Hi, I'm grinding a 7 inch mirror and tonight I changed from 80 to 120 grit. I continued using a 'W' stroke, 1/3 overhang roughly and the sagitta went from 1/2 mm to 5/12 mm! I can't understand how this happened as I thought the W stroke increased the sagitta. The only things different were some paint on the edge of the tool which ground off after one wet(I was using it to seal in any rogue 80 particles) and one wet spent grinding tool on top as an experiment. I am using a porcelain tile grinding tool with asupramax glas blank. I'm aiming for 0.8 mm sagitta so stopped 80 at 0.5 to creep up on the right amount. I grind for 1/2 hour at a time. This is my first mirror.
Ralph
If the sagitta reduced then you are grinding the edge away. In my day I only used the W stroke to parabolize the mirror after polishing. Grinding was mostly centre over centre with the odd, random, variation to avoid zones. Grinding and polishing a spherical surface first is very easy and can be tested very accurately. Then I used the W stroke.
Thanks Charles. I can only think it must have been caused by the one wet spent grinding tool on top. The grit seemed to last about twice as long as mirror on top so could it have been doing enough to counteract the normal action of the W stroke? I quote the stellafane ATM guide to strokes: "Use Mirror on Top to deepen the sagitta and shorten the RoC and Focal Length." At a 0.5mm sagitta the RoC is 7225mm which is longer than what I'm aiming for. I've read that sometimes it can be a good idea to go a few tenths of a mm past the required sagitta and work tool on top for the rest of the grinding. Is this a good idea? Should I go back to 80 for a few hours longer and get to 0.8 mm, I've got plenty of 80 grit left over?
I don't think one wet would change the Sagitta much. I suspect your strokes might be a bit short, try lengthening them a little. If it were me, I would go back to 80 grit and get the Sagitta as close as possible. Then swap between tool on top and bottom for the rest of grinding to keep the RC close.
How are you measuring the Sagitta?
There is a sticky thread on mirror making. You may want to post your questions there.
Thanks Rod. I'm using a bar with a screw that advances 1mm per turn. The short strokes could be the problem, I read that too long a stroke is a common mistake so I could be overcompensating.
Ralph
Hi Ralph.
I assume you hogged out with the 80. Sometimes after hogging out, the first few wets with the next finer grit go "backwards" because the hogged out curve was more of a hyperbola than a sphere. Depends on the strokes used to hog out. The finer grit with more general strokes 'may' be generating a better spherical curve before resuming to deepen the sag.
Dave
Last edited by dave brock; 13-01-2015 at 04:29 PM.
A very quick calculation suggests you're going for close to f/14. Is that right? You might want to check the 'focus' by wetting the mirror and reflecting the sun as a back up to your bar and screw. It may be hard to get repeatable measurements on a 0.8mm sag with a 1mm/turn screw.
Remember, sun test will give focal length, not RoC.
Dave
Yes I did hog out with 80 using chordal stroke up until the last 1/2 hour. Your hyperbola idea sounds like it could be it. Yes, I'm aiming for F/13. I tried measuring the RoC by wetting the mirror and using the reflection of a torch but from 7 metres away it was too hard to aim. I attempted the sun test but the suns reflection was high out of reach so I was hoping to use it once the reflection was at a reasonable height. It's been cloudy all day every day for the last week and it doesn't look like changing any time soon.
Ralph
I agree with Dave, don't go back to 80 and don't use Tool-on-Top unless you really overshoot. I wouldn't have used 80 grit on a 7" mirror f/13 anyway but then I never made one!
F/13 is a long focal length, what are you going to use it for?
I'm hoping to use it for planets, the moon and double stars. It's easier on the eyepeice as well. I did some more grinding last night and the sagitta is back to 0.5mm, the surface is much smoother even after 1 hour of 120 except the outer 5mm which is still quite rough. At this F/ratio, would there be any noticable difference between a sphere and a parabola? Ralph
I hadn't connected that you were making such a long focal length so ignore my comment about going back to 80.
If you are trying to avoid parabolizing I don't think you need to go to F13. I suspect f10 would be ok and easier to mout. If you have a program like Tex or figure xp, just set it up for your mirror and enter the same reading for each zone and see what wavefront error you get.
