View Full Version : OT again hard drive disc reflectors

08-21-2009, 02:55 PM
Just curious- I've been playing with front surface mirrors lately, then I see this hard disc I have laying around. Looks pretty good reflector-wise. I wonder how it would compare in accuracy and reflectivity?

08-21-2009, 07:27 PM
Flatness is very good. Reflectivity is very poor, even with the ones that look very good. As a comparison, polished aluminum is around 88% and polished stainless steel is around 50%. If the hard drive platter has *any* visible colour to it then it will likely be down to 20% or less.

08-22-2009, 04:28 AM

Does it sound to YOU that he has a "hard drive disc" laying around, or more like he has a couple DVDs or CDs laying around?

You gotta tear a drive open to have a HD platter laying around.

Yes, if you have discs with shiny both sides, one might be shinier than the other, the non-write side. Hang it on some mono and it will do some reflecting.



08-22-2009, 04:42 AM
Darryl means HD platter. He likes to take things apart. :D

08-22-2009, 07:26 PM
Depending on how old it is, the disk needs to have the head within a very few microns of the disk.

08-22-2009, 08:09 PM
Depending on how old it is, the disk needs to have the head within a very few microns of the disk.

the old big platter packs were so touchy that a spec of cigarette smoke would make a head crash. A spec of smoke was like a huge dirtclod back in the 80's . now days they are sealed up good for the most part.

Whats his need for the mirrors anyway? you can get mylar cheap and make all kinds of wacky things. I took a direct tv satalite dish and made a fast solar hot dog cooker. I had no problem lighting a stick on fire at its focal point.

08-22-2009, 08:20 PM
Hard drives aren't sealed. They all have a vent with a bit of filter paper in it so they can breath. Since they must have air to function they want to make sure the air goes in and out via the filter.

08-22-2009, 08:46 PM
Hard drives aren't sealed. They all have a vent with a bit of filter paper in it so they can breath. Since they must have air to function they want to make sure the air goes in and out via the filter.

I said "now days they are sealed up good for the most part"

In the old days ,guys with portable super cleaners did all the big computers hard drives, they were called platter packs. you best know what im talking about.

08-22-2009, 09:22 PM
you best know what im talking about

I had a job as an operator on an IBM 360-30 for Branch Motor Express Company in New York City. What do you think?

Ever seen a card restorer?

08-23-2009, 01:13 AM
I do like to take things apart, and this disc (platter?) was one of two in the stack. They are both very shiny with no color at all, unlike cds, etc.

I did a test with a piece of front surface mirror side by side with the disc. The hard disc is less reflective than the mirror piece, but not by much. It looks to have every bit of smoothness, accuracy, or flatness, depending on how you look at it. I couldn't see any aberations in either, and I checked from all angles looking at straight lines at a distance, etc. No patterns showing, no color friinging, etc.

I figure if I ever use a piece of the disc as a mirror I'll protect an area by gluing on a bottle cap or something, then cut a piece out, shape it nicely, glue it in place where it needs to be, then peel off the bottlecap. I figure it's likely that the piece I cut out would warp to some small extent as the full disc is turned into pieces, but maybe not. It will be an experiment. By the way, I'm not looking to re-invent the telescope, just to add a 'yes this can work' to my bag of tricks. A future use in something-

It does come to mind to use the hard disc as is in an experimental optical system by pulling the center of the disc axially while the rim is fixed. A small curve will result, but heaven knows what kind of curvature that would be. Maybe two discs in the system for a self-correction action-

The front surface mirror pieces have been temporarily installed in the telescope tube I made, and I can't see any aberations at all, and there doesn't seem to be any negative effects from not using the binoculars' prisms with the rest of the optics. I have found out the hard way that there's only one orientation for viewing that gives a corrected view of what you're looking at- and it isn't the way I wanted to hold the thing for viewing. Now I'm thinking to just use one mirror, which will correct upside down only and still let me angle the eyepiece so it's comfortable to view the sky overhead without craning my neck so bad.

That's all I had in mind for these 'scopes, a cheap way to view the sky at 7x50 without it being hard on the neck. I don't want to get into image correction lenses, with the attendant aberations that it might introduce. KISS is the name of this game.

I've managed to cut, glue, and angle the pvc pipe I used for the body of the telescope such that the mirrors are exactly where they need to be. That took a bit of head-scratching and careful measuring, but that was part of the fun of doing this. I'll show pictures of the test pieces, the body, and the jigs I used at a later date.

I'm thinking that for the next model I'll orient the eyepiece at 45 degrees to the main tube instead of 90 as I've done with the first one. It should just be a function of proper placement and orientation of the mirror-

08-23-2009, 08:38 AM
It's surprising how little visible reduction in reflectiveness translates to a huge hit in actual reflection efficiency. Just that very slight darkness that distinguishes stainless steel by colour from polished aluminium is a 30 percent reduction in reflection. They won't be suitable for a telescope unless it is for daylight or solar use.

