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Messages - marekc

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Hi Christoph,

I don't know the answer to your question, but I've wondered the same thing. If a person were shooting solely narrowband, would a cheap achromatic refractor suffice? At first it seems like it would, but I'm not really sure.

I guess it would depend on some of the things you've mentioned, like the flatness of the field, and so on. I guess there would be the problem of shooting RGB stars with such a scope, too. But if a person was happy with, say, the Hubble palette and its pink stars, then maybe an achromat would be the way to go.

Hopefully someone will know if there's a hidden optical issue that we're not thinking of!

- Marek

I agree with Mike. All of those images look very nice, the Popov one particularly so! I'd love to have been there to learn about these techniques, particularly the M51 (Leo Bette) image and the Popov image.

- Marek

Gallery / Re: M42, HaRGB
« on: 2013 November 04 17:08:35 »
I like the additional color in the core in that version. What little processing of M42 that I've done has shown me how hard it is to get any color in the Trapezium region. That area gets so easily blown out to grey, that it's gosh-darn hard to get *any* color there! Anyone who has a colorful M42 that *doesn't* have that region as a total grey blob deserves praise. M42 is so tricky! I think you're doing a good job.


Gallery / Re: NGC 253
« on: 2013 November 04 12:35:47 »
Here's another lesson I learned from this tutorial:

If your filters and sensor window are fairly clean - not too many dust spots - and your object doesn't take up much of the frame, you don't really have to shoot flats. Well, you *should*, I mean do it if you can, but if not, it's not the end of the world. Cropping and DBE can deal with a lot of the problem.

This also reminds me that I really need to take my camera and filter wheel apart and get in there with a blower bulb. I have a lot of dust! Flats take care of the problem, but it would be nice to get at the *root* of the problem.

- Marek

Gallery / Re: NGC 253
« on: 2013 November 02 21:44:09 »
Kudos, Alejandro! A beautiful image, a masterful workflow, and a fascinating tutorial. You've hit the Pixinsight trifecta!

- Marek

Hi Georg,

Thanks very much for the update on LinearFit and GradientMergeMosaic. Unfortunately, neither StarAlignment nor GradientMergeMosaic was able to make my mosaic work. No matter what I use, the two panels of the mosaic look very different from each other. One has a reasonably smooth background, without a lot of noise, and the other one has a really noisy background, and is stretched more. Neither Frame Adaptation nor GradientMergeMosaic seems to be able to handle the differences between the images, even with the use of Frame Adaptation.

This is despite following closely the workflow shown by Steve Allan in his `Supermosaic' video on the Pixinsight Youtube channel. That's the video where he shows how to use GradientMergeMosaic.

I suspect that either I've missed something subtle, or a moonlight gradient makes it impossible to shoot a 2-panel RGB mosaic, at least with my particular imaging system. Oh well, it was basically for practice anyway. A darker sky might have yielded more useable data.

- Marek

I think I'm starting to understand what's going on here. LinearFit does, indeed, work on two images of the same piece of sky. That will make it tricky - although not necessarily impossible - to use this tool to make one panel of an image match the other panel more closely.

It looks like Fabian Neyer and Juan had a conversation about this already:

From what I can tell, Linear Fit is designed to work with two images of the same piece of sky, which have been co-registered to each other (using something like Star Alignment). It looks at a pixel in the reference image, and asks `how bright is the corresponding pixel in the other image'? A large number of these comparisons are made, and we could imagine LinearFit as being like a person who's plotting the comparisons as points on an X-Y graph. Pixel values in the reference image might give the X-coordinates, and the corresponding pixel values in the `to-be-fitted' image would give the Y-coordinates.

Since CCDs are, in theory, linear in their response to light, we might reasonably expect that the graph would be a straight line, with some nominal amount of scatter around that line. I don't know if LinearFit does something like a linear least-squares fit, but it probably does something at least a little bit like that. Armed with a knowledge of the slope and Y-intercept of the line, it can transform the brightness values of the `to-be-fitted' image into the brightness values that they *would have had* if that image had been captured in the same way as the reference image. If either image has been stretched into the non-linear realm, then I assume all bets are off.

Given all this, I'm going to need at least some overlap between my frames - which of course I do have, since I'm mosaicking. I think a little jiggery-pokery with Star Alignment, and maybe cropping, can give me frames on which to perform LinearFit. Next, I would need to get LinearFit to tell me the operations it recommends performing on the `to-be-fitted' image. (Maybe those show up in the Process Console?) Then perhaps I could use PixelMath to alter the `to-be-fitted' image.

My remaining concern: Doesn't `Frame Adaptation' do all this? Is Frame Adaptation simply an under-the-hood version of LinearFit, applied to the overlap regions of the mosaic panels? If I'm using FrameAdaptation, and I'm still getting a bad-looking mosaic, maybe the gradients across and between my images make mosaicking hopeless.

Possible lesson that might come from all this: If it seems like a good idea to learn how to shoot and process a mosaic during bright-Moon time... errrr, maybe not so much.

- Marek

General / Re: OS X Mavericks?
« on: 2013 October 25 15:13:26 »
You're right, Lion and ML have a sort of side-by-side version of Spaces. For some reason, my brain just couldn't cope with that - I needed a 3x3 grid of Spaces.

