discardedastro

It will depend on the target, I think. Most video compression/encoding codecs use resolution subsampling at a ratio of 1:4 or 1:2 for colour. For instance, for a group of four "L" pixels, we just encode one pixel's worth of RGB data and mark that as applying to all four of those pixels. Most people can't tell the difference between this and RGB 1:1 data, even on high end monitors/TVs. But this does assume, for video codecs, pretty even distribution of input light source - i.e. we generally have videos that have all sorts of colours in them. For astro targets if your target is highly emissive in R, for instance, you'd be much better off capturing more R than say G or B. But this does require you to appropriately process and combine (because your higher-SNR R data will still suffer from being combined with low-SNR G data, for instance). So the general advice I've seen is usually to keep your RGB exposure times matched for a quiet life, and use R+G+B=L as a good rule of thumb for exposure time. But there are undoubtedly better ways to do it...

Never going to complain about thread hijacks with actual useful content though the angle grinder rig is just showing off! Hah! Well, I have already got a Mitutoyo digimatic indicator much like the photo above - nothing fancy, but enough to measure rough grinding - just need something to mount it in. I think I should have enough bits lying around to make one that'll be pretty stable... but the feeler gauges under a metal rule and variations thereof will do for now I think! The various mirror/tool stands and clamping arrangements are great to see. I think I can do something on the corner of a table with some big clamps - I also reckon I can mount a set of tool/mirror holding buffers in the bottom of the plastic tray I'm using at the moment which will be ideal to avoid mess.

All good tips. Hopefully the tool I've got will work okay. I'm familiar with working with tiny bits of glass in another application (my day job involves lots of fibre optics) so well aware of all the safety precautions to take - hopefully clean cuts will be order of the day!

Oil drum would be good if I had somewhere to put it! I think my best bet is going to be making a simple wooden frame in the garage and mounting a turntable on it. I've got plenty of stuff to weigh it down with to stop it moving around so long as I make something fairly solid. Time to spend an evening in Fusion 360... Did some more grinding this evening and the slower strokes/hand positioning works well - getting the hang of it now, I think. The tile tool is definitely looking worse for wear now so I've ordered a 50x50cm plate glass square from ukglasscentre and I've bought some scoring/snapping tools as well as a diamond wheel for my Dremel - I think between that lot I should be able to make some appropriately-sized irregular bits of glass. I think for my first go with glass I'll repeat the embedding-in-plaster approach and see how that goes - other than the ceramic wearing too fast the plaster tool seems to be working just fine and it's easy enough to make. Just need to use a slightly more dilute mix in the first layer to make sure I don't get any voids that'll trap grit. The worst part is a week-long wait with no grinding for the thing to dry! In the meantime I can at least keep grinding on the ceramic tool and try and get as much material out of the centre as I can. It'd be nice if I could finish the 80 grit before I come to pour the new tool, but that might be a bit optimistic.

I'd quite like to stick to making the tool - it's a skill I'll have to work out eventually if I want to do any more mirrors. Plus with that 21mm being Borofloat it's not ideal. I can get tempered or laminated glass online - I do have a local glazier who might be able to rustle something up though, that's a good idea. Will give them a ring tomorrow.

Thanks again for all the advice. Did another 20 minutes using less grit and slower strokes and I can happily say that there is now a tiny, just-visible gap under my best straight edge. @barkis you're quite right that my work table isn't so solid - I have better options but need to clear them of other projects! Might end up making something akin to the portable mirror making stands I've seen on the Stellafane site and others. In lieu of having the ability to pre-generate a curve I think my best option is going to be getting some 10mm plate glass and making my tile tool with that, either cast or laid on top with epoxy - anyone got any tips for suppliers? Can find plenty of places online who will sell me toughened glass but simple plate glass is tricky to find...

Plenty of noise when grinding so far at least! And yes, expected the outer to go first but quite satisfying to see it. I've uploaded a short (sequences reduced, as the advertisers say) clip of my grinding - well aware my technique needs work (not least rotating the tool more frequently, which I got to around the end) but gingerly uploading this for the purposes of constructive criticism - be gentle! I do think that my next tool I'll go for epoxy-onto-plaster rather than embedding, at least to see how it differs - as much a learning process as anything. The plaster seems to be holding up very well though so far - no chipping or disintegration at least.is I did buy a second (thinner - 21mm) blank as a tool but I figured I'd hang onto it as a spare/second mirror instead. It's Borofloat as well so not got that nice hardness differential advantage you've pointed out!

