ollypenrice

Regarding matt black paint, it must be pigment-based rather than dye based, the dye-based paints being reflective in some wavelengths. Buying stove or BBQ-type heat proof paints will exclude dye-based ones. Olly

First thing, for me, is not to sharpen stars, so they always need excluding. Indeed, using star removal and replacement, I usually blur them by about 0.5 Gaussian. Secondly, as Vlaiv says, don't try to sharpen weak signal and, because it's pointless, don't sharpen any regions with no small scale detail in them. Thirdly, consider the scale of any sharpening. If you set USM with a higher threshold it will only operate on larger scales, working almost like local contrast enhancement. This can be good for structural boundaries in extended nebulosity, for instance. Small scale sharpening, however, works well on small features like galaxy detail. Basically, one USM set of values does not fit all parts of the image. I do this in Photoshop because it means I don't have to faff about trying to get exactly the mask I want. I can just make a copy layer, sharpen the bottom and then erase the top with a soft brush where, and only where, and exactly where, I want it. The Select-Colour Range tool in Ps is also a very easy alternative to masking. (It is a form of masking but generated differently.) Finally, I try not to forget what nebulosity means. lly

I would advise against large numbers of sample points. What you are trying to remove is a broad gradient and the more points you put in, the more you are likely to pick up local brightness variations - or create them. I rarely use more than 8 and, sometimes, half that. Olly

You can balance the scope at present just by sliding it through the rings. If that gives you enough range, well, it just does! The saddleplate-dovetail allows the scope to be removed a little more quickly and the rings come off with the tube. Depending on how you set up and tear down, this might be useful or might be a way of spending money for no substantial benefit. If were using this scope in an observatory, for instance, I wouldn't bother. Be aware, also, that the dovetail rail should have stop bolts at both ends to stop it sliding through the dovetail clamp. This is a classic way to trash a telescope! Olly

By Gad, yes - though the dog has three ears! Olly

That's the fun of it in a nutshell. You are likely to find something you weren't expecting, or a slightly different perspective on an old friend. Olly

Love it! The field gives a wonderfully shapely composition. Olly

My experience, precisely. Olly

Yes, in my previous teaching life I always felt that, when required to take charge of a classroom in which I was not properly informed, the best place to start was, 'I don't know about this, so let's try to find out.' Olly

Even in a 20 inch at our exceptionally dark site (and a good deal south of the UK) I have always found the Eagle disappointing, visually. There is certainly nebulosity to be seen but I never found much to link the EP view to the photographic. (Obviously I wasn't hoping for the tiny Pillars of Creation either.) On the other hand the Swan is spectacular, visually, and the Swan shape is clear and insistent. Olly

I dug this out with a view to using it to enhance our Cepheus to Cygnus megamosaic but, uncharacteristically, Registar refused to co-register the images. However, I reprocessed it anyway, using the X suite. 16 hours Ha OIII RGB with Paul Kummer a few years ago. Dual Tak FSQ106N/Atik 11000s/Mesu 200. Olly

Nice. This has a moody, dusky look and the blue nebulosity hovers dreamily over the scene. Olly

I'm impressed by the image. It's strongest feature, for me, is the way it distinguishes between different levels of very dark, sooty dust. Often they are all clipped down to the same level but here they lie in clearly distinguished layers. What I'd want to work on would be getting it less monochromatic-looking. Most of the image is the same colour. I think this is largely a result of the filter cropping the blues. One thing to try would be increasing the contrast in the blue channel. Regarding playing with ill-understood sliders, I do try to avoid it and we all do it to some extent, no doubt. Just don't make an instructional video saying, 'This is me thrashing about with sliders I don't understand and hoping to find something I like!' There are way to many of those videos already... Olly

That's right, but I'm also suggesting that, if you go for the very minimum on all three components (mount, optics, camera) you won't want to keep any of them if you upgrade along the line. I agree that a second hand HEQ5 would be a sound investment. Olly

If you respect your first budget, very few of your first purchases will pass to the second level. It's best to know this from the start. If you want to build for the future, start with the mount. The rest will follow. This is a fairly sophisticated image but the mount was a basic EQ6 - and a very old one. Olly

