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Here is a brief overview of how it is done, but there is much nicer explanation in document accompanying the software that is used: It would be best if you have access to NB filters - that way you can do exact measurement in certain wavelengths. I have OIII, Ha and also Baader solar continuum filter that is around 530nm or so, but have not done measurement with those. You can also do it with regular RGB filters but results will not be as precise - you will get a sort of "integrated" strehl ratio over respective bands. Same happens with OSC cameras. You will need WinRoddier 3.0 software (or any more recent version, last time I did this, it was with version 3.0). You will also need some sort of application for fast capture - like planetary imaging. Sharpcap will serve this purpose. Last you will need planetary stacking software - AS!3 for example. Measurement is done by recording defocused star pattern in particular wavelength (or filter). You should select fairly bright star - like mag 2-3, and high up in the sky to minimize seeing effects. Night of good seeing will give you more precision in your results (similar to planetary imaging). If you have OSC - you record all three bands at the same time, then split channels later and examine each R, G and B recordings. You proceed by doing short videos of about 1 to 2 minutes. You need one of in focus pattern and one of out focus pattern. There is method to calculate what diameter of defocused star pattern needs to be in pixels - depending on focal length of scope and size of pixels on camera, it is included as separate small piece of software and it is described in how to document / manual. When you finish your movies and calibrate them - you stack them in AS!3. It is important that you don't change intensity of image in any way, so avoid auto scaling of intensity and such. Also - no sharpening must be applied to image. Once you have your stacked images prepared you load them in WinRoddier, set wavelength (for NB filters - pick proper wavelength, and for RGB use wavelength in center of the band for each color). Software does the rest and prepares wavefront images, Zernike polynomials / coefficients and calculates strehl / star profile - look at images that I attached it is screen capture from WinRoddier software. This is process in the nutshell, for more detailed instructions - read the "manual" (or rather document describing workflow). All needed files (both program executables and manuals) can be obtained via Roddier yahoo group. You need to join the group (it might need permission from group maintainer/admin if I remember correctly - it took a day for me if I remember correctly). https://groups.yahoo.com/neo/groups/roddier/info Once you have joined the group look under Files / WinRoddier Ver. 3.0 (Latest)--Apps and User Manuals folder for pretty much everything you need. If you want to check out manuals and "how to" before joining the group - it looks like these are available on different urls on the net, so here are some to get you started: http://www.compubuild.com/astro/download/Roddier_Ver.3.0_Quick-Start_Guide.pdf http://www.compubuild.com/astro/download/New_WinRoddier_User_Manual.pdf

Ususally I was expecting to have a cleaner Halpha channel than OIII, but it's the opposite. I also know that asiderom the optical color correction, in green, you get a better resolution and even better in blue than in red. I'm not going to question how good collimation is on this particular scope, but there might be an issue with correction. Although from your quote we can see that "The colors are very well balanced" - I'm not sure what it means. Triplets are tricky beasts with regards to collimation and proper lens spacing - and I for sure, have no clue about how it is properly done. What I do know is that there is no single strehl ratio for such a scope. If single strehl is quoted - then it is probably in single wavelength - usually around 550nm (or similar - peak of visual sensitivity in scotopic vision I think). Corrections in other wavelengths might and usually are different. Here is an example of strehl vs wavelength graph: This is result for a doublet scope, and while around 580nm it approaches respectable 98% strehl, at Ha this particular scope is going to have strehl around 65% or so. I "tested" my own TS80mm F/6 triplet with Roddier analysis - nothing fancy, RGB image split, and I for example got following results per "band": Blue was around 80% (let's say average), Green around 94% while Red was at around 98%. Such scope would have opposite characteristics - very sharp Ha, while OIII would be softer. Have a look at this post to see results of test: However, not sure if this is the cause of your issues with star bloat in Ha. No, actually, I'm not seeing it. I'm seeing what you are seeing and thinking that best focus position for small stars is as you said somewhere around 32407, but this is just a visual thing. You have no HFR for these stars, and because they are so dim - when not in perfect focus they will be even more dim and their "wings" (wings of star profile) will be below read noise. If you look carefully, you will actually be able to tell that it is spread around. it is very faint, but can be seen. I'll make a screen shot to point it out. Here it is - focus position 32407: Now the same image - ridiculous level of stretch - just look at noise distribution: Noise is more dense in rather large circle around central "tight" points - and this is also light from those stars but very low in intensity due to less than perfect focus, and being at the level of surrounding noise or below it, so it can't be easily spotted. Also compare how much the size of disk of large star increased (it is already stretched) and how much of those two tiny stars.

