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In order to see if polar alignment error or periodic error is going to have an effect on your image - focal length is not enough, you really need to consider sampling rate - in arc seconds per pixel and compare that with drift in arc seconds per second and exposure duration. For example if you sample at 10"/px and you have drift of about 0.5"/s, then you can do 20s exposure before drift becomes 1px in your image - simple math but getting accurate drift rates is not easy - you need to measure those. I had this issue with my HEQ5 from the start and had several enhancements done to it: - I started guiding - found out that I had one cracked bearing that needed replacing, so I replaced all bearings, tightened things up and re lubricated the mount - did belt modification (replaced gears with belts) - changed a few more things on my mount (saddle plate, tripod and such) and now I'm finally happy with the mount to the extent one can be happy with HEQ5 class mount - it guides with RMS of 0.5" on a good night. I guess that above does not mean much to you - but greatest single enhancement I did was that I started using auto guiding. I'm not sure that it means much to you since you have arduino driver and EQ2 and you won't be doing out guiding. Let's see what else I can offer as advice that might help - balance mount as best as you can - EQ2 mount is light so avoid any wind (although you don't have much on that mount - mount itself will catch some wind) - Maybe use faster lens and shorter exposure lengths (but get more of them)? This will minimize trailing per single exposure

Bloating of stars can be related to filter - but can happen only if: - improper filter is used for a given telescope, so bloating is feature of the telescope not corrected by filter - this happens when for example when telescope is not well corrected for chromatic aberration and wide luminance filter is used in place where narrower luminance would block light towards UV and IR part of the spectrum most likely to be out of focus. - there is optical distortion due to filter. This can happen if filter surface is not flat and parallel (front and back of filter to each other) - this is equal to having optically poor telescope - stars are distorted and bloated because of poor optics. Over exposed stars can only happen if you over expose them - no feature of filter or telescope can make more light appear then there is reaching the telescope. -------------------- Halo around bright stars can be associated with filters or other optical elements. Whenever you have something in optical train in front of the sensor that has reflective surfaces (and poor anti reflexive coating) - this can create halos. If filters are causing this - moving them closer to sensor will make things worse. Example of star halo due to reflection: Here is another example - filter is closer and halos are smaller but more intense: Here is diagram of what is (usually) happening in these cases: Light beam is coming down and is focused on sensor (comes to single point - star). There is some surface in front of sensor, or even sometimes sensor itself (micro lens or other feature of sensor) that reflects part of the light back. Even with very good anti reflective coatings some percent of the light is reflected back (less than 1% in case of good coatings). This light goes back - but hits filter from back side (or other optical element like field flattener / coma corrector / reducer - whatever) and there it also some of the light bounce back (again depends on coatings on that particular element - interference filters are the worst because they are made to reject some frequencies by reflecting them) and continues back towards the sensor. When it reaches sensor again - it is very much out of focus and presents itself like large defocused star around original focused star. Sometimes it can be a bit to the side - if incoming light ray was at an angle - star was away from optical axis, and very rarely it can even be on opposite side of sensor - this happens if reflecting element is far enough and only on the brightest stars - but in this case, offending element is usually not filter. On right side of above diagram - same thing, except this time filter was placed further away. Only difference between two cases is level of defocus that halo will have. More it is defocused - it is larger and that means it is spread over more pixels. This means that each individual pixel will receive less light (again - we only have so much light reaching us from particular star - it is fixed per time interval) and halo will be fainter. If it is faint enough - at the level of background noise, it will not be seen. To reiterate - better optical coatings means less percentage of light is reflected (less intensity of effect), and further away filter is - light is spread over larger surface - again less intensity per pixel. We can argue that this sort of thing always happens, and that it's just matter of how much light there is in a star halo that makes difference if it will be seen or not (this explains why it is seen only on bright stars in the image and not all). Btw, there is reason why you might want to place filters closer to sensor and that is related to vignetting, so it is balance - place it close enough to avoid strong vignetting, but not too close to avoid strong reflections.

