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I new I did not get the question Ok, got it now. First we need to understand how telescopes work - I know, such a phrase, but indeed - above question and reasoning suggest that there are still some things that people overlook. Here is "simplified" model. Telescope is device that brings light coming from certain angle to a single point. All light that falls on its aperture coming from a single angle will end up in a single point. This is important thing to understand. All photons coming from angle A1 that are collected by aperture (size matters here) will end up in single point. All photons coming from different angle A2 will end up in a different point. Here is diagram explaining this perhaps better: Green lines represent photons coming from one direction, while red lines represent photons coming from a different direction - different angle. Important point: Every single point on focal plain receives photons from whole aperture. Points on focal plane do not split aperture somehow, each of them gets full aperture - as if aperture represents funnel for every single point on focal plane that works in parallel. What makes the difference is angle at which photons arrive. This is important thing, remember it for later. Aperture is just size of the funnel. Larger funnel, more photons each point on focal plane gathers simultaneously. Larger FOV will not spread light from same aperture in comparison to smaller FOV - it will just add more points to gather light "in parallel" (like having more processors in a computer). What about pixels, you ask? Pixels join/spread angles not aperture. Angles in the sky. Every single point gets photons from just one and only one angle - exact number like 3.212331313..... Pixel on the other hand collects all photons from part of the sky - all distinct angles that end up in that region. Say you have 1"/px. Single pixel will gather angle 3.21212... and 3.33434 and 3.543434... everything between third and fourth arcsecond. Another pixel will gather everything between fourth and fifth arc second and so on. More sky one pixel covers - more photons from all different "individual angles" will add up to form a signal in that pixel. Higher the signal - better the SNR. But pixel size is not alone that determines how much of the sky is mapped to single pixel - there is focal length as well. Now that we understand this, let's see if we can compare RASA8 and RASA11. For purpose of comparison we will assume that both RASA8 and RASA11 have same F/speed when all losses (central obstruction and coatings and all things) are accounted for - let it be F/2.2 or F/2 or F/2.6 - it does not really matter much. Case 1 - using the same camera without pixel binning: - They will be equally fast - RASA8 will image larger FOV then RASA11 - RASA11 will resolve more, under assumption that optical aberrations in both scopes are the same (not manufacturing defects - but design spot diagram) simply because it has more aperture (airy disk is smaller and any aberration will be correspondingly smaller) - provided that mount and seeing are the same and there is no under sampling in both scopes. Case 2 - different pixel size - matching resolution in arcsec/px: - RASA11 will be faster - FOV covered will depend on each camera sensor size and respective focal length - any combination is possible RASA8 > RASA11, RASA11>RASA8 and RASA11 = RASA8 - RASA11 will resolve more provided that there is no under sampling, if there is under sampling - both will resolve as allowed by pixel scale Case 3 - matched FOV - this is really of no consequence because it tells us nothing Case 4 - matched FOV and pixel scale - well that is just special sub case of case 2 (which can have any FOV relations) - same conclusions as Case 2 Does it make sense and does it now answer the question?

