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Yes, that is why all of the above is relevant. - if using mono camera on less than well corrected scope - you will still shoot all the wavelengths at the same time - much like OSC is doing and some of those wavelengths will be out of focus - creating blur. This is why you are limited to LRGB type of imaging (LRGB filters with mono, but using only R, G and B filters - each with their focus position). - even if you do that - there is a chance that one channel will be not be well corrected if scope is not well corrected. Usually that is blue channel as it contains shortest wavelengths - those are bent the most by refraction. Then there is spherochromatism - or fact that not all wavelengths of light have good spherical correction. You can identify such situation from following graph: This is ED scope 150 F/8 - but it shows what needs to be seen nicely. On X axis - we plot defocus - or how much you have to move focal plane for particular point to be in focus. On Y axis - we have distance from optical center to edge of the lens. If you look above graph for say yellow line (620nm wavelength), at very bottom of the graph (center of lens) you will see that it focuses just a bit further away than what we have decided is optimal focus - but as it moves away from optical center - it defocuses more, but then at say 90% towards the edge of the lens, beams start to focus shorter than that and even focus shorter than our designated focus position. When focal length of lens depends on distance from the center - that is what we call spherical aberration. Here is graph from Wiki page on spherical: In perfect lens - upper one, all rays intersect at the same point - have same focal length, but in bottom image - rays that are further away from center - have shorter focal length (intersect sooner). Looking at top graph that shows ED lens performance - we can see that if line is straight - it has no spherical aberration. Further -if it sits on X=0 - there is no chromatism for that wavelength - it has exact focal length. Chromatism or secondary spectrum is when focal length depends on wavelength of light. In any case - in above graph we can see that no wavelength is perfect - they are all bent and they are all some distance way from X=0 at some height (or even the whole time). But look at how bent the green line is and how bent the blue line is. Green is 500nm (green light) and blue is 436nm - or blue towards the violet. Blue line is much more bent than green - which means it has more spherical aberration. In the end - here is another graph: This graph shows difference between single lens, doublet lens, triplet lens and superachromat. Each of these lenses brings progressively more wavelengths into same focus. Simple lens / singlet will have any one wavelength at the focus at any time. Doublet will bring two wavelengths in focus at the same time, triplet will bring three and super achromat will bring four. Each curve is progressively closer to true focus - which means less defocused wavelengths and less false color / secondary spectrum. However - even triplet (orange line in above graph) - won't bring all colors into focus and due to shape of the curve - some wavelengths will be more defocused than the others. Look at defocus at 400nm versus 700nm. Curve shows much bigger distance from 0 on X axis at 400nm (this time Y axis shows wavelength rather than distance from center of the lens). But that is not important bit - what is important is range of defocus for each of R, G and B sections. If we look at 400-500nm range we can see that defocus ranges from -4 to ~1.5. that is 5.5 arbitrary units of focus range. 500-600nm or green part will have from ~1.5 to ~0.5 and that is 1 arbitrary unit of range and red will have from ~0.5 to ~ -0.5 or again 1 arbitrary unit of range. Depending on critical focus zone of the telescope - you might be able to adjust focus for green color 500-600nm range and red color 600-700nm range so that whole parts of spectrum seem like in focus (that one arbitrary unit), but if critical focus is say 3 arbitrary units wide - then you simply won't be able to have whole 400-500nm range in focus as it is 5.5 arbitrary units wide. By the way - optical designer has freedom to tilt that S curve somewhat and this is what we call correction - wider focus range can be put in red part of spectrum - such scope is blue corrected or in blue part of spectrum - so we call it red corrected. Most of the time optical designers opt for red correction - which leaves blue side somewhat softer.

If you shoot luminance you still use the whole spectrum, so it really needs to be RGB (rather than LRGB) and even then, there are reasons why blue might be softer than other colors. One reason is spherochromatism, other is how the scope is optimized and third reason is that atmosphere impacts shorter wavelengths the most. In reality - I think that all of these three reasons combine with different contributions to make blue channel softer than the rest. This is why I sometimes say that probably best way to do true color images is to do LRG imaging.

