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Vlaiv I don't think it is as indeterminable as it may appear. Treating it as a simple adiabatic expansion will get you pretty close to the value in practice - nozzle considerations accepting (choking, throat velocity). Nor is your target of -50 Celsius overly ambitious; for example a common CO2 fire extinguisher (55 bar , 3 litres) will easily get down to that range. Discharge a CO2 fire extinguisher through a close fibre cloth and you will be able to collect lumps of dry ice at -78 Celsius - the horn of the extinguisher will itself drop by as much as 50 Celsius if the extinguisher is fully discharged. Out of interest what is your plan, are you looking to build the setup yourself - interesting project, good luck with it. Jim

For a 1st approximation I would tend to treat it as a reversible adiabatic expansion so T2/T1 = (p2/p1) ^ (Ƴ-1)/ Ƴ . Ƴ (adiabatic heat capacity ratio) for air is approximately 1.4. T2 = (278) * (1/200) ^0.286 T2 = 61 K, but air freezes at around 58K 200 atmo is too high! In practice you would also need to allow for the efficiency of the nozzle used to throttle the gas so the final expanded temperature would be a little higher. Jim

I can't remember exactly Lousie just that it was all very reasonable. I've bought lenses and a beam splitter for an interferometer project and also a set of lenses to build a Rochester cloaking optical train. From what I recall there was no import duty as such or if there was it was pretty insignificant. The benefit of course is compared to the likes of Edmund Optics and similar Surplus Shed prices are way cheaper. It's just a matter of them having what you need. I'd highly recommend them though. Jim

x2 for surplus shed. I use them fairly regularly, very reliable, they may have what you want and if not ask and they may be able to find it. Standard USA postal service but not too long a wait. They had some cracking 5 inch objective lenses a wee while back , would make a nice project. Jim

Secret handshake time Dave , we are in the same club Jim

Without being disrespectful and I mean that sincerely but this type of stuff just makes me cringe - I really cannot get excited about people getting emotional or defensive about their particular way of doing something. Likewise whether we should have dedicated forums or not or call it visual or assisted or whatever - for crying out loud it's a hobby. You do it because you enjoy doing it - If somebody swears by standing buck naked with their head in a tea pot , left foot in a bowl of custard, whistling Dixie and observing through the spout afocal good for them. Do what works for you and don't feel the need to put a badge on it - life is way to short for that folks. For what it is worth I'd use NV at the drop of a hat if I could afford it , I also do visual and astrophotography have dabbled in spectroscopy and naked eye and I have on occasion (many) just stood and marvelled at the impenetrable blanket of grey cloud. Never once did I feel like putting a name to it, I did it for fun and because I wanted to. Jim

Chris would one on those Haynes Manuals be useful, I believe there is one on the Hubble. Haynes Manual - Hubble Jim

Thanks Eric I think I'll definitely try it out. I've played around with the Star Analyser and managed to produce a decent looking spectra of Vega using rspec. I stopped short of getting my head round correcting for instrument response but I've always wanted to go back and pick it up again. The Lowspec looks like a very neat little unit - thanks for the link to your write up, following that now. Jim

Eric, can I ask how has the Lowspec spectrograph performed for you, would you recommend it as a project? Jim

I certainly saw a lot of positive recommendations for as well Steve but pricey compared to others. That said I'd be tempted to try an expensive brand against the likes of Creality own brand which has performed well for me so far . Christmas is coming so I'm drawing up a list now - a reel of plastics , oh how days have changed Jim

Indeed Peter but not with PLA Just out of interest additive layer or even sintering technology would have to progress beyond imagination to meet the performance of Rolls Royce single crystal blade technology. But as you say some very interesting things are happening in the world of industrial 3D printing particularly in aerospace where a range of components are now 3D printed (both metal and non metal) . The method of manufacture, cold pressing, forging, casting, fabrication etc has always had an influence on the mechanical properties of material (advantageous and undesired) , it's an area of engineering science well known to design engineers. 3D printing is no different. JIm

To be honest Gina I think the world of 3D printing and its wide range of filament materials opens up an aspect of engineering design to many whom it may otherwise never really have been a consideration. We both come from engineering backgrounds so these considerations are well understood but I guess the assortment of filaments can be confusing to many. Like any hobby/interest there abounds misconceptions and "urban legends" - the 1 cubic meter pier foundation syndrome comes to mind With specific respect to PLA I think it is a good all rounder: inexpensive, good printing performance , no toxic fumes emitted. It is a good general purpose filament and where it does lack a particular property then other filaments can be chosen with printer capability also in mind. The Ender 3 and 5 models are PLA and ABS capable but at the moment, certainly for my needs, I can't see a need to use ABS yet and certainly not without some form of enclosure and fume extraction. Ill be printing PLA for some time to come before I start on my 3D printed cold fusion reactor Jim

I'll disagree with you on that one gian. The brackets I have printed in PLA do indeed have the mechanical and aesthetic properties for their intended application (adjustable bed for K40 laser engraver). Likewise the PLA printed assembly to hold the beam combiner will also function without difficulty. My Polemaster camera when not mounted for polar alignment functions as an electronic finder held in place by a PLA printed adaptor and has functioned perfectly well over the past 2 years. I certainly wouldn't look to use PLA for a third stage RB199 turbine blade nor a wing box on an Airbus, but neither would I use mild steel, aluminium or carbon composite . However, PLA certainly does have "real world " application. In common with any other engineering/manufacturing project material selection is part of the design consideration - 3D printing is not exempt from that consideration. To say however that PLA is not suitable for real world applications perhaps calls for a broader definition of "real world" - my uses to date are certainly real world and PLA has satisfied the design brief very well. Jim

