vlaiv

I guess it depends how you look at it. Maybe instead of starting at FOV / Focal length / Sensor size - you start by what you think can be achieved in terms of resolution in arc second per pixel. If you say - I'm able to achieve 1.4"/px images with 8" scope, then let's see what sort of pixel size (including binning) will I need on EdgeHD (which I believe is less trouble to collimate than RC). Maybe it will be MN190 instead? Anyway - you start with 1.4"/px - then you say, ah, but most galaxies are in fact less then 10' in extent, and that is only 600" in extent, so I'm looking at 600" / 1.4 = ~430px for a galaxy at best. That will be rather tiny on 3000x2000 px sensor, so all I really need is maybe 1600x1200px to capture galaxy and surroundings. With ASI1600 - that allows me to bin say x3, so my effective pixel size will not be 3.8um but rather 11.4um and I'd need ~1600mm of FL to get there (at ~1.4"/px). So there you go - don't expect very big galaxy images, as galaxies are small. Use "slow" scopes as it is easier to work with them. Compact design has advantage as far as mount goes, so SCTs, RCs and such. Get as much aperture as possible, as it impacts achievable resolution as well as helps with light collection.

I thought we are past F/ratio as indicator of speed :D What is that in aperture @ resolution units?

Actually there is a time limit, but it depends on several factors - and not all factors contribute the same and at the same time. Moon close to horizon is slightly smaller than when it is at zenith. This is due to position of observer and rotation of the earth. When it is close to horizon - then distance includes radius of the earth - but when it is directly above - distance does not include radius. That amounts to difference of about 1.66% If your lunar disk is say 2000px in diameter - that will be 30 ish pixels of difference at those two positions. Not sure if stacking software can account for this (and I think it can't). So first limit is set by how far away (or closer) we move during recording. Second thing is - lunar phase. Full lunar cycle is about 28 days, so sun changes position with respect to lunar face at about 12.86 degrees per day. That is half a degree per hour. Now, if sun is shining directly at lunar face that we are imaging (full moon) - this change is minimal, but if we image the terminator - some really interesting effect might happen due to this. Shadows that are already long can grow quite rapidly due to this. If light is already hitting a crater at shallow angle (like at terminator) - then very small change in angle can make it noticeably longer. For this very reason there is only limited amount of time that Lunar X and V are visible - like just couple of hours. In any case, I'd say that you are safe if you image for less than an hour - as above effects won't be visible.

Or in fact - just clicking on your name in your post should display the same page: https://stargazerslounge.com/profile/1311-beamer36m/

Click on your profile picture to go to your profile page There you will see two tabs - activity and albums Select albums and it should show you your albums

Get 200p to a dark location. Once you do that - galaxies will start popping out whichever direction you point the scope. Seriously. Just get dark adapted and get comfortable. Best to aim scope high up as there is least atmosphere to interfere in that part of the sky. One more tip - try to steer clear of Milky way. There is a lot of dust in our galaxy that will block the view - plenty of stars but not a lot of galaxies to be seen there.

I'd go by what you expect your mount to be capable of doing. You can't expect for example to guide at RMS 0.5" combined - if you won't even issue correction to anything below 0.8" per axis. Maybe half of what you expect to achieve in terms of RMS? I had my HEQ5 set to 0.3px on ~0.95"/px guiding setup - that is about 0.286" - and I was aiming for 0.5" RMS combined.

Note the values of MnMo (min mo / min motion) in that screen shot. It is set to 0.19. That value is in pixels rather than arc seconds (for some reason). From same screen shot I can see that RA error is 0.17px = 0.72", so you are guiding at ~4.24"/px If you set min mo to 0.19px - that is same as setting it to 0.8" You are telling PHD2 not to issue correction if error is less than 0.8". Maybe stars are round, but are they tight (as they can be)?

