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I'm not familiar with this particular model, but I'll venture a guess here on questions you asked. "Standard" Vixen finder shoe often uses two screws to secure it in place - like this one here: https://www.firstlightoptics.com/finders/astro-essentials-finder-shoe.html Although technically - you could secure it in place with just one screw. My guess is that two larger wholes - one marked for finder scope and other of same diameter that you circled in blue are for same purpose - attaching finder scope shoe. Difference between the two is that it allows you to decide if you want your finder to rotate with the focuser or not. In your case - you want it to rotate to properly mount the scope on AZ4. There are cases when you don't want it to rotate - like when imaging - you rotate focuser to frame the subject - but have guide scope attached at fixed finder shoe - you don't want to rotate that as it messes up with guide calibration. I suspect that two smaller holes contain grub screws that are used to adjust the tension / play on rotation mechanism. As for slipping focuser - I suspect that focuser is crayford type and that you should have screws on the bottom side - used to adjust the tension of focuser itself. There should also be focuser lock screw down there. I'm sure there are online tutorials that show how to adjust tension on such crayford focuser. There are threads like these: https://www.cloudynights.com/topic/651495-crayford-focuser-tension-adjustment-help/ for example. HTH

Just realized - if you are using DSLR - than you'll have at least 44mm of distance between prism and main sensor or even more. This is because Canon DSLR has flange focal distance of 44mm - and anything you put on lens mount will be at least that far. You are most likely operating with stopped down OAG because of this. What is the size of the prism on your OAG?

Indeed it is. Some of mine also start showing astigmatism. I have slight advantage as I place OAG closer to optical axis since I'm imaging with ASI1600 - which is 4/3" sensor. With APS-C sensor - you need to place OAG further out from optical axis and it is reasonable that stars will be more distorted further out.

I find that also exceptionally interesting as it can have several implications. One would be that "collapse" of wave function is not random at all and is predetermined - so it really does not matter "which one is measured first". I'm just wondering if we explain the loss of entanglement with decoherence - or interactions of particles with the environment - then particle gets weakly entangled to bunch of other particles and that destroys the phase of original strong entanglement. That implies that all particles are in constant state of entanglement with environment on some level - what if we simply can't perceive all that complexity and it leads to apparent randomness, but in reality - it is all very deterministic? Maybe there is universal reference frame after all and chronology of collapse is tracked in that frame?

At what distance do you keep your OAG? Larger sensor is hard to cover with small pick off prism without vignetting on a fast scope. You need to have prism very close to sensor. I guide with OAG and with ASI185 which is 1/1.9" sensor (tiny bit larger than your QHY with 1/2" size) and at F/8 I get visible vignetting: At F/5 you need to keep prism to sensor distance less than about 30mm in order to have similar level of vignetting as above. If you have it more than 40mm - you'll effectively stop down your guide aperture (in case you have 8mm prism). In any case - get ASI290, bin x2 guide pixels (there is option in PHD2 to bin 2x2 in software - you should turn that on) and yes, use 3s guide exposures - that both helps with seeing and gets you enough guide stars. Another thing - use ASCOM driver instead of native as that enables 16bit data to be downloaded instead of 8bit data. Put your OAG as close to sensor as you can to avoid guider being stopped down.

No effect on transmission what so ever. There is real effect on observation but not related to filter and transmission - but rather our perception of the image and ability of eye/brain system to detect / recognize image. Larger exit pupil (or lower magnification for same aperture size) concentrates more of the sky onto our photo sensing cells in our eyes. This means more photons hit our "eye pixels". Target is easier to detect. Problem with above is light pollution as it also has photons and lower magnification also concentrates those photons so background sky also becomes brighter. Relative change is roughly the same (not quite as our vision is not linear - that is why there is "sweet spot" exit pupil for given conditions). When you add filter - you remove much of the light pollution. More LP you remove, or rather darker the background - more benefit you will see from lower magnification or "concentration of the light".

Too much work as you can get the same result in easier way. By doing that you are effectively interpolating between original data points. There is no more data to be gained - these intermediate stacking steps will have high correlation with other samples - so you might as well just interpolate existing 10 points with some interpolation algorithm to get nice curve. check out this for example (hit example on the page and play around with data)> https://tools.timodenk.com/cubic-spline-interpolation

WO GT81 is doublet so not really an answer to the puzzle that is ~£1000 80 triplet. I would recommend this one (it is a bit higher than £1000 but OP said a bit more could be squeezed if need be): https://www.altairastro.com/altair-wave-series-80mm-f6-super-ed-triplet-apo-2019-457-p.asp alternatively, there is this one with FCD100 glass: https://www.altairastro.com/starwave-80-ed-triplet-apo-travel-refractor-465-p.asp that is well within a budget. First one will be a bit better at imaging - less star bloat while second one will possibly need special lum filters to tame slight star bloat. Not 100% sure about all of that. I have TS80 F/6 APO that is earlier version of first scope listed (at least AltairAstro says it is 2019 model - but I presume it is just batch rather than model year) and I'm really happy with it. I would not however use it as a guide scope . Too good to be simple guide scope. Would rather use something like ST80 for that job if guide scope is required or simply use OAG.

