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In general - noise in astronomical images is related to presence of signal. Strong signal usually is noise free - like galaxy core and surrounding parts. Weak signal like spiral arms and outer galaxy halo and background that is mostly signal free is source of highest noise - or rather noise that is most easily seen as there is no signal to "mask" it. For that reason, I advocate masked denoising. It is fairly easily done. In Gimp - create layer copy of your image and apply denoising on it. Then add layer mask and use image itself (intensity of pixels) as mask (inverted). This means - where signal is weak or image is dark - it will use denoised version, but where signal is strong it will use original version that is not blurred (denoising causes blur). Here is simple example of this principle in action (although it would work better on 32bit version instead on 8bit jpeg):

Start with this: and do this: See what happens? Multiple rounds of curves / levels can produce "cumulative curve" that looks like one I produced and that creates posterization.

Try not to stretch multiple times to avoid "posterization" in M110 and M32. There seems to be three distinct "rings" instead of smooth galaxy: There seems to be gradient present in your image - try to remove it. Also, try to get background neutral. In second version it has red appearance. Try to make it neutral gray. Denoising is your friend when you use it selectively.

Maybe you should think of it this way: If you have your secondary out of collimation - no stars will look good, but if your primary is out of collimation - your secondary can compensate for central part of field. This means that star in center will look ok and when defocused - you'll get concentric rings, but edge stars will be a mess. That is why procedure is as follows: 1. adjust primary so that stars have roughly the same FWHM in each corner (same level of defocus due to field curvature) 2. then adjust secondary so that you get concentric rings in the center of the field then go back to point number 1 (because messing with secondary will change edge stars again but less this time) Hope this helps.

You can do astrophotography with lens - but you can't really capture planetary images with that combination. Len's are really unsuited for planetary work as planetary work requires long focal length telescopes with sharp optics. Lens are not diffraction limited, they are good for wide field work where you don't need all the resolution, but for planets you want all the sharpness you can get and that really means a telescope. 3"-4" of aperture is really only starting to be enough for small amount of detail on planets. For more serious images - you need a lot of aperture to have resolving power to capture detail. Even 4" Maksutov (102mm one) will only capture images like these: And that requires special type of capture and processing - planetary camera with very fast frame rate - exposures of 5-6ms and a lot of them - like 10000 or more. In the end you stack only something like 5% of captured frames and you need to process that with different sharpening routines.

Well, no. Faster isn't always better - and for that matter, it is not always faster either. You won't be able to find any decent telescope at below F/2. Such speed simply requires complex lens arrangements and can only be found in lenses. Camera lens and fast scopes are not diffraction limited either and will not be good in travel scope role. Photographic speed is determined by aperture size and working resolution. That means combination of telescope, corrective element and camera. Say you want to work at 1.8"/px. In that case - I'd say get largest aperture that will give you 1.8"/px with chosen camera. This is with regards to imaging alone, but if you want to take that telescope on vacations - that adds constraints on your choice. Here are couple of examples: 1. Take ASI6200 with 3.76µm pixel size and 10" OOUK ODK 10" with focal length of 1700mm. Bin data x4 and you'll get working resolution of ~1.8"/px (1.82"/px to be precise). That is 10" of aperture providing you with 1.8"/px resolution This scope has 50mm imaging field and can easily illuminate full frame sensor Effective F/ratio - F/6.8 2. Take 6" f/5 newtonian and add best coma corrector - like TV VIP-2010 Paracorr and pair that with ASI2600 camera. Bin pixels x2 this time and you get again 1.8"/px Effective F/ratio - F/5.75 3. Take 80mm F/7 and pair it with ASI1600 and TS x0.79 reducer flattener. Again you have ~1.8"/px Effective F/ratio - F/5.53 In terms of F ratio - third option is "fastest", followed by second option and first option comes in last. However, in term of real speed - first option is clearly the fastest as it uses 10" of aperture vs 6" of aperture vs 3.15" of aperture - but they will all produce the same sampling rate image - target will be covered by same amount of pixels. More light divided by same number of pixels - means more light per pixel - higher signal - better SNR. Now, I would recommend you to think in terms of 1.8"/px - 2"/px imaging resolution and to get yourself nice affordable triplet refractor and pair it with nice camera and flattener reducer to get to your target sampling rate. iOptron mount you linked above should handle such scope and resolution with ease. Here is scope for you: https://www.altairastro.com/altair-wave-series-102mm-f7-super-ed-triplet-apo-102715-452-p.asp Add this reducer flattener: https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html And this camera: https://astronomy-imaging-camera.com/product/asi294mm-pro (comes in both mono or OSC version - so you have choice there). That combination will give you 1.78"/px working resolution. Alternatively, if you want a bit more oomph for your travels - look at this scope: https://www.altairastro.com/altair-starwave-115-f7-ed-triplet-refractor-9198-p.asp But due to focal length, your working resolution will be a bit lower with above FF/FR and camera - 1.58"/px