There are better options out there to do reduction (and field flattening at the same time) for this scope, namely: https://www.teleskop-express.de/shop/product_info.php/info/p11145_TS-Optics-RC-0-8x-Corrector-Reducer-for-Ritchey-Chretien---big-sensors---M68-connection.html that one is x0.8 FF/FR, but I have heard that this one is also very good: https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html (it is cheaper as well and can be used with refractors as FF/FR so it is more versatile)

Made an error in sign in above post - now corrected.

This is simple reducer and it has very mild field flattening effect - so exact distance is not crucial. You can change distance to change reduction factor as per above formula I gave (except I was wrong FL of this reducer is not 303mm, but rather 305mm - it won't change much in terms of results - but it's better to be precise). So if one for example wants to have lower reduction factor of x0.7 instead of x0.67 - spacing needs to be decreased to: 0.7 = 1 - (distance - 16) / 305 => 0.7 * 305 = 305 - distance + 16 => 213.5 - 321 = -distance => distance = 107.5mm 0.7 = 1 - (distance + 16) / 305 => 0.7 * 305 = 305 - distance - 16 => 213.5 - 289 = -distance => distance = -75.5mm We can also calculate needed distance between 2" thread and sensor for other reduction factors following above method. Now you may ask, why would one want to use this reducer with smaller reduction factor? It is important to remember that this is neither corrector nor field flattener. It does have very light field flattening effect - but it will not correct your field. Therefore all aberrations in outer part of the field will just be "compressed" into smaller area. This is the reason why this reducer is recommended for flat fielded scopes with F/8 and above. On TS website for example 10" RC is said to be well corrected up to APS-C size chip. This means diagonal of about 28mm. I see from signature that KAF8300 sensor is likely to be used - it has diagonal of 22.5mm. From that we can calculate what sort of reduction we need to "pack" 28mm diameter field onto 22.5mm diagonal sensor very easily 22.5 / 28 = x0.8. So using x0.67 with KAF8300 will actually image original field with diameter of 33.6mm - that is almost full frame sensor - that level of reduction is going to show both curvature and astigmatism (and possibly other aberrations from reducer since this is no longer flat field). I think it should not be pushed past 0.75 - 0.8 with that sensor on that scope. What distance needs to be for x0.8? We can again calculate 0.8 = 1 - (distance - 16)/305 => distance = 77mm 0.8 = 1 - (distance - 16)/305 => distance = 45mm

If I remember correctly FL of that FR is 303mm, so you need to place your sensor 101mm away from the lens - which turns out to be 85mm from 2" thread at the back. What magnification are you aiming for? equation for reduction factor would be: 1 - distance_from_lens / FL = 1 - ( distance_from_2"_thread + 16mm) / 303

That is more question for Baader marketing team, isn't it On their website - they call it light blue and still picture of the product is indeed the same you have shown - and on box it only says blue 470 band pass

Hi, yes that sort of thing can happen and in principle you can fix it it to some extent. Zig-zag type of pattern in images will happen if two things happen at the same time - first one is polar alignment error. With polar alignment error there is constant drift in DEC - which means that your images will have constant offset in one direction. If there are no other contributions it will just show up as streaks and shifts in that direction. In your video that would be upward motion of stars - if you compare any of your images to star chart you will see that Up/Down direction is DEC direction of the mount. Second part of this motion is periodic error. This happens because gears used to driver the mount and/or worm gear of the mount are not perfect circles. They are a bit egg-shaped or elliptical. In any case- just irregular and not perfect circles. This means that at some points in time mount will trail behind the stars (slower rotation - ellipsis major axis), but at another time mount will lead in front of the stars (faster rotation then sidereal - ellipsis minor axis) - and these will repeat in cycle. This is called periodic error of the mount and it will repeat with worm period. It is often not simple sine or cosine wave because there are multiple gears in the system and each of them will be somewhat irregular. True periodic error is superposition of these smaller periods of all of gears involved. There are two ways to deal with this - one is to disassemble the mount and make sure that everything fits nicely together and change the bearings and tighten everything up - it's called "tuning" the mount. Maybe even replace some gears with belt system if you see that largest oscillations are with that period. Other would be to do Periodic error correction. It might even be possible for you to do since you have custom stepper and arduino controller. You need to figure out main oscillations and program arduion to actually move stepper faster and slower to compensate for periodic error. It will sure be tricky thing to do, but in principle you should be able to do it. You can see similar thing happening here - its an animated gif of one of my recordings (refresh page if animation stops - I did not make it loop for some reason): there is DEC drift due to PA error in left direction and there is wobble up and down - that is RA periodic error (a bit complex because it contains multiple harmonics). It is not fast enough to cause serious trailing, but in some frames stars are elongated. HTH

