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Here are some interesting points regarding exit pupil: - telescope can't make extended objects brighter, only dimmer. The brightest view is with our naked eye. Exit pupil matched to our "entrance" pupil will give the same brightness of extended object as when viewed with naked eye. Smaller exit pupils will dim the object brightness for extended objects. - Sky is also extended object. Contrast ratio, or ratio of target brightness to sky brightness can't be changed by a telescope. It is always the same. Only way to change this is by use of special filters (for example UHC filter if target is emission nebula). - Human vision is complex topic. Perceived contrast ratio depend on amount of light. It also depends on angular features of target - look at this image: Frequency changes from left to right and contrast from on Y axis. It shows that not all frequency components are perceived the same although they have the same physical contrast (ratio of dark and light). This even changes with amount of light and our night vision adoption. This is why we can "dial" in magnification for particular target and why larger scopes show faint fuzzies more easily - it is because of this combination of absolute levels of light + magnification. On physical level - small scope with exit pupil of 5mm delivers same amount of surface brightness as large scope with 5mm exit pupil.

5 at the moment. 8" F/6 Dob, 4" F/10 Achromatic refractor, 80mm F/6 APO triplet, 4" F/13 Maksutov, 8" F/8 RC Well, 5 1/4 I have 80mm F/7.5 achromat being lens in a cell at the moment and trying to grow into full fledged OTA (waiting for me to get aluminum tubing and to 3d print a focuser for it).

Maybe you should try it first with ~40mm 2" eyepiece and a good 2" diagonal? But you really want to wait for autumn for that target. It can be very high in the sky - near zenith (depending on your location). For best results - wait for it to be near zenith and also check this website: https://atmosphere.copernicus.eu/charts/packages/cams/products/aerosol-forecasts This gives you forecast of transparency - you want "white" to be over your observing location for the evening you plan to observe. That is for best results of course. Observing when dark gray is overhead is like observing with 8" telescope instead of 9" telescope - you loose one whole inch (going into yellow territory is even worse and can have effect of loosing 2" or more of aperture size!).

I think that difference would be small if any, so you can simply bin the masters for simplicity.

Very nice, but I think you have much more data in the image than it shows. You could bin the data as it is over sampled to help with SNR, but that is a side note - look at this bit: That is Holmberg IX - irregular dwarf galaxy that is satellite galaxy of M81 - it is there in the data but can't be readily noticed in the image the way it's been processed.

It has a downside though. Such scope has to have really big secondary to avoid vignetting on 2" eyepieces. If you do a little search, I'm sure you'll find someone posting a flat field of their 130PDS with APS-C sensor - and it will show vignetting even on 28mm diagonal.

Maybe this will help? https://all3dp.com/1/best-online-3d-printing-service-3d-print-services/ Most serious online 3d printing services will give you a quote once you upload 3d model and select printing parameters. @Ags If you end up not finding any suitable 3d printing service, I'll be happy to print what you need on my printer for you.

Something like this: https://www.firstlightoptics.com/stellamira-telescopes/stellamira-110mm-ed-f6-refractor-telescope.html (that's my "hope to have M31 and other wide field stuff" scope)

When observing at low power such as M31 requires - difference between 1/6th wave and 1/10th wave is non existent. Only very discerning observers will tell the difference on the nights of best seeing when observing planets at very high magnification.

I think that you'll benefit more from: - having a bit more experience in observing - choosing the right eyepiece - choosing the right night - choosing the right time of the year to observe M31 than changing the scope. I managed to see first dust lane of M31 in 8" scope from Bortle 8 location on particularly transparent night when M31 was at zenith.

You can also reduce CA fairly easily (and cheaply) by using aperture mask. That will trade off some imaging time for reduction in CA, but results can be rather impressive. I'd start with 80mm mask - maybe do a comparison session when the moon is out and you won't be imaging otherwise - find a bright hot star, that way you won't need much exposure time to reveal CA and compare with / without mask results. Even better - make several masks and compare them all - like 90mm , 80mm, 75mm

Well, I''m really not sure. I'm inclined to to recommend the smaller scope - 72ED one. It will simply be easier to use overall and the field of view difference is not that big anyway. I do have similar scopes on HEQ5, or rather one with short focal length - 80mm F/6 APO and one with very long focal length - RC8" F/8 scope, and they both work well and serve different purpose. RC8" is certainly not beginner scope and I had to modify the mount quite a bit to make it usable with such scope (belt mod, tuning, changed saddle plate, changed tripod for Berlebach planet and so on ...) Yes, go with 72ED and also get matching field flattener as you'll need it for APS-C sized sensor. https://forum.astronomija.org.rs/

