ollypenrice

Our cat urinated on my eyepieces - twice. Tele-Vue eyepieces. Twice. The cat is still alive. Miracle. lly

Does this field rotation matter if it is not visible within one sub? All it means, assuming you stack using an alignment which does 'right-left-up-down and rotation' is a slight loss of keepable real estate around the edges. What you gain is a bit of inherent dither. As for imaging 'under the pole,' we are doing this whenever we begin imaging a target which is rising in the east and lying at an altitude below that of our latitude. I'm at a lower latitude than you yet I do this all the time. Since I have dark horizons I certainly don't have to wait for my targets to rise above an altitude of 44.19 to get started. To be honest, I've never given this a thought. Olly

I'm not yet persuaded by this 'half of the sky' theory. The sky is some distance away and yet we're asked to believe that moving the telescope what, 60cm to the right, makes a radical difference to the LP present in the subs. Really? I'm using LP, here, to mean skyglow and I accept that there can also sources of LP very close to the telescope. However, if they are sufficiently close to the telescope for a 60cm relocation to the right to make a difference they must, surely, be close enough to be excludable by some kind of small light sheild? It seems to me that any LP changed by so small a movement in the OTA (which, after all, remains perfectly parallel with its former position after the flip) must be very close and possibly within the observing site itself. I suppose there are other possibilities such as one source of LP being switched off at a time very similar to the time of your flips, but some coincidence is needed for that to be the case. Or perhaps you unwittingly begin your runs before astronomical darkness? You probably don't and this would produce a gradual rather than a sudden improvement, though it would appear as a sudden one if comparing a 'first half of the night' stack with a 'second half of the night' stack. Search for that light source and see if you can block it. Olly

I don't follow this, Wim. The opacity in Ps Layers just decides on the relative weighting of the two stacks. If one stack is better than the other, this will show in real time as you move the slider. Sometimes one stack can be almost useless, in which case the best result will have that image's opacity set close to zero. Note: both stacks have been stretched to the same background value. You can't do this with linear stacks. Olly

If you're going to throw it, tell me your Lat and Long and I'll give you a precise compass bearing on which to whizz it. I'll be ready with a net! This isn't a mono-colour issue, it's an amp glow issue. The amp glow condemns you to using darks and, as I said, I have found darks to be problematic on occasion. (The theory of darks is predicated on the idea that what you actually get in your darks, and what the software actually does with them, are what the theory assumes. Can this be assumed? And, if it can, why do I get better results without them?) Olly

To my eye that looks electronic in origin rather than optical, so I'd put dew at the bottom of the list of suspects. Things to look into would include, 1) Do you see any trace of this type of noise in single subs? 2) Try stacking without darks, then without darks or flats, to see if they are introducing the noise. They may be. 3) Check your stacking parameters. Try different ones, ie Sigma, Median and Average. 4) Try a different stacking program. (You can download AstroArt for trial. It won't save but it's a great stacking-calibrating program. There doesn't seem to be any very obvious object in the image. The stars are tight but what were you imaging? (My thought is that the image shown might have been stretched beyond its limit in search of an object which isn't there.) Olly

My net connection is currently dire so I can't see many of the posted images. However, we have been doing some testing of darks with an ASI 2600MC. They add noise. Theory can say what it likes, but they add noise - and a lot. Without darks the background sky is extraordinarily clean, so much so that I do not use noise reduction yet fear that someone looking at the image will say I've over-applied it. We took the darks carefully, with the metal cover over the chip window. (I discovered in careful tests years ago that, even on a refractor, you cannot reliably exclude light so I always do darks off-telescope.) In fact I don't think much of darks even with a high-noise CCD camera like my Atik 11000. I found that, most of the time, subtracting a master bias and running the hot pixel filter gave the same result as using a matched dark. On rare occasions there was a difference - in favour of using the bias and never in favour of using the dark. When I initially dumped darks I did apply a bad pixel map but stopped bothering when I found that the stacking program's hot pixel filter was as effective or better. (I use AstroArt for stacking and calibration.) Call this a minority report, if you like, but I've come across other imagers who've come to the same conclusion. Images done this way can be found here: https://www.astrobin.com/users/ollypenrice/ Have you tried simply not using darks? I don't know if yours is an 'amp glow' camera but, if it isn't, I'd just try it. It's unlikely to affect a Newt or refractor but the ASI 2600 has a light on the back which played havoc with our flats on the RASA 8. It's now taped over. Olly

I have a file named NGC2403 on my hard drive so I think we must have planned to do it in the TECs but never got round to it. Nice galaxy but, as you say, it could do with more than 3.5"PP to do it justice. Olly

That's a very attractive rendition, subtly processed. Good point about the star forming regions on the companion side. I really think the MN190 should receive more attention than it does. Olly

Talking only about CCD, here I've been told this dozens of times. My experience, however, does not support it. The very long sub exposures, I have found, do go deeper than the shorter ones with the same total exposure. Other experienced imagers I've talked to have found the same - notably Tom O'Donoghue and Sara Wager. The most conclusive experiment concerned M31. I was going after the extended outer parts shown on star charts but not usually seen in images. A deep set of 15 minute subs didn't nail it. When I switched to 30 minutes, there it was. I've read all the arguments against this but... I'm a pragmatist. Olly

