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I would go for 130PDS except for intended mount - it is rather limited. In my view 130PDS has edge in AP and visual. Only advantage 72ED has is portability and small size. It is wide field and travel scope. 130PDS has 4Kg without any accessories. Add CC and 600d and you'll be pushing your mount quite a bit. You also need to think mount precision regardless of the weight. AZ-GTI is wide field imaging mount, so wide field scope is really only suitable option (next to of course imaging with lens - which are inherently wider field since they are shorter FL than almost any scope out there).

There is quite a bit of difference with effects of CO on imaging and on observing. For imaging it is not as nearly as important as for observing. We can and do digitally enhance captured images (contrast and deconvolution / sharpening). We can't do that while observing. In fact, for very technical reasons, I think it is better to image with obstructed aperture than clear one (of the same diameter). As a final note, I don't think people really understand what sort of contrast we are talking about here as it is frequency dependent contrast and it is only partially related to notion of contrast that we use in everyday life.

Not really, or rather - it depends on measurement method used. For example Roddier test (WinRoddier implementation) allows you to select if you are using artificial or real star and in case of artificial star - distance to telescope. It will automatically remove spherical term produced with close target so you can have even 1/4 or 1/2 wave of spherical and it will be accounted for. On the other hand - that much spherical with other tests will mask other aberrations so you won't be able to tell how good optics is. Any test that produces Zernike polynomials of your optics allows for removal of appropriate spherical term - you just calculate spherical magnitude like above given aperture size, wavelength and distance and you remove it from appropriate Zernike term and then synthesize wavefront again. This is common practice - and you will often see some terms remove in optics reports - like coma from newtonian parabolic mirrors since it is inherited in design and will be 0 at optical axis (but can have small value if test was done at an angle and is thus removed).

There are two "points" that you need to worry about when using artificial star. First point is point where artificial star is resolved. You want angular size of your artificial star to be at least half that of airy disk in order not to be resolved. This distance is not all that big with very small artificial star. For example - using 150mm aperture telescope and medium sized artificial star - about 20um (most artificial stars are in range of 50 down to 10um), calculation goes like this: 150mm aperture creates 1.7" airy disk, so we want our artificial star to be at a distance so that it presents about 0.8" of angle with its diameter. http://www.1728.org/angsize.htm handy calculator for those things, we input values - solve for dist a nce, angle in arc seconds and we will put artificial star diameter in millimeters rather than microns - so 0.02 Solution is 5156mm, or 5.15 meters. That is fairly close and in general you need to worry about that only if you have larger artificial star and larger aperture (like 50um and 200mm+) then you need 10m+ for resolving criteria. There is second criteria - and that is amount of spherical aberration we want to introduce or rather not introduce. That is important for optics testing. Here we can do following: Let's say we want to have less than 1/10 wave spherical due to distance and we work with 150mm aperture. What is the distance we want to place our artificial star? 1/10th of 0.5um (500nm green light) is 0.05um. What angle will give difference in hypotenuse and side of 0.05 and what would be sides length? Here is a nice image to explain it. Artificial star is at A. B is center of our aperture and C is edge of our aperture. AC edge (hypotenuse) is longer than AB edge (Adjacent side). We know CB - as being half of aperture, or 75mm. We need AC - AB to be 0.05um long. So first equation is: AC - AB = 0.05um => AC = 0.05um + AB Second equation is Pythagoras AC^2 - AB^2 = CB^2 => AC^2 -AB^2 = 5625mm (0.05um + AB)^2 - AB^2 = 5625mm 0.0025um + 0.1um * AB + AB^2 -AB^2 = 5625mm => 0.1um*AB = 5625000um - 0.0025um AB = (5625000 - 0.0025) / 0.1 = 56249999.975um = ~562450mm = ~56.2m We need at least 56.2m of distance for 150mm telescope to have 1/10 spherical from artificial star.

Not really necessary to do that as collimation issues will be equally seen both sides of focus - only mirror reversed. I don't think so. You certainly need larger or at least the same aperture of "reference" telescope to be able to cover whole aperture of measured telescope for optical quality tests, but wavefront error won't be reduced if one is using only part of the aperture of reference scope - at least not random part. If you know that you have very good section of mirror and you make sure you use that section of the reference mirror - then yes, it will help. Another way large mirror helps is that same mechanical surface quality translates in less relative wavefront error on large mirror (at least I think so), but large mirrors are harder to make and on average I guess you end up with same level of wavefront error (basically you get what you pay for and if you want very good figure, it's going to cost in large aperture).

