Off-gassing

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.

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.

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.

Cons:
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.

Pros:
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.

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.

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.)

Waco Star Party announcement

Here's something my local club is doing:

WACO STAR PARTY

June 5th, 2010
8:40pm to 10:30pm

The Central Texas Astronomical Society (CTAS) will have our Waco Star Party for members and the public on Saturday June 5th starting at 8:40pm, regardless of weather. We will meet as usual at the Waco Wetlands. Bring your telescope (if you just got a telescope and want help with it, this is a great place for that) or just enjoy those of the members as we explore the night sky of June.

We will begin with an indoor program on the life-cycle of stars, even if it's cloudy.

Then, weather-permitting, we hope to see objects both inside our Solar System, elsewhere in and around our Milky Way Galaxy, and outside our galaxy. In our Solar System, we will look at Mars and Saturn. Elsewhere in our galaxy, we would like to look at the Beehive open cluster of stars (M44) which can be seen with the naked eye as a faint glow, the Hercules globular cluster (M13) which is a ball of older stars that orbits our galaxy, and a nebula that is the remains of a burnt-out star (M97). Outside our galaxy, we will look at the Leo Triplet of galaxies, a trio of three galaxies that can be seen in one telescope field of view, and the Whirlpool Galaxy (M51)--a cannibal that is eating up a neighboring galaxy which can also be seen.

Experience it yourself and work with other local amateur astronomers to explore all that the night sky has to offer. This event is free to the public and is sponsored by CTAS.

Directions to observing site:
Lake Waco Wetlands at 1752 Eichelberger Crossing Road.

Directions: From Interstate 35 take the Exit 330 and proceed west (toward Meridian) on Hwy. 6/Loop 340. Continue for approximately ten miles to the intersection of Hwy 6 & FM 185. Turn right onto FM 185 and continue 0.6 miles then turn left on Eichelberger Crossing Road. The Lake Waco Wetlands Research and Education Center will be 1.6 miles on your right at 1752 Eichelberger Crossing Road.

AstroInfo 3.10X

Thanks to hard work by Jochen Hoenecke, AstroInfo has been updated to allow for larger catalogs. No more installing a dozen star catalogs where one will do. The result is that if you want mag 11 catalogs, it's much faster.

AstroInfo is a pretty full featured (except for telescope control) free (GPL) planetarium program for PalmOS. Includes Tycho-2 catalogs up to magnitude 11 (but if you don't have enough memory, you can install smaller versions, like up to 8.5 or up to 10).

Before upgrading, make sure you delete all the old catalogs, as the new version is not compatible with many of the old catalogs.

This should be considered a beta, or maybe even alpha, test version:

https://sourceforge.net/projects/handypalmstuff/files/

Waco area cloud cover predictor

Using NOAA weather maps, some time ago I made a little online Waco area cloud cover predictor, which is now hosted on a better server than before. It shows data for about a week ahead of time (unlike the otherwise superior clear sky clock), every couple of hours, and it shows a mini map of the area. The numbers are percentage cloud covers. My experience is that small numbers are overestimated--about 20% on the map equals to the sky looking clear.

Custom moon map service relocated

I've relocated the custom moon map service to http://pruss.mobi/cgi-bin/moon.pl.  This may only be a temporary fix.

Need hosting for my custom moon map service

It being hot weather, and the air conditioning being in operation, I want to take a long hard look at always-on heat emitters, like our home server. Currently, besides doing some file serving that we can do, and some work-related stuff that I can move to my office server, it serves up custom moon maps--you choose the features you want, and it makes the map. Bandwidth requirements are low as I only have one user about every second day. My current server is a 2ghz P4 with 512mb RAM, so nothing fancy is called for. Just apache+modperl (with Perlmagick). If anybody is interested in volunteering some server CPU time and bandwidth, email me.

Moon and Saturn

Last night, I had a nice time looking at the Moon and Saturn.  The shadow of the ring seemed rather more offset from the ring then I remembered it--must be due to the way things are tilted.  The Moon was lovely, both in my 8" Coulter, and in the 6" F/5 I'm building (where it was super-sharp at 125X, even though I was hand-holding).  I was particularly struck by a crater with a lovely double peak near the terminator.  The deep parts of the crater were dark, and there were two gleaming white peaks there.  Maybe it was Burg?  I didn't check with a map at the time.  Burg looks slightly too far from the terminator in the Virtual Moon Atlas, though.  But maybe.

6" F/5 Dobsonian: Secondary mount and first light!