No, you can disagree about homphones on dry land, well away from boats... lly

Been there, done that! lly

Excellent! However, might there be a wisp of doubt concerning roes and rows (or rose) as homophones? Rose and rows, yes, but does the 'o' in roes not hover for a moment, making it a slightly longer vowel? Perhaps not, because our reading ear is not quite the same as our aural one. My father, a Yorkshireman, could never consider weight and wait as homophones. The former was pronounced way-eet while the long a sound in wait was pronounced as in the word air. Olly

My first thought was, 'The sun rose across the lake,' but I wanted to find something more quirky. Funnily enough, 'rows' across the lake didn't come to me quickly despite the fact that it is perfectly normal to row on a lake. Olly

When combining data from entirely different runs, that's what I do. I stretch both stacks separately and to the same level, getting both backgrounds to the same value. I than co-register them and put one on top of the other in Photoshop layers. By moving the opacity slider and looking at the noise level in the blend I can weight it to give the cleanest result. With the opacity at whatever it is at the point of least noise, I just flatten and save. Olly

Perhaps English is good for wordplay because of its enormous vocabulary, estimated at being three to six times larger than French, for instance. (It depends rather on what is to be counted as a word, English lexicographers being more lax than French ones. That's to say that they are... ahem... laxicographers...) Olly

As topless Cleopatra's oarsmen rowed, I took my repose across the lake. (Can I say that on here? If not, she can be wearing a Barbour jacket if the mods insist...) Olly

Ah, singular collective nouns! Please distinguish between... 1) A lot of binoculars is to go under the hammer at Sothebys tomorrow. 2) A lot of pairs of binoculars is to go under the hammer at Sothebys tomorrow. 3) I've had more than enough of this and will take a hammer to the next binocular, pair of binoculars, quartet of binoculars, quartocular, sextupocular or any other glass object, be it at Sothebys or any other place within reach of my hammer... Olly

It does take time and I think CCD cameras benefit far more from long subs than do CMOS. Olly

So he was giving it like speak the same way they do but I was giving it like whom and no split infinitives, right? 😁lly

It seems to me that you should make a stack of the longer subs, then a stack of the shorter ones, then combine the stacks with a weighting equivalent to their total exposure time. I'm happy corrected on this. Where's Vlaiv? 😁 Olly

If you want to chase the faint stuff you will need to do so, I think. The Hamburger Galaxy in the Triplet, for instance, has a long but very faint tidal tail. https://www.astrobin.com/335042/ The way to get this is to use longish subs and have plenty of them. If this data saturates the stellar cores, why worry? In a Layers-based program you just make an RGB-only version with a gentle stretch for the stars. Ignore the galaxies. Get the background up to the level and colour balance of the over-exposed LRGB image, paste the LRGB over the RGB (which has an identical background) and erase the LRGB stars. (You can do it the other way round if you like, putting the LRGB underneath and erasing the top layer RGB wherever there's anything faint and fuzzy and interesting in the LRGB layer beneath.) There are lots of images in which one stretch doesn't do all that we want to do. Some programs use masks to make selective modifications. I think using layers and the eraser is much easier, which is why I work mostly in Photoshop. Olly Edit; BTW, I think the core and spiral details in your M66 are particularly impressive.

There is no single aspect of astrophotography which is inherently difficult. (It isn't like learning to play a musical instrument, in which almost every aspect of the process is difficult!) The only issue is that a considerable number of fairly straightforward procedures have to be in place at the same time. If you're feeling overwhelmed, start with those procedures which matter most and leave the less critical ones till later. You need - to track the sky, (so driven and preferably equatorial mount, polar aligned.) - to capture focused images with the object on them and to file these in a place where you can find them. (Focus, use Bahtinov mask.) - to have and understand a software program for stacking. - to have and understand a post-processing software package. It used to be Photoshop, mainly, but now there are dozens. If you don't like being overwhelmed, stay away from Pixinsight. You can begin without - Guiding, but make it a priority ASAP. - Dithering if using an uncooled camera. With a cooled one it is no big deal despite all the hype it gets. With a DSLR it should be part of your guiding routine. You do not need - to throw your brain in the dustbin and insist on controlling everything via a PC. Plate solving, robotic focus etc., are luxuries at best and when they work. At worst, when they don't, they are a total waste of time. Use your own eyes, your own fingers and your own brain. Olly

I think Vlaiv's point is that it isn't focal length which determines tracking tolerance. Rather, it's resolution and this is made up from the combination of focal length and pixel size. You can have low resolution from a long focal length and large pixels and you can have high resolution from a short focal length and small pixels. In the past pixels tended to be on the large side and of a fairly consistent size (around 6 to 9 microns). So in those days focal length tended to be the dominant variable. Now pixels come in a vast range of sizes from large to tiny, so focal length is not as reliable a shorthand for resolution as it used to be. Olly

That looks like a promising match. Olly