ollypenrice

When you say, 'combine different images in Photoshop' I guess you mean different finished images, possibly of different exposure times? But question 1 is, Why complicate the issue by taking different exposure lengths in the first place? I have combined different exposure lengths on M42 and... that's about it. I did not find it necessary for the Andromeda galaxy, for instance. Manual aligning in Ps is slow and labour intensive. You might try its automated image alignment, though. I find it works well for daytime photography when I want to combine a set of focus-bracketed macro shots; I've never tried it on astro images. (The path is File-Scripts-Load files into stack.) This will give you a set of layers with each image aligned - we hope. Now, what do you want to do with these layers? You could try Photoshop's automated layer blending. Make all layers active and then follow Edit-Auto-blend layers. Choose stack. I've not tried this for high dynamic range, only for focus bracketing. If you want to be clever, as is necessary for M42, you can take very different exposures. I used 10 seconds, 50 seconds and 15 minutes. The tutorial I used to point out to people has been deleted but this one is similar. https://www.astropix.com/html/processing/comp2.html What I do is not quite the same. I paste the short exposure on top of the long, create a layer mask and copy the long exposure onto it. I then greatly increase the contrast on the layer mask and give it a huge Gaussian blur. Maybe 4 or so. This allows the short exposure to pass through the mask only where the mask is burned to white. I think it is the most natural way to do the blend because the long exposure itself largely creates the mask. As you work on the mask you can see its consequences on the image as it will end up. The beauty of Photoshop! While I have not used multiple exposures on normal targets in the past, I'm thinking of doing so since I've started using StarXterminator in Ps. This lets you remove the stars, process what's left very hard, then replace the stars. I'm wondering if it might be good to shoot a set of short exposures for the stars and use these on the starless image at the end, but I haven't tried it yet so it might or might not be worth it. Olly

RASA users report the absence of dust bunnies, probably because they are too far out of focus to show. We certainly get none at all. It seems like you have a small chip so you may have insignificant vignetting. So, do you really need flats? Just a thought. Your Iris shows only dark grey and blue colour. What has happened to the browns which ought to be there? I'd want to get to the bottom of that question. Olly

I've been skeptical about the realities of super-fast systems (we all know the theory) but am falling in love with the RASA 8. It does not do everything perfectly (does anything?) but what it does, it does in spades. What I would really want to stress, though, is that this super fast system comes into its own in conjunction with a recent piece of software, StarXterminator. When you process a RASA image with StarX you can exploit the RASA's phenomenal grasp of faint signal and dodge its indifferent stellar image quality. RASA and StarXterminator is a marriage made in heaven. (Some say the same about Starnet++ but, for me, it has not done a comparable job.) Olly

What the dedicated astro software packages have are astro-specific routines. Of these, by far the most important is gradient removal, ideally a gradient removal which leaves you with a neutral grey background sky (equal brightness in red, green and blue.) For this I use Pixinsight, after which I work almost exclusively in Photoshop - which probably won't be that different from GIMP though I don't know for sure. When you say that you think your images may have more to give in processing, you will be right - because this is true of any image. It doesn't matter how good you are at processing today: tomorrow you will be better. There is no reachable limit in terms of processing skill. That's its great charm. Olly

Fair point but the data gave little gradient. I'll look at that again though. Olly

With Paul Kummer. Paul pointed at M31 with the RASA 8/ASI2600MC to see what it might give. We both liked the test shots very much and decided to give the galaxy some breathing space in a 4 panel mosaic. The idea was for a change of style to give a simple, natural rendition which would still go deep. After Paul's capture and construction of the linear mosaic, which I gather was difficult, I did the post processing and that was difficult as well! It's ironic that going for a soft look should prove so difficult and complex right through the pre- and post-processing. The faint extension on the right hand end of the galaxy looks slightly detatched, but that was how it was in the data (on a single panel so not a jointing artifact. Olly Edit: only when I'd finished the image did I realize that it has had no noise reduction at all - other than being downsampled at an early stage to 50%.

Nice seamless mosaic. Not always easy! Olly

That's a thought. Olly

Sure. I see lots of roughly parallel dark lines of dense dust. The nebula which repeats the shape of its own edge is arrowed.

A reprocess of this vast mosaic captured by Yves Van den Broek allowed me to pull out the faint nebulosity more fully and I was struck by something strange. It's easiest to see in the nebula to the left of the North America. You'd think you had double vision because its left hand side shows the same outlines twice, slightly offset from each other. Then there's a large body of dust upper left of centre and this has long vertical extensions coming out if it above and below, again showing parallel repetions. There are also some horizontal repetitions in the view as well. The separation between repetitions seems to be roughly the same scale everywhere. In fact these offset repetions make the image a little queazy to look at since they have that offset double vision effect. What might cause this? Some kind of density or pressure wave passing through the dusty medium? Olly

The official method for re-starring in Ps is very strange but the stars can go back fully bright and still adjustable. You paste the starless image on top of the starry, invert them both, set the blend mode to divide and stamp down to produce a third layer on top. Once inverted and flattened this is a stars-only image which behaves just as you'd like it to. Paste it on top of the starless in blend mode screen. Lowering it in levels produces smaller but perfectly good stars. Sometimes a touch of blur settles them into the picture. This method is just incredibly effective, honestly. Olly

