ollypenrice

Agreed, and I think it's probably easier than finding your way around a DSLR for astronomical use. Olly

Have you come across 'The F ratio Myth' in astrophotography? It Googles and is worth your attention. Briefly, your formula is clearly valid when a reduction in F ratio is achieved by increasing aperture but not when it is achieved by reducing focal length. In the latter case there is no increase in 'object photons.' Also, plenty of giant professional telescopes have slow F ratios but capture images in astonishingly short times. That's because they have large pixel cameras in the back. What matters is not the F ratio but the flux per pixel - which goes up if you bin your pixels or use larger ones. Software: In marriage, promiscuity is probably a bad idea. In image processing, it is a good one! APP is very good and exceptionally good at making mosaics from panels which have had gradient removal and green bias removed in Pixinsight, after which a return to the civilized environment of Photoshop is much to be preferred - at least by me! I'll also dive into Registar fairly regularly. You can't beat an exotic processing love life... On one thing, most of the imagers I know agree on this: Russ Croman has changed post processing forever with StarXterminator, BlurXterminator and NoiseXterminator. (I've put them in my personal order of importance.) Olly

Fair warning: I'm often a lone voice on this topic but I always say, Absolutely not a DSLR unless you are cash-strapped. If you can run to a cooled astro CMOS with a reasonably sized chip, then this will roundly beat a DSLR. It will need a PC so, if that's a problem, forget it. On the other hand, managing capture with a decent sized preview and focusing aids makes life far easier. In the days of CCD the dedicated cameras were way more expensive than DSLRs but now, with CMOS, astro cameras are both better and cheaper. Olly

An interesting thing about bandpass is that broader does not seem to be faster. My Astrodon 3nm Ha gives a better result in the time than my 7nm Baader. This may be down to its better transmission but not necessarily. In principle a luminance or a red filter passes the same amount of Ha as an Ha filter. The Ha's advantage lies not in what it passes but in what it blocks. A tight bandpass may block more of what you don't want. Olly

Very magenta on my calibrated screen, suggesting green is too low. On the other renditions I'm seeing a slight excess of green. This is always the difficult axis in galaxies. Since I post process in Photoshop, I measure the background using a 5x5 average sample and look for parity between the three colour channels. This is a helpful way to keep the lower brightness in order but the brighter parts are the difficult ones. Olly

M33 is not terribly colourful if we are honest with the data. Hubble shows a warm central bulge consistent with old Population II stars but the image doesn't reach far into the spiral arms. Adam Block's Mount Lemmon image shows it a mild, silvery blue but his rendition is very colour cold by comparison with Hubble if we compare the bulges Hubble https://hubblesite.org/contents/media/images/2019/01/4305-Image.html Mt Lemmon Lots of imagers push for a decisive contrast between the bulge and this requires conscious processing efforts. Olly

False click. Sorry.

Yes. The alternative is believing that the supposedly clever people belong to some kind of 'they' who have been too stupid to realize that the things that are obvious to you have completely passed them by. I, personally, want to call time on this and say, 'If, as an untutored beginner, you seriously think that you are going to debunk a century of rigorous science, then you are - I'm sorry - an idiot.' This does not, absolutely not, mean you should read the science in an uncritical or non-skeptical way. That is precisely what you should do. Feynman said, 'Science is a culture of doubt.' So doubt away, but do so with a little respect. Olly

The basic choice is between widefield at lower resolution (usually nebulae) or restricted field at higher resolution (usually galaxies and planetaries.) High resolution is more expensive because you need a more accurate mount and, usually, a larger telescope. It is also more difficult because errors as well as objects are shown in more detail and because, as well as good sky transparency, you need good seeing. If you go for widefield then a smallish apochromatic refractor is the easiest because it ought to work out of the box. Buy from a supplier who will replace it without fuss if it doesn't. I have never been a fan of the DSLR for imaging and am even less of a fan since dedicated, cooled CMOS cameras out perform even CCDs and do so at a lower price. In Spain this is going to matter. Mono or colour? At a dark site, colour works well and modern CMOS colour cameras are far better than colour CCDs used to be. Olly

As others have said, it would be good to know all of the proposed setup since the components all interact. Olly

Good post. I reviewed this recent book for Astronomy Now and highly recommend it here, too. https://www.amazon.com/What-Stars-Are-Made-Payne-Gaposchkin/dp/0674237374 Olly

Indeed so, but, when I don't know about a subject, I don't express doubts about the integrity of the hard work done by others - in this case astrophysicists - to inform themselves and the rest of us about that subject. When I became interested in astronomy I found it hard to believe that we could know much about such remote things. Rather than asserting these doubts in public, I started reading books about astronomy and, once I had an understanding of the evidence, I found my doubts giving way to agreement. Olly

Could you tell us what parts you found hard to believe in the books you've read about the astrophysics of stars? If you haven't read any, perhaps you should. This forum is not Facebook. Olly

