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Here are my thoughts on this - mind you, I don't have first hand experience with said mount and scopes of that class on it. You want a triplet rather than doublet for serious imaging. You want as much aperture you can mount. This really limits you to below 130mm refractors as scopes in that class fast approach over 10Kg in weight. Here are two contenders that I managed to quickly track down: https://www.teleskop-express.de/shop/product_info.php/info/p10181_Explore-Scientific-ED-Apo-127-mm---FCD-100--carbon-tube--Hexafoc.html 127mm triplet with ~7Kg of weight https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-Optics-PHOTOLINE-115-mm-f-7-Triplet-Apo---2-5--RAP-focuser.html This scope comes under many labels, and I'm sure there is AltairAstro version as well that could be much easier to source locally. 115mm triplet with ~6.5Kg of weight

With guide scope, I'm guessing that it probably is less important than it used to be, but with OAG - it might be different story. Don't know how much guide stars one can pick up with OAG as FOV is much smaller and also, I'm not sure how much seeing is varied across such a small FOV. From planetary imaging and adaptive optics systems - we know that isoplanatic angle is not that large - so seeing disturbance seems to be the same over say 20ish arc seconds if I remember correctly. That is still much smaller than FOV of OAG even at very long focal length with small sensor (say you have ASI120 which has 1280 x 1024 and you use 2 meters of focal length so you end up at 0.4"/px - you still have more than 500 arc seconds across the sensor). However, I have no idea how much tilt component of wavefront error alone changes with angular distance. I'm inclined to think that selection of stars on small FOV such as one gets with OAG won't have enough of diversity in star position to average them out, but I could be wrong. In any case - more exposure, more average star position approaches true star position. There is also "SNR approach" to guiding. I've read once in some document a very sensible argument (and I tent to agree with it) that we could observe guiding in similar way to imaging - by examining signal to noise ratio. In this instance signal is actual mount error and respective correction and noise is well - noise, inaccuracy in both determining star position, but also in issued correction as no mount is perfect and will not respond accurately to guide pulse - there will always be some backlash, some inertia to overcome, some oscillations due to weight on the mount being moved and its inertia and so on ... Conclusion of the paper is that we increase SNR in part by reducing number of corrections issued and that corresponds to long guide cycle. As long as mount does not accumulate significant error in that time (smooth mount) - long guide cycle is better than short because of this, even if we don't chase the seeing.

Don't know where to start with reply Ok, let's address QE question first. Compared to other parameters of sensor - it is really not that important. QE of modern sensors are very close to each other - they differ by 10-20% in peak performance. For example: Simple ASI120mm has QE of 80% ASI178mm has peak QE of 81% - so (81 - 80) / 80 = 1.25% increase ASI290mm has peak QE of 80% - no difference ASI462mm has peak QE of 89% - so (89 - 80) / 80 = 11.25% increase ASI432mm has peak QE of 79% - so less than ASI120 The latest ASI220mm (I did not even know this one existed) - has peak QE of 92% so that is (92-80)/80 = 15% increase So each sensor is within 15% QE of basic ASI120 However, let's compare some other metric - like pixel size since number of photons per exposure depends on both QE, but also on photon gathering surface - which is pixel area. Let's compare ASI120 and say ASI290 with pixel size. One has 3.75um and other has 2.9um. That is 3.75 * 3.75 / 2.9 * 2.9 = x1.67 increase in sensitivity on account of pixel size alone. Much more than any difference in QE that we observed above. When using OAG - you can bin your camera (and you probably should) to increase sensitivity. Sensitivity of guide camera with OAG vs small guide scope will be much much greater if you have reasonable resolution because large aperture creates smaller star image and larger aperture gathers more light - so star image will be much brighter. Now onto very short exposures. That will simply have you chase the seeing. If your mount requires corrections every 250ms - then change the mount. Best mount is the one that is smooth and that has very slow varying error. Best guide exposures are in range of 4 to 8 seconds if your mount can cope with that. If mount needs to be corrected quicker - that just signals serious mechanical issues like mechanism roughness or some sort of very fast period error (I once had ~11s period error on my HEQ5, but that was due to issues with belt / pulley teeth meshing and I had issue on each tooth - which is roughly 11.4 seconds if I remember correctly). Guiding should not attempt to solve mechanical issues - it is there to correct for periodic error (or declination drift due to inaccurate polar alignment). Anyways, if you are guiding with faster guide cycle than a few seconds, in my opinion, you are doing it wrong. To answer your question anyway, I'd say that ASI220 seems to be very good combination of factors for guiding. It has 4um pixel size which is good size for OAG (you might still need to bin if you guide scope with over one meter focal length), it has 91% QE and it has very low read noise of less than 1e (close to 0.6 and best gain setting).

