The Admiral

Yes, and not being able to work out why I couldn't see any stars on the camera's live-view LCD screen, until the penny dropped, after much fretting, that the shutter speed was set to 1/30th second! Ian

Kind of you to say so, but if they are I'm sure it's down the gear rather than me! I'm lucky to have an APO, and that helps, and I use a focal reducer/flattener. Even so, spacing needs to be 'tuned' and although I've done the best I can with that, I think I have a bit of focuser droop, or else the 'scope's optics are a bit skew-whiff, because I get oval stars in some, but not all, corners. ST can help here, but if I'm honest I'm often too lazy to mess with that. May be I haven't mastered ST's masking yet . I'm not a great fan of spikes that reflectors give, and when they are combined with field rotation things can look messy, in my view. It's hard to beat a refractor for Alt-Az in my view, though I occasionally hanker over a much greater light gathering power, and reflectors steal the show there. But I think the mount needed needs to be in a different league. The use of large aperture dobs on a Go-To mount is an interesting path. Ian

Congrats Nige , but a bit confused . Star discovery, polar aligned, new mount? Must have missed something (there've been a lot of posts!). Do I take it that your EQ3 is the new mount? And you're going over to the dark side? Ian

If one looks at the spectral response for the ZWO ASI1600MM: it doesn't seem to fall off very rapidly around Hα. Also, I wouldn't expect the UV/IR filter to be of the steep-cut variety given the convergence of the light falling on the sensor. I also remember that Leica made a bit of a boo-boo when they introduced the M8, in that in improving the spatial response they ended up with an excessive infrared response which gave an unwanted colour bias in images with a lot of IR. So much so that they had to hurriedly issue purchasers with an IR filter to stick on the front of their lenses! May be, but manufacturers can really only push up the pixel count if they manage to improve, one way or another, the inherent sensitivity of the sensor. Otherwise noise would be objectionable in everyday photography. Even so, I wouldn't be looking at 24Mp sensors for astro, whatever the frame size. I do know though that if I wanted to image some low light scenes with multi-second exposures, I'd pick up my 16Mp Fuji rather than my, admittedly now aged, 16Mp GH2. Also, at the back of my mind I think that there was always the thought that Panasonic (suppliers of most of the MFT sensors) would not give Oly their state-of-the-art devices. I think I'd be very careful about comparing results with a broad-band source and extrapolating it to sources of Hα for example. I would have said that the Astronomik UHC is very much not broad-band, removing as it does most of the visible light except for the bands around O3, Hα and S2. But, for those particular wavelengths transmission appears to be very high however, ~100% I would have said that the didymium filter is a whole different ball-game however. Just my 2 cents worth . Ian

Perfectly true of course, but I've taken the simplistic view that the QE for an unfiltered sensor to Hα will be similar to the QE at other wavelengths, on the basis that if the sensitivity fell off quickly with increasing wavelength there'd be no need for the IR filter in the first place. Might be a totally misguided view of course . What it will show you though is if the filter transmission is very low to Hα then whatever the performance of the sensor itself the result isn't going to be good. And there are some cameras like that. The transmission of 656nm in my camera is over 40%, for ref. Well, I think the 2nd link shows that the sensor in the EM-5 has better noise performance for long exposures than the EM-1, but I don't think it confirms that either is the best choice for astrophotography, as Wrotniak states "The E-M5 and E-M1 are not cameras designed to perform well in long-exposure astrophotography." Though perhaps the noise performance of the sensor is a moot point when you have a very high level of sky background. Ian