I can tell you in very simple terms what theory says (and no - I don' buy into "theory is one thing while in practice ..." notion that many entertain - if theory is properly applied - it works, otherwise it is flawed and we would know about it - it cease to be current theory as new explanation would be offered). Thing is - many respected planetary imagers don't follow it (each for their own reasons) - and that creates a lot of confusion, so I won't go into explaining any of that - just state facts. Optimum F/ratio for planetary imaging is related to pixel size by following formula f_ratio = 2 * pixel_size / wavelength where wavelength is wavelength of light of interest. It can be exact wavelength in case of use of narrow band filters - otherwise we must observe that visible part of spectrum is 400-700nm and one should use 400nm as limit of resolution (as more detail is resolved at shorter wavelengths). However, I often advise use of 500nm instead for regular RGB imaging (with OSC or mono + filters). This is for several reasons, one of which is that luminance carries the most detail information in the image and peak of our luminance perception is north of 500nm. Another is that shorter the wavelength - more distortion there is from seeing (refraction is strongest for short wavelengths - that is why sky is blue - strongest scatter). This leads to two very simple formulae - you can pick which one prefer: F_ratio = 2 * pixel_size / 0.5um (500nm) = 4 * pixel_size F_ratio = 2 * pixel_size / 0.4um (400nm) = 5 * pixel_size With C11 and 678 - you don't really have a choice, as C11 is F/10 scope - and ASI687 has 2um pixels - so you can pretend that you used 5 * pixel_size formula Btw, don't do drizzle (except bayer drizzle, which is done by default with AS!3 when processing OSC data). Back to original question ASI224 vs ASI687 There are only 3 numbers that you should compare: 1. QE 2. Read noise 3. Frame rate Most of new USB3.0 cameras are capable of frame rates that depend on your exposure length (which should be around 5ms - so around 200fps), so third point is not as important as it used to be (USB 2.0 vs USB 3.0 for example). In the end - there is little between these two models. Both have QE around 80% (maybe ASI678 has slight edge - but do note that QE is concentrated in blue part of spectrum - and blue carries the least information as far as luminance is concerned) - so I'd give them a tie there. Read noise is also a tiny bit better on ASI678 - 0.6e vs 0.8e, but I've found that quoted read noise is often too optimistic anyway, so there is question if those numbers are accurate. Regardless of that, if ZWO uses same methodology - then relative difference should still hold. ASI678 has very small edge there. Only advantage (in my view) is that ASI224 can use F/15 (which is 4 * 3.75) while you don't have that choice with ASI678 and need to use F/10 - which is 5 * 2um - so you are stuck with that formula instead of having a choice.

Might be too low aggressiveness, or low min-mo. Most of your hits are on one side of DEC axis (vertical axis is DEC and hits are below axis). There are two (or maybe even more explanations) for this. First is low aggressiveness. Guide system detects that DEC is not correct and issues correction - but due to decreased aggressiveness - correction is "too weak" to correct in single cycle so a bit is left to do in next guide cycle. However, due to polar misalignment - there is constant drift in DEC. If that drift is high - then it will "eat up" any correction in previous guide cycle and instead of giving final nudge in next cycle - guide system starts again - detects larger deviation, but issues weak correction with hope to again compensate in next cycle and so on.... If min mo is set too high to combat the seeing - then similar thing can happen. Guide system detects error, but issues too weak correction. This correction brings mount position closer to wanted position - but this time mount lends in min mo zone. When mount is in min mo zone (minimum motion) - no correction will be issued if it is less then minimum desired motion, so guide system does nothing and subsequently drift (does not need to be high) pushes mount further of target, it goes out of min mo zone, correction is issued but weak - and mount again lends on same side but not quite at target. Similar thing can happen with RA - but not completely. RA is subject to periodic error, so it does not have constant drift like polar alignment error. However, it can have sort of constant but small drift, due to various reasons. First is inaccurate tracking speed. Even if you set to sidereal, it might not track at precisely that rate. That depends how accurate clock is in electronics of the mount system (it does not have real time clock and instead relies on quartz crystal - which might have some offset compared to designed frequency). Another thing that can happen is difference between sidereal rates - there are several sidereal rates and apparent position of stars does not track perfect rate but instead tracks differently depending on part of the sky (atmospheric refraction and apparent position of stars thing). Third thing is just large component of periodic error that is simply drift over duration of session. Worm wheel of RA turns once every day. That is about 1/4 of revolution per 6h session. It can be out of shape (ellipse) and that means it will have two "fast" and two "slow" segments - roughly 1/4 (for ellipse it is like that, but for egg shape it will won't be exactly 1/4, but still there will be long periods of faster and slower rotation). In any case - it is similarly drifting due to one of above, but there is also periodic error component. Same thing can happen as with DEC - if above combination makes correction less frequent than needed - it will mostly under correct, but sometimes due to periodic error it will switch to other side naturally - so you'll have more hits on one side of RA then on the other. This is actually a good sign - it means that your mount is responding to proper tracking issues. If hits were equally distributed - then I would say that maybe it is all due to seeing to be random like that. Maybe you don't want all hits to be on one side - but some imbalance is a good thing - so if frequency is higher in one quadrant - don't worry too much about it, but if all hits are mostly there - maybe increase aggressiveness and/or reduce min mo.

