Fine grinding

Powder	Pressure	_oT	Stroke	Time
120	No	ToT	W	7,5 min
120	No	ToT	W	7,5 min
f = 1m25
120	No	MoT	W	7,5 min
120	No	MoT	W	7,5 min
120	No	ToT	W	7,5 min
120	No	ToT	W	7,5 min

Friday evening December 10th: after grinding for 15 minutes with the tool on top, the focal length is 125 cm. Another 15 minutes with the mirror on top follow, and finally 15 minutes with the tool on top.

Powder	Pressure	_oT	Stroke	Time	Remarks
120	No	MoT	W	15 min	In the middle is a zone without water and only a few grains of powder, which do not move.
120	No	ToT	W	15 min	Air bubble in the middle is gone (maybe because in the first wet I didn't use enough water?)
f = 1m17
120	No	MoT	W	10 min	I want to make the focal length longer to 1m20, so the MoT wet is a little shorter
120	No	MoT	W	15 min
f = 1m22

Wednesday evening December 15th: The last time I did two wets of 7min30 at a time; today I'll try out what the result of one long 15 minute wet is. It seemed to work out fine. First 15 minutes Mot, then another 15 minutes ToT gives a focal length of 117 cm. To make the focal length a little longer (since I want 120 cm) the next MoT wet is a little shorter. After a final 15 minute ToT wet, the focal length is about 122 cm.

Wednesday January 4rd 2000: the spherometer has arrived! Curiously enough, we find a radius of curvature of 189 cm at the middle of the mirror and 216 cm at the perimeter. A check of the focal length gives 128 cm (strangely enough we found 122 cm the previuos time).
***PENDING: explanation of spherometer usage***

Powder	Pressure	_oT	Stroke	Time	Remarks
240	No	MoT	W	7 min	At the start of the wet, I feel some counter-force while moving the disks. After some time there's again an air bubble in the center.
240	No	MoT	W	8 min	Powder is distributed more equally, too in the center; at the end of the wet there's again a central bubble, but smaller than with the previous wet.
240	No	MoT	W	8 min
f = 1m25
240	No	MoT	W	9 min
f = 1m29

Wednesday January 5th: despite the strange spherometer readings, I decide to proceed to 240 grit. First I checked with a looking glass the surface of the mirror, which seemed nicely homogenous to me, so it seemed safe to switch to a finer powder. I do four wets of 7 till 8 minutes each. The focal length doesn't seem to change much (two measurements give 1m25 and 1m29). At the first wet, the powder is distributed equally across the disk, except for a large bubble in the center of the mirror. Maybe this is an indication that the mirror is curved differently in the center than at the rim. As I do more wets, this bubble becomes smaller, and appears after a longer time. Whether the shape of the mirror became better is not doubtable, since the spherometer still gives the same radiuses of curvature.

Monday January 9th 2000: I pay a visit to the VSRUG to buy finer powder (320 grit) and check my mirror with their spherometer. Alas, it gives the same result: the mirror is more curved at the center than at the rim.

Powder	Pressure	_oT	Stroke	Time	Remarks
120	No	MoT	lange W	9 min	A large bubble in the middle appears. It is a long bubble, in the direction of the motion of the mirror. Around the bubble is much powder; more to the rim, the powder is less dense. Sometimes the disks don't move smoothly over each other, especially when the center of the upper disk moves over the center of the lower one.
120	No	MoT	lange W	7 min
120	No	MoT	lange W	7 min	Moves more smoothly
120	No	MoT	lange W	5 min
120	No	MoT	lange W	6 min
120	No	MoT	lange W	6 min
f = 1m20

Friday January 14th 2000: since switching to a finer powder doesn't seem to solve the problem of the unequal curvature, I decide to return to 120 powder. At Mel Bartels' page I read that the rough grinding is best finished with some long W-strokes followed by a short center-over-center stroke. So I decide to do first some long W-stroke wets. This is a stroke that resembles the W-stroke I used till now, but the W is much higher (the center of the mirror is moved almost up to the rim of the tool). After 7 wets of 5 till 10 minutes each, the focal length appears to have shortened to 1m20. Alas, the spherometer does not indicate much inprovement.

