rotatux

Nice one for a first. With your barlow it somewhat looks like my first shots with a Mak127, your progress will be fast and easy once you start using longer stacks. 10-15s per sub seems fine given your setup, though you will want longer ones at some point About your 130 not focusing enough out of the tube... maybe try a lower power barlow such as 1.25X; I don't think ocular projection would gain you anything as with a DSLR the projection distance would be too long and induce a focal multiplier bigger than your current barlow. Or try one of the primary-mirror-raising mods :-P

Makes sense, but if I had to design such a filter as an engineer I would make it such that it only lowers undesirable signals to unnoticeable levels at the sensor's output. Which makes me think the sensor cells are already not very sensitive to IR (and H-alpha), when I see the slope of all the curves on the IR side. For ref, my filter should be at about 35% at 656 which is nearly comparable to yours. All things being relative, as he compares to "full frame" sensors. One should actually look at pixel size rather than whole-sensor size, to get a hint of capture range. Of course FF are ahead of the race, but latest APS DSLR models with 24mpx and more have equal or smaller pixels than the E-M5. My darks were fine, so the problem didn't come from them. But I think I've pinpointed the "problem". I did a few camera flats, with just the camera, T2 and M48 adapter, and various or no filters, against daylight (or rather, what clouds transmit :-/). Then did stats on the resulting images, all taken at same exposure (ISO and duration). As Ken hinted, the filters eat on red and green, and let a growing part of blue pass (as show by the relative "B-V" shift). One could object this is on visible light, not H-alpha or O3, but it probably just has the same effect on them, as these are supposed to be wide-band filters. IMO this explains the severe blueish shift I have with the UHC, not only there is much less total light -- though I wouldn't bet on bad transmission as those stats are with indirect sunlight so much of the spectrum is impacted, not just star/nebula light -- but there is 3x less red transmitted. So my 20s of Rosette, given it essentially comes in the reds, are equivalent to 6.7s without filter or 10s with Didymium. I would bet that's why I don't get enough absolute signal. I have yet to find a combination of exposure that gets more signal, but that UHC filter may not be part of the solution, as strangely as it sounds.

With all that new gear for many people, no wonder why it's been rainy or cloudy for several weeks ;-) Congratulations both, all nice buys.

Yes your setup with APS-size sensor and no flattener/corrector has nearly exactly the same FoV as me with M4/3 and ComaCorr. The Rosette framing of both images indeed look the same. Nice picture Ian BTW ;-) Looked at kolarivision site. I found a cam with same sensor as me, the E-P1. However it's only the characteristic of the filter, not the sensor. I know from sensorgen.info that my sensor has 38% quantum efficiency (though I don't know how to mesaure and check it myself), but I suspect that value also varies with wavelength, so I can't get any conclusion (the net light sensitivity should account for both the filter and the sensor). About noise: The first generation sensor (12.3mpx) is from Olympus and indeed quite noisy, though manageable. But the 16mpx sensor found in E-PM2/PL5/6/7, E-M5/10 is from Sony and much better (3x less noisy, and 1.6x more sensitive at 60% QE) -- that's what I'm willing to get, maybe second hand. Your 2nd link conforts me in this opinion. Interesting... makes sense given my technique, I need to double check my "NG darks": as the target was rapidly climbing in the sky, I may have captured the wrong gradient and mismatched a bit with the lights background. Oh damn, I have yet to write a blog entry on that "NG dark" technique of mine :-P Thank you Nige. I don't think separate bias would help as I already use traditionnal (not scaled) darks dedicated to each session or even subject. I hope it's LP and it can be sorted out, the filter was here to help. I would like lower ISO if I could achieve longer subs, but the mount is so erratic I'm afraid I wouldn't capture any signal as lower ISO + 20s. Will retry on occasion though.

