DuncanC

Looks good! how did you bond the parts together? Epoxy resin?

1/2hr ~1/4d TOT and 1/2hr 1/4d MOT and I think the overall shape is pretty good. scratches remain though obviously. I still think the edge is a bit turned down. Would appreciate anyone else's thoughts on this . Plan at the moment is to try and get the edge better before starting parabolising. Probably another hour of 1/4d strokes in a couple of sessions. Foucault at ROC: Ronchi (Inside focus):

Today's progress: 4x 1/2hr 1/3D CoC sessions Mirror on Top, with at least 1/2hr cold press after each one. Also did one 1/2hr session Lap on Top with shorter stroke as I have a slight turned down edge I think. Looking a lot better, nearly at a sphere. Current images (mirror cooled for a couple of hours). Note the scratch is reduced a fair amount: Note the sling I mount my mirror in for testing obscures the edge a bit over the lower 150deg or so, hence the wonky shape. It's just the sling masking the image a bit. Going to do a couple more sessions with the shorter strokes to improve the edge, and hopefully be nearly ready to attempt parabolising again.

Thanks for the encouragement. Had another "interesting"couple of days! Decided to try and pour more pitch on the lap to increase the thickness a bit and cover any grit that might be stuck in it. Mirror was wet when I pressed but I hadn't put any cerium oxide on... Ended up with it stuck hard to the lap. Managed to get it free with lots of pressure and progressively hotter water. Got it clear in the end then had to melt off as much of the pitch as I could befpre wiping with turps to clean the rest. Had a second attempt to repour the whole lap again. Went better but I hadn't let it cool enough before pressing so had some seepage out the sides. Had made it pretty thick so think I got away with it. Channels didn't last though so ended up redoing with the soldering iron and scraping of the excess earlier. Did a long press (no weight) for a few hours and 1/2hr with a counterweight before doing a 1/2hr 1/3d centre over centre stroke mirror on top. Seemed very smooth without sticking etc. I could probably be guilty of a lot of your suggestions, especially through the initial polishing sessions. I was almost definately using too much pressure, strokes too fast, sessions too long between pressing, maybe rocking the mirror during the stroke, making the wets too long / cerium too watered down etc. I did note some warmth generated during polishing back doing the inital polish, so that won't help. As I resued the pitch and had a couple of attempts it should be a little harder than before, buy could still press a finger nail in a cold lump of pitch. From now on strict 1/2hr max sessions, with 1/2 press between them. Light pressure being carefully not to rock the mirror, very slow smooth strokes keeping to 1/3d (+/-1"), 1:7 cerium to water mix, well shaken. Hopefully will fix my issues. Hopefully get a couple of sessions done tomorrow and will give it a chance to equalise before testing. Wish me luck!

On my way back to a sphere I thought. I just seem to be making ring even worse. Very confused at this point. I am doing slow, steady 1/3 centre over centre strokes MOT. Hopefully the new lap will be better. Big scratch quite evident in the Foucault.

Thank you for the interest in my project. Been a bit up and down recently! I started a few W strokes and realised I'd gone a little past a sphere and dug a whole in the centre so decided to keep going into a parabola. It didnt go well! I still have this issue with a ring about a third of the way out not being polished. Tried a few different strokes (chordial and circles centred at this point) to remove but nothing was working. I've given up on that and decided to go back to a sphere. Also had a few issues with my testing. My ronchi grams have never been great and a grating printed on acetate was no better, even trying different light sources. I tried manual measurements using Foucault and a pin stick for the zone but found it very hard to see the zones. I've since added a dial gauge to my tester aligned it better and started using Foucault unmasked which seems to work well, though I want to do a test to see how repeatable it is. This is where I was before going back to a sphere. Getting rid of the peaks just wasn't working as I expected. Unfortunately I picked up a two inch scratch at the weekend. I'd hoped it would polish out but further polishing has introduced a few more (though small). The first scratch does seem to have reduced from the polishing done so hoping I don't have to go back to fine grinding. I've decided to pour some more pitch on top of my lap as it's down to only maybe 4mm thick. THis should also seal in any grit causing the scratches and maybe stop the problem with the ring being created. After that I'll do a bit more polishing to a sphere and do very slow (5min a session) parabolising sessions with testing in between to try and get a good figure. I really hope the revised lap will allow me to get more predicable figuring and I can get rid of the scratches. If I get a chance I'll upload a Foucault of the latest session of polishing back to a sphere to see where I am.

