rickwayne

Until I actually built myself an autofocuser, a Bahtinov mask was pretty much a mission-critical item for me. You get to a point, focusing by eye, where you can't really tell that moving the ring is having any effect. But the Bahtinov knows. I personally had some trouble judging when the spike was exactly centered, others don't seem to struggle with that. But any movement at all of the focuser has a visible effect on the spike location. The other bit of advice I'd give is not to go for a really bright star, you will often see a cleaner pattern on dimmer stars. The fatter spikes from brighter stars obscure the last little bit of accuracy. A bright star is great for getting in the ballpark, then fine-tune with a dimmer one. Note that focus can vary across the field, so if you use a bright star in the center and try to fine-tune with a dim one in the corner...that way madness lies, my friend.

Lunar is pretty easy relative to DSO, since the Moon is a daylight target. But also relatively small, there are quite a few DSO targets much bigger than the Moon. The thing about deep sky is that the objects are dim, requiring long exposures or lots of them or both. But if your ambitions are modest, I think you can get something you like. Your mount has a stated payload capacity of 22 lbs.; standard practice for astrophotography is to load the mount to no more than half capacity. You will just squeak under that, I think. Your scope is also pretty long at 700mm and pretty dim at f/7 for a beginner's DSO tube. Focal length magnifies any mount imperfections, and a higher focal ratio requires longer exposures, making it harder still to get sharp pictures. A flattener is necessary for almost any scope, if you want edge-to-edge in-focus images. The focal "plane" for both refractors and reflectors is in fact a curved surface. And since the zone of critical focus is measured in microns, it doesn't take much curve to make it impossible to get both center and edges in focus. Flatteners unbend the focal surface, but only at a very specific distance (you may have seen the term "back-focus"). Even more attractive, if you want to do DSOs with that scope, is that frequently flatteners are combined with reducers, sort of a negative magnification lens that concentrates more of the light coming from the objective onto a smaller image circle. A field flattener/reducer would make your scope a bit less long and a bit brighter, simplifying your problems getting good tracking performance out of a low-cost mount. Frequently FF/Rs have a backfocus such that the T-ring and bayonet adapter for a DSLR puts the sensor just at the correct distance; 55mm is the standard. Absolutely crucial: Buy and read one of the basic astrophotography books. I totally feels you WRT roof-crying! There are so many options and tradeoffs that it's really hard to know where to start without wasting tons of money trying to jam together gear that Mother Nature never intended to be together. I cannot speak to lunar or solar photography, but here are three very-well-regarded books for DSO: The Deep-Sky Imaging Primer by Charles Bracken Making Every Photon Count by Steve Richards A Beginner's Guide to Astrophotography by Jerry Lodriguss Seriously, if you don't smack your forehead and mutter "Oh, so THAT'S what they were talking about" at least six times while reading any one of these, I will give you back the price of this post :-). Bracken's was/is my bible, he builds you up from basic concepts to the gear and techniques that stem from them, with gear selection, capture technique, and processing all covered. Welcome, and as I often say, I wish you joy in the journey! To calibrate your expectations, I'll leave you with a quote from a Texan visual astronomer I encountered in the Bortle-1 Big Bend National Park: "In my experience, an imager is a guy standin' there in the dark swearin' at something that ain't workin'".

Orion shot looks much better focused than the Pleiades. What are you using for focus assist? A simple brute-force antinoise tactic: simply to pile on the integration time. But it's a square-root relationship, so four times the integration merely doubles the signal/noise ratio.

Differential flexure is pretty easy to diagnose: If you're getting great guiding but the stars are still bloated or egg-shaped (and you know it's not the optics), you've got it. Since the pieces from the back of the finder to the camera will stay the same, I'd say go ahead and try the finder mount if you already have the bits in hand. Chances are adding guiding to your routine will improve matters regardless; if you feel the need to push further, you could buy a pair of rings (e.g. these Stellarvue ones), assuming your main scope's rings have the necessary threaded mount holes on top.

