Saturday, July 24, 2010

Eyepiece case

I've found it a nuisance to take eyepieces in and out of bolt cases.  Moreover, the amount of time it takes to pack up everything at the end of an observing session limits the length of the session.  So I decided to make one of those eyepiece cases where the eyepieces sit naked and can be used right out of the box.  I initially planned to make a plywood box, but decided that would be too heavy.  Instead, I picked up a Harbor Freight steel and plastic toolcase (like this one, but maybe slightly smaller) for a low price, and made a plywood insert.  Unfortunately, the case was stinky.  The plastics in it were off-gassing.  It took about seven weeks of ventilation until it became usable (obviously, I don't want off-gassing onto optics).  Finally, today, I put the insert in--gluing it in place with silicone glue and putting some screws in.

It includes space for 1.25" and 2" eyepieces (the laser collimator occupies one of the 1.25" slots).  In the middle there is room for filters.  I drilled two holes through all my slide-mounted Rosco gel filters, and put machine screws pointing upwards on the insert for the filters to sit on.  The fit is good.  My ND and OIII filters are in a plastic box attached with Velcro (or maybe a generic hook and loop fastener) to the insert.  Finally, on the right side, there is room for my battery box (for driving fans or giving extra power to PDA), geen laser, flashlights and spare batteries.  I am quite pleased.  But since the silicone is still off-gassing, I took everything out after taking the photo.

Sunday, July 18, 2010

Fun in less dark skies

Last night, I went out to my usual just outside of Waco site.  Not as dark as the observatory in Clifton, and not only was the moon out, but there was a fair amount of traffic.  But I had my 13" which compensated to some degree.

The moon looked OK at 500X and better at 375X.  Venus showed its phase, but that was all.  With the exception of the Splinter Galaxy (NGC 5907) which I hadn't seen before but which happened to be on the Best of NGC/IC list I was using and with a high altitude, I was looking at Milky Way objects.  M57 looked rather nice--no central star (never seen that), but a nice eye shape, not just round.  M13 looked decent as did M92 (which was hard to find for some reason;  maybe because AstroInfo wasn't set for the current time).

Most of my time was spent going through objects in Steve Coe's What's Up Sagittarius and Scorpius articles, which I keep plucked on my PDA (with permission;  I even have it set up in a cool way so that when I click on an object in the text, it takes me to AstroInfo's search, though this wasn't working always last night).  I did all of the Sagittarius objects (Coe only covered a small portion of each constellation) except B91: M8, M20, M21, Collinder 367, NGC 6544, NGC 6546, NGC 6559.  I thought I had B91, but I had overestimated its size (to remove that problem in the future, I added Barnard's catalog of dark nebulae to AstroInfo).  And then I did a few of the Scorpius ones: M80, M4, NGC 6144.  There was a clear dark lane in M80.  6144 was hard to find.

I ended with a look at the Veil Nebula.

The views in the mediocre conditions in the 13" were similar to those in the 8" in the better conditions of my previous session, though M13 was better resolved.

Focuser tube baffling

I've been having bad light control problems with my 13mm Hyperion eyepiece in my 13".  Whenever there is anything even slightly bright opposite the focuser--even if it's just skyglow at a pretty dark site--it was flooding the field of view.  So I made a simple focuser baffle.  I just cut a cardboard ring with inner diameter just over 1.5" and outer diameter 2.2".  The inner diameter was arrived at by ray-tracing.  The outer diameter fits the focuser tube.  I then just slip it into the inner edge of the focuser tube.  Because the focuser tube inside is threaded (or just ringed) it stays in.  I also added a wide ring around the outside of the focuser tube, inside the scope's OTA, but I don't think this was what made the difference.

Result: Great!  Last night, I was able to observe with the 13mm despite the moon, a family fishing with a light hanging over their stuff, and boats in the water.

I also have a large foam light-shield, which I occasionally used, but the baffle ring seems to render it unnecessary.

I expected the baffle to vignette the field of view of my 30mm Super-View, but I left it in for convenience even when I was using that eyepiece, and the views were fine.

Tuesday, July 13, 2010

Quick but fun dark sky session

I went to the Meyer Observatory last night to learn a bit about the operation of the 24-inch telescope.  Unfortunately, the camera shutter wasn't working.  While it was being fixed, I sneaked out and pulled out my 8" F/4 telescope, which I brought along.  I had a rather nice and very fast session under really dark skies--about 20-25 minutes.  Actually, my first impression of the skies was that it was cloudy and the clouds were lit up by city lights.  In particular, Sagittarius looked really clouded over.  What an embarrassing mistake!  The clouds were clouds of stars--the Milky Way!  Unfortunately, I forgot to bring my 30mm 1.25" eyepiece, and the scope only has a 1.25" focuser, so I was stuck with a 13mm, which rather took away the scope's wide-field advantages.

I started with M13, which resolved fairly well, but not as well as in my 13" from brighter sites, though I had a hard time finding Hercules--too many stars in the sky!  I had a nice look at M51.  It had its companion visible, and there definitely was a connection between them.  I got a vague hint of spiralness.