Thank you Nigel. I think I understand what you mean by the diffraction rings, but will research further. When I have the knife edge throwing one side of the mirror in shade, I can still see the highlight round the edge of the mirror. Is this a similar thing? Another few hours done to get the mirror spherical. I'm defianetly finding a small side to side action in addition to the Normal CoC stroke is needed otherwise I create rings on the surface. Example below. This is after 1/2hr of pure 1/3D CoC strokes, MOT. Knife edge coming in front right. It's very close to spherical now, just trying to get the last bit of the centre done without damaging the edge. I might try a few W strokes for maybe 10mins to see If I can smooth the general surface and take the last bit of the middle. This is inside RoC

Thank you both! I really appreciate the comments. I've been working to remove the TDE ahead of sorting out the middle of the mirror. I ended up doing a total of 1hr 20mins in 3 sessions of this stroke as suggested on a cloudy nights thread: fixing a tde on a primary mirror This did a good job of fixing the edge. I've since moved onto to sorting out the centre without too much luck. Tried a few 20-30min sessions of 1/6d centre over centre MOT slow with no extra pressure. Tonight I tried 20mins of the same, but with a slight w to see if I could smooth things. This is the mirror as of this evening (Ronchi Inside focus): Im sarting to wonder if the strange central shape is due to an issue with my lap. Maybe I don't have enough offset to my channels. I might try some 1/4d coc strokes until the channels close again, then recut slightly more offset. Edge is a bit wonky, I might need to trim the overhang a bit. I think the reason for my TDE was either the lap being a bit soft, or that it's a little undersized. I made the lap on tool which is 152mm diameter, where as my mirror is 155mm. I guess it will help when figuring! Nigel, It's possible the TDE is caused by finger curling as you say. I try not to do this, bit is difficult with big hands and a 6" mirror!

Results after 30mins 14 CoC MOT and an hour or so cooling. Please ignore the finger prints (since cleaned). Inside RoC. Not much change but maybe slighty reduction in the centre hill. Bit worried I might be increasing the TDE. Foucault Test. Noticed a lot rougher surface since the last 30mins of short strokes. This is just inside RoC. Central hill smaller but more pointy... Odd. Outside RoC. Bumpy. Lap is pressing now. I might do a little bit more 1/4 CoC MOT later. Not sure if I should try to fix the TDE or the central hill as I'm oblate at the moment.

After 10.5hrs of polishing I decided to to call it done. Laser pointer test seems good as long as the mirror is cleaned with a lens wipe first. Did my first tests of the mirror - I found it very hard to align the mirror and tester to get decent results. I need to sort a better Ronchi screen as it's currently homemade from 0.1mm fishing line. Initial tests below. Inside ROC Roughly ROC Outside ROC Looks like I have okay polish, but slight hill in the centre (under corrected sphere) and a slight turned down edge. I've recut the facets in the lap and cold pressing it. Plan to do 30mins with 1/4 Centre over Centre strokes and see where that gets me.

I totally agree with your thinking Nigel, in fact I've seen similar written elsewhere. It's not my intention to test earlier, more that I will polish until I'm convinced I'm finished, but am likely to have a good safety margin. Another 2 1/2hrs completed, taking me up to ~8.5 hrs. Laser completely invisible on the front surface nearly all the way to the edge. I'd say it's just visible around 3-4mm from the edge and there's a slightly hazy ring of about 1mm at the far edge. I'll do another 2-3hr's and see where I am. In between polishing I built a mirror stand over the weekend. There's a single foot at the back for adjusting the angle. I made it a little oversized, big enough for a 12" if needed. Mirror support by the sling at the front and stopped from falling forward by a plastic head wing bolt.

Now upto 6 hrs polishing 50/50 Mirror On Top / Lap On Top. Polishing is nearly completed. I was seeing resisdue on the glass with the laser pointer not a lack of polish. Things improrved a lot with a clean with an IPA lens wipe. It's only the outside 3-5mm that is unpolished now. Will do another couple of hours, but not panic too much as I'm expecting to have a lot of porabolising attempts which will polih further. Couple of images below. Centre Edge

Sorry for the delayed replies again. Over the weekend I opened up the channels and re-pressed the lap with the net in place. I did another 1.5hrs MOT, taking me to three hours. I then opened the channels again with the soldering iron and pressed again. The mirror looks really polished out. I checked with the eyepiece and could see much but there were a few odd isolated pits. I was holding the mirror up to the window, and it was easier to see where my fingers blocked the sunlight, illuminating the surface obliquely as you say Nigel. When grinding I supported the mirror on a couple of blocks of 3x2 and slid a garage inspection lamp underneath which worked quite well as a simple light box, might do that again. I did do a laser pointer test - I thought I was polished out. I really wasnt! I still have a spot on the front surface of the mirror that is slightly stronger toward the edge as expected. The mirror doesn't look frosted at all however. I will complete the planned 6hrs polishing and check again. Not going to worry too much though- I suspect parabolising may take a few attempts so plenty more time to get it's totally polished! Another hour completed this evening taking me to 4 hours. This time Lap On Top. I will do the rest like this to make it roughly 50/50. My lap is 152mm diamter vs 155mm for the mirror, so I have to move the cleats each time I swap over. I will probably build a tester sometime this week before the polishing is finished. Intending to build a mirror support similar to the one in the BBC video, but with a strap to support it. My tester will be based on my smaller milling machine so I can take advantage of the x-y table. I'll use a magnetic dial indicator stand as a variable height prop for the head. I'll probably 3d print a couple of heads, one with an LED and stanley knife blade (slitless Foucault) and the other with some form of Ronchi set up. I had better get busy!