Could be many reasons, but let's list a few: 1. changing seeing conditions between OIII, Ha/SII - you could analyze this maybe by examining what time of the night did you do each set, was it on same or successive nights and what was guiding like for each session. If OIII was first or last in single night - it is more likely that seeing had something to do with it, if it is in the middle of the night - it is still possible but less likely. If each was taken on a separate night - highly likely that it was the seeing. Poorer guide RMS often means that it was either the seeing or maybe wind? 2. Different signal levels will cause different level of "auto stretch" / "auto develop". If you want to see if stars are similar in size (or different), you should check actual FWHM of respective stars to see change in "bloat". 3. It might be that optics + flattener is better corrected in green than it is in red. I doubt that you would see this from scope alone, but with flattener - it might be an issue. 4. In principle it could also be due to filters, but I don't think Baader filters are to blame here. Many people use them and no one (including me) reported this. In general it is other way around - Ha/SII tends to be sharper than OIII because longer wavelengths are less impacted by seeing than shorter ones.

For smaller sensors you don't need flattener. You might want one for APS-C sized sensor (I'm not sure). RCs have rather flat field, but not fully flat - for large sensors they need flattener. I use RC8" with ASI1600 without flattener and field is good on that size (21mm or so diagonal). This scope has 1370mm FL and with 460ex this will give you 0.68"/px - and that is rather high sampling rate and you will be oversampling natively. There are two things that you can do - add FF/FR, or bin. I think that you should bin x2 - that will give you 1.37"/px - and that should be rather good for HEQ5 and regular guiding results (something like 0.8" RMS). When you bin x2 it's like having twice shorter focal length so you will effectively have "F/4.5" scope - which will be fast enough (F/ratio and "speed" of scope is not measure of how fast you will take images - it's aperture at resolution, and in this case you will be using 6" at 1.37"/px - which will be quite fast). Here is an example for you - this is two hours of lum in rather heavy LP (mag 18.5 skies) - 8" scope (F/8 natively - also RC but 8") at ~1"/px: I would not call that slow ... You can "speed up your scope" with use of FF/FR, and a good one would be Riccardi x0.75 FF/FR - but that would give you 0.91"/px - and I think you will still oversample on this resolution. Binning with FF/FR will give you 1.82"/px - and that is going to be rather low resolution - comparable with 80ED - so you will gain nothing (apart from speed, because of almost x4 light gathering for same sampling rate).

Maybe this: https://www.rothervalleyoptics.co.uk/altair-astro-6-f9-ritchey-chretien-astrograph-telescope.html

I did read once a review of this unit, and author did get tighter stars by using it, but can't remember what mount it was on. Will try to find it again and post a link Yes, you are right that some mounts are rather expensive, but why consider GM2000 and not something like Mesu 200? EQ8 + AO device will place you in Mesu 200 price range, and I have sneaky suspicion that Mesu 200 will deliver at least as good performance if not better without complexity of running an AO unit.

In my view - completely useless thing. It won't correct for seeing for different reasons: - it only corrects for first order aberration - tilt - seeing aberrations, or specifically tilt is very local phenomena - seeing happens in higher in atmosphere at at least couple of kilometers of altitude. It is enough to move couple of arc seconds away from a guide star and that translates to significant length at that distance (for example 20 arc seconds at 5km is already about half a meter - seeing cells tend to be less than that). Any sort of wider field will be distorted by different amount of tilt - you can easily see this effect if you look at planetary recording or maybe Lunar recording - distortions (jumping around of the features - mostly due to tilt) is different at different places. Correcting for tilt at location of guide star will do nothing for rest of the image and probably cause more harm then help. - it does corrections on time scales that are most of the time above time scales of seeing changes. It does something like 30hz max if I'm not mistaken, and seeing often goes as fast as 100hz or more. It does have one use, and it is at best limited at that - correcting for a rough mount. If you want good guide performance but your mount is rough and it has small time scale error large enough (like 0.5"-1.0" or similar jitter) that can't be corrected with guiding because guide exposure is longer and mount is not as responsive enough - this will catch it and react provided that seeing is good enough that any guide star deviation from true position is due to mount and roughness rather than being "masked" by seeing. On the other hand - why buy such expensive unit that might not perform as expected when you can instead sell current mount + invest that money into purchasing a new better mount that will be smoother and guide better.

