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It looks like I'll have to design reduction as standard. I'm trying to assert if 3d printed rack and pinion system is feasible. Problem is precision and I can't print gears of very small diameter. From what I've gathered online - lowest that I can go is module of ~1. For pinion gear if I say use 9 teeth, that is going to be 9 * 1 * pi = ~28.3mm per turn (sort of inline with that SW focuser). However, I'd like to use that on slightly faster scope 70/500 comes to mind as potential next DIY scope, so I'll probably add 3:1 reduction by default so I get 10mm per turn - that is right between coarse and fine focus, and might work good?

I've found that Baader Contrast Booster works best for achromats. It is "cross" between Neodymium and Semi APO, and reduces CA nicely while boosting contrast. https://www.firstlightoptics.com/baader-filters/baader-contrast-booster-filter.html Here is interesting read for those Baader filters: https://www.firstlightoptics.com/reviews/baader-contrast-filters_US-ATT_review_0414.pdf I second use of Wratten #8 to reduce chromatic aberration - however, it does impart distinct yellow cast on the image.

I'm using same equations for critical focus zone, but I'm not really sure how applicable that is for visual?

FLO lists it being in stock in two weeks: https://www.firstlightoptics.com/baader-filters/baader-neodymium-filter.html

How much would you say is optimum travel of focuser draw tube per knob rotation? Recently, I played with non rotating helical focuser that I printed. I used 1mm pitch thread for focuser motion, and it has travel of 18mm - that translates into 18 full rotations to move it between two extreme positions. On the other hand - I measured stock Skywatcher 2" refractor focuser (single speed) - and it moves about 30mm per revolution. Interestingly, that skywatcher focuser is from F/10 refractor. Critical focus zone for such scope is 1/4 of mm (249um), That would make something like 3 degrees of a knob turn. Way too small? I'd say that critical focus zone should be maybe 1/16th of a turn for easy focusing. That would make 1mm / turn for F/5 scope, ~1.45mm / turn for F/6 scope, ~2.56mm / turn for F/8 scope and ~4mm / turn for F/10 scope. Would that be too fine for a single speed focuser?

Opinions are not relevant. Facts are on the other hand straight forward, no ambiguity there.

Aperture is the key at any size. If you fix arc second per pixel (sampling rate) - aperture dictates speed

Problem is that "photographic" knowledge is limited in its context. Most who have photographic background will assert that say F/5 telescope is faster than F/10 telescope, but in reality: F/5 can be faster than F/10 F/5 can be equally fast to F/10 F/10 can be faster than F/5 There is important part of equation missing when you only mention F/ratio. This is deliberately done in daytime photography, because most talk about F/ratio of objective lens assumes you'll be using it on the same or similar camera. In astrophotography - we have a luxury of large range of different sensors and thus pixel sizes, as well as binning - which can make all the difference.

Only if you want to observe very faint objects. For planetary viewing you actually want to avoid getting dark adapted. Dark adaptation reduces sharpness and color perception. Have a look here for more detail:

Because important step of RGB ratios is not performed Any function that looks like this: or to put it into words - operates on domain [0-1] and transforms it in [0-1] range and is monotonically increasing, will give you results that you need. It does not have to be arcsinh - it can be any other function. It can be "free formed" - that is what curves do, and no, you don't have to do multiple rounds - you can only do a single round to reproduce exact result, or you can do analytical function. What is important is to calculate "ratio" or coefficient c for each pixel that is in form - transformed_lum / lum, or to put in words - "how much we amplified lum for that pixel", and then we use that amplify each component of RGB. As a final step - I would advocate exact same thing be done not in RGB space, but instead in XYZ color space, and then result transformed into suitable working color space (usually sRGB, but could be wide gamut like Adobe RGB or other if need be).