That is quadruplet scope so there is "rear" element that can be cause of reflection, but that is far fetched according to diagram on Askar website as it is very far away. Maybe try it with some other scope, but if you intend to primarily use it on Askar - then I'd consider returning it (or just seeing what sort of magnitude will make halo visible - weaker stars simply don't produce enough light after all those reflections to show halo).

Yes it is. It is of course filter related. I'm not sure however if we can say it is produced by filter. Using different gear and same filter - it might not be there. Here is a diagram of what I believe is happening: Blue line is surface of sensor. Black vertical line is filter and red vertical line is some optical element - probably field flattener / reducer that sits in front of filter (filter sits between that optical element and sensor). Black converging rays are incoming from the left and they hit filter. Some of the light is reflected of that filter - as it is reflection rather than absorption filter type. In above diagram it is represented by orange line. This light should go out in front of the scope - but as it hits rear of flattener / reducer - some of it (very small percentage, like less than 1% - depends on AR coatings applied) is reflected once more towards the filter. However this time light passes thru filter and ends up on sensor. How can this happen? Well - there are two things that can cause light to be reflected first time and to pass thru second time. - Filter is not blocking light 100%. This is normal, no filter is blocking light 100% out of band - but very small amount of it passes. In first contact very small amount of that light passes and is properly focused and majority of light is reflected. In second contact same thing happens - but this time light is out of focus and ends up in halo rather than in star core - and can be seen. - Second reason why light might come thru in second contact and not in first contacts is angle of incidence. Rear element of flattner is curved and will disperse light rays more. They won't hit filter at same angle as in first contact - and filter QE response curve depends on angle of incidence. Interference filters work the best when light is normal to filter surface, but when light is at an angle - it "sees" dielectric layers as being thicker then they really are (much like sloping armor has more stopping power than vertical one). And this changes dielectric layer thickness compared to wavelength of light - which in turn leads to CWL shift. In any case - we can say it is due to filter - but not solely fault of the filter. Filter by itself won't produce halo. If you remove field flattener and repeat the experiment - you probably won't see halo, although you'll still be using that filter. Put in different field flattener and halo might change or disappear. Even changing F/ratio of the scope (aperture mask) might cause effect to disappear or diminish. Another thing you might try is changing filter position / distance. If you can - if it is 2" filter - put it in front of the field flattener rather than between field flattener and sensor.

I guess it can certainly be used - but I'd use it only in cases where I would not worry too much about flex - meaning short exposures and "loose" guiding requirements - short focal length scopes and low resolution work / wide field.

Maybe image with single filter for longer than one exposure?