Yes, one of the greatest problems with telescopes is managing expectations. People want to see more, see better and one way to improve what you can see is size of telescope, or more precise - aperture of telescope. Larger aperture gathers more light and resolves finer detail (as long as atmosphere is cooperative) but also weighs more - and is hence harder to use because of that. My personal view is that for beginner scope 8" size should be really only used in dobsonian configuration. Mount with tripod (either EQ or Alt-Az) to hold 8" of scope will simply be very large and very heavy. Dobsonian base for 8" scope is no light weight category either - 8" dob weighs in at about 25-26kg total, 10-11kg being the telescope tube and additional 15Kg for the base. Advantage to dob mount is that base is as low tech as can possibly get - no precision gear that need to be handled with care - you just lug it around like luggage, drop it onto flat ground and just carefully carry OTA (optical telescope assembly - or telescope tube) and place it on the base. That is only time you have to be careful with dobsonian type telescope assembly - handling the OTA. Otherwise, if looking at Catadioptric type scope - 6" is probably much more manageable. Yes, EdgeHD has special optical elements that make sharp view over all of the field. This is very important for astrophotography, but not as much for visual. With visual observation, couple of things happen that lessen the effect. First is - we tend to place objects of interest in center of the field of view - where image is the sharpest and we concentrate on that part. Since our vision is such that we can only concentrate on one part of the image - we don't notice blurry image at the edges as much if we don't look intentionally there. Second thing that happens is that we are not nearly as sensitive to this at low magnifications - as these imperfections tend to be very small and only become noticeable when magnified. With low power views where these tend to show - they are small and much less intrusive. At higher magnifications - we are simply looking at central part of the image and image is sharp in that region and when we have magnification that would otherwise obviously show optical issues - there are no issues to be shown (they are outside of field of view since we are "zoomed in"). Standard SCT suffers from two most obvious aberrations - coma and field curvature. Coma in F/10 SCT is comparable to F/6 Newtonian. I have 8" F/6 Newtonian scope and never really noticed coma, even in wide field eyepieces. Mind you - once you start searching for these and you see it - you can't really unsee it later. So if it's not obvious - don't go looking for it . Point being - for visual, you won't be much bothered by off axis aberrations from telescope, but they do show up in images and you need correctors to take good images with such scopes.

In that case, I'd say, ok, why not, go with EdgeHD version. Keep in mind that astrophotograpy is very difficult and demanding. It's really not - attach a camera and snap what you are looking at. Even very modest images require at least dozen minutes of exposure as things in space are very very dim. Exception to this rule is the Moon and planets, but imaging those requires radically different approach (it consists of recording a video sequence with specialist cameras and then cherry picking good frames - software does that for you. In planetary it is all about catching that moment in time when atmosphere is stable enough to offer sharp view). Starting AP with 2000mm focal length scope is not something that I would recommend though. Only drawback of having EdgeHD 8" is relatively narrow field of view / high magnification, otherwise it will be very nice scope. Computerized version will find objects for you once properly aligned so narrow field of view is not that much of issue. Before you decide, do look at few videos featuring that scope. Pay attention to size of the mount and factor in that you need to set it up each time you observe. It is Equatorial type mount that weighs 40lbs for mount head, 20lbs for tripod and another 17lbs for counterweight. Telescope weighs another 14lbs. In total that is 90lbs or about 41kg that need to be assembled in order to observe. Best telescope is the one you use most often. If setting up each time feels like a chore or takes too much of your time - you'll be wanting to watch your favorite TV show rather to go out and observe.

Here is 60mm https://www.firstlightoptics.com/specialist/masuyama-ultra-premium-japanese-eyepieces.html and I think I saw 80mm somewhere - it is 3" eyepiece and you'll need suitable focuser and diagonal (also 3" ones).

That one might not be total waste If you ever get into Solar Ha observing and you happen to get Daystar Solar Quark combo, you'll be able to turn your small F/6 refractor into very decent F/30 solar Ha telescope.

Yes, indeed, thank for this. I totally was not aware that they will be slowly coming together over next 7-8 days (and similarly growing apart after), so that they can be seen together in FOV with lower power eyepieces. I was afraid that I won't get the chance to observe this event due to poor weather - but this gives much more chance for catching a glimpse of these two planets in the same FOV. They are now spaced (as of tomorrow) - about 1° - easy target with 50° plossl at about x40 magnification. Over next few days they'll just keep getting closer enabling higher power while still having them in the same FOV. Current forecast looks promising on next Thursday evening.

Hi and welcome to SGL. If you were astrophotographer doing long exposure deep sky astrophotography or experienced amateur observer of deep sky that enjoys pin point stars in their widest 2" eyepieces, then I would say - obviously 8" EdgeHD version. On the other hand if you were keep planetary observer or imager - then I would say, without a doubt - C9.25. For someone about to use telescope for the first time - I simply can't say one way or the other. Both scopes are not really suited for all round use as they have very long focal lengths, and I can't help but recommend something like 8" dob mounted newtonian (maybe even motorized) as much better all around scope that will cost much less than two above but provide the same level of enjoyment and very similar views to scopes you suggested (in areas where two said scopes excel). Maybe some more background, expectations and possible future interests could help give better recommendation?

