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Oh, Biliostatus Rexus, why for art thou numbers not updating? CISS

It must’ve been a problem on GoDaddy’s end. My web populator component that pushes the stats to the website reported that it couldn’t connect from 5:30 until about 8:30. Looks like everything’s back to normal now, though.

Wait…didn’t I just see that black cat walk by a second ago? 😯

[ravnostic]

Oh, Biliostatus Rexus, why for art thou numbers not updating? CISS

It must’ve been a problem on GoDaddy’s end. My web populator component that pushes the stats to the website reported that it couldn’t connect from 5:30 until about 8:30. Looks like everything’s back to normal now, though.

Wait…didn’t I just see that black cat walk by a second ago? 😯

[ravnostic]

Oh, Biliostatus Rexus, why for art thou numbers not updating? CISS

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More than awesome. I’d be proud to have taken any one of those photos. The juxtapositions are excellent. And…somewhere in my archives, I know just the face to place against yours. I’d post it, or link to it, but the location escapes me. Nice work, seriously.

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By “Good” I’m thinking of shots that will give me an outside shot to place well in a Farktography contest. There’s plenty of shots that are interesting, and some that I even like, but I also know the vast vast vast majority of them have zero zilch nada chance of placing anywhere near the top in a contest where U-Man, orionid, kestrana, Mopsy, and at least a half dozen others are just consistently awesome.

Never underestimate the whim of the voters! They’re unexplainable, you know. I’m going out this morning to catch an Iridium flare, and there’s this diner I know that just moved north a block, but the old location is still set up like they’d be opening in the morning; I think I’m going to see what I can do with it before dawn, with the streetlights coming in through the windows.

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That’s completely awesome–but it’s time you start claiming your work so that next time, you’re compensated!! But isn’t it nice to see out there? Did you at least get the photo credit on the album sleeve? (err; CD jacket).

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Love the truck shot Barra. Great monsoon pic, too, Caradoc

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So I reworked that Eagle Nebula (nest) shot. I 2x drizzled, to make it bigger (cropping down to it, of course). I went for true color, and as much detail as I could eek out of it, sacrificing a good many shots (only 14 lights now) that were the best of the best. Stacked it against 12 flats and 7 darks. A little noisy, yes, but far more detailed; you can make out several Bok Globules. I will need to revisit this again, and soon. I think next weekend (well, Sunday) I may go out with the scope, or perhaps the weekend after (when I hope to have my 2Ti again, far better resolution/less noise). But in the meanwhile, I still like this better than the one posted earlier, for the added detail and trueness of tone/color.

https://plus.google.com/photos/107857888121727893520/albums/5799203425257274321/5799203430624633938

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Very pretty location, beautifully shot, Yugo

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You’re quite welcome; learned some things myself in the process. One more note, though, and that’s dynamic range. Let’s say you’re shooting Orion’s nebula. If you want the detail in the nebula, you’ll blow out the Trapezium. If you want the Trap, you’ll miss the nebula. It really does take HDR to capture it all, so to speak.

//Currently restacking the Eagle Nebula, till I’m satisfied with improved results. Using 2x drizzle for more detail/structure; working on finding the right parameters. Will post when I’m happy. If I get happy.

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Simply put, in an equatorial mount system, the more accurate the polar alignment, the better. I use the CPC 1100 from Celestron (this one http://www.celestron.com/astronomy/celestron-cpc-1100-gps-xlt.html )coupled with a focal reducer (making it f/6.3 at 1728mm equiv, bumped back up to 2800mm with my APC sized camera sensor but retaining the f/6.3). Basic polar alignment is easy enough, and drift aligning really isn’t THAT difficult or time consuming. That being said, I’ve taken as long as 7 minute exposures with very little blur, but that comes with a caveat.

There’s an oft-talked about rule (the 600 rule) when taking star photos without guidance. Divide the lens mm’s by 600 to get your maximum exposure time, in seconds, without star trailing. (This is effective mm, so on my APC with a 50 mm lens I have to treat it like it were 85mm). I can tell you, from my standpoint, it’s B.S. When I zoom in using that rule, all the stars are ovals. I go with 1/2 the recommended maximum and then I’m happy. Obviously using higher ISOs results in more stars.

With guidance, though, you’re stretching that window. Polar (equitorial) guidance is a must, or you’ll have what’s called ‘field rotation’–that’s where the stars appear to rotate around the center of the image; you’ll find this happens with alt/az mounts (i.e., ‘fork’ mounted with one axis parallel to the ground) You can take a longer exposure, sure, by maybe a factor of 2, possibly 3. Now how much you can stretch that window, polar mounted, is a factor of how well aligned you are, because field rotation is eliminated. I find it’s pretty easy to get within about 1/4-1/5th a degree of ‘true’ alignment, and this allows me to get good shots of about 2 minutes. But I’ve had times when buy luck or perseverance I’ve been much closer, and I’ve taken those 7 minute exposures then.