You must have amazing skies where you shoot - very dark. I was at first baffled of how you managed to get such a shot, but then I did some calculations and in fairly dark skies it is possible to have a glimpse of Running man in 11s exposure with this gear. Fact that you took a jpeg image and it is only 1920x1280 helps a lot to get that level of detail and relatively nice looking stars. Please don't take what I'm about to show you as taking a stab at your original image or anything. I'm just doing this to show how much both processing but also presenting image at certain scale can help make image look nice. Here is a piece of original raw image showing couple of stars. At this scale we can't really say they are nice looking stars - they look more "pacman" like. But you don't need to show your image at that scale - you can bin your data (similar to resizing down - same thing that your camera did when converting to 1920x1280 jpeg) and then stars no longer look out of shape: Now those same stars don't look out of shape any more (this above image is what you get when you reduce image size to 1500 x 1000 - or about x4 less than original resolution). Let's now look at what Running Man looks in blue channel in original resolution: Hm, nothing much there, is it? Just noise, and some more noise. But bin that and smooth it out and you will actually start to see the shape of it. Here is what I've got as synthetic luminance from that single sub, after I did both binning and noise removal: Looks rather good? So yes, data is there and if you know how to process it you can extract quite a bit out of it. People that work at Canon and with other companies that make cameras are in fact experts in data manipulation and no wonder automatic Jpeg from camera almost looks as good as this image above. Data is in fact there - and I believe in great part due to dark skies at site this was taken at.

Now when I think about it - it is rather strict isn't it? What do you think should be reasonable criteria? Should we still maintain SNR5 20/20 in one minute or relax it in terms of SNR, or time achieved or something else? And what should be criteria on FOV and resolution? I mean - I've figured out really fast setup but it fails on couple of my points above. AzGTI + Samyang 135mm F/2 + ASI178mc cooled. It is 3.67"/px and FOV is something like 3° x 2° That one is SNR8 for 20 / 20 / 15 x 4s exposures. If we swap cooled for regular ASI178mc (as cooled is no longer produced and to save some cash) we are looking at about 1300e and SNR7 (assuming dark current as high as 1e/s/px - although I have no idea what it might actually be - I doubt it is as high as that).

That Borg is rather fast at F/4.3 and even triplet would struggle with color correction at that F/ratio so no wonder that double shows a bit of unfocused light. L3 could help, but not sure how much. In any case - this is one of the reasons you are struggling with focus - fast scopes have very thin critical focus zone and it is rather hard to get good focus - although your image seems to have that aspect covered, I have not seen any focus issues (even in fast dropping temperatures).

I was under the impression that it is other way around L1 being widest and L3 narrowest of the three: In any case this scope would benefit from the narrowest of the three (L3 according to above graph).