I glued the secondary mirror to its support the standard way, with silicone sealant and some toothpicks to ensure spacing, which toothpicks I removed after a couple of hours of drying.  It's a nice-looking 1.5" minor axis mirror.

I had a hard time deciding on the secondary mirror mount.  I wanted two-axis collimation done in Coulter-style, by having the support rotate along one axis and the secondary mirror along the other.  But I was having a hard time figuring out what the support should be.  I considered a two-vane support going across the aperture, using maybe a hacksaw blade (I went to the length of grinding teeth off it) or a steel ruler or even a hacksaw blade epoxied to a steel ruler.

But I also had some 1/8" thick 6" long 1" wide steel strips.  I bent one near an end--sandwiched it between two others, and hit it with a mallet, and then did the final fine adjustments by grabbing the end with an adjustable wrench and just bending by hand.  It was hard finding the exact right place for it and the exactly right bend.

In the evening my friend came.  We put in the primary.  And then tried to mount the secondary.  I was very nervous about dropping something on the primary.  My initial solution was to tie a safety line to my secondary support strip, and tie the other end to the focuser so if I dropped it, it wouldn't hit the primary.  I never dropped it, fortunately.  Then I had an idea for a better safety measure--I gently put a scrunched up T-shirt on top of the primary.  Good idea!  I dropped a fender washers two or three times on it in the course of the evening.

With my friend there, we measured off half of the diameter of the tube and cut a piece of wood to help center the strip.  Moreover, we cut a small piece of hardwood to act as a small carpenter's square, for squaring the strip against the focuser tube (which I racked in all the way).  Also, to get the vertical positioning right, I put a laser collimator in the focuser and used Inkscape to draw a little centering target which my friend cut out and we taped over the front of the secondary.  (My laser collimator emits a cross, so I aligned it with one of the crosses on the target.)

Eventually, I gave up trying to get it completely perfect, and mounted it.  Then I drilled a hole for the secondary mount on the support strip after aligning the target on the secondary with the laser (sorry, no offset).  And mounted the secondary to it.  (By the way, that big screw will have to be trimmed slightly.  And I will glue its head down so as to make the collimation entirely tool-free.

The secondary support is mounted to the tube with a carriage bolt.  One end of the carriage bolt is capped with a plastic wingnut, and the other goes into a plywood disk on the outside of the tube.

I collimated with my laser, and it was time for first-light.  The sky was clear.  I plunked my "30mm" Rini (probably 26mm or so) in the eyepiece for 28X with an approximately two degree field.  And, hurrah, stars came into focus, while handholding the tube.  With another friend's help (he saw us over the fence working away, and dropped in to kibbitz), and the scope leaning on the earlier mentioned T-shirt on the trunk of my car in the driveway, we saw a lovely but tiny sharp image of Saturn.  We then looked at Mizar and Alcor, and split Mizar.  Didn't look at any deep sky objects.  At one point it looked like I saw a dark nebula, but it was just leaves on a tree.

The diffraction spike from the single vane support didn't bother me and more importantly didn't bother my friend--it's his scope, after all.

So, the tube is done is done.  Now it will be time for the mount.  We'll make a hexagonal ring, about 7" wide, around the middle of the tube (the center of gravity with my 13mm Hyperion plus a Barlow is in the middle;  it'll be even further front once my friend attaches a Telrad), attach the altitude bearings to that, and make a standard Dobsonian box, probably on a circular base.

6" F/5 Dobsonian: Wooden Crayford focuser

Update: I now prefer the much simpler design of this focuser.

Like I said, I hate making focusers.  If I were making this for myself, I'd probably stick with a push-pull Crayford like on my travel scope.  But I'm making this for a friend, so I made a more normal Crayford.  The draw tube is aluminum with 1.5" outer and 1.25" inner diameter.  Actually, my eyepieces and laser collimator didn't fit when I got it, so I had to sand the inside of the tube.  The method was to take a screwdriver, wrap some sandpaper and foam around the handle, affix it at one end with duct tape, and spin the screwdriver with a drill.  Took a while, but eventually I got the bore large enough.

The main part of the focuser is a particl board plate, with two 5/16" dowels along the sides to fit the curvature of the tube, and three posts.  The two of the posts on the right are, I think, oak, and the one on the left is cut from 1"x1" poplar square rod, and they all have little plywood thingies behind them to keep them from falling over.  They are both glued and screwed to the plate.  The two posts on the right get ball bearings (the cheap ones for skateboards that one can buy on ebay, with wood screws JB Weld'ed inside them), and the one on the left has a hole  cut out in it for a holder for the focusing shaft.  There are two screws on the holder for adjusting tension.

