skybadger

Cant agree more. I actually measure from out to in and then offset in a spreadsheet. I have a digital vernier to remove user error , I just need to work out a way to fit it.

Always good input. thanks Nigel. That was why I was asking. I plan on making a 50mm lap tonight to do that job. The latest set of figures after polishing in the above method are below. The surface error plot tells me I have a conic of about -3! Which might explain the difference in sizes of figures betwween ideal and measured. Overall Im happy that I am getting a better understanding of what is going on, getting better at manipulating laps and the test tools and starting to get some repeatability in my foucault measurements. The Foucault sequence is shown below moving from zone 0 to zone 7 outwards. : zones 1, 3, 5, 6, (8) And finally a ronchi just outside RoC

The pin locations are from both the stellaphane web calculator and the calculator in figureXp. I marked a sheet of polystyrene plastic using a compass and used a scalpel to cut the fingers out by allowing some materialaround each mark. Means a new mask per mirror but styrene sheet or just plain card is cheap.

Further progress According to my grinding diary I took these after the last two wets of 5 mins which are digging out the hill from the centre and attempting to smooth out the trench before moving on to take the curve out to the edge. The pin mask lies across 7 zones and shows the trench being on the inner edge of zone 2 I used the small lap across the centre to reduce the hill and added some offset to blend that outwards. I also took some actual measurements but not all make sense - I clearly wasn't reading the micrometer correctly. more practice required in the dark garage. Here's the table data from Figure XP First Q: the difference in range between measured zone focus distance and expected is real. Why is it so different ? I haven't polished away all those mm since I started near spherical. I set the zone settings to moving slit. I will double check with a second micrometer to validate. Regarding the Dall null test- I read up on the test on optics.net but I think I have enough contrast to see the shadows as I currently am. I am aiming to being the line of shadow bisection to the centre and measure the null when its on a zone marker. for example. the null in the right hand picture is between zones 1 and 2. I am getting more familiar now with what I am looking for! Second Q: Considering Zambuto describes the most effective polishing 'sweet spot' for the lap as being where the lap has greatest effect at 80% diameter of the lap and then uses offsets to target the zone requiring work, I should be offsetting the lap to make the sweet spot coincide with the zone I want to reduce - which means for an 80 mm lap , using a -30mm offset for the central hill and a +15mm offset for the zone 3 reading. Sound about right ? I did some more wets last night . the first 4 for small lap fixup and the second for blending across the mirror. Cold pressed for 5 mins with 5Kg Time Offset Eccentric (Overhang) Notes 80mm lap 5mins 0 25 0 forgot to rotate blank 40 mins in total 5mins 15 20 0 forgot to rotate blank 5mins 30 10 forgot to rotate blank 5mins 45 5 rotated blank Moved to 120mm lap - warmed for a warm press before use. channels not closing up yet. Probably not warmed enough yet. 5mins 5 20 ~10mm at extremes rotated blank 5mins 15 15 ~10mm at extremes rotated blank 5mins 25 15 ~10mm at all times rotated blank 5mins 30 10 ~10mm at all times rotated blank I hope to measure the outcome of this, this afternoon and I'll show those measurements here..

You can move from resistance to capacitance measurement using an AC signal , water will change the capacitance. The AC current reduces corrosion.

Once you get past hotspot in Win11 and power and usb issues it's all fine.

Great stuff. I have a stack of 4 CNCv3 onsteps on my bench waiting for the WiFi interface boards to be made up and on already driving my alt az mount.

Isn't one of the issues with the Dall null technique the necessity to very accurately place the lens at a certain distance from the foucault ? I'm beginning to regret not picking up the testing frame I was offered when I went to collect some glass and grits from a retiring ATM. I just didn't have the room for it then. Nowadays I have room but its full of other stuff instead. I wonder if I could build one upside down in the roof of my garage ? Ie suspended from the ceiling. Feels diversionary.

Mine too.

I think I would summarise progress so far as introducing and now trying to remove the central hump. The hump was introduced by a poor lap which had poor contact, was too tall and was too small in diameter, introducing the central hump and ditch. That was addressed by creating a larger lap (120mm) with much better contact, less tall so good drag on the mirror but still unsuited for removing the hump. So I made a new lap of 80mm, softened the pitch a tad and moved to focusing on reducing the hump which seems to be progressing slowly but positively. I'll cut out and prep a 2" lap and give that a go as well. As part of that process I've started capturing measurements and gaining practice in the Foucault method in order to make them repeatable and representative so I can track the figuring progress. My problem there seems to be capturing a reliable zone 0 measurement and making sure the mask is symmetric for repeatability. So I think I'm making progress slowly, even if its not showing well in the mirror yet, mostly because I'm snatching half hour sessions in the garage between other activities. cheers Mike