I found he brilliantly explains how stacking can get you additional bit depth (though there seems to be a off-by-one error in bit gains for each of his stacks). (reading links from your last post, will follow up later ;-) )

Two new pics finished (actually one has to stop somewhere), one half-success and another failed. First is my #2 try at nebulas around Alnitak. Much better than my try #1, still progress to be done. I guess I can't catch more unless going to a darker site (was taken from Paris suburbs). Capture: 101 good of 123 lights x 25s x 2500iso, 30 NG darks, Olympus E-PM1 with Skywatcher 130PDS on Celestron Nexstar SLT, Skywatcher ComaCorr and TS-UHC filter. Processing: Regim, Fotoxx. Second is Rosette on which I failed to capture enough SNR (sorry for your eyes ;-)). I had to stretch so much I had to process the noise, even then the result isn't satisfying. Surely I was too ambitious when reducing the sub length to 20s, I suspect the UHC filter dims the overall image and requires at least 25-30s; Stars were burnt in my previous 30s try, so I decided to reduce it, but I might need HDR on it (or stop trying to image from Paris suburbs from which I can only barely see Orion's belt). Capture: 50 good + 42 average lights (of 122) x 20s x 2500iso, 56 NG darks, Olympus E-PM1 with Skywatcher 130PDS on Celestron SLT mount, Skywatcher ComaCorr and TS UHC filter. Processing: Regim, Fotoxx. A few days ago Ken gave me the idea to try to calibrate the color levels coming on the sensor through my filters, so I'm in the process of making some kind of flat in different filter configurations. Hoping to understand my filters characteristics and those bad star colors.

Maybe you mean that one ?

Nice... I don't know that (Android) app, using Stellarium to plan my framing, what is it ?

As Ken reminded us, the longer focal length requires more precision in tracking. I may just as well retry my MAK on small objects, as mine is somewhat shorter than Ken's. Yes aperture and/or focal ratio (also based on aperture). 80/400 is f/5 so you would have to close your 135 past 5 (e.g. f/5.6 and smaller) for your scope to bring you more photons on the sensor than the lens. Which you probably won't do, as most 135 are good enough from f/3.5 or f/4 (I dream I could find a f/2 one :-P).

BTW, is there such a thing as Alt-Az guiding ? Field rotation put apart, it could be a way to achieve longer subs. Or said otherwise, achieve subs as long as field rotation allows, without being annoyed with balancing or tracking errors. Thinking about it, that would severely impact transportability and ease of setup though (add laptop, guide scope+cam, cables, etc), but I wonder whether anyone has done it.

Pretty obvious indeed. IMO this helps to bring an answer to a long asked question about the equivalence between various apertures. In all cases it's agreed that the bigger the (true) aperture, the more photons you get and the less exposure time you need. And the longer the focal length, the more magnifying and less photons (*) you get and the more exposure time you need. True aperture is focal / focal ratio, so 150mm for your scope and 48mm (2.8) or 39mm (3.5) for your lens. (*) actually it's less photons per pixel, since the same quantity of photons flowing through the same aperture are just spread over a wider surface. Question is by how much those two factors combine and compensate each other, and which one wins over the other. I've read many articles about it. Some say / demonstrate the captured light level per pixel is proportional to D²/F (or F/R², R being the focal ratio F/D), others say / demonstrate it's proportional to only D/F (= 1/R). If the former was right, your scope (150²/750 = 30) should capture more light at the same sub length than your lens (135/2.8² = 17.2). Your shots seem to prove the latter is right, i.e. 1/5 < 1/3.5 or 1/2.8. Of course post-processing vary, you should really compare out-of-sensors RAWs, or JPEGs processed strictly equally. But it also correlates with my own experience as a wide-field imager.

Good luck. A fixed observatory certainly alleviates the burden of EQ setup, when it's mostly done once. Hoping your observatory will get enough clear skies to enjoy.

