rickwayne

Thanks man! All good advice. (Um...3 hours? 180 x 137 is just shy of seven hours, no?) I'll try the Photoshop stuff again and see what I get. Thanks too to Geordie. <sobs>"Not stretch it so hard ? But I LOVE drama!" ;-}

I'm usually working toward a print for the wall and so want the maximum pixel size I can extract. Although I might have to settle for smaller with this one. With a CMOS camera, binning may as well be done in processing, no? Unlike CCDs they don't bin in hardware, so you'll wind up doing some kind of software downsampling anyway. I had to double-check, but no, no sharpening at all was applied. I did use starnet++ (the python command-line script) to eliminate the stars and 99% of the processing was done on the starless image, recombining at the end. That definitely left some haloing artifacts, which I actually hand-edited a bit to reduce. This FITS is a crop of the original crazy-quilt mosaic (I was messing with the rig and no two nights had the same camera orientation). This is about 55% of the original frame size. Thanks for having a look. Sh2-155_6_nights-H_Alpha-session_1_session_2_session_3_session_4_session_5_session_6--34degCCW-1.0x-LZ3-NS-mod.fits

So here's an image I've been working on lately. I leaned pretty hard on it for noise reduction, so for this example I'm pulling all that out. 137x180", ZWO ASI183MM at gain 178, ZWO 7-nm H-alpha filter, f/4.6 StellarVue SV70t-IS. (It's Sh2-155/Caldwell 9, called by some the "Cave Nebula".) I had a suggestion that 3 minutes was way too short for narrowband, and yet that's within the ballpark for my bright urban skies with the moon out (when the 183 first came out there was a good thread on it). Also that it was "overprocessed", which I took to mean stretched too hard. Perhaps. I'm concerned that with the residual periodic error in my CEM25P, if I go much longer I'll really have monster stars, probably egg-shaped too. Your thoughts? Vlaiv, I oughta be able to go by total integration time so long as I'm not down in the read noise for individual frames, right?

It is a misconception that mono cameras with filters require longer to shoot an equivalent image. You can actually get away with shorter total integration times, especially if your skies aren't super-dark. Since our eyes discriminate fine detail in monochrome, concentrating on luminance gives you better detail quicker. Bonus: Luminance is pulling all frequencies at once, so the photons accumulate faster to begin with! RGB data can be at lower spatial resolution without compromising perceived image quality. So you don't need as anywhere near as much time on the color filters. It's definitely more fuss and gadgetry, and a slightly more complex processing workflow. But if a duffer like me can pull it off... And of course, mono+filters enables narrowband imaging, which is a whole other ball game WRT good signal/noise ratios even in bright urban skies, in addition to being fun in itself.

The way I can tell that I REALLY love platesolving is for it to break somehow, and dump me back into the bad old star-hopping days -- or nights, rather. Nights when I would strive and sweat and curse for 45 minutes to an hour straight and never get the target in the FOV. I don't miss those days even a tiny little bit. It boggles my mind that a computer the size of a cigarette pack can look at a random image of the sky and figure out EXACTLY where its center is without even having to access the Internet. It blows my tiny little mind completely that it can then robot the scope to an exact point in the sky based on that. Next Big Thing, if you haven't already: Autofocus. I twiddled and bashed and hacked code and soldered and started over and it was still unquestionably worth it. Now that I've figured out the params and such it really does Just Work, even with my laughably crude-looking setup.

Nice. I was just commenting on another NAN image that this particular target is so bestrewn with stars that some sort of reduction technique IMO helps emphasize the nebula. YMMV, it's an artistic rather than technical decision.

As well you should be! Care to detail any of the issues you encountered making the mosaic and how you solved them? I'm also an APP user, and when the Moon quits squirting light all over the sky I'm going back to my M31 mosaic.

And of course one astronomer's "parfocal" is another's "what the heck is this blurry stuff trying to pass itself off as an image?" Just in case vlaiv's point is insufficiently clear, when you change focus, the vignetting changes too, invalidating flats taken with other filters. Sadly, no matter how much I clean them and am careful with them, my filters always have griblies, and parfocal? It is to laugh. YMMV, of course.

