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To the first approximation, color calibration can be performed on any star on the image. First thing to avoid is - stretching individual channels (curves / levels on individual channels). You should really stretch luminance and leave color as is. Other thing is - perform color calibration while data is still linear. It really depends how complex procedure you want to follow. Photoshop might not be the best tool to do this with as you need something to calculate pixel values (or rather star intensity) correctly. Here is a handy tool that you'll need: http://www.brucelindbloom.com/index.html?ColorCalculator.html Set values that are marked with red arrows as they are in screen shot, then enter star temperature in CCT field and press it (marked with blue) and you'll get linear RGB triplet values in fields marked with green. You need to scale your R, G and B channels until you get that exact ratios for star intensity (measured by photometry or simply selecting star with circular mask, and doing stats on all three channels and comparing sum or average pixel value in each channel). Alternative is to use some software that attempts to do something similar for you automatically. Best and simplest choice would be to use Siril (as it is free) and does it on single click. https://siril.org/tutorials/tuto-scripts/ (checkout photometric color calibration section)

Yes, ground needs to be shared between boards - it is "reference" thing and all boards must have the same reference. Check out schematics you linked - there is black wire going from arduino to all other boards - joystick and both drivers. Not sure if I can find one. In any case, you define pins that you want to use for particular driver, so you have choice of pins on your board. All those pins that are marked with cyanish blue (digital) can be used to command your stepper driver. In program you'll define which port number will do what (stepper1 dir, stepper1 step, stepper2 dir, stepper2 step). So you can for example use, Pins 6 and 7 for one driver, and 8 and 9 for other

Just FYI, in above schematic - ground/GND also must be connected to Arduino, otherwise it might not work. May I ask, why don't you use something simpler like DRV8825 and say raspberry pi pico as controller? (that would be my choice of the top of the head for motorizing SkyTee2).

Does this help? As far as I can tell, you only need two pins per driver - that is direction pin and step pin (at least I think).

If I'm not mistaken, this combination should give x2.25: that is barlow element + "adapter A" - screwed into 1.25" filter thread. I don't know how big things are in above image - but ~75mm does sound sensible as barlow and 1.25" nosepiece are about as tall as zoom body and from this image: that seems about right +/- we don't know exact location of focal plane of the zoom EP.

Formula is simple one - but application of it often not - as there are so many factors that we don't really know. Magnification of the barlow is given by its focal length and distance to focal plane: Magnification = 1 + distance / focal_length (it is actually the same formula used for focal reducers except barlow is negative optical element and then it turns minus in above equation to a plus). Problem is - we don't really know focal length of the barlow element, and we don't really know where do we need to start measuring (as optical elements are inside housing and it's not always clear where the nodal point is - or center of the lens). It also not always clear where the focal plane will be located as that depends on eyepiece used. Eyepieces should place focal plane on "shoulder" level - but not all do. Otherwise all would be parfocal. In any case - increase in distance - higher magnification factor, and decrease in distance - lower magnification factor. If one finds approximate FL of barlow element and has approximate distance - then approximate calculation of increase in magnification is simple. Say that we have barlow that is x2 normally and we judge that there is about 50mm between barlow element and eyepiece adapter top (where we judge focal plane will be for most eyepieces). How much magnification would we gain by adding 20mm extension? Mag = 1 + distance / focal_length => 2 = 1 + 50mm / focal_length => focal_length is about 50mm If we increase distance by 20mm we will get 1 + 70 / 50 = 1 + 1.4 = x2.4 We go from x2 to x2.4 by adding 20mm extension (in this example case).

To be honest, I have no idea how much our acuity is reduced in mesopic / scotopic conditions, and since there is no real way of determining "percentage" of photopic / scotopic attributions to mesopic vision. Even for scotopic vision - just vague accounts on lowering ones visual acuity are given - like this excerpt from wiki on scotopic vision: and on the end of the page: and It looks like our brain and construction of our eye perform sort of "binning" of signal in order to boost sensitivity in low light conditions thus reducing how much we can resolve. As for neighbors - here is a "trick" that you can perform. Record sounds of someone working with tools - like hammer and saw and play that with the lights on, giving your neighbors explanation that since moon is very bright there is not much you can do except do some upkeep on observatory

You know - this thing: often seen used by professional photographers on photo shoots. "What is he on about now..." I hear you say Well, not long ago, I tried to explain on my local forum that moon is not very bright in telescope and that there is no need for ND filters. In fact - moon is not brighter in telescope than naked eye (this is true for everything except point sources like stars), it can only be dimmer (surface brightness) as we bump up magnification and exit pupil goes down from 7mm to smaller size. But why do people get "blinded" by low power views of the moon? For same reason people get blinded by car headlights at night. Go outside during the day and look at those same headlights - you'll feel no discomfort of any kind, no blinding at all. In fact - they will look rather dim in a sunny day. Similarly, if we observe moon in daylight, close to sunset - there is no chance you'll feel any discomfort even at low power, yet moon is shining with same amount of light as always. Then I ran across a post by @Louis D - showing difference between achromatic refractor and reflector on planets. I felt that what has been shown is somewhat unjust to achromatic refractor. There was too much purple halo around the Jupiter. I knew that camera sensors are more sensitive in short wavelengths than human eye and this is one of the reasons. While visually we can hardly see purple halo around stars in fast ED doublets (unless they are very bright) - in images, those halos are clearly visible. This is of course due to different sensitivity. I just did not know what is the actual difference in sensitivity between the two, so I went online to look it up. What I've found is very interesting: Sensitivity to shorter wavelengths (below say 480nm or there about) dramatically changes with vision mode. I mentioned above issue with lunar observing - because it also heavily depends on Photopic vs Scotopic vision. In Scotopic vision we are far more sensitive to light levels and are easily blinded because different receptor cells are active. Not only that - but there is added sensitivity at lower wavelengths and also - there is issue of visual acuity - that is reduced in Scotopic vision. I also found out that there is "middle ground" - Mesopic vision - where there is transition between Photopic and Scotopic - and this transition is gradual and effects above are "interpolated" between the two: https://en.wikipedia.org/wiki/Mesopic_vision This means that we start to adapt to dark already at street lighting condition levels - and certainly get Mesopic vision in our back yard at night time. It will impact things like: - color perception - sensitivity to light levels - sharpness of our vision - sensitivity to chromatic aberration I'm saying all of this because conclusion is very obvious - if you want to get good planetary and lunar views - don't get into Mesopic and Scotopic vision regimes. Stay in Photopic. Observe with bright light around you. Wiki states that Photopic vision is above ~10 cm/m2, Mesopic vision is between 0.01 to 3 cd/m2 and Scotopic occurs below 10−6 cd/m2 I have no idea how to judge certain conditions to figure out cd/m2 - but apparently there is device that will do that for us - hence the title of this post. If you have one of these - measure light levels on yourself and if EV number is 6 or greater - you are good to go.