Seeing is expressed in FWHM and guiding in RMS so two are not directly comparable. What you can do is convert seeing FWHM into RMS and then see how much larger it is than guide RMS. For Gaussian distribution, conversion factor is x2.355, so FWHM of a Gaussian curve is x2.355 times larger than RMS or sigma of that curve. 2" / 2.355 = ~0.85 or in another words - having guide RMS of 0.85" in ideal conditions (no atmosphere) will produce same blur as having 2" seeing and absolutely accurate mount that you don't need to guide. I'm always advocating for as low guide RMS as possible - but there is good rule of thumb which says that you need to have it at least half of imaging resolution. This rule of thumb is particularly well suited for 2"/px and 1" RMS case. It's also worth noting that impact is not as large with small scopes as it is with larger apertures. Aperture again acts to blur the image and we can also "convert" it to RMS. 4" of aperture is roughly equal to ~0.95" RMS. When we say that seeing is 2" FWHM - that simply means that with very large telescope (so that aperture does not play a part in star image blur) with perfect mount / or rather in 2 second exposure so that mount issues don't come into equation star image is blurred to the level that it has 2" FWHM in its profile. That has nothing to do with how well we guide. That is average of star images over two second exposure. Some of it is due to star bouncing, but some of it is just because of other types of distortion (star bouncing around is just "first order" wavefront error - or tilt, but star image is distorted by whole wavefront error and that wavefront error changes roughly every 5ms so in two second exposure we have average of about 400 different distorted star images). Guiding RMS simply means average error in mount position compared to true star position. Sure, when there is seeing we have a bit of trouble determining true star position because star jumps around - but: 1. in longer exposure this jumping around tends to average out and star centroid is very close to real star position 2. We can always use longer guide exposure to be more precise about star position (if our mount has smooth enough error) 3. Advances in guiding algorithms now allow to guide on multiple stars - which again adds another layer of "averaging" things out thus getting more accurate star position

Good point. I forgot to say that. My HEQ5 is very far from stock mount. Had it tuned, replaced all bearings for SKF ones, belt modded, changed saddle plate clamp, put it on Berlebach planet tripod

I had my HEQ5 guide as low as 0.36" RMS. Better thing to ask would be - which mounts guide below 0.5" RMS on regular basis. Here are a few that can do it: Mesu 200 E.fric 10Micron mounts Astro-Physics mounts (not sure if all, but I'm certain that some will do that) ASA direct drive mounts

Back spacing is just like you understood it - you need to either increase or decrease distance from sensor to FF/FR. Increasing is easy - you add some thin spacers. Decreasing distance is a bit more involved - you need to add shorter extension and again add some spacers. Alternative is to get variable length extension that you can tweak / adjust. Tilt on the other hand is much worse thing to deal with. It really means that something is not perpendicular to optical axis. It can be sensor or field flattener / focal reducer itself. Often, cause of tilt is in the way everything is connected together. For imaging best type of connection is threaded connection. Clamping mechanisms are not very good (like when you insert eyepiece into receptacle and clamp it down with thumbscrew and possibly compression ring). I think that Baader click lock is exception as it centers the accessory. As far as tilt is concerned - I'd first look into how things are connected. Next thing to check is if camera already has tilt mechanism. Some newer models already have tilt plate in front that you can adjust. If not - there is tilt mechanism that you can add to your optical train - but you must check to see if it's sensor, field flattener or perhaps even focuser that is not squared with the rest.

I'd say that there is some slight tilt and in general - you need to fiddle with spacing of field flattener to get the best result. It is certainly not guiding - that impact whole field in the same way - so you would see same distortion in all stars in the field. In the end, you might need to accept slightly astigmatic stars in the corners - as some field flatteners / focal reducers produce such results. It is more obvious in good seeing when stars are tight and sharp.

@PeterC65 Are you using UV/IR cut filter with that sensor? Most of these astronomy OSC cameras come with just AR (Anti-reflex) coatings so they need additional filtration in UV and IR especially if one is using refractive optics with them. What you are seeing in images might be poor correction in UV/IR part of spectrum.

