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What will be reduction of that assembly, and what sort of backlash and precision do you expect from it?

Not quite sure what you are saying here, but yes - star color is consequence of their emission that is very well approximated with black body curve. In principle, you can derive transformation for Hb, Ha and OIII data that will produce proper star color - simply because you can produce temperature from three points on this curve (I think that you can with two as well). Yes, it needs only two points: Here we can rearrange expression so that T is function of B_lambda (measured value, all other are constants) and we can calibrate on a star of known temperature - and it would take only one channel to derive star temperature if all stars were the same brightness, but since they are not, we need two measurements to solve for two variables - T and brightness.

Forget DSLR and just ASI120 to capture SER movie with SharpCap. Use AutoStakkert!3 to do lucky type stacking of moon images. ASI120 is rather small sensor and you'll probably need to do mosaic out of couple of panels. Just capture a crater or two to practice first. Search youtube for lucky planetary imaging tutorial to get the idea of what is involved.

Yes, gearing is essential for this sort of thing and by far the most difficult part to get right as it must be machined with very tight tolerances in order to track properly for AP. If you still want to try DIY-ing star tracker - take a look here for some ideas about gearing: That is another good combination. 150PDS is just a bit bigger version of 130PDS (they share design and focuser - just larger aperture and focal length). I would say it is a bit more geared towards visual than 130PDS and a bit less for imaging. Actually it is very good imaging scope - but it is larger and heavier. All that makes it a bit harder to work with as longer FL makes all tracking imperfections easier to see and more weight puts more strain on the mount. On the other hand - larger aperture gathers more light so it will show slightly brighter images visually and will show a bit more objects.

You should be able to use RPI. PHD2 is usually weapon of choice for guiding (it works under linux as well). Main problem with RPI / linux and PHD2 is that you'll need some sort of terminal to RDP or VNC into RPI session and calibrate and operate PHD2. Look into astroberry for full featured setup. Alternative is to use IndigoSky https://www.indigo-astronomy.org/indigo-sky.html It works on RPI and you can use smart phone / tablet to access simple web interface and operate mount / guider / camera (not sure if DSLR is supported). Guiding software works even with modified web cameras (just removed lens so it can be put in prime focus of guide scope). As long as there is driver - it can work as guide camera.

Ok, so there you go - start with EQ5 and look at DIY kits for full goto mount solution. Get a pair of stepper motors, 3d print brackets to mount them, 3d print hand controller. Get arduino and stepper drivers to drive the mount. There are few already made open source DIY solutions that you can look into. https://www.astroeq.co.uk/about.php https://onstep.groups.io/g/main/wiki All those small newtonians have issues and won't be anywhere near suitable for astrophotography. They all have very flimsy/cheap 1.25" focusers. There is no coma corrector available in 1.25" format - you would have to build one yourself (I've found once design for it - you need to order two particular lenses and assemble them in some sort of housing that will fit into 1.25" - spacing is very important). Focus point is not optimized for imaging with those scopes and most won't even reach focus with DSLR type camera. Cheapest / smallest option is really 130PDS. It has nice dual speed focuser (which you can later upgrade with DIY auto focus unit once you get hooked ) Alternatively - print your own EQ mount? If you are into programming / robotics and 3d printing - than this will be a challenge for you: And maybe start small - with camera and lenses rather than telescope. Purchase one of those nice dobsonian telescopes for visual ...

Bare minimum for anything serious in long exposure AP would be: Eq5 manual mount Single stepper motor with controller for tracking (can easily be DIY if you have basic skills, or purchased as separate kit) 130PDS Coma corrector That is already close to 1000e If you don't have the budget - maybe look into getting a star tracker or DIY-ing one (look for barn door tracker) and getting some longer FL lens. Samyang 85mm and 135mm are very good for AP. You can also check here: https://www.teleskop-express.de/shop/index.php/cat/c270_Travel-mounts-for-astro-imaging.html if you don't want to mess with DIY - there are some fairly basic solutions based on EQ1/EQ2 mount and small trackers. Do note that none of these are capable of handling telescopes or longer focal lengths.

