Frame rate considerations

[iceman]

In response to Gary's question about frame rate, he said that he was always brought up to believe 10fps is best..

When I started pondering this question, I asked some of the experts at CN land and Bird amonst others replied, and the general gist is this..

The ToUcam uses the USB 1 interface, and so if you're using 640x480 and 15fps or higher, the only way for the ToUcam to send data quick enough down the thin pipe is to compress it.

So ideally, you want to use 5fps all the time, as this won't compress the data and your raw frames will be cleaner.

But, if the seeing is bad, you want to take a lot of frames so that you can find the good ones amongst the mush. In this case, you might use 15fps to get more frames in a shorter amount of time, accepting that you will lose some quality due to compression.

So, seeing = good, use 5fps. Seeing = bad, use 15fps. A happy middleground = 10fps, which is probably why most people recommend it.

If the ToUcam worked on USB 2, you could probably use 15fps or 30fps without compression, but to my knowledge, a USB 2 compatible ToUcam doesn't exist.

As for Bird, he uses a firewire camera on a firewire card, which can transfer data much quicker, therefore he's able to use 25fps and gather lots of frames in a short amount of time without compressing the data.

I hope i've got all the facts right, i'm sure Bird or someone else will correct me (please do) if i'm wrong.

[Comet Hunter]

yep, I understand what you guys are saying and that bird is using a different camera etc to what most of us are using... hence why i said "if your camera + system can handle the higher workload ....." The main reason I mentioned bird using 25fps was to highlight that 10fps wasn't the be all end end all for everyone...its something everyone needs to find out themselves to get the most/best out of their setup :)

[[1ponders]]

Ditto to you Mike, which is why I rarely record at 640. I trade of size for speed.

[Striker]

It sounds to me we need USB2 compatable camera's...make sence.......

I thought the new Toucam was USB2....if not...I'm not getting one.

[bird]

Hi guys. Even at 5fps the ToUCam compresses data. I measured it a couple of years ago when I got curious. The "raw" frames from the camera that come down the bus at 5fps are about 1/2 the size that they should be. If you're curious then dig aropund on the net for the Linux driver and you'll find the source code to a decompression routine thats needed to recreate the frames.

The only way to get non-compressed data from a ToUCam is to use 320x240 image size. All 640x480 frames are compressed.

regards, Bird

[iceman]

If I had tracking I could image at 320x240 because at 2500mm FL my Saturn/Jupiter isn't that big anyway.. Certainly no bigger than 320x240.

But with the drift-recording I do, I need a bigger framesize so I can get more frames in each pass.. otherwise it just doubles the amount of work I have to do trying to align it with a tiny piece of CCD chip :/

[Robby]

Hi,
I always thought that the ToUCam was actually a 320x240 chip, but they interpolate to get 640x480. I think 5fps is uncompressed raw with compressed interpolation. I could be wrong & it wouldn't be the first time either!!
I've always used 10fps and it seems ok.
ToUCam 740 is not USB2, but will work in USB2 port (at USB1 speeds). Not sure about the newer 840 ToUcam though, but II suspect it is also USB1.
Cheers

[[1ponders]]

Thanks for that bird and Robby. If you're right (and I'd be surprised if you weren't:P), it explains why no matter what I do, I'm rarely happy with my 640 images. Even when the 320's look good, I cant get a 640 image I'm really happy with. Hence the 352 option. Relatively good quality with respectable size with an acceptable frame rate. But hey isn't that what lifes about - compromise. :D

[gbeal]

OK you fellas (and fellesses, as applicable)
Thank you for the 5fps/10fps/whatever debate.
I will continue to use 5fps if possible and at worst 10fps, as I have always done.
To try this I shot another bland Jupiter early this morning.
Same 10" f5 newt (my deepsky scope, not my "Planetary").
Using 5fps, and shooting 60 seconds, through the 5x Powermate.
I have decided to only use the 5x, rather than accept a smaller image scale, as if the image isn't up to the 5x, then I am better off not imaging.
Any different?
Gary

[bird]

The ToUCam is definitely a 640x480 camera. The ccd that it uses is the ICX098BL. Dig up the spec sheet from Sony if you're curious. (google is your friend).

The 320x240 mode is created by combining pixels in a 2x2 fashion.

regards, Bird

[bird]

Not bad Gary! Looks like the red plane might have to come down a pixel or so...

Does it look a little over-exposed to you? There's some detail missing in the equatorial region that should be there on a good image like this.

regards, Bird

[gbeal]

Hi Bird,
God knows what it is. I was all happy to have tried something new, and couldn't work out why the image kept needing over 50% brightness, and similar gain, with a measley shutter speed of 1/25th (10" f5 with 5x Powermate). I wondered if I was dewing up, but it was merely the interference of early morning fog, and eventually I gave up.
The raws, both on the laptop monitor, and on the desktop the next day were very dim, in fact the desktop was a guessing game as to where to plonk the alignment box, it was that hard to see.
However once stacked etc, the resulting bmp was fine, bright as even.
Sound OK? Thanks for all the advice.
Gary

[bird]

Hi Gary - a 10" f/5 should be perfect with a 5x powermate! Gets you to about the same brightness and magnification as me with my 10" f/6 newt + 4x powermate.

The image shouldn't be dim @ f/25 so you must have had some nasty morning fog or dew! I've got a portable hairdryer that I use to remove dew when it strikes (almost every morning at some stage).

Now I understand the low contrast in your image - not enough brightness in the raw frames. Even so, it's quite a nice image!

regards, Bird

[gbeal]

Bird,
thanks again, especially for the time taken to pass on little tit bits like this, I appreciate it.
Agreed, the setups, without a few of the refinements, are similar.
It was foggy, and normally is during the winter, where I live.
So, you are saying a "brighter" raw/monitor image? If so, I can only assume the final stacked image will be significantly brighter still (than what I ended up with this time).
Thanks again, gotta keep practising.
Gary

[bird]

Gary, registax will automatically do a histogram stretch on your images, "normalising" their brightness regardless of the brightness in the raw frames.

You might try turning off histo-stretch in the stack page to see what it would otherwise look like...

If you have too little or too mugh brightness in the raw frames then you lose information, and the histo-stretched images have fewer discrete colour levels than they ought to have.

Bird

[gbeal]

Thanks again Bird, I never knew this. I will keep at it. Maybe tonight, but certainly in the coming week(s).
Gary