M57 was a nice little donut.  Better than I expected from this very fast scope with a damaged mirror.  I then tried to find M20 and M8.  While trying to point to M20, I hit M8, which was very nice.  In fact, M8 was naked-eye, with a nice dark lane down the middle.  M20 was a little ways up.  I don't think I saw all of its three dark lanes, but I did get an impression of at least two.  I tried for the American Nebula at some point, with no luck, because the field of view was too small.  And I also tried for the Veil Nebula, and did in fact get a nice line, which may have been the broomstick, but it was hard to tell--the image wasn't as sharp as when I used my 13".

Still, it was a very nice session squeezed in a short amount of time.  I rather like a scope that weighs less than 17lbs.  I am looking forward to taking the travel scope by plane to Canada.

Saturday, July 10, 2010

Fun with meteorite and magnets

About two years ago, I got a free L6 meteorite sample from Inner Planetary Products (now, alas, apparently defunct).  Here was a fun thing to show my son: one can pick up the meteorite, which looks like a small stone, with a magnet.  The L chondrites are about 4-10% nickel-iron according to Wikipedia.  A fun science project for a kid would be to check how magnetic different meteorites are.  If one could get good-priced samples.

Sunday, July 4, 2010

Cross-hair laser collimator: Part I

I'm making a cross-hair laser collimator for my friend's 6" F/5 scope--actually, the final steps will be done with him.  I've already made one for myself.  Instead of having a laser dot, it has large cross-hairs.  This looks cool, and is in some ways nice to use (e.g., the cross-hairs light up the central donut a bit in the dark, and you can align them more precisely on the return target than a single dot that might disappear inside the target hole), though it's not so good if one prefers a Barlowed laser collimator.  The costs are low.  The laser module itself costs $4.06, including shipping, and most of the cost is a couple of inches of aluminum tubing from Speedy Metals.

The body of the collimator is made of two pieces of T-6061 aluminum tubing, the larger of 1.25" outer and 0.75" inner diameter and the inner of 0.75" outer and 0.5" inner.  This is very hefty and solid aluminum tubing.  I need a 1.25" outer diameter to fit in the telescope focuser.  But the laser module has a 0.4" diameter (approximately, going from memory).  So one needs to bridge the gap, and as I couldn't find laser tubing of 1.25" outer and 0.5" inner diameter (the last 0.1" is bridged with electrical tape, and one needs some wiggle room for adjustment), I had to use two.  Plus, I only want the 0.5" diameter in the last 1.5" of length, so there is more room for the cross-hairs on the return beam.  So nesting the tubing is the trick.  Unfortunately, 0.75" outer diameter doesn't fit in 0.75" inner diameter, as the tubing is not made to tolerances that make telescoping possible.  Moreover, aluminum on aluminum friction is very high--as high as rubber on dry cement.

Hence, one has to sand the larger tubing from the inside and the smaller tubing from the outside.  I ended up sanding the larger tubing from the inside by attaching sandpaper (with duck tape) to the handle of a screwdriver, with some foam in there to add pressure, and spinning the contraption with a hand-drill to sand.  I went through a bunch of sandpaper that way, but finally the small tube started sliding in.  And then it stopped.  Aluminum on aluminum friction is very high.  I hammered it in a bit more, but an inch was left sticking out.  Which is perfectly fine.  There was not going to be any problem with sliding away, because of that amazing coefficient of friction.  Anyway, so I had about an inch in and an inch out.  Also, the outside of the 1.25" tubing needed sanding to fit inside the focuser tube (which was also aluminum, so I was afraid it would get stuck there if there wasn't some wiggle room).

I then cut a 45 degree slice out of the tubing.  A little too close to the end--I should have done it in past where the smaller tubing was.  (As it was, to allow more room for the cross-hairs, I ended up expanding the hole with a Forstner bit.)  I used two pieces of steel bolted around the tubing at 45 degrees as a guide for the hacksaw while cutting.  Took a long time.  Aluminum may be relatively soft, but cutting through such thick tubing at an angle still takes a while.

Next, I drilled three holes for adjustment screws--the collimator needs to be itself collimated.  I then planned to tap it by driving #6 screws into it.  One went in fine.  Another broke as I was screwing it in--it just sheared its shaft.  I managed to remove it.  A third broke off flush with the tubing.  That one was nasty.  With a friend's help, we tried to remove it in various ways.  Finally, a punch made a small hole, we drilled a ways in, and then I just drilled a new hole right through the screw shaft.  My friend also told me that when one uses a screw to tap a hole, one needs to go two partial turns in, and one out, and so on, while I was foolishly screwing straight in.  I tried that, and broke a second screw in that same shaft.  Oops.  Around then I realized I had drilled the holes one size too small, and that's why the screws were breaking.  I sanded it flat again, drilled a slightly larger hole than before, and tapped it successfully.  Whew.