Thanks Nigel. There's a grey polycell mat under the mirror which is the material used for underlay for wooden floors, so moderately dense and very even. I've made a smaller 80mm lap to focus on the small zones and using direct strokes over the centre, moving out increasing the offset and eccentric to bring the central zone down and spread it to the outer zones. Then once I get positive change in the desired direction I'll follow it up using the larger lap to smooth it out. For the MoM, the direction is to bring the platter speed down from 60 to 6 and bring the eccentric up front to 15 which moves from polishing to figuring configuration. The place I'm at now is trying to get good quantitative measurements after each wet using the Foucault to confirm the qualitative interpretation of the surface shape. The micrometer stage is starting to give good readings and I'm using an Everest pin mask to identify the null zones reliably. In short, taking your input and adjusting accordingly.

I spent another 30 min session trying to reduce the central hump yesterday. I say its a hump because the ronchi looks like this , outside of ROC. The kink to the outside in the central zone , outside of ROC, indicates the hump ( by reference to this: https://nicholoptical.files.wordpress.com/2016/11/the-ronchi-test.pdf ). The ronchiimage isnt very clean because the one I have to hand is 250LPI and I need to find my other, more coarse one (~100lpi) to remove all the edge diffraction effects. Two wets of 15mins each with zero offset and 20mm length strokes across the centre Knife edge is from right. I'm having a hard time recognising when a zone is nulled. Its either bright. dark or gray/nulled as I understand it. If I was using a coude mask - the 50% zone would be grayed in the picture below ? So I think the central hump is reducing and blending - ill give it a final hour of polishing to try to confirm and reduce out and then blend with the larger lap before making a final decision on going back to grind. I also spent yesterday re-making the foucault measurement device. Its still tripod mounted since I haven't got a bench for this. The radial movement is now on a micrometer driven slide giving +- 7.5mm at 0.01mm resolution, the tangential direction on a photographic rack mount. If its still too sensitive to movements I'll mount it on a XY table I have for the mill. The led I need to move closer to on-axis and so eventually I'll move that in front of the slit. I want to move to making measurements to graph the shape out objectively. I have FigureXP and sixtests. I'm also looking at FoucaultUnmasked. Any recommendations ? Cheers Mike

For me going to regrinding is a practical act after learning what happens with the initial lap too small. If the hump can't be polished down because it's too large, grinding is the only option. The next step after grinding would be to polish with the 80% lap and see how things differ there. I do want to master the mom..

This is the first photo of the last session - taken after 15 mins of polishing using a 120mm diameter lap set to swing across the centre from edge to edge so that the 80% zone of the lap sits on the starting zone of the TDE. As far as I can see I have smoothed out the central peak and trough to some degree. Platter was 60rpm and eccentric 8rpm. I used a 1.5kg weight on top and movement was smooth. I had also cold pressed it for 10 mins with a 5Kg weight before starting while I did other stuff. This picture is after the second session, continuing the same settings but adding more weight - now at 2+1.5=3.5Kg. I think the smoothness is suffering due to the weight - I need to reduce it. Other parameters remain the same. Outcome: Hill is slightly smaller and trough better blended. This seems a slow process which probably indicates hill is quite large. No apparent change to starting zone of TDE . Surface now rougher than before. Plan: Continue and observe This is after two more 15 minute wets later with the same settings as before. I didnt leave the blank much time to cool down but did give it a good wash in cold water. Outcome: I may have deepened the trough while trying to remove the central hump. I can't see the hump reducing and the surface remains rougher than when starting the session. Plan: Finished this session . Re-measure after back from travels and cold press. Remove the central hump using a 50% (80mm) lap operating across just the centre hump ( i.e. 10 mm eccentric, 0mm offset from centre) and then smooth out using the 80% lap with wider eccentric. Check for hump reduction then take curve out to edge using tangential strokes on the eccentric, increasing offset, starting at 10mm going to 35mm with a 80 mm lap. If the hump doesn't show reduction, re-grind to spherical 'cos polishing ain't enough.