According to the PHD troubleshooting doc, the best practice is to calibrate on a star near the equator/meridian, as has been noted. If your setup tells PHD the mount position -- as yours does -- it can work out the correction factors for other parts of the sky from there. In fact if you don't change anything (e.g. orientation of guide scope or camera), you should be able to reuse the same calibration for quite some time, no need to shift-click every night. It is a good idea, however, to run the Guiding Assistant pointed at the target. This will read the actual behavior of the mount while it's tracking (but not issuing guide commands) so that it can fine-tune the parameters. I've had it tell me some surprising things. One night the seeing was particularly good and it advised that I could use a 1-second exposure for the guide camera; I had always gone with the conventional "don't chase the seeing" wisdom and selected something in the three-second range. Bang, the performance went from 2-3" RMS to under 1. At my image scale, that's plenty good enough.

Bias frames are important for the math to work out correctly for flats. The 1600 is known to have squirrelly performance at very short exposure times so dark flats are the way to go with that camera. Fortunately you can take the dark flats any time with a cooled camera, no need to take them at the end of the night. Just be sure to use the same exposure as your "light" flats. As for shooting flats on just one channel, I suppose if it works, that's hard to argue with. Assuming your filters have all the dust bunnies in the same spots, that is. I don't enjoy that happy circumstance!

It's worth spending extra time on luminance because of the physiology of our vision. We discriminate fine detail largely in greyscale, so the visible structure of your image will be much improved by more luminance integration time. This reduces both what we commonly think of when we refer to "noise" in a digital image -- small artifacts analogous to grain in a film image -- but also smoother, more finely-delineated detail in both small structures and in sweeps of tonal variation. Noisier RGB data are tolerable, in fact some imagers commonly blur the RGB a bit to reduce grainy chrominance noise.

Awwww! We're also going back to the days of buying the stores out of toilet paper here in Wisconsin. Trips are pretty much a no-go (so to speak). Although outdoor activities are way up, you basically can't find a used kayak or bicycle in Madison. The premier local paddle shop says his business is up 30% over last year. So sorry the trip is on hold, but just think how much more ready you'll be to do astro by the time you go! And please do say hello to the Lake District for me, my wife and I spent a goodly chunk of our honeymoon there and it is still one of my favorite memories, all these decades later.

To play around with focal lengths and deep-sky objects, you could go to telescopius.com -- that will allow you to previsualize what a target will look like for a given sensor size and focal length. Or you can download Stellarium (free), and set it up to do the same thing. Planetarium apps like Stellarium are also great to help you find the objects in the sky, it's available for phones as well as for computers. For Andromeda, you'll definitely want the long end on the zoom. Here's a screenshot out of Stellarium showing the galaxy and a 300mm focal length on an APS-C sensor. If you get a chance, play around with the mount and your camera beforehand, you'll want to minimize your "fiddle time" in the dark and also should get an idea of what's practical for exposures. For example this is a 500mm lens, APS-C (image is cropped) on a simple camera tracker (iOptron SkyTracker). It won't win Astronomy Picture of The Day but for one of my first ones, I was pretty pleased! These were two-minute exposures on an f/8 mirror lens, so I had to process them with a pretty heavy hand to get the details out of the data. You will have to focus manually -- your camera will almost certainly refuse to autofocus on a starry sky. Magnify the live view by a bunch and you should be OK. Note that that image is composed of 19 frames of the same thing, "stacked" using software like Deep Sky Stacker (Photoshop also has a very basic mechanism for doing this via setting the stacking method for a Smart Object). Without going into brain-numbing detail (references available upon request though!), taking a whole bunch of short exposures allows the software to stack up the exposures as if it were one long one. Once you have the images captured, I for one would be delighted to help you through the processing part. 19 two-minute images is actually pretty short, but a total exposure time of half an hour to an hour should let you get something pretty good out of a bright object like M31. At a shorter focal length, the left end of Orion's Belt has some truly wonderful molecular clouds just jumping with color. Again, this is not anything to submit for prizes, but I did it with a simple 50mm lens on my APS-C camera and an electronic widget that simulates a tracking mount (sorry, only Pentax cameras have it!). The 12mm should let you get some really stunning images of the Milky Way. The Lonely Speck tutorials on the Milky Way got me started, he has a 5-minute video on learning to shoot the MW, and the Astrophotography 101 series goes into a little more depth. One last tip, on exposure: Don't be disappointed if what comes up on your camera screen doesn't look like much. You'll need to enhance the contrast pretty hard (stretching), and stacking helps you do that without so much noise. However, do examine the histogram on a test shot. Since most of an astrophoto is dark sky, the biggest peak will be from the sky background. You want that to be somewhere in the range of 1/4 to 1/3 of the way from the left, but at a minimum it should be mostly clear of the left side (not black-clipped). Welcome, and please don't mind if I'm a little envious. I wish you joy in the journey!

