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It will be the same. Guide sensor will be roughly (1mm give/take) the same distance from prism as will be imaging sensor. This is because the beam that would otherwise converge on imaging sensor is diverted at 90 degrees - and it will converge after the same distance, but this time on guide sensor (that 1mm give or take is due to field curvature and such, we are taking beam at extreme edge of the field). If prism holder is 31mm long, then yes, you'll have to put OAG slightly further away from imaging sensor than those 31mm to get focus on guide sensor. This means that you won't place guide camera at optimum distance for prism size and speed of your scope, but don't worry, it will still work even if it has a bit less light. It will be as if guider is operating at - F/5 or F/6. A bit less light then it would otherwise receive from F/4 scope, but still plenty of light to find guide stars.

That sounds like a good plan - put OAG close to sensor, just make sure you have enough space to bring your camera to focus. There are really only two things that you need to be careful with. First is length of prism stalk. You can't bring your guide camera closer to prism than this distance. This is also minimum distance from prism to your imaging sensor. Second thing is total distance between prism and guiding sensor. This is limited by speed of your scope if you don't want to loose light for OAG. Here is diagram to help you understand what is going on: In top image - prism is closer to sensor and can pick off whole cone of light. In bottom part of the diagram, prism is further away - and since light beam is wide - it only pick off part of it - effectively reflecting less light. This acts as aperture stop and can cause significant loss of light if you have fast system and move prism too far away. Critical distance is easy to calculate - F/ratio * size of prism. In this case it is F/4 * 8 = 32mm. With 32mm distance, only central point gets 100% illumination. You want a bit larger illuminated field so you need sensor to be 30mm or less. If you can't place camera close enough - don't sweat it, I use OAG at F/8 on my RC scope and still manage to get guide stars.

I think you won't have any problems with sensitivity, just remember - with OAG and fast scopes, you need to pay attention to distancing in order not to stop down aperture with prism itself. What OAG will you be using? Do you know the size of pick off prism? Let's say it is something like 8mm. At F/4, you need to be less than 30mm from prism to guide camera (and similarly to imaging camera as these two are about the same in distance) to fully exploit aperture. I guess that your imaging train will be something like CC -> OAG -> camera. If you need to dial in CC / sensor distance - put extension rings between CC and OAG and make OAG closer to imaging camera as there is already something like 17.5mm from T2 to sensor.

It is just method of debayering. Instead of interpolation, it just uses bayer matrix of 2x2 pixels of different color (RGGB for example) to produce single pixel with all three colors - R, G and B. It is available in DSS and I'm sure it is also available in other software. With DSS, you just need to select it: As you see - resulting image size is divided by two, so you'll end up with images that are 1504px x 1504px in size. Hence, resolution will be two times smaller as well - instead of 0.78"/px it will be 1.56"/px. In reality, because of Bayer matrix, you are sampling at twice lower rate than pixel size suggests, so this is convenient way to get actual sampling rate of color camera - which suits you better.

533mc pro is OSC camera, and if you use super pixel binning mode, you'll be effectively imaging at twice that - so 1.56"/px. That is reasonable sampling rate for 10" scope if your guiding is somewhere around 0.6-0.8" RMS. Main issue will be small FOV of course and your main targets will be planetary nebulae and smaller galaxies with that combination. In fact, it's not even that small: This is M13 - it fits nicely in FOV. So does M81: I would say - just go for it!

We can argue that exposure under the full moon is better than no exposure at all and hence all the discussion about avoiding the moon is moot point. However, this requires use of sophisticated stacking algorithm that can take into account different SNR of each subs (and no, I don't mean simple weighting available in PI ) - and such algorithm is not readily available. I personally don't stress about moon until it shows above horizon as that coincides with visible brightening of the sky (if moon is over about 50% of illumination). If you want to be more pedantic about all of that - get yourself SQM meter (directional one), and do measurements of your sky - with / without the moon, just before moonrise and just after moonset. This will also help you asses sky brightness N degrees away from full moon / or different phases (quarter moon and so on). As a rough guide - two magnitudes of difference in sky brightness is equal to roughly x6 exposure time for faint stuff, but you can use SNR calculator to get the idea what is equivalent imaging time under different conditions depending on your gear and target brightness.

That looks more like astigmatism to me than coma. What scope is that? Third one is astigmatism and if it is off axis - cross tends to be shifted to one side. Could be due to coma corrector or perhaps mirror is too much tightened in mirror cell. How many clips are there, 3 or 4?

That one is going to be tough to reproduce I have no idea what sort of effect that would produce.

Look at these stars - there is little point in the center of each of them - that is part of spectrum that is in focus. This is star from first image. Only difference is that you have wider "skirt" of the star because of UV and IR light that has not been filtered out. Filter out far parts of spectrum (reduces skirt) and stretch a bit more and you have the same star as above.