The non-fixed parts consist of a little H-shaped poplar thingy that fits in the slit in the left post.  It has some V-shaped holes cut in it and lined with bondable PTFE.  Between the H-shaped thingy and its post there is a folded rubber thing to add some spring, cut from a bicycle inner tube.  The adjustment screws on the post bear against the rubber.  The shaft is 1/4" stainless steel.  The knobs are cut from pine boards.  They're not as straight as they could be, but they're light.  I cut the knobs by first drilling about 1/3" deep into a pine board with a 1.25" paddle bit.  Then I cut the pieces out with a larger (1.5" or 1.75") hole saw.  Result: cylinders with a recessed hole that lightens it and makes it look better.  I then put a bolt through each, and spun it against sandpaper on a drill.  The shaft then fits in the holes that the hole saw mandrel made.  I put some PTFE rings on the shaft, and I filed the ends of the shaft to make it rougher and less round, and glued the knobs in placed with, of course, JB Weld.  (I use JB Weld for most bonds other than wood-wood.  I use Titebond II for most wood-wood joins.  Except that the dowels at the bottom of the focuser, to make the focuser hug the tube, were attached with Gorilla Glue, because I wanted a glue that would (a) dry quickly and (b) make little gussets.)

Motion is delightfully smooth.  The next major step will be mounting the secondary.

6" F/5 Dobsonian: Drilling a focuser hole

I was really nervous about making the focuser hole--drilling in a cardboard tube can be messy.  On my Coulter 8" when I replaced the focuser, I had to enlarge the focuser hole, and it was a quite big mess.  I thought of trying to use a rotary tool, but I couldn't figure out which bit was appropriate, and had no scrap to try it out.

But I decided that the glue-impregnated cardboard just might take a hole saw, and risked drilling with a hole saw.  Success!  There was just one little bit of tearout on the inside, but I glued that down while impregnating the hole with water-diluted Titebond II.

The focuser is going to be a Crayford.  It's almost finished--it just needs some 5/16" dowel glued on the underside for mounting on a curved tube, and I'll buy that tomorrow after dropping the kids off at school.  I'll post on it when it's finished.  This morning I got to do some trigonometry to try to figure out where to place the dowels so the center of the focuser board would be tangent to the telescope tube, and the dowels would be tangent to the focuser board and the tube.  Somehow the calculation didn't work out.  Then I found a simpler method with Pythagoras' Theorem, which did work out.  Geometry is fun.  I don't know how some people manage to do serious woodworking without using trigonometry.  I guess they might be really good at scale drawings, or just able to eyeball things in ways I can't.

6" F/5 Dobsonian: Mirror cells

I used a router and a hand saw to make a simple secondary cell out of 1" square poplar rod.  It will eventually attach it to a simple two-vane hacksaw blade spider (I used a cheap Harbor Freight rotary tool with a cut-off disk to grind the teeth off the blade), and it will adjust about two axes by direct rotation.

The mirror came with a plastic primary cell.  Unfortunately, that would have required quite a bit shimming to fit in the 8" inner diameter tube, as the cell was rather small in diameter.  So I made one.  As I often do, I laid it out in Inkscape.  (The annotations are for the benefit of the blog--they weren't there in my original file.)  I am very bad at measuring (but getting some cheap digital calipers for $8 from a Hong Kong seller on ebay helped a lot--and then I got $4 back as one of the jaws was chipped and a set screw was crooked, so it was a great deal) so my best way for precision is to print out a cutting template, attach it to the piece and drill and cut right through it, or at least make marks with a pencil or drill bit through it.

The plan was to use scrap material.  The cell itself was to be a triangle with corners snipped off made of two layers of 1/2" Baltic Birch plywood, cut from the circular inner waste generated by routing the tube reinforcement rings.  I cut the cell with a jig saw, not very precisely, and cut a ventilation hole with a hole saw.  I then drilled and countersunk holes for collimation screws in only one of the two triangles.  I JB Weld'ed 2" long machine screws into their countersunk holes, and then I glued the other piece of plywood on top of this one (with Titebond II), covering up the screw heads, and clamping hard.  As a result, the heads of the screws are trapped within what is basically a solid piece of plywood (two pieces of plywood glued together are basically one thicker piece of plywood!)

