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Thanks all for input on the topic. Luckily, I'm not trying to be very precise about all of that - just wanted approximation to see if there would be cooling with about 40K delta T. From above two approaches - energy conservation / isothermal process and second one - adiabatic expansion, I think safe bet is that there will be some cooling in range of 8K to 200K delta T (although I'm well aware that this second figure is very unlikely). It did show however that this topic is rather complicated and that there is no single formula that can be used to predict what will happen with good accuracy - it actually needs a bit more complicated simulation to get close. I think it is easier to actually test it out. For all interested, here is what I had in mind: I'll probably move to a new house (yet to be built - currently in phase of project and getting required permits) on piece of land that I purchased recently (yes there will be obsy too ) - and there is a lot of fruit there. That area is known for fruit production. Although you can buy frozen fruits and vegetables in supermarket at decent prices and there is no real commercial incentive to do that sort of thing - I fancied a way to do quick/deep freeze in home that does not involve expensive equipment and dangerous materials (like handling liquid nitrogen and such) - so I came up with above idea - use compressed air to quickly lower temperature. Grow your own food kind of approach (and store it for the winter). Had no idea if it would actually work, but recently I stumbled across these: (I'm posting links to commercial items - but have no intention of promoting them in any way. If it is deemed against SGL policy - please remove them) https://www.minidive.com/en/ And also a kick starter project that seems to offer similar thing: Both of them have 0.5L bottles (that can be filled with hand pump up to 200 atm pressure). That is why I used those figure in above calculations to see if it would work. In any case - I fancy idea of small scuba units which I can use on holidays , and if I could use it to do above as well - that would be nice (I have not found small pressure tanks anywhere else).

Ok, this is deeply confusing (as is most of the physics if one does not know it ) Here is another approach - adiabatic expansion (probably closer to what will happen): Taken from this page: https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Map%3A_University_Physics_II_-_Thermodynamics%2C_Electricity%2C_and_Magnetism_(OpenStax)/3%3A_The_First_Law_of_Thermodynamics/3.6%3A_Adiabatic_Processes_for_an_Ideal_Gas We have following: and hence this: If we have 200 atm pressure and we quickly release gas to ambient pressure (1 atm) we will in fact have about 17 liters of gas out of 0.5 liter container (Cp/Cv for air ranges from 1.4 up to 1.58 at 100 atm, so I took it to be 1.5 for approximation). And according to ideal gas law it will be at temperature of about 50K! That sort of makes sense if PV=nRT then at the same pressure if temperature is five times lower, so it will be volume. That is seriously different answer from above one.

Best I could think of right now is to do some really crude approximation. Problem is that every quantity depends on bunch of other quantities, and one ends up with really complicated differential equations. Basis of my crude approximation would be: - What sort of energy do I need to compress 100 liters of air to 200 atmospheres? - What is heat capacity of air? These of course are not constants (which makes things complicated) but let's do some approximations to at least see what sort of magnitude do we get as an answer. If we assume isothermal compression, then work done is: Let's say that we compress at 300 kelvins, and R (specific) of air is 287.058 J/kgK. 100L contains about 0.1276kg of air. Work done is: ~58KJ (Not much energy needed in fact). Now I'm going to just assume that heat capacity of air is something like 1KJ/kg K. This is really complex topic, but I've found some tables, and this value ranges in 0.7-1.5 depending on what you keep constant (constant volume or constant pressure) and vary other thing. I reckon 1KJ/kgK is a good approximation as any in this case. So we have about 60KJ of energy, we have 0.1276Kg of air and we need to see how much kelvins can we get out of that 60 * 0.1276 / 1 = about 7.6K? Not sure if this approximation is any good but it shows that this sort of thing will not get me delta T that I'm after.

Tried that one - and it is for ideal gas - which does not exhibit that behavior. Both @andrew s and I posted "right solution" for this problem above. Well I still don't have a solution, but at least I know that it can't be derived from ideal gas law.

