rickwayne

Yeah, I was really struggling for a response to this and eventually I had to drown my Inner Nerd and just make my response "he's right". 🙂 I think where I was having trouble was conflating some similar issues. In case anyone else has the same confusion, here's me: "It's analogous to blowing out highlights, but in the other direction, right? Once you get to zero, you're at the floor, you can't discriminate values below zero. So you're losing differentiation among the darkest tones." Which is true...for a single exposure. So, other things being equal, six photons are six photons, whether captured in one exposure or 200. As vlaiv is careful to caveat, not all other things are, in fact, equal. But once you get past read noise, they're equal enough.

It's really amazing, isn't it? I still haven't gotten over the thrill of seeing something appear on the screen.

The 1/3 rule is a guideline, nothing more. If your exposure is too short, the electrons generated by the dimmest parts of your target are too few, and it's hard for stacking to discriminate their values. If your exposure is too long, your sensor's "well" will be full of electrons and so the signal for that part of the image will be undifferentiated full-scale values, regardless of how many subs you take and stack. Between those extremes, there's a lot of wiggle room! And it's not always right to begin with. For many targets, the peak of the histogram corresponds to the background skyglow -- the most numerous pixels are those representing "black" sky (which isn't really). But if nebulosity fills most of your frame, it's a different story. It's a really complex topic, but to criminally oversimplify, for a sensor with an infinite capacity to record and discriminate photons, a single long exposure will have a better signal/noise than a bunch of stacked ones adding up to the same length. However, longer exposures are more prone to satellite trails, aircraft flying exactly between you and your target, coyotes bumping the tripod, clouds, cable snags, cars illuminating you with headlights, wind gusts, periodic errors due to mechanical imperfections, fireflies alighting on the dew shield...and simply filling the well of the sensor. (If you class me as a coyote, I've had every one of those problems.) So practical matters force us to shorten our exposures and do lots of them. I like Robin Glover's explanation of these issues. Deep Sky Astrophotography with CMOS Cameras

If you're not running off mains power, another counterweight suggestion is simply batteries. A dedicated dovetail for your camera+lens is a great way to start -- that way it's easy to swap out when you want to do visual, and as you point out it's easier to balance. You also get the benefit of your mount's electronics for finding targets. If you edge deeper into the deep-sky waters, you can get a USB-to-serial adapter and control the mount with a computer, possibly the camera too (depending on what DSLR you have). That opens up a whole new range of possibilities, such as using plate solving to radically simplify pointing at targets too dim to make out visually.

You can use the two-eye technique to ensure that Polaris is more or less within the field of view of the scope: Start well back from the eyepiece, with both eyes open, looking at Polaris. Move so that the polar scope occludes one eye, but (if possible) keep Polaris in view with the other. Get as close to the eyepiece as you can and still see Polaris with the other eye. If you find yourself looking straight up the scope, it should be within the FOV and it's a focus problem. If the scope is at an angle to your line of sight, you need to adjust where it's pointing.

That's a beautiful M31. Did you do localized contrast enhancement to bring out the dust lanes, or is this just overall stretching?

Yep, I think more integration time will significantly improve this, enabling more stretching. Wish my first Horse had been this good! Where is the histogram peak on your subs? You can do a lot with total integration time, but you have to have enough photons in the first place on each frame. If it's crammed to the left, you may wish to see how much you can extend your exposure time and still maintain a good "keeper" rate when it comes time to stack.

Files for astrophotography will always be bigger than the JPEGs you see most of the time. JPEG is a very clever compression format but it throws away some of the data necessary for getting the absolute most out of astro images.

Not to mention Pentax, recent models of which have a built-in "star tracker" feature called AstroTracer that can obviate a tracking mount for shorter focal lengths (around 200mm).