Rule of thumb for astro imaging is not to exceed 1/2 the mount's rated capacity. So we're talking a max total optical-tube-assembly+camera weight of 5.5 lbs.. The Rokinon 135 f/2 is very highly recommended. If you want to go longer without shattering the bank, you might be able to pick up a Pentax 300mm f/4 for a not-absurd amount of money. There are quite a few beautiful images you can make at pretty modest focal lengths. I would recommend starting with kit that's lightweight and not too long, to minimize basic problems, and honing your acquisition and especially your processing workflows and skills before going deeper. Especially with Andromeda and the Orion Molecular Cloud Complex easily visible, you can start pretty wide and amaze yourself. One example of difficulties that mount with the focal length: unless you're already an accomplished visual astronomer you will not believe how difficult it is to simply get a dim target framed with a long lens or telescope. Thirty dollars spent on one of these helped rescue my sanity: red/green dot sight for hotshoe mounting. Until I got a GOTO mount I used it every time, I got so that I could point a famously-dim 500mm mirror lens and only take half a dozen tries or so to frame something with it. (A less famously-dim astronomer would no doubt have helped, perhaps you're one of those lucky ducks.)

I'm using a 120MC with my OAG. I can't say that I have the finest performance yet but I do get one- or sub-second guiding when things go right.

Having a two-axis mount is really nice. Having GOTO is even nicer. Getting pointed is a big part of the battle when you're starting up -- and even now, years in, if my automation takes a dump...I shock myself at how long I have to work to get the target in view! I respectfully disagree with those who counsel you to "future-proof". You don't know what you're going to want in the future. May as well start with something well within your budget and your ability to transport and set up repeatedly, and build up your skills to the point where you're ready for the next step -- if you ever are. My friend the Finn sees no need to go beyond his iOptron SkyTracker. I went upscale for a CEM-25P and don't regret it, but I went in for deep sky in a major way. In fact for quite some time I hardly took a photograph that wasn't a deep-sky image. You will have plenty of hills to climb with getting good data and especially learning processing. No need to complicate yet.

I would find it really hard to visualize all the steps without missing any, lacking the equipment in front of me. I would concur with doing a dry run or two, slowly and carefully, perhaps in the daylight so that you're not pressured to get imaging. After you've been through the whole thing once or twice, then write the steps down. For example the excellent detailed list from Padraic would be useless to me. Set the scope outside? Hah -- my RGB imaging site is 45 minutes away! If you're in the same boat, a packing list is absolutely crucial, at least until it becomes second nature. I have been stymied more than once by forgetting spare batteries, for example, or failing to charge my phone or whatever device I'm using to control the imaging. I recommend that you keep a field notebook and write down what sorts of things you tried, settings you used, and what seemed to be a problem. Future You will thank you.

The guidescope mount has a lot to do with it, or so I've read. At only 362mm with a flattener/reducer, my scope wasn't exactly in the "gotta have it" range for an OAG, but I found one inexpensively and figured I'd give it a whack. LIghter and easier to balance than the guidescope. If you will have a rigid two-ring mount for the scope, you'll be up and running more easily with that. If you were planning to use a finder-scope shoe, yeah, an OAG might do you right instead, especially at 650mm. An OAG can also be more fiddly to keep up if you're a nomad like me and are always boxing your scope, driving it about, and setting it up. Hard to keep the focus, the guide cam keeps getting bashed down onto the stalk. And if the spacing works out, consider a helical focuser for the guide camera, it beats trying to adjust a friction-fit fitting on a stalk in the dark with frozen fingers, where moving it in or out 2mm is enough to go from stars to vague doughnuts.