You can do collimation on defocused star pattern so no need to bring it to focus On a side note, if you want to do collimation or anything else indoors - you can use another scope to do it. Place artificial star at focal plane of a second telescope and turn that telescope towards the telescope you want to collimate - sort of aperture towards aperture setup. Second telescope will act as collimation lens for artificial star and it will provide collimated beam - like real star at infinity. You should be able to reach focus with primary scope quite easily in this case. Mind you, if you want to measure quality of optics of the first scope - you need seriously good second scope that won't produce very aberrated wavefront, or alternatively you need to know wavefront of second / collimation scope and subtract that from result.

Have you tried to figure out if you are missing out focus travel or in focus travel - trick with the bright star? What telescope is this?

Barlow lens usually moves focus plane further out so you would need additional extension to reach focus. You can check this by aiming scope at the bright star and using longer exposure. Star will be a circle rather than point. Take a shot and note how big that circle is. Move focus a bit and observe how star diameter changes. If it is getting bigger you are "going the wrong way" with focuser, but if it's getting smaller - you are moving focuser in proper direction. Continue doing so until you turn circle into a point. If you run out of outward focus travel - you'll need additional extender to make it work. I highly doubt that you have issue with inward focus travel - but that can happen when using focal reducer rather than barlow lens, as focal reducers move focal plane inward rather than outward. If you don't have suitable extender, you could perhaps try with diagonal mirror? I don't recommend imaging with diagonal mirror all the time, but for testing purposes it will add quite a bit of optical path - about 60-70mm for 1.25" and about 100-110mm for 2" version. That should give you plenty of distance and you should be able to reach focus with barlow.

With Mak you might get slightly shorter FL or longer FL, I'm not sure. That is because FL depends on distance between mirrors and using diagonal with less optical path means you need to take mirrors further apart to find focus (again, I'm not 100% sure on that). On other telescope designs, using diagonal with shorter light path won't have any impact on FL of objective what so ever. In either case - I doubt you'll see any change in magnification.

Problem with measuring of apparent field of view of eyepiece is in the way it is measured. It also depends on what we mean when we say apparent field of view. For example you could measure apparent field of view by using true field of view (probably easiest and most common method) - like drift method. It measures time it takes for star to drift across the true field of view - thus giving one true angle on the sky that view covers. Then it is simple matter of multiplying with magnification. There are couple of issues with this approach: 1. Are you certain you know exact focal length of telescope as you need it for magnification calculation? 2. Are you certain that manufacturer of eyepiece did not round up/down their EP focal length - is it really 24mm or maybe 23.8mm or perhaps 24.1? 3. Is there geometric distortion in image that eyepiece renders? All of these can change AFOV result. Then there is an issue of what you mean by field of view? That can also be defined in two different ways and actually have different values for same eyepiece. First way to define it would be: take eyepiece without telescope and just look at it against white wall. You will see a white circle surrounded by field stop. Take white circle and put it on the wall and look at it without eyepiece. Once these two circles appear same in size to you - you can calculate what angle circle subtends. Other is actual TFOV that you see in night sky - again subject to magnification. These two can be different due to point 3 above - but which one is more important to you? One has to do with "immersion" - how big image is compared to framing (field stop). Other is to do with how wide true field of view you can achieve. All of the above is just to explain why there are different measurements of AFOV of eyepiece and what they mean - depending on method used. For your particular problem - if you think that 40ish degrees at 24mm will be too narrow - I suggest you start looking at this eyepiece as being 8mm - 20mm zoom instead of 8mm-24mm. Get yourself 24mm or longer FL eyepiece to serve you as wide field / finder EP and use Baader zoom for everything else.

