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That is correct. Shoot flats though to correct for small difference in sensitivity (although it is not uniform, so you can't really correct it in any way, flats will help somewhat).

You think it will suffer from spherical aberration? I don't know, I think all of those mirrors on AliExpress come from the same factory / factories as those mirrors in cheap newtonians. If whole scope with EQ3 mount (not really EQ3 they just like calling it that): https://www.teleskop-express.de/shop/product_info.php/info/p5546_TS-Optics-Beginner-Telescope-150-750-mm-with-mount-EQ3-1.html can cost 235e prior to tax - that is with shipping from China, tube, mount, counterweights, finder, eyepieces and of course profit margin of TS - I don't see why it could not cost 25e for mirror alone. We can see a lot of small refractors in same price bracket or even cheaper - and sure enough you can purchase achromatic pair (even select blue or green coating for small price premium) in 20-30euro range.

Neither of the two scopes is suited for astrophotography, or maybe I should phrase it like this: both scopes require extensive measures to be able to take even simple astro photos. 200p is massive beast and you'll have to purchase something like EQ6 mount and some rings to be able to track the skies. Alternative is using EQ platform - which just simply tracks and can't be guided. It is also not very precise and special technique needs to be employed in order to get anything decent (lucky type DSO imaging). That also requires special type astronomy camera with low read noise for best results (DSLR won't quite cut it). Upgrade to focuser will also be needed to be able to steadily hold imaging equipment. I actually imaged with 200p on Heq5 mounts - but quickly abandoned it as rather unsuitable imaging platform. ST120 is short achormatic refractor and it suffers from large amount of chromatic aberration. This will affect your imaging. I also used ST102 for imaging and here is example of result of that: yep, that is crescent nebula. You can see blue halo around bright stars - but also red halo around all other stars. This is spherochromatism - spherical aberration depending on wavelength of light. You simply can't get sharp image from this scope unless you do again some serious things that make imaging less than enjoyable. If you want to start with astrophotography - maybe it would be best to start another topic and ask for advice depending on your budget (it can be really costly enterprise). More, but not a lot more. Difference is really hard to quantify and it will depend on your observing experience. Beginner might not notice a lot of difference, but seasoned observer will say that it is night and day simply because 8" scope will show some small very faint galaxy while 5" will fail to render it visible. View will be much more different (dramatically so) on planets. 200p will be planet killer compared to ST120. Not really. As is - it will provide very poor view of planets. Fast achromats are simply not suited for high power views. You can however do couple of things to improve views - namely stopping the aperture and using filters. This will improve views somewhat at expense of other things (filters make colored view and aperture mask resolves less than full aperture without aberrations), but it will never reach what is possible with 200p. Out of the two 200p is much more all around scope and is better unless you have very specific observing as a preference. ST120 is better at wide field views and is lighter and easier to carry and store.

Maybe best way to practice looking thru the eyepiece is actually during daytime with eyepiece only. Take your eyepiece and hold it against white wall or sky or some bright uniform surface. Whole FOV will simply be uniform white (or that other color like pale blue or whatever). You easily adjust distance by moving eyepiece in your hand rather than your head - which can stay put for this. This will give you sense of best placement - one that presents whole FOV and there is no blackening of any sorts. Each EP will behave differently so you can sort of practice this way and get muscle memory to how should each eyepiece feel. Most telescopes will keep exit pupil roughly in the same place - but some scopes will move it. I have some issues with my Mak102 and 32mm plossl - eye relief is just too long in this combination. That 32mm Plossl is otherwise extremely forgiving and comfortable EP to use in other scopes.

This is quite interesting. If you are far away - you will only see center of the field of view - and this is normal because "pencils of light" diverge after they form an exit pupil: In above diagram - if you place your eye too far away - red and blue "pencils" will miss your iris and you won't see that part of the FOV. You will only see green. There is one place where all pencils intersect - this is where you should place your eye and you will see whole FOV without any blackening. Be careful not to mistake field stop which is the edge of the field with blackening of the edge of the field. If you push your eye too much while having long eye relief - you will start to see similar thing - edges of the field will again blacken. This is again because in above diagram blue and red pencils can't enter your eye. Slowly moving your eye towards the eye lens will let you hit the sweet spot - where you can see entire AFOV without any blackening. Long eye relief eyepieces often have eye guard that you can adjust for height. This is very handy so you can move it to represent correct eye placement height. If you never hit the sweet spot - that is very strange, but in principle can happen with eyepieces that have SEAP and very big exit pupil while your iris is very contracted.