Why is this "method" special? It is very clear that it is simply "repacked" simple RGB ratio method. If R' = (R/L) * f(L) (I'm omitting that parameter Z as it is really unimportant for the argument) that can be rearranged as R' = R * ( f(L) /L ) Similarly if we do the same for other two components we get: G' = G * ( f(L) / L ) B' = B * ( f(L) / L ) f(L) / L is simply some g(L), and can be considered constant on pixel level (single luminance value per pixel) We are simply multiplying every component with a constant (on pixel level) which preserves R:G:B ratio of that pixel (this is desirable quality). Choice of f and consequently g is irrelevant, and we can use power law (gamma) or any other histogram transformation on luminance we desire and still preserve color in the same manner.

I was hoping for some real time shots of star testing done with planetary / guide camera? Maybe that would be worth adding. Important part of star testing is knowing how to distinguish seeing and tube currents from the rest. Video of actual defocused star in different conditions might help there. You should also make slightly more distinction between pinched optics and astigmatism. Astigmatism can be inherent in the optics or caused by improperly supported mirror. That kind can't be fixed with adjustment of the clips. Similarly - pinched optics is more likely to show up as triangle in case of three clips used to hold mirror. Astigmatism due to sagging mirror will be worst when telescope is pointed horizontally (near horizon) and usually happens with larger / heavier mirrors

It is, but it also depends on rest of geography. Sometimes being near the sea can be really beneficial for seeing, rather than cause poor seeing. Some of the best observatories are situated at vulcanic islands. In these cases large area of ocean makes air flow rather laminar in nature (smooth) - which gives very nice seeing. Shallow see next to large land masses on the other hand is acting like heat source. It causes air to swirl. Especially if there is ocean current passing near by. That causes poorer than otherwise seeing.

@kingsbishop I've read your question in latest closed thread, so I'll give you brief answer on that one. https://en.wikipedia.org/wiki/Angular_diameter There is handy table containing some angular diameters that might be of interest - like Uranus, Neptune, some minor planets and moons of larger planets. In any case, objects that you are talking about - are about 4" or less in diameter (in fact - most are less than 1" in diameter). Largest amateur telescopes are up to 20 inch or so in diameter, while most are 8" and below. 20" telescope will have 0.51" Airy disk diameter, while 8" telescope has 1.28" Airy disk diameter. Even if you could mount visual adaptive optics system on amateur type telescope - limiting factor would not be the atmosphere but rather resolving power of the aperture. In order to fully start to exploit adaptive optics with its small isoplanatic angle - you need to have aperture large enough to be able to resolve detail over such a small angle. We are talking here about diameters expressed in meters and not inches. As far as MATX "adaptive optics kit" - I don't have anything special to say except that it looks like a scam from very sketchy information that I've been able to find. I've just seen one youtube video where two blokes - don't even give their full names although they do say that they work at University of Riga in Latvia (so we can't fact check that) - talk about their "product" - next to what they refer to 12" scope, although it is clearly 8" skywatcher F/5 newtonian with some gadgets and ZWO camera mounted on it. Yep, that is "12" newtonian - see 0:25 of this video: https://www.youtube.com/watch?v=hAfi1yT-gSw (note call to investors at the end the video)

Observe next to a bright light - like under street lamp or on your balcony with the lights turned on.

I started fiddling around with building 3d printed scopes, or rather using 3d printing to assemble telescopes. One of the biggest hurdles that I foresee for time being is - focuser. Commercial units are either very poor performing and cheap, or too expensive for cheap DIY scopes. I made little 3d printed non rotating helical focuser, and to be honest, I'm rather impressed that it works at all, but one of its major drawbacks is printed draw tube. Draw tube is printed in "standing" orientation due to threading involved - which puts layer lines on sliding surface. That obviously makes sliding motion seriously rough. All that made me thing think - what about using linear rods or linear rails as sliding support for DIY focuser? Linear rods can be sourced very cheaply and PTFE bushings + PTFE dry / spray on lubricant for rod itself - should create very smooth motion of the draw tube. What are your thoughts and does anyone have experience with building such focusers?