There is another option that currently exists only in my head - but it is cheapest of the lot and most portable. Finder scope converted to guide scope with 3d printed adapter + web camera, Raspberry PI Zero 2 and custom guide software (or maybe even adaptation of PHD2) that is controlled via smart phone

Depends what you want to achieve. If you just want to add Hb data and get some star colors - well, they will still look artificial / strange / maybe ugly. You can extract true star color with Hb + the rest, but the thing is - you already have enough data to extract actual star color with only two components - Ha and OIII. That is enough data to calculate proper star color, but that involves quite a bit of math and understanding how things work. Principle would be like this: - derive temperature from Ha / OIII ratio for your camera + filters (this includes calibration from actual star as well as using Plank's law - approximating stars as black body radiators). - Create starless version of the image in NB and compose that. - Use stars from Ha exposure (nice, tight not overly stretched stars) - Color them according to Ha/OIII derived temperature and temperature to sRGB ratio - Blend In fact, as a first step - I'd recommend just using white stars. That will remove dodgy star colors and make NB image more natural. You can even color them the same - slightly yellowish and you won't be far off from actual color.

Dynamic range is something you get from stacking and should not be concern in single exposure. Even stack of two images will have 40% higher dynamic range compared to single image - let alone stack of several hundred of subs.

@powerlord Can you stack your images without alignment using max as stacking operator? That will create "star trails". You can then see if there is actual drift between first and last exposure. If guiding is working properly - stars should be roughly in the same place even if you dither - as dithers are random. If you see stars streaking - that is sign of differential flex (which is, by the way, very hard diagnose / pin point what part of setup is causing it).

Another important point - you want to raise gain so that your read noise drops. At least gain of 100+ (maybe even 350). Also - check out AutoStakkert!3 instead of Registax for stacking of planetary images - newer and better. Registax is good for wavelet sharpening and basic post - like channel align / white balance and so on.

How do you control your mount? Do you have means to perform PEC (periodic error correction)? EQMod + computer control has that feature but I'm not sure about latest handset (there might be option). In any case - belt mod is worth it, but there is a chance P2P on periodic error will in fact increase - but will become smoother and easier to correct with periodic error correction. +1 to guiding.

Yes, forget the histogram and set exposure below 5ms. Set it to lower if you are over exposing but don't increase if image looks too dim - stacking takes care of that. Your main concern is to freeze the seeing with short enough exposure to avoid motion blur created by atmosphere.

Sure thing I made an error - need to use half of primary diameter rather than diameter.

You also need to consider position of focus plane above the tube. It is best to use low profile focuser in this case. If you want to cover only 0.4° with full illumination - that is about 23mm fully illuminated field. A bit of trigonometry similarity of triangles should then be used to solve the problem: Y is equal to tube radius + focus position above the tube and X is smaller radius of secondary (or X * sqrt(2) = larger radius) You can use proportion that goes: (X-11.5) : (254 - 11.5) = Y : 1600 (X-11.5) * 1600 = Y * 242.5 X = 11.5 + Y *242.5 /1600 = 11.5 + Y * 0.1515625 (X-11.5) : (127 - 11.5) = Y : 1600 (X-11.5) * 1600 = Y * 115.5 X = 11.5 + Y *115.5 /1600 = 11.5 + Y * 0.0721875 Voila! (someone please check my math - I did this very fast without careful analysis - back of the napkin sort of thing)

I took a look at that guide graph - and it really does not look that poor as is. You have very decent RMS values of around 0.5-0.7 and even P2P error is not that bad really. Problem is - I'm convinced that poor star shapes are due to guiding, and I think that log does not match what I see in the subs Look at this animation of your subs Guiding shows that most of your errors are below 1" and that is reflected in RMS (you can't have total RMS of ~0.5" and have lots of errors higher than 1". However, stars in your images are distorted by much larger amount. You are imaging above 1"/px (ASI1600 + 750mm of focal length or less) - so single pixel is 1". Look at this star profile: Two centers are separated by 4px - that is at least 4". Guide log shows no such error. I think there are few possible explanations: 1. Wrong guide scope / guide camera parameters entered (like pixel size and focal length) - which results in wrong arc second reading from PHD2 (but that is not very likely) 2. some sort of differential flexure. How is your guide scope mounted? Is it on OTA or attached separately to rings or somewhere else? Could it be that mirror is loose in the cell and moves around (although unlikely - as wind would have to be strong to move it and it would be seen in guide log)?

