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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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My 8" F/3.93 travel scope uses a primary with a clamshell chip in it. It came with about 15-20% of the primary's area covered up by duct tape (which I then covered with black felt). The actual chip is only a 1" x 2" area near an edge, as I can see from the underside.
Not so hot for planetary use, but otherwise works nicely, and I appreciate having a scope small and light enough (17lbs) that I can carry scope and eyepiece case in one trip.
It would be nice to optimize the amount of mirror that has to be masked. I have no idea how whoever decided to put the duct tape on the mirror figured out the area to be masked. I am thinking of dissolving the duct tape (naphtha? turpentine?) and trying to minimize the masked area. Any suggestions on how to figure out which area has bad figure? Or is this something I can't do without serious optical test equipment?
I definitely don't want to put a significant amount of money into it. I got the mirror for $35, after all. But if there were an easy way to optimize, I'd like to.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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If you have, can build, or can borrow a Foucault tester you can determine the area of the mirror affected by the damage easily enough by looking at the surface with the tester. It's not likely to be much larger than the chip, though.
Another way would be to through star testing the unmasked mirror - if the chip affects the image significantly you can see it in star testing, just like any other error. Pull far enough away from focus and you should be able to measure the area involved, at least close enough to mask it off.
Best,
Mark
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8320
Loc: Bremerton Washington
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Quote:
If you have, can build, or can borrow a Foucault tester you can determine the area of the mirror affected by the damage easily enough by looking at the surface with the tester. It's not likely to be much larger than the chip, though.
Another way would be to through star testing the unmasked mirror - if the chip affects the image significantly you can see it in star testing, just like any other error. Pull far enough away from focus and you should be able to measure the area involved, at least close enough to mask it off.
Best,
Mark
Mark..I would think masking off all that mirror for one clamp chip is killing more then needs to be killed.
Reducing the mirrors aperture for one clam chip is limiting the mirrors ability significantly..IMV.
Blaken it and move on...I would think.
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Thanks for the advice! I now need to try to get all the gunk off, and then go the star test route, assuming that there isn't something really obvious under the duct tape. When I peeled an edge of the duct tape, I did notice some permanent marker lines, so maybe whoever put the duct tape on had done some testing after all, and then marked the area carefully.
Do I need to remove the permanent marker lines for the star testing, or will they just behave like spider vanes?
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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Rob, I said only mask off as much as needed - the key is to figure out how much that is, right? And I sure didn't mean mask all around the mirror, just the area near where the chip is. 
Because of strain release in chipping, if you chip the optical surface and look at it under Foucault you'll see effects from the change in strain beyond where the chip is/was. That's why you need to mask those portions as well, leaving them intact is far worse than masking them off, due to the effect on the wavefront.
A chip that existed before the mirror was finished won't have this effect, BTW, as the figuring would have accounted for the change in the surface.
Arpruss, you can use acetone to clean all the gunk off, followed by rinsing in tap water to take the acetone off while hydrated, then distilled water before drying the mirror off. Take the marker lines off, maybe photograph them first?
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Quote:
Arpruss, you can use acetone to clean all the gunk off, followed by rinsing in tap water to take the acetone off while hydrated, then distilled water before drying the mirror off.
It's currently glued to a hardwood cell with silicone glue. Will the acetone affect the glue if some gets on it?
Quote:
Take the marker lines off, maybe photograph them first?
Good idea to photograph! Any suggestions on how to remove the marker lines?
From what I recall the seller saying, the chip resulted from the mirror's being dropped, if that's relevant.
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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Acetone won't affect silicone adhesive unless you soak it for days. Marker lines should come off with acetone. Just soak the mirror upside down in a bowl or something that you can partially fill with acetone to cover the area affected and seal off the top, leave it long enough to take the gunk off and then rinse it. Don't rub the mirror physically - this should get it clean without touching it.
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Dumb question: If I do this in a glass container, will the container be rendered permanently unusable for food preparation purposes?
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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Acetone? Not likely. Acetone will dissolve most plastics, so glass is a good idea. I'm thinking, like, a mixing bowl? Wipe it out with acetone (mind the fumes) afterward, and then scrub it good with sink cleaner and wash it normally, should actually end up cleaner than when you started. Don't tell anybody you used it for that purpose... 
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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OK, I bought 16 oz of acetone. How long should I leave the mirror in? I think the duct tape is dried up.