Powder	Pressure	_oT	Stroke	Time	Remarks
120	No	MoT	1/3 CoC	8 min
120	No	MoT	1/3 CoC	8 min
120	No	MoT	1/3 CoC	7 min	large central bubble
f = 1m43 (??!)

Monday January 17th: the next step in Mel Bartels' documentation is, as has been mentioned before, a short center-over-center stroke. Here one moves the mirror only forwards and backwards across about a third of the diameter of the disk; so one does not move the disk to the left or right. Three 8 minute wets give a focal length of 1m43! Despite the mirror lying atop while grinding, the focal length has increased a lot! The spherometer provides a possible explanation: the middle of the mirror has become flatter.

Wednesday January 19th: Via Filip Rooms I find an interesting new site about telescope making and mirror grinding. Here also is an explanation of what the precise effects of a short stroke and a long stroke are.

Powder	Pressure	_oT	Stroke	Time	Remarks
120	No	MoT	1/3 CoC	8 min	Central bubble
120	No	MoT	1/3 CoC	10 min	Central bubble
120	No	MoT	1/3 CoC	10 min	Central bubble
120	No	MoT	1/3 CoC	8 min	Central bubble
f = 1m32
120	Yes	MoT	1/3 CoC	4 min	large central bubble
120	Yes	MoT	1/3 CoC	4 min	central bubble has diminished a little
120	Yes	MoT	1/3 CoC	4 min	only after some time, a small central bubble appears; later some long (stretched in the direction of the grinding motion) bubbles in the middle
120	Yes	MoT	1/3 CoC	4 min	same as previous wet
120	Yes	MoT	1/3 CoC	3 min	same as previous wet
120	Yes	MoT	1/3 CoC	3 min	central bubble has disappeared; bubbles are more or less equally distributed across the whole mirror!
f = 1m28

More specifically, long strokes cause the mirror to be too curved in the middle. So I decide to continue the short center-over-center stroke. After four wets of 9 minutes, the focal length appears to have shortened to 1m32 and the middle of the mirror has flattened again a bit, so it really seems to have effect. Then I try what the effect would be of applying pressure during the short center-over-center stroke, and what do we see: after 6 wets of 4 minutes, the middle of the mirror has become flatter than the rim, so we've actually gone a bit too far! The focal length has shortened again to 1m28.
So it appears that I missed a really important aspect of the grinding process: the finishing of the rough grinding. Grinding using the chordal stroke does not result in a perfectly spherical shape, like I wrote earlier. The center of the mirror becomes too deep (for some reason one calls the mirror hyperbolically curved). This can be remedied by using a short stroke, while still applying pressure. And surprise surprise, one of the symptoms of a hyperbolical mirror is a central bubble in the middle ...

Powder	Pressure	_oT	Stroke	Time	Remarks
120	Yes	MoT	chordal	4 min	Sagitta mirror: 0.2775mm at the center, 0.29mm and 0.2925mm at the rim
120	Yes	MoT	chordal	4 min	Sagitta mirror: 0.29mm at the center, 0.29mm till 0.295mm at the rim; the center has deepened indeed! Sagitta tool: 0.25mm in the middle and 0.24mm at the rim
f = 1m27

Thursday January 20th: in order to curve the center a little more, I use the cordal stroke again (this is the same stroke as the one I started the rough grinding with). The first wet (4 minutes) does not seem to cause much dufference, but after the second, the mirror has become more or less sperical: the spherometer indicates sagittas between 0.30 and 0.29 mm. When determining the focus, the light spot at the focus has become remarkably much smaller and brighter, which indicates that the mirror sends more light to the same focal point, as opposed to the earlier situation where not all parts of the mirror reflected the light towards the same point. Hopefully the fine grinding can start for good now!