It does precisely the opposite, which is the whole point. If galaxy redshifts are the result of the proper motion of galaxies then they are all moving away from us and we are at the centre of a kind of explosion. This offends the Copernican Principle which urges us to doubt any observation which requires our position to be special. If, instead, the distance-redshift relation is taken as an indication of the expansion of space then all galaxies observe precisely what we observe and our place is not at the centre of anything. Olly

Could you describe in detail what an observation of such half galaxies would reveal, bearing in mind that galaxies are gravitationally bound systems? Edit: And bear in mind, also, that we can detect remarkably little of the far side of our own galaxy, let alone of those at the limit of what is observable. Olly

I'm just a superannuated English teacher but I have tried to find a way of distinguishing clearly between the two forms of 'moving' which we seem to need when discussing the universe. In the normal sense of moving, a thing needs to be accelerated in order to move. It then moves in a direction relative to things around it which have not been accelerated. It will get closer to some and further from others. This is not at all what happens in the expanding universe. None of the galaxies moves towards some of the others and further from some of the others. Every galaxy finds itself to be at rest and sees all the others as moving away from it. None experiences acceleration. It's tempting to say that there is a difference between moving through space and space expanding, yet I thought we were not supposed to think in terms of any kind of fixed grid of space. That's why I focus on acceleration as a way of distinguishing between the two kinds of movement. Feel free to put me out of my misery! Olly

Of all the things that will affect the quality of your results, the glass type will appear nowhere near the top of the list.* You would do better to look into focuser quality, standard of collimation and risk of cold weather pinching. Don't fret about the name of the glass: have a look, here and elsewhere, at what a certain telescope produces in terms of images. Pay more attention to broadband because that is a far harsher test of optics than narrowband. I'm not saying glass type is irrelevant. I'm just saying that it is not the primary determinant of image quality. Olly *Edit: I mean it won't make the top ten. And it really won't.

I shouldn't worry. There is no need to be near the pier when you launch the imaging run. Olly

What do you need this torque for, though? A balanced mount offers very little resistance. Olly

Thanks. I hadn't come across this refinement but I last took a Cosmology course over 20 years ago so either I've forgotten, or the thinking is new - or they were just keeping it simple for people like me! Olly

You're right. I use a program called Registar to align images because it does other things as well and is devastatingly accurate. However, you can align Layers in Photoshop. Paste one on top of the other, make them both active, and then go to Edit - Auto Align Layers - and choose to stack them. Your layers will now be aligned and you can proceed as explained above. I think you can auto-blend layers in a high dynamic range mode as well but I don't do that, I use the method explained earlier because there are things like colour saturation which can be manually adjusted as you go along. (Bright parts often need a colour stauration boost.) Olly

It doesn't sound anything at all like a theological argument and there is very good scientific reason to believe the universe is 13.8 billion years old. Almost a hundred years ago, convincing evidence was found to suggest that the universe was expanding. Since then, this evidence has grown considerably. If you run that expansion in reverse you find that the universe reduces to a point source which began its expansion 13.8 billion years ago. Unlike other objects, the universe does not have a centre. This may seem absurd but consider this: we can observe all other objects from the outside. When we do so we see they have a surface, even it it's a diffuse one like a cloud's. This surface lets us think of the object's centre, a point at some kind of average distance from all parts of the surface. Since the universe cannot be seen from the outside (because any outside would be part of it and therefore inside it) we cannot define any kind of centre. Your second point has been discussed already by astronomers and cosmologists. Fritz Zwicky, in the 1930s, proposed that the redhift might be explained by his 'tired light hypothesis' but the evidence was against him. Light from very distance objects can never reach us because its source is receding from us a more than the speed of light. (Note that this does not violate the 'speed limit' of the speed of light because the galaxies sending out the light have not been accelerated by the expansion of the universe, it is simply that the medium of space between us is expanding.) That part of the universe which lies within the volume not receding faster than the speed of light is known as the observable universe because it is observable in principle. Olly

If you have Photoshop, I'd use that for working on the combination of layers (stars on top of starless.) Everything is adjustable and visible in real time. Olly

Clearly much better. I've always imaged M42 using radically different exposure times, with 10 seconds for the Trapezium itself. I then place the short exposures on top of the long, copy the long onto a layer mask, blur the mask, increase its contrast and then blend the layers. Olly

More focal length projects a larger image onto the chip. This much we know. Does this larger projected image contain more resolved detail than a smaller image? This we do not know, because it depends on your seeing. Larger aperture collects more light. This much we know. Does this increase in light collection increase the amount of light per pixel? This we do not know, because we don't know your pixel size. Larger aperture, in a diffraction limited optic, resolves more detail. This much we know. Is this increase in resolution sustained by the seeing? This we do not know. That makes for a lot of unknowns, two of them coming down to the seeing. Olly