The edge test is a good trick! I'll have to give that a try. Spent some more time grinding today, tool and mirror definitely feeling more closely aligned now. Tool showing wear already - getting through the glazing on the ceramics pretty quickly, wear distributed largely on the edge. I also realised I'd forgotten to put a bevel on my edge, so did that this evening. Still nothing measurable in terms of sag, but I've only done ~15-20 minutes of grinding, and the way the table's set up I can't put loads of force on things. I've taken about 4 layers of Sharpie off, or thereabouts, and while the whole mirror's getting some abrasive action the sharpie in the middle is going first, which I take as a good sign.

Well, today the mirror tool's epoxy had cured and I thought I'd have a go at this actual grinding lark. The epoxy pour had worked well in terms of coverage but had leaked round just a little bit (I elevated it on three more tile segments) leaving a couple of raised spots - I ground these off with a sanding block prior to starting. I marked my mirror with sharpie just to give me a view on coverage, added some 80 grit and some water, and off we go... I only did a short session this evening before rinsing it all off, just to get my head around it all and go re-read all my texts/books with some practical knowledge in hand. The contact seemed pretty good by the end of the 10 minutes or so. I was quite gentle with the amount of force I applied, so wasn't expecting a lot of material removed. I used a ~40% chordal stroke, randomly moving the tool and more frequently randomly moving the mirror (mirror on top). This was the surface I was left with, which was quite uniform. So that appears to be working okay - just need to do some more now I think! The fact I'm now removing material has brought into slightly sharper focus my lack of measuring tools. I've got straight edges, of course, but my spherometer is still in my friend's lathe backlog. I'll have to fab something else instead at this rate. I need to get a surface plate in either case, which will have to wait for next month (for boring, practical financial reasons) - I'm aiming for a decent-spec 400x400 granite plate which should do for all the potential mirrors I'd make.

Perhaps worth mentioning that ArchiveTeam (a fine bunch of data hoarders and collaborators with people like the Internet Archive) have a form to request archival of groups: https://docs.google.com/forms/d/1Z-lODnyXsE2kiu8uL01L--10nDq3b_lowT88aB447-E/viewform?edit_requested=true If you want something to persist, it's worth submitting it. https://archiveteam.org/index.php?title=Main_Page is their site if you're interested in learning more or helping out (if you've got enough internet spare - my gigabit symmetrical line does its part!).

Bit of an unplanned capture of 27x300s (just over two hours) 7nm Ha which came out alright. Nothing special but thought I'd share nonetheless. Capture was with my usual 200PDS rig, processing was PixInsight following a pretty standard cal/cosmetic correction/subframe selector/registration/integration process, with some light deconvolution to sharpen up stars followed by some MLT denoising and MureDenoise, histo+curves to finish. I'm planning to do a reasonably long duration mosaic of the Heart in general, my first mosaic - some test overlaps below (forgot to crop before I registered, but it's a start).

I couldn't get any better tiles but got some more promising (much finer) plaster, so mixed that up and poured a fresh tool; the plaster is Dentstone KD yellow. It mixed up a bit thicker than I'd anticipated and set fast, so I had to do some sanding to get the back reasonably flat once it had hardened. My 80 grit and tungsten carbide sanding tool both appear to have gone walkies so I ended up using some 120 grit - took some time but got it pretty well tidied up. Weatherproof plaster with a slightly lower hydration is probably ideal - I'm not sure how the Dentstone would hold up very wet. Might mix up a small billet and soak it, see how it copes. I've done a first coat of epoxy using a polyester resin for the sides/back, so that should do for waterproofing and help level the back - I'll see how it looks after that coat and give it another coat if it needs it. Messy process so far, so the potting tray is paying off; the respirator, while not (as many have noted) a critical safety tool does help a bit with all the dust kicked up by sanding and the smell of the epoxy. I've gotten plenty of use out of my stash of nitrile gloves. If the epoxy's set well and completely covering the tool then tomorrow will be the first day of grinding!

So I've never done any solar imaging but have always fancied giving it a go. The Mercury transit next month seems as good as excuse as any. I've got a 200PDS/183MM setup and my thinking is that I could take a standard Baader solar filter and with that use my existing setup, which means I get to reuse everything. That seems like the cheapest option at ~£120 for a filter and a tilt adapter to mitigate Newton rings. I've got a 7nm Ha filter in the wheel as well as some LRGB. Would this be a sensible approach to do some basic imaging of the sun and spot Mercury? What else should I consider getting for my budget solar setup?

https://www.canford.co.uk/CANFORD-RJ45-CAT5E-SCREENED-PATCHCORDS-Using-Cat5E-F-deployable-cable https://www.canford.co.uk/Products/31-850_CANFORD-CAT5E-F-CABLE-Black is the cable used. It's standards-compliant and will carry a gig at the usual distances, but it's stranded-conductor and so flexes really easily.