There would be a rectangular baseplate made from a piece of scrap red oak that I had.   The baseplate would be mounted in the tube, near its bottom (not quite at bottom, as the plywood reinforcement ring on the tube would   I rounded the corners of this scrap piece with the router using approximately the right diameter to fit in the tube, bearing in mind I could always sand it smaller (as I indeed had to) but couldn't make it larger.  (By the way, here is a hint for how to measure things off from the router.  I cut things with a 1/4" up-cut spiral bit.  For measuring, I replace the spiral bit with a small length of 1/4" stainless steel rod, and measure from that with the calipers.)  Notice from the diagram that the rectangle would be somewhat off-center in the tube--this was on purpose, to fit better with the collimation screws.  I drilled holes for the collimation screws (actually, I did that while drilling them in the triangular cell, because they needed to line up exactly).

I also made a bit of a countersink around the holes, so that the collimation springs that would go on the screws (I had some springs that someone once sent me) would be countersunk, which would make the assembly have a lower profile.  I didn't have a paddle bit of the right size for these countersinks, and I didn't think I could control my router for such small work, so I did something wacky.  I chucked my 1/4" spiral cut bit into my cheap Harbor Freight drill guide, locked its height, and used a drill to route out the countersinks.  It wasn't very neat, but it was good enough, and the lower speed was less scary.  May not have been good for the bit, I already somewhat damaged the bit in an earlier episode when it was accidentally cutting against cement.

I also cut three ventilation holes in the baseplate with a hole saw.  The result coincidentally (honest!) looks like a certain rodent from a company that protects its intellectual property with a zeal that one may think violates the Greek maxim meden agan.  In defense of this design, I will say that the placement was entirely functional.  I made the inner hole as large as I could while keeping strength around the collimation screw holes, and the small holes are also located in such a way as to be fairly symmetric, and not too close to any of the alignment holes or the inner hole.  Four wood screws hold the base plate to the tube.

The mirror is glued to the cell with three 3/4" blobs of silicone sealant, with the positions optimized with Plop. I used some removable 1/4" particle board spacers to make sure the blobs wouldn't flatten out.  One wants the mirror to float on the blobs, and not have stresses put into it by the differences between thermal expansion of wood and glass.  By the way, when I removed the mirror from the original cell, I was horrified to see it was attached with two blobs of 1/16" thick rubbery stuff.

6" F/5 Dobsonian: Primary and tube

I've been missing making a telescope, but didn't need another one.  So I persuaded a colleague to make one with me.  The optics were cheap--a 6" F/5 mirror (with a 1.5" secondary) from a Cloudy Nights seller for about $60.  In classic style, this was going to be mounted in a Sonotube, with most of the rest made of 1/2" Baltic Birch plywood.

Before I even got my hands on the mirror, I cut altitude bearings from 3/4" red oak.  They are slightly more than semicircles.  It's traditional for altitude bearings to be circular, but using semicircles (or slightly more), the radius could be made larger for semicircular bearings without wasting money on a wider board, thereby making for greater stability.  Moreover, it's easier to cut less than full circles on my fixed-base router.  And I cut lightening holes with a hole saw.

I measured the focal length of the mirror at around 29".  The time before that I measured the focal length I projected the moon onto a piece of wax paper, and then measured the distance.  That wouldn't work this time, as we didn't have a moon in the evening sky.  So instead I put a red LED light some distance (d₁) away from the mirror surface, focused it on a piece of wax paper, measured the distance of the wax paper from mirror surface (d₂) and used the formula 1/d₁+1/d₂=1/f, where f is the focal length.  I did this twice and averaged. 

The mirror did have a chip on the back side, and I was worried that this may have set up stresses and distorted things, so I tested it as described in this post, and it was fine.  Hurrah.

The first hitch was that the Sonotube that my friend managed to get was only about 0.095" in thickness and very flimsy.  Following Cloudy Nights advice, I did a couple of things:

• Cut four plywood rings with a router for outside reinforcement.  Currently, one is installed on an end, and I'll put another on the other end, and I don't think more is needed.  A pair of plywood rings rigidify a tube by a lot.
• I only needed a 29" tube (that's the focal length; one can't assume the tube length is the same as the focal length, but that's how the calculations worked out for me).  Since the Sonotube was 48" long, I had 19" to spare.  I cut that into three rings, two 7" wide and one 5" wide.  I cut snips out of these so I could roll them into a smaller diamater (snip width: 2(pi)(thickness)) that fits inside the larger tube for reinforcement.  I would eventually glued the 7" wide rings at the ends of the 29" tube, and the 5" ring in the middle, but first...
• I impregnated all the cardboard--cardboard reinforcing rings and tube--on both sides, with a 1:1 mix of Titebond II and water.  Titebond III would have been better as it's waterproof, but II is water resistant, and it's what I had at home.  After impregnating, I inserted the rings inside the tube (the waterly Titebond was good enough for gluing them in place).  When I impregnated the insides, I replaced some of the water with DecoArt black acrylic, to make for an initial coat of dark (smudgy blackish gray!) for the inside, which eventually will have to be made much blacker.