Yes, and it works well. I've done it on short & fast ST102 F/5. I made couple of aperture masks, and did some testing. Even managed to take a decent deep sky photo without any blue bloat around stars - by using 66mm aperture mask and yellow #8 filter. Just be aware that putting aperture mask on will reduce maximal attainable magnification - good rule as always is aperture in mm x2. 80mm scope will be good for up to x160. Of course image will be dimmer. Btw, I've now got Evostar 102 F/10 and Baader contrast booster filter removes almost all CA while keeping things almost natural looking (there is small color shift). If you go for something like 70-75mm aperture mask, you will in principle eliminate chromatic aberration pretty much completely (CA index >5). Btw, here is "a study" for photographic purposes I did at a time with ST102: Rows contain same image, but stretched to different level (first one the most obviously as hot pixels / noise start to show). From top to bottom - no mask, no mask+#8, 80mm, 80mm+#8, 66mm, 66mm+#8, 50mm, 50mm+#8. Exposure times were scaled appropriately.

Just to throw in a wrench or two ... Since OP almost decided to go for Mak180, I'm wondering if this could be something worth considering: https://www.teleskop-express.de/shop/product_info.php/info/p10753_TS-Optics-8--f-12-Cassegrain-telescope-203-2436-mm-OTA.html As far as I know, Australia has a dealer of GSO equipment as well, so worth checking there for availability as it would certainly be more affordable then shipping it half way across the globe. Quick comparison: 200mm vs 180mm - more resolution and sharper image at lower magnifications (given same optical quality). Price about the same - slight edge for 8" Cass in TS pricing. Slight weight advantage for 8" Cass (7.5 vs 7.8kg). Much less of a dew magnet since no front corrector plate. Cool down time is consequently less in 8" Cass Probably better focuser as it has 10:1 reduction and does not cause mirror shift when focusing. CO size - Mak180 probably wins - I've seen figures around 30% quoted online - 33% for 8" Cass F/ratio and focal length - again +Mak180 as it is F/15 vs F/12 and 300mm additional FL - easier on EPs, easier to reach high magnifications. 8" Cass - diffraction spikes - might detract some people, better baffling of the tube - probably very small boost in contrast because of this.

Don't get AZ3 - I had that exact combo and later switched to AZ4 on steel legs. Very big improvement. AZ3 has couple issues that annoyed me quite a bit. Stiff to move telescope by hand (although there are adjusters - it is not very smooth mount). Slow motion controls needed rewinding after a bit of use - they are not 360 degrees but rather section of the circle so you when you reach the end - you need to move scope manually and "unwind" slow motion control back to middle (if you plan to pan around slowly) or all the way if it is only for tracking single object. Looking at zenith was next to impossible on that mount. If you want slow motion controls then AZ5 is better option than AZ4. With AZ4 it is pretty much only feature that is missing (and I believe AZ5 is slightly lighter).