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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An hour will do it well. Don't breath the fumes - do this in a well ventilated room. I once gave myself a case of chemical pneumonia working too close with acetone...
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Got it. I did it in an open garage, right by the door, so the fumes weren't an issue. I had to rub a bit--the gunk was dissolved but wouldn't simply flow off. At most angles, one can still see most of the places where the duct tape was, and there seems to be one small area that used to be clean where some of the dissolved gunk must have flowed, and which has a slight stain. On the other hand, all the marker came completely off.
After I re-spot the center, I'll star test.
If I can't figure out from the star test what area is disfigured, is there some rule of thumb I can use, like "probably the figure is fine more than an inch away from the chip"? If it helps, the mirror is 3/4" thick, and the chip is inside the glass, about 3/16" from the coated surface. It is about 2" x 7/8" wide, stretching along the edge of the mirror. Looking from the coated side, there is no way of telling where the chip is.
p.s. I was amused to note that the acetone sold in the paint-stripping section of the store has a very scary safety label, while the pure (except for the bitter taste additive) acetone sold in the fingernail care section has very little in the way of warnings, and even tells you to put your fingers in it for 10-15 minutes. Hmm.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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Nope, no rules of thumb for chips - that's why you need to test. A picture might help though. The stress could go a long way.
Acetone isn't absorbed very well through the skin, and because it evaporates quickly you have to have a lot of surface area to get the vapor concentration high enough to cause much harm. That's easy if you have a pint or two, pretty hard if you have an oz... You can use it clean gunk off your skin if you don't leave it on too long.
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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OK, so I was naively trying if I could do a Foucault test without a Foucault tester, just with a knife edge and a point source of light (LED flashlight with lens replaced by aluminum foil with pinhole), both hand-held. At one point, while experimenting, I forgot about the knife, and I noticed that with my head at around 2*FL, and the point source further in, I could see a reflection of the point source. But as I moved the point source towards myself, it grew into this funny looking oval (no doubt due to the fact I was viewing off-center), with funny patterns on it (kind of like the patterns one sees on a bright star defocused into a wide doughnut), and shadows of my eyelashes over it. What was cool, though, was that as I moved the light around, in one area near the edge, shaped like the chip but bigger, the oval visibly distorted, both in shape and in internal patterns. (I didn't actually check that that was the area where the chip lies, since it's kind of hard to see with the mirror in the mirror box. But it would be too much of a coincidence if not.)
There were no distortions anywhere else. As you can no doubt tell, I know nothing about optical testing, so I am wondering if what I was seeing was in fact precise enough to map out the damage. What kind of a test was I doing?
Update: Tried to photograph the spot, then realized that I couldn't do that because the spot was an image of my eye (retina?). In any case, the spot seems to nicely map out non-uniformities.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
Edited by arpruss (10/28/09 11:17 AM)
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Further experimentation: When I move the light source somewhat further out, and get my eye in the right place, the oval expands to cover the whole mirror, and is full of various fringes, with the fringes neatly showing up an oval area precisely around where the damage on the underside is.
1. Should I just mask out that area (maybe plus half an inch to be safe)?
2. Is there any advantage to using one shape of mask or another? Should I just choose a mask to match the areas I see with these experiments as closely as possible, or should I go for a somewhat larger but more regularly shaped mask?
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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Just mask the area where you can see that the patterns are irregular. Ideally the mask should be shaped like a portion of an ellipse to cover the damaged area but cause the least amount of diffraction damage...like this:
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Thanks for the help!
The project is basically completed (the scope needs some tweaks, but it's basically usable and done, though I haven't tried it yet).
The mirror before:
The mirror now:
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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Man, some chip! This looks a lot better - note that diagonals routinely obstruct large areas of the mirror with a circular (elliptical) outline and are accepted. Let us know how this works out!
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Still a problem--when I defocus on Jupiter or a bright star, I see a narrow fan of light emanating from the side of the mirror almost directly (but not quite directly--maybe at 170 degrees) opposite to the area I covered up. When I focus on Jupiter, the fan becomes a wide bloom of light emanating from Jupiter. The bloom disappears when I cover up an area about half of the radius of the area I covered up around the chip. However, I can't detect any damage using the point light and looking at 2*FL method--it looks pretty much normal around there. What could it be?