I think I'd gotten pretty seriously hooked on Spaces when I was working on my Swinburne degree. I used Space #1 (upper left) for my bibliographic reference management software, Space #4 (middle left) for my PDF-reading software and for writing in LaTeX, Space #5 for my browser, etc...

My brain has been stuck in an `I need a 3x3 grid' mode ever since  :-[

Pixinsight lives in Space #3, BTW. I can't imagine PI being anyplace other than the upper-right. Adobe programs live in the middle-right Space... one spot below PI  :tongue:

- Marek

General / Re: OS X Mavericks?
« on: 2013 October 25 14:41:49 »
I had been using Snow Leopard for a long time, when I got a new MBP at work, with Lion on it. The main thing that (at first) freaked me out about Lion and Mountain Lion was the lack of Spaces and Expose. I can't live without Spaces!

Fortunately, I found a 3rd-party solution, which gave me back my Spaces:

Looks like it's been updated for Mavericks.

I need to get around to updating my home machine to Mountain Lion, at least, and PI 1.8.

- Marek

Once again I find myself running into a problem with LinearFit, and I'm curious to see if anyone might know what's happening.

I am trying to make my first two-frame mosaic. I shot the R, G, and B frames for each panel last weekend, when the Moon was pretty bright, so all of the images have fairly heavy gradients. I've worked DBE pretty hard, trying to flatten everything as much as possible. I think I have removed the gradients reasonably well.

Whether I use StarAlignment or Gradient Merge Mosaic, the results are poor. Both of those methods are able to align the panels and fit them together, but one panel looks reasonably smooth, and the other looks very noisy and grainy, in the final mosaic. It's much worse than just a visible seam. This is despite the fact that the background levels of both panels are pretty similar. (Also, I've used Frame Adaptation at all of the points where Steve Allan recommends it in his `Supermosaic' video.)

It occurred to me that since the images are still linear, maybe I should try to Linear Fit them, prior to building the mosaic. Problem is, I keep running into the `Incompatible Image Geometry' error, no matter what I do.

I have tried cropping one panel so it's the same size as the other... no joy.
I have tried Linear Fit on RGB images and on greyscale images of one color channel... no joy. (In other words, I'm avoiding the problem that I caused for myself the last time I posted about this:

No matter what I do, I get `Incompatible Image Geometry'.

As usual, I'm probably missing something fairly basic. Does LinearFit only work on images that show the same part of the sky? Did I create an impossible situation for myself by trying to shoot a test mosaic under a moony sky?

- Marek

General / Re: OS X Mavericks?
« on: 2013 October 23 23:13:35 »
Oh man, now I really feel like a caveman  :P

My home computer (the one I use PI on the most) is a MacBook Pro purchased in spring 2009, and it still runs Snow Leopard. Heck, I'm still running PI 1.7 on it! Haven't gotten around to Mountain Lion or PI 1.8 yet, even.

(Although I have installed ML and PI 1.8 on two computers at work that I occasionally use for PI.)

- Marek

Gallery / Re: Hubble Legacy Archive
« on: 2013 October 23 11:51:38 »
That's a really nice M51!

At this year's AIC conference, Rob Gendler gave a talk about processing HLA data. I found it really interesting, and I'd say inspiring, too. It made me want to dig into some HLA data and work on them.

He gave some recommendations for potential first projects, one of which was NGC 253. I've been playing around with some of the HST ACS data on that, although I'm still trying to figure out how he got the colors that he did. There are basically B, R, and IR data for NGC 253. He used all three, but I've been trying to come up with a synthetic-green method, kind of like Davide De Martin does with DSS data. So far my HII regions all come out yellow. I'm still scratching my head as to what to do.

On another HLA-related thread here on the PI Forum, I complained about HLA images being noisy. That's probably true in some cases, but upon looking closely at the NGC 253 data, I think some of what I thought was noise was actually resolved stars! That was a `wow' moment, if I'm right about it.

At any rate, after hearing Gendler's talk, my interest in processing HLA (and DSS, etc) data certainly increased!

- Marek

Gallery / Re: IC 443 in Ha and RGB
« on: 2013 October 19 16:53:21 »
Hi Albert,

I think this image is simply lovely. Lots of nice stars, but they're subdued enough to avoid being a `star carpet'. Intricate, colorful nebulosity, but not garishly `neon-ish'. The stars are colorful, but in a nice pastel way, rather than aggressively glowing with color. And I, personally, like the background level you've used. As I'm viewing the image, on my monitor, it doesn't look like you went for a really dark background, which to my eye could look clipped. I know I'm not missing any information in the background when I look at this image. I think the word I'm looking for is `elegant'.

I should hasten to point out that there are plenty of `brighter', `more-saturated' images out there, which I like a lot. Many imagers go for brighter, more saturated colors, contrasting strongly with a darker black point. And I think a lot of those images look really beautiful! But something about this image struck me as having an `elegant, understated spectacular-ness' (to coin a particularly un-elegant phrase). I really like looking at this image.

- Marek

Hi Rick,

I think your efforts paid off. Those look like good RGB stars to me. I haven't shot narrowband yet, but when the time comes for me to tangle with putting RGB stars into an NB image, I'd be happy to get a result like yours.

- Marek

General / Re: Help with Veil Nebula
« on: 2013 October 12 10:10:31 »
You may also want to check out RBA's tutorial on star-shrinking:

He demonstrated this here at AIC yesterday (using a different image), and we were able to see it work well in real-time.

- Marek

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