Yes, almost all 6" F/5 scopes are in fact with parabolic mirrors - but there are a few exceptions. Interestingly - one can get 6" F/5 spherical mirror of AliExpress for something like 25e - so it might not be nearly as expensive for a test project? I wonder if simple sub aperture corrector / reducer can be made to reduce spherical aberration to acceptable levels for something like EEVA? Will have to play around with some optical design software to see what I can come up with that can be cheaply ordered online (simple lens and mirror combinations).

Aberrator to the rescue! Right one is 150mm F/5 system with 34% CO, while right left one is the same system with 1 wave of spherical aberration and -1 wave of defocus (to balance spherical). Not as drastic difference as would seem from above equations, but still quite significant when compared to 2" FWHM seeing blur: I guess it is not as feasible as I expected - too much blurring. Maybe only feasible for very low resolution work like 4"/px and below (instead of 50-200mm camera lens - but one would need to bin pixels and use mosaics).

Same page gives formula that I was using (although I found it on page about spherical aberration of one mirror systems) which is: W = 0.89 * D / F^3 (See above post - actual formula used was 0.888 * D / F^3). This formula results in one wave of spherical aberration. I'm not sure what blur diameter represents but 47 microns does seem too much (blur resulting from spherical aberration does not have definitive diameter). It does however go on to say this about 150mm mirror: Estimate that they give is that it acts as 1/5th of perfect aperture rather than 1/2-1/3rd (although 150 / 5 is 30 and not 20mm). I do need to run actual tests to see impact when combined with average seeing.

Let's call brightness of Star A - a and brightness of star B - b If stars are side by side, total brightness will be: a+b But when star B is fully eclipsing star A - total brightness will be just b. No light from A will reach us as B is in the way so we will only have brightness of B - which is b Magnitude system represents ratio of two values so we can write b/(a+b) and magnitude of that ratio is 1 (change from 6 to 7). 1 = -2.5 * log(b/(a+b)) => b / (a+b) = 0.39794 0.3981 b = 0.39794 0.3981 * (a+b) b = 39.81% of total flux (a+b is total flux)

I would advocate a bit different approach. If you already know limitations of your scope - maybe you could circumvent those and get better images by changing approach? For same sort of money as ASI533 - you can get considerably larger sensor - something like this: https://www.bhphotovideo.com/c/product/1508687-REG/canon_3699c009_eos_m200_mirrorless_digital.html That camera costs about the same as ASI533 - but you'll be able to bin your data x2 - to compensate what you are going to do with the scope You are going to stop your scope to 80mm of aperture to make it F/7.5. That will reduce level of chromatic aberration to more manageable levels and sharpen up image. Binning data x2 - will make any chromatic blur show less in final image, and you'll still have 3000x2000px images (a bit loss compared to 3000x3000px of ASI533).

It looks like effect might not be that significant as long as Airy disk is significantly smaller than seeing induced blur. This shows 0.75 waves of spherical: with different focus positions. I think that from graphs we can approximate resolution of the telescope alone as being 1/2 - 1/3 of aperture, so about 70mm scope. Maybe 2"/px would be a bit far fetched, but 2.5"-3"/px certainly possible with spherical mirror without corrector.

I just came to that idea because someone on our local forum made an inquiry about Omegon 150/750 on "EQ4" mount - it is this scope: https://www.omegon.eu/telescopes/omegon-telescope-n-150-750-eq-4/p,22465 I did a quick search online and found several sources that claim it is fast spherical mirror. Omegon does not state it is parabolic mirror - something they do for their other newtonian scopes - like this one: https://www.omegon.eu/telescopes/omegon-advanced-telescope-150-750-eq-320/p,61021 And according to this resource - it is most likely fast spherical mirror: https://star-hunter.ru/en/black-list/ In any case - it seems that 6" scopes with fast spherical mirrors are available - there is one from TS as well (looks very much like that Omegon one on EQ3 mount): https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5546_TS-Optics-Beginner-Telescope-150-750-mm-with-mount-EQ3-1.html That got me thinking about performance of such scope. According to https://www.telescope-optics.net/reflecting.htm P-V spherical aberration is given by this term: That calculates to 1.0656 waves of P-V error, so quite a bit of spherical aberration - but I can't tell how much that is on MTF diagram until I run some calculations / simulations. My gut feeling tells me that it won't be as bad when we take into account seeing and tracking errors. Maybe it will be possible to shoot 2"/px images with such scope.

