For those of you that tried Isolda, thank you. Seems like most of us had one difficulty or another but it’s good to get that our of our collective systems early. I haven’t had a chance yet to review all of the files uploaded. I really have to get this automated more. Being on the road non-stop isn’t helping either. Last week I was in Flagstaff for a Planetary Defense Conference. Saturday I was at the bottom of Meteor Crater. Today I’m in Baltimore serving on an advisory committee for the Hubble Space Telescope.
I wanted to share some reflections on last week’s Isolda occultation event. First, I have to apologize for one of my mistakes here. I didn’t check on the Moon for this event. It was really close and pretty bright on event night. It gave me a lot of trouble with getting setup and finding the field. I was not really able to use anything on the star hop list fainter than Alhena. If it wasn’t for PreciseGoTo I would not have found the field at all. In the end, the moonlight caused me to take longer than anticipated to get on the field and I was very rushed for time to get the data recorder started.
Aside from the obvious reminder lessons floating around that night, I learned something really important about our cameras. The concept is a little tricky to explain but the bottom line is that if you use an exposure time (senseup) that is too short, you can fail to detect your object at all. That meant x12 was a really bad idea. Kudos to the Carson City folks in figuring this out and running with x48 instead.
Here’s the details in case you are wondering. I took a lot of data a couple of weeks ago getting ready for the Pluto event. Normally you can take an image with one set of camera parameters and then scale to what you’d expect to see at other settings. I do this all the time, even for working with the Hubble Space Telescope. In our case, this calculation doesn’t quite work right, as I found out. You see, today’s digital detectors are a lot more capable than cameras were at the time the video signal standard (NTSC in the US) was developed. Video is designed for a fairly limited range in brightness, far less than what a good camera can deliver. That means you have to do something in the electronics to match the camera signal to the video output. This is normally labeled “brightness” and “contrast”, same as you’d see on an old TV.
If you were designing the perfect system, there would be a control that would let you set the signal level for the background of your image. There’s always some background, either it’s from the sky brightness directly or it’s from the noise floor of your detector. Now, you can think of a video signal as having 256 levels of brightness — 0 would be black, 128 would be grey, 255 would be white and you have shades in between. I always prefer to see my background. That means I’d set the background to be a signal of 5 to 10, depending on how noisy it is. That means any source in the sky you can detect will be seen as a brighter bump on the background.
Our MallinCAMs have other ideas about how to set the background, unfortunately. Now, I have to say that there’s a chance I just haven’t figured out how to configure them to do what I want but with my current recommended settings this is a problem to watch out for. As I was saying, the MallinCAM doesn’t have a problem with black sky (signal=0). That’s what I had for the Isolda event. The problem with this is that you can’t tell the difference between a signal level of -100 and -1. It all comes out as 0. So, not only could I not see the sky but the star to be occulted was at a signal level below 0 and I only got a few of the brightest stars in my field.
How do we deal with this issue? I’m not entirely sure yet. I can say that x64 for the upcoming Pluto event is safe. I really need to characterize the camera better so I know how to better predict its output. This will be an ongoing effort in the coming months. All of you could help if you like and I’ve also got a couple of bright high school students that are going to work on tasks like this as soon as school lets out.
Oh yes, there’s one other thing that I’ve noted. The DVR screen makes your images look darker and less useful than they really are. I put an example of this on the Pluto event page. This makes it a little tricky to ensure that you are really seeing the sky level when you are in the field.