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Martin Meredith

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About Martin Meredith

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  1. Hi Richard Just tried this on Catalina 10.15.4 and can confirm that it crashes for me too. Martin
  2. I understand you have found a solution using sky flats but in case anyone else comes across this thread, here is my experience of collecting dusk flats last night and monitoring the timings. I have an automatic routine that checks the exposure required to reach a target ADU (in my case I set it to 80% of the ADU range based on a robust estimate -- i.e. at the top of the expected linear range of the sensor, but I allow a range so e.g. even 30% of the ADU range is deemed OK). Essentially it collects 5 exposures in the range 0.5-2.5 seconds and checks whether it is too early, too late, or just about the right time to collect the flats. I was able to collect H-alpha flats with the sun 3 degrees below the horizon, but couldn't start blue until it had reached 5 degrees below. Then it was a bit of a rush to do blue, green and red but there was time to collect 91@1s for blue, 39@2.25s for green and 70@2.5s for red (not the most sensible distribution I know). Then I had to wait until the sun was around -7 below to collect luminance flats (98@1.5s). Yes, there were some stars -- pretty much unavoidable with dusk flats -- but retaining the central 80% of values at each pixel was able to remove them effectively. It has been mentioned above that changing exposure times is one approach to handling the rapid drop in light levels at dusk. An easier approach, for me at least, in software, is to use the same exposure for each sub, but to scale the individual subs so that they have the same overall level (for normalisation I use an outlier rejection scheme operating in the central third of the image, but I'm sure other approaches would work as well). It is important to either scale in this way, or adjust exposures during the capture, not for aesthetic reasons or ensuring theoretically the same SNR in each sub, but for the practical reason that outlier rejection techniques don't work well if the subs are not normalised. What is an outlier in one sub and ought to occupy an 'extreme' brightness value might not do so if the brightness level is changing across the set of subs. Actually, this doesn't affect the obvious outliers, but it does affect the fringes of stars that creep into the flats, leaving ghost trails on the flats. In sum, it is possible to collect flats at twilight for multiple filters in sufficient quantities to use outlier rejection to remove stars, but the timing is quite tight for the RGB filters... Martin
  3. Hola astroburning It is hard to find a scope/camera combination that is optimal for both DSO and planetary work and is portable. The software/hardware requirements and techniques are quite different for the two (unless you are thinking of the outer planets/dwarf-planets in which case DSO-style techniques are actually quite appropriate). I guess the biggest limitation you are facing is the need for portability since this defines the mount and therefore limits the range of scope weights. Certainly choosing an ED refractor is a good idea as achromatic refractors do produce horrible star bloat that is hard to process out in an EEA context (I speak as one who started with such a scope!). I would check out the EEA images that others manage to produce with small refractors and the kind of relatively small sensors you are considering. There are a few more questions I would be asking: 1. What apparent field of view are you aiming for? You might plan for a large FOV to cater for bright and dark nebulae, a few larger galaxies (M31/M33) and the larger open clusters, and maybe some of the larger galaxy clusters, but 99.9% of interesting deep sky objects easily fit in a small FOV. All other things being equal, you can achieve a small FOV with a large megapixel camera used via region of interest, or via a camera with a smaller sensor. The cameras you mention are quite suitable for the majority of objects. Other cameras such as the Lodestar are also worth looking at as they are very sensitive and simpler to use (no need to set gain) and produce excellent results. I know the trend is towards CMOS, but there is a lot of life left in CCD guide cameras. 2. Do you want colour? It is actually much harder to start off in EAA with colour than with mono for lots of reasons. Mono is much more sensitive and doesn't require spending a lot of time achieving satisfactory-looking colour, especially with software that makes it quite difficult to do so in the EEA context, where the goal is to observe and not process. I started with colour but soon 'graduated' to monochrome, and have since added an electronic filterwheel for when I want colour. 3. Are you sure you need portability? You might be surprised by how much EEA can cut through LP; the difference between your country house and city sites will be far less using EAA than visual. Saludos Martin (near Vitoria)
  4. Very good detail in that capture Mike. It might be interesting to see some H-alpha superimposed on it. I do have an image of the Cocoon from years ago with my achromatic 80mm scope but the chromatic aberration makes it far too embarrassing to post... (I soon dumped that scope in favour of my Newtonian -- well, I still have the scope but it doesn't get much use). Martin
  5. If I may add a couple of post-prandial globulars to your feast, here are two rather understated clusters, both discovered by William Herschel, that I observed a couple of weeks back. Both have small apparent diameters (although Herschel described NGC 4147 as very bright and pretty large). I found the colour contrast rather appealing. NGC 4147 is said to contain 23 blue stragglers. The Hubble image of NGC 4147 on its Wikipedia page doesn't look as blue as mine but nobody's perfect. It is well described in this article: https://arxiv.org/pdf/astro-ph/0508650.pdf, where the authors point out that it doesn't suffer from much reddening since it is located near to the North galactic pole. These are live-combined 1-click LRGB captures using LAB colour space. Thanks for looking Martin
