As promised, I can finally give you the lowdown on how good the TeleVue Plossl is.
It ain’t worth it.
I observed Jupiter tonight, for almost a solid hour, with a couple of breaks for the Geminid meteor shower. (I only saw one, out of the corner of my eye.) I switched back and forth a couple of times between the 9.7mm Meade 4000 Super Plossl and the 8mm TeleVue Plossl. Through both I saw pretty much exactly what I saw two days ago: the two equatorial belts, with some detail in both. I saw faint third and fourth bands closer to the poles. All four Galilean moons were on display, at a good distance from the planet.
Yet again, the Great Red Spot was nowhere to be seen – it wouldn’t reach meridian on Jupiter until about 2:30am, 3 hours away from when I was observing. Since Jupiter’s day is 9 hours and 50 minutes, that means that the Great Red Spot is only visible for half that time, less than 5 hours at a time. The GRS simply hadn’t come around to the side that faced the earth yet when I packed it in at midnight.
And that’s it. I have two possible explanations as to why I’m not seeing any difference. The first is that Jupiter was still fairly low down in the sky, only elevated about 25 degrees off of the horizon. This means that I was observing through much of the muck and mire of the atmosphere. I’m also not completely sure how good the seeing was tonight, although I was observing after 11, even after 11:30 on Sunday night, and plenty of lights were turning off as the hour went on, so it was fairly dark. But I think this thing about the atmosphere is an important consideration, and for that reason alone, I’ll rerun this test in about a month or so, when Jupiter is two hours – and 20 additional degrees – higher in the sky.
The second reason is focus. Just for fun, in addition to comparing the TeleVue with the Meade Plossl I also put in the 9mm Kelner that Celestron sent stock with the scope for a couple of minutes. I didn’t see quite as much detail with the Kelner as I had just seen through the TeleVue Plossl. Then I switched the Kelner out for the Meade Plossl. I know from previous use that these two EPs are basically parfocal, even though they’re from different companies. When I put the Meade in, the focus was slightly better than it had just been with the Kelner. Io was closest to Jupiter and it was tack sharp – a little pinprick. I still couldn’t see any more or less than I had just seen with the TeleVue
It is difficult getting a completely precise focus in the Mak. The focus mechanism for a Mak, and on an SCT as well, is that the mirror is mounting on a track, and as you twist the focus knob, you’re actually moving the entire mirror back and forth on that track ever so slightly. There is a little play in the track, so that when you go past the focal point, you have to twist the focus knob back the other way past where you think it should be to get that focus right.
One solution to this is something called the Bahtinov mask, named after its inventor, who only made it generally available less than 10 years ago – so something I had never heard of 30 years ago when I was observing as a kid. This is a similar to a solar filter in a way, in that it’s something that slips over the front end of the scope and effects the light coming through. But that’s where the similarity ends, because it has numerous slits for the light to get through. It’s sort of like a radiator grille, or an air conditioning vent – lots of little slits, all parallel to each other. There are three sets of parallel slits, at different angles. You first point the telescope at a bright star, focus as best you can, then slip the mask over the front. The mask creates what looks like three sets of diffraction spikes. When you get them all lined up, you’re in perfect focus. You can either buy a mask or make one.
However, there’s also something much, much simpler, called a Lord mask. Instead of lots of little slits, this consists of one big skinny “Y” shape that you put in a mask that goes across the aperture. The same effect is achieved – three sets of diffraction spikes, that you have to line up by focusing properly. However, because the Y covers so little of the aperture, much more light gets through. I have to try this out and see if it works.
So again, it is possible that because of the lack of precise focus, the comparison still isn’t completely fair. As for the TeleVue, I feel like Natalie Wood in Miracle on 34th Street: “I believe, I believe, it’s silly but I believe.” I want to believe that the TeleVue is better, but I’ve still got absolutely no proof. If it is actually is better, it’s certainly not showing up in my observations. Yet. Another test and we’ll see.
Another upgrade I just ordered is a new dielectric diagonal. When I initially bought the scope, I investigated as to whether I should or shouldn’t upgrade my diagonal. My understanding is that the diagonal that comes stock is actually rather poor, reflecting only about 90-91% of the light that hits it. There are diagonals called dielectric diagonals, specially coated, so that they reflect 99% of the light. When I did my research, I specifically inquired as to whether this 8-9% made a difference. The answer was no. But lately, doing more research, I’ve been finding a different answer – that you can see the difference, and you can see it easily. It’s almost like the Bible: you can find support for practically any position since there’s just so much material. But I thought, since I want to try to optimize the entire optical system so that there aren’t any excuses, why not? Right now my order is processing with Agena; maybe I’ll get it next week.
Next time – What is light-polluted astronomy, anyway?