I think that confusion comes from terms faster / slower and lower / higher. I believe that terms lower / higher are referencing actual number in F/number expression - hence F/6 is lower than F/10 because 6 is lower number than 10. Phrase F/6 and higher should be therefore interpreted as F/6 and slower

In fact, we could argue that it is not suited to faster than F/6 because of exit pupil. Say you have F/5 scope and want to use 40mm Plossl. You'll get 8mm exit pupil and since odds are that your pupils can't dilate that much - you'll see less light than your scope is capable of delivering. Your eye will act as aperture stop.

In principle - yes. However, don't despair, even mounts that are designed for astrophotography (some decent EQ mounts) - could easily end up with such exposures. That is the reason people guide. With 2000mm FL - any modern DSLR with relatively large pixels is going to be oversampling - or in another words - "very zoomed in". I measured your star trails on second image and they are about 200px long. With modern DSLR with say 4.3µm pixel size, you'll be imaging at 0.44"/px. That means that your star trails are 88 arc seconds long. Mounts like EQ5 or HEQ5 and even EQ6 which are considered good for AP - can easily have 45" peak to peak periodic error. If periodic error is fast moving (usually it takes 5 minutes for mount to drift 45 arc seconds) - you can get star trails of 30-40 arc seconds long in matter of minute or two. For visual - you would not notice the difference, even at planetary scales. Imagine you are observing Jupiter - 88 arc seconds means that it will drift back and forth about two diameters over minute. That is small portion of field of view in the eyepiece and you would not even notice it. For astrophotography - yes, it is a problem - but most people have this problem and if they want to do long exposure - they guide. You can't do that on AltAz mount so if you want to do imaging - you'll have to limit your exposures to maybe 5-10 seconds to avoid excessive trailing - and even then, be prepared to loose some of the subs as not every frame will be equally affected by this (as you can see from your example).

As long as you don't saturate your target - bright background is really not important - you remove it by setting black level in processing (or in case of gradients - with gradient removal tool). However, bright background is telling you that you really don't need to use longer exposures. Exposure length should be set to whatever exposure length is enough for LP noise to swamp read noise. Cameras with higher read noise - benefit from longer subs. As long as you use same total exposure length, once any other noise source (be that thermal, or LP noise) swamps the read noise - you don't need to use longer subs - results will be roughly the same (very small difference that won't be seen in final image). In above example - you won't be able to tell the difference between 30 x 120s stack versus 20 x 180s stack (same total imaging time - but 120s is already swamping the read noise so there is not much point in going longer).

Cone error is just positioning error. It will impact your goto accuracy, but with good alignment model - it will be accounted for. It is best if you get into plate solving early on. Connect your DSLR to laptop via USB cable and use software like APT for acquisition. As far as I know APT has plate solving, or it can be added easily. With plate solving - you don't have to worry about accurate positioning - you'll get exact coordinates where scope is pointing in matter of seconds.

Well, that would let you mount scope as suggested above - below the scope on vixen dovetail. You could mount both guider and counter weight - just put guider scope in front so it does not get blocked by counter weight. I don't know if weight of guider alone would be enough to balance the scope - but you could always add that counter weight behind it on the rail for additional balance. And you get free finder shoe for RDF that simplifies alignment (I use OAG and put RDF on my RC8" when doing initial alignment). Btw, here is tip to get good alignment: Mount everything and put scope in home position (pointing to NCP). Unfortunately you can't be 100% sure if scope is really at home position - it could be a bit off in both RA and DEC, but there is simple method to deal with this. Prior to doing any 2-3 star alignment or anything - tell it to slew to bright star. It will do that. Once on the star - use clutches to center scope / RDF on that star rather than handset / slew. Then move back to home position by slewing. Now, when you start your alignment - gotos in alignment and afterwards will be more precise.

Could it be a dropped scope that landed dew shield first? Lens would be still in lens cell so impact would be evenly distributed along the edge of the lens with retaining ring being sort of "soft" - but stress would cause internal fracture as you suggest.