Rod
I'm not doing F/13 to avoid parabolisation, I chose it from the start but if it can be left spherical then it would be a very nice bonus. The mounting is a non-issue as I have a fairly solid looking undriven equatorial mount that I intend to mount it on. According to google, a 6 inch mirror can be left spherical at F/8 - F/12, depending on the source and an 8 inch can be F/9 - F/13. I also like the idea of using long focal length eyepieces for planetary and lunar views so I can have long eye relief with plossls etc.
Ralph
That's going to be quite an instrument when finished. You'll need that mount to be really solid to avoid wind induced vibration spoiling your view. I assume you'll have a truss tube rather than solid...also to avoid wind buffeting?
I live in a valley so the wind isn't too bad but it restricts the view low to the horizon, especially to the northwest. I don't know about the tube yet but a two metre long solid tube would be very heavy, a truss tube would be lighter as well and easier to get out. Ralph
I live in a valley so the wind isn't too bad but it restricts the view low to the horizon, especially to the northwest. I don't know about the tube yet but a two metre long solid tube would be very heavy, a truss tube would be lighter as well and easier to get out. Ralph
As a DIYer myself am very interested in the mount. Is it GEM? A horseshoe or split ring would give you ultimate stability. Or perhaps an English Yoke...
Yes, the mount is a GEM. I did consider a horseshoe or yoke mount but I decided it wouldn't be portable enough, if a 2 metre long telescope can ever be considered portable. Another idea I had in mind was a dobsonian with the azimuth axis pointed at the SCP but when this popped up I thought I would use it instead. Sorry, no pictures at the moment but I should be able to take some tomorrow. When I got it there was a box full of goodies, one of them being setting circles from astro-optical supplies so the mount could have originally come from there too. I've had a play around with it but I can't see how to put them on, or even anywhere to disassemble it so the setting circles could be unrelated. Possibly a surrurier truss would be good, it would have a mounting point in the middle for the GEM, ultimately the tubes purpose is to hold the optics in alignment. Another possibility stolen from garyseronik.com is a simple metal bar with the mirror and focuser at each end.
Ralph
Vibration will totally kill the pleasure in using it if you make it so long ...I'd recommend no longer than a 60" tube for that diameter. Modern eyepieces even basic Plossles work really well at F8. Aside from that at F26 at the knife edge it will be very hard to see what your surface looks like especially with the extra air currents involved in the light path at the longer Radius of Curvature.
Mark
On the vibrations, do you mean vibrations from wind buffeting or vibrations from accidently knocking the telescope, focussing etc? I'm not necessarily going to do a solid tube, I haven't decided yet.
I'm assuming you mean F/26 at the RoC; At that distance the mirror surface would appear just over 2 degrees across so the size it appears should be ok. Air currents will likely be a problem at a 5 metre RoC but I'll see how it goes. Is it possible to test at the focus using a star or very distant artificial light as the light source, meaning that the mirror would go grey all at once for a parabola, not a sphere or would that actually increase the light path from the distant source and worsen the problem?
For me for the eyepieces it's a question of eye relief as the eye relief of plossls scales with the focal length. An F/13 would have 13/8 of the eye relief at the same magnification using plossls as an F/8. I own two kellner eyepeices, a 15mm and a 9mm which is close to the 13/8 ratio. I love the 15mm, in fact it's one of my main eyepieces but the 9mm requires me to jam my eye up against the lens which detracts from the comfort of the view. I'm obviously not going to be able to sit down while observing unless I go to F/5, possibly F/6 and I don't mind standing on a chair to observe. A barlow would be a partial get-around but no matter how good it is, adding more glass to the optical train will scatter more light.
For Les, I've attached images of the mount, sorry they're blurry but it shows the general form of the mount.
A 7" f/10 mirror has no need to be parabolised, it may as well be spherical.
As for f/13, not suitable for a Newtonian - buy an f/15 maksutov.
I appreciate your comment about eyepieces but IMHO the optimum is f7 - the maximum useable exit pupil is achieved with a 42mm eyepiece (lowest power) and the maximum useful magnification is achieved with 4 or 5mm eyepiece there are many to choose from as f/7 motors are not hard to make (but will need to be parabolised).
I only used the W stroke to parabolize the mirror after polishing. Grinding was mostly centre over centre with the odd, random, variation to avoid zones. Grinding and polishing a spherical surface first is very easy and can be tested very accurately. Then I used the W stroke.
