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Martin Meredith

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About Martin Meredith

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    Sub Dwarf
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  • Gender
    Male
  • Interests
    Mountains, cycling, growing stuff
  • Location
    Northern Spain

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  1. Great work Don! That looks like a bright one (and maybe relatively brighter in your shot than the discovery image?) Martin
  2. Hi Miguel A warm welcome from just up the road in Vitoria (where today it snowed ;-) Martin
  3. Check out this matrix here which Dr D sent me some years ago. Martin
  4. Hi Bill There should be no need to dump FITS to Nebulosity. How much adjustment of the colour channels did you do for the SLL shot? I've not used the LodestarC software (I don't think it runs on my Mac), but perhaps it has some kind of automatic colour balance built in, whereas with SLL you will need to do some manual adjusting. Martin
  5. In case you need more, here's a PDF I made from the Washington Double Star catalogue with 22k+ double stars sorted by magnitude of the primary. If you click on UMA you'll get just those for Ursa Major, etc. Columns include separation in arcsec, position angle, magnitude of the secondary, mag difference, first and last observation years and spectral type. DSMagI.pdf Martin
  6. I think its a great concept and maybe something that could be repeated in Europe? Land is very cheap here in Spain and entire villages have been abandoned in some of the regions with dark skies (not where I live though...). I don't think anywhere could rely on 300 clear nights a year but maybe 200. In any case, 300 would be exhausting, at least for observers! I was in the north of Arizona last year and spent some time in Flagstaff, the world's first dark sky city. Along with neighbouring Utah its a part of the world I'd love to return to and highly recommended for an astro-related visit. Martin
  7. Anyone fancy 300 dark clear nights a year? https://www.theguardian.com/environment/2017/apr/13/stargazers-arizona-sky-village-home-observatories Martin
  8. Sure, but if not using identical length subs one would use a weighted-average to get the same effect. My point is that it can be confusing to provide both sum and average options as if they were fundamentally different beasts... Martin
  9. As Mark says, sum and average are the same in floating point, and are theoretically optimal in reducing noise. However, there are occasions when you want to do something else (but not median, which loses a lot of SNR). That something else might be sigma clipping, which gets rid of outliers (defined as values that are more than a certain number of sigmas (= standard deviations) away from the mean), then recomputes the mean. This does a good job with satellite and plane trails for instance. Martin
  10. That's really great! Congratulations. One fast-moving comet. Are you simply superimposing saved pngs (or similar) to produce the animated gif, or did you have to do any star alignment? I guess no need if you're in EQ mode. Martin
  11. Hi Don The audience must have been blown away by these. I've not seen so much colour in an EAA image as your HH/Flame. I wonder how many are inspired to take up the hobby as a result of seeing these images during one of your sessions. Love the F2.0 Needle, especially the very faint edge on aligned similarly (NGC 4565C, 5 times distant re the Needle). You've got a great sense of depth in that shot. Seeing your M83 reminds me that I'm heading to latitude 36 in a few weeks and must take advantage of the location to revisit one of my favourite galaxies. Martin
  12. Thanks all for the comments. Yes, NGC 4274 definitely has something emanating from the central bulge. This image suggests it is indeed the start of the spiral, but rather than the spiral being gradually unwound, it makes a quick break for the inner ring -- most unusual! Martin
  13. Here are a few captures from a badly-collimated session last night. (Lodestar X2 mono, Quattro 8" f4, alt-az, StarlightLive, no filters; seeing around 3.3", moonless, slight wind, village lights). 1. Leo I dwarf. = UGC 5470. This is a right tough one due to its low surface brightness (24.0) and proximity to Regulus (the last few sessions I've finished my 2-star alignment on Regulus and it hadn't occurred to me until last night to take a look). Stacking doesn't help much as the Regulus glow remains. The dwarf is towards the top-centre of the image in case it isn't obvious... 2. Hickson compact group 68 in Canes Venatici. = NGCs 5350, 5354, 5353, 5355, 5358. This group of 5 spirals in various orientations is somewhat overshadowed by the mag 6.5 star in their midst (I sense a theme emerging here). 3. NGC 5371. Less than a degree from Hickson 68 is this interesting spiral with some detail appearing in its arms as the exposure builds up. The faint starlike object at 4.30 is a mag 18.5, z=1.6 quasar. The stars on the right are displaying pretty bad coma by this point (the close pair looks like the prototypical double star atlas representation!). 4. Hickson 61 in Coma = The Box = NGCs 4173, 4169, 4174, 4175. One of the most perfect galaxy groupings in my opinion. I've observed it before but never for long enough to notice the thin tail extending from the faintest (long thin) NGC 4173. I let this one stack for a while to get a better look at the tail. While it looks to be interacting with the other similarly-oriented edge-on (NGC 4175), the distances are wrong: NGC 4173 is listed at 62 million LYs, and the other three at around 197 million, so the perfect configuration is just a happy accident. 5. NGC 4274. Also in Coma, a few degrees from The Box. This is classified as a SB-spiral with an inner ring and an outer pseudo-ring, if I've interpreted the classification correctly. This was the find of the night for me, based on a random excursion between planned hops. I was so keen to observe the various ring structures that I left the stack running for quite a while as you can see. The bright inner ring is obvious on a single 30s sub, with a mere ghost of the outer material. With time this reveals itself first as a faint outer disk (around 4x30s),which then resolves into a ring with clear dark gaps at the north and south by 8x30s. Subsequent exposure leads to the emergence of detail in the central bright bulge, which appears to contain yet a third ring, but which I believe is actually a bar. This galaxy is quite close, at 52 million LYs, and just a few tenths of a magnitude fainter than the 6 Messier galaxies in Coma. The fuzzy ball at the top-left is IC 779. The beauty of EAA is that it brings into reach any number of fascinatingly different galaxy types. Thanks for looking Martin
  14. One standard approach is 1. Find a star with spectral type G2V that is at an altitude of 60 degree or more (to minimise differential atmospheric extinction per colour channel). I attach a list of relatively bright examples. 2. Collect exposures of the same duration in each of RGB, ensuring that the star is not saturated in any of them (stacking is fine) 3. Calibrate (darks etc) as normal, separately for each of RGB 4. Using the calibrated channels, measure the brightness values in the pixels at the centre of the G2V stellar image (averaging a few pixels at the centre is ok) This gives you ADUs for each of RGB. Selecting one (say the highest) as a reference, compute the ratios with the other two colour channels. When imaging, you can either expose the channels with the least bright values for longer (i.e. multiply the exposure duration by the ratio of brightest channel to this channel), or use them in software to weight the contribution of the different channels. This only needs to be done once for each sensor/filter combination (description summarised from Charles Bracken's The Deep Sky Imaging Primer) Another more approximate approach that I've used in the past is to take a shot of an open cluster with a known range of (B-V) colour values and attempt to match them. Cheers Martin
  15. Great results! M101 in particular has some fine details. I don't think I've seen the 'trailer' faint arm at the right of the image in an EAA shot before. It looks like the coma corrector was worth using judging by the stars. Have you checked out the coma without it? I have one but rarely use it, but on the occasion I did it definitely improved star shapes at the periphery. Its a compromise between that and shoving more glass in the way. Martin