Not very long I'm afraid. Two major things that you will be facing is periodic error and polar alignment error. Out of the two, I think that periodic error is going to give you more issues than polar alignment error. Let's run some numbers so you can get the idea of what sort of exposures will be attainable. You are imaging at ~ 1.6"/px, so let's set upper limit to 2px, or about 3" as maximum drift per exposure that will produce acceptable stars (a bit elongated, but still round enough). With regular polar alignment, you are looking at something order of 5 minutes of arc or PA error. In worst case scenario this translates to drift of about 1.3"/minute. Stock HEQ5 can have as much as 30-40" P2P periodic error. Since you've done belt mod, this tends to drop quite a bit, but it is still in range of let's say 12-14" P2P. Period of HEQ5 mount is 638s. If we assume perfect sine wave then max RA drift rate due to PE will be 4.13"/minute - in reality PE is never sine wave and you can expect it to have a bit higher drift rate at some point, but let's go with 4"/minute because P2P might be even less like 7-8". This shows that PE will be limiting factor, rather than PA error (unless you did a very poor job of polar alignment and error is something like 16 minutes of arc - which is quarter of a degree, so yes very large indeed). With drift rate of about 4"/minute you are looking at about 45s exposures. This is worst estimate, so not all frames will be distorted (it depends where on period of mount you are at the moment, what is the DEC of the target, etc ...). I think that you can use 1 minute exposures and expect to throw away something like 10% of frames if your target is close to equator. Higher up in DEC you can maybe do 90 seconds and still keep most of your subs.

1um will be provide 0.1" resolution with about 2 meter radius. Might be feasible to put it in 1-1.5m diameter if you have sub-micron resolution and are happy with about half to quarter of arc seconds in angular precision. Not sure if those linear encoders can bend?

There are couple of ways you can determine mount pointing with encoders: Motor shaft encoders / tick counting - this is for example how Heq5 mount does it. It measures motor shaft position / number of revolutions. It would work perfectly if gearing from motor to last stage was perfect - no periodic or non periodic of any sort. But there is difference between what motor shaft outputs and the position of the scope in the sky, and while you think you are pointing at certain spot - you are pointing to somewhere else. Double low resolution encoders - it's like low bit counter / high bit counter, or if you are not familiar with that, best way to explain it would be - one encoder keeps "hundreds" and other keeps position within a hundred (0-99) when you combine them you get actual position - it's a bit like old analog clock - small hand will give you hour and large hand will give you minutes - combine the two you will get exact time. A bit more precision over precious - motor shaft option, but how much precision depends on resolution of encoder on main shaft (hour hand). Absolute encoder on the main shaft. It does not need to be absolute encoder, but it needs to have sufficient precision to determine exact shaft position - something like 28bits to have arc second / sub arc second precision in pointing. I assume that we want to reach a sort of precision that is needed for given requirements - that means imaging at around 0.75"/px. This means that you want guide performance of about 0.1- 0.2" RMS. You want to be able to have something like 10 seconds between guide exposures. In these 10s max that mount can deviate from true position should be around 0.3". Which then translates in max drift rate of 0.03"/s. This is very precise tracking. Problem with Alt-Az type mount is that your speed in Alt and speed in Az are not constant - they change every second, and depend on where scope is (or better - should be). This is not so with EQ type mount. In normal operation, RA motion is constant, and DEC motion is 0 - wherever you are pointing. If mount makes tracking error - and goes a bit "forward" or a bit "backward" that will not impact DEC rate - it will remain 0. Similarly if there is drift in DEC because poor polar alignment, RA rate of motion will not be affected by this. With Alt-Az type of mount - change in position requires correction of both Alt rotation rate and Az rotation rate to properly keep correct pointing. If there is error in any of these two, scope will be pointing at the wrong place, but will think it is pointing elsewhere and will change Alt and Az rate accordingly which in turn will make it drift more - further from wanted position, and again it will calculate improper tracking rates and drift more .... For this not to happen, you need most precise encoders that you can have - and those would be full resolution encoders (either incremental or absolute) on Alt and Az axis of the scope.

I'm going to expand a bit why absolute encoders are necessary for Alt-Az type mount if you want accurate tracking - it is not the same as with GEM mounts where you increase precision of tracking - it is for accurate tracking with AltAz. Maybe diagram is going to explain it better - I'm going to exaggerate curvature of things so you can see more easily what is going on. Imagine scope can't precisely determine Az position - there is certain error. Scope is just a tad past meridian, but "thinks" it is pre meridian in position. Software will tell scope that Alt should increase a bit, but in reality needed Alt position will decrease a bit - tracking will create error in Alt because it is going "the wrong way", and guiding will combat this instead of only correcting things it should correct. Similar thing applies to guiding as well - it needs to know direction of vector - where Alt is pointing and where Az is pointing in order to give proper corrections - if orientation of scope's Alt/Az is different from that of guide system - wrong corrections will be given and guiding will not work to optimum - it is what happens in GEM mounts when you have wrong guider calibration. Difference between GEM guide system and Alt/Az system is that Gem guide system in principle needs calibration only once (although people do it every time when changing target / part of the sky - good practice because of cone errors and fact that RA/DEC axis are not always perfectly at 90 degrees to each other). With Alt/Az - you need constantly changing calibration, and software needs to track this and change things, but in order to do so accurately it must know exact pointing of the scope. This is why you need absolute encoders. Not sure what is needed precision of such encoders, but we might be able to calculate drift rates depending on guide command cycle length and position in the sky.

