Wednesday, September 29, 2010

Comet Hartley 103P

Finally I had a decent view of it.  Took a long time to find, partly because I didn't realize it was moving fairly quickly, so an ephemeris even 20 minutes out of date had a visibly off position.  What I could see with my 8" at around 68X and 150X was a diffuse oval glow, about a minute of arc in diameter.  The center of the glow was about a minute of arc away from the predicted position, which also threw me off.

Thursday, September 23, 2010

Project updates

I'm at various stages of four astronomy DIY projects.  Projects 2 and 3 are linked in that the eyepiece will have narrow usable field of view, and hence will benefit from the equatorial table.

1. Digital setting circles: I bought a bluetooth-based module that can communicate with encoders.  Unfortunately, I can't get the magnetic encoder to work with it.  I think an oscilloscope might be needed to figure it out.  So I may just need to save up for two optical encoders (360 cpr, and then attached via a 1:4 timing pulley setup).

2. Equatorial table: I had routed the sectors, but it turns out that I had miscalculated the center of mass of the telescope, so the cut radii weren't good.  I've been procrastinating recutting them.  Need to get back to that.

3. Hastings triplet eyepiece: Today I ordered a super-cheap (about $1.80 shipped) ebay 30X triplet loupe.  I think it should be a 8.3mm Hastings triplet.  If it works, I hope to drill out a 1.25" wood dowel and mount it inside as a nice planetary 8.3mm eyepiece.  If it works really well, I might try to sell them for $15-20 each.

4. Laser collimator: This is for a friend.  I still need to print and cut out a collimation target, and then collimate the collimator.

Monday, September 20, 2010

How small stars in the sky are

For optical calculations, it is usual to approximate stars other than the sun as basically point sources of light.  How good an approximation is this?

Well, Alpha Centauri is about four light-years away, and a little bit bigger than the sun.  It is very easily seen, being the fifth brightest star in the sky.  How much apparent space in our naked-eye visual field would the size of Alpha Centauri's disk take up, if we didn't count diffraction, atmospheric effects, etc.?  Answer: Very, very little, about 0.01 seconds of arc.  In fact, it would take up approximately the same amount of space in our naked-eye visual field as an HIV virus seen from one foot away!  The stars take up tiny spaces in the sky.  But they're very bright for their size.

Saturday, September 18, 2010

Easy and cheap small thumbscrew

The set screw on my digital calipers has always been bent, which was a nuisance as it didn't let me tighten them.  I finally got fed up and fixed it.  Turns out that it used an M2.5 screw, and I had a whole bunch of these that I bought to fix our laptop.  But it would be nice to have a knurled head.  Well, I still had a lot of #6-40 nylon socket cap screws that I needed two of for my Daisy finder improvement. So I cut the heads off one of the socket cap screws, and drilled through the hex key hole in the cap.  I then screwed the M2.5 screw into the cap, gluing it place with a package of some cheap no-name super glue that I picked up at a grocery store and that I've been very happy with because somehow it never gets stuck in the nozzle like it does with other brands.  I then decided I wanted a bigger knurled part to turn, so I beheaded another socket cap screw, drilled it, and glued it in place.  Doesn't look perfect, but works great.

Binocular mount

I finally made a binocular mount for my faithful Celestron 15x70s.  It's a parallegram mount with five degrees of freedom: (1) the whole thing swivels where it attaches to the tripod, and (2) goes up and down; the binoculars then swivel (3) up and down and (4) left and right, and (6) can rotate a bit side to side.  It sits on a very short tripod as it's designed to be used by seated me and standing children.

Total cost was quite low.  The long dark pieces of wood are pieces of a recalled cherry crib that was being given away for scrap wood on Craigslist.  Most of the rest of the wood is 1/2" Baltic birch leftover from the 6" F/5 made for my friend, for some parts laminated to approximately 1" thickness.  There are a bunch of 1/4-20 carriage bolts and knobs from Amazon's Industrial and Scientific store.  I had some leftover PTFE for the main azimuth bearing, and purchased some cheap PTFE washers from Amazon for a low price (the price has now gone up by an order of magnitude).  The hideous counterweight is a bubble wrap envelope full of sand, duct taped all around.  The first iteration used a brick, but I was worried that a kid might whack someone on the head with it, and so bubble wrapped sand is better.