Star adventurer will do the job, but if you want to get anything decent out of it - think of putting a small scope on it rather than using lens and using guiding / planetary camera instead of DSLR. Even something like this will be miles ahead of simple lens: https://www.firstlightoptics.com/maksutov/skywatcher-skymax-90-ota.html

If you work at same sampling rate with and without focal reducer - in principle you will see no difference at all as far as detail and time taken is concerned. Only difference will be in size of FOV. Say you work with ASI1600 natively at 2000mm - for 0.39"/px and of course since that is hugely over sampling - decide to bin x3 your result for ~1.18"/px. Adding focal reducer will get you to ~1340mm and that means ~0.58"/px. Again you are oversampling so you can bin x2 this time - and you'll end up with ~1.17"/px. There you go, in both cases you'll be around 1.18"/px with 8" of aperture - same "speed" in both cases. If reducer is optically good - there will be no difference in the detail as you did not change - your aperture, nor your mount nor your skies. All three important ingredients will stay the same regardless of your use of focal reducer.

ASI1600 can do 0.39"/px just fine. Question is what are you loosing by doing so? Let me show you something. This above is image of M51 taken with 8" RC and Atik 314+ at 0.82"/px (taken from here: http://www.astrosurf.com/topic/134977-m51-au-rc8/ ). On that image I superimposed image that matches 0.82"/px with full resolution. You can clearly see that 0.82"/px image taken with RC is not as sharp / detailed as image that matches 0.82"/px. 0.82"/px is simply wasted on level of detail produced with said scope and mount. Now have a look at same thing but this time at 1.6"/px (I just binned both images x2 and made same flip animation): Now there is virtually no difference in the detail in galaxy except that one image has tighter stars. This means that actual resolution of that image is closer to 1.6"/px and definitively not as low as 0.82"/px You want to image at twice as high resolution at 0.4"/px - and that is fine except for two things: 1. You simply won't record detail to match that resolution 2. You will be wasting your imaging time since you need to achieve certain SNR and going higher in resolution means that you are spreading light over more pixels and each pixel receives less signal. SNR will drop. For each doubling in resolution (like using 0.8/px instead of 1.6"/px) you need to quadruple imaging time to reach the same SNR. If you go for 0.4"/px over say 1.6"/px - you'll need to image for x16 as long in order to achieve same SNR (for same aperture size of course).

Probably best thing to do is to try to understand limits of DSO imaging in terms of achieved sharpness and detail. Many people have this idea that they want to "get in close" to galaxies. They want big image of small galaxy. In most cases - you can just take small image of the same galaxy and enlarge it in software - you'll get the same result as spending money and time in trying to image it. There is a limit to what can be achieved and that limit depends on several factors - seeing, mount performance and optics. In most cases, that limit is at about 1.5"/px. If you have larger scope and very good mount - it can be as low as 1.2"-1.3". In ideal conditions you might be able to reach 1"/px - but that happens in like 1% of the time - only with best mounts and large aperture scopes. You have 533 camera that has 3.75µm pixel size. Your baseline focal length is about 520mm. With that focal length you have 1.5"/px sampling rate. You can double that focal length or triple it - then you'll bin your data x2 or x3 to recover lost SNR and you'll simply reduce your FOV. Having said all of the above, if you want good and affordable telescope and you don't shy away from mirrored systems - take a look at this scope: https://www.firstlightoptics.com/ts-telescopes/ts-photon-6-f4-advanced-newtonian-telescope-with-metal-tube.html It will provide you with plenty of aperture and 600mm of focal length (somewhat longer but still within limits - 1.3"/px sampling rate). It will be a bit fiddly to get it going - collimation of such scope is not easy, you need to purchase good coma corrector for it as well, and you need to shield from stray light to avoid light leaks (look at back of the scope - you'll need to put some sort of shielding over mirror as it has holes around it so light can leak in - similarly add a dew shield type extension to ota at front side).