I'll try to throttle the "Oh it's nothing really", but it really WAS supposed to just be a test of my mount's repairs. Pleased to report that between the repairs and running PHD2's Guiding Assistant, I was guiding down around 1-1.2" RMS in RA and 0.8" in DEC, which sadly is WAY better than I was getting before. Hope I can get better still. But since I had to point it at something, I pointed it at the Bubble and the M52 cluster. This is maybe a 50% crop of the FOV from the Stellarvue at 368mm and the 183; plate solving was cranky on the second night when I was collecting OIII data so there was only maybe 60-70% overlap. Full deets on Astrobin if you like. Focusing still a challenge through the NB filters, the Bahtinov image is more like a line of dots than a spike and of course having to wait 5-10 seconds between images makes it harder to run the feedback loop between my brain and the focuser. Also, I think there were high thin clouds degrading it somewhat. I ran the OIII at 270 gain and not quite double the exposure time (Ha was at unity gain of 110), but there still seemed to be very little there, there. I've seen bicolor Bubbles with a lot more color so maybe I just need to expose even more, and just remove all the stars from that channel. (Hmm. Unsuccessful throttling, there.)

VERY nice. Great detail in the disk, while not blowing out the core (at least not anything but the very center, if that).

I agree -- while people do manage to do deep-sky and planets with one scope, they are much more advanced imagers than I, with bigger budgets. Planets are tiny, so you need a lot of focal length, and greater aperture helps resolve finer detail. But they're bright, sunlit objects, so exposure times can be minimal. And successful planetary imaging leans heavily on software that processes hundreds of frames, picking out the transient sharp bits of each to assemble a high-quality image. Deep-sky objects can be tiny too, of course, but many of them are HUGE compared to planets. But they're very very dim, so imaging them involves sensitive cameras, low noise, and absurdly long exposure time compared to planets. That's why everybody is banging on so about mounts. For a good "starter" equatorial mount with goto, you're already spending around $1000. HOWEVER. You can get started for less. A LOT less. Since computers let us stack multiple short exposures into a picture equivalent to one long one, you don't have to have something that enables two-hour exposures. A minute or two suffices, and you can get there several ways: Use a camera tracker. I'm more familiar with iOptron products so the SkyTracker and SkyGuider are what I think of, but there are others. You can go even cheaper with "barn door" trackers such as the Nyx mount. Use shorter focal lengths. At 500mm, tiny angular deflections are pixel-sized and visible. Plenty of excellent targets are out there for shorter lengths, however (North America nebula, Milky Way...). You can get old "prime" telephotos for just about every DSLR for a song on the used market, if you hunt. Accept some image flaws. No, really. Astrophotographers tend to be really detail-oriented pixel-peepers, but normal people won't notice or care about tiny tracking imperfections. They'll look at the glorious nebula on your wall and say, in that wonderful slightly-confused tone of voice, "...you took that. Yourself." The book recommendation is excellent. Understanding what's going on is a healthy immunization against wasting money on the wrong gear. I also really like Bracken's The Deep Sky Imaging Primer. Example: This image has all manner of flaws, so (3) applies. Boy, does it ever! It was taken with a DSLR, a $125 500mm mirror lens, and a $300 SkyTracker. But I tell ya, it wows the normies.

Worse? Man, that is ART!! No, seriously, I know that's not at all what you wanted but it's a super-cool effect! Oh jeez, now I'm hijacking this to one of those threads that are all "You think that's a screwup? Pah! THIS is a screwup!", aren't I? So be it.

Oh come now, what bride or groom would prefer a nice wedding to a nice mount? ;-} Actually, I know what mine would have said: "That's a really beautiful piece of equipment, sweetheart. Well done you! It will look lovely standing up at the altar with you." In any case, congratulations, whether it's your own wedding you're paying for or an offspring's. I am all about this marriage thing. If you find a way to upload your original M31 data someplace (Dropbox e.g.), if I have a few minutes I'll have a go at them with Astro Pixel Processor. Finally, don't let Invisible Polaris stop you. Pretty much all the EQ mounts you'd be looking at for astrophotography can use their sky model to iterate back and forth between other stars, allowing you to adjust the ALT and AZ until the polar axis is dialed in. Then you can do a drift check (a couple minutes, tops) to ensure that it's spot-on. More trouble than squinting through a polar scope, but eminently doable. One of the small advantages to living in light pollution is that your sub-exposures have to be shorter in the first place, so one needn't obsess over polar alignment.