It can happen. In that case - image will be steady but you won't be able to focus properly. FWHM is measure of average displacement of star from its position and it is good mostly as indicator of seeing for long exposure. It tells you nothing of how "violent" that disturbance/displacement is. If such disturbance is bending image slowly - then we call that a good seeing (since it is small in magnitude and moves slowly). You can then freeze it in short exposure and have no blur (only mostly distortion) in single frame. If it is very high frequency motion - it will simply present as blur, as even at very short exposure, you won't be able to freeze it - point will move left/right/up/down multiple times although with small magnitude - but enough to cause blur that can be frozen (at least not with reasonable exposure time). FWHM can be though of as sound loudness, while seeing can be both low and high pitch sound. Low pitch is better for imaging and observing than high pitch sound - even if not it is not that loud.

It is usually due to lens cell being misaligned. Focuser being misaligned usually shows as tilt on focal plane - center being in focus and ok - and sides being defocused (one inside other outside).

Only lunar software that I've used is this one: https://ap-i.net/avl/en/start or source forge download page: https://sourceforge.net/projects/virtualmoon/ But I have no idea if it will identify features from images. I know that you can specify your images folder and maybe it will let you overlay images so you can manually identify features (or maybe it even does it automatically - but I have no idea about it).

Sky Watcher AZ4 is an option. It can handle 6" newtonian very well - there is only slight issue near zenith as longer OTA can hit tripod leg and then you need to rotate legs depending where scope is pointing. Another option is SkyTee2. That is much more heavy weight alt az mount - and it comes with slow motion controls (above AZ4 does not). As a bonus - you can mount two scopes to it side by side (not sure if that will be interesting to you).

This is actually another thing to consider. Quite true - in daytime pupils contract and there is less curvature of the lens that can be "wrong" so prescription indeed changes - which can shift focus range of eye alone (but it still does not let you focus to infinity - it won't correct your eyesight completely).

To be honest, I have no idea. Much of it is related to medical terms like diopter / dioptre https://en.wikipedia.org/wiki/Dioptre Ideally, eye should be able to focus on range of distances by itself - form infinite focus down to say arms length (although my arms appear to be shorter as I get older ). I can draw you a diagram of what geometry of rays look like in this case: Left is object at infinity and right is object closer. When object is at infinity - this simply means that lines are parallel and that they eventually come to same point - but at infinity. You can't focus when rays are like in left part of this diagram without corrective optics (glasses / contact lens). You however have no problem focusing on something that is closer even without glasses (there should be a distance where you can read without any issues even if you don't wear glasses at that moment). This means that your eyes also have range of focus - but that range is shirted compared to person that does not need glasses. They can focus from infinity to some distance, and you can focus from some distance closer than infinity to some distance closer than that other person. Ok, back to scope. Scope focused at infinity for person that does not need glasses has following ray diagram: Both exit and entrance beam have parallel rays - as if both images are at infinity (real and projected image). When you are wearing glasses - you can also observe in above configuration, but when you remove glasses, you must have following configuration in order to be able to observe: This happens when eyepiece is moved forward to focus point - exit rays are no longer parallel and it looks like object is closer. Most people have to strain their eye to be able to look like that - but this comes naturally to you as you are nearsighted. In relaxed state our eye is tuned for far and we need to work with our muscles to focus to near (this is why there is old age farsightedness - lens gets less elastic and muscles weaker and can't bend it into shape to focus real close). When observing local objects - above ray diagrams are just a tad different People that don't need glasses usually focus like this: Or maybe with slightly diverging outgoing rays. That depends on how their eyes are tuned and how they observe - if you look at target without using spotter scope and then switch to spotter scope - I think that eye never really relaxes like in night time when you don't have reference - it is always tuned to some distance. Similarly - when you observe - telescope is focused like this: Now when you remove glasses - rays are not parallel, and but are already a bit bent. Now you can refocus with your eyesight as it does not require you to do something that is physically impossible for your eyes - to "over relax" in order to achieve infinity focus. This is "conceptually what is happening - but actual angles and when you can refocus with your eyes and can't - that bit I don't know. It depends on distance to object, focal length of telescope and eyepiece - your diopter (and range of distances you can focus to) and so on, but should not be hard to find the answer as long as you apply basic geometric optics formulate (see thin lens formula) and figure out what diopter really means in term of lens power.