I think that we are talking about couple of things here, so let's leave details (resolving power or the telescope) aside for now and concentrate on SNR, shall we? Here, things are fairly simple - we need to fix one variable and see how other variable contributes. Let's fix sampling rate first. If we fix sampling rate - so both scopes sample at the same resolution - be that for example 2"/px - given all else is equal (sky quality, sensor QE, read noise, filters used ...) - larger aperture wins. Here we should really calculate clear aperture equivalent - by taking into account central obstruction, air/glass surfaces (hopefully coated) and reflectivity of mirrors (regular, enhanced, dielectric ...). If we fix aperture - larger sampling rate wins. At same aperture 2"/px will be faster than 1"/px. If we fix pixel size (by using same camera and not binning) - F/ratio wins, faster F/ratio will be faster than slower F/ratio. Fixing FOV is not meaningful for example - as it will depend on further things (like how many pixels you divide that FOV into and what aperture you use). Back on resolution / resolving power. In long exposure imaging, 3 things determine resolution of the image provided that distortion of optics is below this level. Seeing, quality of tracking / guiding and aperture size. I purposely used term "distortion of optics" rather than "diffraction limited". Most telescopes are diffraction limited only on or close to central axis and once you start introducing field flatteners / coma correctors and such - you are compromising diffraction limited performance. In any case - as long as blur from optics is significantly smaller than that of atmosphere / guiding above holds There are optical systems where above does not hold - like camera lenses - which might be sharp for day use but are far from diffraction limited. I suspect that RASA is similarly not as sharp - which is not really much of a problem for it's intended use - low resolution wide field imaging. Did I help with above? Why do I have feeling that although what I've written is correct and informative - it did not help much to this discussion?

Hi and welcome to SGL. Maybe best course of action would be to do some reading on astrophotography first. Get a book, often recommended one is "Making Every Photon Count" https://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html Thing with astrophotography is that it is very demanding and it is expensive. Having one telescope that will do it all - both visual and astrophotography is probably wrong approach. Single telescope will hardly do it all for astrophotography (many end up using small apo for wide field and larger telescope to get in closer to subjects). Get a book first and see what is needed to do astrophotography, and then think about getting two scopes instead of one. Telescope like this will be all you need for observing for many years: https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-200p-dobsonian.html It will take just a bit of your budget - and then look into getting small apo telescope and good mount (often recommended ED80 and maybe EQ5 mount would be within budget) for astrophotography.

It should, but those wavelengths that are not in focus - I wonder how much out of focus they will be. Both ED and regular achromat are just doublets, and have two wavelengths in focus while others are out of focus. ED glass helps those out of focus wavelengths be just a bit out of focus - in some cases below what could be visually observed (but does show on long exposure photographs like slight violet halos around bright objects). I wonder if regular glass types could achieve in triplet form what ED doublet achieves and why it was not attempted - or it was, but it did not do well?

Or to make it easier - both at 100% of their respective photos:

I used to have ST102 and I now have Mak102, so I do have first hand experience in planetary views between two such scopes. We could say that F/12 or similar 80mm refractor is going to be contender against good Mak90, but ST80 is certainly not. Here, maybe moon shots will help: Different technique and equipment (ST102 + DSLR vs Mak102 + planetary camera), but you can see chromatic issues straight away on the moon with ST102 - look at yellowing around the limb. Above is of course ST102 Here is Mak102 version: Here is Maksutov version - which you can zoom in quite a bit (right button + open in new tab and then click with magnifier to get 100% zoom).