I think that x0.6 reducer might be a bit too much for this optics. Resulting F/ratio is F/3.6 - and that is tall order. I do know that other FF/FRs do work well with short F/6 optics. For example, I used TSRED279 - which is 2" version of x0.79 reducer by TS on my 80mm F/6 triplet. It works fairly well. Makes stars a bit astigmatic in the corners of 4/3 sensor - but that shows only if seeing is very good. I'm sure that x1.0 Field flattener will work well - as long as flattener itself is good optically. I've heard that TSFLAT2 works very good - people tend to use it for visual as well with small scopes as it has plenty of working distance (needed for 2" diagonal and accessories). I just saw this as well: https://www.teleskop-express.de/shop/product_info.php/info/p10127_TS-Optics-1-0x-Refractor-Flattener-for-APO---ED-with-70-72-mm-aperture.html Maybe that would be best option as it is designed for ED/APO scopes of 72mm aperture? Then there is this as well: https://www.teleskop-express.de/shop/product_info.php/info/p12208_TS-Optics-REFRACTOR-0-8x-Corrector-for-ED---Apo-with-70-72-mm-Aperture.html That is FF/FR with x0.8 reduction - maybe worth a try?

That is not saying very much. It can be refractor, reflector, SCT or even MCT with those specs (although I've only seen F/10 Maksutov in form of photo lens). Main negative aspect can be that you don't get what you are hoping to get with focal reduction, but that really depends on what scope you have, what focal reducer and what you intend to do with it. For example - I've mentioned that I have x0.67 focal reducer. I've primarily intended that for imaging use. There are some people that find this focal reducer not usable for that exact use case (same scope, same focal reducer) - because they want to use it with too large sensor. Scope simply does not provide corrected field large enough to cover large sensor after "squeezing" of the field. On the other hand - I'm using that reducer happily on refractor telescope for visual with particular eyepiece. It might not work as good with different eyepiece though. I even used said focal reducer on my 4" Maksutov - because I tried with very small sensor that I knew won't vignette although back port on 4" Mak is very small. Besides these issues - focal reducer can amplify optical problems if is not suitable for particular telescope design or is used on very fast optics. For example - I tried very strong x0.5 focal reducer for EEA with F/5 fast achromat. Results were horrible - stars were distorted and I was even not able to bring them to focus properly. On the other hand, same focal reducer worked very very well in EEA role with F/6 newtonian with very small sensor.

Btw what scope is this and what focal reducer are you considering using?

I'm not sure that Maks and SCTs have as much vignetting due to size of rear port as people think. Beam is very slow and focal length very long, so I don't think that much of light gets blocked in percentage - and drop of about 10% is just on the edge of being perceived as difference (just noticeable difference for light is 7% if I'm not mistaken).

That is ok, just remember one thing we will need for further explanations - maximum field stop diameter with 1.25" eyepieces is ~27mm. It can be both good and bad thing - it depends. Larger exit pupil makes object both brighter and smaller in size. Up to a point. If exit pupil is larger than the pupil of your observing eye - then you are wasting light. Look at following image: It describes what happens when you have exit pupil larger than your pupil (in this case, observer is not dark adapted and their pupil is only 2-3mm wide, while exit pupil from eyepiece is 7mm wide). Disregard "result" comment at the bottom of the image as it is actually wrong in this case . In any case - some of the light will hit iris of your eye and will fail to enter your eye. That light is lost. For this reason you want to keep exit pupil of the eyepiece below or equal to how much your pupil dilates in the dark. On second note regarding exit pupil size - I've mentioned that object observed gets brighter when you use larger exit pupil. Same thing happens with the background sky. If we were floating in outer space and we did not have this background light from the sky - largest exit pupil would be ideal, but since we have atmosphere and that atmosphere scatters some light and is not completely dark, then it is matter of contrast. As we increase exit pupil size - we brighten both target and background sky. At some level of target brightness vs sky brightness - contrast will be the best (this also depends on the size and shape of target). For this reason, largest exit pupil is not always the best - we need to try different exit pupils depending on our sky conditions (level of light pollution) to find one that will show target the best against background sky. Ok, yes - I will explain this a bit better. Telescope and eyepiece are really the same thing but in reverse. Telescope takes angle and projects it onto a plane - focal plane, and eyepiece does the opposite - it takes point of focal plane and makes light ray out of it at certain angle. This angle is related to distance of the point from optical axis (center of focal plane) by simple equation that depends on focal length. This diagram explains it nicely - there is entrance angle alpha which gets projected onto focal plane (little vertical arrow between two lenses) and then eyepiece turns that into exit angle beta. alpha = tangent of length of small arrow over focal length of telescope beta = tangent of length small arrow over focal length of eyepiece This is why magnification of a telescope and eyepiece combination is given by ratio of their focal lengths - it is actually the same as ratio of beta / alpha (magnification is change in angles). Ok, so now imagine that this little vertical arrow in the middle can only be of a certain length. This is what field stop / field diameter is. It is diameter of black ring in the image you see: This is usually limited by physical dimensions of the telescope and eyepiece - or to be more precise focuser tubes and telescope tube. You've mentioned that you have 32 and 40mm plossls and that they show the same amount of sky just at different magnifications / AFOVs and exit pupils. This is because both of those eyepieces are limited by 27mm of their field stop. 40mm plossl will show same part of sky limited by same black ring - only smaller (less magnified). Focal reducer helps with following: - if telescope "field stop" (or illuminated field to be precise) is larger than eyepiece field stop - it helps to "squeeze" that field produced by telescope into field shown by eyepiece Hope above makes sense.