Here is interesting idea. Would you use fast newtonian - like 6" F/5 with spherical mirror as astrograph for wide field? I know that there will be some spherical aberration, but how bad will it be if one samples at say 2"/px - resolution that is very fine for wide field work. No messing with coma corrector, not nearly as sensitive to collimation as fast parabolic. Only remaining issue would be field curvature and I don't think there would be much of it with spherical mirror on 750mm of focal length? What do you think?

Yep, my bad - I expressed myself wrong - images are in fact the same dimension - but content is "zoomed" in. This is clearly software artifact. You can see it if you open both images in browser - each in its own tab and blink between them. They are the same size, but contents is a bit zoomed in in second image. Here is animated gif made from two images (Baseline and your Lanczos x3 resampled example): Don't know why it happened though.

I don't use PI so I can't comment that particular script, but I do know that FWHM measurement is very sensitive to background levels and if background have been removed. After all - it is about maximum and maximum will depend on removed background. AstroImageJ uses aperture photometry and as such it automatically removes background influence regardless of any background removal done prior to measurement.

I was referring to enlarged image: this is enlarged with use of nearest neighbor interpolation - stars are not "pixelated" even when undersampled - that is artifact of interpolation algorithm. What do you use for FWHM measurement? Different software will give you different results. Try measuring with AstroImageJ. I believe it gives most accurate results. In any case - if your image has FWHM that is ~1.6px - that is properly sampled image.

@Mandy D Here is your proposed experiment with original image and x3 Lanczos resampling in IrfanView - with care taken that resampling is each time to 50% and back to 100% Left is x3 resampled image and right is original.

That is to be expected. For FWHM of 1.75" - adequate sampling rate is about 1.1"/px (FWHM / 1.6 is close to optimum sampling rate), so if you resize to 50% and enlarge to 100% - you will loose some data as sampling rate of 1.32"/px (half that of 0.66"/px) is equivalent of FWHM of 1.32 * ~1.6 = ~2.1" FWHM (you actually got 1.97" - but that is close enough given that 1.6 is not exact number but rounded approximation). I can't say because you used nearest neighbor interpolation. This does not look like undersampled star. It has at least 5-4px in both width and height. I don't know how much data is stretched and where is FWHM - but you only need ~1.6 px per FWHM to properly sample the star. If you want how that star signal really looks like - use appropriate interpolation.

Your Lanczos resampled image is slightly different dimension than other two - you probably used different scaling up and scaling down factor? But it is in any case much sharper than other example: Left is Gimp resampled and right is Lanczos resampled one - difference is obvious. Try applying Lanczos 3 times - but keep dimensions the same instead of changing them. No harm in doubting - that is the way we come to truth and true understanding. Everything must have an explanation.

That is not Shannon's sampling theorem states. Nyquist-Shannon theorem states that for perfect reconstruction of band limited signal one should sample at twice highest frequency component of that band limited signal. One needs to examine signal in frequency domain and determine cut off frequency for that signal. It is related to FWHM if we assume Gaussian profile - but not as simply as "half that value". You are correct that one can sample at higher frequency than that - and nothing wrong will happen with the signal itself as far as sampling is concerned and that is called over sampling. It has no ill effects as far as signal reconstruction is concerned. It does however have ill effects on SNR of the image as you needlessly spread light over more pixels than necessary and thus reduce signal per pixel and hence SNR suffers. As far as diagonals of pixels are concerned - you are wrong. Diagonal is longer than side of square and sampling twice per diagonal will be at x1.4142 longer wavelength - and longer wavelength is lower frequency. Highest sampling rate for 2d case with rectangular lattice - is side of that rectangle. Ideal sampling pattern for 2d case is not rectangular grid but rather hexagonal one, but since we don't have sensors with hexagonal patterns - we are using pixels instead and we need to make sure that we sample with two pixels per cycle of highest frequency component in X and Y direction (twice per wavelength of that highest frequency component - or at twice as high frequency). I had sharpening turned off for Lanczos interpolation. It was simple interpolation that was used. Drizzle works only on undersampled data and is probably most misused algorithm in amateur imaging. We can also do this on linear raw sub - results will be the same.