Well, at least we agree on minimum accuracy needed, that being 1/10 of arc second . But from what I can tell, you have very high gear ratio after that. I'm not sure if you were recommended 9000:1 because of motor torque - it needs a lot of reduction for small motor to move such a large scope. But let's run precision calculation again with this new info. So motor has about 10:1 gear reduction (and it better be smooth since encoder is pre reduction from what I understand in your quote). 500 x 4 x10 x 60 x 60 x 2.5 = 180000000 (and that is quite a lot) - that is 555.555 ticks per arc second (if I'm not mistaken). As far as precision goes - that is exceptional, but more precision you add, lower your slew rate. Do you have any idea of what the max RPM is on this motor? Found it - it says here: http://siderealtechnology.com/MotorSpecs.html "Maximum RPM with SiTech ServoII controller (24 volt Supply Before Reduction): 10,000" That is 166.66 revolutions per second. One revolution is 1.6666 arcminutes, so it's 277.777777 arcminutes per second, or 4.63 degrees per second. Ok, that is actually quite fine (if my calculation is correct). As far as I can tell, Alt drive calculations are quite ok. What I'm worried about next is smoothness of the drive components (any little bump is going to be amplified significantly at those reduction ratios), and lack of absolute encoders. We should discuss how much exact positional information impacts on tracking and guiding rates in alt az. This is also very important, as you can't expect 0.1-0.2" RMS tracking/guiding error if drive itself makes wrong move because it does not know exactly where it is pointing.

Could you go a bit more into detail regarding precision of motors. 9000:1 is total reduction of motor spin right? Let's say that you need to move thru 90 degrees in altitude. You also want at least 0.1" precision in altitude position (to be able to guide properly, and possibly you will want more precision). This gives you 100 revs per degree, or 1.6666 revs per arc minute, or 0.027777 motor revolutions per arc second, so we are looking at 0.002 revolution of a motor per "step". That is something like 0.72 degree precision on motor shaft, or 1/500 accuracy. I guess that should be doable with 10 bit encoder on shaft if we are talking about servo motors.

Having thought about this, I'm worried If you are going to go for something like 0.79"/px, and aim to properly sample at this resolution, you will be wanting something like 0.1-0.2" RMS error in tracking/guiding. I'm not so much worried about the drive (although that is also major concern), I'm worried about smoothness of Alt-Az mechanism. It needs to be extremely rigid, yet so smooth in motion that it does not have even slightest "stiction / jerk" anywhere along the arc of motion. All of that holding something like 200kg+. I believe this calls for exceptional machining precision and knowledge of materials. You would not want your mechanics seizing due to temperature change or becoming jerky or whatever, and there is also potential for forming a slack in hotter conditions - at this scales, with such large parts - it can easily happen. Things can even get out of shape if load is not spread evenly .... Another thing to consider is that you are going to need custom software written for this. No guiding software, as far as I'm aware, guides in alt-az mode. Software needs to know exact pointing of the scope to properly calculate needed shift depending on guide command. Same goes for tracking software - it needs to know precisely where the scope is pointing at any given time - that means either full precision absolute encoders on both axis ($$$) or some sort of split configuration with calibration - meaning lower resolution encoder on both axis and lower resolution encoders on drive shafts. With Ra/Dec system it is fairly easy to determine tracking rate and needed "resolution" of the motor (provided you are using steppers) to keep things within certain limits. For Alt-Az, this is not so easy calculation, as rotation rate will change with respect to where the scope is pointing. You will also be giving up best position in the sky for imaging - near zenith, as alt-az has trouble properly tracking in this region of the sky. Just some things to consider if high resolution imaging is one of your goals.