That is on filter (my guess). Use calculator to determine distance of dust from sensor to give you idea what surface it sits on. https://astronomy.tools/calculators/dust_reflection_calculator

It has to do with the fact that you are using refractor for close objects and not because there is some difference between refractor and reflector in focusing - there is none. There is something called depth of focus and there is also ability of our eye to accommodate different focus positions. Fact that you are near sighted means that you can't "relax" eye enough for infinity focus and you need to compensate with glasses (or refocus when you remove glasses). When focusing on close object - people with normal vision when relaxed will focus as if object is at infinity - meaning exit rays will be parallel. Given that you are near sighted - for you relaxed observing is when exit beam is diverging. When looking at close object - you will set focus position differently then someone without nearsightedness, and when you take glasses of - you eye will be able to accommodate change in focus without you realizing (like when someone with good vision switch from near to far object - their brain refocuses without them really doing anything or noticing that). Bottom line - you will always have different focus position then person that uses no glasses (has no nearsightedness) but your eye can't compensate at infinity focus and can compensate for near focus, while people without nearsightedness have range of compensation at both points - at infinity and at close focus (younger people more so than older because eye lens hardens with age and it is harder to refocus with muscles).

I'm a bit confused by this. I can't really understand what sort of image your scope is presenting when in focus and did you view star image defocused? Maybe by mistake you said Airy disk when in fact you meant out of focus disk? Is this what you are seeing? either side of focus concentric rings are not quite concentric and in focus image like you say - fans out to one side? In any case - it could be thermal issue. By thermal - I don't mean what we usually think of when we say thermal - like scope not cooled down properly and seeing influence of circulating air inside scope - nor local thermal issues like local seeing. By thermal I mean telescope tube and lens cell expansion / contraction issues. If cell is made out of two different types of metal - they can contract or expand at different rate. This means that they will be of different size while scope reaches ambient temperature and this can introduce tilt of some sorts. Such effect would be only visible while scope is cooling to ambient temperature - and would present as miscollimation (not wavy like regular tube currents).

Yes, that is one example where going with lower temperature can show actual benefit. Exposure is long enough for dark current to build up to significant level and filter removes LP so LP noise is not most significant factor any more. If you can, in that particular case cool down more.

It does not have much to do with being fussy - there is really a point where going cooler simply does not matter as much. That depends on use case though, and is different for different setups and conditions. Dark current doubles roughly at every 6C difference. If you are using -5 and someone is using -20, then that is (let's round up) three times doubling, or x8 more dark current. Now, by itself dark current is no biggie - it is dark current noise that is sometimes issue and it is equal to square root of dark current, so we increase dark current noise by factor of about x2.83. If for exposure time you are using, read noise is x10 larger than dark current noise at -20C, then you won't see much if any difference if you cool to only -5C as read noise will swamp dark current noise by factor of x3 and a bit. This is also related to LP noise. One should aim for exposures that swamp read noise with LP noise. Even if at -5C dark current noise is comparable to read noise - it will also be swamped by LP noise. According to this: -5C has dark current of about 0.012 or something like that? In order for dark current noise to be the same as read noise (around 2e) - dark current must be 4e and exposure time therefore around 4/0.012 = 333.333s If you are swamping read noise with LP noise with exposures below 5 minutes - yes, with this camera model, you can even cool to -5C, it won't make significant difference (yes, there will be improvement by going cooler, but barely detectable from measurement and certainly imperceptible by eye).