I'm sort of trying to invent some criteria for "speed" of live stacking rig and I came up with SNR5 "standard". I'll briefly describe what that "standard" is and how it is calculated and want to see what rigs satisfy it - as being at least SNR5 capable, and how much such rigs would cost - not including computer, but including scope, mount, camera and all bits and pieces needed to tie it up together and make it work (well maybe not power source and things like that?). Here is what goes into SNR5 "standard" - it is rig that: 1. has at least one degree diagonal coverage of the sky 2. has sampling rate around 1.5"/px (I guess we can say +/- 0.2 or so but I think we really want to say at least 1.5"/px or if we allow it to go higher - at least 1.6"/px - lower res benefits SNR but looses detail, and in reality it is unlikely that we will achieve higher detail than 1.5"/px with these setups). 3. Is capable of doing color rendering of the target 4. Needs to achieve SNR of at least 5 in following conditions: - one minute total exposure in 4s subs - in 20mag skies - of 20mag target (20/20 - easy to remember) Calculations include following: Mag 0 sky / target produce 880000 photons per second. All telescope losses are accounted for - This means we calculate clear aperture (central obstruction is subtracted if present, mirror reflectivity is accounted for, and glass transmission also). QE of sensor is taken to be 40% of that declared to be max QE and final result is multiplied by 1.41 - this is because of the way color sensors work and the way we can obtain Luminance layer from the data (we have two pixels of green color - hence 1.41 factor, green is about 1/3 as sensitive as peak QE, but other two channels contribute as well so we use 0.4 instead of 0.33 - this is rough estimate but enough for "standard"). For mono sensors + filters - only mono is used and "average" QE is used (this means if peak QE is 80% we will use something like 60-65% depending on where endpoints sit). Read noise which can be achieved is used - not baseline, since anyone doing 4s exposures is likely to use high gain. If there is binning - we will treat it as software binning and read noise is increased with factor of "base" of binned pixels - for 2x2 binning - we have x2 read noise, for 3x3 binning - we have x3 read noise and so on.... Dark current is also taken into account - either at -10C if cooled camera is used or 25C if regular camera is used. Here is my candidate: EQ3 class mount 130PDS + CC ASI294 cooled model. This combination will achieve SNR5 with no binning. Working resolution is 1.46"/px, assumed peak QE is 80% (although ZWO says it is TBD, I read somewhere that it is estimated to 80%) - so used QE was 0.32. I used CO of 36% (47mm) and mirror reflectivity of 94%. I also used read noise of 1.4 (although it can go down to 1.2e according to ZWO - I wanted to be safe there). In any case stack of 15x4s subs gives SNR of 3.583 and multiplying that with 1.41 gives 5.05 All of that for only 1999.99e (give or take) Can we do this cheaper?

You are right - being that much processed does not help with determining the cause of problems. In fact - I'm not sure what exactly is your problem as having stars like this: screams color aberration to me - but it could be just down to processing. Btw, what filter did you use with this camera? You do know that 183mc is only AR coated and you need UV/IR cut filter with it? If you in fact used UV/IR cut filter with it and this issue with color is not down to processing - then you might want different UV/IR cut filter - something like L3 from astronomik. Since you are mentioning distance of field flattener - I suspect you are actually concerned about star shapes in the corners? In that case - I would suspect tilt as stars in bottom of the image are just fine, while stars on top have astigmatism to them: Top of the image: Cross / seagull shaped stars (although it is hard to tell due to level of processing) Bottom of the image: mostly round - but again, it's hard to tell

How about a good book then? Something like this: https://www.firstlightoptics.com/books/turn-left-at-orion-book.html