I'm sort of struggling to see justification in such a project on science side. I understand the appeal of such a large aperture for visual, so that is legitimate requirement, but on the science side, especially if we are looking at DSO imaging, I would look at other options. Don't know what is expected budget for this project, but such a large aperture is unlikely to give you significant advantage over smaller aperture in terms of resolution unless you construct very precise tracking platform. It is large telescope and it needs to be tracking very well to have edge over for example 16" RC on a good mount. There is of course huge light grasp of such aperture, but let's do a comparison against alternative that can be used for "quick" acquisition of DSO images - like small galaxies for photometric / astrometric measurements for example. You expect to be tracking for at least couple of minutes, so I presume that you will be doing multiple exposures and stacking anyway, and not working with fast transient phenomena that would benefit from short exposures and "concentrated" light gathering. Here is a quick calculation: 16" RC with x0.75 reducer / field flattener will provide you with very large corrected field at ~2400mm FL. It costs about 6K? Put that on Mesu 200 mount (another 5-6K), attach suitable camera / filterwheel whatever you like and repeat 4 times. That is about 50K of investment and you will have total of 800mm aperture, 2400mm FL and very multi hour tracking / guiding. I'm not sure if there will be any significant resolution advantage in 32" vs 16" aperture, given even very good seeing and tracking (there will be some, but not sure exactly how much, we just had a discussion on seeing impact vs aperture size in another thread, and concluded that that topic is out of reach for our level of understanding as is). Best you can hope to achieve in my estimate is about 0.75"/px practical sampling rate (FWHM of about 1.2"). If you can get acceptable tracking, field correction at F/3 with some CC in this custom solution for less money then yes, it's worth it. It's worth anyways if you like the challenge of custom making and see it as open source project to be repeated by others, and of course to be used as awesome visual scope. As for me, and my usage of such system, well I would use it the way I use smaller scope - to do what ever comes to mind, with addition of crazy imaging speed - or rather large SNR for given imaging time. I do have couple of ideas for some processing algorithms that require higher SNR than usually achievable, and would like to opportunity to test those, so such scope would be good for that purpose.

What sort of science are you looking at with this scope?

Well, I just did some calculations above. I agree that noise goes down as square root of subs stacked, but in general - it is not how low you get your read noise, it's how that read noise impact the rest - if it is not significant to start with (there are other noise sources high enough) then you won't make much difference, but if it is comparable to some other noise source - then you'll make a difference. Much larger difference is obtained by dithering then going with large number of subs, but still, I advocate going with large number of subs because you won't be loosing anything. No imaging time to be lost, you can do it during day time and over multiple days.

Here is simple reasoning behind number of darks. You can either do read noise alone if dark current is low, or you can do combined calculation. I'll do combined calculation in two extreme cases: a) perfect dither (no subs have aligned pixels) b) perfect opposite - every pixel is stacked against the same pixels (here pixel means X,Y position on sensor rather than on sky). Let's examine atik 460 as an example, read noise is 5e and dark current is 0.0004e/s/px at -10C. Let's do reasonable sub length - let's say 15 minutes. Dark current in this case will be 0.36e per exposure - which is low enough not to impact flat calibration much. Associated noise will be 0.6e. Again not as significant, but let's include it anyways. Total noise per sub will be - 5.034 (so really not much difference to read noise alone). But here is important thing. Stacking 20 of darks will lower this value by sqrt(20), so it will be 1.126e With each calibration you are "injecting" 1.126e of noise back into a sub. This means that in a) - each sub will not contain 5.034e of noise (read+dark) but rather 5.1584e of noise - slight increase but not too terrible. b) - you will end 1.126e noise to final stack instead (because this values will be constant on each pixel they won't be random and can't add as random, but rather are "pulled out in front of parenthesis"). Imagine you did 4h worth of imaging, that means 16 subs of 16 minutes, so your read+dark noise in final stack will be reduced from 5.034 down to 1.2585, but when we add 1.126e to that, we will end up with 1.6887e of noise. That is like we stacked a bit shy of 9 subs as far as read+dark noise is concerned. Let's be a bit more aggressive with number of darks and see what the difference is - let's take 100 instead. Now master dark will have 0.5034e of noise instead of 1.126e (less then half of original) in case of : a) single sub will have 5.059e instead of 5.034e - almost no increase this time b) 1.2585e of noise + 0.5034e of noise = 1.3554e of noise, or it's like we stacked 13.8 frames instead of 16 as far as read+dark noise is concerned. Much better. So yes, dither and use a lots of calibration subs. I use as much as 256 of each darks, flats and flat darks because I use shorter exposure time.

