vlaiv

You can test your eyesight right now if you know pixel pitch of your computer screen. Make image that consists out of three pixels - two white ones and one black in the middle. See what sort of distance from your screen you need in order to see two distinct pixels. Use angle / size / distance calculator to figure out angular size: http://www.1728.org/angsize.htm also look at this article: https://en.wikipedia.org/wiki/Snellen_chart It is about this chart: Letters in that chart are each 5x5 elements - look at first E on the top - space between top and bottom notch on right side of E is one element - top side is 5 elements - white space between top and bottom bars are 3 elements. Each letter is like that - and if you can recognize the letter - you have certain vision - for 5' letter size - you have 20/20 vision - which means that you can see all the notches and spaces on 5x5 grid that is 5' on the side - smallest element that you can see as distinct is 1' with 20/20 vision.

What sort of fly is that? If we take that fly's wing is 5mm in length on average fly - at 700 meters it will subtend about 1.5" and Dawes limit for 90mm scope is 1.3". I'm not sure if you'll be able to resolve the shape of fly's wing at that distance let alone detail on it

If we take Dawes limit - say 5" will have resolution of 1" - or two stars will be seen as two stars if they are separated by 1" and we take visual acuity into account: https://en.wikipedia.org/wiki/Visual_acuity Person with 20/20 vision has 1 arc minute visual acuity - that means they can see two stars as being two distinct stars if they are separated by 1 arc minute. This implies that magnification of only x60 is needed for 5" telescope to resolve two stars at 1" apart - or x60 is all that is needed. That is just x12 as minimum needed magnification in inches - much lower than x50.

Not sure how to best answer that question. I don't do much astronomy at the moment at all. Neither imaging nor observing. I don't really consider myself to be experienced observer, but I do like to observe and have done so on many occasions. What I don't understand about image break down is this - This term sounds to me that image somehow changes. This does not happen to my eyes. I've pushed my 8" to silly mags like x500+ - and only effect I ever saw when pushing magnification higher is just that - more magnified image, but image remains the same. Once level of detail is reached - it remains like that and image just gets magnified - like images on computer - image gets bigger and no additional detail is seen. Detail that is there is blurry - simply because image is bigger without additional detail. This process usually starts at about x1D in my scopes to my eyes. Up to x1D image is very sharp. Past that point - image sort of stays the same in terms of detail seen. I do find it more comfortable to observe at say x1.4D although no additional detail can be seen - but what is there is just a bit bigger and still not soft enough to be distracting. That is why I don't understand the term break down - for me, image never really changes - it only becomes larger and dimmer but it stays the same in terms of contents.

What does high mag reveal to you about the telescope? I really don't understand that - "holds up well at high mag" and "breaks down at high mag". Can someone explain that to me? Here, look at this as an example. Contender is this, probably one of the most affordable 80mm triplets on the market: When I got it - I did a test shot of some chimneys on house down the street (from where I lived at the time): That is FOV with APS-C sized chip. Here is close up at 100% zoom level: Second chimney - top part / gravel detail And this is part of it zoomed further 300%: Does this fall under definition of "falls apart" or "holds together"? I know that this is "digital" zoom - but in reality, image formed at telescope focal plane has certain definition irrespective of what sort of focal length eyepiece you use to magnify it (how much you magnify it). If EP is good enough - it will be just magnified as is - it can't turn better than it is?

I'm not even sure what does "start to break down" really mean. I've seen that used countless times - but I never really understood the term. I do agree that 1D is all that is needed for sharp eyed, and I personally enjoy the most that sort of magnification (maybe a bit more - at about 1.2-1.4 D - maybe I'm not as sharp eyed as one can be ).

That 2D for D in mm or x50 per inch in inches - is really not founded in science. Even term - maximum useful magnification is very strange. If you want to go by science - you'll actually get much lower values for "magnification that shows all there is to be seen". How much more magnification is needed for comfortable observing - depends on observer. It is a bit like text on computer screen. At certain font size - you'll be able to read the text. This depends on your eyesight. But that is not font size that you'll use to read the text - you'll probably want a bit larger text than smallest one that can be read. There is no upper limit of how large text can be - similarly, there is no upper limit to how magnified image can be - with one exception. Magnified image gets darker and that can start to cause issues at some point (sooner with smaller apertures than with larger).

