, if the seeing isn't great, your final image will be.

When I image Jupiter at f/40 on my C-14 (that's a 14m focal length) I keep my captures down to 30s. However, I'm working with a mono camera so I've got three channels to complete for a single RGB.

If your focal length is short you can get away with longer and I wouldn't be too worried at going for 2-3 minutes with a colour camera. At the end of the day, the rule of thumb with a rapid rotator is to keep the exposures as short as you can get away with. In this respect it's better to use the figures bandied around as a general guide. Try some variations and see what works best for your setup.

Mars is another planet which falls into this category but it's not as rapid as jupiter so your capture times can increase a bit.

Saturn should also be treated to short imaging bursts but there's a tendancy amongst some imagers to go for broke and simply grab as many frames as they can to get a nice smooth result. This is great from an arty point of view and can produce a lovely smooth, banded planet. However, from a science perspective it's a disaster as it'll blur out any low contrast features on the planet's disk.

The bottom line is that you should use the information given to give you a general feel for what you need to do. Experiment around this area to find what works best for your particular set-up. The final best time for capture will always be something of a compromise.

[Kokatha man]

Hi "Thing" - just limit your avi's to a maximum of 150 seconds on Joop and you'll have no problems.....I contribute hi-res images to Jupos, BAA etc and always use 150 seconds: that said, I utilise an OSC unit which is what an LPI is I believe (one-shot colour, ie. it creates colour images by having a bayer matrix on the video cam.....)

Check some of my images on SGL to see the results of said.....if you were using a mono camera and having to do 3 seperate "runs" with RGB filters then you'd have to reduce each run to (approx) 1/3 that total time.

Saturn, as one of the posters here notes, needs a max of similar length to reveal cloud and storm structures.....but these aren't easily resolved at the best of times (actually very difficult with the smaller apertures) hence why many people run the avi much longer which brings out the various cloud bands and their colouring much more distinctly.....but of course any faint and delicate features will be totally "lost" at that amount of video time!

The maxim for Mars is around 4 minutes.....I am unfamiliar with the LPI's use but all cams can "drop" frames during videoing - but ending up with 5 frames out of 2 minutes (even at 5fps) does sound mighty strange....!

[TheThing]

Thanks for the info guys. I'll try that over the w/end.

[GazOC]

Also, is that total length or length of usable frames? My LPi only keeps good frames and gets rid of duff ones automatically. If seeing is bad, out of two mins I might only get 5 good frames!

IIRC using Autostar/ Envisage to run the LPI theres an option to keep all the frames. You can then check the quality yourself or get some other software like Registax to do it for you.

[TheThing]

Thanks Gaz. I've not come across that yet, although the manual isn't quite as detailed as I hoped. I'll have a play tonight and see what's what.

I'm planning on imaging Jupiter (again, but with a barlow this time!) using the f/5, 750mm Soligor (that's if the clouds clear!)