Donnie’s Blog

At long last, I am finally getting back to building my 13.1″ telescope!

The last time I did anything with that beast, I think I had just finished the 320-grit grinding. The surface looked great; reflections from the wet mirror were really beginning to look sharp. But, there was one little problem: the mirror’s focal length was awfully long. I was looking at a telescope that would probably be 6-7 feet tall, and for a 13.1″ mirror that is a little silly. I didn’t want to have to stand on a stool just to look through my little telescope. For an 18″ aperture, that might be completely reasonable, but for a 13″ it’s just silly. Going back over my mirror-grinding e-mails, I think my technique was causing the problem. Regardless, at the time I was pretty frustrated with the thing so I wrapped it up and put it in a box for a future time.

Nearly five years passed… and in the meantime I decided that what I really needed to do was to just go back to the beginning and deepen the curve of my mirror, then go through the intermediate stages again with a little more care. It took a while for me to warm up to the idea though. Mirror-grinding is hard and painful work. I think I needed to forget how bad it was before I went back to try again!

So, I finally got around to ordering more mirror-grinding grit from Newport Glass Works, a pretty cool company that I definitely want to use again once I get this telescope done and begin working on the next one. I ordered a grit-kit that contains all of the different grit-sizes, along with cerium oxide and pitch. The grit-kit showed up today (my last day of lectures, quite convenient), so I should be able to get back into the mirror-grinding pretty quickly. This should be a lot of fun!

Now that I have had a couple of serious astrophotography experiences with my new Canon 40D, I figured that I should actually write a bit about it. Before the 40D I used a 20D, which was certainly better than film, but I never was really able to achieve the level that I wanted to. The primary problem is invariably achieving a good focus. There are several techniques for getting a good focus, the best of which is probably using a focusing aid like the Kendrick Kwik Focus. The concept is simple - when the telescope is out of focus, the light takes on the shape of the aperture of the telescope, central obstruction and all. Thus, if you put a mask on the front of the telescope with two or three holes, you see two or three spots if the scope is out of focus. But, once the telescope is brought into focus, all of those spots converge, and you see only one image.

Aside: Most focusing aids use circular holes, but if you are making your own focusing aid (and why wouldn’t you?) you probably would do best with triangles at different orientations, rather than circles. This way you get some diffraction spikes as well, and when they are all centered around one point in the field of view, you know you are in focus.

There is one feature on the 40D that hands-down makes it a sheer joy to work with: Live View! Live View lets you get a really good look at what is coming into the camera, before you actually take any photos. This means you can point the telescope at a bright star, switch on Live View, zoom in on the image, and get a really good focus on that star before taking any photos. I have used Live View for both piggyback-mount photography and prime-focus photography, and it is just great for getting a good sharp focus. (I do have to say though, nothing shows the ugly realities about the quality of your camera lenses like focusing on a star. Oh well…)

The Canon 40D has a number of other nice features for use with astrophotography. I always switch the camera into timer-mode and set the mirror to pre-fire so that there is as little motion as possible when the photo is taken. (Even with that, there is still vibration from the shutter, but you can see my previous post for details about that.) I also use the TC80-N3 timer-remote to control the camera without touching it. Of course, the main feature I use is the timed-exposure functionality; the other features of that remote aren’t so relevant to astrophotography. If you switch the camera into M (manual) mode, turn on the camera’s 10-second timer, and set the timer-remote to your desired exposure length plus 10 seconds, everything works great. The only hard part is sitting there waiting to see the results.

That is about all I have to say about using the 40D for astrophotography at this point. I don’t know much about “frequency-response curves,” how suitable the camera is for imaging hydrogen-alpha wavelengths of light, or anything like that. I’m an amateur. I just know that this camera is a whole lot easier to use than the 20D, and it really has me motivated to get out of the city a bit more often to take some pictures of the sky. I’m happy with that!

For the last couple of weeks there has been a strange, tailless comet in the constellation Perseus. It started getting more attention in the news when it suddenly brightened, so it’s one of the more popular things in the sky at the moment. Hopefully it will be around for a while; you don’t normally have things that easy to see from the city.

Now, usually I would be whining about how much I hate comets, but this time I was actually able to find it myself, so this one is okay.

Here is a wide-field photo of Holmes, taken with my camera mounted piggyback on my telescope, using a 100mm macro lens:

And, here is a close-up taken through the telescope:

You can see some vibration in the close-up photo. I am pretty sure this was caused by the shutter when I took the photo. Normally I set the mirror to pre-fire, and I take photos on a timer, so I can’t think of anything else that could cause any vibrations.