I think it is still relevant to this thread since I gave that advice in context of the thread and I'm happy to expand on it. Histogram as guide is inherited from daytime photography where it ensures that there is proper exposure. In context of planetary imaging, you are right about that, it can serve to ensure that there is no over exposure, however, I have couple of things to note and couple of objections to that approach. 1. If we are utilizing lucky type imaging properly, it is very unlikely that planetary imaging will produce saturated pixels. Solar and lunar can, but Jupiter and Saturn are almost impossible to saturate pixels if approached properly. 2. Over exposure is bad even if we can't see it in histogram. Due to nature of light and Poisson distribution - we always have varying level of signal in the image. Even if most pixels are not saturated - some can be (say signal is 95% - some pixels will shoot above 100% intensity and will be clipped). 3. In light of point number two - there are other ways to warn about saturation. If I'm not mistaken, SharpCap has this - you can turn it on and it will mark saturated pixels in preview. It can also warn if any of the pixels in image are saturated. This is better indicator than histogram alone. When we have saturation out of the way - then we can deal with "70% histogram recommendation". This is really not a good recommendation when we utilize stacking. We can image planet in 8bit mode or in 16bit mode. In these two modes - 70% of maximum is different number of photons, so which one is right? How long do we need to expose for to get it? Exposure length should be set with regards to seeing as to avoid motion blur of atmosphere. Each sub will be distorted by seeing and each distortion will be different. We want to get to a point where two consecutive subs are almost the same - to expose so that we don't give seeing a chance to "combine" two distortions, as such combination of distortions is much harder to deal with than single distortion. In turn this gives us much more frames to choose from and select ones that have "good" kind of distortion. Now we get to stacking part. Here read noise is important factor. It is only thing that causes distinction between 1000 x 1s stack vs 1 x 1000s in terms of SNR. If we had zero read noise camera - these two would be the same in terms of SNR. Since in lucky imaging we go for very short exposures - we want read noise to be low in order to get best possible result - one that is closest to single exposure in terms of SNR. Gain can alter amount of read noise and therefore we should select gain as to have the least amount of read noise - and not to make our image "brighter" - which does not really mean anything in context of stacking. In the end, that is rationale behind - "forget histogram rule" recommendation. It is used to avoid saturation and to get "correct exposure". However, there are other tools to avoid saturation and saturation is often not even an issue, exposure length is dictated by seeing and aperture size (coherence length and time) and gain should be utilized to select lowest read noise (which does not need to be highest gain settings - some cameras have lowest read noise at different gain setting - there is graph for each model and it's worth experimenting to find best gain setting for particular camera). Aiming for certain histogram value will just cause us to select either exposure length or gain that is not optimal for lucky imaging.

What would be intended usage? In any case - I think that selecting telescope based on focal length is not very good way to do it. If you are interested in planetary - then look for largest aperture that you can mount and afford. Here correction of the field of telescope is not important as you'll be using only central part of the field. F/ratio and focal length are not important - you'll be adjusting those with use of barlow lens depending on pixel size of your camera. If you are interested in DSO - I suggest you set realistically your working resolution first. Do you want to image at 1.5"/px, 1.3"/px or perhaps try 1.2"/px and do you have the mount and the skies for it? Then look at your cameras and see what sort of focal lengths will give you your target sampling rate - either natively or with some way of binning. In the end - go with largest aperture for selected focal length that will give you corrected field large enough to cover your sensor (that you can mount and afford of course).

Not quite, there are a lot of variables in play here. There are also different types of contrast to be discussed. Planetary type contrast when there is enough light and we are talking about small features on planets are one type of contrast. There is other type of contrast - in extended objects. First one has to do with aperture size, seeing and central obstruction. DSO contrast depends on other things. Here refractors do have edge - but not all refractors. Achromatic refractors lower contrast because of defocused light. Good apo (visual) will have edge there. Mirrored systems have less light throughput than would be suggested by their F/ratio (central obstruction and mirror reflectivity). Mirror surface can scatter more light around - that reduces contrast. Newtonians are often poorly baffled scopes. For example it is recommended that tube extends above focuser about x1.5 tube diameter. For 200mm scope this means at least 300mm above focuser position on the tube - and this is just to prevent stray light hitting area behind secondary and causing contrast issues. Refractors are well baffled by design - they have dew shields that also shield objective lens from stray light and they often have baffles inside OTA to improve contrast. Back to original topic - could you fit M31 in FOV with large newtonian? Could you take your large newtonian in one hand and mount in other? Could you mount Herchel wedge on large newtonian to do some white light solar observing? Maybe put camera on the other end and take some images with it? Add quark and do Ha solar observing? For some tasks - large dob is better, but for other tasks - well, small apo is simply more sensible solution.

That TS scope is just branded by TS. It is also available in other brands like Altair Astro. https://www.altairastro.com/altair-wave-series-125-edf-f78-apo-binoviewer-ready-37-inch-rp-focuser-454-p.asp If you intend to observe mostly seated - then newtonian type scope is feasible only on dobsonian mount. Targets like M31 - Andromeda galaxy are wide field targets and are best observed with short focal length - up to say 600-700mm. Planets on the other hand like aperture and also focal length. You'll really have to choose between refractor and folded design like SCT or Cassegrain. Maksutovs probably won't suit you. Compact scopes like SCT or Cass will have more aperture and be better at planetary - but won't allow you to get wide field views. Refractors will be good on planets - but less aperture for DSOs. Will allow for wider field of view. Maybe @johninderby can help here because he owns two of mentioned scopes - 125mm frac and also 8" Classical Cassegrain.

