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For some reason, "at" mention did not trigger notification - and I was going to suggest the same method I already wrote about. Something similar is done when collimating the RC scope primary mirror - one is looking to get same FWHM in each corner. It is fairly easy to do - just fire up SharpCap - and check corners to see what sort of FWHM you will get. CCD inspector really does the same thing - except it relies on star field to provide stars all over the sensor - and then it measures FWHM of them and uses their average. You don't have bunch of stars - but you do have one that you can "move" around the sensor by simply slewing the mount in needed direction.

So here are "pros" and "cons" of each. Zero gain does indeed increase FWC and some might think that is important to capture star color of bright stars. I'd personally use different approach because no FWC is big enough to capture all the bright stars. There are always some targets that will have brighter star still that will clip. Best way to deal with above is to use few short "filler" exposures - it is much easier to capture very bright parts with shorter exposure then to rely on large full well capacity. FWC is important in daytime photography - where we take only a single exposure, but in AP - we stack our exposures and we can use all sorts of tricks in stacking to compensate and circumvent limitations of FWC. On the other hand - when using zero gain - one is likely to destroy faintest signal in the image. Any gain below unity will do this to some extent. I'll give you an example when using e/ADU value of 2 (so lower gain than unity). Say you capture 11 electrons and convert that to ADU units. You will (or rather camera will) do that by dividing the two 11e / 2 e/adu = 5.5 ADU - but we always record ADU as whole numbers so we must round it somehow, and it is usually rounded down - so we end up with 5ADU If we try to restore original signal - we then should multiply with gain or rather e/ADU value so we have 5 ADU * 2 e /ADU = 10e We were not able to fully restore our original number of captured electrons. This always happens when we digitize our measurements if we use whole numbers - but there is a difference. For gain values other than 1, but e/ADU lower than 1 - we get this "step" error - that is sort of random. It is not truly random but in presence of read noise it almost becomes random. Same thing happens with e/ADU larger than 1 - but here error is both more pronounced and also harder to shape - it impacts more. There is reason why most cameras have distinct relation ship between read noise and gain - read noise is always greater at lower gain settings and designers of sensors - don't bother to minimize / correct that - because it suits them. You need larger read noise to "mask" this quantization error that is larger on low gain settings. In general use in most cases - you won't see much difference, but if you gather large number of subs and you hunt for faintest signal - my recommendation is to go with unit gain and avoid most of issues with quantization error.

Gain is just numeric conversion factor - it won't magically increase number of captured photons per pixel. Same signal will be captured when using gain 0 or unity gain or high gain. There are however some other differences between these gain settings that will affect your final result: - full well depth. As long as you don't saturate some parts of your target (unlikely with such short exposures and targets of "normal" brightness - i.e. not M42 or core of M31) - this parameter is irrelevant as stacking increases effective full well depth. - read noise. As long as you expose long enough that you swamp read noise with some other noise source - usually light pollution noise, but can also be thermal noise if camera is not cooled (both grow with exposure length) - it does not matter how big it is. It will have very minor impact on total result if you keep above rule. - there is additional error introduced due to quantization - which is masked by read noise. I would personally use unity gain, but feel free to use any gain setting you wish, and no - single exposure at unity gain and 0 gain will show same level of signal when stretched properly - you won't be left with "blank screen" after exposure at low gain settings.

This is quite true - similar effect happens when tracking is affected, but it affects all wavelengths of light equally - so there is no separation of wavelengths like in above image where red is concentrated and tight and blue/green is bloated to one side.

Don't waste time - this is not sensor or focuser tilt. Sensor or focuser tilt shows distinct pattern: - there will be spot or rather "band" in the image where stars will be tight - usually corners suffer and besides stars being slightly out of shape in some corners - they are also slightly out of focus. In your data - you have teardrop shaped stars all over the frame equally. Effect it the same as atmospheric dispersion - which separates wavelengths of light in one direction. If you want to understand how this effect might be formed in objective lens of a telescope - check out atmospheric dispersion corrector - which has two prisms and changes their relative position to cancel the effect of atmosphere. You have two lens in your objective (that on small section act as prisms) and if those two are similarly displaced (or one is displaced with respect to the other) - you will get the same effect. You have 3 modes of displacement. Tilt of one of lenses with respect to the other. Change of spacing between the two. Axis displacement (one lens being shifted "up"/"down" so that two axis are still parallel but don't align any more). Out of these three - first and third can produce above effect. Second one usually produces increased spherical aberration (just bloated stars but still round). Why don't you do a simple star test - take image of slightly in / out focus star - to see what sort of pattern it presents - and then we can know more.