The result is a very solid and strong tube, though right now I only have one plywood ring in place.  (One problem: the inner diameter of the rings is too small by about 1/16", and it took about 1.5 hours of sanding to get the one that I installed the right size.)

My 8" F/4 travel scope: Light shield

The light shield is made out of a coat hanger and some foam board (found by a dumpster when students were moving out last year) tied with wire ties. For transportation, the foam board removes so everything can go flat. The coat hanger hooks into a hole on the strut and focuser board.

And I think that's all I have to say about this project. Feel free to ask questions. I am really grateful to all the nice folks on the ATM forum on Cloudy Nights for their suggestions, and I did a lot of looking around the web to get ideas.

My 8" F/4 travel scope: Strut

The telescope started out as a two-strut design (the picture shows an early stage with the struts not cut to size). The struts are 1" square poplar, and attach inside the mirror box with carriage bolts and knobs (unfortunately, one has to be careful not to bump the carriage bolts against the primary--space is tight). However, I removed one strut and found that it was basically just as stable with only one, so in the end there was only one.


I then cut the strut in half, with a diagonal cut, so it would easily fit in a small suitcase. Four machine screws hold the halves together, and it is just as solid as if it were a single piece. I used JB Weld (yes, again) to capture the nuts for the machine screws, and to reinforce the thin edges of the wedges. In general, I don't like dealing with nuts flopping around, so I like to JB Weld them.

By the way, I finish wood with a mix of Titebond II and water (about 1:1, but I am not very precise) and when I want to paint it with somewhat glossy paint, I replace some or most or all of the water with DecoArt Crafter's Acrylic paint. So, that's the black paint on the strut. It's a cheap and easy finish. It doesn't look great, but I am all for cheap, easy and functional. And, as a bonus, it's a low VOC paint. Inside the mirror box, I then went over with the acrylic paint again to get a flatter layer.

My 8" F/4 travel scope: Focuser

I hate making focusers and am too cheap to buy. This is a push-pull Crayford: the draw-tube moves against PTFE runners. The draw tube is made out of super-cheap PVC conduit (something like two or three dollars for 12 feet). Unfortunately, the conduit's inner diameter is significantly more than 1.25", so I lined it with PTFE strips, which I sanded down to fit eyepieces.


The focuser's tension is adjusted by having a piece of metal glued on its bottom part (with JB Weld), and with a thumbscrew bearing against the top part, and with PTFE on it, on one of the three posts.


Here is a picture of the focuser board at an early stage in the production. The elongated hole is for the secondary stalk.

My 8" F/4 travel scope: Secondary mirror

I get confused by three-screw collimation arrangements, and I couldn't figure out how to make one. So the secondary mirror collimation is done like in my Coulter scopes, by rotating along two axes (and one can adjust the position up-down because the support stalk fits in an elongated hole in the focuser board). Basically, the cell is two pieces of softwood that swivel around a screw. The gray square knob at was made by putting a nut in a square made out of painter's tape, and pouring JB Weld into this cast.


The stalk started as a piece of threaded rod. Unfortunately, the support wasn't solid enough--collimation shifted too much with elevation. So I JB Weld'ed (there is a theme here, isn't there) a small square steel rod in parallel with it to reinforce, which helped.


And here is a picture of the cell before it was painted and the mirror was glued to it (with silicone glue). I later cut and sanded it smaller.
I did have one problem with the scope--when I viewed Jupiter, I would get a long smear in one direction. It turned out that the secondary mirror had a distorted edge on one end. I tested by looking through the telescope at an LED flashlight with aluminum foil and a pinhole, if memory serves me. I blacked out the bad part, and the smear disappeared. It might be a good idea reorient the primary so that the blacked out part of the primary aligns with the blacked out part of the secondary, but currently the blacked out part of the primary aligns with most of the secondary stalk, which is also good.