I see. How far can you move the scope in the saddle? I remember that I could move mine scope (I have old version with only one vixen dovetail bar) all the way so that back of the scope is flush with saddle on Heq5. Maybe you could also consider moving guider on top dovetail if you have two of them? That would solve two of your problems at once. You will be left with spare finder shoe for RDF (light weight / cheap one) that you will be able to use for visual alignment and you'll be able to balance the scope properly.

There is really no "up" or "down" in space. However, there is convention that is more of a guideline than hard requirement - most people orient sensor so that width is aligned to RA axis and height is aligned to DEC axis. In your case - that would mean putting camera "on the side" with respect to dovetail bar (90° from what you have on the image). Depending on framing (if you want "portrait" orientation) - you should use it like on above image. Although you can set any orientation that you want in order to frame your subject properly - above two are recommended as are fairly easily repeatable between sessions. This means that your framing on the same target will be the same between consecutive nights which is important if you don't want to crop away much of the image due to rotation (if you are not careful and shoot at arbitrary orientations in different sessions on same object). You can be very precise with your alignment by using a simple trick. Find very bright star and start exposure. Now hit slew key in RA for example. Star will create a line on your exposure. If line runs horizontal then you have "landscape" orientation with respect to RA. If it runs vertically - you have portrait orientation. If it runs at an angle - well, rotate your camera to correct it unless you want exactly such orientation for framing. Orientation of guide camera is absolutely of no importance. What is important is to have it exact same way for each session if you don't want to calibrate your guider every time. I assemble and strip my setup for each session and I run guider calibration each time (it's not that long - like 5 minutes or so, so not a big deal).

If it indeed uses 0.965" and as I now pay more attention to details - it indeed does look like it - it won't be able to use regular eyepieces. Alternative would be: https://www.teleskop-express.de/shop/product_info.php/info/p6042_TS-Optics-45--Hybrid-Erecting-Prism---upright-and-correct-image---0-96-and-1-25-inch.html but that still would not allow for 30-32mm plossl and highest FL eyepiece would probably be around 20mm (if 50° afov). It makes more sense to get one of these instead: https://www.firstlightoptics.com/startravel/skywatcher-mercury-705.html or perhaps this one: https://www.firstlightoptics.com/startravel/skywatcher-startravel-80-ota.html and see how to mount it on photo tripod or similar.

That sounds sensible. You won't find 32mm wide angle eyepiece in 1.25" format. Most you can get is around 50-52° AFOV with plossl (maybe you meant eyepiece for wide field observing and not necessarily wide apparent field of view eyepiece?). Do be careful when selecting long focal length eyepiece for fast scope - at F/4.3 (70mm with FL of 300mm is ~F/4.3) you'll quickly get very large exit pupil and light will be wasted if your pupils don't dilate that much (some people say - well, a bit of light wasted, so what - and that is ok - but on reflectors there is issue with central obstruction - not so on refractors). 32mm plossl will give you exit pupil of 7.44mm and young people have pupils that dilate up to 7mm (sometimes even a bit more). Older can hit 6-6.5mm. Maybe go for EP up to 30mm? Most people find this to be a decent zoom eyepiece: https://www.firstlightoptics.com/ovl-eyepieces/hyperflex-72mm-215mm-eyepiece.html this to be decent barlow https://www.firstlightoptics.com/barlows/baader-classic-q-225x-barlow.html here is decent 30mm 50° Plossl design: https://www.firstlightoptics.com/vixen-eyepieces/vixen-npl-eyepieces.html (select 30mm option)

I'm not 100% sure, but I think so. CC is refractive element but situated much closer to focal plane and bends lights minimally (just corrects for coma rather than bend light to focus it). Although there will be refractive effects - I think that they are too small to be noticeable, even on images (who cares if star bloats 1/10th of a pixel due to CC, right?).

Hi and welcome to SGL. Maybe best way to start would be to give us more info about the scope - maker and any markings on it? I don't think that you should worry about F/ratio and eyepieces at cheap end of equipment prices. Eyepieces that people talk about that are well corrected in such fast scopes tend to be more expensive than cheap scopes themselves, so I'm guessing that is really not an option for you.

Depends on the scope. Refractors will require some sort of UV/IR cut filter as lack of it creates bloated stars. These scopes can't properly focus UV and IR light if visible is focused. For reflectors - you don't need one. In fact - when you image DSOs - there is a benefit of not using one. Many targets have significant portion of their signal in IR part of spectrum - this boost signal and thus SNR. Color processing is a bit harder when not using UV/IR cut filter - so that is something that needs to be considered as well.