Ah, yes, here is excerpt from wiki page on Joule-Thompson effect: It also "explains" how to calculate this (no idea where to start with that): https://en.wikipedia.org/wiki/Joule–Thomson_effect

I'm a bit stuck and would appreciate some help on this one. Don't know much about this part of physics and as a consequence - I found myself rather confused While contemplating device that would flash freeze fruits and vegetables, I came up with following idea: Take old fridge (just as an insulated container - it won't do any cooling), put one atmosphere pressure release valve (or a bit above one atmosphere) on one side of it, and inlet on the other side of it. Take 0.5L gas canister capable of storing 200 atm pressure and fill it up. Let it cool to some temperature like 5C. Connect it to inlet and release the gas. It should expand into fridge, and security valve should open up to release (ambient temp) air that was inside. Common sense says that there will be drop in temperature when air is released from the canister. I'm trying to see what sort of difference in temperature I will get with this procedure (roughly) - hoping for about 40C delta T. However when I try to fiddle around with ideal gas law - I get exactly 0 delta T - and that is what is confusing me. I'm probably not doing something right, but I can't figure out where I'm making mistake. Here is what happens: P1 * V1 / T1 = P2 * V2 / T2 As far as I understand, if I fill 0.5L canister with air up to 200 atm pressure - it will contain 100L of air. This means that 0.5 * 200 / T1 = 100 * 1 / T2 Or T1 = T2 - there will not be change in temperature when I release the gas from canister? I can see that there is likely error in my reasoning that 0.5L canister will contain 100L of air at 200 atmospheres - but how do I go about calculating temperature drop?

Darks are fairly simple to do. You need the same settings as lights for them to work, and that means: 1. Same gain / iso / offset (depending on your camera if it is DSLR or dedicated astro camera) 2. Same exposure length 3. Same temperature Only difference is that you have to block all light coming into the scope. You can take them while camera is attached to the scope - by covering the scope with lens cap, or you can take them when camera is detached from the telescope by putting camera cover on. Be careful about any sort of light leak - even in IR part of the spectrum (plastic can be somewhat transparent to IR wavelengths). In case that your camera does not have set point temperature control (not cooled) - try to take darks at approximately same temperature as your image subs (lights). If your camera has stable bias and is not cooled you can use something called dark optimization (there is option in DSS for that). Flats are done with camera on the scope and focus position roughly the same as when doing light subs. You need either flat panel or flat evenly illuminated white surface in front of the scope. You can even use sky when it is brighter (not during night, but early in the morning or early in the evening when sun is not out but it is still bright enough) - some people put white T shirt (or other cloth) on aperture to diffuse the sky light. You need to be careful not to overexpose when doing flats. Histogram peak needs to be at about 75-80% in value. Take flat darks as well, and rule with those is the same with regular darks, except that they need to match your flats - so same exposure as flats, same settings as flats ...

Have a look here: There is 10" version as well if you want a bit more oomph since it will be observatory mounted.

There is another way to go about this - doing real test on this scope. It's going to be a bit involved, but we will get detail on that particular sample. One test that can be done by owners themselves that is a bit involved but not out of the reach of amateurs is Roddier analysis. If you have planetary camera and a bit of planetary imaging skills you can do it.

Just realized that Aberrator has simulated Ronchi test, and that can't really be used to distinguish the two (or maybe it could by someone really experienced), here are sims of parabolic mirror and 0.35 wave spherical :

Btw, 130mm to have 1/4 PV with spherical mirror according to above formula, needs to be at least F/7.84, or about 1020mm FL.

Question is can we tell the difference between parabolic and spherical mirror at that aperture and F/ratio? According to telescope-optic.net here is expression for low order spherical aberration of mirror with a given conic constant: Where K is mirror conic - in case of sphere it will be K=0, and therefore PV error will be: 0.888*130 / 6.9^3 = ~0.35 This is for 550nm, and that is more that 1/4 PV, or about 1/2.85 PV. From expression it is obvious that parabolic mirror will have 0 low order spherical aberration (K=-1 for paraboloid). No way that this sort of spherical error could be mistaken for parabolic mirror in a star test. Here is what Aberrator shows for defocus patterns, first the control - parabolic mirror: Here is what 1/2.85 PV of low order spherical produces: Star testing should really reveal that level of LSA and image will be soft - such scope is not diffraction limited! Only "evidence" that I can provide having owned that scope (I don't have it anymore), would be simulated and real Jupiter images. Aberrator produces this image of Jupiter (from high resolution base image) under ideal conditions (no other aberrations and no atmospheric turbulence) with that level of LSA: Here is what image with parabolic mirror would look according to Aberrator under ideal circumstances: This is actual image taken with said scope in real circumstances: Surely not conclusive test - but I like it It definitively shows that simulations are quite realistic. I would say that real image maybe, just maybe leans towards parabolic mirror simulation than to spherical one.