The laser assembly is much easier than the housing.  The laser is a <5mW unit, and I assume is safer than the typical unit as the crosshair grating diffuses the beam quite a bit.  The laser comes with two nice wires.  I attached a switch and a battery pack.  The battery pack was made of three 1.5V LR44 batteries taped together, and some scrap pieces of thin metal for contacts for the wires touching them.  The laser also has a focusing ring.  (If one removes it all the way, one can remove the crosshair grating, and get a laser dot.)  I focused it for roughly the length the beam would have in the scope, namely focal length times two, though it'll probably get defocused before I'm finished and I'll have to do it again.  I used some heat shrink tubing (nice stuff) to make the connections more solid.  And I transferred the warning label from the laser module to the housing.  The batteries and switch will stick out of the back of the collimator.  Not so pretty, but usable and easy.

Steps still left: wrap bottom of laser in electrical tape to fit snugly in housing, insert, add adjustment screws, adjust laser, add a cardboard target, mark cardboard target, and then put in something to keep it from falling into a focuser tube (maybe a little screw? maybe a dab of silicone?).  That'll be another post, and these last steps will be done with my friend.

I should say that I've made one of these for myself, so none of this is breaking new ground for me.

Friday, July 2, 2010

Wooden helical Crayford focuser

I've done this project a while back, but in case anybody is interested, I am documenting it.  My Coulter 8" scope came with a friction focuser made of pipe fittings that Coulter described as "helical", I think because you aren't going to do very well just pushing the focuser tube in and out unless you give it a twist as well.  With practice, the focuser was usable for light eyepieces, but it really didn't work for something as heavy as a Hyperion 13mm (and there are eyepieces heavier than that).  So I decided to make a helical Crayford focuser, because I was too cheap to buy a KineOptics one.

The idea behind a helical Crayford is that you have ball bearings against which the draw tube rotates.  However, the ball bearings are slightly twisted so that their axes make a small angle with the angle of the tube.  Consequently, as you rotate the draw tube, it moves in and out.  If the ball bearing angle is larger, the movement is faster, and if it's smaller, the movement is slower.

I mounted the ball bearings on two T-shaped supports made by taking a #12 flat-head screw for the stem of the T, and JB Weld'ing a segment of 1/4" square rod to the head of the screw for the line.  I then JB Weld'ed a  skateboard ball bearing (I expect from VXB's ebay store) at each end of the T's arm, trying as best I could that everything would be square and the arms of equal length (not that I am very good at that sort of thing).  I then took a piece of 3/8" particle board (Baltic Birch plywood would have been better, but I was being cheap) and mounted three posts on it.  Two of the posts would sport the ball bearings, with the stem of the T going through the post, and with various nuts and spacers to hold the bearings in place.  The third post had a little flap adjustable with a thumbscrew.  The flap was made of a piece of bondable PTFE JB Weld'ed to a piece of metal, with the thumbscrew bearing against the metal, all as in my push-pull Crayford.  For the draw tube, I used the aluminum draw tube from the Coulter.  In the picture, the hole for the focuser tube wasn't cut yet.

Here's another picture, with a better view of the T-shaped ball bearing supports.  You can see the gray JB Weld all over the  place.  Two things to be careful of when gluing are (a) don't get any JB Weld on the bearing surface, because then movement will be bumpy and (b) ensure that the JB Weld stays on the inner ring of the ball bearings, so it doesn't stop the ball bearing from moving!  Note also that the posts are located in such a way that the focuser tube would fit snugly in.  At this point in the process, the posts are, I think, screwed in place.

Eventually, I unscrewed the posts, cut a hole of the focuser with one of my trusty cheap Harbor Freight hole saws, sanded the faux veneer off the particle board, glued the posts with JB Weld to the particle board, with screws for added strength.  Or something like that--I don't remember all the details.  (It's possible that at some point the posts were weakly affixed with craft glue to help with layout and then pushed off.)

I did have one worry, and that was that the posts might bow outward with time, thereby decreasing tension, and generally making the focuser not work too well.  So I JB Weld'ed some strips of hardwood that I had lying around in my scrap pile, joining the posts as if with a fence between fence posts.  I was generous with the JB Weld.

Finally, I cut the particle board down to size, removed the old focuser (cutting the sonotube around it, ripping it out, etc.--it was a messy procedure), and attached the particle board with screws and some hardwood rails to fit to the curvature of the tube.  I also painted the wood and JB Weld black.  It's pretty heavy, but somehow fits the home-made styling of the Coulter scope.

The focuser has had a bit of image shift, which has increased with age.  Lately, it wasn't doing too well, so I had a closer look at it.  It turned out that the T-shaped supports, under pressure, had rotated further, so that the angle between the axis of the ball bearings and the draw tube was too big for good movement.  I made the angle smaller, added washers under the nuts (did I forget? or maybe I just ran out of washers?), and tightened it up.  I am guessing they'll rotate away again.  If they do that, and maybe before that, I may replace the nuts with locknuts, and/or add some toothed washers.

The focuser weight is an issue--I've had to increase the counterweight--but that's part of the price of making stuff out of wood.  And the price is hard to beat: a couple of bucks for the bearings, and then some screws, washers, nuts, square rod, and various pieces of scrap.