Larger aperture does collect more light - but longer focal length spreads that light over larger area. If you don't do anything to offset this (like change pixel size by using different camera or binning pixels) - then 90mm at F/4.8 will be faster than 102 at F/5.6 but it will also have different sampling rate - namely 1.8"/px vs 1.35"/px At those aperture sizes (around 4") - I would prefer closer to 2"/px than to 1"/px - so 90mm seems like logical choice both in terms of speed and in terms of sampling rate.

First recommendation for planetary observation is not to get dark adapted. As soon as you get dark adapted - changes happen to your vision and what is very faint starts to look very bright (think car headlights during night and during the day). For filter performance and usability - check this: https://www.firstlightoptics.com/flo-guides-colour-filters-to-improve-lunar-and-planetary-visual-observing.html

Thanks. I have no idea when the obsy will be finished. Probably during the next year. At the moment, I'm waiting for the guy that did my roll off roof to find time to come back and fix what the messed up It does not roll all the way back, so he needs to extend guide rails about 1m or so more in order for it to move all the way back. After that - basic construction, roof and steel pier will be done (had to switch to steel after workers botched concrete pier). Oh, yes, I also need to add quartz sand to pier to dampen that ringing, but that is easily done. Then, I'll need to do electrical installation, door and window and some inside finishing (it is in raw state at the moment). I won't be doing much now as it is wintertime and I'll leave it all for next spring / summer.

I think that AZ pronto will be more usable of the two. AZ3 is really not very good mount. It is much more suited for terrestrial observing than for astronomy - as it's having tough time when the scope is pointing high up (OTA hitting tripod legs and in general poor motion). Although it has slow motion controls - those are not fully usable. AZ motion is only limited to small section of "left/right" motion and needs to be "rewound" every so often (returned to center position). Focal length will affect chromatic aberration, FOV but also general sharpness of the scope due to something called spherochromatism (also aberration and related to wavelengths of light - but different from simple chromatic aberration) which affects overall sharpness of the image. Chromatic aberration is when not all wavelengths come to same focus - so when you bring image of star or planet to a focus - there some light (most notably purple being on far end of spectrum) - that is defocused and presents itself as "halo" around the object. Spherochromatism on the other hand is spherical aberration that depends on wavelength. Even if color/wavelength is brought to focus - if it suffers from spherochromatism - it will be blurry and not sharp (like regular spherical aberration). For this reason shorter (faster) achromat scopes are not suited for high power viewing. Having said that 90/660 is right there on the edge of not being overly fast and will probably show very decent high power images if stopped down to say 70-80mm (you can use aperture mask to stop down fast achromat and make it slower - which reduces chromatic aberration and sharpens up the view).

I present to you my first 3d printed scope named Quick LUNAtic mk1 (that is DeeDee howling at the little scope) It features "massive" 38mm multi uncoated objective lens that came off old Russian binoculars. More astro dogs and unfinished obsy in the background. Long stroke (18mm), ultra rough non rotating helical focuser. What it lacks in smoothness - it more than makes up for in backlash (about 1/6th of a turn). 7.5-22.5 premium zoom (branded AngelEyes) sports less than 30 degrees of AFOV at far end of focal lengths and fabulous internal reflections that contribute to "sparkly" view 1.25" eyepiece adapter comes with two locking screws (I could not come up with anything witty for this part as it is actually quite usable as is - it even has T2 thread printed) Everything is 3d printed except lens with its cell and eyepiece (even eyepiece cap is 3d printed as it lacked caps on both ends when I got it second hand).