Too red and too linear to be IFN - classical LP gradient

Depends ... If we are talking about tripled or ED doublet refractor with good figure vs SCT with good figure - refractor wins in almost all areas. With achromatic refractor - it will depend on F/ratio of the scope. Problem is that it is hard to find refractors with matching aperture to that of commonly available SCTs. C5 is smallest SCT, and next is C6. You can find apo refractors in that range that don't cost an arm and a leg. They will still cost more (even twice or three times more in 6" range) than SCT. In 8" and above - apo refractors will be seriously expensive. For same aperture - they will provide more light gathering. SCT has two mirrors, central obstruction and corrector plate. Mirrors have lower reflectivity than losses in the lens (with modern coatings). Good lens will also have better sharpness than SCT telescope, and better correction (SCTs have some spherical aberration depending on focus position due to moving primary mirror - only one distance between mirrors has best correction - but you move mirror to focus). No central obstruction with refractor will provide better low contrast (better detail) performance. SCT will be easier to mount due to shorter tube and lighter for same aperture.

It seems to be popular these days - we have one of our bridges lit up as well, similarly distasteful, luckily much mess LP produced.

Just a couple of points. I'm having hard time identifying hot pixels in this image - star field is so dense that I'm never sure if it is hot pixel or the star. Hot pixels will show in your darks - stretch your master dark and note position of hot pixels in it - they should roughly correspond to pixels in final image (there is some alignment in final image). You can't really do cosmetic correction at this sampling resolution because stars are tiny so that is out of the question. Did you dither your subs? If you plan to use sigma reject stacking you need your subs dithered properly - that means quite a shift because of sampling rate. If you move your sub by just a few arc seconds - that is no dither at all when your imaging resolution is about 4"/px.

First check if you got your debayer settings right. Most of the time, LP is dominant in red part of spectrum and most people have issues with red cast rather than green. I'm assuming you in fact have Altair 183C rather than 138C and above is typo - altair website says proper bayer pattern is: GBRG It is either that or GRBG edit: RGGB (image can only be flipped upside down by software but not left to right, and that depends if one observes Y coordinate to go up or Y to go down. X always goes to the right). If you are sure about your bayer settings and get those confirmed, here is "in depth" explanation on how color works and how to get proper color balance. Even when you get your debayer settings right, it is important to understand this. Each sensor has different sensitivity curve regardless if it is mono + filters or OSC sensor. R, G and B from sensor do not map to R, G and B for display directly. Here is what published sensor response looks like for 183 model: And here is what "response" for sRGB (color space most likely to be used by computer and therefore you should transform to this color space when working with computers / computer screens): Now, don't be confused by the fact that there is "negative" part of the curve in sRGB matching functions. sRGB color space has lower gamut than human vision and three primaries used for sRGB (type of blue, type of red and type of green) don't allow for all colors to be reproduced. Here is sRGB color gamut compared to human vision: sRGB color space can only display section in inner triangle, while humans can perceive everything that has been shown as color - much larger "area" in this chromaticity diagram. You need to "subtract" some light to get things that eye can see but sRGB can't reproduce. In any case, that should not worry you much. Important thing to note is that red and blue are "higher" then green in this graph. If you examine 183 spectral response - green is higher than other two. Raw color information from the sensor is not going to represent true color. You need to do color calibration to transform raw color information into sRGB color space. Most people don't do this, and they adjust color balance as they please (reduce green, boost blue and things like that - until they get result that they want). There are other ways to accurately transform raw color into sRGB (linear - it needs gamma adjustment later) color. One of the ways is to do star color calibration. PI has script for this for example. Another way is to use precomputed matrix (from someone who did general color calibration for this sensor - this works only if you use same filters as that person), or alternatively doing color cards. You can purchase standard color cards that you can use for color calibration. In fact that is what they are designed for. Photographers use them to calibrate equipment. It looks something like this: you take such card, provide uniform daylight illumination to it (or specific illuminant like D65) and shoot a picture of it with your sensor. From actual raw color values and expected color values for such chart you can derive transform matrix. Btw, transformation is as simple as channel mixer and it goes something like this: sRGB_red = a * raw_red + b * raw_green + c * raw_blue sRGB_green = d * raw_red + e * raw_green + f * raw_blue .... so you need nine numbers a, b, c, ..., i that represent your color transform matrix. Bottom line is - don't expect raw color information to be properly white balanced, you need to do white balance on your data to get good results. Some ways of doing color balancing will produce accurate color results (star color calibration, color cards, ....), while others will be only approximate.