We don't actually know what was it like - but all points to type of event similar to recombination (source of CMB). We can't say that it was in single point as at that time - space was not well defined. What we can say is that it most likely happened "all over the universe". This implies sort of causality arrow which I would prefer to be pointed the other way - something like "universe was created all over the big bang" - but point is - there is 1:1 mapping between big bang and "all over the place". As such - it is different than single point event and it leaves effects similar to happening in every point in space.

To me, term snap to focus means following: It's defocused - you turn focusing knob, turn, turn and then - suddenly it's in focus - snap to focus I'd say effect has more to do with mechanics of focuser and seeing conditions than quality of optics.

Not quite. If event lasted 1 second and was localized in single point - we will now see it lasting 1 second. It is a bit like this: So event happened at some point and light (information) from it starts to expand in every direction. We are some distance from that point and eventually that front hits us - and then we see the event. If event lasted 1s - we see front for one second. This all holds true for localized event that happened at single point. We need to be at right distance to see event now at some point and that event will last 1s to us as well. Big bang is not like that - big bang did not happen at single point. Big bang is more like this: It was happening in every point and in every point we have a circle that started out - so we don't have to wait for particular moment for front to arrive - at any given moment, some front will be passing us. In each second - front from points one light second further will be passing us. That is why we see cosmic microwave background radiation in every direction - in every second light from event that happened everywhere is passing by us - but each second we have front from a different place passing us. Makes sense?

https://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Timeline "Big bang event" is continuously reaching us. This is because big bang was everywhere. We can't really see the big bang - but we can see first event that can be seen after that - and that is cosmic microwave background. It can be seen now and it was seen 1 billion years ago. However - that does not mean it lasted billion years. If there was planet at the moment of big bang - or for sake of having any sort of merit - say we have localized event that had finite duration. Such event "projects" a sphere of certain thickness that travels at speed of light and expands. Thickness is given by duration of event x speed of light and radius at any point in time after event is - how much time passed after event x speed of light (this is simplified as you really need to do the same in 4d space-time to get correct results). In order to observe such event - you need to be at certain time space coordinates when this event shell is passing by. It's a bit like agreeing to meet. Say we agree to meet in pub in Island Inn pub in West Bromwich. Odds are - we will miss each other. If we agree to meet at 4:13 pm - we will miss each other. But if we agree to meet at the pub at 4:13pm - then we will meet. You need both space and time coordinates to coincide in order to meet. Ok, back to the big bang - big bang is not event that is localized in space, although it is localized in time. This means it is everywhere - and therefore "sphere" of detection is no longer expanding sphere. If we could detect big bang - we could detect it any time in all directions. This is in fact how we detect similar event that is causing cosmic microwave background. It was also localized in time but happened everywhere.

That is great input. Having that experience, do you think that current price difference between two scopes is justified?

It is indeed interesting topic to think of about how we make purchasing decisions, and you are right - I also like when I experience that "I made a good purchase" / "I'm happy with this item" sensation Sometimes it happens that I'm initially not overly happy - but as time goes by, I start to appreciate the actual item more and more (sometimes this second part never happens ) - that sort of shows that our initial happiness with our purchases depends in part on our expectations rather than on actual quality / usability of the item.

How do you know that "best you bought" will really provide the best view you can have? What gives it away as being the best? Most expensive in price bracket that you can afford? How much of "goodness" of item do you prescribe to actual tests, how much to subjective reviews and how much to price of item ("we are not rich enough to buy cheap items")?

It will really show what camera sees, but I think that it is the upper limit of what can be seen. I don't really think it is possible for us to see something that camera (if used properly) can't record. I don't have problem with reports that differentiate camera and personal experience, but reports of finer detail than camera captures to me signal that eye/brain system is misinterpreting what it sees. This is not uncommon and this is not bad thing - it is just what happens. Take any optical illusion as example. We really do see things differently then they are in reality in some cases. I understand this is sensitive topic, and I mean no offense to anyone - but this is a real thing. Just see this: https://en.wikipedia.org/wiki/Confirmation_bias I also provided possible mechanism of why we are inclined to believe that more expensive is better (we are simply evolved that way). Again I need to stress this as I fully understand that it is sensitive topic - if there is confirmation bias involved - it is really natural thing with humans and we should see it like that.