Not my post, gov! You are responding to Fabien's post. Ian

I'm not sure exactly what's going on here Fabien. Here's an image of the Rosette I made when I first got into astrophotography, and which was posted ages ago on this thread. Now this was taken with a 102mm f7 refractor, so from an imaging point of view very similar to your set up. I used 130 x 10s lights, so about half the exposure you used. Admitedly, I'm not in the suburbs of a huge city, and I'm not using a pollution filter, and Orion's belt is clearly visible to me (at least, when this never ending cloud shifts out of the way!). I've found my camera to have a good red response, even though it is unmodified. May be your sky background is just too large. What effect the filter will have I don't know, as I've not used one, but one might hope it would give a significant improvement. It would be interesting to see what the deep red response of your camera is; this site might give you a clue https://kolarivision.com/articles/internal-cut-filter-transmission/, though it doesn't have your actual camera you may be able to identify others with the same sensor. Also, as a previous Olympus user (for general photography) I am aware that the MFT sensors are quite noisy, even the latest ones suffer a bit with long exposures. I fear you may be fighting against the odds here. As for stars blowing out, I think that is to be expected with exposures used to capture the tenuous nebulosity, particularly with a sensor not specifically designed for well depth. Personally, I don't worry too much about it, and I think if you were to try HDR to capture them then the exposure differences needed would be quite large, much more than a 30s to 20s change would accommodate. I guess the other alternative in your viewing location would be narrow band imaging, but that would require a dedicated camera such as the AS1600 4/3. Ian PS. You might be interested in this review which discusses noise levels in the EM5 and EM1 http://www.wrotniak.net/photo/m43/em1-em5-dark.html

I think I get your drift here Rotatux, though I admit it's pushing my knowledge envelope somewhat! Of course, it's worth adding that stacking works by pushing down the noise levels at the bottom end and so extends the DR that way, but I don't think it does anything to affect the point at which either the wells reach capacity or the DAC saturates. The 'fullness' of the wells when the DAC saturates (which will also affect DR) will be governed by the gain between the sensor and the DAC, i.e. the ISO value. I can see that if the gain is too low then one will not be able to discriminate the feinter parts of the target, so there is an optimum value of ISO to be used. There has been much written about that I think. I presume that you've read Craig Stark's article on "The Effect of Stacking on Bit-Depth"? (http://www.stark-labs.com/craig/resources/Articles-&-Reviews/BitDepthStacking.pdf) Ian

Well here's my go Herzy. Not as sensitively processed as Ken ; I've been more aggressive, as is my wont. Finished in Lightroom. I wanted to reveal the dust clouds as these are so often not seen in M42 pictures. A lovely amount of detail in the nebula away from the core, rather envious of that!. But as Ken pointed out, the core is totally washed out, (this target seems only to take seconds rather than 45), and there is banding just visible. Did you take bias, and/or darks? Cheers, Ian

Nice shot Herzy, bags of detail of surrounding dust clouds. I agree with Nige, you could probably afford to bring that out a bit more. As to the blown core, if there is some more detail available in the core with a much less stretch, you may be able to blend the low stretch image with the full stretch image. Otherwise, I think that 45s is likely to have over-cooked it, and perhaps if you have another opportunity you could try a much shorter exposure, say a few seconds, to blend with it. Ian

Congrats Nige! Ian

I suppose that I tend to work the other way and expose for a convenient maximum of 30s irrespective, on the basis that the longer I image the more photons I'll collect, which must be a good thing as it improves the photon statistics and gives more signal from the sensor. The big problem of course is saturation of stars/particularly bright parts of the target. Where I want to retain as much information about the stars I've tended to drop down to ISO400 in order to give more headroom before the DAC. With M42 though, I used both ISO400 and reduced the exposure down to 2s for a proportion of the subs in order to blend later. With my camera, ISO1600 is the max analogue gain. The other thing, as I understand it, is that it is not strictly necessary to have sufficient gain to get from 0 to 1 out of the DAC. First, sky background will take you above the 'floor', and averaging the signal from all the subs will, provided you are not doing integer arithmetic, allow interpolation between DAC units. The reason is that the photon and other statistics will sometimes give a zero, and other times 1, and perhaps occasionally 2, and the average will be somewhere between. So may be it might be possible to use a lower ISO to advantage. Ian

That's a great start, and you've got 7 mins worth of subs with a 250, which means 6x more photons that my 102mm frac would get in the same time. So I can't help feeling that there is a lot more data in your image still to be teased out. It's all down to the processing now. I know many use Photoshop and Gimp, successfully, to do the initial stretching, but I'm not one, but it might be worth trying some astro specific software instead. Most can be had on free trial so you might want to investigate further, though do bear in mind not to expect them to give you what you want straight out of the box. They all have a significant learning curve, but are well worth the effort in the long run. Also, if you rotate the camera you can fit more of M31 in across the diagonal! Interesting to see a GoTo dob being used for deep sky imaging. I wonder if we'll see more of that in these pages; no reason why not. Ian