It was pretty easy to make.  I used a router for some round pieces, like the semicircular binocular attachment, but polygonal pieces cut with a hand saw would have worked just as well.

The azimuth bearing was the most complicated part.  It's basically scaled-down Dobsonian azimuth bearing: there are three PTFE pads inside, which ride on a CD (two out of my three Dobsonians ride on vinyl records).  This then bolts to tripod platform.

The tripod is entirely home made, too.  I once ended up buying twelve feet of 1.25" PVC conduit at Home Depot or Lowes because I needed a few inches worth for a focuser.  I expected I would eventually find a use for it, and hereby I did.  The tripod platform is a round piece of 1/2" Baltic birch, with some square poplar rods glued underneath, between which the PVC legs sit, attached with a carriage bolt.  To keep the legs from sliding apart too much, I glued three pieces of webbing, I think scrapped from a car seat or feeding seat, glued to the legs with Shoe Goo, and then screwed into place for additional safety.

I may later make a post with more detailed photos, but it was all fairly simple.  One thing that took me a bit of time to figure out is the brown block of wood that the semicircular binocular attachment attaches to.  It is a rectangle of cherry, with an up-down 1/4" hole and an across 1/4" hole.  The latter hole has a permanently mounted carriage bolt.  I used a 5/8" Fortsner bit to dig a deep well for the head of that bolt so that the up-down bolt would be able to sit to the left of that head.  I then filled the well with JB Weld for strength.

Thursday, September 16, 2010

Solar system evening

I pulled out my 8" Coulter, and spent a long time looking at Jupiter, trying different Rosco gel filters.  I think I had my most detailed views of Jupiter ever--it really looked good.  I found the emerald-green and sky blue filters the most helpful.  I then spent about half an hour trying to find Comet 103P/Hartley 2.  It was tough, because in my backyard there are only small patches of sky showing, and in that direction is a lit up street, so I had a hard time identifying a star that I could star-hop from.  Finally, I went for Zeta Cephei, which was 15 degrees away.  I then went field of view by field of view, matching the view through the eyepiece with the chart in AstroInfo.  I found the stars where the comet was supposed to be.  And then I experimented with different eyepieces.  I kept on getting glimpses of something in the area with averted vision.  And once or twice with an O-III filter, also with averted vision.  I was using magnifications between 65 and 150X.  So I guess I saw it.

I rounded out the evening with a look at the moon, another quick look at Jupiter (not as good as earlier) and then a look at Uranus.

Saturday, September 4, 2010

Altitude brake

My Dobsonians move very smoothly.  The problem with that is that if I am showing things to the public, it is easy for people to push the scope away from the object, especially in altitude.  Before the last public star party, I made a little altitude brake, based on ideas on Cloudy Nights.  I used the router to cut a plywood disc the size of the altitude bearings (7" diameter) on my 13" scope.  (Actually, one could just cut a triangular piece with a hand saw.)  A plywood strip (with a plywood pad on the other end) is glued to it as a handle.  I attached three grippy rubber pads (one could put silicone sealant pads, too) near the circumference.  A bolt keeps the handle immobile, and a big 1/4"-20 carriage bolt runs from the inside of the mirror box, through the altitude bearing, through the disc, and then there is a fender washer and a knob to adjust tension.  The handle is bolted to the rocker box.  At low tension, movement is slowed down, but still smooth, despite the grippy pads.  At full tension, movement is stopped.  The original design I was following used a spring, but that doesn't seem to do anything.

I also made a smaller 2.5" version with Velcro hook pads just to slow down, but not stop, movement.  I didn't try it in the field, though.

Daisy red dot finder modifications

The Daisy red dot sight (the best price I've seen was about $8 at Walmart, but last I checked, my local stores didn't have it) makes a decent non-magnifying finder for telescopes. I have them on two of my reflectors, and the third reflector has the Galileo version.  Mounting requires a dovetail.  One option is to take a piece of wood of the right thickness, and then file a roughly shaped dovetail.  Another option is to take a piece of wood, and screw on washers of the right size for the dovetail to fit around.  It's a good idea to mount it offset from the scope for comfort.

A few modifications make the Daisy finders nicer to use.