Mak90 will outclass ST80 on planets. ST80 has masses of chromatic aberration - and that is out of focus light. Most of the light of planets is slightly out of focus and thus blurry. Central obstruction or not ST80 behaves more like 40mm scope than 80mm scope with respect to planets. Indeed, if you want to see color free images of planets, then put aperture mask of 40mm over it (or use supplied cap and remove central part for 50mm of aperture). 90mm with central obstruction is simply going to run circles around it.

Anyone ever seen triplet with regular glass? I wonder how good such a scope would be?

I don't know - I sort of prefer the top version. It is much less aggressively pushed and it has much more detail / more resolution. Look at these groups of stars as an example: These are almost joined into single structure with RASA - they are not resolved. Here in Esprit image, they are very nicely resolved as individual stars.

I have slight concerns about CGEM mount. It used to have well known 8/3 period error that was giving people problems as it was hard to guide out. Not sure if was fixed in the meantime. Apparently it is related to motor gear box and not the mount, so changing motor assembly could have been part of the solution.

I did not even notice the third one - but yes Saiph looks like culprit for that one.

Center of the prism, but don't worry about it much, you won't be able to dial in exact distance due to field curvature - just get it reasonably close and then you'll focus guider properly under the stars. First you focus imaging camera and then adjust guider to match it. By the way, you don't need to be perfect in your guide camera focus - you can even guide a bit better when slightly unfocused. Algorithm will still calculate proper centroid but seeing effects will be smaller.

RASAs only offer wider field of view. You can have the same speed with F/5 newtonian or F/8 RC - if you select appropriate pixel size. Same aperture gathers same amount of light and then it is only the question of gathering/funneling those photons - in buckets. If you gather the same amount of photons in a bucket - you'll get same SNR, but if you gather more photons in one bucket than the other - first one will have higher SNR than second. Pixel size is not the bucket size here, bucket size is combination of focal length and pixel size - what is called sampling rate or arc seconds per pixel. RASA has large aperture but very short focal length - which means it utilizes that aperture on very large bucket (when paired with normal sized pixels). If you want to get same size bucket with say 8" F/4 newtonian - you need to bin your pixels x2. If you want to do it with 8" F/8 RC - you need to bin them x4. Binning will not provide you with increased FOV - RASA will have the largest FOV, then 8" F/4 newtonian and last will be 8" F/8 RC. RASA scopes are used with OSC more then mono - for practical reasons. Camera sits in front of the scope (in front of aperture) and OSC camera can have round body and fit behind the central obstruction, but Mono camera + filter wheel can't. You either need to find round filter drawer and change filters manually or just simply use OSC cameras with RASA. I suspect most people opt for second option and for this reason RASA scopes are better with OSC - not because they are magically "better" with OSC cameras

I'd say this is freak occurrence of a lens flare. Two very bright stars are in direction of centers of circles. There are other brighter stars than Mintaka in belt, but they were not in "the right place" to produce lens flare. It seems that Rigel and Mintaka were about the same distance from optical axis and in the sweet spot to produce some sort of lens flare effect. I could be wrong with this, but it does seem like likely explanation. It will not clear with stacking - it will compound, so you'll either have to live with it or process it out.

Have you ever seen shadow of secondary? I'm not sure how that is supposed to work? As far as I can tell from optics - you should not be able to actually see shadow itself - only some sort of "vignetting cone" - opposite from regular vignetting where brightness falls of towards the edge - here it would fall off towards the center of the FOV but it would never be completely dark unless "pencil of the shadow" is larger than dilated iris. Something like this: (as far as I can tell - above is actual image of shadow of secondary).

