New Horizon Cameras Options

[Guest_vjkane2000_*]

I've been wondering what the focal lengths are for the two New Horizon cameras (Ralph and Lorri). Curious to know how much Alan et al. were able to squeeze into their weight budget.

[edstrick]

The Mariner 4 images of Mars were badly degraded by a design-defect light leak. They were salvaged by digital image processing. The 6 bit data was inadequate for lower exposure images and after picture 11 of 21 1/10, the imaged degraded rapidly into digitization contour patterns

The Mariner 6 and 7 Mars images were badly degraded by analog tape recorder noise (the oxide flaked in flight and accumulated on the tape-heads), and electronic interference noise patterns, together with considerable geometric distortion of the images, severe shading across the images and bad residual images.

The Mariner 9 images were all-digital once the vidicon image was readout, but the cameras still had distortion, shading, and severe residual image problems. In addition, the color filter wheel stuck some 70 days into the mission, ending up at a polarized-orange position which was acceptible, but didn't help. The narrow angle camera also had severe defocus in images that had high exposure levels (not saturated)

Mariner 10 (Venus-Mercury) fixed the residual image problem by putting lightbulbs INSIDE the camera to create a uniform saturated-frame residual image (after the saturated frame was erased), permitting for the first time reasonable decalibration of the images, but the cameras had real problems with dust specks and lint shadows, and the spacecraft stability was inadequate for the narrow field of view, causing severe pointing wander. Look at Mercury flyby-2 mosaic segments for example. The real time 100,000+ bits/sec communication (experimental) was barely adequate at Mercury so the majority of images (other than the relatively few that could be taped for later playback) had severe salt-and-pepper noise.

Viking orbiters used twin cameras with silicon vidicon detectors instead of selenium-sulfer vidicons (I believe). The data storage system couldn't handle the 2 second (approx) raw readout rate from one camera while the other was taking an image in a rapid-fire left-right-left-right sequence for 50 meter/pixel landing site mapping from periapsis. The 7 bit (inadequate... orbiter images often have digitization contouring) data were split into several pixel channels and dumped in parallel tracks onto the tape recorder, which were then read out sequentially. Pictures (and there were many) where all tracks didnt' make it back to earth are missing columns of pixels in varying amounts which have to be filled in by interpolation. The zero-level of the camera ended up negative, instead of small positive numbers, so a blank exposure of black space was all ZERO's, and it was impossible to directly measure the dark-image shading of the cameras. Inflight calibration of shifts in the camera shading was difficult and generally inadequate. As each picture was being readout the other camera was shuttering it's exposure, then resetting the shutter. The mechanical shutter slap vibrated the cameras, resulting in a series of sine-wave interference patterns for a number of lines in the other camera's image, one near the bottom of the frame, one near the top.. due to "microphonic' vibrations of the photocathode in the camera that was beign read out.

Voyager's cameras were again selenium-sulfer vidicons, using the same lens as Mariner 10 for the narrow angle camera (with improvements). Dust specks were minor, calibration techniques reasonably well worked out. The cameras could be operated with light flood on for optimum calibration, or light flood off for low-light-level sensativity. Inflight calibration between encounters greatly helped maintain decalibrated image quality, though a lot of the cal data has annoying levels of bit errors and missing lines etc, and I had poor luck using the calibration target images to decalibrated flat-field shading from Voyager images (they had an aluminum plate they could get sunlight on by reorienting the spacecraft and then point cameras and other instrumets at). Image stability and pointing was better than Mariner 10's but still less than ideal. Stability was improved after Saturn for the extended mission by reprogramming the attitude control system. The vidicon cameras still had a problem with image distortion, and the images are literally "stretched" toward bright objects by a pixel or two due to electron-beam deflection by the charge pattern on the vidicon surface. This made truely precise geometric measurements in navigation images and the like quite difficult. CCD utterly don't have that problem!

but: "The sad thing about the optical calibration issues that appear to be cropping up on so many current/future missions is that 35 years ago, they had the thing done and dusted"... Uh.. done and dusted?.. no way!

[Bob Shaw]

I take the point regarding the steep learning curve on past missions - but it doesn't change the fact that nowadays the problems are known yet dumb mistakes keep getting made. I have every sympathy if it happens once, but after the Hubble mirror fiasco (and it's expensive recovery, for which all credit to those involved) you'd think that the word 'focus' would be at the forefront of everyone's mind...