ollypenrice

What I would do (what I do do!) is make a basic stretch from linear till my background sky is just a little darker than I'd like it in the end. I de-star that. I take the starless and open Curves (This is Ps but I guess GIMP is the same). I place a fixing point on the curve at the brightest point in the background sky. I place a second fixing point below that. If I now lift this curve from just above the two fixing points I'm stretching only the galaxy, which is what I want to do. Once I've done this, I can do the opposite for the background sky. Again in Curves, I pick the brightest point in the background sky and put in a fixing point. I'll then put a series of fixing points in above that, this time, so all the galaxy stretching will be unaffected. Now I can pick a point below the brightest background fixing point and lift it, so reducing background sky noise. Don't eliminate background noise, it will look oily and false. Olly

Let's not forget that RGB or OSC does have Ha signal but it only has it in a proportion which is about the same as seen by our eyes. For me, galaxy Ha is not compulsory. Olly

I believe it's best to think through what it is that your calibration frames actually do. Darks record camera noise and no light reaches the chip so you can do them in any situation in which no light reaches the camera. Flats record the illumination arriving at the chip, which should be perfectly even but won't be. Why won't it be? Mostly because of vignetting, which is not affected by which filter you use. Also by dust somewhere in the light path. The closer to the chip the dust is, the worse the 'dust bunny' it will create. This dust might be on a particular filter but is more likely to be closer to the chip, perhaps on the chip window. This means that you might need flats per filter but might not. With observatory based rigs I used luminance flats for all filters and had hardly any problems with this. If I did - once every few years - I'd shoot a filter flat or clean the rig. If using an uncooled camera you will probably do best without darks entirely and with a large dither between subs. With a CCD camera the only dark-for-flat you need is a master bias. With a CMOS camera you need a matching dark-for-flat. My real point is that there is no need to make a mystery of calibration files, nor is there any need to treat them as a religious ritual to be followed to the letter every time. If you think them through and look at how they work for you, you'll be fine. With my CCD rigs, flats per filter would have been a total waste of time and luminance flats lasted 6 months to a year. Olly

What he said^^ lly

I don't think I gave Chris this info but it's Lum 21x15 mins, R,G and B 8x10 min each. TEC140/Atik 460CCD. Not all that much, really. Olly

I didn't know I'd captured so much tidal stuff! Good job. Olly

Now that's a very good idea, Tony - especially in my case because we don't have to offer UK weather, here! My thinking cap is on... Olly

Be aware that dedicated CCD cameras are now considered old school next to new CMOS cameras and represent stunning value on the used market. Autoguiding is one of the great triumphs of amateur astrophotography, turning a mount like yours into a premium performer. Don't be put off by guiding, rejoice because it's there! Olly

Good going from Cornwall! Olly

The 2600 chip is making everyone involved down here very happy! We have it in ZWO and Telescope Service variants. Olly

The only time you'll notice the shutter is in taking flats: you need to make them long enough in exp. time to avoid 'shutter wipe.' From memory, 3 seconds is about right. Olly

There's little difference in pre- and post-processing methods between CMOS and CCD. I'd have thought that the only significant one involves calibration of flats. With CCD you could just use a master bias as a dark-for-flats whereas, with CMOS, you'll want a matched dark-for-flats. CMOS can also be fussy over flat exposure duration so there may be a bit of experimenting. As for post-processing, it's easier in general with CMOS because the data are cleaner. I find that my workflow is far more consistent with CMOS than it was with CCD, where there was more variation between one image's needs and the next. Olly

All those great images made with CCD remain great images and many are still the best ever taken of certain objects. I'd say the cameras will only become obsolete when these images are obsolete and that won't be happening in the next ten minutes. What I would now regard as obsolete is the advice to start with a DSLR because used CCD cameras are incredibly cheap on the used market. I think they must be the best buy on the market, though some would-be vendors have yet to accept just how little they can expect to get for them. I do think that OSC CMOS is much better than OSC CCD but I wouldn't consider mono obsolete. The dual and tri-band fiters, however, have had a great impact on OSC flexibility and the small CMOS pixels play really well with short focal lengths. Very long focal lengths haven't become obsolete but you can now do great galaxy images with FLs of under a metre. Olly

I realize that the Doppler redshift is a product of velocity, not acceleration, but my point is that, to have a velocity, an object must at some point have been accelerated. The cosmological redshift, however, is not introduced by velocity but by the expansion of the space through which the light is travelling. While following the same formulae, these redshifts are not of the same origin. Olly

I discovered this when painting a steel handrail in front of the house. On a hot day it flowed out nicely. Olly

I'd just use Hammerite. No real surface preparation needed. They only seem to do the liquid form here but the trick with Hammerite is to put it on warm. This is easy with a counterweight, which can just be put in the oven till warm (not hot) and its large thermal mass will let the paint flow nice and evenly after application. Olly

This was my point. The galaxies do move apart from each other without the need for proper motion. It seems to me that there are two ways of looking at the 'motion' imparted by the Hubble flow. We can take the point of view of an observer on a single galaxy and see radial recession on the part of the others, or we can conceive of galaxies within a universal expansion. We cannot observe this conception because an observer, of course, is pinned to a single location. Olly

I agree that redshift/blueshift don't need acceleration (as Jim also points out) but a body's proper motion does, surely, need to originate with an acceleration? What I'm getting at is that the OP's question is about movement since he asks in which direction the stars are moving. Is there not a fundamental difference between a body 'moving' because of the expansion of space and 'moving' because it has, at some point, been accelerated? If not, why do cosmologists distinguish between Doppler and Cosmological redshift? Olly

But where did this uniform motion originate? Olly

I'm always nervous around Vlad (despite his being such a nice chap! ) but here goes... There are two kinds of redshift. Though they are mathematically equivalent, their origins differ - because there are two kinds of 'movement.' 1) The Doppler redshift. This is created when the star or the observer (or both) are accelerated by some force. This kind of acceleration must have a direction and we are familiar with 'movement' in this sense. In this case we would talk of the star's proper motion. (It's own motion.) 2) The cosmological redshift. This applies on cosmological scales, outside gravitationaly bound systems, and does not arise from an object's being accelerated. Although the galaxies on this scale are moving away from each other, each one considers itself to be more or less at rest (ignoring the fact that they are all whirling around to some extent in curved spacetime.) In this unfamiliar situation each galaxy experiences no movement of itself but sees all the others as moving away from it as the space expands between them. Describing the way in which they move apart from each other requires a metric for the expansion of spacetime. Any one galaxy will find it easy enough to describe all the others as moving away from it along radiating lines like shrapnel flying from an explosion. That description, however, will not work for more than one galaxy because each one will see itself as being at the centre of an explosion - and they can't all be right! Olly

It's an edge artifact from the Ha layer and I thought I'd fixed it in the latest version! I'll check it out again. Thanks, Wim. Olly Edit: Fixed.

Managed to squeeze a bit more out of the Ha: Olly

That's really good. Olly

Very good indeed. For me the colour is a bit 'green high, red low.' Olly

From Kit Marlowe's doomed Dr Faustus, perhaps? See, see, where Christ's blood streams in the firmament! One drop would save my soul, half a drop: ah my Christ! Olly