This is very busy star field and when stars are bloated like that - you can't really do much about it. Also, don't attempt to stretch data further than it can handle - there will be too much noise. Was the target too low in the sky? If not, we can rule out atmospheric dispersion, but then you should look into collimation of the scope. Star bloat is easily resolved by using special luminance filter - like Astronomik L3 - that cuts a bit off ends of spectrum to prevent that bloating, but stars should be concentric for that. If it is optics - it is either collimation of decentering of lens. I'm not entirely sure what is the cause, but tilt is not - it's something to do with lens itself.

Data does not support what you are trying to do. Stars are too bloated for image to be used at full resolution. Besides that - it looks like there is some issue with optics or maybe atmospheric dispersion was too high - but channels don't seem to align properly. Stars are not round but rather "tear shaped". This is normal for ED doublet (I'm assuming image was taken with 72ED) - these scopes don't have the best correction and there will be some bloating in blue part of spectrum. However - stars should be concentric - meaning bloat should be around the core and not to one side. Given that stars are as they are - when you remove them - you'll get very poor background as software tries to guess what is behind each of those bloated circles. I'll try to process the data - the way I would process it, so you can see what can be accomplished (by me, I'm certain that others will have different approach and outcome).

Problem with this approach is that I'll have 3D printed pulleys to tension next - and those are not very stiff, or rather - they should be supported on both sides of their rotational axis before attempting to put tension on the belt - otherwise axis will tilt. I'll probably solve it by building "cages" that will hold pulleys and then those will move in slots in the base.

That is very similar to idler - or in fact - it is an idler with screw to pull it back. Two more bearings and in principle, like Louise said - no need for constant tension adjustment, nor for very big pull on belt. Here, belts are rather small compared to those on 3D printer. Standard way is to have grooves instead of fastening holes for motor and other things and you manually pull them back until there is sufficient tension on belt so that it engages teeth on pulley properly and that is about it. Here is solution that I employed in above example: Motor is fastened thru those slots - but only barely until everything is assembled - then motor is pulled back for tension to form on belt and then bolted down properly. In fact - I might even put a bit more tension than needed in this proof of concept - top and bottom plate do have some flex in them and it seems that whole assembly is ever so slightly bent because over tensioning the belt.

Belts arrived, and I made first test of printed pulleys and how they work with belts: I'm happy with results so far. Idea is to have 3 stages. Two stages will be 5:1 reduction (80 teeth to 16 teeth) and last stage will be 10:1 (160:16). That will give total of 250:1 or 0.81"/step. 3 belts are required - 2x200mm and 1x360mm - those are supposed to be standard sizes. Whole thing sounds like it's playing space invaders - very distinct "8bit" squeal is heard from the motor, but I guess that is from 18 steps per second or step about each 50ms. One can almost hear pitch changing like sine wave as motor is doing micro stepping. Major hurdle now is to design easy to assemble and rigid box with ability to adjust tension on belts. I won't be using idles idlers unless I absolutely must - as those require additional bearings.

That is interesting idea, and I've seen some people do it. I'm not sure I'd do that for my work though, for two reasons: 1. I think it is fairly easy to remove in dedicated software (not that I know how to do it at the moment - but I've seen software that can do it) 2. With my work, I intend to publish "source" - or original work, so that people can modify it to suit their printers. In order for some things to work properly - certain tolerances / clearances must be met. Not every 3d printer is tuned the same, and sometimes people might need to tweak some hole size or part size to fit properly. I'd rather have them do it easily then think about copyrighting the work. Publishing under certain license is just about right amount of work that one should do. If someone is set to steal / reverse engineer or exploit - there is not much we can do about it (maybe press charges and so on - but that would make sense only if large damages result from such actions).

That will lead to some sharp statements being said ...

I just asked, because I printed some stuff for our local astronomy forum members and I really did not pay attention to the license agreement. At first - I did not charge anything for that, but since they insisted on paying at least something - I made quick calculation of material and time cost and that's what I charged on several occasions. Now that I think of it - most payed stuff was actually drawn by me in FreeCad because there were some issues with original parts

Just out of interest, If there is an item that is specified as non-commercial in license agreement, and someone prints that for someone else and charges for - material and time expenses - is that violation of license agreement? It's not being printed for profit - but rather as a service that is payed for - customer specifies what is to be printed.