1. Resistor.  The stock sight is designed for shooting in daylight, so the red dot is too bright for night use.  You can solder in a resistor (I think probably 2K-5K ohm, depending on taste), or you can fold a resistor in half, put a piece of paper between the leads, and put this resistor sandwich between the battery and a contact.  Even better, you can buy a 5K or 10K ohm trimmer pot (e.g., at Radio Shack), drill holes for the leads, and solder wires in place.  Then you can adjust brightness.  The photo shows the version with the pot.

2. Altitude/azimuth adjustment.  The stock sight needs a screwdriver for adjustment.  Since the sight needs to be adjusted from time to time (or even every time, as on my 13" split-tube scope), and using a screwdriver in the dark is no fun, it would be nice to have tool-free alignment.

On the Daisy that's on my 13", the azimuth adjustment screw has a hex nut (attached with some thread locker, I assume) that can be easily turned by hand.  Only the altitude is an issue.  What I did for the altitude adjustment is I removed the screw and turned it upside down so the hex nut is on top.  That nut wasn't big enough to turn with comfort, so I used Super Glue (or some other cyanoacrylate glue) to glue a larger hex nut on top of it.  Works just fine by hand.

The Daisy on my 8" is an older model with round (!) nuts.  The adjustment screws are #6-40, a bit less than an inch in length.  Amazon's Industrial and Scientific store had a really cheap pack of 100 black nylon socket cap screws (about 50 cents), so I got that.  I was a bit worried if nylon screws would work well.  But they did.  The azimuth adjustment was straightforward: I just replaced the stock screw with the nylon socket cap screw, and the knurled head of the screw nicely turns by hand.  The altitude adjustment was moving poorly with the first screw I put in.  I removed it, and put in another, and it was fine.  Maybe there was something wrong with the thread in the sight, or maybe the screw wasn't good.  But, hey, I had 98 others at that point.  Looks good, too.

Finally, I trimmed the screws right outside the nuts by cutting them with kitchen scissors.  That also spread out the ends of the plastic screws, which should keep the nuts from coming off them.  But for good measure, I took a soldering iron and squashed the part sticking out of the nut, so the nut wouldn't spin off.  (If I need to exchange screws, I can always trim with box cutters.)  I went for nylon screws in the first place because they were so cheap, but in this application, they're also easier to work with (no need for any thread locker or hacksaw), and they won't rust.

The only issue is that (as of Sept. 4, 2010) the price on the screws has gone up to about $6.  Fortunately, they still have a pack of 50 of white versions of these screws for 82 cents.  If you have Amazon prime, or are doing a $25 order, the shipping will be free.  (If you can't get free shipping, you can paypal me the price of a stamp and if I still have enough, and there aren't too many requests, I can send you two of the black ones.  And maybe even a resistor if you want.)

3. Removing coating.  The stock sight comes with a lens that's coated with a dark reflective coating.  The sight is still usable, but you can't see dimmer stars through the lens.  That's not a big problem as you can keep both eyes open and see the stars with the other eye, but stripping out the coating is a nice idea.  This was hard work--about an hour for each sight I did this to.  And one of my sights now points differently from before--I don't know why.  To get it aligned with the scope, I had to angle the dovetail mount on the scope quite a bit.  The image also isn't as neat a dot (and some have seen ghost images).  All in all, I still think it's an improvement, though just barely worth it given the work.

The first step was to remove the lens--I can't remember if there was more than one.  There is a plastic retaining ring holding the lens in place.  Remove that first.  I did it by use of a jeweler's screwdriver, wedging it in behind it.  Once when I did it the ring got creased--it's not so strong--but it was still fine.  Be careful that the lens not fall out, as you will need to note the direction the lens is pointing.  Then remove the lens assembly.  Make careful note of the direction in which the lens is pointing--one side is coated, and the curvature is only on one side.  If there is more than one lens, do it for all of them.

Now, it's time for the hard work of polishing off the coating.  I don't have a satisfactory method.  I used two different polishing compounds: toothpaste plus water, and baking soda plus water.  Generally, I first did the toothpaste, and then moved on to the baking soda.  I applied them with denim.  I did this by hand, which was slow and inefficient.  The other thing I tried was to wrap the cloth with polishing compound around the handle of a screwdriver, duct tape it into place, load the screwdriver blade into a drill, and run the drill.  Eventually, you notice that the coating in the middle is disappearing.  It's really frustrating to get every spot off near the edge.  Some you may just want to live with.  Once done, wash, dry and put back in.