andrew s

The simple answer is we don't know what happed before the first few microseconds. Our best theory has the Universe in a very hot dense initial state. It was and remains spatially infinite (or very close to infinite) . It has been expanding and cooling ever since. As the Universe undergoes metric expansion it is not expanding into anything as it is everything. There is no outside perspective. Regards Andrew

I fully agree @vlaiv that most would dismiss the Mak however, what you describe in the example is exactly what I believe Etendue tells you. I.e. Area of aperture x Area of field of view, is the throughput and the Mak wins. I also agree that it does not cover SNR directly but for conventional imaging you are normally in the photon shot noise dominated region so telescope aperture is dominant. If you have a bright sky then if your targets are extended and polychromatic then I am not sure what you can do except move to a darker site. I think we fundamentally agree. Regards Andrew

I think that is what etendue encapsulates. Am I missing something? Regards Andrew

Going well, hope it continues well. Regards Andrew

Not sure I fully understand you. However, I think that you just end up with the aperture being key. As you change focal length the solid angle of the sensor changes but so does the projected area of the sky. If two systems capture the image at the same resolution ( I.e. arc secs ) only aperture determines the signal level per arc sec squared. Regards Andrew

Yes we have reinvented Etendue. What it says in broad terms is the bigger the aperture (to capture more light) and the larger the FOV (to capture more sky) the more efficient the system. However, if your target is significantly smaller than the FOV the it's just aperture that counts. As @vlaiv points out noise sources also play a role in detecterbility . Regards Andrew

Quite right, I had forgotten about that. Although I am not sure it quite mirrors the same idea. If I have remembered correctly it measures across the filed of view not per arc second. Regards Andrew

For me "aperture" is D*D so that it measures the energy collected and hence how much gets into one arc second . Then you have to decide what limits the resolution. Scope seeing, guiding . Need to think it through. Regards Andrew

Thierry Lagault the French astro-photographer has an excellent web site. Regards Andrew

As a retired person I rarely no what day it is and without my regular badminton, table tennis etc its even harder than normal. Regards Andrew

Not clear how I join this can someone point me to the Zoom meeting link? Thanks Andrew Too much wine it's still Saturday panic over!

Good start. Regards Andrew

I like the idea but I am not sure that the right unit as it depends on the focal length and pixel size. Not sure I can think of anything better but might go for aperture per FWHM . However, I know that has issues too. Regards Andrew How about aperture per arc sec.?

As someone who studied physics and tries to keep up to date with Astrophysics and Quantum theory I found your post difficult to understand. I never try to give beginners advice on equipments as my experience is too far removed . If I do try to help I tend to recommend the site sponsor FLO as they have the experience to help. Regards Andrew PS could you explain what you mean in a short clear statement. SGL is difficult for beginners because... and list few points.

Sorry about you accident. I suspect you could repair it with a little care or maybe a new tube from OO? Good luck however you proceed. Regards Andrew

Thanks Jeremy but it's just a dream as SWMBO has ruled it out and in reality I don't think I would use it. My eye sight is not that good and I never mastered star hopping. Interesting review, however, cool down was from a warm room, a silly place to keep it. I am unconvinced by central obstruction voodoo. The MTF of a suitably sized obstructed telescope can easily exceed that of the largest realistic sized refractor. However, I am not going to be drawn into an argument about it. I know there are strong views especially amongst refractor aficionados. Thierry Legault the French astro-photographer who uses both has a very balanced view on his web site. Regards Andrew

@Scotti G you have some major misunderstanding of galaxies , gravity and black holes. The spiral form of galaxies is not due to material falling into the centre. A black hole has the same gravitational effect as a similar mass object that is not a black hole. Given the expansion of the Universe it is not likely that all mass will coalesce. Indeed current theory suggests it certainly will not. Objects can have stable orbits around a black hole. Although no orbits are ultimately stable including the solar system. Regards Andrew

For me to find a nice nice tight double I would have to add the encoders and Nexus DSC. So Back to the slippery slope that led to full automation. Regards Andrew

I did consider a OO VX 6L or 8L 1/10 PV on their Dobsonian mount plus half the orthoscopics as a practical back to basics system. Again a classic optical design and the Dobsonian mount is not too far removed from one William Herschel would have recognised but, in mahogany. Regards Andrew

Actually @JeremyS I went for the Mewlon as it's a pure Dall-Kirkham design and also the Abbe orthoscopics are (as I understand it) the original design. Not to mention an unhealthy love of quality optics. Regards Andrew

You obviously missed the "basics" reference 😜. Chocolate and alcohol high. I think the Tak diagonal is a prism 😏 Still looking for all reflecting eyepiece design. Not sure Tak mounts are in the same class as their optics? Regards Andrew

... or if you prefer my posers system. Having fully automated my main setup and shipped it to PixelSkies I have nothing to get my hands on. This together with the BAA back to basics drive made me consider the ultimate solar system manual rig with a bit of clusters and bright M and NGC objects thrown in for good measure. The logic such as it was, was to avoid long cool down and dew hence the Mewlon (no refractors for me) . The lack of availability of Monocentric eyepieces led to the Abbe Orthoscopics. Portable. But, mainly it was just about how good things looked as well as working as near perfectly as possible! Its a personal choice and no chance of getting it as SWMBO decreed no more telescopes as the cost of shipping the kit to Spain, hence, the lack of a budget. So here it is costed by and all available from FLO. What would you choose? Regards Andrew PS Missed of the telescope cost £4545.00. I wounder if FLO would give a discount for a one off purchase!

That I suspect is what we don't know. Regards Andrew

I would agree theoretically with this and similar above provided the Airy is resolved by the detector. If the detector is a eye/brain then I don't know. One key question would the intensity of the star. I.e is it easily seen or near the limit of observability. Regards Andrew

For what it is worth my view is that we don't know for sure. Not withstanding the lack of clear agreed definitions of what it all means, I don't know of any modern experiments on this. Yes even a single photon can be detected certainly 5 to 7 at the limit. However, I am not sure about localising that in the field of view etc. Controlled experiments would be needed to decide for sure. Regards Andrew