Hi and welcome to SGL. With that sort of budget - I think that most sensible option would be this one: https://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p-ds-ota.html together with this https://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html Alternative would be to get ~70mm ED doublet refractor with ~400mm of focal length with matching field flattener. Something like SkyWatcher 72ED for example. There will be some differences between two setups: - Newtonian will be more "all around" scope - meaning that it will give you better field of view on most targets, refractor is more a bit wider field instrument that will render most galaxies as very small - Newtonian is a bit harder to setup as it requires maintenance - like collimation (sometimes - that really depends on how often you move the scope and how you handle it), and arguably it is a bit harder to get spacing for coma corrector right - Newtonian will also produce diffraction spikes - so that is something you may or may not like. This model also sometimes shows issues from protruding focuser tube and mirror clips - but there are solutions for that - like 3d printing mirror mask and shortening focuser tube a bit. - with added focal length - you will likely want to guide sooner with newtonian than with refractor - which is additional cost (guide camera, guide scope and computer).

That's something I haven't seen before.

Just a tad confused here ... MC is color version, but used with RGB filters?

In principle yes, but there is clever way of stacking that negates this advantage. It is implemented in AS!3 - it is called Bayer drizzle. While regular drizzling requires very specific conditions to be effective - Bayer drizzle works almost always with lucky type planetary imaging. It requires image to constantly move (be dithered) - which happens in lucky imaging due to atmosphere. There is no need to "artificially reduce pixel size" - as pixels are already at the size they ought to be (same size as with mono camera) and so on. With this approach color cameras are much more effective in general color planetary imaging - there is no need to do separate runs for each filter and to do filter change and refocusing and all of that. However, for some types of planetary astrophotography - mono is better choice - like Solar Ha or calcium line, UV or infrared, or lunar with different NB filters.

Nope.

Then, there is this thing: https://www.firstlightoptics.com/equatorial-astronomy-mounts/sky-watcher-eq-al55i-pro-go-to-astronomy-mount.html (not quite up to HEQ5 level and to be honest, not even sure how it compares to EQ5, but it is capable of going all the way from 0 to 90).

I missed that part. Was it the use of $ that gave it away? I can imagine number of people expressing value in $ while not being in US.

If we are talking that sort of money (and that is over OP stated budget for setup) - I would personally choose this scope: https://www.firstlightoptics.com/stellamira-telescopes/stellamira-110mm-ed-f6-refractor-telescope.html over both F/7 4" Starfield and F/11 4" ED - It is easier to mount than ED - has more light grasp than 4" F/7 - will offer widest field of view than both - has very good CA correction (not as good as either of those scopes - but scope has potential to go all the way up to 0.997 Strehl in green part of spectrum)

I think that it is still very good buy in it's price range. FLO currently sells it for £194 - together with 2" diagonal, x2 eyepieces and 30mm finder. 4" F/7 ED doublet like StarField goes for £899 - that is more than x4 as expensive. If one has the budget - then sure, ED doublet is the way to go, but I really don't think anyone can complain at 4" F/10 achromat at current price.

I agree about ST / short versions. It's not only CA - there is quite a bit of spherical aberration (spherochromatism). My ST102 showed fuzzy blob on planets when used as is. However - I would not say that F/10 version has plenty of CA. CA index of F/10 4" achromat is about 2.5 - which is in "filterable" range, and close to Sidgwick criteria of CA index being >=3. Interesting thing about F/10 4" achromat is that you can easily turn it into CA index 5 instrument that is virtually color free - by simply putting aperture mask on it. I actually managed to get rather good (color free) image of Saturn with ST102 - by placing 50mm aperture mask in front of objective cell - that created 50mm F/10 achromat that has CA index of ~5. Of course - resolution suffers because of decreased aperture but you can see where you have sweet spot - the least level of CA versus the best level of detail with aperture size (I use my 4" F/10 unfiltered and without mask because level of detail is best that way even if there is some level of CA present).

For Mak to win over refractor on planets, it really needs a bit more aperture. That is, if both instruments are fairly decent in optical quality. I have both 4" F/10 achromat and 4" F/13 Maksutov, and to my surprise - I could tell the difference. In one head to head comparison on planets, frac gave slightly better views. I don't know the cause of that - was it down to thermals (although there was enough time to cool down for both instruments) - or if diagonal was to blame - I used different diagonals - both GSO 99% dielectric ones - but 2" version with frac and 1.25" version with Mak. Since then - refractor gave me even better views, but although Mak gave me nice views - I was never really impressed with what I saw (unlike refractor on one particular night). I guess that above is sample of one and that conditions were not controlled, so should only be viewed as anecdotal evidence, however, physics also slightly favors refractor for visual, even if it has some residual CA (and F/10 4" has some obviously, but it was not really that obtrusive to my eyes - I even preferred unfiltered view versus Baader Contrast booster). To match (or even surpass) 4" frac in planetary view - 5" Mak is needed. Then it comes down to other things - like this one: 4" F/10 refractor is capable of showing x3 wider field of view over 5" Maksutov.