This one as well: https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-200p-dobsonian.html Just be careful. While both 6" F/8 dobsonian and 8" F/6 dobsonian are excellent all around instruments that will show you plenty - they are large telescopes. Best to see a few videos on youtube of said scopes to get the idea of how large those instruments are. Check out the specs as well - for weight. They need to be transported - and for example 8" dob can't be transported longer distances in single go. One can wrestle it maybe 15-20 meters in one piece - but if you need to carry it further than that - you'll need to take it apart and carry each piece separately (easily done though - both disassembly and assembly). In this video for example - you can see it split and carried as well as size of 8" version: https://www.youtube.com/watch?v=oj9TnVu_vAA

@Mr H in Yorkshire and @markse68 I think that you are right. I've spent some time visualizing what is going on ("riding the gear" in order to change perspective) - and yep, you are right - it won't deal with backlash. Here is how we can see that. If we have situation that two motors are rotating in opposite direction and output shaft is standing still - that can happen even if there is zero engagement between planet and annulus. We can imagine that backlash is equally spread in forward and backward direction. look at above gif - this animation shows situation when the mount is standing still (we don't have planetary carrier show in the image - but it is output shaft). Now look at annulus gear and any of planets. Planets are driven by sun and annulus is driven by other motor (or gear assembly - irrelevant - we can just use motor with different speed setting) - so each is driven independently - they don't even need to be in contact and can still have same tangential velocity. In any case - backlash can be "split" between forward and backward direction like this: whatever we do next slow or speed up outer gear or slow down or speed up planets (via sun) - it will need to clear some of that backlash - either side of it. Ok, so we came to conclusion that this drive is useless for mounts - it won't deal with backlash and it wont increase stepper resolution

Good idea - I can actually use larger pulley on RA shaft for additional boost in resolution and use smaller ones in between - two of them in total. People can then opt to purchase smaller pulley sets or print them (but they will need to print large RA pulley anyway). I still need to make room for electronics - or that might go into separate box - since all we need at tracker is just power to stepper motor and that is just a single cord with 4 wires. I think that AZ-EQ5 has something similar going to DEC motor: Yes, just making mechanical side of things without motor assemblies will be a bit of a challenge. It will require some additional DIY skills. I think that most will be done with aluminum tubing which will need to be cut to size and some small holes drilled in. Then there is matter of counterweight shaft and counterweights. I know how to make that - in theory, but never done in practice. Say brass or even steel rod is taken and then on one side is drilled with drill, tapped and piece of threaded rod is screwed / glued in. Similarly on other side, short nut is screwed in similar hole to act as stop. Dumbbell weights can be used for counter weights - but those also need drilling and tapping and possibly 3d printing some sort of centering insert as most have 30mm bore diameter and we'll be using something like 20mm shaft. That sort of project is really on next level, and I do hope to do it one day - but it will present whole set of new challenges.

You might be right - I'm having tough time visualizing it. I don't think it is as simple as regular gears - it probably depends on relative motion of components prior to change and the way you change them in order to achieve wanted result. Here is an example Say that two gears - annulus and planetary have same speed and that mount is still - it does not move. You can initiate mount movement in two different ways - you can either increase sun (and hence planet) speed - or you can decrease annulus speed. Difference will change in the same way and mount will move the same. Question is - will backlash work the same in both cases?

I don't think there will be backlash. I'll try to make a diagram to explain things. this system depends on planetary gears - it can't work with ordinary gears. In planetary gear system - there is sun gear (central small one), planet gears (3 same gears) and outer gear. We usually use sun as input and planets as outputs (their centers are "rigid" and rotate so when connected they form output "body" that we attach shaft to). During this time outer gear / annulus gear is stationary. What this system does is to rotate that annulus gear instead of keeping it stationary. We have 3 choices: 1. Move it in same direction as rotation of planets - or up arrow 2. leave it stationary 3. move it in opposite direction What happens here is a bit like using escalator or those person conveyor belts / moving sidewalks - we will walk faster if "ground" is moving in the same direction - then we move at speed that is our speed + "ground", but if we walk in opposite direction and we match the speed of moving sidewalk - to someone looking from a side we would appear not to move. Ok, so as we now have analogy with person and a sidewalk, let's introduce backlash analogy. Effects of backlash happen when we walk and we want to change direction. At some point we need to stop and turn - that is our backlash. If we need to turn in order to change direction of motion - then we will have backlash. Let's do our escalator / moving sidewalk example again. Mount is standing still - in our case that is me going to the right at some speed and moving sidewalk going the opposite direction at the same speed. For the rest of the world I appear to stand still. If I want to move to the right for the rest of the world - all I need to do is to keep walking as I did and we reduce moving sidewalk speed in the opposite direction - net speed will be to the right. Did I turn to do that? No. We want to reverse the mount, what do we need to do? We need to increase moving sidewalk speed to above my walking speed - and I will start moving in opposite direction to the rest of the world (but I'll keep constant speed to myself - I'll just pace at the same pace all this time). Notice that - neither me nor sidewalk changed direction of motion whole time - we never stood still and turned around - no backlash. But this analysis pointed something very important and that is resolution. I don't think that I understand fully impact on resolution if one is using stepper motors. My gut feeling says that we won't have improvement in resolution. Here is what I'm thinking - if we have minimum step size - then our position can only change in these steps - regardless of how many steps we do per unit time. It's a bit like that old saying - one step forward, two step back. No matter how many steps we perform forward and backward - total movement will be integer number of base steps as we subtract the two. You can't move half a step back if you move integer steps forward and then another integer steps backward.