I use a mathematically more complex system in Ps but I don't do the maths myself! Essentially it's invert and divide. 1 Log stretch the image to about 80% of full stretch and set the black point not too dark. Save as Stretch1. 2 Run StarX. Save as Starless. Process Starless as you see fit. I use Noise Xterminator as a bottom layer and erase the bright, sharp bits which don’t need it. I also fix the background and stretch a little more above that, using Curves. Do all contrast enhancement and sharpening to the starless image. Save. 3 Paste Starless over Stretch I. From here on I have made an Action: 4 Ctrl I to invert both layers. 5 Top layer active, set blend mode to Divide. 6 Stamp down. (Alt Ctrl E) This gives a new top layer. 7 Ctrl I to invert that layer. 8 Flatten image. (Do this under the layers palette from the top toolbar. Ctrl E does not work for me.) 9 Save as Stars. End action. 10 Paste Stars onto the processed starless, blend mode screen. 11 Use Levels mid-point slider to reduce stars. Small stars benefit from contrast reduction. Large soft stars benefit from contrast increase. We were working with an image of 552MB after stacking in super-pixel at 80%. This is much smaller than huge telescopic mosaics in which I've participated. StarX worked reasonably quickly. I wandered off and came back, but 20 minutes would cover it, I'm sure. No big deal. Olly

Good points. However, I wouldn't say that SXT has any role in preserving or discarding stars. In order to create the starless layer it obviously removes them. However, when they are replaced, they are replaced by stars from the original image, untouched by SXT. There are different ways of doing this but the simplest (which I no longer use) just involves putting them as a top layer in blend mode lighten. If Photoshop is doing what it says it is, these stars will come into view as the stretch renders them brighter than the background. This means that the imager is likely to halt the stretch before the faintest stars have become visible. I'll see if this method gives a greater range of stellar brightness. One thing I have done is experiment with ways of varying the star stretch according to the brightness of the background around them. While this is not true to the data, it may be more true to the visual impression since stars of a given brightness otherwise look way brighter against a dark than a light background. I didn't do that on this image. Olly

This is a false dichotomy. It's both. Almost all astrophotos that you'll see anywhere will have been made from stacked and post-processed multiple exposures. It is possible to shoot something as bright as the moon in a single, untracked exposure provided the focal length is not too long and the F ratio fairly fast. (F ratio is a simplification, here, but it will do for now.) This was with a Canon 400L lens, a 250D body and hand held at F5.6, braced against a house wall. Your focal length is more than three times this, meaning that the moon's apparent movement will make it track across more pixels in a given time, introducing blur. While your telescope can resolve more detail with its longer focal length, it will lose what it has gained because of the motion problem. Basically, astrophotography is not a point and shoot activity. If that's what you want, I'd stick to a conventional camera and appropriate lenses. They don't have to cost as much as the Canon 400L! Olly

No, yours is a great image - as I said at the time and repeat here. Olly

Interesting question. The stars have been given an absolutely bog-standard log stretch using the mid point slider in Ps Levels, so the size/brightness range should be perfectly normal and typical of most astrophotos. (Indeed, since there has been no star reduction*, it should be a classic AP stretch.) However, the stretch itself was much lighter than that given to the background. If you use one stretch for all you end up with this: Now some people might like or prefer this and, as an image demonstrating the richness of the MW starfield, it's fine. It's just not what our image is about. Modern processing allows a telescopic look to be extracted from a lens image, meaning smaller stars and more visible nebulosity. A consequence of the separate stellar stretch is that small/faint stars will not reach the level of the widespread nebulosity and will remain invisible, so diminishing the range of the stars we can see. This is exaggerated when faint nebulosity is lifted well clear of the background sky but, again, that is the whole point of an image like this. Perhaps this accounts for what you are seeing? Olly *By star reduction I mean the reverse-processing of a star of a given size to make it smaller. Our stars have simply never been stretched far enough in the first place to need it.

I realize that there are such distortions, inevitably, but I was only speaking approximatively. Olly

It's put together in APP which is very clever with the geometry. The printer's running as I type. My largest option is 'extended A3' but I'm thinking of joining two A3s together. I no longer have a friendly neighbourhood printer with a roll paper machine, unfortunately. He's retired. Olly

New image from our Samyang here. Love that lens!

Our largest mosaic to date. Capture, pre-processing and construction by Paul Kummer using gear jointly owned by Paul, myself and Peter Woods. (Avalon M Uno, Samyang 135 wide open at F2, TS 2600 OSC CMOS camera. My post processing. I gently gently enhanced ten extensive regions of interest using existing telescopic images but only the nebulosity was enhanced. All stars are Samyang, for consistency. Paul's construction of the linear data was outstanding and this was remarkably easy to process. I do like finding out unexpected relationships between well-know objects and also seeing their relative sizes. The North America, for instance, is smaller than I thought. The target per panel was 21x3 minute subs, captured between Aug 21st and Sept 23 this year. Stacking was in super pixel and then downsized to 80%. Larger verison here. https://ollypenrice.smugmug.com/Other/Emission-Nebulae/i-TL48wrC/A Olly

There are better ways to beat low signal than clipping. This is a big subject but it is worth looking into. Olly