Cutting up the disks can be a problem. I have done experiments using the thinnest ones to make galvanometer laser scanning mirrors and I haven't found any way to cut them that doesn't introduce major distortions in the flatness. The one method I haven't yet tried is chemical milling as I haven't found a suitable resist to sodium hydroxide that can also be easily removed without leaving any trace.

I'll be very interested if you you can find a way to cut them without ruining the figure.

Lew Hartswick
08-23-2009, 10:22 AM
Darryl, I suppose you are a bit too young to remember "Sky Watch".
In 19 57 when The US was preparing to launch their first satelite, and
Russia beat us to it, we had already formed groups to track it and
report times it crossed our meidian. The set-up was a tall pole with
a cross arm and lined up below the observers with small telescopes
(like a 7 x 50 monocular) with a mirror in front to do just what you
said (keep from getting a stiff neck) [and of course to accurately
locate the scope ]. Worked a charm. I was out there with the rest
when the warning went out for Sputnick. Later the same day I got
married. :-) Makes it easier to remember an anaverery.

08-23-2009, 12:51 PM
Thanks, Lew, for making me feel at least a few years younger :) I don't recall skywatch, but I did follow the space sciences as much as I could. Sputnik, Telstar, Dynasoar- Chuck Yeagers flights, Kennedys plan to land a man on the moon- exciting times, with the fear of the Russians too-

Evan, I'm going to try cutting one of these discs in as much of a non-destructive way as I can. We'll see what happens. I'll take your advice that these won't make good mirrors for astronomical use. Hmm, maybe a shaving mirror for the backpack-

08-23-2009, 02:28 PM
Also you need to watch out, some hard drive platters are glass.

08-23-2009, 05:05 PM
Glass, hmm. In that case it's easy- I've had pretty good luck on the cutoff wheel, grinding a line where I want the break, then having the pieces separate right in my hands. Did another front surface mirror like that just a few minutes ago. I think the heat from grinding expands the smaller part of the glass, the parts around the periphery that I'm clearing away, then those parts just fall off. I'm only grinding a bit into the surface, probably 10 to 20 thou, so it doesn't take much. Starting with the larger piece and breaking that into four pieces, I have to snap it. Not much force required anyway, and so far the breaks have gone fairly cleanly. This is done with the front surface up, so I'm not even touching that surface to anything, or with anything except at the edges.

I did a ring test with the hard disc, and it sounds like aluminum. Glass of that size and thickness would ring at a much higher frequency, in my estimation at least. All it's going to take is a moment with the hacksaw to tell the story, or a momentary touch to the cutoff disc.

Curiosity got the better of me just now, and I determined that it is al. I clamped the disc between thin strips of mdf, on edge so only 1/4 inch width of mdf touched the disc on both sides, then cut through the whole package with a high tooth count new hacksaw blade (by hand I might add, I don't know where my hacksaw frame is). That's a length of cut of 1 1/2 inches through the mdf, plus the thickness of the disc, about 60 thou I'm guessing. Worked great. I can't tell where the mdf touched the disc, and the cut is clean.

Two things I think worked in my favor. One is that the aluminum disc is not soft, so it had less tendency to jam the teeth, secondly I think the dust from the leading piece of mdf lubricated the cut and also helped prevent the teeth from clogging with aluminum.

I cannot tell from the 'reflection of straight lines' test that there's any warpage in the pieces. I believe it remained flat right up to the edge of the cut.

Contouring the pieces will be another challenge. Most likely I'll be using squared pieces, so my method will continue to work.

By the way, the mdf I used is fairly hard and rigid. Some of that stuff is relatively soft and probably wouldn't have the strength to support the disc where it sticks up out of the vise for the cut. It might work as well or better to use hardwood pieces.

08-23-2009, 05:10 PM
Another question if I may. When I tried to score the front surface mirror with a glass cutter, I couldn't even leave a mark. Is it possible that this is a harder glass, or maybe quartz? These mirrors came out of polaroid cameras, if that helps at all.

08-23-2009, 08:19 PM
They are overcoated with silicon dioxide in amorphous form. It hard as heck and really hard to scratch. I have a bunch of old photo copier first surface mirrors and you can scrub them every month for 5 years to clean them and they don't scratch.

The problem with those mirrors that really limits their usefulness is that they are 1/4" thick.

The only consumer drives that I know of that had glass platters were the 60 and 80 gig IBM DeskStar line. The pulled them from the market because they had problems with the platters exploding. I'm not sure there are any still being sold with glass platters.