Don't know. I have one with collimation screws. I specifically wanted that one so I purchased it as OTA while getting AZ-GTI at the same time (so it would have made sense to purchase bundle - but I did not want to not to be able to collimate scope in case of need).

Note absence of collimation screws in bundled version: also front corrector holder is differently shaped This is not exclusive to skymax - newtonians bundled with new line of mounts - like 130mm model, come without collimation screws as well. (notice absence of collimation screws at the back).

Just do search for plastic (polyamide or bakelite) "U handle" or "Pull handle" on the net and you'll get variety of sources for things that you can use as telescope handle. For example: https://www.amazon.com/MroMax-Plastic-Household-Cabinet-Cupboard/dp/B081GVS1WR/ref=sr_1_37?crid=U1BIV702Q0KC&keywords=u+handles+polyamide&qid=1668718260&sprefix=u+handles+polyami%2Caps%2C333&sr=8-37

Needed to look up what the pot metal actually is - but yes, it makes perfect sense.

Under magnifying glass - it certainly looks like some sort of sand/aluminum mix with actual aluminum just being very thin coat on the outside - you can see that bit in the image here: under hand it feels like rough stone rather than anything metallic

I just remembered that I still have piece of that tube ring left, so we can inspect it and if someone could guess the material @900SL ? It certainly looks more like some sort of rock then metal

I would not mind nicely designed and machined aluminum mount with same / similar performance, instead of cast "something" housing.

Indeed. I once had SW tube ring crack. In fact - it saved the scope. I dropped the scope by accident when fiddling with the mount as one does, and I quickly put my foot under it to soften the fall (not sure where I've got that reflex - but its not very good one. I wonder if brain can autonomously judge the weight of falling stuff and decide not to try to soften the fall of say 20kg rock :D). In any case, scope did hit the floor and my foot simultaneously and it actually hit tube ring first where it hit floor tiles - which as enough to split that ring into several parts. They call it aluminum ring (or maybe they don't and it was just my expectation for it to be aluminum) - but it more looked like slag that was removed from melting aluminum then metal itself. It certainly looked like some industrial process byproduct rather than pure metal (and pure metal would not crack like that).

Actually, I have my eye on desktop CNC router. Properly built one will cut aluminum with some precision Then there is always need for small precision lathe as well - all those T2 / M48 adapters ....

Indeed, not long a go, member of local astronomy forum asked me if I would print him some EOS clip in filter holders for 1.25" filters. He purchased this one, but was not happy: https://www.teleskop-express.de/shop/product_info.php/info/p14030_Wega-Clip-Filter-for-1-25--Filters-on-Canon-EOS-APS-C-Cameras.html That is total of 70e per filter holder after shipping, customs fee, and tax I printed him 4 of those and total cost was less than 2 euros in plastic and electricity

Priceless!

Sure, but only for those that love DIY. It really is not as expensive as people might think. I started with about 250e, and I did put in some more cash towards improvements - but for most things, stock machine would have been enough. I remember times when regular 2d laser printer was more money.

No You can go about this in several different ways. Average will be comparable only on single target and only if you don't shift a lot with your FOV (regular dither is ok). If you want to measure background - probably better way would be to use median. Alternative would be to take stack with removed background (just make sure you use sensible stacking algorithm that won't do weird stuff like scale output in 0-1 range, but leave ADUs as they are) and then subtract from each sub and then measure average. For this to work - you need to have your subs aligned and cropped to have some data in all subs (trim edges that would otherwise cause stacking artifacts). In any case ImageJ should be your go to tool

ImageJ can do all sorts of nice features. If you want to measure stack in one go - just select a region in stack (stack being sequence of images in ImageJ terminology) and perform command. This puts results in table that you can export or copy/paste in spreadsheet for further use.

What would be ADU value for each image?

Maybe this would be solution to your needs then? https://www.teleskop-express.de/shop/product_info.php/info/p13750_TS-Optics-50-mm-f-4-ED-travel-refractor--spotting-scope-and-guiding-scope-with-Crayford-focuser---perfect-optics.html It is a bit expensive for my taste, but it does all you want it to do.

By the way - if you look at above numbers - you'll see where guide at 1/2 of imaging resolution rule of the thumb comes from. You can achieve 1.5"/px with 0.8" RMS guiding You can achieve 1.15"/px with 0.4" RMS guiding In general range of sampling rates and usual conditions (here I mean 1"/px to 3"/px and 1.5" to 2.5" FWHM seeing) - it turns out that guide RMS is in ball park of half of resolution, but lower RMS is always better.