If you use same camera on both scopes and process your data the same way (no binning or other things done differently between two scopes), then despite having scope with only 72mm of aperture - it will be "faster" than 6" scope. It won't be as fast as F/stop speed increase from photography says. That rule holds only for daytime photography when there is plenty of light and again - for same sensor and processing. In astrophotography we are dealing with very low light levels and everything else starts to make big impact on final image - thermal noise, read noise, light pollution. All of those mean you won't get as reduced total exposure time as difference in F/ratios would lead you to believe from daytime photography rule. You might have read that F/ratio does not describe the speed and that is correct if you have ability to control third parameter - sensor size and pixel size, whether thru hardware (choice of camera), or even in software - pixel size can be changed with binning. Things get more complicated then and F/ratio does not represent speed of imaging anymore.

So, hypothetically in 100-130mm refractor line you want this: 10-12" RC + ASI6200 mono Three of those of course, or maybe Quad setup would be even better - so you can do LRGB and Ha, OIII, S2 + Quadband for luminance at the same time?

Don't see it as major upgrade step from 224 / 385 / 290 It has similar read noise characteristics as 224 / 385 models - around 0.6-0.7e and pixel and sensor size as 290. Peak QE is TBD, but I suspect it will be around 80% mark, maybe a few points more - again nothing significant. Most interesting feature is IR capability of this sensor. It can be considered monochromatic sensor at 825nm and it has peak QE there - provided one uses rather narrow filter - like 20-30nm. That is very specialized usage that does not have general appeal.

Not sure how to answer that. Scale has 9 steps and that is smack in the middle of it But then again, if you consider difference between Bortle 9 and Bortle 5 and on the other hand Bortle 5 and Bortle 1 - perceived difference / enjoyment improvement is much greater on 5-1 side.

We can do waves as well Here is ripple tank simulation of this phenomena - how waves "bend" behind edge. This is why I said above picture is not very precise technically - but to my mind - it is easier to grasp - you can't have both things at the same time - "closeness" to the edge and "direction". Much easier than trying to explain why waves bend behind edge

Maybe easiest way to understand it (but also not very true technically) - is that it is quantum phenomena and related to Uncertainty principle. Once photon passes very close to straight edge - it is defined in position (closer it is to edge - well edge is not fuzzy so photon must be at certain distance to it) with respect to edge - but it does not have well defined position "along the edge" - only at direction towards the edge. Uncertainty principle says - you can't have both speed or rather momentum and position known at the same time. Photon moves at speed of light - so speed is fixed, it also has known energy, so only component of momentum that can change is direction - closer to the edge - more defined position with respect to edge - more deviation in direction with respect to edge (or in that direction - not direction parallel to the edge). Very similar thing happens when you have water running from the tap and you put your finger - it will start bending perpendicular to your finger. Mind you - this happens for completely different reason - but effect is the same. Here is illustration with comb - this time, third reason - electric charge - but again same effect water jet bends perpendicular to comb edge: Another example would be dual slit experiment / diffraction grating - where light spreads "horizontally" if slits are vertical - in any case at 90 degrees to slit orientation.

Star spikes are actually perpendicular to original spider supports - this is why you have 6 of them on 3 prong spider support.

Or in another words - projection system from angles to linear distance, where certain angle always forms same distance (to a good approximation)?

I had that scope and I measured it long time ago but I can't remember exactly what it was. About 3.5kg or something like that.

I'm afraid I don't really follow your logic / argument here. You say that imaging system has linear magnification - but that means that any object of size X will be mapped to image with size Y. If you check, both the Moon and the Sun, while having vastly different diameters will form images of the same size. Hopefully OP will find good enough answer in non technical stuff as I'm certain that all participants set off to answer original question the best they could. I do agree that it is rather easy to slip into much more technical discussion that underpins issue discussed. @inFINNity Deck maybe we could continue this discussion over PMs once I read the article / post you referred to, or perhaps @Stu could be so kind to split technical part into thread of its own if there is enough valuable info to be found for other members of SGL?

