Sunday, June 27, 2010


About three weeks ago, I bought a plastic and steel tool case from Harbor Freight to use as an eyepiece and accessory case.  The price was right, and I made a nice plywood insert with holders for the eyepieces.  But the case turned out to be stinky, no doubt due to the plastic off-gassing.  I can't put optics in it while it's still off-gassing as then they might be coated with the residue.  I've kept the case for most of the three weeks in the garage, open, hoping that the Texas temperatures would help, but the effect has been fairly small.  Yesterday, I went some steps further.  First, I left it out in the sunlight for a couple of hours.  Second, I filled the plastic bottom portion with a mix of water, dish soap and hand sanitizer (didn't have alcohol in another form) and left that in for a while.  Then I poured that out, wiped it, and left it in front of a fan for an hour or two.  It didn't smell much then.  But then after its spending another night in the garage, it's smelly again, but pretty definitely less than before.  Annoying.  But I think once I get all the smell out, I'll have a nice eyepiece case which will cut pack-up time by a few minutes, so I can stay at dark sites for a few minutes longer.

Sunday, June 20, 2010

Back from trip

I've just come back from a conference in Ireland.  I didn't do any stargazing there, but as always it's amazing to see how long the summer days are up north (latitude 53 versus the 31 of my home).  Here's a photo of the moon (left) and Venus (right--to see it you may need to click to blow up the image, or adjust the contrast on your screen) over a foggy golf course in Maynooth (Canon G7 braced on a bridge railing, set to 1600 ISO and 1/10 seconds), at around 9:50 pm local time.

Friday, June 11, 2010

Using three cent Rosco gel filters for planetary observing

As an experiment, I bought the Roscolux Swatchbook from B&H Photo for $2 plus $5 shipping. One can also get them free from a Rosco dealer if one lives near one (Lee also has a version). Anyway, what you get is about 250 gel filters, each 1.75" x 2.75" (with a hole that mounts it in the book) and each with a light-transmission curve. Anyway, one can then hold a filter between eyepiece and eye for planetary observing, assuming sufficient eye-relief (a reason to use long ER eyepieces). The filters are meant for stage lighting, and the small format are sometimes used for coloring a camera flash or strobe. I didn't know if they had the optical quality for actually looking through them, but thought it was worth trying, and so I ordered the set.

I had a bad experience in the past with a basic Celestron color filter set. While they occasionally may have teased something out, they scattered the light too much.

Here are my initial observations of the Rosco filters. The standard Roscolux gel filters look optically good to the naked eye--no obvious irregularities. (There are also included some special filters that are intended to scatter light. Obviously, these are unusable for astronomy purposes.) When I looked through them indoors naked-eye, I saw some light scatter around light bulbs (a small halo), which worried me a bit. (I hadn't done this test with the Celestrons.) But otherwise the image was very sharp.

I then took the set outside with my 8" F/4.5 Coulter, with a 6mm TMB/BO for 150X (I actually should have used a higher magnification, but I was in a bit of a hurry). Targets: Saturn and Mars.

I initially couldn't see any detail on Saturn without a filter. Occasionally, I got an intermittent hint of a band in the north.

I started with red filters. I could barely see Saturn through #27 Medium Red (4% light transmission). #26 Light Red (12%) was better, and occasionally--the seeing must have been quite intermittent--I got hints of a sharply defined Northern band. What worked best, however, was #23 Orange (32%). The band only appeared from time to time, but when it appeared it had a sharply focused appearance. Moreover, the filters removed the annoying CA that my TMB eyepiece contributes. I tried Moss Green (45%) but it did nothing for me.

I then moved on to Mars. Orange helped control the bloom that I get from the 1/8" thick double-vane Coulter spider, and it highlighted a dark feature near the south pole.  The ice cap was visible, too.  I eventually was able to see the feature and the ice cap without the filter, but the view was much better with the filter. Moss Green again did nothing for me.

