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Thought of that, and not sure what would be suitable scope for it? With 52.2mm we can go almost to 750mm - 747mm to be precise. Maybe 150mm F/5 with upgraded 3" focuser?

Actually - I'm talking visual only - hence all that mentioning of eyepieces and field stops

One of possible solutions is AP 27TVPH photo visual x0.75 reducer. It has has 56mm of clear aperture and it squeezes that into 41mm illuminated circle. With 56mm we can go up to 800mm of FL for 4°, and this reducer has 118mm back focus - which is consistent with 2" diagonals. 130mm F/6 scope has FL of 780mm - so good candidate. I'm not sure if above reducer will work fine on such scope though, but that would give 130mm of clear aperture (give or take - mostly take due to number of elements) and 4°

Yes, amount of scatter depends on wavelength - that is why sky is blue - because blue light (shorter wavelength) scatters more than red (longer wavelength) - IR is longer still so it scatters the least.

There are several "projects" where I think IR is essential part. For example M42 - there are number of young stars that you can't really see due to dust in optical part - but will show up in IR part of spectrum.

Sometimes I just think of what M31 scope to get (and yes, mostly when cloudy). I have bunch of different combinations that might work well if I compromise on field of view a bit. So far, best combo I think is this one: https://www.teleskop-express.de/shop/product_info.php/info/p14000_TS-Optics-152-mm-f-5-ED-Rich-Field-Refractor-with-4--RAP-Focuser.html + some EP with up to 35mm and as wide AFOV as possible Price is ridiculous and I realized that it won't fit veil properly - and it would be shame to have wide field scope that can't frame veil well - so I wanted to know what is the most aperture one could squeeze if there was hard 4° limit imposed.

In either case - I would recommend UV/IR cut filter to be used with OSC / newton combination - because of color accuracy.

Well, if it helps, here are some calculations: 46mm field stop: 650mm FL (658mm to be precise) 45mm field stop: 640mm FL (644mm) 44mm field stop: 630mm FL 43mm field stop: 615mm FL 42mm field stop: 600mm FL 41mm field stop: 590mm FL Answers so far have been up to 100mm of aperture so let's spice up things a bit. ST120 with ES82 30mm with field stop of 43mm fits in above table, and that sort of beats 100mm by 44% (aperture area) - although I have no idea what the effect of achromatic scope on contrast is 150 F/4 TS photon newtonian with ES coma corrector will work with 42 or field stop eyepiece. Problem is - not sure how large illuminated field is ES CC does correct for 30mm - but it does not show vignetting (I think it also extends FL by 6%). Accounting for central obstruction and reflections is bound to bring effective aperture down to 130mm or so.

I'd use UV/IR cut filter with OSC camera simply because of getting color info correct. For mono + filters it makes sense not to use filter for luminance if system is well behaved in IR simply because many sources out there give of significant signal in IR part of spectrum and one improves SNR for luminance that way. RGB filters should cut off IR anyway so that is not concern. As far as above diagram - it shows that said coma corrector does not do very good job. It does correct for coma up to about 4/3 sensor size (11mm away from optical axis, on 14mm we still see coma but to much lesser extent than system without coma), but it makes scope not diffraction limited very quickly. We don't have closer data, but spot diameter 7mm of axis is ~10.3µm (geometric radius is ~5.16µm) and airy disk diameter is 6.2µm for 200mm F/5 system (5.9µm for F/4.75 system). One of definitions of diffraction limited system is that all rays hit within airy disk. I would be more worried by star bloat from coma corrector even with IR/UV cut filter than from IR/UV itself. Take for example this spot diagram: Yes, indeed, it has slightly larger spot in the center 2µm RMS vs 1.2µm RMS - but keeps that spot all the way to the edge of the full frame sensor at 22mm. Btw, this last diagram does not say which scope is it for, so that is a problem, but let's assume it is indeed F/5 of moderate aperture.

So here is a puzzle. Using existing gear on the market - which scope / eyepiece / reducer / coma corrector - in simple words - kit combination would provide the most effective aperture for 4° TFOV (or more). Things to consider: 1. Should not be significant vignetting - max 10% in outer field 2. Exit pupil should be kept below / up to 7mm (or if it is larger than 7mm - it will be treated as aperture stopped down) 3. Realistic field stop should be taken into account when choosing max FL - we want 4° TFOV irrespective of any AFOV claims 4. Mirrors should be treated as 94% reflective unless specified otherwise by manufacturer (like 97%). Each air glass optical surface should be treated 99.5%. Central obstruction should be taken into account. Diagonal if used should be taken to be 99% efficient. Comparing of effective aperture is used. Not sure how to account for lower contrast of achromatic scopes though, so for the time being - let's treat them as equal. 5. Stars should be "OK" at the edge. Some distortion is acceptable, but for example fast newtonians do need coma corrector (and CC transmission taken into account). This also restricts use of 4/5 element wide field eyepieces in fast scopes.

"In order to provide normative data on light sensitivity as a function of age and sex, some 17,500 Ss, ages 16 to 92, were tested for both form recognition ability and glare recovery time under scotopic levels of illumination. The results show: (a) a progressive deterioration of performance on both tests with increasing age, (b) a very low correlation between form recognition ability and glare recovery time, and (c) no consistent difference in performance between males and females" https://journals.sagepub.com/doi/abs/10.2466/pms.1967.24.3c.1279?journalCode=pmsb As we get older - we are less sensitive to low levels of light. I guess MW looks the best to young people.