Not sure what to answer here. Yes, there are several possible routes for you to take. 1. Get different type of scope that you'll be using on HEQ5 - no need to go all Takahashi on that - scope like this will be excellent and cost effective choice: https://www.teleskop-express.de/shop/product_info.php/info/p10133_TS-Optics-Doublet-SD-APO-125mm-f-7-8---FPL-53---Lanthan-objective.html, or maybe this one: https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-8-f12-m-lrs-classical-cassegrain-telescope-ota.html depending on your interests 2. Get another mount like SkyTee2 to use with 200PDS - or as you suggested, build a dob mount for it. If you want goto capability - maybe exchange HEQ5 for AZEQ5 or AZEQ6 3. Sell all that and simply get 8" F/6 dob In order to give meaningful advice - you need to tell us more about your expectations, targets, way of observing (seated, standing, tracked, manual), portability issues, etc ...

I've heard that those FPL53 masks provide better COVID correction than FPL51 ones

Level of periodic error is one side of story. Smoothness of it is another. If whole 10" error is covered in said 20s - then it will create 7px+ trailing in that image. If we assume that worm period is say 8 minutes - that is about 500s, and that PE is uniform - in that period 10" is covered twice ("up and down"), so 10" for 250 seconds - that makes something like 0.04"/s of drift, or in 20s exposure - 0.8" or less than half pixel. If that is true nature of the PE - well, one could use much longer exposure, but question is - is mount up to specs and is 10" P2P error condition met (SkyWatcher mounts often have 35"+ P2P error), and what is the shape of error - it does not need to be sine wave - it can be multiple ripples and still be less than 10" in amplitude. I think that it is at least hour. Each up down wobble (whole cycle) is about 10 minutes as worm period on Heq5 is 638s. I think that video has at least 4-5 cycles or so.

Here is small animation I made from one unguided session. This is Heq5: Steady drift to the left is due to PA (in DEC axis), up down periodic motion is of course in RA due to PE. You really need to mess up polar alignment in order to get trailing in short exposures like 20s. According to this: http://celestialwonders.com/tools/driftRateCalc.html if your PA is half a degree out (width of a moon in the sky), you'll get two pixels of elongation in 20s exposure (at DEC 0°): Canon550D with 4.3µm pixel size and 130PDS with 650mm of FL gives 1.36"/px

That really depends on periodic error of the mount. I've found that in most cases - Periodic error is causing larger drift than polar alignment.

Tried once CCD47 with RC8" and did not like what I saw. I need to try it few times more, it could be down to improper spacing or tilt / collimation issues. I'm also interested to hear how x0.75 Astro Essentials reducer works. There is one FF/FR that seems to work very well and reduces to x0.75 - but it costs pretty penny - Riccardi FF/FR x0.75 (m63 version for example): https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

If you don't guide - well, start guiding. Mounts have imperfections in gears and that results in something called periodic error. Sometimes mount tracks a bit faster and sometimes a bit slower (think ellipse instead of perfect circle). It happens on a period of revolution of out of shape gear - hence the name periodic error. You can try periodic error correction if your mount is capable of it (either in hand controller or if you connect it to computer), but that is limited in scope of what it can do. It can minimize the error - but not remove it completely in most cases. Solution is to start guiding. If you are already guiding - well, you are either not guiding good (wrong parameters) or there is differential flexure between guide and imaging scope. Former can be addressed by setting proper guide parameters while latter requires better connection between components.

I've had small level of such ability. It was not actual control of pupil size, but I could rather voluntarily change focus position and given certain lighting conditions that would change pupil size. Haven't tried that lately. I also had very strange condition happen to me sometimes. After night of heavy alcohol use - next day I would have pupils of different size! Now that is strange to see in mirror reflection. I asked my sister who studied medicine about it and she said it can happen in some neurological conditions. In my case, I believe, it was due to the fact that I don't see particularly good at one eye.

Eye dilation drops maybe - these are used when doing serious eyes exam.

Indeed, I have 85mm T1.5 which is cine version of 85mm F/1.4 De-clicked aperture ring that is a bit light for my liking, but I've found that front aperture mask is better solution for stopping down the lens as blades produce diffraction spikes on stars.

That is a circular ring that those cables are attached to? Seems to fit the orientation in the image as it is at 45°. What is the height difference between camera rear and dew shield? Tilted circle is ellipse.