Pardon if you already know much of this, but it's hard to tell. One really important thing to understand is that different kinds of targets respond best to different approaches. For example, a reflection nebula like the Pleiades or much of the Orion Nebula provide light that an unmodified DSLR will "see" just fine. Galaxies and clusters, same deal. A narrowband filter will, along with light pollution, knock out a lot of the broad-spectrum colored light from such targets (I HAVE NEVER PERSONALLY USED AN L-ENHANCE). If you want to image emission nebulae (North America, the red bits of Orion), you need something with good frequency response to the hydrogen-alpha transition, emitting at 420 nm. That's getting close enough to infrared that a consumer camera's IR cut filter will knock it down a lot. By no means to zero, however! I've done emission-nebula work with an unmodified, unfiltered DSLR. The ZWO lineup is indeed quite extensive, but it's manageable if you categorize. There are so-called "planetary" cameras, with small sensors, high frame rates, usually uncooled. There are cooled and uncooled deep-sky cameras, which are optimized for high quantum efficiency and low noise for dim signals. Except for folks with fairly specialized needs, IMO an uncooled DSO camera doesn't offer enough of an advantage over a DSLR to be worth the extra price. (THAT statement may well be contradicted by someone -- note "in my opinion" there!) Next up would be sensor size and pixel pitch. For a given optical train, the image circle often limits the useful size of the sensor. (For example, my SV70t-IS covers an APS-C sensor nicely, but vignettes badly on a full-frame one). So that might knock out some of the big-sensor cameras. Then you can look at the size of target that you want to image, and see how it would do on different sensors (I use Stellarium but there are oodles of tools for that). Finally you can look at imaging scale, which relates the optics and the size of the pixels on the sensor to determine the angular size (seconds of arc) of the smallest sky detail you can usefully image. There's a great discussion and a calculator at astronomy.tools, I linked the "CCD suitability" calculator but there are other useful ones there too. Of course, there's the distinction between "one-shot color" cameras and mono ones, as well. Mono cameras tend to be more expensive, not less, because of the smaller market for the sensors. If you're doing full-color imaging, mono cameras have some technical advantages, especially in more light-polluted environments, but are admittedly fiddlier since you have to expose and process the channels separately. Mono cameras do enable optimal narrowband imaging, which is crazy-resistant to light pollution. And if you don't mind starting with some black and white images, you can delay the expense of multiple filters and a filter wheel -- just buy a hydrogen-alpha filter and image emission nebulae, without a care in the world about moonlight or city lights. E.g. this, or this, or this, all shot in Bortle 8. Ish.

The devil is in the details with deep-sky. Guiding does help keep the image centered, but "centered" is a relative term. For your scope and camera, a pixel represents about 2 arcseconds. Let's be generous and say that anything under two pixels' error would be undetectable in the image. (Try processing an astro image with a blur filter set to two pixels -- I bet you can tell the difference pretty easily.) So that's four seconds of arc, one nine-hundredth of a degree. Which means that a displacement sufficient to move the end of your scope 0.005 millimeters -- five microns -- will be visible on the image. Guiding definitely helps with this. Unfortunately, guiding only corrects for errors, it doesn't prevent them. Since seeing -- the fluctuation of the image due to the atmosphere, about which we can basically do nothing -- happens at a fairly high frequency, and since stars aren't all that bright, usually the exposure time on a guide camera is 1 second or more. Doesn't take much slop or imperfection in the gears to produce a 5-micron swing over 1-3 seconds, and guiding can only sense and correct it after it occurs.

Thanks! I went back and completely redid the calibration and integration, and am now working from a much better baseline. One key was to do light-pollution correction in APP on the mono images, for some reason I thought it only operated on RGB ones. In fact now I have too much color, I have these weird pink highlights along the dust lanes. I mean, it's really pretty but not exactly scientifically accurate! As for Windows updates -- got you covered there mate, I work on a Mac 🙂

Right, there are certainly cameras for which darks just don't contribute much (and can make things worse). It should be really simple to test whether the problem is amp glow in the camera -- just cover it up to make absolutely sure no light is leaking in (doesn't need to be attached to the scope), and take an exposure as long as the ones you do for astro. Take that "black" frame and stretch the living heck out of it, however you prefer to run up your contrast (e.g. Curves or Levels in Photoshop). If the glow is there and in the same place as in your lights, then you'll need darks (or a new camera!). If not -- and it probably isn't, with a DSLR -- you can look elsewhere.j

Actually it's dark frames that should be used to calibrate out amp glow. I mean, I'm glad flats worked for you! But if this really was amp glow, then it should show up during an exposure of equivalent length with no light hitting the sensor, primarily from thermal effects. A flat should have much less glow, since they're almost always much shorter exposure time than your lights. If there's some light leak or other optical problem, then the flats might calibrate that out. Turning the camera on the scope should differentiate the issues. If you turn the camera 90 degrees and the glow shows up in the same part of the frame, it's in the camera and should come out with darks. If you turn the camera and the glow is in the same part of the sky, it's light pollution, or something bizarre going on with your OTA. Of course, if there was some systemic problem with your flat frames in the first place, rejiggering those would fix it.