And of course the rabbit hole goes deeper than that -- small sensors tend to have smaller photosites in order to keep the pixel count up, which tends to increase noise, other things being equal. The really good answers have all been given, but to summarize: Get an idea of what you can frame with your DSLR at various focal lengths with Stellarium or Telescopius.com or something similar Find out the pixel size, in micrometers, of your particular sensor so you can run the image-scale calculations Know that the classic error of the beginning imager is to put too long a scope on too small a mount. Pick up one of the several good books on the subject to get a handle on these interrelated concepts Mount, mount, mount, mount. Much more important than optics, for anything over 100mm Making Every Photon Count is very well-recommended, my personal favorite and my introduction to this art/science/obsession is The Deep-Sky Imaging Primer. You will learn not just about the techniques, but why they matter, in a structured and sensible way. Doing deep-sky work means that you have to learn to do a lot of things well, many of which aren't very intuitive (e.g., scope aperture matters way less than in visual astronomy; there's an amazing number of targets suitable to APS-C sensors and 200-500mm focal length). Since exposures have to be long, tiny imperfections in tracking the sky can ruin the image. We are talking single digits' worth of arcseconds, here, aka 1/3600th of a degree. Here, for example, is a shot from my attempt at Andromeda the other night; the error here is on the order of 7-9 arcseconds over a half-minute interval. And that's at a mere 360mm F/L. So again, the mount is absolutely crucial, and you should literally spend the majority of your budget on that. If the mount is wobbly, ain't nothin' gonna fix that. If the scope is short, you just image big things for awhile. All hail the people who started with big scopes and made it work for them. Me, I have frustrations enough with a short one!

Well, that's all right then! The infinity stop is usually worthless on modern autofocus lenses -- why bother making it accurate when the punters will never use it? If you're not already using live view, a magnifier, and a Bahtinov mask, those are three good ways to ease achieving good focus. But maybe you already knew that.

To bring out the details and color in a dim subject like the Milky Way -- it may be bright compared to some distant nebula, but it's dim compared to a more-typical landscape or portrait shot -- you need to enhance the contrast dramatically. You'll often see that referred to as "stretching" the image. Trouble is, when you amplify the signal in this way, you amplify the noise too. "Noise" means more than just colored griblies that look like grain from old-fashioned film. It's any departure of the image from the actual scene. So it might appear as "chunkiness", or overly harsh contrast, or weird bits of color. Shooting a very long exposure helps smooth the noise out, since as the signal (actual photons from the distant object) gets bigger, the noise is a smaller and smaller fraction of it. Stacking is just a software-enabled means of breaking one long exposure up into lots of short ones, obviating shot-wrecking problems such as satellites, fireflies alighting on the lens, tripod kicking, and "filling the well" of the sensor by maxing out the number of photons it can count at once -- to say nothing of the motion of the stars during the exposure. But before you try anything sophisticated, try simple things, and see how far they get you. Take that Curves tool or contrast slider or whatever you're using and really yank on it. You might not like the result, but you'll likely be amazed at just how much more detail is hiding in your image. You're on your way. Possibly on the way to madness, mind you, but you're going.

You may find that e.g. Vega is good to get roughed in, but that dimmer stars will allow you to better discern small focusing errors. Use ISO eleventy-million for the shortest exposure times possible, and definitely use magnification of the live view to really peer at the details.

You want to do all your proc in PI, right? If you were using Photoshop you could just layer the original starry image above the nicely-processed nebulous one and set the top layer's blend mode to "Screen".

You can pick up a used Ha filter somewhere and get started right away. You don't have to have a filter wheel, the ZWO cameras let you mount a filter easily. With just Ha you can do things like this, or this, or this.

Welcome! Holy BUCKETS that's a lot of millimeters for starting out! Certainly won't say it can't be done, but DSO is a real course of sprouts to begin with -- you're setting yourself up for some challenges with that mount and that long a scope. But if you're on a budget, the conventional advice ("Just get a short refractor to start with") is, um, unlikely to appeal. 🙂 I would heartily second the reducer advice; someone with experience with SCTs will have to recommend which one, or if you need a flattener/reducer. The advice about image scale is also spot-on. Really, if the pixel size is appropriate, a DSLR lacks but one advantage over an uncooled astro camera: Better hydrogen-alpha response. So I doubt the increment over your existing camera is likely to be worth the money; my advice would be to shoot with your existing camera until you can save up for a cooled astro cam, which will give you better H-alpha but also lower noise, and allow you to shoot dark frames at your convenience and reuse them, rather than wasting dark-sky time shooting them in situ. I will give you my "conventional advice" for two purchases, though: Charles Bracken's The Deep-Sky Imaging Primer and Astro Pixel Processor.