  6. Good question! I've no idea... What I do is move the scope outside about 1 hour before it is dark enough to start doing serious observing, and I guess about 30 mins later is when I would start to think about taking flats. Next time I will check how far the sun is below the horizon at that point. Not far below, I reckon. What I actually do is take test exposures and observe the histogram, aiming to get the peak around 70%. Part of what I hope to do in software is to estimate the best time to start taking flats based on analysing the test exposures automatically. And of course the best time will vary with transmissibility if you're using filters. Martin
  7. Hi I occasionally take dusk flats with the scope pointing at around 70 degrees and open (e.g. no t-shirt diffuser) and aim for a large number of subs (~100) of up to a couple of seconds each, which I then combine using a percentile-based outlier rejection method to get rid of the inevitable stars that appear (I usually find keeping the central 70-80% works well). Timing is critical, especially if you are planning to collect flats for more than one filter. I reckon there is a sweet spot of 10-15 minutes when you can get good flats this way i.e. when the exposure times are not too short to be noisy and not too long that you can't collect enough subs to apply outlier rejection successfully. I reckon someone should publish 'flats' timetables like the tide tables! Actually, I'm trying to automate the procedure in software at the moment. Others may have a different process. This is what I use for EEVA-style observing rather than AP and I find it works well. cheers Martin
  8. As Vlaiv said, but an easier way in to simbad is via aladin: https://aladin.u-strasbg.fr/AladinLite/. Click on the Simbad checkbox once you have M13 in view and identify your star if it has a box around it. It might be in there GAIA database in which case click the GAIA DR2 checkbox. Martin
  9. Nice haul there Mike -- not sure how I missed them until now. VV 364 is particularly intriguing. Here's a very appealing VV group, VV 170 in Virgo. It reminds me of Hickson 50 (or is 55?), except there are just 4 galaxies in this chain. As a bonus we also have Shakhbazian 357 in the same field of view. In fact, the SHK galaxies are the central part of Abell 1773, estimated to be around 1.1 billion LYs distant. The VV chain is less than half that distance. The faint galaxy just off to the edge of the chain seems to belong to the Abell cluster, so the proximity is accidental. Also, one of the 4 VV 'galaxies' may well a stellar interloper? While VV and particularly SHK galaxies have a reputation for being faint, this is just a 1 minute exposure (6 x 10s) with my 8" Newt. Even so, I'm just about picking up a mag 19.9 galaxy. The attached chart helps in identifying the fainter reaches. VIR3549.pdf Martin
  10. Thanks for those links Bill and that precis, Mike. When I get a little time I am definitely going to plot a GAIA figure of this cluster. This is a screenshot of where I am placing the boundaries based on Harris' data on GCs. It isn't very visible but the pdf is chart SER3414 if anyone has the Pretty Deep Maps downloaded. It seems to cover a wider area than our shots (the star you indicate is the brightest star in the lower-left quadrant of the GC on the chart) My interest was piqued in part by the presence of the Abell cluster next door and also the scattering of galaxies 'within' the boundaries of Pal 5.
  11. Mike, I was inspired by your Pal 5 capture to have a go myself tonight during another very brief test session. It was really hard to find this one! I rotated the shot to agree with yours. This must be one of the loosest globular clusters out there, more like an open cluster really. There are quite a few faint galaxies visible through the cluster as well as quasars, including a mag 21redshift 3.73 (won't be visible on my shot). Just a screenshot tonight as the software is in an in-between state.... this is 15 x 15s, captured natively. It would be interesting to look at the GAIA data release to see just how extensive yet sparse this cluster is. I get the feeling that there are not many more stars than are being picked up here but I could be very wrong. Certainly it has whetted my appetite to dig out some articles on this cluster. Martin
  12. That is a fascinating object. Thanks for the back-story! It amazes me how so much variety can emerge from a few simple physical principles. Martin
  13. That's a really nice result in terms of tight stars and good resolution, so I think your approach does work. My feeling about binning is that it is nearly always better to do it in software. The only benefit from doing it on the sensor is to save a little noise -- how little depending on the sensor. Although it is almost never mentioned, on-sensor binning also introduces aliasing noise, because the energy from those high spatial frequencies that are present in the unbinned image has to go somewhere, and ends up being aliased at lower spatial frequencies, thereby contributing spurious photon counts i.e. noise, although I've seen no studies of whether it makes a noticeable contribution or not. One advantage of downsampling in software is that one can apply a spatial lowpass filter to remove aliasing. Another advantage is that one can downsample by non-integer factors, which permits precise matching of the resulting reduced resolution to seeing, giving the best possible noise reduction while not oversampling relative to seeing. I'd be interested to hear other opinions! Martin
  14. Hi Mike Lovely shots. You never really know what you are going to get with these groups but their forms can be quite compelling and always distinct. Whenever you post a Shakhbazian I check to see if it is amongst my fairly meagre collection and in this case I turned up some observations of SHK 122 and 123 (noisier than yours): I see that I observed these in late July. In spite of new sensors, my kit hasn't changed at all in 5 years (6 actually). I was also thinking about how late one needs to get out to have a good dark session at this time of year, although at my latitude it isn't quite as bad. I've observed 3 quite interesting SHKs in the last couple of days -- now where's that Shakhbazian thread?! Martin
  15. Hi Bill That's a fine capture! The purple cross is where Aladin tells me it is located on the DSS image (the RA/Dec correspond to the object). In this zoom it is marked with the green square. It is very faint and somewhat reddish on the DSS image. Now I must quickly fix a few bugs and get the scope out to cool..... Best Martin
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