Friday Januari 21st: I start with 240 grit powder.

Powder	Pressure	_oT	Stroke/TD>	Time	Remarks
240	No	MoT	W	6 min	central bubble. The disks stick strongly together sometimes
240	No	MoT	W	6 min	at first a ring-shaped zone with bubbles, which unite later on to a central bubble. Sticks less after a few strokes.
240	No	MoT	W	5 min	Idem, but the central zone of powder is bigger. After a while, the bubbles are distributed about equally.
240	No	MoT	W	6 min
f = 1m27 sagitta mirror 0.29mm with only 0.005mm difference (hurray!) sagitta tool: 0.24 mm
240	No	MoT	W	6 min
240	No	MoT	W	6 min
240	No	MoT	W	6 min
240	No	MoT	W	8 min
f = 1m27 sagitta mirror 0.295mm with only 0.0025mm difference sagitta tool: 0.245mm with 0.0025mm difference

Powder	Pressure	_oT	Stroke	Time	Remarks
320	No	MoT	W	6 min	central bubble, probably because I used too few powder/too much water
320	No	MoT	W	7 min	Sticks during the first few strokes
320	No	MoT	W	6 min
f = 1m27
320	No	MoT	W	6 min	Switched to the plank with bolts
320	No	MoT	W	6 min
320	Nee	MoT	W	5 min	Attempted a sharpie-test, but I discovered that my marker does not work well when the mirror is still wet. Next time I'll let it dry first ...
320	No	MoT	W	6 min	Not sure if the mirror is ground enough already, so to be sure an extra wet.
f = 1m25 sagitta mirror 0.285mm in the middle, 0.29mm at the rim sagitta tool: 0.25mm in the middle, 0.245mm at the rim

Tuesday January 25th: I start with 320 powder. This powder is coloured more green than the previous ones (which looked more grey). With these fine powders, the disks really stick together strongly at times; you can no way lift up the upper disk, except by moving it horizontally, and even then sometimes there is much resistance when there's much powder between the disks (e.g. at the beginning of a wet).
When I used 240 powder, I used a plank without bolts upon which I laid the disks, but after three wets I decide to use the bolted plank again. This is a lot handier during the first few strokes of a wet, where both disks tend to stick together firmly. During the rest of the wet, the disks still glide smoothly over each other.
At the end, we see that the mirror is a little flatter in the middle than at the rim. The solution for this is using a longer stroke, but that's for another day.

I also try out the Sharpie-test. This test is named after a brand of markers, the idea being that you put lines on the mirror with a waterproof marker before grinding, so you can see which parts of the mirror are ground away fastest.

The disks with the marker lines

Powder	Pressure	_oT	Stroke	Time	Remarks
Sagitta mirror: 0.285mm in the middle, 0.2875--0.29mm at the border Sagitta tool: 0.25mm
320	No	MoT	long W	5 min
Sagitta mirror: 0.2875mm in the middle
320	No	MoT	long W	5 min
Sagitta mirror: everywhere about 0.29mm Sagitta tool: 0.25mm f = 1m28

Wednedsay January 26th: finishing the 320 powder. As said yesterday, I use a long stroke to get the mirror more spherical. At the end, I find a sagitta of 0.29mm everywhere; sometimes the value differs by 0.005mm, which maybe could just be the inacurracy of the spherometer itself?

Monday February 7th 2000: at the VSRUG I again buy some gridning powders:

• 20 grams of 600 grit (45 fr)
• 20 grams of 800 grit (55 fr)
• 20 grams of 1200 grit (70 fr)

Donderdag 10 februari 2000:

Powder	Pressure	_oT	Stroke	Time	Remarks
600	No	MoT	CoC	7 min	sticks together a little when both centers are close over each other. Markings are about equally worn away.
600	No	MoT	W	7 min
600	No	MoT	W	7 min	Markings on the mirror are almost completely gone away; on the tool they're more visible.
f = 1m27, sagitta 0.29mm everywhere Magnifying glass inspection: all looks well.
600	No	MoT	W	8 min	A last wet, just to be sure.
f = 1m28, sagitta 0.29mm everywhere Markings have worn away everywhere; at the border they're as good as gone.