Ok, so here are couple of measurements that I took from some of my subs: Ha filter, night of good seeing - 4.7px at ~0.483"/px (I round that up to 0.5"/px when talking about it, but it is closer to 0.48"/px), so that gives: ~2.27" FWHM OIII filter, night of good seeing - 4.63px - ~ 2.24" FWHM Ha filter on a night of poor seeing - 7.87px = ~3.8" FWHM Lum filter (actually IDAS LPS P2), poor seeing - 5.91px = ~2.86" FWHM We could say that it ranges from 2.2" FWHM to 3.8+" FWHM, or expressed in effective resolution - 1.375"/px to over 2.275"/px. One could probably fare worse if seeing is really bad - but who would want to image in those conditions? Just for a comparison, here is theoretical expected FWHM for my conditions - 8" aperture, around 0.5" RMS guide precision, and let's say seeing in 1.2" - 2" range. Star FWHM for ideal optics under 1.2" seeing and 0.5" RMS guiding should be 1.78" FWHM, while for 2" seeing is 2.39" FWHM. My values are a bit more than this, and while I get rather OK seeing forecast, I think it's down to local thermals that bump up my FWHM values (surrounded by houses and bodies of water - Danube river is right in my imaging path - about 1-2Km away). This is what meteoblue forecasts for this evening, for example (gray column is seeing in arc seconds): Like I said, even in best conditions, I'm still in 1.3"/px zone. Maybe things will change once I move to the countryside and to a bit higher elevation, and get better mount. I have CCD47 reducer, tried it once, did not like it, but that could be due to tilt. In the mean time I upgraded my focuser on RC to 2.5" one with threaded connection. I will have to try again, but I'm not expecting much from it. It turns out that it will not correct fully if placed at distance that makes x0.67 reduction, so most people place it closer to get x0.7-x0.72 reduction. It's not corrector and if you apply x0.67 reduction, then 22mm diagonal will be close to 33mm circle, and anything above 30mm on that scope requires field flattening and correcting, even below 30mm might not be very good. If I were to look at reducer for RC again (and I will at some time in future) - I would look at x0.75 Riccardi FF/FR. It seems to work with this scope as well and I think it works good (at least that is what I read about the combo).

Catadioptric is general term for all telescope designs using both mirrors and lens, or rather reflective and refractive elements (in base design - that means without eyepieces / barlows / detachable reducers / finders). Other two basic designs are reflectors and refractors. SCTs, Maksutovs (both Mak-Cass and Mak-Newt), and some other designs - like Bird Jones are all Catadioptric designs.

Very good estimate of optimum sampling rate is about FWHM / 1.6. That is based on Nyquist sampling theorem and Gaussian approximation of star PSF (and it's Fourier transform). If you for example have star FWHM in arc seconds be 2.8" in your stack (individual subs will differ on average FWHM, but you can do "test round" of stacking to measure FWHM in resulting stack - just convert to arc seconds based on your initial sampling rate), then optimum sampling rate is 2.8 / 1.6 = 1.75"/px. Most of the time you won't be able to bin to exact value based on FWHM, and I advocate going "a bit over" rather than "a bit under". If you for example have 0.5"/px sampling and your ideal sampling rate is 1.2"/px - I would rather bin x3 and go for 1.5"/px then bin x2 and go for 1"/px. First - it will raise SNR by larger factor, and second - image will need less sharpening if any.