Well, this is indeed true. I've recently experimented with 4" refractor and focal reducer to see if I'd be able to get full view of M31 in the eyepiece. There is couple of ways this can be achieved with said scope, and I've chosen one that was available for me. I already had (rather expensive) focal reducer that I knew would work well with this scope, and I also had eyepiece I wanted to try this all out with. If that was not the case - I'd probably go the following route - get one of these: https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-super-plossl-eyepiece.html 2" 55mm Plossl. In any case - when trying to get wide field of view from such instrument - there is really only two things that you need to be careful about: 1. Exit pupil must not exceed your pupil when fully dilated / dark adapted. This is usually quoted at 7mm but people tend to loose ability to dilate their pupils with age and it's best to actually measure it. 2. Fully corrected and illuminated field of the telescope can't be enlarged and is subject to restrictions. For example - in 2" system, max field of view is around 47mm, while in 1.25" one that is about 27mm. Try to "squeeze" more field into that - and you will have vignetting and poor stars at the edge of the field. In my case above - 4" F/10 can provide good field up to 2" size - so 47mm and 2" 55mm Plossl is one of the few eyepieces that has field stop that large. Also, that eyepiece would give 5.5mm exit pupil so it is a good match (and only drawback is 50 degrees AFOV of plossl - for those that love their wide field EPs). On the other hand, same scope with x0.67 reducer will "squeeze" 47mm down to 47 * 0.67 = ~31.5mm. Eyepiece that I already have is Explore Scientific 28mm 68 degrees. It has field stop of 31.8mm - so again very good match (maybe tiniest bit of vignetting but it was not noticeable when observing). It also produces 2.8mm exit pupil so that checks out as well. They both achieve the same (or rather very similar) thing - one with and one without focal reducer. This is because the scope is capable of showing that much. If I tried to use 55mm plossl with that reducer - it would not work (at least not very nice) - as vignetting would be very pronounced. So I would not see more of the sky (then the scope is capable of showing) - that is physically impossible. Focal reducer just makes it easier in some cases. Similarly - when you try to match sensor size for photography with illuminated field - focal reducer can be used to widen the field of view if scope is capable of otherwise rendering such image on larger field of view. Same effect can be achieved by using larger sensor - so it is matter of economics and convenience - what is affordable and what works well for you - larger sensor or smaller sensor + reducer. There are a few more minor things that need to be taken into consideration. Reducers often work at prescribed distance - so you must take care of that. I for one had to remove nose piece and eyepiece adapter from my diagonal and 3d print direct adapters for reducer and eyepiece - because of distance required by reducer. Reducers also move focus point inward and you might not reach focus if your focuser does not have enough inward travel.

Yes, that recommendation 5 x pixel size is directly derived from formula I wrote and article on wiki I linked to. Given your solar setup and work you do with it, I did suspect that you have academic background. That is why I just linked the article in the first post. Math for calculation of spatial cutoff frequency is straight forward. Math behind wave nature of light producing that cut off frequency is not as straight forward, but it's not very difficult for someone with masters degree or higher in sciences. It boils down to interference effects which turn out to have the same form as Fourier transform of aperture (there is whole field of optics named Fourier optics because of this). This is similar to how Fourier transform represents filters (convolution in spatial / temporal domain is the same as multiplication in frequency domain and vice verse - convolution theorem). In any case - there is clear cutoff point due to circular aperture in frequency domain and there is limit to how much optics can resolve because of this (this is why we need to use radio telescopes in different spots around the globe to be able to get good resolution since radio waves have much much longer wavelengths). Sampling finer than this limit produces no additional detail - same data will be recorded, but using higher F/ratio simply spreads the light (aperture gathers only so much light per unit time since photon flux does not change) over more pixels and signal per pixel gets lower - so does SNR (for same exposure time).