I thought so too, but, check this out 4" Evostar F/10 is listed at 300 euro with TS: https://www.teleskop-express.de/shop/product_info.php/info/p2684_Skywatcher-Evostar-102---102-1000mm-Refracor---optical-Tube.html Throw in another 150 for replacement focuser and 50 euro for adapter - and you have 500 euro refractor. But, for that sort of money (516 euro), you can have this scope: https://www.teleskop-express.de/shop/product_info.php/info/p4964_TS-Optics-ED-102-mm-f-7-Refractor-Telescope-with-2-5--R-P-focuser.html Granted, it will have some color issues - but I think it will be less than 4" F/10 achromat. It will probably give equally good views of the planets and it will show wider field of view. It will generally be better all rounder scope than 4" F/10. I think this is down to very strange situation on the market - some scopes are old stock, some are new, there is general market upset due to pandemic and I think that replacement focusers are probably overpriced because they are replacement units (well not overpriced as such, but same amount of overhead in transportation, handling and retail goes in both whole scope with focuser and focuser alone). Some scopes have very strange prices probably due to demand. For example that Evostar 102mm is 300 euro, but look at this: https://www.teleskop-express.de/shop/product_info.php/info/p3309_Skywatcher-Evostar-120-OTA---Refractor-120-1000mm-optical-tube.html 120mm refractor for 314 euro - only 14 euro more! I think that price of that 4" is very inflated. If we look at prices for F/5 versions (which are somewhat less money for some reason - as if it is easier to figure faster objective lens), we get "proper" ratio of prices. ST102 is ~200 euro and ST120 is ~280 euro. Something tells me that if Evostar 120 is 314 euro and above F/5 versions have prices that they have, Evostar 102 is overpriced by at least 80 euro I need to take a deep breath here, realize that I'm perfectly happy with 5 scopes that I now have and it's just time under stars that is the issue

Could be that most of the cost of the scope goes into labor rather than materials? Cost of transport, retailing costs, profits? They sure look nice, and that focuser looks like a proper unit. I think I read somewhere that people were not happy with such focusers. This was not in relation to Opticstar scopes, but SVBony models that look remarkable like those of Opticstar, so I'm guessing that they might have same source factory? Here is SVBony model: and Opticstar They might be decent units and at same level of quality as other focusers I have been looking at. This information came from Cloudy nights discussions on these SVBony scopes - some people report grinding feel to focusers, while others say their units are fine? In any case - these sort of scopes come at right prices I would say? That Opticstar AR90S should be around 180 euro without vat - and that is rather nice price for 90mm aperture scope with 2" focuser with 1:10 micro focusing. There is also 90mm F/5.5 version (SVBony and also Omegon has it with single speed focuser). I don't know if there is smaller version, though? Above report of lens quality is something that also needs to be considered, however, as far as I've heard this AR90S model should be quite good optically?

Well, certainly, if one likes the focuser - why change it. I, on the other hand really like idea of having 2" capability, smooth action and 1:10 reduction. I added 1:10 micro focusing to my 8" F/6 newtonian and it is really nice feature to have on planets for example.

I agree. What replacement focuser would you put on ST80? Maks are good the way they are, I would not replace anything on Mak and use it as is. We could argue that Mak 102 mm and ST80 could combined replace single scope with ED glass. Mak would excel at planetary / high power, while ST80 would be in charge of low and medium power DSO and wide field. Would we save money in doing so? According to TS prices, we have: ST80 OTA - for 120 euro Mak102 OTA - for 206 euro Together they add up to 326 euro. Add focuser to ST80 - that GSO one dual speed already has 86 ID flange, so it will be direct replacement - no adapter needed, it costs 145 euro. Total of 325 + 145 = 470 euro But then again, you have this: https://www.teleskop-express.de/shop/product_info.php/info/p7169_TS-Optics-ED-APO-80-mm-f-7-Refractor-with-2-5--R-P-focuser.html For 426 eueo and also this: https://www.teleskop-express.de/shop/product_info.php/info/p8637_TS-Optics-Doublet-SD-APO-80mm-f-7---FPL-53---Lanthan-Objective.html for 499 euro First one probably has FPL-51 glass, while second is virtually perfect with respect to color correction with FPL-53 and lanthanum glass. I suspect both would put up good fight against Mak102, which although has 102mm of aperture, has central obstruction, so it would be tough call on planets between those. Both of these scopes would easily win versus ST80 on low / medium power for deep sky.