So yours is fully conformant. With the SWCC mine gives 565 or 590 depending on the spacing (shortest or longest, about 18mm between the two). Pretty cool actually, as it gives more FoV on demand to my smaller sensor

Maybe it's actually loading the JPEG "thumbnail" embedded in the RAW, rather than the RAW itself ? It's a feature I have in my "ORF" RAW format, so other RAW formats may have it too.

I'm afraid you must not have measured the right stars :-) In my Stellarium Propus and Tejat are 1.865° apart, so the approximate corresponding sensor distance should be on the order of tan(1.865°)x650mm = 21.17mm. That's way off your measure, so either you missed the right stars in your image or you are measuring a resized image (maybe onscreen display rather than true-image pixels ?). Apart from that I agree on the formula. The /2 division on both sides isn't really necessary because of small angles.

Hi all, my first post in this long-time read thread. 1 year ago it finished convincing me and I bought a 130PDS, and don't regret it. These days I essentially use it for Alt-Az imaging though I also have an Eq (but don't use it often). Incidentally I used some of my past months images to check and calibrate the pixel resolution with different combinations of filters, CC position, oculars (I also do projection imaging sometimes). It's pretty easy: knowing the sensors dimensions and number of pixels, and reference angular distances between stars (thanks to measure tool of Stellarium and catalogs), and just apply some basic trigonometry. For reference I also calculated the resolution with no additional optics (the cam barely adapted to the OTA), and I found 632.5mm ± 1.3mm. This is also confirmed indirectly by other optical combinations. I know for normal that actual focal varies a bit from theorical / sold specification, mine is away by ~2.7%. Is it a normal value and what's your own values ?

You are on a good way, maybe not aggressive enough in your stretching. Look at what I get with only an 8-bit stretch of your JPEG, this was with only levels black point and mid point. So you must be able to get far more from your 16-bit image. The main idea is 1/ strech with levels (gamma / mid point), or brightness/contrast, or curves, then 2/ trim the backgroung with levels black point. Ok, now unless I'm wrong you will see you some apparent (strong?) vignetting, so you will need flats. But keep that for after you are comfortable with processing

Exactly, that was the "saturation-limit" part of my reasonning. Though a physicist will argue the sensor's pixels keep the same "full-well" capacity whatever the ISO, only the maximum representable electronic level drops when ISO raises. I've read many similar, but maybe not this one. I learned many of those articles have to be taken with care as there are often gotchas in the reasonning, sometimes small sometimes big, so you need to keep awake. Always interesting, gonna read it, thanks Ian.

My go too Not as stretched as others and I tried to calibrate the color and get back some core (failed at the latter). I saw core is not totally saturated but didn't find the right tool to get it back. Most stars are saturated too so had to resort to background stars for B-V calibration. Was funny, thanks again for letting us play And Nige... WOW you got the core back and nice nebula color changes.

Not so poor ! That RGB did much good to your image, with nice star colors (even if it lacks a bit of blue) and delightful and delicate color change between red, pink and orange in the nebula. Eventually it's not all red ;-) Thanks for that superb image. I knew all this region is dust and feature rich, but seing it all revealed like this blows me off. Don't mind blurring too much, some images (including this one) are better appreciated as a whole rather going into the details.

Thanks for pointing that Ian. It was already discussed in the thread (which I have read once entirely) so I was aware of it, and is actually one of the triggers which made me come back at Alt-az imaging But it's also a matter of precision: if you get one bit (literally) of signal out of each sub, it would take 256 subs to gain 8 bits of precision (not taking noise into account) and have something to stretch -- maybe less bits would do, I take 8 as a known reference. Also, depending on sub exposure, you could also get less than a full bit of signal (lol! I like quantum physics ), needing even more subs. There's also the matter of image depth through imaging software chain: if I get subs with 1-bit of signal each it means they are at position 12, so to get 8 bits of precision requires software to process at least up to 20 bits. That's where I may be hitting Regim's 16-bit depth limit (of images). On M42 all was fine but on Rosette and Horsehead I feel I'm at the stretching limit. With the number of subs I take, the remaining noise should be near level 0 or 1. I stop stretching steps when noise becomes significant, but at the same time I understand there's no more signal to come because I've used all the bits within available depth. Feels like a dead end -- though I'm in the process of trying to "pre-scale" the images, which *will* saturate more stars but I hope allow stacking to expose more bits of signal.

