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It is exactly like in terrestrial photography - in this case, F/4.8 will be faster than F/5.9 This is because pixel size remains the same. There seems to be so much confusion on this topic. Don't know why is that. When camera remains the same and pixel size remains the same - daytime photography rule applies - faster F/ratio is indeed faster - but only when paired with that particular camera. In light dominated regime - rule from daytime photography also applies here - rule of F/stops and how much faster system will be. What we can't extrapolate from above are following: - F/4.8 scope is faster than F/5.9 scope. That does not hold in general - it does hold for same camera and if we keep pixel size the same, but if we start binning or using different pixel size or whatever - it is no longer true. - F/stop rule can always be used to calculate how faster system is. Again - this only works in above case where we have the same camera and same pixel size and we operate in light dominated regime. Once we start imaging very faint targets - other noise sources start to dominate single exposure and that math no longer holds Back to original question. Benefits of using reducer while keeping the same camera and same pixel size: 1. larger FOV 2. faster imaging (where speed of imaging is defined as time needed to reach target SNR)

As long as noise is in the same place in each of the images - above extraction will work. If noise also shifts for some reason but pattern stays the same - you could first align on noise rather than on image detail and then perform above. Then you would need to align that master to each image again when calibrating.

Maybe you can even extract FPN from those 16 images? As first approximation, I would do following: - Take each of 16 images and apply 3x3 median filter on them. - Subtract each median image from corresponding original image - Average residuals or maybe even do sigma clip stacking on them

good point.

Well - you can take 6" F/4 Newtonian + coma corrector and you'll get the same thing - well almost. Not sure how well corrected the field will be - but for low power visual - probably good enough. 6" with lower mirror reflectivity and central obstruction will provide you about same light gathering as 5" refractor. Then, there is real deal, but with a bit of false color https://www.teleskop-express.de/shop/product_info.php/info/p7788_Bresser-4827635---127-mm-Refractor--f-635-mm--OTA.html It is 4 lens elements design - although that is probably just to lessen chromatic aberration it could also be that field is fairly flat?

While all three of listed telescopes give nice views - only two of them can easily provide you with large exit pupil. As for rich field telescope / high etendue telescope see this: https://www.bbastrodesigns.com/HET.html Another way to put it was my recent quest: Give me largest effective aperture telescope that will fit M31 and some surroundings into FOV. You might think that eyepiece plays a part here - but it does not really - what does play a part is illuminated and corrected field (diameter - up to 47mm - max for 2" eyepieces) and largest aperture that will provide that while still keeping exit pupil at bay so no light is wasted. A lot of people report that best views of M31 they had was with binoculars - and indeed - you want something like x10-x15 magnification and large exit pupil. 127mm F/5 refractor telescope is capable of providing that low magnification with ~42mm eyepiece while giving you something like 4.3° of FOV.

Not easy question to answer. Depends on your eyes and on levels of light pollution. F/5 scope will let you easily achieve maximum exit pupil - which is about 7mm in young people - but is highly individual. You need 35mm eyepiece with F/5 scope to achieve this. With F/10 scope - you can't achieve such large exit pupil and best you can hope for is about 5mm - 5.5mm (55mm plossl for example in 2" variant). What does exit pupil have to do with anything? Well - it makes target smaller in size because it provides lower magnification - as long as you keep exit pupil smaller than your pupils - otherwise you'll be wasting light as not all light exiting eyepiece will be able to reach the eye. Smaller target means more concentrated light - which is in principle easier to detect - if there is no light pollution involved. If there is light pollution involved - it behaves the same - it also gets more concentrated and background sky looks brighter at lower magnifications. In light pollution it is all about contrast - but that does not change much with exit pupil. Best way to get better views of galaxies is to go to a darker place. Really. Even stepping to next aperture size or one after that is not going to improve things as much as going to bortle 3 skies for example. I saw more galaxies in 4" aperture in Bortle 4 than I ever managed with 8" aperture in bortle 7-8

Not very likely unless people learn to do their own tests. Say you had your scope shipped to a professional and you get report back and you do Roddier method on it and you get same result. Then you are fairly confident that a) Professional did their job right b) your scope is as per measurement If you were to just do Roddier test - you would always wonder if you did it wrong. If you have your friend perform same Roddier test on your scope with their camera - then you would have more confidence that you did it ok - but is it accurate enough? Maybe test itself is not as good as people say?

That is not quite what I had in mind. Thing that we measure needs to be the same - but tools used or person doing it can change. If two unrelated people perform same technique measurement on object and get the same result - I have more confidence that none made an error in doing the technique If two different measurement techniques are performed on an object and we still get the same result - then I have more confidence that measurement reflect object properties.

I think that best test of the "test" is repeatability. Can the same result be obtained by another test and can two independent testers using that test obtain same results. If so - then I'd say test is valid.