Thanks, Rodd. 2170 was all TEC 140. Only the Cocoon close-up was partially ODK14. I had a real pantomime installing Starnet++ into Pixinsight and in the end asked my IT-savvy stepson to do it for me. This was an earlier version and it was very slow, sometimes stalled and sometimes worked well enough to use partially. I then found it had vanished from my Pixinsight tools and read that the developers had removed it. I wasn't about to repeat this experience so jumped at the chance to install StarX into Photoshop. I added Noise Xterminator at the same time. StarX works either perfectly or almost perfectly even on fully stretched images but some say the same about Starnet. So I don't know. We ought to set up a side by side test on an assortment of datasets. Olly

I wouldn't put too much Ha in the L but the project is sound. Olly

Not for sale: 26mm TeleVue Nagler. lly

You have a top to bottom gradient, dark at the top. This is fine if it is also there in your astrophotos, but is it? An easy way to find out is to open a linear image in Photoshop and go to Image, Equalize. This will give an extreme stretch in which the gradient will show, assuming you've used an image which has more stars than nebulosity. Other programs may offer the same thing. However, I'd be very suspicious of that as a flat. It's too flat, no vignetting, no bunnies. That can't really be right. Olly

Decathloàn have red USB charged head torches but the red is of fixed brightness. One thing to look out for is that many head torches require you to scroll through the white light to get to 'Off.' This is not good for our purposes and my Decathlon torches can go straight from red to Off. Olly

Iris to ghost built around a two panel mosaic from the Tak 1O6 but including TEC 140 data for both Ghost and Iris, the cameras being Atik 11000 and 460. I've had the image for a while but wasn't happy with it. Star Xterminator finally made it possible to process as I'd imagined it. Olly

Truly original view of a familiar target. I particularly love that transparent window of, I guess, sky swept clear by the shock front. Olly

Super. I never think of this as a purely narrowband target but you've shown that it can be. Lovely restrained use of colour, too. No shouting. Olly

I don't know whether a linear image or a soft stretch is best before star removal, not having done enough testing. However, the thoughts you list after this question are the same as my own. I do like to have eyes on what I'm doing, which is why I prefer Photoshop. I never sharpen stars. I only ever soften them - and especially when recombining them after removal. I tend to think they look too hard when seen as a top layer prior to recombining. I'll certainly sharpen nebulosity on the starless, though, to bring out structure. I'll also make heavy use of local contrast enhancement. This might be LHE in Pixinsight, Noel's Actions or stretching through a Layer Mask made from an equalized, blurred copy of the image. This may be an inverted mask or not, depending on where I want the extra stretch. If a de-starred image has residual evidence of halos (as sometimes happens) I make no bones about using the spot healing brush to get rid of them. I think you just need to check whether the halo is coming from the star layer or the starless before thinking out an approach. I use Noise Xterminator on a boottom copy layer of the starless image before going for the hardest later stretches. I'll then use Colour Select to pick up the noisy parts and erase them. I'm impressed by the lack of damage to detail in Star Xterminator but I'm still fussy about only applying it to parts which need it. As you say, I really don't like too much NR. I'll even add nose to small parts of an image which have become too smooth in processing. I've just had another great result (well, I think it's great! ) by de-starring and re-starring a fully processed image, reverse-processing the stars once separated. (ie de-stretching them) This makes me think the process is very tolerant of when we remove the stars. One thing to watch, though: if you de-star a mosaic in which the background isn't perfectly seamless, StarX will make it worse. The best way to check mosaic backgrounds is to equalize the image for a check Olly

I don't know about 'way off,' but 'off,' yes. (Sorry! ) What we see in your image is not what you were aiming for, I don't think. You have quite large stars applied at what looks like partial opacity. What you want, I imagine, is the reverse, small stars of full opacity and brightness. I use StarXterminator in Photoshop rather than Starnet++ in Pixinsight but the underlying principles must be the same. If working on an existing processed image I remove the stars and then follow the method explained by Xiga in the thread linked below. When I have the original stars as a layer on top I have to reverse-process them to make them smaller. Basically this just means using Levels and moving the grey point slider to the right to de-stretch them. It can also help to give them a mild Gaussian blur to integrate them into the image more naturally. Although de-starring a finished image probably isn't the best way, I find it works astonishingly well. If starting from scratch I've had good results by stretching the image gently until I feel the stars are as I would want them at the end. At this point I de-star and save the stars as they are. I then continue to stretch the starless and can push it considerably harder than I'd be able to do with the stars in place. As Xiga suggests, I put them back as a top layer over the starless in Blend Mode Screen. So I guess what you're looking for is the Pixelmath equivalent of Blend Mode Screen. This will certainly be available in PI and is probably what Budgie 1 is suggesting. I'd certainly persevere, though, because this is the biggest breakthrough in astrophotography that I've seen since I began thirteen years ago - and by a country mile. Olly

What a program. Can I make it four? Or maybe that's too much?

It's a great field. Olly

The standard trick to lose the magenta halos in Pixinsight is to invert the image and then run SCNR green. This in effect becomes SCNR magenta. The halos apart, I think the first one is absolutely beautiful. Olly