It is worth noting that LiFePo4 batteries are much much safer to handle than regular lithium ion batteries - so they are good choice where premium power density is not required (they have slightly lower power density than lithium ion batteries - while they might not be the best choice for say a drone, they are perfectly fine for this application among others - like electric cars and such).

Check to see if your device is on the list of compatible devices: https://letsencrypt.org/docs/certificate-compatibility/ FLO website uses Let's encrypt certificate that is signed by root certificate authority ISRG Root X1 If device does not trust that root certificate (meaning that developers of the platform omitted that Root CA for some reason - maybe lack of support and upgrade on part of developers or whatever) - you will get above message.

https://www.firstlightoptics.com/evostar/sky-watcher-evostar-102-ota.html + https://www.firstlightoptics.com/alt-azimuth-astronomy-mounts/skywatcher-skytee-2-alt-azimuth-mount.html or https://www.firstlightoptics.com/equatorial-astronomy-mounts/skywatcher-eq5-deluxe.html depending on whether you prefer EQ or AZ type mount With rest of the budget - get some additional eyepieces ...

Since I moved to the country side - I often get to see very prominent rainbows. Here is one image I took with my phone that shows both first and second order diffraction spectrum:

I would argue that no mount needs that high guide rate. What is wrong with guiding at x0.25 for example? Just to put things into perspective - even if you guide at 0.5 second intervals (which is very very fast guide cycle and probably unnecessary) - you can still correct for 1.875" of error in single correction - that is almost two arc seconds of drift or whatever needs to be corrected. As you see - there is plenty of "corrective power" in x0.25, but such slower guide speed is much easier on the mount and your setup. From physics we know that F=ma, or force is equal to mass times acceleration. On the other hand, acceleration is change in speed per unit time. If your pulse is x0.9 of sidereal - it is very high speed change and very short pulse - that means much more acceleration (or deceleration in case of RA axis and correction in different direction than motion of the mount) which causes jerk on your setup and introduces oscillations that need to dampen down (and in general cause issues with FWHM of your stars).

You are quite right about being careful not to over expose in some cases, but in principle - it is never ok to underexpose. We should really say that one should always strive to do as much exposure as "sensible" or needed. In photometry that you mentioned - under exposing will produce less reliable results because SNR suffers. There is even technique to avoid over exposing star core and to still capture as much signal as possible. Star is defocused a bit so that star profile is no longer sharp in center but rather spread in doughnut. That allows for longer exposure and accumulation of signal and it avoids over exposed parts. Similarly - in planetary / lucky imaging. One can argue that subs are really under exposed - but they are not in general sense. They are not needlessly shorter than they need to be. In order to freeze the seeing one is indeed using very short exposures - like 5 ms or less - and that can look like very dark / under exposed single frame - but max exposure is governed by seeing and no one would go lower than is actually needed to get the sharp image. On the other hand - for say Lunar imaging - one might go lower than what is needed to freeze the seeing - but that is because signal is so strong (moon is very bright target) that shot noise swamps the read noise and shortening the sub duration does not have as much impact on final SNR, but shorter exposures allow for even more stable subs with respect to seeing.

No. In AP we use stacking - so many individual exposures. Rules that apply are: - SNR of stack depends on total integration time - more time you spend - better the image - Individual sub duration is primarily determined by level of read noise of your camera compared to other noise sources (I say primarily, because you might choose based on some other criteria like - ability of your mount to track precisely, storage space availability, likelihood of individual subs being ruined for your setup due to wind or some other effects ...) - Over exposed parts of the image (mostly star cores, but sometimes bright parts of targets as well - like galaxy cores or very bright nebula parts) are handled by using set of shorter exposures that capture data for these parts only and you blend those in in processing to replace over exposed parts in regular stack.

Problem is with the predictive power of the theories. If it walks like a duck and quacks like a duck - it is easy to think that it is then accurate description of underlying physical reality

You can image Jupiter with 3-4 minute imaging runs. If the planet drifts thru the FOV in 25 seconds - you'll need to move scope couple times to re acquire it. This will probably be hard at first - but I'm guessing that you'll get the hang of it. You can use special software to discard frames where the planet is outside the FOV and you are in process of putting it back on sensor. Alternative is to make DIY dob platform and have tracking for up to hour without need to manually move the scope.

Light pollution atlas 2022 also features click for more information when opened in OpenStreetMap I'm currently at 20.34 according to that data source (which I think is very much the case). It also provided me with new possible observing location that I must relay to my local astronomy buddies