Thank you for the interest guys. Agree that anything smaller than a 6" would be a struggle. My hands are struggling with a 6" as it is. 8" would be easier! Was thinking it would be a nice job to help tone the old muscles without a machine, but not sure they agree! After a busy week where I didn't get anything done I did an hour and quarter earlier MOT. I'm doing 1/3 centre over centre with slow speed and lots of pressure. 6-10 strokes, rotate the mirror ~10-30deg, repeat maybe 3-4 times, then move to the right a step. Once Ive gone round the corner of the table I rotate the lap 30 deg or so. Does that sound about right? Photo of the lap before polishing below. I cold pressed for 45mins with a net from a pack of oranges and a counterweight prior to polishing. After an 1.25hrs the lap looked like this. I had a few chunks of pitch come off round the edge. I think contact is okay, but time to open up the channles maybe? Given how much my hands hurt I'm wondering if I'm using too much pressure which is why the channels have closed up so quickly. That or the lap is a bit soft. Just as you predicted Nigel, the mirror almost looks polished out to the naked eye bar a slight duller area in the outside 3-5mm. I fully intend to polish for at least 6hrs before testing though. Apart from anything else I need to build a tester!

Hi Nigel, Thank you for your post. I will keep an eye out. From what I've read polishing is quite hard work so taking a break occasionally to see how the polish is developing won't be an issue! It appears that 50/50 Mirror on top / lap on top is the way to go? Will try to monitor it.

I did a good hour on each of the 225, 175, 125 and 95 (22.5 -9.5 micron?) optical powders. Probably didn't need a full hour, but no problem to continue for a bit longer to make sure it was ground out. Was very lucky (I think due to the glass tool and keeping everything clean) that I didn't get any scratches during fine grinding. With fine grinding done I'm moving on to polishing. i spent the day making a pitch lap for polishing the mirror. The 30yr old black pitch from Beacon Hill was very hard and failed the thumbnail test. I bought some proper turps from HobbyCraft and added a third of a cap full into the pitch. Made a massive difference and was now probably a bit on the soft side. I raised the heat on the pitch a little and let the volatiles boil off till it was a bit harder. Pouring the lap seemed to go okay and I think I have contact. Hopefully get some polishing done this week. Expecting 6+ hours of polishing prior to figuring.

A grinding machine would be nice, but it seems you can do a better job by hand due to the random nature, and it would take longer to build than grind my mirror. I have to admit to looking at a few for future mirrors if I decide to make another one. I wouldn't want to do much more tha an 8-10" by hand! I think the cerox is the right colour, but haven't tried the consistnacy as I dont want to open it before I have too. Measured the focal length in the sun earlier. Appears to be as close to 680mm as I can measure, so sagitta was correct as far as I can roughly measure. Did a good hour this evening with the 320grit ali oxide. It looked to be pretty much ground out after half an hour but I decided to do the regulation hour anyway to be sure. Image below. Ive been lucky for stray grit so far, but there's been the odd bit floating around I've managed to spot before grinding. Hopefully be able to do the finder grinding and polishing outside to reduce the chances of contamination.

Hi, Thanks for the interest. I can see this becoming quite addictive! It seems nobody ever finishes just one mirror. You either don't complete it, or if you do finish it you go on to make several more! Quite rewarding I have to say. Just wish the sun would come out today so I could check the focal length again. I have a tub in the second hand kit I bought labelled "Cerox" - but no idea if it's still any good or what grade. Will buy some more if needed - luckily there's a couple of suppliers here in the UK. From what I've read in India getting blanks and grit isn't too bad, but pitch is difficult to come by.

I measured my focal length earlier using the wet mirror in the sun. It had gone up a bit from where it was, upto around 750mm, with a reduction in sagitta of about 0.15-0.2mm during my grinding with 120 grit. I must have had the tool on top too long during grinding. I shall grind with mirror on top from now on to increase the sagitta back to where it was. Did about another 1 3/4 hr this evening with the 220 grit and happy to have gotten rid of those pits. Surface is pretty uniform now and I've used all the 220 grit I had in the it. Not going to worry and move to the 320 grit. Measuring the sagitta using feeler guages after the 220 grit it appears to have gone back to ~2mm, which should put the focal length to ~680mm/F4.6 ish (taking the bevel into account). Increased the bevel again on the tool and mirror. Should be the last time now.