Where does it say that? I think that best results can be obtained provided you do following: - use barlow and "dial in" F/ratio of your optical system to match pixel size you are using. Formula can easily be derived from above spatial cut off frequency and goes like F/ratio = 2*pixel_size / minimum_wavelength - where pixel size and minimum wavelength are in same units (meters, nanometers, micrometers ...) and minimum wavelength is smallest wavelength in range of wavelengths you are recording. If using OSC sensor - use stacking software that supports bayer drizzle (AS!3). - use as short exposure length as possible. This will be governed by QE of sensor and its read noise. Use highest QE and lowest read noise sensor you can get and don't look at histogram to set your exposure length, just use as short as possible. Only use longer exposure length if seeing allows, but most times it will be around 5-6ms per exposure.

Of course, but that is very much related. Spatial cutoff frequency is well defined for perfect optical system (so perfect telescope and absence of seeing aberrations) and sampling above it just has negative impact on final image. It is simply waste as no detail above spatial cut off frequency can be recorded - but it lowers SNR per pixel and thus forces longer exposure. Longer exposure often goes above atmospheric coherence time and along seeing aberrations we also get motion blur (different seeing wavefronts end up superimposed on single exposure). For 2.9um pixel and if imaging in visible light - meaning 400-700nm, following applies: F/ratio = 1 / lambda * sampling_frequency Sampling frequency is 1 / 2 * pixel size (two samples per wavelength corresponding to cutoff frequency), so we end up with: F/ratio = 2 * pixel_size / lambda => 2 * 2.9um / 0.4um => F/14.5 (we use 400nm as lower bound for most detail). That is highest you really need to go in order to capture all the available detail that perfect telescope can provide in ideal conditions using this camera in visible spectrum. You are sampling at ~F/25 (C11 is F/10 and x2.5 telecentric amplifier gives F/25), so your exposures need to be (25/14.5)^2 = ~3 times longer to achieve the same SNR / signal level per sub. This can easily push you over coherence time for given seeing and you enter region where most of the subs are not only distorted by atmosphere but also blurred by moving atmosphere (motion blur).

Do read this short wiki article : https://en.wikipedia.org/wiki/Spatial_cutoff_frequency , and try to spot the problem in above sentence

Best to measure with a set of calipers. M4 screw will have thread diameter (not body but threads) a bit less than 4mm, depending on tolerance class. From your image, right screw seems to be M5 because shaft of the screw is more like 4mm than whole thing with threads. If you look at this diagram: Major radius differs from minor radius by factor of 5 * H/8 where H is related by pitch by pitch * sqrt(3)/2 M5 has pitch of 0.8mm so H in this case is ~0.693 mm, and hence 5 * H / 4 which is difference of diameters (twice difference of radii) is ~0.866mm - or close to whole mm, so if you measuring shaft to be ~4mm - then actual thread diameter is ~5mm

That is a bit loose statement How about hexagonal or maybe octagonal sensor? In fact, best non circular sensor having all straight edges would be N-tagonal regular polygon where N -> infinity

No reason not to use it. It is optical element - it does not care what comes after it (if it is DSLR or some other type of sensor). Only thing you have to worry about is size of corrected field and most dedicated cameras are either same size of DSLR or smaller (there are only few full frame dedicated astro cameras and they cost an arm and a leg). No, it won't be much of a problem. Even if you are over sampled - it will be by small amount.

With 2um pixel size and Mak127, you really don't need barlow for planetary imaging. This will help keep the exposure length lower to freeze the seeing and help you make better image. This is the size of image you can expect 5" telescope to produce: (yes, that is your image slightly tweaked and resized to "normal" size).

I recently acquired Svbony 9-27 and had a chance to compare it against Baader Mark III and Mark IV. Comparison was done in daylight, and while Svbony has narrower field of view - it provided me with better contrast in Svbony. This was in Mak102 and in F/6 80mm triplet. I did comparison to decide if I'm going to get Baader zoom to be my zoom eyepiece, but I decided not to. I had a chance to compare Svbony 9-27 on moon against ES82 11mm and 12mm plossl, and I had the impression that it was lagging behind those two by a hair - but it was hard to tell as seeing was changing moment to moment and it might as well be effects of seeing that I was experiencing. In my mind it was ES82 > Plossl 12 (GSO version) > Svbony, but like I said - exceptionally small difference in sharpness - almost imperceptible to regular observer.

Here it is: https://www.1728.org/angsize.htm