Those RC battery chargers should work but won't be "automatic" - they'll require you to configure the charger and start it. You'll need to make or buy an XT6 to 12V cable. If you're using a cigarette lighter socket then you'll normally be limited to around 6A anyway.

So each link in that chain will have some losses - DC-to-DC can be 90%+ efficient, but DC-AC-DC is much less efficient. Add to that the fact that your AC charger is just going to assume it has limitless power whereas a well-behaved DC-DC charger will make allowances for the fact its input is imperfect because the car+alternator will be doing various things that they consider normal. The DC-DC stuff has lots of inbuilt logic to protect both batteries and charge/trickle-charge efficiently, whereas the AC charger doesn't know there's a DC battery feeding it and so will just sit there trying to pull power from the AC supply. You can find Sterling and others on eBay, don't know about aliexpress.

Figured I'd do a pour with the too-thin hex tiles and my non-preferred plaster (some "herculite" from Amazon, mixed to 42% hydration as recommended) just to get a feel for things and let me cock everything up on something that doesn't matter much! It all worked out pretty well for a first attempt, I think. I clearly need to make the clingfilm over the mirror perfectly flat - if you zoom into this photo you'll spot there's some gaps and voids in the surface, which isn't ideal. I'd put down a layer of Castrol multipurpose grease (what I happened to have lying around) underneath the clingfilm just to ensure there was a surface between the glass and everything else but I think I'll omit this next time as it made positioning the clingfilm taut across the mirror quite tricky - air bubbles were hard to avoid. Otherwise the mold didn't leak at all and the depth feels pretty good - might do a little more for the next tool, but it's there or thereabouts (~1kg of plaster, so I'll probably go for 1.5kg or so in the real tool). The sides are pretty good - not perfect, but perfectly acceptable.

I've actually done just this before at work for a mobile mapping system (laser scanner, camera, and computer) in the back of a Nissan Navara. The whole system could use a dozen amps at peak so we opted to keep it isolated from the vehicle battery but didn't want to have to recharge the battery independent of the vehicle. The 12V-to-12V charger option is the route to go. Most modern cars won't engage the alternator and pull the float voltage high of the charging level unless they think the vehicle battery needs it, so if you just hook it all up in parallel or similar then you'll find you don't often get the required voltage. The efficiency of the DC-DC approach far outstrips the DC-AC-DC loop. We went with the Sterling DC-DC chargers - the one we used was https://www.12voltplanet.co.uk/sterling-pro-batt-ultra-waterproof-battery-to-battery-charger-12v12v-60a.html but they have cheaper options. This can either be attached to the ignition indication circuit in the car (so it only tries to charge when the engine is running) or a separate switch. I'd recommend the ignition approach, though I think we included a separate isolator switch for that in series so it can be switched off entirely (there's also some chunkier isolation switches to let us disconnect the feed busbars from the batteries without having to remove connectors). You do need suitably chunky cables between your charger and batteries, especially considering voltage drop at the current levels you might anticipate. If I were trying to hack something up while being incredibly lazy I'd do the inverter+charger route, since that requires no wiring, but it will be slow and generally suck.

Thanks Nigel - tons of useful tips! Ref dust, absolutely - I'm mostly taking precautions for handling of abrasives before they're wet and on on the tool/mirror, cleaning up, etc. Won't be wearing my mask while grinding. I'll definitely try and get my hands on some thicker tiles. Glass tiles I hadn't considered but should be able to get those in quite good thickness. I've got some epoxy on the way to coat the tool with once it's had a week or so to dry out. The light box trick sounds interesting - essentially a sort of contrast imaging source. I'll have to give it a go.

Figured I'd have a go at laying up the form for a tile tool even though I don't have all the bits yet. I'm leaning towards Gordon Waite's method of laying up tiles in the base of a form on the mirror (plus a layer of clingfilm) and pouring directly onto that, since it'll fit the tool to the mirror exactly. I've got some Dentstone KD dental plaster on the way as I'm a bit wary of the "best hobbyist stuff from Amazon" option. I've used some waterproof 3M tape and the remains of dinner to make the form around the mirror - I think it'll hold.