Yes, that will happen if sub optimum resampling algorithm is used. Try using IrfanView and its resampling - choose Lanczos resampling to repeat your experiment. Lanczos resampling is closest thing to Sinc resampling needed to fully restore properly sampled data. It is in fact windowed Sinc method (Sinc is just SinX/X function: https://en.wikipedia.org/wiki/Sinc_function - it is used to restore sampled data to actual function). In fact - choice of interpolation algorithm is very important, and we should always use advanced algorithms if we don't want our data to suffer low pass filtering. I've written about this topic before here on SGL:

Ok, I know something might look one way or another, but that is why we have science. Sampling theorem is mathematical theorem with concrete proof and we have very good understanding of the physics of light and processes that go into forming an image at focal plane of the telescope. In fact - we don't have to have it, we can measure FWHM of stars in the image and that is all we need to know about resolution and needed sampling. Above image is over sampled by at least factor of x2 and it is very easy thing to show even without explaining all the math behind this. You can simply take a crop of your image like this: Which contains enough detail and stars and then you can resample it to 50% of its size: which will contain all the data in above image, although sampling rate is twice as coarse. How can we tell that it contains all the data in the image above? We can simply enlarge that reduced version and we will get the same image as base line above - without any loss of detail: It works although we actually did this on stretched and processed (probably even sharpened) data, although this test is best performed on linear data. Another way to test is to examine FFT of the image: this is log stretched FFT of the image above. All the data is centered in the middle at just a bit less than half the frequency range. There is one outer ring that is consequence of processing / sharpening and is not there in linear data. In fact - I can repeat above experiment, but this time in frequency domain. I can completely erase all the frequencies above about 4.4 pixels per cycle - like this and do inverse FFT and this is result: Again - no difference in the image (although we are processing stretched data at 8bit per channel). So while it may seem adequately sampled to your eye - it is over sampled by at least factor of 4.4 / 2 = x2.2 or 0.66"/px * 2.2 = 1.452"/px is sampling rate for this data that will record all the information.

Google moon screen shot: See also: https://en.wikipedia.org/wiki/Rille

That won't work. I mean - RGB part will, but luminance at video speeds wont. It works for planetary because planets are really bright and single 5-6ms or even shorter sub contains enough signal for software to be able to align subs for stacking. If you try that sort of exposures, or even a bit longer - like 20-30ms - all you are likely to get is a lot of noise and maybe 1-2 brighter stars in the FOV. Not enough to judge quality of the sub (how tight FWHM is - since these stars will be distorted by seeing and polluted by noise) and not enough to compute good alignment information. In theory, you could do the following: Take bunch of very short subs (video speeds) that fit in some short time frame and then do brute force approach - stack without alignment all combinations of subs that contain at least some percentage and then select best combination. You will get 2s frame that you'll be later able to stack with other such frames. But that would require massive processing power to do all the calculations in any sensible time frame.

Yep, HEQ5 will happily carry RC8, even unmodified one. Mine is extensively modded, but maybe most important mod for RC is replaced saddle plate for longer one with surface clamping. As far as original topic - it comes down to following: 10" Newtonian will be larger and will require coma corrector. RC8 is lighter, shorter (smaller mount will handle it), won't be wind sail - but people complain about collimation and it has less aperture. RC8 does not require any sort of corrector with 4/3 sized sensor - which is a plus. Both of the scopes will require binning. I'd say that RC8" will require x3 most of the time on ASI294 (native / not unlocked) - with 4.63um pixel size. Natively it will give ~0.59"/px and that is oversampling. Even if you bin x2 - for 1.2"/px, that will also be over sampling 95% of the time. 1.8"/px will be good sampling rate most of the time. 250PDS at F/5 and if CC does not change FL will give 0.8"/px - and you'll want to bin that x2 for 1.6"/px If you unlock ASI294 in full resolution with pixels of 2.31 - then you'll be able to choose better bin factor depending on seeing conditions. Either of two scopes will give you plenty of FOV for galaxies (or should we say for most galaxies except maybe M31 and M33).