jetstream

Last Christmas we bought a pair for the eldest grand daughter, not only are they warm but are really high quality and look great.

This is surprising as this is a reflection nebula. Ive had zero luck with filters on it but do see much without.

No but Vlaiv is helping me to be able to image someday, trying with 90mm first if I ever get going on it. No, not driven other than by the wheel barrow handles

lol! Dragon flys among others sitting on an island across the bay, btw my eyes are 20/10 the doc says. Does the Dawes limit apply to flys?

We are forgetting about "MTF" resolution when it comes to planets and the moon. More detail can be seen than Dawes limit IMHO.

Years ago I bought an Astrsosytems dob out of Colorado and had Terry Ostahowski make me a nice mirror. Its actually "low spec" 1/5 PV but also tested at .92 Strehl ( I believe Terrys numbers) and is obstructed 21% and with no offset. This thing gives wicked lunar and planetary views under good conditions. The mirror is very smooth, a requirement of mine.

Part of getting the most out of observing is finding the best avg mag seeing will support and then buy a scope that gives that mag at "your" optimum exit pupil. Herein lies the reason the 100mm Taks perform so well in the UK.IMHO. Btw, I usually observe Jupiter at around 300x-400x in my 15" when the planet is high and the seeing good. Staggering views actually.

I dont believe in hard and fast rules when it comes to visual observing or the eyes. Years ago I had a great article with a chart showing the effect of exit pupil/entrance pupil vs resolution. I then found my "best" exit/entrance pupil (.8mm) and then designed my 15" dob around it which gives about 460x. I can go lower in illumination but after time this will strain the eyes so I dont go there other than to test.

ps, my 90mm will show detail in a flys wings at 700 meters lol! and across the lake, under vg conditions.

Take the sharpest, clearest view you see through your scope and keep upping the mag, if the view worsens in any way youve reached the limit or breakdown. This needs to be done under vg seeing IMHO. OR the limit of your eyes have been reached.

So what mag? most every design scope can be comfortable at high mag, given proper execution of the design. I evaluate my optics many times, under my best seeing before drawing conclusions.

How high razor sharp?

Back to expensive scopes- many fracs will support 80-100x aperture and my own 15" dob supports 60x aperture on the moon, with the very fast in and out "snap" focus- thats 900x or so. The little Chinese optic 90mm Raptor goes over 80x aperture on the moon and has a vg but not the best snap. I think that expensive scopes should ensure vg optics and the process of ensuring this adds cost, not to say less expensive scopes like my 90mm arent right up there either. Would I pay $15000 for a refractor around 150mm-160mm? thought about it, but no, I'd go for more aperture in different design scopes, but to each their own.

In my 2 best scopes, one f4.8 the other f7.5 the focus is BAM in & BAM out... my SW120ED has a broad focus range and also doesnt support really high mag, 300x on the moon, less on Jupiter, 250x or a bit better maybe.

Thanks for bringing this up Magnus, so I didnt have to. Yes there are other factors at play with this issue IMHO.

You need 2 scope of the same focal length and aperture- one that has a fast snap focus and one that has a longer focus range. This will eliminate f ratio as the factor and confirm or deny this idea. Eagerly waiting test!

👍 Optical quality can effect whether the focuser needs to be moved a lot to find best focus, possibly, IMHO.

I dont know much about this, Glenn on CN does however https://www.cloudynights.com/topic/455942-what-factors-affect-depth-of-focus/ "I take it we're discissing depth of focus for visual instruments. If so, objective focal ratio has nothing to do with it (but it does for prime focus imaging, and that only.) If we assume good quality, at given object distance depth of focus scales inversely with both magnification and exit pupil diameter (or the eye's iris diameter if the smaller.) Double the power, dof is halved. Double the exit pupil diameter, dof is halved. These two factors compete in any one instrument as eyepiece focal length is changed. Halving the eyepiece focal length doubles magnification and halves the exit pupil diameter. And so the depth of focus remains the same. Note: The foregoing assumes an inability of the eye itself to focus (which applies to those having artificial lens implants), which is necessary for understanding how the optics work. Considering eye accommodation adds an additional element, wherein eye accommodation varies inversely with exit pupil/iris diameter. But for any one individual accommodation is a fixed quantity. Fundamentally, depth of focus depends on the focal ratio of our eye's own lens. This is why the exit pupil is important in this respect for telescopes in the afocal configuration (where it directs essentially parallel light to the eye as from a great distance.) Note the implication; the telescope objective's f/ratio is of no import, while the eye's own f/ratio is. "

One more myth as it was alluded to earlier. ie PV equates to about .8 Strehl, no necessarily so... IMHO http://www.rfroyce.com/standards.htm which one would you rather hav "The answer is obvious. And so is the inadequacy of the peak to valley methodology as a measure of optical quality. It looks at only two points, the high and low, and ignores all that lies between. Important issues such as roughness are totally ignored while a very small high or low point are exaggerated totally out of proportion to their significance. A mirror showing a peak to valley value of lambda/2.5 (.4 wave) might in fact be an extremely good mirror with the vast majority of its surface at better than lambda/10 and very smooth. The reason that the peak to valley criterion is so popular is due to a combination of factors that have transpired over the years."

One of the biggest myths is that OIII filters will not work with small apertures, it is simply not true. Another struggle many have or resist, sometimes vigorously is the concept of using exit pupil and not magnification. IMHO.

I think that focal length and focal ratio has little to do with visual astronomy as related to seeing and depth of focus. For imaging the depth of focus vs f ratio might be real... As far as an f6 triplet having a greater focus range , yes its possible but nothing to do with the f ratio IMHO.

"Conclusion Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio. The error in the hypothesis is that it was assumed that the same atmospheric distortion will cause the same shift in the best focus position in the two telescopes, and this is not true. While the high f-number telescope does enjoy a greater depth of focus, unfortunately the shift in best focus caused by turbulence is also greater. In fact, the two are locked together; the instrument with four times greater depth of focus also has a four times greater linear shift of the best focus position." https://www.fpi-protostar.com/bgreer/seeing.htm

One more thing before I fade back out... it doesnt matter the brand or price as long as the optical specs are met IMHO.

If a scope has really performed, even once, it confirms its ability IMHO. Thing is, if seeing is superb for say a 120mm refractor it will also be superb for a 15" (in my case) so obviously I reach for the larger aperture, lower strehl and all than the 120mm. My best scopes replicate their performance over and over again. They are not the "best of the best" but right up there.

My best scopes have a very fast "snap focus" and eyepieces will influence this as well.