May I chip in? "Wrong" calibration can work in some instances, but not in all, and therefore I think that we should always advocate for proper calibration. I have nothing against using "wrong' calibration if that works for someone, provided that they understand why it is working for them. The minute it stops working for them (with any particular data) - if they don't know why, there will be trouble. Once one understands why it is working in certain circumstances, it is ok to use it if you accept approximation that you are doing. Using bias as dark in calibration can work if you have low dark current and exposure such that low dark current does not build up to certain level, and if dark current is uniform across the sensor. If it builds up too much - it will mess up your flat calibration. If it is not uniform, you will introduce a sort of "noise" that is predictable in nature (unwanted signal) that you could have removed with proper calibration. Yes, calibration increases random noise but there are ways to deal with this, and often benefits of removing predictable unwanted signal outweigh a bit more random noise. Take a lot of calibration frames, and dither and you will minimize impact of added noise in calibration step.

Worth checking out is "honeycomb" cast of blank - it can save quite a bit of weight while still maintaining figure. Here is image of it: It is pretty much same as regular blank with a difference that casting was done "over" honeycomb pattern so one side of blank is flat and used for figuring while other is sort of "hollow" with support structure that gives it enough rigidity to hold shape.

Yep, check your calculations. No way such a large mirror can be 25mm thick - most of that thickness is going to be ground away if mirror is to be fast, and it ought to be. F/3 800mm scope is going to have 2400mm FL - or be almost two and a half meters long. Anything slower than that is going to be as tall as a house Next thing to worry about is impact of mirror thickness on ability to maintain figure. When you have a chunk of glass that thin it is going to bend by its own weight and that will produce astigmatism in virtually any part of the sky apart when looking straight up at zenith. I know that newtonian seems like easiest thing to do, but you might consider two mirror system instead? Instruments of that size are probably best made in Nasmyth configuration. Here is schematic: Such telescope does not need central hole in the back and hence no hole in primary, but uses two curved mirrors and one flat. It can be mounted on dob type motorized mount - alt/az, with addition of camera rotation device if you need to counter field rotation (not needed for visual, but probably needed for imaging and science stuff). Flat mirror and "focuser" are mounted on alt rotation axis, so it is pretty much stationary. It is actually fixed in place if you have your scope on large platform where observer is rotated with the scope. In any case it is easily accessible - no need for ladders or anything.

I think that @JamesF gave that advice in light of long exposure AP, where you can cycle between filters for each sub (or couple of subs). With planetary imaging this is of course not feasible since exposures are really short, but you can actually use this approach - shoot 1 minute video in each filter then change and do next filter. When you finish all three (or four if you do LRGB, but not sure if it is worth it - maybe just do RGB to start with) then return to first filter and again do another set. This way you can choose best 6 minutes of video - two for each filter, depending on seeing as you can cover as much as half an hour or more by cycling thru the filters. I'm not sure if you will be able to use all recorded material due to motion blur even if you derotate your videos. Yes, each channel will have lower SNR compared to lum, but that is simply the way things are if you want to shoot in color - same thing happens if you use OSC sensor since only 1/4 of pixels pick up red and blue and 1/2 green color - again meaning less signal and lower SNR. This of course should not worry you much as there is no other way to do it that is reasonable (you could in theory use multiple scopes and shoot all filters simultaneously, but like I said, let's keep things reasonable ). Yes, wavelets work on both mono and color data. In fact wavelets work on mono data "exclusively" - but when you think about it R, G and B when you look at them individually are mono data - they become color only as consequence of "interpretation" by hardware capable of displaying color.