Question of how "clearly" you can shoot the object is very complex. There are a lot of variables that come into play, and quality of optics is not at the top of the list. In fact - it is probably one of the last ones if we consider importance. When we are talking about clarity of deep sky objects / long exposure astrophotography - following things are more important than quality of the optics: - how bad atmospheric seeing is - what is the precision of the tracking / guiding of your mount after those two come aprerture size and quality of the optics. Nowadays, most, if not all telescopes come with very decent quality optics. It's not premium, but most do have diffraction limited optics. Without going into too much detail, with D3200, you should really keep your focal length at about 500mm to get the sharpest image at 100% zoom level. This is why I said to use either refractor of about 400mm or reflector at about 600mm (650 + x0.9 coma corrector gives 585mm). Here is handy tool to help you visualize what sort of image you'll get: https://astronomy.tools/calculators/field_of_view/ For example, let's look what M51 would look like imaged with these two different scopes: As you can see - yellow rectangle is field of view with refractor and red is field of view with reflector. In both images this galaxy looks small. This is because your camera has 6000x4000 pixels (give or take), and for astrophotography - it is not so much the FOV as something called sampling rate - how much sky is covered with single pixel. For your conditions and skill level - this figure should be around 1.5 - 2"/px - which means that every pixel should roughly cover 1.5 arc seconds x 1.5 arc seconds of the sky. Depending on conditions - even this might not look sharp enough (poor seeing, or poor tracking). Formula for calculating this value is pixel_size * 206.3 / focal_length so for D3200 with pixel size of 3.84um each scope will give: 3.84 * 206.3 / 420 = 1.89"/px for refractor 3.84 * 206.3 / 585 = 1.35"/px for reflector Now, given that M51 has apparent size of some 12' x 7' and if you want to frame it right you can add another 10' around it so overall it will be something like 22' x 17' and that is 1320" x 1020" - that is apparent size in arc seconds. When we map this to pixels, for refractor you get: 1320 / 1.89 = ~700px and 1020 / 1.89 = ~540px - so whole "interesting" image will be 700x540px and for reflector it will be 1320 / 1.35 = ~980px by 1020 / 1.35 = ~755ox - or a bit larger at 980x755ox I know that this looks small - but that is what you can realistically get to look sharp. In fact - most amateurs are going to be limited to about 1"/px. Sky simply does not allow for finer detail, so you'll be already very close to that limit with 130PDS and your camera with 1.35"/px I think that it is a good scope - but I would not recommend that to you. For imaging you want a stable platform. Larger scope is simply going to strain the mount more as it is heavier. It has larger cross section so wind will shake it more. It has almost the double the focal length - which translates into "empty' magnification with your camera (there are techniques to get around this, but are probably too advanced for beginner). If you really want a bit bigger scope - then get this one: https://teleskop.rs/reflektori-ota/117-150750-skywatcher-newton-tubus-sa-110-mikrofokuserom.html Just make sure you get PDS version of Skywatcher 150 newtonian and not regular, because PDS comes with dual speed focuser and larger secondary mirror and is aimed at astrphotography rather than visual. In any case - do keep in mind that you will likely want to start guiding at some point. Have a look at this: That is a "video" that I made with EQ mount (not sure which one, could have been HEQ5) - but it shows two important aspects of the EQ mount. First is polar alignment (PA for short) error and associated drift - in this video shown as drift from right to left, and second is periodic error (PE for short) of the mount - that is jumping up and down of the image. First one is caused by less then perfect polar alignment of the mount and second is inherent to the mount because of the way reduction works - it uses machined gears and those gears are not perfect in shape - they are a bit "egg" shaped due to manufacturing errors (very small errors but at telescope magnifications - it shows). Both of these limit your exposure time and if you expose for longer - you get star trails. You can even see some star trailing in the video as each frame was about 1 minute exposure with the mount. In order to avoid this and have round stars each exposure, and to be able to expose for longer - you really need to guide. Lack of guiding will really be the biggest source of blur in your images when you start in all but poorest seeing. I'm telling you that so you can choose between scopes - in essence, go for smaller scope that has shorter focal length and is lighter on the mount and save the money for guide camera and guide scope as you will need them soon enough. Yep I'm also active on our local astronomy forum

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.