But the caveat: A guidance system is only as good as the gears–and any gear system has it’s quirks, at some point, that will show up in a long enough exposure. A gear system will ‘slip’ or ‘tick’ or hit a seam not dealt with in manufacture or elsewise cause either a shift or a bump in the exposure. When you’re dealing with my scope, it’s about a 3 minute limitation to my exposure, as currently configured. I WILL have some type of movement/shift in the starfield on longer exposures. If I went with the CCD system, where I’m at f/2 with a much wider FOV, this may not be noticeable. But where I’m at now, limited to about 1/2 a degree FOV, it is.

Frankly, it’s one of the reasons I’m no longer happy with my telescope; the other being the limited FOV that would cost me about $800-2000 to address and would not be an easy thing to switch away from if I actually wanted to look through the telescope (I’d be stuck with it, and forced to use a laptop to see what the telescope sees, always–and I’d be on a CCD system instead of a DSLR system to boot.)

I’d have to go back to find where you posted what rig you’re building, but keep that in mind for larger aperture mirrors. That, and every little shake (wind, footsteps, etc.) will mess up your shot–and it really doesn’t take much at all to do so.

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That’s an excellent synopsis, Chupa; I’ll add my 2 cents (grabs a Budweiser; I’m lowbrow). I use Deep Sky Stacker myself. It does pretty much what you described, in that order, only I didn’t have to do any programming (I merely select my parameters–but I learned to do it without anything close to instructional aids beyond ‘the best way to learn to use “x” feature is to play with it’ {and I did, a lot}.)

Stacking has 1 goal: Reduce the SNR (Signal to Noise Ratio). It doesn’t brighten the image or add any more color, it just reduces the noise. But this means you can stretch the histogram more without degrading the photo, and that brings out color (think along the lines of the grain of dry wood versus wet {or lacquered} wood) and finer details within very closely aligned tones.

Chupa mentions shooting as many light frames as you can get away with, and that’s correct, to a point. Let’s say you shot 1 minute frames. For ease of de mathz, 64 would be great, and 121 would certainly be better. But your SNR in stacking them (darks, flats, and biases aside), would be improved on the order of 8 to 1 for 64 frames, versus 11 to 1 for 121. 225 frames would be 15 to 1, nearly twice as good as 64, but now you’ve nearly quadrupled your frames (and the processing time). It’s a square root deal.

There is no easy formula for what’s worth it and what’s not in regard to how many you’re willing to stack. How much time do you have to take the frames? Under what conditions? How long will you spend stacking them? That’s all up to you. You can certainly stack stacks, and improve things, which is good if you’ve done frames on multiple evenings under different temperature conditions. But I get ahead of myself.

Also, 100 1-minute frames is not the same as 10-10 minute frames. This is a little harder to explain, because in one way, they are. Imagine light being rain in a bucket, and one minute equals one inch; either way you’ve caught 100 inches. But in reality, it’s a little different. In stacking, you’re enhancing (creating depth) in each set of buckets. So in 100 1 inch buckets, you’ll have a finely detailed ‘map’. But the depth, while detailed, will only vary in thickness by 1 inch. In 10 10 inch buckets, you’ll have much greater depth. But it will be more coarse (more grainy, which in this analogy equates to the SNR). Another way to describe it is that in the 1 inch bucket frames, you miss the deeper details; they may be there in some frames (splashes, if you will), but in averaging all the buckets in most of which they don’t appear in the same spot, they disappear. In the 10 inch buckets, You’ll get the steady deeper ‘drops’ (detail), but then you have fewer images in which to average them out; the result looks more like a Lego ™ structure than a finely curved clay sculpture.

Basically only the details that are captured in most of the frames will be in the finalized image, so depending upon what you shoot, you have to balance exposure time and number of frames against increased SNR that comes with longer exposures.

Back to the combining stacks; the square root deal still applies. Combine 4 stacks (all things being equal between the stacks, exposure, frames, etc) and you’ll double your SNR. 9 will treble it. But if all things aren’t equal (and even if they are), the worst image among them will be the limiting factor in the result. Same, incidently, is true in any stack. So review your frames before stacking. Get rid of the ones with a little camera shake (or, as in Deep Sky Stacker, program things to only use the best x percent of frames).