But the really happy news was that I had none of the blurring that I had with the Celestron filter-thread filters. The rings and their shadow stayed sharp. At three cents per filter, this is a great deal.

The filters are very flimsy--I don't know how long they'll last. Moreover, they are hard to use when in the book. I need to find some way of getting them out--simply trying to flip them out of the book doesn't do the job well. Moreover, if I hold the whole book with the filter in front of the eyepiece, the filter wobbles and the object moves, which of course makes it harder to see detail.

Sharp images. Very large selection of filters, with a large variety of light transmissivities and for fine-tuning the choices. Includes light a transmission curve for each filter. Super cheap: get about 250 (not all astronomy-usable) for the price of one filter.

Future work:
I want to make a holder for these in my eyepiece case. I am thinking of putting up two steel rods sticking out, one for permanent storage of the filters and the other for the filters I am using during the session. There would be Velcro'ed webbing to keep the permanent storage filters from flopping away.

I also want to experiment with the gel filters as full-aperture filters. One can buy them in 20"x24" sheets for $6.50 plus shipping. In fact, one of the main reasons I got the swatchbook was so I could figure out which colors I wanted in the full-aperture dimensions. Orange is definitely going to be there.

Saturday, June 5, 2010

6" F/5 Dobsonian: Finished (at least my part)

We cut up the 1/2" (actually a little less Baltic Birch) on a friend's table saw (thanks!), and finally glued most of it up yesterday.

There is a hexagonal box around the center of gravity of the telescope tube.  That was hard to do.  I thought I had the table saw set to very close to 30 degrees from vertical (with a precisely cut piece of cardboard) and I thought we were getting 0.01 inch precision on the length of each cut, but the box was a bit too big, and the angles didn't quite close up.  To compensate for the size being too big we cut two sides a bit smaller, and we shimmed between the box and the tube where needed.  Here is the hexagonal box drying, held together with a giant rubber band (red) and Duck tape (silver).

The rocker box, on the other hand, came out really nicely.  The round parts were done with my fixed router.  We did need to use a home-made drum sander (plug cut by hole-saw on a bolt spun by a drill, with sandpaper hotglued--unfortunately, eventually the sand paper came off) to expand the sides.  I also glued 1/16" bondable PTFE pads to the sides under the rockers, and used the rockers themselves to press the pads down.  The clearance is pretty small--we'll see if any problems develop as the wood ages.

The rocker base is very simple: just a round piece of plywood, with three round legs (2.5" plugs from using a hole saw to cut the lightening holes in the rockers), and PTFE pads near the legs (for stability).  Bricks used for clamping both the pads and the legs.

Finally, today, with the kids' help I added the rockers to the tube.  I also added little wooden thingies, with PTFE lining inside the rocker holes on the box to keep the rockers from falling off.

And here is the finished scope.  Well, finished, except for finishing the wood, which is for my friend.  Interestingly, the tube and the primary cell are really stable.  Even though we did all that stuff with the tube, and the primary cell was removed for much of it, when I put the primary cell back in, although secondary collimation was off (I think that single-stalk secondary holder may need an upgrade eventually), primary collimation was almost exactly spot-on.  I also put some silicone on the outside of the inner edge of the focuser tube, where the Crayford focuser's bearings go, so the tube can't be racked too far out.  To prevent it from being racked too far in, the set-screw for eyepieces is located so it'll hit a wooden piece eventually.  To remove the focuser, I guess one either has to twist the focuser tube past the rockers (not good for the aluminum of the tube) or remove the secondary and get it out from the inside with the set-screw removed.  Now my friend has to finish the wood.

Wednesday, June 2, 2010

Galaxies and a comet

Last night I went out to my local darker sky site (yellow skies, not nearly, but still much better than in the city; and it's only 24 minutes away from home) with my 13" F/4.5 Coulter split-tube. And it was really quite delightful. But I was surprised by how late it gets dark. Sunset was about 8:30 pm, but at 9:00 pm it was very bright, and it was only around 9:20 that it got decently dark. I looked at Venus, Saturn and Mars first, since it doesn't need to be dark for them, and aligned AstroInfo to my setting "circles" (the azimuth circle is actually a square). Not much detail.