Thaks for the encouragement guys. To answer the questions I'm working in the workshop I built a few years ago. You need somewhere you can make a little messy I guess during grinding. It's not ideal though as I've done lots of GRP work in there recently so lots of dust around. I think I'll do my polishing outside if I can to avoid contamination. I did another hour or so mirror on top with 80 grit using a reducing chordial stroke down to a normal 1/3 centre over centre stroke. This got me up to around 2mm where I decided to check the focal length. Luckily the sun was shining earier so I was able to measure the focal length and it's about 700mm, basically where I wanted. I decided to call it a day with the 80 grit and do a good clean up. After lunch I added a bevel to the tool (didn't know this was needed), and increased the bevel a little on the mirror. I switched to the 120 grit to clean up the pits left by the 80 grit. I started with a quick sharpie test to see if the shape of the curve was okay by seeing it it grind out evenly. I think it was okay, but it only took like ten strokes before it had gone and seemed to be even, so not worried. Will check again at the next grit. The fine grinding continued with a mix of tool on top and mirror on top to avoid changing the focal length too much. I did a couple of hours like this and seem to be pretty much done with the 120grit. Unfortunately the first owner of the grit used up most of the 120 grit (the notes say 2 tsp per wet!) so I've run out. The images below show the surface with the worse imperfections I could find. Am I okay to move to 220 or do I need to order some 120 grit? Thoughts? Ignore the out of focus dust on the old 25mm eyepiece I used for the photos.

Hi, After about 15years of thinking about it, I finally got started on my first mirror grind today. I bought a used mirror grinding kit from AstroBuySell which includes a 6" 25mm plate glass blank, matching glass tool, pitch, selection of grits etc. The kit appears to be a 30 year kit from Beacon Hill Telescopes. The first owner had taken copious notes while rough grinding out to ~F11 (0.9mm), then switching to 120 grit and smoothing. Theres a couple of small chips on the back of the blank and tool, and a tiny chip on the edge of the mirror. They are not mentioned on the notes so suspect they may have happened during storage. I realise I'm aiming for a fast mirror for a beginner. This is really driven by the fact that I want to use this mirror in a telescope optimised for astrophotography at prime focus and like the 700mm focal length. Whilst a faster mirror like this might be a stretch I'm going to try and make things as easy as possible for my self during the figuring stage by making the best tester I possibly can and a sub-diameter lap if needed. Being 6" I'm prepared to take lots of attempts to figure if needed as returning to a sphere should be quicer than larger mirrors. First task was to bevel the edge of the back some more as this was a little small. This was a lot quicer than expected using the grinding stone. It's around 3mm. Is it better to have this as a 45 deg chamfer, or a smooth radius? I'm assuming the former to avoid too much light scatter? The little chip has been reduced by the increased chamfer and I also blended it a bt using a wet diamond file to avoid further damage. After that was done I started increasing the sagitta from the current 0.9mm up to my target of ~2.1mm. After just over an hour of the chordial stroke at 40%, reducing to 25% I got the sagitta upto about 1.6mm. At the moment I'm measuring using feeler gauges and a straight edge. Maybe I'll make a spherometer at some point. Chip much less noticable.

Other posibility is the filters aren't par-focal. Did you check focus between filters? Could it be a simply focus issue due tot he colour change?

Sorry my mistake. Damn dyslexia! Looks like the D3500 can connect, but doesn't support bulb mode, like my D5100. I had to solder up a bulb cable to control the shutter. Works well, but needs a bit of solering. Digicameracontrol can control a usb relay shutter cable( I tried but the USB relay boards from china were nasty with missing transistors and a mini usb that fell off the first time I tried to plug a cable in due to lack of solder), or a serial shutter cable via the RTS pin. This is the way I went, but needs some soldering. Commercial cables are available for £100 or so, but really I'd just buy a used D5300 rather than faff with two camera cables etc.

Everything I've read suggests the D5300 is one of the best DSLR's out there, and gives better performance than most of the Canons. There's a very high majority of Canon users doing astro as they were first to open up the cameras for external control, but the sensors haven't been as low noise as the sony sensors used in most of the Nikon's. The D5300 is a brilliant camera if you can sort out the tool chain. I use my two Nikons with DigiCameraControl which works really well. I have a D7200 (similar vintage to the D5300, might even be the same sensor) which I control via USB and an older (full spectrum modified) D5100 which I control via USB and a home made bulb cable. With digicameracontrol you can focus on screen using live view, set up interval shooting, shutter delays and it will link to PHD2 guiding and do dithering. It's really good. I tried backyardNikon and didn't get on with it. It will mean you are linked to a laptop, but that's going to happen eventually anyway in this world. Hope this helps.