And if we're being really picky, unless the filters are actually parfocal the vignetting will be slightly different between filters, since the focuser had to move. I just set up a standard sequence, one for narrowband and one for LRGB, and run it at the end of every session (well, every session when I'm not too blind from sleep deprivation to remember). Ten flats takes maybe a minute, most of which is download time. So, ten minutes to do 20 on all 4 channels -- probably less. Heck, if I pulled the scope out of the saddle and propped the LED panel up against the end instead of pointing at the zenith and laying the panel on top...by the time I had the mount torn down and put away the flats would be done.

That's a really nice tutorial! If you're using Photoshop you can also stack right in the program by using the "Maximum" function (one way is to put all the images into a Smart Object and set its stacking mode.

If you'd like a quantitative but VERY pragmatic discussion, video search for "Robin Glover exposure times and temperatures".

I have tried that previously with good results. Thanks! BTW my redo is cooking in Astro Pixel Processor as I type. I set local normalization correction degree and iterations very high and am reaping the result...5 hours on and the progress bar is at 10%...

Yeah, doing it on different nights really raised hob with the backgrounds. I don't think it was APP's stitching that was the problem so much as the data itself. I composited it with Astro Pixel Processor with local normalization set to 1st degree and 2 iterations, with multiblend-background set to 5% as I recall. The LNC adds a LOT of time to the processing so I didn't want to turn it up any higher than I had to. I experimented with almost firewalling both of those and while the gradients didn't go away, they receded far enough that the portion of interest is better salvageable, and probably APP's light-pollution removal tool will be able to deal with it. Oh well, what's another nine or ten hours of processing! Thanks for the help. I don't know why I'd forgotten about subtracting the starless from the original to leave a star layer, I used to do that. Age, I guess.

Are you at the limit of your focuser's travel? It's not uncommon for Newts built for visual use to be unable to achieve focus with a camera. Some folks have even resorted to cutting the tube so they can shorten it by a few centimeters!

Well, I'm not exactly crying in my beer over it, but the whole idea is to get better, right? Vlaiv, the weird gray stars are partly an artifact of how I blended the "stars" layer back in, via Screen blend mode in Photoshop and the layer opacity. I was going for a more subdued look so that the stars didn't distract so much from the galaxy, but absolutely gave up too soon! Having the stars gray makes them "pop" less, to be sure, but I agree that's not acceptable. I've got a new version coming that should be a lot better with that. As far as I can tell I don't actually have any good nebulosity in that awful black region, but maybe I'm wrong. I don't know why the bigger stars are so fuzzy -- I was reasonably strict in culling my subs. Maybe it's an artifact of the Screen mode again -- if I'm going to be culling the stars in front of the galaxy for artistic reasons anyway, perhaps I should just mask in the surrounding region unchanged instead of relying on a blend mode. I should probably figure out how to shrink the stars, a technique I've never mastered. If anyone would like to play with the integrated channel stacks, please feel free, they're available from my Google Drive. I'll warn you that there are some pretty awful gradients due to (a) spreading the imaging over nights with wildly varying Moon phases and (b) struggling with Ekos's mosaic scheduler so that some nights were entirely missing parts of the image! If anyone would find useful the details of how I set this up in Astro Pixel Processor, let me know. Thanks again for the help. I'm delighted to have informed criticism.

That's one of the reasons it's so appealing to me -- a target that literally anyone with a camera can get a picture of, but a near-infinite well of challenge. M42, another "beginner" target, is exactly the same way. Wax on, wax off.

Yes, the galaxy image is out of starnet++; I used Screen blending mode in Photoshop on the original image to put the stars back, then messed around till I liked the look. One thing Screen mode did was brighten the galaxy intolerably -- it's a technique that seems to work better on dimmer nebulae -- so I just masked it out. And probably went a little too crazy on the mask in doing so, leaving the objectionable black borders. The color...I dunno. If I apply much stretch at all to the original image the stars all wind up near 250 for red, green, and blue. I tried exporting the combined image from APP with no stretch, and starnet didn't have enough to work with. Tried again with just a teensy smidgen of stretch so that the nebulosity was just detectable, and by cranking the Saturation slider on the result in PS I was able to see color in stars, but it didn't look natural. And the core of the galaxy gained some color, but it was more yellow-rose than straw colored. That also induced some REALLY ugly gradients between the panels of the mosaic, since they were shot at different moon stages. APP's light-pollution/gradient reduction tool knocked back a lot of that, but by no means all. I'll see about posting some of the data. Thanks for the help folks!