You must be right, though I could not find the specific response curve for my filter. When I look through it during day, I see all seems red and blue, something like a mix between 2 eyes of a red-blue 3D glasses. At first I thought it would remove essentially yellow and a bit of red, but it's narrower actually and removes green as well (confirmed by looking at trees folliage and grass, which turn brown or dark red). So Regim's calibration must be trying to compensate for the lack of green in stars, and the image ends with too much green. Sounds correct. Now, what's the best and more practical way to get back a correct green channel ? If I play with balance to raise green level, I may end with the same green cast as auto-calibration since there's not enough green from the start (but will try). Capture green from another session, like you do with L+R+G+B(+Ha) ? Apart from the inconveniences of multi-session and number of subs, I would have to do it with a narrow green filter to be efficient -- if it exists and that would somewhere negate the use of a UHC filter. Or synthetize a fictive green channel from the two others ? Would be wrong as the red+green/blue ratio is precisely what changes from star and star and enters the definition of the B-V index. Back to home to think and try :-P

Thanks. I hoped some shots I've seen had real colors rather than mapped, but maybe not after all. As Ken reminds us, the Rosette is very bright in red. Just wonder then why the sensor captured 2 different colors. That's a matter of camera. Actually I've tested 800, 1000, 1250, and 1600. With only 12-bits depth and 30% quantum efficiency (taken from sensorgen), most if not all deep sky (not clusters) subjects' data just happens to be below the minimum first value above zero with short exposures. Said otherwise, lower ISOs just don't give me enough data to stretch within my exposure limits. So I need to amplify more than all of you 14-bits sensors owners, and 2500 is my camera's maximum of analog amplification (3200 and above is digital). Would be different if my mount accepted to do 40-50s subs Good news is my read noise is about the same at every iso, so I prefer to always use the same ISO level and vary the length and number of subs. Bad news is my noise is quite high so I need about 80-100 subs to tame it. BTW don't be fooled by the DR such as indicated at sensorgen.info, it's for a single shot: If you consider stacking removes the noise, you get full DR for any analog-scaled ISO from a sensor-only point of view; What matters then is to avoid saturation of your subject wanted parts (you may saturate some subs for HDR to get faint parts), and hence match the subject brightness range to your camera by adapting the exposure. It's good if you have the choice to adapt the exposure, to lower ISO in addition to varying sub length, I don't think I have that choice.

Just to not let unfollowed my horrible Alnitak shot proviously shown, this is my 2nd try at Alnitak and its nebula friends. Much better I hope, though still not on the par. Capture: 101 good of 123 lights x 25s x 2500iso, 30 NG darks, Olympus E-PM1 with Skywatcher 130PDS on Celestron Nexstar SLT, TS-UHC filter. Thanks to much more subs, there's much more detail. However I had difficulty with color calibration: I had to revert to the semi-manual version, the full automatic gave green stars rather than blue. Again I suspect the UHC filter to be the cause, just as if it was ripping out some information essential to the calibration But I've no idea what yet. On my calibrated displays this results in white/grey rather than blue stars, including Alnitak itself (and some nebula too), but I'm happy I eventually managed to avoid getting green stars. Gives me a headache: On one hand the filter would allow me up to at least 30s subs (if not limited by mount accuracy) and catch many targets, while without it I am limited to 8-10s subs by light pollution and unable to catch Rosette or Horse Head. On the other hand the filter seriously reduces Regim's ability to auto-calibrate colors. And I love good (correctly) colored stars...