The rest–dark, flat, and bias frames, are calibrations. Darks average out the noise and subtract it. Flats average out the vignetting and subtract it. Biases average out electronic noise and subtract it (note: I’ve never dealt with biases, to this point). The idea is to take the light coming in and fix all the things wrong with it for an image without calibration flaws, much like we all want to have calibrated monitors that translate to our printers so our images print out like we see them on the computer screen.

The same square root deal applies. A dark isn’t supposed to remove graininess from the image–that’s what the multiple lights do. A dark frame is to remove noise inherent to the camera sensor, which is dependent on temperature, ISO, etc., and thus they should always be taken at exactly the same settings, under (ideally) the exact same conditions. But you can actually add noise to your lights by having to few of them. Using 4 instead of two will decrease this by double. 8 instead of 2 by treble. Etc. I like using at least 10 darks myself, though I haven’t always had the time or patience or mind to do so, especially early on.

Flats, on the other hand, serve the purpose to remove flaws in the lens system. Vignetting is the biggie here (but sensor dust, or dust anywhere in the optical mirror/lens assembly, plays a role). Here’s where the light bucket analogy kinda fails. All the light we see comes from (in essence) one direction, right? Yes, and no. It does, but using either a mirror or lens, we’re bending it. It’s bent less in the middle. More so at the fringe. Besides CA (which we don’t have with front-surfaced mirrors, of course), this leads to less light contributing to the final image from the edges of the frame.

A flat, then, is an image that takes a look at (preferably) a white-or-greyscale field with the identical optic setup. Temperature and ISO don’t matter here–optics do. Change a lens, add a focal reducer, there go your flats. I took my flats with a foamcore whiteboard in the early evening. There’s a complicated formula for what an ideal flat should look like, but I’ll admit; I guessed and got a pretty good set. Take 10 of each at different exposure times and play with them. Once you have a set that removes vignetting (and lens/sensor dust), you’re good till you get new lens/sensor dust. Again, the more the better–you’re averaging out the flaws.

Bias frames, I read, relate to digital cameras pixel-to-pixel variations. However, since dark frames contain the same bias as light frames, a dark frame takes care of them. Assuming the dark is equal to the light in exposure, etc, you don’t need to worry about them, but if it’s not, you could use the bias to equalize things.

However, I’ve recently learned of the importance of Dark Flat frames. (Egads–there’s always a ‘however!!’.) And I’ve never used them, even though I’ve used flats. But here’s the principle. When you shoot the darks, you subtract the biases from the lights, since they are the same. But to calibrate fully (because no lens system is perfect), you need to subtract the flats as well. But if the lights are bias, and the darks subtract the bias, and the flats, of course, contain the bias, then in using both you’ve subtracted two biases from one! In other words, you re-introduce an inverted bias. So by taking dark flat frames–which, of course, have to be taken at the same exposure, ISO, focus, etc as the flats (just with the lens cap on), then you eliminate the bias from the flats, and you come out with two positives (light and flat) and two negatives (dark and dark flat), and you wind up with true calibration.

Phew!

If you use bias frames, however, they substitute for dark flats, so you need one or the other, but NOT BOTH.

The program I use, Deep Sky Stacker (as mentioned) (also; I agree with chup regarding converting to TIFF–works much better than the actual RAW, though that’s what I shoot in for the same reason). First it determines the offsets (which include the natural bias, then creates a master dark upon which to apply them, subtracting from that the dark flats (if available) and then the flats themselves (cross-canceling the bias issue). All that gets applied the the images, which then result in a calibrated stack.

//Damn I need to take my scope out, even if my 2Ti is still out for repairs. I bet I can betta’ myself now.

I think my 2 cents has grown to a dime, or perhaps a quarter. I’ll leave you be now.

A lot of this, but not all, I got from the deepskystaker.free.fr/english/index.html site. But it misses as much as it covers, so I relied on a few other sources as well.

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errr, I mean fluffy

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One way to do it, chupa, would be to use filters, and live view focus to get the chroma pin-sharp for each R,G&B. The difficulty factor would be barrel distortion (which would be slightly different at each wavelength) and a steady center of focus so it could be corrected manually. Me? I often just crop down as needed to eliminate the worst of it.

You can at least get filters that would transmit only the correcdt wavelengths, but that also means you’ll need longer exposures at least, and probably more stacking images.

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Just a reminder again that we have a large amount of theme suggestions in the Themes forum that you should look at and vote on ones you like!

Reminds me of that scene in Galaxy Quest, when Brandon is in the midst of saving Commander Taggert from certain doom, and then his mom comes in and tells him to take the trash out now…

😛

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