Oh thank you, thank you, it does my flabby little black heart good to hear those lovely things. Of course like all of us I adored my image for maybe thirty whole seconds before its flaws started to pulsate and flash, but hey, I have stars that aren't little arrowheads in this one! This is my very first commissioned astrophotograph -- my wife wanted an Andromeda for the wall but knew how much I shuddered and twitched every time I looked at my old one. She agrees that it needs more margin, but is not as bothered as I am by the puzzling lack of color.

I suppose I'm not really a beginner anymore, since I've been whacking my head against this stuff for years now. Still, this is where I mostly hang out, so here you go. I've spent more like seven nights on this, but these two nights gave me the best data, it would appear. Just shy of five hours total of LRGB. Tech deets at astrobin.

Hey, it beats aviation, which was like standing in front of a giant noisy fan holding out hundred-dollar bills and releasing them to be confettied, one by one. BVVVT! BVVVT!

I join my welcome with the others'. Enjoy your plunge down the rabbit hole. In my experience, learning deep-sky astro is a bit different from doing it. That is to say, there are so many things to learn that it can be immensely frustrating, and it's good to minimize the challenges when you're starting so that you can get results pleasing enough to keep you going. The dimmer the object, the more exposure time it needs; the more exposure, the more chance that some tiny wobble in your mount will make the stars dance a merry little dance* across your sensor during it, and long focal lengths only magnify that. I'd counsel you to do two things: Start with something shorter and faster while you're building up your game, and take the time to read a well-rounded introduction to the art and science. It's not like terrestrial photography, and in fact many of the lessons and habits we learn for that are exactly backwards for deep-sky. There are some nice big targets which are also bright enough to not require eons of capture time, such as the Andromeda Galaxy, the Pleiades, and the Orion Molecular Cloud Complex. Those are amenable to lenses as short as 50mm for the latter. You can mount your Nikon with one of your lenses on your AZ-EQ6GT and have a solid base from which to learn polar alignment, pointing, focusing, and processing. You will need to be proficient at all those to run a 1200mm scope on deep sky. I am a huge fan of Charles Bracken's The Deep-Sky Imaging Primer, while this forum's perennial favorite seems to be Making Every Photon Count. You will get a lot of great advice here, but what these books will do for you is give you a good general grounding from which you can move forward with fewer "unknown unknowns". Of course, you could just bung your Nikon onto your 250 PDS and start shooting, too. Some folk learn better that way. *Lest you think I'm kidding, allow me to point out that I've been doing deep-sky for a good five or six years, everything's computer-driven, autoguiding, autofocus, yada yada yada, and I'm running a mere 362mm of focal length. This is from the night before last, with the Andromeda Galaxy and its pals doing the cha-cha. Or perhaps it's more of a samba.

Well, I know that I tried. Ekos and my CEM-25P mostly play nicely together, but every once in a while they have a fight about where the mount is really pointing. Parking frequently engenders an apparent attempt to do a slow roll in RA. I really do love Ekos, but it does tend to have settings kind of scattered about the place and you have to build up a mental model of what it thinks it's doing.

Or, you know, slews your scope into the tripod and spends a couple hours trying to hammer a bend into the leg with your thousands of dollars worth of imaging gear. That's fun too.

I know for sure that at least one night was terminated by clouds. Probably should have looked at the SNR more carefully but I was going exclusively by the APP "quality" score and eyeballing any frame that had a low score. Pretty sure the darks were spot on. I only use two gain settings these days, one offset, one temperature. So if I pick the right gain and exposure time -- and I did -- there's not a lot to go wrong there. The darks were created with same setup, 100 frames. I shot them and the bias (500 frames) in May of this year. The bad-pixel map is older, from last year, probably ought to replace that one. Oh, and I reprocessed the image last night. I still leaned a little heavy on the "drama" side but what can I say, I like bold images. I'll just attach the full-res image, pardon if the 9 MB file is annoying.