How much vignetting, if any, will there be from filters on a particular sensor depends on two things. F/ratio of the converging beam and distance of filter to sensor. It also depends on their sizes - you can't use filter that is smaller than sensor itself as that will cause vignetting in any case. 1.25" filter have about 28-29mm of clear aperture and that is large enough to cover 23mm diagonal of ASI294 if mounted fairly close. To roughly get max distance of filter without vignetting - you need to do a bit of math. First you take free aperture of filter and subtract diagonal of sensor. let's go with 28.2 - 23.2 to get nice round number of 5mm. Divide this value with 2, resulting in 2.5mm. Next thing to know is F/ratio or speed of the beam. If you are using ED80 that has F/7.5 natively, and you use something like x0.8 reducer you will get F/6 beam. Now it is just a matter of multiplication max distance without vignetting = 2.5 * 6 = 15mm (roughly). With asi294 you already get T2 ring with 1.25" filter thread that screws in camera nose piece (2" outer and T2 female inner diameter). That way filter sits less than 10mm away from sensor, and you are fine down to about F4.5 - F/5 with this combination. Quick search online gives this image that shows configuration: You can still use remaining T2 thread or 2" nose piece for further connection to telescope. Out of filter choices - you will need at least IR/UV cut filter because you are using refractive optics (and doublet). Might not be needed much for SCTs, but I would use it anyway just in case corrector plate causes a bit of color bloat. CLS/CLS-CCD is rather aggressive general purpose filter and will cut deep into LP but will make havoc out of your color balance. UHC filter is even worse / better - depends how you look at it - it is very good for emission type targets but should be avoided for star clusters / galaxies and reflection nebulae. It will also throw off color very much. I don't know about Optolong L-Pro filter, but looking at its response curve it looks like good general LP suppression filter. I do know that Hutech IDAS LPS filters are very good - I use P2 version for my LP levels and type of LP. If you live in very light polluted area then it is worth having LPS filter, however, because all filters block light - at some point it will do more harm than good, so if you have mild light pollution where you shoot - I would avoid LPS filters. As for field flatteners and reducers with ED80 - I think they will provide very flat field over 4/3 size chip, so you don't have to worry about that. As far as SCTs are concerned - if you don't have Edge versions - these scopes have quite a bit of coma and sometimes mild spherical because of focus position (moving mirror changes distance between primary and secondary and only one exact position is free from spherical if surfaces are figured well). Focal reducer designed for SCT does take this into account and I believe they reduce coma - so I would recommend them if you plan on using SCTs for small targets. However if you do plan to use them - I would suggest that you get OAG and make sure your mount really guides well and that you understand software binning as part of processing workflow. At these long focal lengths you will oversample and you need to recover back SNR by binning your data and have it at proper sampling rate for detail available - which is usually about 1.3-1.5"/px with these larger scopes, 1"-1.3"/px requires excellent seeing and excellent mount and optics.

Your flat subs look good - both of them together with flat darks. In fact - this is what they look like after flat dark subtraction and stretch: Both look the same. Short one is obviously more noisy but they contain the same dust shadows and vignetting. This is of course when we inspect them by eye, but there is much better way of inspecting two flats if they match - we do what is called - flat/flat calibration - or we calibrate one flat with the other - result needs to be pure noise and gray image without any obvious features. Here is what it looks like when I divide two flats together: This is sort of text book example what flat/flat calibration should look like - histogram is beautiful bell shaped curve with value of about 0.1 (this is because we divided 4s flat with 40s flat - a bit less than that because it was 3.9s flat in fact) and the image itself is just pure noise - no features to be seen. Let me see what happens when I try to do calibration of that one sub to see if there is something else wrong (it's not flats usually when flat calibration is not working - it's darks or light leak or some other thing but rarely flats, unless of course flats are clipped - but they are not in your case). Ah yes - everything is fine with your flats - they calibrate out ok, except one thing - dust is no longer where it used to be: If you look at this image - which is calibrated single sub and then binned x6 to get enough SNR (this of course looses the color, but we don't care now - we want to see what is going on with calibration) - it looks fine with exception of perhaps two dust particles - that moved. vignetting is fine - it is neither under nor over correcting, and if you look at above flat - let's do another screen shot of it: I marked here with two red arrows - tow shadows of dust particles that moved in the mean time, and with blue arrows - all other dust particles and their shadows that simply calibrated out fine and can't be seen in light sub above. This happens sometimes and there is pretty much nothing you can do about it - except calculate distance of surface that has these dust particles (either coma corrector or filter) and then use blow bulb from time to time to blow away any loose dust particles so this would not happen again. Sometimes people have this issue with filter wheel if their filter wheel can't reposition precisely each time - any small shift will create above effect - it's like "double" or "embossed" dust doughnut. Here it is on one of my images - sometimes it looks so strange that people ask if that is some sort of planetary nebula they have not spotted before Hope this helps

I would probably go for ASI294 myself.