A super image Ken, this colour one is a hundred times better than the your earlier one! It's nice to see some variation in colour, coupled with a feeling of solidity about the object, yet at the same time retaining that certain nebulous-ness. That is something that Ha seems to bring to the table. Mind you, seeing your other posts, Ha seems to also bring some processing challenges! And yay! 100 pages Ian

I can understand why mounts do better when the 'scope isn't perfectly balanced, but I've never quite understood why it matters whether it is front heavy or back heavy, for Alt-Az mounts at least. The gears will either be pushing up or pulling down according to the balance situation, but either way they will be under constant load and not give rise to backlash. So why is there this perceived difference? Ian

That's pretty reasonable for only 7 minutes exposure Fabien. Looking forward to your 120 x 20s set. The transition from red to blue is, I think too, unreal, and your B-V calibration is probably giving you nearer to the truth. A lot of images one sees with red/blue are of course NB images with colour mapping. I wonder also whether you really need ISO2500, as you are reducing the DR significantly I feel. I used 1600 on my Rosette, and have used 400 on M42 and M45 where I wanted to retain as much information in the bright areas/stars. Ian

Well I think you've squeezed out a decent image there Nige, considering the distinct lack of data. Certainly very identifiable. It'll be interesting to see what a decent number of subs with your modded camera will produce. I've also found that my mount micro-tracks better towards the west than the south. I put it down to the fact that towards the west both altitude and azimuth are increasing steadily and both drives are working continuously, but towards the south the altitude isn't changing much so that drive seems to less frequently operate (bearing in mind it's a stepper type motor so the drive will make discrete jumps), and I find that it seems to play catch-up with consequent streaking whilst it does. I also wonder whether the grease gets a lot stiffer in this very cold weather. Who knows. I've also found that the macro-tracking (i.e. keeping the object centred) has not been at its best of late (when I've been imaging towards the south), even though I've tried to keep the 2 alignment stars well either side of south and separated by approx. 90°. Certainly I wouldn't say the tracking is consistent, but I'm not really sure what causes it. Ian EDIT. I meant towards the East, not the West. Doh!

That it may be, but.... Ian

That's true* Neil, and I'm not convinced that the Cone cannot be made out in your image either. At least, with an eye of faith. There is a dark 'blob' where it should be! Ian *Sorry, I didn't specifically mean the comparison of class

As you say Neil, there is certainly a lot going on. If one Googles 'Rosette' images the amount of nebula that is revealed in long exposure Ha images is staggering. For example, 'Christmas Park' at http://deepskycolors.com/deepspace.html, or here http://www.caelumobservatory.com/gallery/rosetteabtp.shtml. Ian

That's very nice Ken, and I'm amazed at what you've done with such a short exposure. I'm not sure whether I prefer the monochrome version; to me the colour one looks a bit flat colour-wise. Do you think a greater exposure in RGB would give more colour variation, or would one need to use other narrow-bands as well and colour map? Or is that just a natural consequence of using Ha? Ian

At the end of November last year I had a go at M36 and M38. I tried processing in ST but I found the results quite messy, too many stars really. Anyway, I thought I'd have a go using AstroArt to process. After all, although I don't really know how to get the best out of an image with the software, at least with star fields it's not a question of teasing out feint details. M36. I stacked 60 x 30s at ISO400 M38 (with NGC 1907 I think). 70 x 30s, ISO400 Ian

I meant, if sensor cleaning had been switched off. Otherwise, I agree there'd be no point in doing another. Ian

I think that the Alt-Az vigilantes would allow it, just this once mind Ian

Yes, a point worth remembering, though I've not been that diligent in this regards I must admit! I've been lucky in that the dust hasn't been that troublesome. It'd be worth doing a sensor clean at the beginning of an imaging session I'd have thought. Ian