I guess it is down to transparency and SQM reading from lightpollutionmap.info is questionable at best. It can vary by 0.2 or more on SQM scale - depending on factors. According to above wiki link - MW is nearly or totally invisible in 18.38–18.94, and that is my experience - on most nights one would not be able to see it, but on some nights, yes, there were hints of it - I could clearly make out direction in which it was going across the sky for example.

I was also able to spot M31 from same location in good conditions using averted vision. This seems a bit more accurate comparison table: https://en.wikipedia.org/wiki/Bortle_scale

Just to add to above post - not sure if that chart is actually true. I've been able to spot MW at zenith - extremely faint and barely noticeable as different shading of the sky - from Bortle 8 location - edge of white / red zone, and SQM 18.5 according to Lightpollutionmap.info According to above - it should be invisible

I always thought that this is relevant:

If you want it to double as terrestrial and planetary scope - then I guess 102mm Maksutov is a fair choice? Either than or 80mm ED refractor. ED refractor will cost more but will allow for wider field of view / lower magnification. Otherwise maksutov + 45° amici prism is very good option, it will be better on luna/planets, however, you'll struggle to get below around x40 magnification with it. Get good zoom eyepiece to go with it and there is your spotter. Pair it with nice alt az mount - maybe zero https://www.firstlightoptics.com/alt-azimuth-astronomy-mounts/scopetech-mount-zero.html

Sell your SW 100ED-Pro and focal reducer and get 100-115 F/7 triplet and matching reducer. That will have the same effect as getting 0.6-0.7 reducer for SW 100ED. Alternatively - you might try this for example: https://www.teleskop-express.de/shop/product_info.php/info/p7943_Long-Perng-2--0-6x-Reducer-and-Corrector-for-APO-Refractor-Telescopes.html It says it will illuminate and correct up to 30mm, but I'm skeptical. If you have smaller sensor like 4/3" (KAF8300 / ASI1600) or even smaller - 1" (ASI183) - then it should work. This will make your scope F/5.4, so you'll be imaging with 100mm of aperture at 540mm FL. If you take 115mm triplet scope (yes, it is 50% more expensive than SW100ED) and pair it with Riccardi FF/FR 0.75 - you'll get 115mm of aperture with 600mm FL - or ~F/5.2 system. It is really down to what FL you want to image at / what FOV and sampling rate you want to achieve).

I can't find it either if I'm looking in wrong direction!

I see that being nothing more than marketing statement. In most nighttime and dark-location images that they refer to, main source of noise is shot noise. If they managed to lower read noise, then yes, that is accomplishment - but more interesting to planetary / lucky imaging approach scene then to deep sky / long exposure. With read noise around 1.5e already - DSO imagers can't really complain much. Lowering dark current / dark noise is again not very useful - neither to long exposure AP nor for lucky imaging. First use cooling, second use short exposures. I never thought that dark current is main issue with non cooled cameras - for me, it is always lack of set point temperature and problems with dark calibration resulting from it.

Well - they are not Many people thing that having bigger full well capacity makes camera better - but it does not really. Dynamic range is just extension of that - FWC divided with read noise. In AP - these concepts are related to stacks and not single exposure. If we imaged objects with single exposures and did not stack - then yes, FWC and dynamic range would be important. They are important in daytime photography because of that - almost all images are taken as single shots. With stacking - we control dynamic range. We can choose what sort of exposures we stack so we can have any FWC we choose in final stack. We can also have any dynamic range we choose in final stack (given enough imaging time of course). That is why 12bit cameras work every bit as good as 14bit or 16bit cameras in AP images they produce.

Why exactly is "dynamic range" and full well capacity important in astro imaging?

I actually don't mind it at all I'll probably be covert agent of Bortle regime that infiltrated rebellion?

Any description of sky brightness is bound to be flawed in terms of matching experience - since experience depends on other factors as well. Transparency plays vital role in what can be seen - regardless of sky brightness. In same sky brightness - two people can have very different experience if they have different transparency conditions. How about SQM? I find SQM to be useful in many ways. First - it is not estimate, it is measured quantity. It uses same system we use to compare brightness between astronomical objects (magnitude system). It is useful for imagers because they can estimate signal levels from SQM given their equipment and so on ...

These are probably due to filter or sensor cover glass (multiple bouncing close to sensor). They are very common on fast mirror systems like Tak Epsilon 130 and alike - look here: It is the same type reflection - but this time being "pink" - due to IR light involved. Image discussed was taken with ASI533 and SharpStar F/2.8 hyperbolic newtonian (similar to Tak epsilon). If you look at ASI533 QE curve and in particular IR part of spectrum (700nm and above) - you'll see that red is still a bit dominant over blue and green: Same amount of IR light is likely to produce pinkish color.

Ok, I see it now. Pink circles are due to reflections. Anti reflection coatings are usually optimized for visual part of spectrum and will now work well on say IR part of spectrum. These are however not chromatic aberration - they are simply filter reflections (or maybe other optical element) - one that has coatings for visual part of spectrum and these reflections are in IR part.