Try observing with your glasses on. If you have cylinder mentioned in your glass prescription it should sort (at least some) astigmatism. I have severe astigmatism in my right eye - I can't even observe at high power with it, but left is perfectly good - so I exclusively observe with my left eye.

Only issue with that is - take set of measuring devices and if calibrated properly - they produce the same answer each time. Take set of Mark I eyeballs - and no matter the calibration - you get whole range of answers

I don't think there was intentional fooling. I also think that good cheaper scopes are recent thing and that there is still some expectation bias going on. We heard so many times that Tak beats cheaper scope - and I'm sure that was true many time before. That makes it hard to stay unbiased. Here is review of AstroTech 115 ED by Ed Ting: https://www.youtube.com/watch?v=sE18PBQRzDQ There is one particular sentence (or two) that stuck with me. Fast forward to 10:10 (or a bit earlier if you want to hear about whole head to head vs Tak):

Well, to be honest - I think we see it all the time. Just look at fashion industry. They fool so many people in wanting that new look this summer / winter / whatever

I'm not sure if it can be called delusion. Many people are influenced by optical illusions and alike - are they delusional? No, its our senses that trick us. Expensive = better, probably has simple roots in biology / evolutionary science. We crave sugar as it is fast way to get energy. Similarly - we are evolved to value more things that are sought after - as they must be useful / provide benefit, otherwise no one would care much about them. Expensive = wanted / sought after. See the connection forming there?

One of the reasons I advocate objective testing. I know it's possibly a dull subject to many - but it would produce some credible results (at least I hope so). Having those next to first hand accounts would help form better image on the matter.

Anyway, I did not want this to turn into technical discussion either. Anyone care to offer their view on four point list - perceived difference on all 4 counts for scopes they had a chance to own/use?

I know very little about common methods of testing, but I do have few ideas of how it can be done. One of those is rather easy to do (in principle). "Simple" in house strehl measurement. This one can't really test telescope by itself. It needs some sort of optical aid - either a barlow or eyepiece. I think good sharp ortho is better option. It is very similar to what I've described above - it involves taking an image thru the eyepiece, but this time instead of using an object (image of planet or a nut ) - one would image artificial or real star (artificial placed far enough is probably better choice). Image is taken and then circle that represents airy disk is selected on the image - pixel values in circle and outside of it are summed and ratio is found in_pixels / (out_pixels+in_pixels) - that is Strehl ratio or "encircled" energy (or photon count in this case). Another method would produce actual wavefront - but it is much more complex and requires specialist software to be written. It works on the principle that I though Roddier works - but now I'm not so sure of that (I haven't read Roddier's paper). It would involve taking several defocused star images and then running software that "solves" what sort of wavefront would produce those kind of out of focus patterns.

That one is for bookmarks surely

Or translated to other values: Interestingly their RMS is much lower than these calculation suggest. Not sure if that is possible. Above calculator probably only calculates spherical aberration and not general aberrations, but I think that spherical is rather smooth variation of wavefront and thus RMS should be small for given P2V (RMS is measure of how much on average wavefront deviates from perfect, while P2V is just two most distinct points - but if we account for "smooth" transition between two most distinct points of error - we should get rather small RMS). That is actually very good point - can we trust telescope test results and to which extent? I was really pleased to find out about Roddier analysis. Idea seemed sound and I did some testing on my scopes - and got very good results. Whole concept of DIY testing is very pleasing - you don't have to take anyone's word for it - just do it yourself. Problem is however that it is not reliable. When I created synthetic images that should be very easy to "solve" and calculate wavefront errors (like as simple as pure 1/4th spherical) - it gave wrong results. If it can't produce accurate results in ideal conditions - how on earth will it work with actual data that has noise in it and is far from perfect.