There is very small benefit in terms of pixel blur if you process your subs right. However, that is not the main reason why I image at that resolution. I image at that resolution because that is what my setup gives me, and I can bin image to get to resolution that my guiding and sky allow. When I was choosing my setup, resolution was only one of concerns, and in hindsight, I probably over estimated things - 1"/px is really hard to reach (FWHM of 1.6") even if sometimes I do get seeing in 1" FWHM zone (0.8-1.2" FWHM). I might be able to utilize this resolution more often once I move away from the city and get better mound (aiming at mesu 200). Good thing is that one can use focal reducer to widen the field in this combination (up to 30mm of field will be usable and sensor is only ~22mm diagonal), and I'm working on fractional binning - that will get me optimal resolution for a given night (or target). There are other reasons why I've chosen that setup, and I'll name a few: - I wanted 8" aperture that could be used on HEQ5 mount. I tried 8" F/6 newtonian and while it worked - it was not stable platform. This means that I needed either F/4 scope - which is very demanding both on collimation and on coma corrector (design and placement). When I did my research, I found out that coma correctors don't really provide perfect correction - some have issues with spherical aberration, some correct coma only to a certain distance of center. Most are designed with very short back focus suitable for DSLR type cameras (55mm) and not for OAG + filters, etc ... Another option was SCT - but I don't like those in imaging role - RC on the other hand is very good imaging instrument - it is well baffled, won't dew up easily (just happened once to me - dew on secondary), it is compact so easier to mount and guide. Does not need any corrective optics on sensor the size of ASI1600 - it has fairly flat field and round stars. There are additional things about this scope that I value not related to imaging (but rather as science instrument). That is just a combination of scope and camera that I'm most pleased with and it forces me to use 0.5"/px when shooting and bin later, and that is fine with me. I do have another scope that I use with ASI1600 for wide field imaging and resolution of 2"/px - a small frac.

Only issue I have here at the moment is that there is quite a bit of fog in the evening - high humidity and probably high air pressure. With LP, I can hardly make out Orion's belt in the evening. Don't know why is it so humid - we haven't had rain in quite a while. That is just because I use yr.no as my weather provider - they give forecast for much of Europe and are precise enough

Over here it's been quite interesting 25-26C + during the day. We had warmest October day in past 60+ years couple a days ago with temperatures over 28C. Heating season starts around 15th of October (for those on remote / central heating) and most people that do their own heating start even earlier than that - second week of October. Average daily temperature for October here is about 12.4C. Last two weeks we did not get down to that temperature even in the early mornings. It's also much drier than usual. Warm weather will apparently continue until beginning of the next weak. Btw, I'm also currently in my shorts

It could still be Bird Jones type telescope - spherical primary and corrector lens, only F/5 primary instead of faster F/4 like TS example above. Maybe in this slower configuration it is better optically? Or it could be regular newtonian with a barlow. But what would be point in that? I mean you can buy regular 6" F/5 and use barlow on planets and use it without barlow to get wider field.

We could argue that their description is another "clue" to design of that specific telescope. Bird Jones does not use simple barlow lens. It has spherical primary (rather fast - F/4 perhaps) and corrector lens. It is a bit similar to Mak-Newtonian - except Mak-Newt uses full aperture corrector plate and modifies wave front prior to primary mirror. Bird Jones modifies wave front "in converging beam" (after primary) - so it has different curve on corrector lens, and also as a consequence corrector lens needs to extend focal length to correct it. In any case it is not simple barlow lens, and it should not be called so. There is no reason why regular barlow lens can't be mounted on parabolic newtonain before focuser at proper distance. Technically it would also be considered Catadioptic design - combining reflective and refractive elements.