You can get AstroImageJ to accurately measure FWHM but I'm not sure it will do batch measurement (like multiple stars across multiple subs). I do know that you can CTRL+click or something like that to get FWHM profile on any star. I find AstroImageJ more accurate than other software. Yeah, I think that rule of thumb is from old days when things were measured in focal length Nowadays it is measured in resolution / sampling rate irrespective of focal length used. Back then - if you were using short FL that meant you were working on low resolution and if you were using long FL - that meant that you were working at high resolution. Now we have both small and large pixels and we know how to bin - that means you can use 650mm FL and still work at very high resolution of 0.76"/px for example. In my view, OAG is better way to guide than guide scope for number of reasons. People are sort of afraid of using OAG because of "difficulty of setup" and "trouble with finding stars". I did not experience any of those with my OAG setup - it is as easy to use as guide scope and more effective. In order to achieve good guiding results that your mount is capable of - you need to provide it with good resolution of star position from guide system. Short FL guide scopes paired with larger pixels don't really offer good resolution for that. Even smaller pixels struggle. Say you want to guide with F/4 50mm guide scope and ASI120 - that is 200mm of FL and 3.75µm pixel size. Single pixel covers 3.87" by 3.87" and you want your mount to guide at say 0.5-0.6" RMS. You want average measured error to be something like 8 times less than single pixel size. Although centroid algorithms can measure star position down to say 1/16 of a pixel, in this case ~0.241875" - can you really reliably measure error in mount position down to 0.5" or 0.6" of average error if your best position estimate is limited to every 0.25"? Using OAG let's you use much longer FL to guide on - and that simply means better precision in determining error / correction and average error, resulting in tighter stars.

Not sure what you mean by this - but even if stars seem small or large in the image (bright or faint) - they will have fairly similar FWHM value. That is why FWHM is used as a measure of things. FWHM will depend on where in the sky your scope is pointing. Out of 3 things that impact resulting FWHM the most - two depend on where the scope is pointing. Seeing is usually the best in direction of zenith and degrades as you move down towards horizon. This is because light has to travel thru thicker piece of atmosphere so it has more chance to get distorted along the way. Guiding performance also depends on where the scope is pointing. RA error is the worse at equator - but it gets smaller as you increase declination and move towards NCP. If you think about it - Polaris takes 24h to make very small circle on the sky - you can fit that circle on your sensor with ease. Path that Polaris takes is something like maybe few thousand pixels - and it takes 24h for it to make it. Close to north celestial pole - large movement in RA results in very small movement in pixels on sensor - this also means that any error in RA will be very small when translated into pixels. Star won't be smeared as much due to RA error close to NCP. At 60° declination RA error is effectively halved because of this effect and continues to drop (times cos declination). Math is more complex than that. In fact - with same sensor both scopes are F/5 so both scopes will be equally fast. This is because of fixed pixel size. Although aperture is half in diameter - so is focal length and that means that sampling rate is half of larger scope. Light gathering is for smaller scope is 1/4 of larger one but effective pixel size (how much of the sky pixel covers) is 1/4 with larger scope - so each pixel in principle gets the same amount of light. 250PDS indeed gathers more light, but you need to pair it with matching sensor (both sensor size and pixel size) to take advantage of that. Or - you need to bin your data (which is in effect the same as having larger pixels) to match the resolution between the two scopes. If you match the resolution / sampling rate / effective pixel size - how ever you want to call it - thing that is expressed in arc seconds per pixel for both scopes - then you have the math right - x4 larger aperture will be x4 faster.

Not sure that I can. It is really basic concept. It is taking group of 2x2, 3x3 or larger of pixels and summing their values. Result is the same as having x2, x3, etc larger pixel size than original. There is hardware binning - that is inherent in CCD design, and there is software binning. CMOS cameras due to their design are only capable of software binning. Even if you bin at capture time - it is effectively done in firmware of the camera and not in silicon. Difference between the two approaches is level of read noise. With CCDs and hardware binning, summing is done on electrons them selves - they are "counted" together, before ADC is performed. This results in single "dose" of read noise added to binned result. With software binning - each pixel gets single "dose" of read noise and total read noise on 2x2, 3x3, etc bin is equal to 2 * read noise, 3 * read noise, etc ... Binning works very similar to stacking and is equivalent to having larger pixel size in terms of sensitivity (both ways of viewing it yield same SNR improvement - so you can look at it either way). I prefer to bin my subs in software as that lets me choose bin factor. If I judge that 2x2 bin is enough and I have resolution in the image to match - I can leave image at x2. Sometimes atmosphere is just too unsteady and I need to bin x3 or even x4. ImageJ has that option - simple command will bin your sub by chosen factor. PixInsight has it also - called integer resample.