Sure. These two attempt to tackle two very different things. Astronomy.tools formula attempts (but fails) to address sampling rate of long exposure astro photography. One that @ONIKKINEN gave is related to critical sampling of planetary imaging. Two differ by how they threat atmospheric and mount influence. With planetary imaging all we are concerned is to capture all the detail that aperture can provide. We don't care about mount tracking / guiding performance nor do we care about atmospheric seeing effects. These are dealt with in different way (by using very short exposures to "freeze" the seeing instead of letting atmosphere create additional level of blur due to its motion - kind of motion blur on top of distortion, as well as selecting the least affected frames - often only few best percent out of tens of thousands of frames and also by means of stacking - that differs from long exposure stacking in how it is performed). With planetary - it is size of aperture that dictates level of detail, and since arc seconds per pixel depends on pixel size and focal length - it turns out that for certain pixel size there is optimum F/ratio (combination of aperture size and focal length) that will let you capture all the detail. This is based on Nyquist criteria and also on cut off frequency due to limited aperture size. Exact formula for that is: F_ratio = pixel size * 2 / wavelength_of_light (where pixel size and wavelength of light are in same units of length - either micrometers or nanometers, and 2 is from Nyquist). If you put in 500nm as being relevant wavelength for visual (400-700nm) - you get F/ratio = pixel_size * 2 / 0.5um = pixel_size * 4 There ya go - as far as planetary is concerned. Back to long exposure imaging. I'll briefly go over what is involved, and why Astronomy.tools calculation is wrong. In long exposure imaging there are several factors that contribute to total FWHM of the stars in the image - that is seeing FWHM, guiding RMS (mount performance) and Airy disk diameter (or telescope spot diagram RMS if telescope is not diffraction limited - surprisingly enough most astrographs are not as they trade field flatness and corrected imaging circle over central field sharpness). Sampling rate is related to FWHM by factor of 1.6 - so sampling rate should be FWHM/1.6 If you are able to achieve 3.2" FWHM stars in your subs - correct sampling rate is 2"/px You can measure your subs for average FWHM to see if you are sampling properly, or you can try to estimate it (but that is more involved) from seeing FWHM, guiding RMS and airy disk size (or spot diagram RMS). First you need to "convert" all to same thing and there is relationship of FWHM and RMS that goes like FWHM = 2.355 * RMS (everything should be in arc seconds) so your final FWHM = sqrt( seeing_FWHM^2 + guiding_FWHM^2 + airy_disk_or_spot_diagram_FWHM^2) This shows how all contribute in non trivial way. Factor of x1.6 is also related to Nyquist - but FWHM does not directly relate to frequency so it must be calculated from Fourier transform of gaussian approximation .... (complex stuff).

I'm not sure you do fully understand. RA and DEC coordinates are part of coordinate system. In our 3d space (which is orientable, and not all spaces are) - there is notion of handedness - coordinate system can be left or right handed. RA and DEC system is right handed coordinate system. Maybe best explanation is this - if you look at an image of a hand - you can always know if it is someone's left or right hand. hand in the image is left. No matter from what angle and in which orientation you image it - everyone will be able to tell that it is left hand. Only operation that "messes up" this information is mirroring the image (either by use of real mirror - or flipping on axis).

No, seriously, they are all good in most important aspects for planetary. Here, have a look (I'll use ASI examples as they are easy to find, but other brands should have same/similar specs): Read noise: 224 - 0.8e 585 - 0.8e 678 - 0.6e 662 - 0.8e Peak QE: 224: 75-80% 585: 91% 678: 83% 662: 91% All provide very fast readout times. It is just the matter of selecting pixel size (although - you will use barlow to get wanted F/ratio anyway, so maybe only advantage is if you can have setup without a barlow, but good barlow will be virtually invisible in optical path as far as quality goes), and selecting sensor size that will suit you - larger for lunar and solar, otherwise, size does not matter much for pure planetary (but might for other uses like EEA and such).

Yes, but there is something called Chirality, and one should not invert it. It is "handedness" - pretty much like the fact that we have right handed screws. If you turn that screw clockwise - it will move "in". No amount of rotation / orientation and direction of observing can switch that screw to be left handed. For that you need to use mirror. Regular objective (like refractor or primary of reflector) - swaps up / down and left / right - effectively rotating image by 180 degrees. Add one diagonal mirror to the mix - and you now have changed Chirality. That mirror can be secondary of newtonian or diagonal mirror for refractors and MCTs/SCTs for visual. That could be thought of as changing the nature of object - as no "normal" way of observing will produce such image - only use of a mirror will.