Friday February 11th 2000: I start with 800 grit. It looks much the same as 600 powder.

Powder	Pressure	_oT	Stroke	Time	Remarks
sagitta spiegel: 0.2875mm; hulpschijf: 0.25mm
800	No	MoT	W	6 min	Markings worn away equally everywhere
800	No	MoT	W	9 min	Sticks together a little in the middle (i.e. when both centers are close to each other) Markings worn away a lot at the border, rather little in the center
f = 1m30, sagitta = 0.24mm
800	No	MoT	CoC	6 min	Markings are now worn away equally about everywhere. Still sticks together in the middle.
800	No	MoT	CoC	5 min	Markings almost everywhere completely gone on the mirror; still faintly visible on the tool.
800	No	MoT	CoC	6 min	Last wet, to be sure.
f = 1m28, sagitta = 0.2875mm mirror, 0.25mm tool

We continue with 1200 powder (it's blue-grey coloured):

Powder	Pressure	_oT	Stroke	Time	Remarks
1200	No	MoT	W	12 min	Again much more worn away at the border than in the middle
1200	No	MoT	CoC	6 min	Midle is now much more worn away
1200	No	MoT	CoC	5 min	I didn't wash off the lower disc during the last three wets; the result is that a fine layer of fine-ground powder has settled on it. Just rinsing doesn't help any more; it really needs to be swept off. In the middle the markings are only faintly visible.
1200	No	MoT	W	9 min
f = 1m20 (!!), the focal point is markably less sharp than before sagitta = 0.2875mm at the border and 0.29mm in the middle
1200	No	MoT	W	8 min
1200	No	MoT	W	8 min
1200	No	MoT	W	9 min
1200	No	MoT	CoC	5 min
f = 1m32 (!!); sagitta 0.29mm Lookingglass inspection: all looks well.

As you can see, I follow the same pattern as happened before a few times: grind with a W-stroke, and if the middle appears not enough ground (which you can detect by looking at how much the marker lines wear away), use a few CoC-stroke wets untill the situation is fixed.
I also suspect that the spherometer readings are not fully correct: the mirror sagitta is about 0.29mm and the tool sagitta about 0.25mm, whereas whe would expect both to be equally curved. So maybe there's a 0.02mm error on the sagittameter, so both really have a 0.27mm sagitta ...
Those who have been calculating well, will have notice that sagittas and focal lengths don't really match very well. Besides a possible error on the spherometer, there's another possible cause: at the last measurement of the focal length, I decided to find out what happens when I move the lamp much further away than before, and all of a sudden I get f=1m20 (whereas it was 1m32 when the lamp was closer). The rays the lamp emits are of course not fully parallel, and by moving farther away, they get more and more parallel. It seems that I've been placing the lamp too close, and as a consequence, I've got longer focal lengths than the real focal length. Also note that when you fill in a sagitta of 0.27mm in the formula, you get a corresponding focal length of 1m20. So it's very likely that the real focal length of my mirror is very close to where I wanted to be: 1m20!
To finish (and also because I fear a little that the real focal length maybe could be even less than 1m20) some ToT wets:

Powder	Pressure	_oT	Stroke	Time	Remarks
1200	No	ToT	W	8 min	Markings have worn away a little everywhere, at the border even completely; somewhat visible in the middle.
1200	No	ToT	W	8 min	In the middle, the markings are only very faintly visible (on the tool the're still more clearly visible)
1200	No	ToT	CoC	5 min	Markings almose completely gone (only a little bit visible in the middle of the tool)
sagitta mirror: 0.29mm