I must admit I like the Ender's magnetic bed it gives excellent adherence without further preparation and it makes removal of the print a doddle. Having looked at the brackets I made for the K40 laser bed I can't see that I will have any problem with PLA for the majority of my practical applications (not really into toys and models); currently printing out a beam combiner again with PLA and so far all is well. Jim

Experiment with the gap Louise, position it so that there is a real noticeable drag on the paper. You will know if it is set right as soon as the printer lays down the first few lines of the raft/brim bed. I had a similar problem as you described when I did my second print - it just wouldn't adhere to the bed. I reset the level this time lowering the gap and it worked fine. For filament I've been impressed by the Creality supplied sample so I'm going to stick with that at the moment £17 or thereabout from Amazon. Jim Jim

Lousie as Gina said your bed is too far from the nozzle. I would start by going through the bed levelling procedure again. Send the printer to its home position, disengage the stepper motors, then place a slip of paper between the nozzle and bed. Adjust the height until you can feel a noticeable drag on the paper - it should still be able to slide out though. repeat for each corner and again if necessary. Don't overthink it , and if you end up with the gap being smaller than it should be then (show in the deposition of the filament) it will at least adhere to the bed - you can fine tune it later. You should be able to alter and save the temperature profiles here. Jim

The Reddit forum looks like a good source of info Lousie , it's not something I've used much. It looks like as you say a known problem then. I hope you can get it sorted. Jim

I noticed the same thing on mine Louise, I m guessing a strategically placed diode would have sorted that out. That said, I don't think it's causing any problems mine has worked flawlessly, I really can't fault it. I can upload a video showing how the encoder and screen on mine behaves if that would be of any use for comparison. Jim

That's looking really good Lousie you will be pleased with that. It's very satisfying especially when its something that you are going to make use of. I haven't noticed the problem you reported with the encoder not responding. Having said that if I am at the top of the menu and turn it the wrong direction (anti clockwise) it does nothing - I half expected that it would scroll from the end of the menu upward if you know what I mean. When Im in a mid part of the menu it will operate fine in either direction. Happy printing Jim

I would caution against looking to do astrophotography at the outset, certainly not for a whole class experience. To be honest sitting waiting for multiple exposures to be taken and then processing will be tedious for pupils and they will quickly lose interest. However that is definetly something to work up to in teh future. The best option I would suggest is either visual, certainly for the moon, solar observing, supported by some form of video capture if you want to view real time images in the classroom. Astrophotography is a bit of steep learning curve which brings in a whole set of additional requirements and where it is something you can work towards as a growth project I would not recommend it for a class activity. That said, astrophotography is perhaps something that a smaller group of more dedicated and patient group of pupils such as an astro club could engage in. As a secondary school teacher myself (Physics) I have offered both whole class and astronomy group activities in school and by far the most successful has been the visual experience. I have used a video setup for solar observation including our last partial solar eclipse and will be looking to do similar for the transit of Mercury in Nov. The advice offered by Pete above regarding seeking help from a local Astro club is definitely something you should consider. For what it is worth there is a lot in a project like this for a school to benefit from with respect to cross curricular engagement; you could engage pupils who are interested in design and manufacture, electronics, programming, art and their associated departments. Definitely worth pursuing. Take a look at the ATIK Infinity , search it out on YouTube to better understand its capability, If it is within budget something then like this would be excellent for remote viewing (whole class or small group). I would also try negotiating a supply deal with a retailer for single supply, most are willing to offer some for of "educational discount" especially for a whole set up. Jim ATIK Infinity - Video Astronomy

Based on what you have said above then I would go for the Skywatcher AZEQ6-GT (room for growth) and an 8 inch reflector something like the Bresser-Messier NT 203 or a SkyWatcher 200p (the one in the link is bundled with an EQ5 mount which whilst a budget mount will still allow it to be controlled remotely - this will save some money for you). Usually refactors are favoured for planetary use but I'd suggest that your pupils would be delighted to see the Andromeda Galaxy some of the globular clusters and nebula so a reflector would be a good choice. As far as software is concerned to allow remote operation you are in lucky as most of it is free and excellent at what it does. For controlling the telescope download and take a look at EqMod and Stellarium or Cartes du Ciel. Jim Eq Mod Youtube DemoCartes du Ciel Stellarium Skywatcher Explorer 200P Bresser-Messier NT 203 SkyWatcher AZEQ6-GT

3d printed pizza Jim

No need to be over cautious it simply works and works well - Creality get's my thumbs up. If problems arise as surely they will they will be investigated and overcome; but so far, it's on its 8th print and storming through with excellent results. I'm quite pleased now that I have the combination of the laser cutter and 3 D printer they complement each other very well. * Manual adjustable bed for Co2 Laser (K40) by dgferrete, Published on April 2019, www.thingiverse.com/thing:3595259 Jim