Since it's my first one I'm sticking to an easy focal length - the one I've already got a telescope for, 1000mm, so an f/5. So if all goes well I can pop this in my 200PDS as an initial base.

It's all arrived! Grinding and polishing supplies, two blanks, and a sharpening stone. I've also got almost all the bits for making a tile tool, so can have a crack at that this coming weekend. The two mirror blanks (I've already smeared fingerprints over the 25mm, which I got flattened). I got 10" supply kits, just to give myself a bit of a supply buffer. Was all very well packaged from Germany in about 6 layers for the glass and 4 layers for everything else - very well protected.

Tempting - if I manage to make an 8" I'll probably want to leap to 12" thereafter though! The blank kit has arrived in the UK, so might even arrive this week. I've been buying supplies in the meantime: 3M 6500 half-face respirator and P3 particulate filters - don't fancy silicosis from handling fine abrasives and glass dust, though obviously wetting everything is the main defence there... Nice big tupperware boxes to keep the mirrors-to-be in Some small colour-coded bottles I can put abrasive/water mix in Clean spray bottle for water dispensing Sponges for wiping down/removing swarf/chips Big potting tray - this I figured was the easiest option for a good waterproof tray to do everything in. I'm not 100% sure it'll be big enough and might take some adjustment, and I might have to fit some rubber matting or clamp points to secure either the tool or mount 40x optical loupe for surface inspection - I actually want to get a stereo microscope which would be ideal but it's on my "when I have some money in the bank" list so a £7 little LED-equipped inspection magnifier will do for now Thick waterproof painter's tape While I bought a 25mm and 21mm set of blanks with the intention of using one as a tool I've decided I'll start off by making a tile tool and see how I fare with that. Couldn't find dental plaster on Amazon but I found some "herculite" "high-strength" plaster which I'll try for starters along with some bog standard (hexagonal, for easier tessellation) ceramic tiles. If the tile tool works I can either make a second mirror from the 21mm once I've cocked up the 25mm, or pass it on if by some miracle I don't make a mess of the first one! Also starting to shop around for the bits required to assemble a Focault tester and Bath interferometer, though it'll be a while before I need them - mostly nice and easy, but I want to make my measurements nice and repeatable/recordable, so designing everything around cameras. I managed to nab a Mitutoyo digital indicator off eBay for £50 (new in box!) - it's only good to 0.01mm but that's plenty for basic measurements of mirror curvature. My friend with a CNC machine is going to turn up a basic spherometer (i.e. reasonably thick bit of metal with hole in middle and grub screw to clamp the indicator) with a few sets of holes I can use for different measurements. Failing that I'll rig something up from some carbon fibre pultrusion stock I've got lying around which should be nice and stiff.

https://www.protocase.co.uk/ are a thing for "bespoke" cases for projects, including control panels. Very flexible, but not the cheapest. For your synth-in-a-flight-case, though, I'd find your nearest laser cutter owner. Engrave labels, cut holes, single sheet of alu. You can always paint the engravings and buff back the top layer to leave a really good high contrast finish.

Since my observatory project is on hold pending funds and some warmer weather, and the setup I have is working okay, I figured I'd try my hand at making a telescope. My plan is to do this in stages - first a mirror, since that's quite cheap to start on, and then I can use my existing tube etc (same mirror size). Then I could get a carbon tube and new mirror cell and assemble that later down the line as a further upgrade. I wanted to do a mirror because the process has fascinated me and I do like the idea of doing my imaging through something I've made from scratch. It's also one of the cheapest, longest-running "projects" I can think of in the hobby! So I've done my research and ordered a 25mm thick 8" borosilicate blank from http://www.stathis-firstlight.de/spiegelschleifen/material.htm as well as a 21mm 8" blank as a tool, and a set of abrasives etc. Stathis stocks good Schott borofloat glass - I didn't want to do plate/regular glass since the price difference isn't significant and if I'm going to put the effort in I'd rather end up with something as close to perfect as possible! I've been watching Gordon Waite's videos (and others) on YT and doing my research. For coating I figure I can either use Orion Optics or I have some contacts at Viavi who might be able to arrange something, well down the line. I've also gotten a few books on the topic, so I'm reading up. Feels like it should be a fun little project and I can start to build up my mirror test/inspection tools - a cheap little Bath interferometer, focault tester etc. I'll post in here as I go and keep a bit of a record...