ollypenrice

The great square is a famous test of naked eye observation and sky quality. A number of us, here, tested another guest, famous for his eyesight, and we were all convinced that he made mag 7, naked eye. This is not unprecedented, but t is rare, and we do have a good sky. Olly

The difference is that I've swapped out a fair number of failed mini-computers on behalf of their owners, but never a full sized PC. This is anecdotal and doesn't suggest that all mini PCs will fail. However, when setting up our jointly-owned rig in my robotic shed, it won't surprise anybody that we went for a destop, based on this experience. Olly

It took about 200 years for astronomers to detect stellar parallax (shift in apparent position) when the Earth was offset by 2x its distance from the sun (ie when it was six months away from its previous orbital position. Call it 200 million miles.) Even this offset was impossible to detect till they factored in the effects of changing atmospheric refraction. I think you can relax about a few cm of offset. Olly

Big second hand desktops with lots of USB ports are quite cheap. Put it on a trolley with damp protection, ditch hubs and mini-computers and live happily ever after. Yes, I'm old-fashioned - but I also host six robotic imaging rigs so I have some insights into what actually works. Olly

More resolution in the Redcat but far less speed/depth/FOV. It's the imager's choice. On targets with fine detail the Samyang lacks resolution and is at its best making mosaics where each panel will be downsized so that this doesn't matter. With the Redcat you'd get more detail in more time. Is that what you want? I think that it takes a certain amount of experience to find out what you want and what will provide it. What I want is irrelevant. What do you want? What I really wanted to say is that the Redcat's numbers are out there: Focal length, aperture, F ratio... The question remains, Does it do those numbers well? The one I met today does them very well indeed, to my eye. Olly

With a RASA 8 and a Samyang 135 I don't feel I'm missing out on widefield, but the Redcat strikes me as being damned good. Olly

I'll admit to a lingering wariness when it comes to William Optics instruments, suspecting a risk of style over substance, a pretty finish over good mechanical engineering. Maybe I'm just out of date on this, as on many other things. However, today I processed some data taken here by a guest using a Canon 5D Mk3, so full frame, in a Redcat with a FL of 250mm. These are, of course, great numbers for anyone wanting to shoot widefield. We didn't have flats and the image came out with a very red bias, easily subtracted using ABE. The gradient map was a flat sheet of red so there really can't be much vignetting and I saw no sign of any in the stretching. Better still, We ran the data through Blur Xterminator and the stars seemed tiny and tight, corner to corner. Once we started stretching, they remained tiny and tight. Star Xterminator produced a nasty grid pattern this time, as it occasionally does, but then we asked ourselves why we wanted to use it anyway? The image, centred on Sadr, had such small stars that we didn't want them any smaller and they were still perfect into the corners. Sadr itself was better than it was in my Tak FSQ106. It had the smallest of soft halos and was OK as it was or it could be tightened up in 30 seconds in Ps if preferred. Frankly, I was mightily impressed by these optics. I'm already using two short FL systems, both F2, but, if I weren't, Id be raiding the piggy bank, I think. Olly

There is a lot you can do in post-processing to reduce star bloat. If you use Star Xterminator you'll get to the stage where you are ready to replace the stars and have them as a top layer over the starless image. Simply increasing the contrast on the star layer can drop the bloat below the level of the nebulosity in the bottom layer so it won't show. The stars themselves become brighter but then you can adjust the global brightness of the star layer. The key thing is contrast in the star layer, which you can adjust using Photoshop's contrast slider or have more precision using an S-Curve in Curves. Olly

That would make sense. Olly

In which case I'd give the rig another run under a crisp sky. Olly

Hmmm, I'm a little dubious. What seems odd is that, the bigger the stars are, the softer they are. Yes, this is normal enough but, here, it seems exaggerated. We seem to jump from small, tight stars to large soft ones. Was there any haze when you shot this? On the basis of just this image I wouldn't, personally, sign off the optics as all OK just yet. I'd try another dense star field. Olly

What you are describing doesn't just apply to science teaching, but to all teaching. It's sometimes called 'decontextualized teaching,' and leads to something fairly useless called 'school knowledge.' That's to say knowledge which will never play a part in students' lives outside school. It is, as you suggest, the enemy of real and useful teaching. But... it is very important not to misunderstand this problem. It is not resolved by finding cutesy little applications of what is learned in class. (I'm not suggesting that you think it is, let me stress.) There is no value in conducting a decontextualized chemistry lesson and then tagging on, at the end, the assertion that 'This is how we make nail varnish.' Personally, I think that a contextualized lesson is one which delivers the pleasure of truly understanding something. Olly