Name is Vladimir, but feel free to call me Vlad With regards to critical sampling rate, 2.4px per Airy disk radius or 4.8px per Airy disk diameter is based on Nyquist sampling theorem. In fact, I did not do the math, but rather set of simulations with different airy disks and FFT to determine cut off point in frequency domain. Many people assume that x2 from Nyquist has something to do with spatial features - like Rayleigh criterion or similar - it does not. Sometimes I read that people say x3 or even x3.3 is better as we are dealing with 2d Nyquist instead of 1d case - again not true, x2 max frequency is again correct criterion in 2d - grid sampling case. This applies to band limited signal and indeed telescope optics provides band limited signal - point is, finding what frequency represents cut off point. If you generate airy disk PSF (that would be FFT of aperture - so simple circle or maybe obstructed circle with/without spider support) you can also generate MTF of that - which is FFT of airy disk. That represents airy disk PSF in frequency domain. It is also description of blur that optical system introduces - how much attenuation there is for a certain spatial frequency. In this case, X axis of MTF that I marked is in relative frequency units, but in general - it is in absolute frequency units that can be expressed as cycles per arc second. At one point, here marked with 1 - MTF falls to 0 and there are no higher frequencies - all have been attenuated to 0. This is our cut off point. Mathematically, you need to take Airy disk function and do Fourier Transformation of it and then equate that with 0 and find at what frequency it reaches 0 in order to find cut off frequency. I might do that some day, but it is not easy thing to do as Airy disk function is not trivial. In any case, I did simulations with different airy disk sizes and "measured" where resulting cut off point is (using FFTs to generate both airy disks and MTFs). It turns out that relationship is about x2.4 of airy disk diameter. Later I found out that this value could be related to 1.22 constant of first zero of corresponding Bessel function and that actual value that I measured above is 2.44 rather than 2.4 - but I'll need to see some math to confirm this. As for relationship to FWHM - theory behind is the same, except that I used gaussian profile to model PSF and then did Fourier Transform of Gaussian (that was easy to do), and looked at cut off frequency - which I took to be when attenuation is such that frequency is attenuated to less than 1% of original value. I then took that frequency and based on that and Nyquist criterion determined factor for "optimum" sampling rate. I wrote about both of these approaches here on SGL. Let me see if I can find corresponding threads.

I was thinking of messing with mesh size - that is obviously way to go. Regardless, I might need to touch it up by hand as it mistakes some bright nebula patches for stars and removes those as well.

I have not forgot about this thread, but I did run into difficulties for some reason. Starnet++ is not doing very good job on star removal with this data. Although on this scale it looks nice (still linear, but stars removed so much more nebulosity is seen): there are quite a few artifacts once zoomed in - which currently prevents me from doing decent processing - like these:

There is no such thing as magnification when imaging. Magnification amplifies angles and is applicable when talking about visual for example - so telescope / eyepiece combination. When you view image on your computer screen - zoom will depend on how close you are to that screen - place it at 50cm away and it will be reasonably "zoomed in", but stand at 10m - and it will be very small. This shows that image does not have magnification. Telescope + camera is projection device. Telescope projects image onto the sensor. Two things are important here - field of view, which depends on telescope focal length and sensor size - how much of the sky are you projecting on the image? Second thing that is important is sampling resolution. That depends on focal length and pixel size (or sensor pixel count and field of view if you will - these things are connected because quantities depend on one another). With sampling rate - you can, over sample, under sample and sample just right (Goldilocks anyone? ). Under sampling is not so bad, it won't record all the detail that could be recorded, but it improves SNR. Over sampling is rather bad - you are "spending' resolution on empty detail (no detail to be captured) and in doing so you are lowering your SNR (signal to noise ratio - probably most important thing in imaging after mount ). Sampling just right is the way to go of course. Ok, that strange F/ratio > 3 x pixel_size_in_um formula is just wrong. If you are after optimum sampling rate of diffraction limited system (Without influence of atmosphere - for planetary / lucky type imaging), then correct sampling rate is 2.4 pixels per airy disk radius (4.8 per airy disk diameter). On the other hand, if we are talking about optimum sampling rate for long exposure imaging - we need to look at star FWHM, and in this case, close to optimum sampling rate is FWHM / 1.6 as a resolution arc seconds per pixel (if FWHM is expressed in arc seconds - so 1.6" FWHM needs 1"/px sampling rate). You are correct about color sensor sampling at twice the lower rate than monochromatic sensor - but many people don't process color images / separate color information in the way that respects that anyway (they interpolate instead of just splitting color planes).

If it's not too much to ask - could that be just linear stacked (32 bit fits / tif) without DBE and deconvolution or any other processing?