[X] My Assistant

[Toma B]

26 Nov. Close Flyby (500 km.) of Rhea...(Size: 1538 km.)
27 Nov. Distant Flyby (108,000 km.) of Enceladus...(Size: 512 x 494 x 490 km.)
27 Nov. Distant Flyby (86,000 km.) of Helene...(Size: 36 x 32 x 30 km.)

This should be interesting...

Can anybody give a bit more information on the upcoming flybys???

Is this the best image of Helene that we have???
Vojager 2 - distance :341000 km)

[pat]

Just to try and clear up some misconceptions. The 'frame time' is the time it takes the ISS camera system to prepare to take an image, actually take the image and then send the data to the spacecraft's solid state recorders. The system can't begin taking another image until one 'frame time' later. The 'frame time' can be thought of as how long is needed to take an image.


Now the frame time is calculated the following way:


The frame time for each image = prepare cycle + readout cycle

prepare cycle = NAC filter wheel window + WAC filter wheel window +
flood/erase allowance(950 ms) + exposure window + pad (525 ms)

readout cycle = (NAC readout window + WAC readout window)*(48 packets per sec
telemetry rate / actual telemetry rate) + pad (525 ms)

shortest prepare cycle = 4.475 seconds (1s + 0s + 0.95s + 2s + 0.525s) (NAC or WAC only)
shortest readout cycle = 6.525 seconds ((6s + 0s)*(48/48)s + 0.525s) (NAC or WAC only)

shortest possible frame time = 11 seconds

But this is for the shortest of the 4 allowed (quantised) readout times (6s, 14s, 25s and 50s). You can only use the 6s readout time for 4x4 summation (i.e. 256x256 pixel) images and all but one type of 2x2 (512x512 pixel) image, for 1x1 (1024x1024 pixel) images you have to use at least 14 seconds. That means for a 1024x1024 pixel image the minimum frame time is 19 seconds in telemetry mode S&ER 5 (48 packets/sec). In the more usual S&ER 3 (24 packets/sec) the minimum frame time for a full image is 33 seconds. This assumes that you have losslessly compressed 8 bit images (i.e. 12-->8 bit lookup table has been used). For losslessly compressed 12 bit 1x1 summation mode images its recommended that the readout time is 25 secs so the minimum frame time is 30 s (S&ER 5) or 55 s(S&ER 3).

You can use a shorter readout window than is necessary to readout a full image, this results in image truncation i.e. losing lines at the bottom of an image (on occasion the person planning an observation doesn't use the recommended readout time and uses a shorter one instead and this can result in all the images being truncated, especially if they have underestimated how well the images will compress). Oh and the reasaon there are different telemetry modes is that the available bandwidth to the solid state recorders is divided differently between the 12 Cassini instruments in each mode. The modes where ISS gets alot of bandwidth are S&ER 3, S&ER 5 and S&ER 5A.