I think Vlaiv has answered that far better than I would. I also think it's worth saying that undersampling may not be a problem at all on some targets. Large, diffuse nebulae, for instance, often contain little fine structure to resolve. Galaxies, of course, contain a great deal and these are the subject of your thread. However, galaxies also produce tidal tails and streams which are inordinately faint. Arp 94: I imaged this, or tried to, in the TEC 140/CCD rig at 0.9"PP. In the end I gave up. The interest in the image is the tidal streaming and I just couldn't get this to separate reliably from the background. I had nice detail in the galaxy cores but they were not what I wanted. When I tried again with the RASA 8 I could not resolve fine core detail at 400mm FL but the tidal streams were plain sailing. With finite resources there will always be compromises. Olly

It remains a wonderful program for astrophotographers. I refer to it as 'civilization,' to which I return after carrying out adjustments in more barbaric environments! lly

Interesting! What are you going to put it in? Olly

Bravo. Ten times bravo. As for the 'justification' of science through technology - no. There is a hell of a downside to technology but to science there is none. Olly

I don't know if I'm a serious imager or not but I do share Vlaiv's point of view and have found no significant difference in resolution between 0.6 and 0.9"PP. In the end I went for the 0.9 option (TEC 140/Atik 460 mono.) Indeed, I wrote a feature article saying as much for the British magazine Astronomy Now. I was half expecting a good deal of comeback from it but there was none and, I must say, many of my imaging guests do feel as I do. Arch enthusiast of all things technological, the late Per Frejvall was persuaded by my TEC 140 and bought one himself. It's still here in other hands. If you do find more real resolution from 0.6"PP then you do. I looked at my data carefully and concluded that I didn't. We can both be right. Careful! I don't think anyone's picked up on this but, to realize the resolution of 0.5"PP your guiding RMS needs to half that. (This is a rule of thumb but good enough for government work...) You are very unlikely to reach a guide RMS of 0.25". I can get about 0.33 out of my Mesu 200. Olly

It's a really great image, we can agree. Now let's ask what's great about it. The resolution of small scale detail? No, that is unremarkable. Plenty of M51s have that resolution, or even better, but it is representative of what I regard as roughly 'what you'll get' out of an amateur system, seeing-limited. What is remarkable, very remarkable, is the depth of signal on the faint stuff. The outer halo shows modelling and structure which I have never seen before and the Ha feature just beside the pair is also new to me. These don't require high resolution. Indeed, this image supports the thrust of my argument perfectly: as amateurs we can bang our heads on a wall in search of scarcely perceptible improvements in resolution or we can go for what matters, what will really allow us to show something new, and go for depth of signal. The headline of this image is 225 hours. Exactly, and it shows. It doesn't show in resolution, it shows in depth. Space telescopes are not primarily, or even significantly, created for making pictures. Their use is primarily spectroscopic. Amateur scopes are for whatever the amateur wants to do with them. Hubble and James Web could not/cannot take widefield images but amateurs can. There are PNs out there, still being discovered by amateurs but new insights into the night sky, in the amateur domain come, as it seems to me, from images which combine depth with breadth of field. Olly Edit: Ironically you have brought Adrian and me into agreement!

I'll become really interested when I'm shown an amateur image of M51 which is significantly better than those shot at around an arcsecond per pixel. I don't want to be shown calculations and measurements, I want to be shown a picture in which I can see a worthwhile difference. Please do rattle my cage when this image is available! Olly

You could copy the passband graphs for both and put one above the other to see which wavelengths get through both. Olly

I meant 'host permanently.' One of the four I host permanently has never gone wrong and one I host permanently has been back twice. I didn't say that every 10M I host permanently has been back... Naturally, I'm delighted that yours has been so good but, on my sample, I would have to stay with Mesu myself. Olly

I host or have hosted four 10 Microns and five Mesus. Returns to manufacturer stand at 10 Micron four, Mesu zero. I'm hardly likely to want to change... Olly

Possible dew - or ice - on the chip window. The source must be fairly close the the chip since an obstruction at the front of the tube would be blurred out of visibility, as is a secondary mirror, for instance. Olly

Well, as I said earlier, I've shot the same targets at 0.6"PP and at 0.9"PP, the former with a far higher theoretical resolution (350mm aperture versus 140mm) but I could find no significant or consistent difference in real detail captured. I think we are, quite simply, seeing-limited and that shooting at less than an arcsecond per pixel is a waste of time. Olly

Absolutely so! Olly