You should actually move top slider. Balancing is done in linear mode. Moving top and bottom slider preserves linearity while moving middle - changes gamma / does non linear stretch.

Not really sure what you are asking, but this is what I would recommend with regards to PIPP and AS!3. Capture raw data and save as SER In PIPP do your preprocessing / calibration / frame rejection and also save as SER. Make sure you have following PIPP options set: Don't debayer and protect bayer pattern. This will make sure bayer pattern makes it all the way to AS!3 which will then use Bayer drizzle for debayering (best resolution / detail).

Yes it will be green because of relative responses of each channel. This is easily fixed in post processing with color balance. For accurate color representation, you need to do full color correction with color correction matrix - but that is probably too much work / too complicated Registax provides very decent results with auto white balance. Here is one of my captures with ASI178: It has that strange green cast after stacking. In Registax I do white balance and it makes nice color: then a bit of wavelets / resize ... starting to look nice ... If you want full color correction thing - it goes something like this:

Depends what you are after. For casual planetary imaging - it sort of is. I use ASI178 in that role and it has more read noise than 533. For best performance possible - all things must be taken into account.

Slightly higher FPS for same ROI and lower read noise. QE is about the same.

I just wanted to share few ideas and not complete solution. I still don't own a 3d printer (hopefully that will change some time soon), but I do from time to time think about what can be 3d printed astronomy related. Star tracker + DSLR and lens is very cheap way and easy way to get into astrophotography. Most people, when thinking about DIY star trackers - think barn door tracker as a cheap solution - but I thought - what about 3d printed options? There are several full fledged 3d printed EQ type mounts, but I had in mind something simpler. Worm and worm gear are not really 3d printable. Sure you can make them, but they are not very smooth. While watching youtube videos and prompted by recent thread on a new harmonic drive available - I found 3 rather interesting and 3d printable gear reduction ideas. First a bit of math: 360 degrees x 60 arc minutes x 60 arc seconds = 1296000 arc seconds in full circle 360 / 0.9° * 64 micro steps = 25600 micro steps in full circle if we get 0.9° per step stepper motor and do microstepping with 64 micro steps. 1296000 / 25600 = 50.625 We need x50.625 reduction to get 1" / step resolution. We can use more than that - say double to get to resolution of AzGTI which has 0.625"/step. How to easily get that sort of reduction? It turns that there are several 3d printable solutions - which can get that and much larger resolution in single assembly (no multiple stages required). 1. Strain gear drive / harmonic drive - very low / no backlash - can't be back driven (like worm / worm gear arrangement) https://www.youtube.com/watch?v=-l7FbsJSteg https://www.youtube.com/watch?v=fGI-4dHYz9U and there are of course several more interesting designs that you can check out on youtube 2. Cycloidal drive https://www.youtube.com/watch?v=tgEOpl880KM https://www.youtube.com/watch?v=7uXN_y7JdyM This is probably a bit more expensive as it involves quite a bit of bearings and shafts to be added - but I suspect it can be made real smooth even 3d printed 3. Ring split compound epicyclic gear https://www.youtube.com/watch?v=-VtbSvVxaFA https://www.youtube.com/watch?v=66MlWxoQE1s I love this one because of crazy reductions available - but it does suffer from backlash as most gear based systems. Good thing about this design is that there could be multiple teeth in contact at the same time which helps with backlash and smooths the transmission. If we use helical gears instead of spur gears things get better as well. It is also very simple design in terms of bearings and shafts / pins needed. In the end - it is fair to mention that x50 is really not difficult reduction ratio to achieve with synchronous belts - all we need is two 1:7 stages so even that design should be working well.

Well, it does not look bad to me What were your stacking settings?