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Aargh. Instead of looking at the primary's reflection of the point light as before, I tried looking through an eyepiece, and there was indeed a nasty dented area. I eventually noticed that the area wasn't on the primary but on the secondary, which is why it didn't show up when I was testing the primary.
I blacked it out with marker, and when the marker dries up, I'll go over it with flat paint or glue some construction paper over it. The secondary was only about $8 anyway. :-)
Defocused star images are now going to look like somebody's been nibbling on the primary.
I might consider trying to save up for an undamaged set of F/4 8" optics. I am guessing that trying to make a mirror this fast wouldn't be the right way to start out with mirror making?
On the other hand, it's just a travel scope, and I already have a nice F/4.5 8" for use in my local area. So I may just forget about it all.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2158
Loc: salem, OR
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Well, the diagonal can't cost that much to replace, sounds like it has its own issues (maybe they were a matched set?).
Best,
Mark
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Quote:
Well, the diagonal can't cost that much to replace, sounds like it has its own issues (maybe they were a matched set?).
Yeah, but my current spending priority is an equatorial platform. :-)
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Wow, that is one big chip. It is definitely reducing your image contrast relative to a perfect mirror. This first photo is my digitization of your chip in Excel, with the X,Y coordinates.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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This is the ZEMAX analysis of the chip, using the chip coordinates as a user-defined obscuration. I didn't know your diagonal size so I assumed a 2.6" minor axis for the center obstruction. The chip blocks a little under 15% by itself, and the combination of the diagonal and chip block about 25%.
The point spread function does show a slightly broader central diffraction peak and strengthened ring structure, which reduce contrast.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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This is the pupil function and PSF with the chip removed. The 2.6" diagonal blocks about 10% of the light to the primary. The PSF has the usual light distribution for a centrally obstructed aperture without the additional scattering.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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PSF plots are fine to look at, but the modulation transfer curves give quantitative analysis for the effect of the chip on image contrast. The chip darkens the image another 15%, and though it boosts image contrast at midrange detail around 270 cy/mm, overall it reduces image contrast relative to the diagonal by itself.
Bottom line - as long as the mirror figure is good, and didn't spring up around the edge of the chip, the mirror is probably fine for use except at higher powers, with some reduction in image intensity.
But that's probably nothing you didn't already know, right? I agree with the assessment to just blacken the chip itself. That's about all you can do unless you make or buy a replacement mirror.
Happy viewing,
Mike
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
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Thanks for the analyses!
Correction: The secondary is 1.75" m.a.
The chip itself is takes up less than half of the area of the masked portion. However, my experiments with a point source of light and viewing the magnified reflection of my eye at 2*FL showed that the mirror was distorted up to around twice the width of the chip (the chip is about 3" long and 1.5" wide--I had to mask out a slightly gibbous area of diameter 3.5"). I didn't build a proper Foucault tester because the distortion was showing up so nicely as it was--the blob would stretch into a teardrop when it got nearer to the chip, but not nearer all the other edges.
It's usable at 200X on the moon, and up to about 133X on everything else, and best below 100X. Last night, Jupiter on this scope was much less sharp than on my Coulter 8" F/4.5 (though Jupiter was a bit lower down--not a controlled experiment), but I did clearly see two cloud belts at all magnifications I tried up to 133X, but they were hard to see at 200X. (The Coulter was showing sharp images at 150X, and good images at 225X.) Still, like I said, it's a travel scope, and I think it'll pull in faint fuzzies sufficiently well for my purposes. (The only DSOs I've tried it out on so far--and that was with the old, larger mask--were M31/M32/M110 and the double cluster. The Double Cluster looked really nice--the second nicest view from my red skies backyard ever. M31 was core-only, as is normal for an 8" in my backyard. M32 was obvious and definitely non-stellar. I may have caught a glimpse of M110, which is better than I've done with my Coulter 8"--but I am more experienced than when I last turned my Coulter on M31/32/110 in red skies. In any case, the views of M31/M32/M110 were no worse than in my Coulter, while the Double Cluster was decent, though the latter comparison is skewed in that the only memory I have of the Double Cluster from that site with the Coulter was of an exceptionally lovely view of it.)
To add to the mess, now a semicircular area of 5mm x 10mm along one edge of the secondary, on the side opposite to the chip on the primary, is blackeded out. As Mark said, it sounds like a matched set. :-) (When I was buying, I knew about the problem with the primary but not about the problem with the secondary.)
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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