I think it acts a bit like "east heavy" setup. Motors and gears always move in same direction, so there is always load on them and backlash, although present because of gears - is always "eaten up" - or gears are always in contact on one side - in the direction of motion. I'm just not sure if that is true on connection of annulus ring and planets though. It's a bit like relative velocity thing. If I'm moving in one direction and you are moving in the same direction but faster than me - it will look from my point of view as if you were approaching me and I'm standing still (sort of). Similarly - if I'm faster than you - it will look like you are moving away from me although we are moving in the same direction. Now, I'm not really sure what it means when gears are involved - if some of the gears actually start to spin in opposite direction with respect to some other gears - and if they mesh - there could still be some backlash.

He has more videos worth watching. Has some other interesting designs too. There is a community of makes on youtube that mostly design reduction boxes for robotic purposes - but one can learn a lot from their videos on this topic.

When you say accuracy - what exactly do you mean? Timing issues or step / micro step accuracy?

I haven't but from that video it seems that it will give lower of the two in "addition" mode. I think that it actually might have issue with holding torque rather than with slewing. Steppers have lower torque with higher speeds - so holding torque is greater than torque at higher RPM. If both steppers are running at same but high RPM - we will have effectively "holding torque" as mount will be at rest - this is interesting in case of DEC and resistance to wind for example, it will be as high as either of two motors at that RPM - so can be lower than holding torque of either motor.

What would you consider to be deluxe version? I'm certainly going to explore behavior of that split ring compound planetary system - and see how it behaves from periodic error and smoothness point of view. I was thinking of maybe doing "poor man's EQ5" kind of thing as one of my next projects - something that will have ASCOM control and be completely motorized with ability to be guided and of course decent payload. As far as calculations for direct drive go - it might almost be doable with 256 micro steps and 0.9 degree motor - but do keep in mind that torque at those settings is seriously small and one would need to "rev up" things before switching to such fine control. Anyway - here is quick calculation 360 x 60 x 60 = 360 x 3600 = 1296000 arc seconds per circle 400 steps x 256 micro steps - 102400 steps per circle Ratio of the two is 12.65625" / step Now, that might be better than using x500 rule and static tripod, but I'd add at least one belted stage like 5:1 to improve things.

Probably something like that. AzGti has resolution of 0.625"/step - so that is very close to 2 steps per arc second. I think I'll be taking a step back and going with 1"/step instead - so a little bit coarser than commercial star trackers, after all - this is, as you say homebrew / DIY stuff. That way, I can simplify things considerably and simply go for 3 stages of 80 / 15 GT2 belt / pulley combinations. 80 tooth gt2 fits in 50mm diameter - so the whole thing should not be large - and it will certainly be slim enough. It turns out that it is quite hard to do so efficiently. Tooth height is about 2.25 x module, so for say module 1mm - we need more than 2 tooth difference - just to achieve clearance on the opposite side. Then there is issue of interference. It turns out that if we have internal and external gear that have small difference in tooth count - we will have interference unless we use large pressure angle and introduce all sorts of backlash and additional clearances. For above demonstration I used outer gear with 80 teeth and inner with 75 teeth and I needed to use 28 degrees pressure angle and to shorten the teeth somewhat to avoid interference. That makes whole thing something like 1:16 reduction, if I'm calculating things correctly as 80 / (80-75) - each wobble of "runner" gear (one revolution of input shaft) advances it by 5 teeth on a circle of 80 teeth. Cycloidal drive of the same size can be made to much higher reduction ratio and it operates on the same principle - except it does not use teeth but rather "lobes" and semi circles. so same thing - except for the "shape" of the "teeth"