Most of my session was deep sky stuff.

Nebulae: Just M97. At one point I thought I saw a hint of the eyes (it is the "Owl Nebula"). But it really was a very faint blur.

Open clusters: Just M44. The sky in that direction was still bright, so it wasn't particularly spectacular. It was rather pretty in a delicate sort of way in my 70mm finder, and it was nice to pan with my SuperView 30mm in the main telescope.

Globular clusters: M3 and M13. I thought M13 wouldn't be so great because it was still fairly low, but it was quite nice, and indeed nicer than M3. Both resolved well with a 13mm Hyperion.

Galaxies (my main focus this time): Whirlpool (M51) looked decent. I could see a connection between the main galaxy and the smaller one being eaten, and I could see portions of the spiral, though it looked more like a concentric ring. I couldn't see the black eye in the Black Eye Galaxy (M64) very well. I think I once saw it, but faintly. I guess the seeing wasn't as good as last time I looked at M64. And the Leo Triplet looked delightful as always. Then I went around browsing for fainter NGC galaxies. First, 3593 (mag 11) and 3559 (12.8) in the vicinity of the Leo Triplet. Then on to the Coma Cluster. I've looked at that before, but without much luck. This time, I didn't have all that much luck either--I picked out a bunch of faint fuzzies, but some needed averted vision: 4944 (12.9), 4874 (11.9), 4884 (11.5), 4898 (13.6 -- yay! this is the dimmest galaxy I have ever seen), 4895 (13.2). Then I moved closer to Virgo, because I was looking for comet. I moved the scope around where the comet was supposed to be and picked up a faint fuzzy. But I was afraid it might be something else, so I compared with the chart to see what galaxies were in the vicinity, and while I was at it, I picked up the 5427 (11.4) and 5426 (12.1) pair. I then moved on to the comet (below), and ended my observing session with Centaurus A which was pretty low (I should have caught it an hour earlier) and all I could see was a bit of fuzzy with a lane.

The comet: 81P/Wild. My Palm TX downloads AstroInfo-compatible positions for brighter asteroids and comets every night (using a script that grabs data from Harvard's ephemeris service). Unfortunately, AstroInfo doesn't do orbital movement, so the positions are really only valid for one time in the evening, but unless the object is pretty close to earth, they'll get one pretty close. Anyway, 81P/Wild was in Virgo, at magnitude 11, so well within the range of what my scope could do, and it did show up as an elongated blur. Nothing particularly impressive, but fun to see--only my third or so comet so far.

On the less happy side, I found two bugs in AstroInfo. After I added a fourth alignment object, the computations went haywire. And I couldn't tap on the screen to select a minor planet--instead I got an object far, far way. I'll have to try to duplicate these at home and fix. But I shouldn't get distracted from my nature of modality book.

I have the entire Tycho-2 star catalog on my PDA. But it's still not enough--there are way more stars in the field of view than in the catalog. (Occasionally, too, the catalog misses something really bright, even in the mag 11.5 range. It's amazing just how bright an 11.5 magnitude star looks when one's been hunting for magnitude 13 galaxies.) But I shouldn't complain. The catalog was deep enough to help me find all the objects I was hoping to find.

(When I was a kid, and early in my amateur astronomy hobby, I found the night sky spooky. But now I feel relaxed and at home in the starry dark, with a telescope and (electronic) charts. There is a pleasant familiarity (though I still don't know the constellations by name very well; I often just see the shape on the chart and think of them as "the one that has such-and-such a squiggle"). I wonder if the feeling of at-homeness in the universe isn't some evidence that the universe is created for humans. On the other hand, we are made for heavenly life and exiles here. So maybe the feeling is pernicious. But the "here" where we are exiles, maybe that is not a spatial here. Perhaps the new earth and the new heavens will include--as in the last Narnia book--all the best features of the present earth and the present heavens.)