From what I've read so far in this thread - I think it would be best if you started with your goals and expectations and build on that. In the first post you said your interest is in AP of galaxies and nebulae. With full frame sensor and small refractor there are in fact just a couple of galaxies that are good fit for that. M31 - our closest neighbor and M33. Such setup is very wide field (nothing wrong with that - as long as you know it will be). For that reason, maybe best thing to do would be to go to: https://astronomy.tools/calculators/field_of_view/ And check out some targets with scope of your choice and build on that. For example - Let's take famous pair M81/M82 and see what sort of FOV can you expect if you use 70mm scope with FF/FR and your camera: You see that small smudges in the center? Those are galaxies that you will be imaging. Btw - these specs you should use when working with above field of view calculator - 4.88 um pixel size and 7360 x 4912 resolution with custom camera as Pentax K1 is not in database: Ok. Once you have that covered and decide on best FOV - or Scope to match your camera - then you have further parameters for choice of your mount: up to few kilograms of gear (that means 3-4) you can go with Star adventurer or AzGti mount in EQ mode - this is really suited for small focal length and scopes up to 70-80mm that are light - wide field imaging - but are highly mobile platforms. Eq3 - Eq35 class mount is still good for wide field imaging and scopes up to 5Kg (maybe up to 6Kg on Eq35). Eq5 you can push up to 8kg. Heq5 you can use with - 10-11Kg of gear. As you go up in "class" of mount - mobility goes down as bulk and weight of mount go up. But if you want to work with longer focal lengths and get close up images of targets - that is really requirement. EQ6 type mount is really heavy. I've got Heq5 mount and it is manageable - I set it up every time and tear down and it is a chore, no question about it. If you need mount in Heq5 class and have the funds - there are other lighter options available if you put high value on mobility and plan to image from remote locations. Have look at iOptron mounts like https://www.firstlightoptics.com/ioptron-mounts/ioptron-cem40-center-balanced-equatorial-goto-mount.html Btw - if you see mount weight limit - use about 60-70% of that as weight limit for imaging. For example Heq5 can hold something like 15kg and indeed I've mounted close to that weight on it but for smooth operation you really need to limit the weight on it to 10-11kg Mount is the most important thing in AP.

I've found that I like guiding better when I use ascom driver and download 12 bit data. Never had SNR over 50 or so with 8bit and native drivers in Phd2 but regularly have SNR over 100 or even few hundred with ascom driver and 12bit

Have no idea. I know that Altair Astro sources scope from same supplier (only different branding, but could contain different coatings and or glass types - can't be sure). Check their offering: https://www.altairastro.com/altair-wave-series-80mm-f6-super-ed-triplet-apo-2019-457-p.asp and 115mm model: https://www.altairastro.com/altair-wave-series-115-f7-ed-triplet-apo-453-p.asp

That very much looks like QHY5LIIc that I used to have - I was happy with that camera except for some driver issues (a bit quirky drivers). I eventually replaced it with ASI185. It uses well known sensor and for guiding purpose you don't need high speed interface. I would only be worried about drivers quality and possibly lack of raw format. Does it support 12 bit RAW output? I can't tell from the link you provided:

Using full frame sensor is going to be an issue as not many scopes are fully corrected for that format - especially smaller scopes. Are you limited by your mount in some way (star adventurer or AZGti or similar?) - if not - look at 80-100mm range of refractors with good field flattener. I have TS 80mm F/6 APO and it is indeed very nice little scope. With Riccardi reducer you will get nice F/4.5 scope and about 360mm FL - however, for imaging of galaxies I would choose something with a bit more oomph - maybe this scope: https://www.teleskop-express.de/shop/product_info.php/info/p11871_NEU--TS-Optics-PHOTOLINE-115-mm-f-7-Triplet-Apo---2-5--RAP-focuser.html I linked that one on purpose (there same model without discount price) - because I believe discount price is worth having since these are supposed to be showroom models at lower price. One of member recently purchased one and impressions were good (search the SGL - there is topic about it).

I think easiest and most cost effective thing would be to just try it out. As long as Samsung 1000nx can produce raw subs, there is setting for needed exposure length (at least 30s and up to few minutes would be good) and you can turn off any "advanced" processing - I think it is just matter of trying it out (if raw or long exposure are missing - then maybe there is no point in trying). Only thing that I can think of that will cause issues is if camera is doing some "enhancements" - like noise reduction or whatever and you can't turn that off. That is going to create issues in stacking later on and you won't get the quality of images that you would otherwise get if you just downloaded pure raw files without any processing done to them.

Both.