Like you, yourself noticed - it is biased towards Takahashi refractors. I don't mind that, Taks are certainly fine scopes (I have not looked thru one myself, but I have heard/read about them so many times), however, text contains quite a bit of half truths and even some things that are just plain wrong. Let's start from the quote you posted above. This is "half-truth". Apos, if properly executed, indeed produce better high power visual images - inch per inch of aperture, but as we have seen, things are not as clear cut when we take in consideration larger aperture. My 8" dob, although around 0.8 strehl, walks all over my 80mm APO that has strehl over 0.96 on planets. Small Apo simply can't deliver detail / resolution that 8" scope can. This is of course for visual. For photographic high power applications - obstructed scopes even have slight advantage over unobstructed. We can go over that in detail, but here is simple explanation: In planetary photography, we perform a step called frequency restoration (or rather, it is commonly called - sharpening, but proper term would be frequency restoration) - we restore below curve to be 1 or close to 1 on whole interval. Lower it is in the graph, and further to the right - harder it is to bring it back. Obstructed telescopes, have this curve lower than unobstructed in left part of the graph and at "higher" values - which is easier to restore and have edge on right side of the graph. One should not mix visual and photographic when it comes to sharpness and level of detail, as for example, this was taken with 4" telescope that costs ~ $200 (and has central obstruction): No 4" telescope in the world, Takahashi or not, will give you that level of detail at the eyepiece. Or to put it in more "scientific" terms - eye can't correct above curve, while computers can with appropriate mathematics involved. For dark backgrounds and quality of the view, I suggest you read this: https://www.scopereviews.com/best.html To get the idea of what Newtonian can deliver in comparison to premium APO refractor. When photographing deep sky objects - we are really not concerned about contrast, or quality of telescope. It is all about SNR. Quality of optics, comes 4th after - seeing, mount quality and aperture. Yes, sure. We all know that top planetary photographers all use high quality Takahashi 4"-5" refractors, right? So I started reading the article to see what is else written, and I found this: and this: and then I stopped reading further ...

Well, yes, that is the problem with lens that have chromatic aberration - always some wavelengths of light out of focus.

Have you tried Roddier analysis to confirm these results? It is rather easy to do - you need either artificial star, or you can do it on a real star in good seeing and planetary camera. Test consists of shooting defocused pattern both sides of focus and then analyzing it in software written for that purpose WinRoddier. Only difference to above will be that it tests system performance not the mirror itself (but secondary should not make much difference if it is good).

I'm sorry to say - that article is not very good.

I would say dither always if you can. Not going to go into math - but dithering reduces noise regardless of other things (dark calibration, etc ...), so it's worth doing always. I know it adds time and if it's that much time - maybe dither every other or every third frame. Btw, I also advise full/proper calibration (which means set point cooling, or in case it is not available - at least dark optimization in case camera has stable bias).

Skip that one as well. These compact reflectors - which are really catadioptric scopes (means they combine lens and mirrors) are usually very poorly executed design and views tend to be blurry and collimation nightmare. Just search google for Bird Jones telescopes to see what sort of problems people have with them (design itself is ok, but at those low prices - it simply can't be executed properly).

This might help you overcome "fear of the quark" Want to have both full disk views, medium magnification and high power Ha viewing, all in one package? Get quark combo. Quark combo is a bit different than regular quark in that it is: 1. a bit more expensive 2. does not have integrated barlow element (less things for more money - perfect , isn't it ) What is the catch? Combo quark has a bit wider blocking filter and allows full disk views up to 1800mm of focal length. However, you yourself need to provide means for it to operate on F/15 or higher. You have 805mm of FL natively, and let's say you want to do nice full disk views. Get x2 telecentric amplifier like this one: https://www.firstlightoptics.com/barlows/explore-scientific-2x-3x-5x-barlow-focal-extender-125.html With x2 telecentric, you'll be operating at 1610mm of FL - still within 1800mm, and you need to work at about F/15-F/20 - higher F should give you slight boost in contrast. How to make your scope be at F/20? Make 80mm aperture mask for example. Now you have 80mm F/20 full disk capable solar scope for the price of 50mm unit. Want to have higher power still? Add x3 telecentric and use it at full aperture. That will also be F/20 but you won't be able to do full disk any more. In any case, you get the point, combo quark let's you dial in your own F/ratio and focal length, and can be used with small / cheap grab&go refractor in the same way - even simple achromatic doublet - something like 80mm F/8-F/10 will give you range of options for different magnifications and full disk included.