Since optical quality really isn't an issue -- at all -- a dirt-cheap reticle eyepiece like the SVBONY is well worth the whacking great $35 for a multi-star alignment. That also allows you to do a quick DEC drift check if you're so inclined (and don't want to do the camera-enabled version).

I did not spend a lot of time on it, but I did apply an arcsinh300 and an arcsinh10 stretch to your single sub and I gotta admit...I couldn't get it going on either. Nor could I get much when I wonked on it manually. Strange. I still maintain that it ought to work... 🙂

Aww, thanks! I am a total affirmation hog, so that is richly appreciated.

Thirty seconds might be a little shy on sub-exposure time, but over an hour of total integration time should be sufficient to bring out a lot more detail, even in fairly light-polluted skies. Do you happen to recall what the histogram looked like for any of your sub-exposures, or can you re-summon it? If the big peak from the sky background (gazillions of pixels that represent "black" -- ha ha! -- sky) is something like 1/3 of the way along from the left edge, you should be able to dredge more out of your data. I get that you can't go much longer on sub time with an alt-az mount -- you're just edging into eggy-star territory, there -- but you may be able to crank up the data you've already got. If you're using Photoshop, you can give Mark Shelley's arcsinh curves a whirl, they do a nice initial "stretch" for you that should be in the ballpark. This image is not a brag, there are so many things "off" with it that I almost hesitate to post a link. (Stars that are little arrowheads! Blue where blue Should Not Be! Etc., etc.) Nevertheless, it's got a lot of detail in the galaxy. The total integration time was less than yours, 19 x 129 seconds at ISO 1600, if memory serves, and I shot it with a stupid-slow mirror lens. Skies were maybe Bortle 5 (dark yellow on a light-pollution map). M31 with Tamron 500mm f/8 lens, 19 x 129s

Sequator is commonly recommended for this. Since I do other work involving masking, I have a copy of Topaz ReMask, which is AMAZEBALLS at letting you select the boundary between the sky and foreground, even with trees, brush, what have you. Super-useful tool. I would have gone bug-nuts insane trying to mask all these little leaves and branches without it: Example 1, Example 2 Absent ReMask,I think the Lonely Speck guy has a useful video tutorial on the process. (Yes, I screwed up on the horizon in Example 1. But look at those trees!)

Both companions, dust lanes, yep you're getting there. Concur about the focus. What sort of mount were you using? Stars are a bit egg-shaped, axis about 20° from vertical in this image. Which, now that I check, is about aligned with the RA axis, I think. Heartily recommend a Bahtinov mask for focusing. MUCH more precise than doing it by eye for most folk.

Nice work! Your artistic choice to really saturate the reds works quite well here IMO.

OK, so that gradient from Ha to OIII wasn't just me -- whew! I have gotten some fair-decent Ha on this target but my OIII was way underexposed (autoguiding inop) and I thought the overall balance shift from the "bottom" to the "top" of the trunk was some kind of artifact. Pretty wild colors here -- cool palette choice. Agree that the softness is likely due to focus. Suppose you could also do star removal to process them independently to get the diameters down, but I kinda like the look.

"The dog ate my astrophotography session." THAT'S new. [Sorry, man.] I don't recheck focus nearly often enough myself, but even if you have it nailed and don't touch the equipment, temperature change can still affect it.

What s/w tools are you using? Makes a big difference.

My SWAG (Scientific Wild-A** Guess) is that focusing won't. Vignetting is unlikely to significantly change with tiny focus moves. Changes in dust on the sensor will affect the result both ways (new specks since flats were taken, old specks that got knocked off since). Dust on the glass will be diffracted into invisibility anyway. You can definitely reuse the bias frames. In fact you should only have to redo those periodically, no need to run them every time.