There is this technique called the “hat trick” (at least, that’s what Michael Covington called it in his great book on astrophotography), which is the simple solution to this problem. Put something large and dark in front of the telescope - say, a hat - start the exposure, then move the hat out of the way. That way there will be absolutely no vibrations during the exposure.

It’s strange but true. The more sophisticated things get, the less you can actually do with them.

I tried setting up my Canon 40D with the new telescope to see how it does with prime-focus astrophotography. “Prime-focus astrophotography” is where the telescope functions as the camera’s lens. There are other ways to connect a camera to a telescope, but that is the simplest one, and best for deep-sky objects. I wanted to see whether there was any vignetting, how sharp the images look, and all that kind of stuff. Instead, all I get is:

    Err 01
    Communications between the camera and lens is faulty.
    Clean the lens contacts.

Um, of course the camera can’t communicate with the lens. The telescope mount certainly doesn’t have any electronics built in.

So, I read the manual to see if there was a way to tell the camera to ignore a “missing” lens. Nope. Search around on the Internet for a way around this? No dice. Call Canon tech support. That was the infuriating one - they acted like this was a silly thing to want. To take a photograph without a lens? Who would want to do something like that??!

Guess what - the 20D can do it! I’m sure glad I still have my 20D…

I have been looking for a portable telescope that I can take on trips that involve air travel. This is no easy task, especially with increased airport security measures. Although there are certainly hard cases that can protect telescope optics from a certain amount of trauma, are they up to the challenge of protecting a telescope through checked baggage? Especially if one of the workers is having a bad day and is being a little rough on things.

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Anytime I am thinking about putting together a star party I have to do some research so that I can figure out some interesting things to look at. The sky is a big place after all, and there’s a lot of pretty lame things to look at. Also, “here’s a globular cluster that’s 4000 light-years away, with an estimated 3000 stars in it” is a lot more interesting than “ooo a cotton-ball blob!” when you are talking to people who aren’t so excited about astronomy. Or, if someone asks if you can see Neptune, it’s a lot nicer to say “Oh, it’s already set,” rather than “Hmm I don’t know where it is right now.”

I used to use the Sky and Telescope Interactive Sky Chart, but recently they changed it to require a user account, and even though the account is free, I don’t want to have one more account to remember. It’s just an extra hoop to jump through. So I looked around for some free star-chart software and I found Stellarium.

The neatest thing about Stellarium is that a lot of attention was given to visual appearance, and everything is rendered in true 3D with OpenGL. Sunrises and sunsets are rendered in a pretty realistic way, which is good if you want to get a feel for what is visible at a particular time. The band of the Milky Way is rendered for night displays, and the Small and Large Magellanic Clusters if you look from a location in the Southern Hemisphere. Constellations, including artwork, can be turned on as well. It was definitely an “ooh, ahh” moment, the first time I used it.

Anyway, if you are looking for a simple, free program for seeing what is in the sky, check it out. It’s quite nice.

Pluto is like any other large asteroid now.

It didn’t even get a good low number like all the other well-known asteroids, since it’s so late in being classified as a non-planetary body. Pluto’s designation is 134340.

Well I guess it’s an easy number to remember, at least.

Last night I actually scraped together the energy to get out the ol’ telescope. Jupiter is out early at night, and the weather has finally started being clear at night, and I had never used my new Canon 20D camera with my telescope, so I decided to take advantage of the opportunity.

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Today I came across this very interesting article on the Sky and Telescope website:

• Solar Cycle Solved?

It’s really quite an interesting thought, that scientists have developed a much more accurate model for the solar cycle! It is also personally interesting to me, because I happened to buy a solar telescope just a few years before the solar cycle hit its minimum level of activity. Which was quite a bummer for me, needless to say. All that money, and bupkis to look at! “Note to self: Before buying the next solar telescope, check the solar cycle…”

But now it looks like we are getting back to the up-swing in a couple years, so it will be good to hang onto my solar telescope and see all of the increasing activity through it. Maybe I will even get some good photos. I’ll have to go up to the mountains if I want to get any really good photos; there’s just too much atmosphere down here near sea-level.

It also got me to thinking that, just like telescope sales jumped right before the 2003 Mars opposition, maybe we’ll see a jump in the sales of solar telescopes over the next 3-5 years. I’m not exactly sure how I could take advantage of that, though…

Last night I actually felt like getting out the ol’ telescope for once. Saturn is high in the evenings now, as is M42, and this is the only time of year that I get to enjoy those guys without getting up at crazy hours.

Unfortunately, it was hazy all day, and by the time I went out to start setting up the telescope, there was a thick cloud-cover.

So, maybe tonight or tomorrow night will work out.

This is one benefit of moving my schedule to match the students’ schedules - I’m more apt to pull out the telescope, because I’m already going to be up late!