Indeed. If it helps, here is how I put my imaging train of my TS80 F/6 apo with all the bits . I have 2.5" R&P focuser with M63 connection. I used this rotator: https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html I also used this adapter: https://www.teleskop-express.de/shop/product_info.php/info/p5144_TS-Optics-Adapter-from-M68-and-M63-to-M48---Riccardi-Connection-Adapter.html This one has M48 on both sides - male towards the camera and female towards the scope. Handy for attaching 2" LPS filter in front of whole optical train for example. It also means that 2" flattener / reducer can be sunken into focuser if there is need for that. Then I have reducer and after than OAG , filter drawer and camera. Reducer/flattener requires around 61mm of distance for my scope, so OAG gets much closer because with this scope after reduction I'm working at roughly F/4.8 (I use OAG on my RC8" as well - but there focuser has rotation feature so I don't need separate rotator and have much more optical path for different bits).

It matters. It is not just what we understand as "color correction" that is important - purple halo around bright stars. It is the fact that doublet scopes have only two wavelengths of light in focus at the same time, while triplets have three. That fact alone means that there will be wavelengths of light that are more out of focus in doublet that there will be in triplet scope (provided that they use appropriate glass types - good ED doublet will outperform triplet scope made out of ordinary glass - although I have not seen one yet - tripled made out of ordinary glass, but I wonder why there aren't any). Any unfocused light causes blur / lowers contrast. More of it you have - less sharp / contrasty image is. Only case where it does not really matter and you can use even singlet lens / and regular achromat works well (if it is well corrected) - is narrow band imaging. Since every type of scope can bring at least one wavelength of light to focus - in narrowband you are effectively shooting only one wavelength (or wavelengths at very small vicinity of it) of light.

No it would not, guiding can work with stars that are not perfect in shape, but you have to be careful not to eat too much of the light cone. This depends on the size of pick off prism and the speed of the beam prior to flattener/reducer. I also vote for OAG - it is actually cheaper and better solution if you can fit it in your optical train. I use this one: https://www.teleskop-express.de/shop/product_info.php/info/p8319_TS-Optics-Off-Axis-Guider-TSOAG16---stable---length-16-mm.html There is even shorter one at 9mm optical path. Just be careful that you need adapter on camera side - I use T2 one: https://www.teleskop-express.de/shop/product_info.php/info/p1649_T2-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html In any case, say you get above OAG - it has prism that is about 8mm wide. Say you have F/8 beam. This means that at 64mm prism to guide sensor distance - you'll have only single point being illuminated at 100%. If your flattener requires 90mm of distance to sensor and you place OAG in front of it - it will operate at much lower "aperture" because of this distance - prism will stop it down. I'd say that at F/8 you want your guide sensor to be about 50mm away from prism (and hence main sensor as well from OAG). Here is example of using slightly larger sensor with this prism and about 60mm of distance: you can see illuminated section and you can see that central 1/3 is brighter than the rest - there is some vignetting due to distance from sensor.

I did write that - and I explained why: 1. Because it does not work with scopes of different aperture: 5.25 contrast index of 4" scope means more blurred image than 3.17 contrast index of 8" scope and higher contrast index should mean sharper image. 2. Because you can't really tell how much more blurred image you'll be getting (if at all) by examining numbers - sometimes same number mean almost no increase in image blur and sometimes that same number means visible increase in image blur. If you want good measure of how system behaves - you have one, it is MTF diagram. It is specifically designed to work with contrast - function on that graph is contrast, 1.0 being full contrast and 0 being no contrast and all the values in between acting accordingly. It is based on physics, hence not arbitrary, it is absolute (as in not relative) and hence will show you differences between any two scopes, regardless of aperture. It also includes other aberrations as well, so you'll be able to compare telescope with spherical aberration to one without, for example. Single number simply can't give you that information.

I was not aware there is actual gear box in there! Luckily, mine does not seem to suffer much backlash in those (but in reality, I'll need to see how it's guiding as final word on that).

I actually managed to tune out backlash on mine, but on my version, gears are made out of metal, same as in this video: Btw, here is whole youtube video on how to tune out backlash in this mount: https://www.youtube.com/watch?v=hgEzeXrhhEg Good to know that this mount will guide at 1.5" RMS - I guess that is about as good as can be expected from it.

Just to add to answer already provided by Stu, prism diagonals are better suited to slow scopes. When used with fast scopes, due to angles involved there is possibility of introducing chromatic aberration. For this reason, mirrors are preferred for faster scopes and prism diagonals for slower scopes. For fast scopes, there are options like this: https://www.teleskop-express.de/shop/product_info.php/info/p9870_TS-Optics-T2-90--Star-Diagonal-Prism-with-28-mm-free-aperture-for-Observation-and-Photography.html which you can equip with low profile adapters to shorten optical path.

Most of the things you listed could be the cause of blue bloat. Possibly weather conditions could be the worst offenders - hazy skies. I doubt it is due to tilt. Fact that some stars have it and some don't is likely related to stellar class if anything - hot bluish stars emit much more light in blue/UV part of the spectrum. That big star is particularly nasty - here it just outside of the frame causing havoc - also very hazy conditions.

I think that is sensible course of action. Narrowband filters help a lot - regardless of the type of light pollution - as long as you image targets that give off light in that particular band - which is emission type nebulae.