Understood re the rotation, that's where I am at right now. The same for the spin polish, adding some eccentric to blend the zones. Spent an extra hour this afternoon with a large eccentric to blend right across the face. Have some more pics to load. Seems to have reduced the hump and pushed out the tde. I'll have to see how much effort is required before considering a regrind to sphere

Here's a foucault of the mirror from this am, so its had all night to acclimatise. At RoC, knife from right , appearing from left ( so a touch inside RoC ? ) . I think this shows : - rolled edge at 90% zone - some shallow concentric zones from the spin polishing that isnt ripple or dog biscuit. - a raised central hump but with a surrounding trough. What I'd like to get is input on the relative scales. In order to get a view on whether this is worth continuing to polish ( vs return to grind) I need a view on how large each component is. Can I treat the turned edge as partial parabolisation and excavate the centre using CoC strokes to smooth the zones out from there to the edge ? Is the reason the whole mirror isn't nulled due to not quite being at at Radius of Curvature ? Still feeling my way to finding that spot. Currently using the change in direction of the shadow ingress as the indicator of moving through the centre of curvature. Is there a better way or do I just need to practice more ?

I should be able to photo the foucault , I'll give it a go.

I re-poured the pitch lap once I saw that the backing disk for it was too tall- it had a concrete pad bonded to the plywood, done at the same time I made the grinding lap. When I add the pitch, its just too tall and tips and wobbles across the machine. So I junked that and made cut some new lap backing disks from scrap plywood. I chose a 120mm diameter for the 150mm mirror , so I could do spin polishing ToT on the MoM machine I have In the process I also re-jigged the gearing for the pulleys driving both the eccentric and the mirror platter. The platter now runs at 60rpm and the eccentric at 8.. The overarm wasnt quite centered on the platter, so that is also now fixed. I'm 4.5 hours into polishing using cerium and my stock of pitch. I recycled this pitch from a 16" lap I acquired, it seems to be very hard and resistant to the fingernail test, which might explain why its taken as long as 4.5 hours to get fully polished out to a sphere. Also, I'm still learning when to press and whether to do it hot or cold. I did it hot pressed this am and then cooled and brushed with a steel brush which seems to have made a decent difference both in degree of contact and extents of good contact - all the way out to the edge. I'm doing 15minute wets and rotating the mirror roughly 90 degrees every whet. I've waited for a good polish before putting it on the tester. The polish goes out to the edge but could bear some more according to the laser test. Since this is a practice piece, I'm pushing on. Here's a short vid of the mirror grinding machine: Shows a 16" platter, 12"step and then an 8" slotted disk with mirror stops. The lap is 3/4" plywood, varnished in 10% diluted varnish for penetration and counterbored for a rotating foot to drive. The overlap below is 25mm on a 120mm diameter. The mirror under test is suspended in a sling mounted on a frame on my garage wall and retained using an aluminium catch plate at top. . Heres the ronchi from inside ROC: At ROC outside ROC Outcomes: Based on the hooks at the end of the ronchi lines inside ROC t looks like I have turned down edge. That seems unexpected for a MoM machine. If anything I was expecting TUE. You can also see a zone at the centre , about R/4 . Closer inspection reveals a raised zone, a dip ands then another raised zone very close to the middle. Assuming Im reading this right, they could be dips. Reading the MoM operations guide, I expect some zoning towards the centre. I'd like to confirm whether its a raised zone or not. I rotated the mirror through 90 degrees and inspected it again - I didnt see anything different . Maybe I wouldnt. I think this looks like a smooth surface - bar the zones - Looking for confirmation and feedback of reading. I also used the foucault test to view and with the knife from the left, the highlight was to the left - is that raised or dipped ? Her's a close up of the central zone. 20230326_161530.mp4

These are they. They are rigid ish but stretch to fit snugly in eyepieces and then flip up and out of the way. Similarly priced at 3-4 each. Found on Amazon and ali.

Also look for flip top caps you can leave on, typically sold for rifle sights but perfect for eyepieces of various dimensions. I leave them on my common use eyepieces all the time.

++for onstep and stay away from the A4988 drivers, they are not man enough or as well controlled as other step controllers that the onstep is known to work with. The difference being the level of current control they support and the speed they can drive at. My vixen clone mount slews at 3 Deg/sec using on step and tda6210 drivers with nema14 steppers . That has the 9 Deg stepper belt driving a 3:1 external gear and a final 144:1 worm.

You could look up the dtfringe software by Dale Easton and its use with a home made Bath interferometer. There's lots of info on the web. I have one in bits waiting to be assembled on the bench right now, behind a long list of other projects.

Is that an auto collimator, ie a mirrored internal surface ? I have a 1.25 and a 2" I use as part of my checks, alongside a Cheshire and a baader laser. When you mention that they don't need collimating, you are assuming they are sufficiently well setup at time of sale... I'm not sure that's always true. But they are useful.

Not at all flimsy but ... The rubber cups on the end of the feet allow some squishing as weight moves, so need swapping with the hard feet. Mines carrying about 10kg.

Not using an oil bath then, for 2c ?