That is from wiki article on photo tripods. 1/4" is 20 TPI while 3/8 is 16 TPI Maybe this will help? https://www.firstlightoptics.com/skywatcher-star-adventurer/astro-essentials-3-8-photo-adapter-for-heq5-az5-tripod.html This has M10 female on the bottom and 3/8" on the top - if that is sort of the thing you need. I've used one of those to adapt my AzGTI to standard steel skywatcher tripod. It also accepts my Ball head mount as well - which has 3/8" connection:

Why is this better than say - Roddier analysis - which is essentially free if you have a camera?

yes - same fov will be visible - only "squeezed" in the center with dark ring around it - vignetting.

No, that would not work. Pan 41mm has field stop diameter of 46mm. With x0.75 reducer, you would be asking telescope to deliver 61mm of illuminated and corrected circle. Telescope itself illuminates only 45mm - so Pan 41mm natively without any reduction is pretty much all the scope can deliver - about 1°. Other than exit pupil being small (at F/12) and narrower field of view - I would expect 8" CC to perform as any other 8" on deep sky - very good.

I figured out why plugin was not working - it requires dark side of edge to be located on left side of the image. Here are results of perfect PSF for clear aperture at edge of sampling frequency - synthetic data, it should be the same, but it is not: Top one is plugin, bottom one is standard approach that I've been using (directly derived from math).

@alex_stars I think that we are not yet close of having proper working method for this. For your test data from few posts ago, this is what you got: This is what I've got with 2D fft approach: and this is what ImageJ plugin that you linked above produces (plugin is a bit buggy - sometimes it works and sometimes it does not - not sure if selection size has something to do with it): Their curve is much smoother and much more sagging.

Well, I would like to explore what sort of results do we get from under sampled data. There is well known drizzle algorithm that deals with this in imaging, but I believe it is often misused since people don't have control over dither step and I don't think it works good for random dither (original Hubble documentation implies that telescope is precisely pointed for dithers of a fraction of pixel - like 1/3 pixel dithers) In this case - we can use edge at an angle to give us wanted consistent dithering step - but we need to compensate for the fact that edge is tilted. I'm wondering if you implemented drizzle bit - or anything similar in your python code? Method that I proposed above with afocal and mobile phone - will mix in both optical properties of eyepiece and phone lens (although not sure how much at that scale will be picked up) - but it will provide wanted sampling rate. Camera in prime focus is likely to under sample. If we want a method that is reliable and easy for amateurs to use - we need to explore both options.

Forgot to post. For anyone interested in playing around with scope parameters and generating MTF - there is Aberrator: http://aberrator.astronomy.net/ It will pretty much do automatically what I do in ImageJ - given set of parameters it will generate various things for you: One of the reasons I'm interested in all of this is to make modern version of Aberrator (last version is 3.0 from 2002 if I'm not mistaken)

Phone at eyepiece is not as easy to calculate. You want sampling rate to be able to produce accurate MTF (properly scaled for cut off frequency - general shape will be the same), and you also want to make sure you are not under sampling but rather oversampling (in this test it is ok to over sample). Issue with phone is that you don't have phone lens focal length and often you don't have pixel size of your mobile phone - that leaves us with only measuring sampling rate by shooting object of known dimensions at known distance. Otherwise, for spherical aberration - same rules apply - you need to place it at large enough distance for spherical to be minimized. You don't have to do it - you can test with that spherical term, if you want to see close focus performance of your scope or you can make comparison ideal MTF to contain spherical term as well - just to compare things (but again - you can't tell how much spherical to introduce without measuring). I think I'll get myself that phone adapter as well. There is another project where I might need it - EEVA with smart phone thing.

Probably accurate, however, scope is very bulky / long / has high moment arm I'm not comfortable how it sits on my AZ4 - version with steel legs - which is otherwise very solid mount. I have AzGTI as well - and I would not even think of trying to mount Evostar 102 on that little mount head. Just to give you idea of the size of it: That is the picture of a friend of mine once I first got that scope - he came over for as I tested the scope to have his first views of the Moon (so I took a picture for documentary purposes - yep, he really looked thru a telescope). In any case - he is not short - probably about 5'10" or something like that. That scope is rather long. It has been a while since I last used that scope and I haven't had a chance to use it much, so I'm going to refrain from giving any serious comment on its performance. I remember being surprised at levels of CA - I believe moon had less than I thought scope will have but stars had more than I thought they would have. Baader Contrast Booster does help quite a bit with CA. If I was to choose planetary / lunar / quick grab'n'go scope - I would choose SkyMax102 instead (have that scope as well - and that one sits perfectly on AzGti mount).

Depends more on wind and humidity then on actual temperature. If everything is quiet and air is dry - with proper clothing couple degrees below zero can be quite comfortable. Check out wind chill factor and "feels like" column in forecast.

I think suggestion has nothing to do with recording time - it has to do with processing time and creating of masters - in which order to create those.

If anything, my question should be understood as a compliment - given your level of knowledge, for a moment it seemed very plausible that you were working on something like that - and maybe it was me who was not fully understanding the topic

No offense taken - I was just confirming if I got it right

Could be. I just responded to what was written - don't know how Shack Hartmann sensor work - will need to look it up. Ah, ok - fair enough - it is easy to understand and it is simple in its operation. Sure - just checked it. My bad - I thought that you were talking about pixels in focal plane of telescope - but here Pixel refers to Lenslet sensor as a whole - or subdivision of aperture.