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FraserClarke

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Everything posted by FraserClarke

  1. Nice. In the interests of minimising weight -- what purpose does the top-ring serve in the design?? Seems like you could lose those grams without affecting anything??
  2. Wikipedia gives a formula here; https://en.wikipedia.org/wiki/Heliocentric_Julian_Day It depends on the distance to the sun. Depending on the accuracy you need, you either use the mean value, or calculate it for the specific time (few % difference only) What language are you using? There may well be a library which does all this for you already of course...
  3. Have you got an OAG in the train? Looks a bit like something like that just creeping into the beam...
  4. Some of my favourites from the past 20 years; 1) On an old (late 1980s) computer with just enough disc space for one night's worth of data, zipping up the previous night's data to transfer, and then doing; rm *,fts (rather than: rm *.fts) before I'd done the transfer.... Deleted the entire night's data... 2) Taking a 20 minute exposure of a flat field lamp because I forgot to move a mirror (on the Palomar 200-inch) 3) Leaving the flat field lamp on in the observatory overnight and wondering why the background was so high... (last night) Nothing to do with being a newbie -- being more experience just means you have a longer list!
  5. In principle yes. Not sure about the specific optics combinations. We use this kind of set-up to demonstrate spectroscopy indoors. We set a small telescope up looking at a pair of pinholes ~20m away (with an eyepiece). They look just like a pair of identical stars. Then we flip in an SA-100 grating; and hey-presto -- you clearly see that one of the pinholes is illuminated by an incandescent bulb (continuum spectra), and the other by energy-saving/CFD bulb (whole bunch of emission spectra). They're not identical at all, despite what they 'look' like!
  6. Well I have a lakeside controller driving a moonlite stepper -- so you can combine both if you want! The moonlite stepper itself has not had a single problem in the past 4--5 years for fairly intensive operation (50-100 nights/year). The moonlite stepper controller however developed an increasingly frequent problem with communincation dropping out and needing to be reset manually (pull out the USB lead and plug it in again -- pretty inconvenient for a basically robotic observatory). I think this was down to a dodgy joint somewhere in the power connector electronics, but I could never track it down. I didn't bother going back to moonlite to see if they'd replace it, as we'd had the unit for 3--4 years by then. I replaced the moonlite controller with a lakeside controller -- purely because it was easier to buy in the UK (in stock) and (I think) was a bit cheaper. I'm very happy with it, and now the whole system runs very reliably. The lakeside controller feels a bit more 'handmade' than the moonlite one. Don't take that as much of a criticism -- it works incredibly well and robustly, it just needs a few more manual configuration steps to set it up. The upside of that of course is that once you've got it configured correctly (and the 2 page long manual is all you need to do that), it runs robustly. I'd certainly be happy to buy either again.
  7. X axis of the chart is the Abbe number, which as Nigel says is a measure of dispersion. Low Abbe number corresponds to higher dispersion. Combining glasses with high and low dispersion is how you achieve chromatic correction in lenses. Unfortunately glass isn't evenly distributed across this diagram, so you don't have infinite choice in the combinations... There is a general (negative) correlation between Abbe number and refractive index.
  8. Have you spoken to diffraction limited? If you have the licencing information, they might well be able to get you a copy of MaximDL5. I think they have a good list of all the licences/customers, and they've been pretty helpful sorting out issues with multiple licences I've had with them...
  9. That looks like a pretty classic polar alignment error. The field appears to rotate around the guide star (presumably you're using an off-axis guider, which is located just "above" this image?) because the guider forces the telescope to stay pointing exactly on the guide star, and therefore forces the scope to wander around to keep it in the right place. How "pretty much bang on" are you with the polar alignment? The fact the trails are not the same all across the image is probably telling you how far off, and in which direction, the polar alignment is -- but I'm afraid I'm not quite clever enough to work that out from scratch!! :-\ Given they change substantially over the field, it's probably on the order of the field size; within a degree I guess??
  10. Yes, this also happens on our (old) ST8 camera. The cooling system pulls the CCD chip down fastest, so any moisture condenses there first. As the rest of the system gets colder, the ice sublimates off the chip and ends up on whatever is the coldest bit. It's a good sign the desiccant needs replaced/regenerated. I do the same as Zakalwe -- cooling in steps to try and make the chip not the coldest bit. I also leave the camera cooling running all the time (permanent observatory), only letting it warm up to -5C between observing sessions. In professional instruments we spend a fair bit of effort making sure the chip (typically running at -230C to -120C, depending on the type) is never the coldest part of the cryostat, so that it doesn't get all the crud (present even in a very good vacuum) deposited on it by the same process. Pipnina: you're right, but the idea is that the cold electronics and chip are kept in a sealed and dry compartment, so there shouldn't be any moisture to condense on them.
  11. ACP also uses astrometry.net as a back up when the local solve fails. I'd still rather download the catalogs once and during a daytime than waste time/bandwidth repeatedly uploading images during nighttime though...
  12. A google search returns; http://deepskypedia.com/wiki/List:Terzan All near the centre of the MW, so late summer/early autumn objects...
  13. The cameras on professional telescopes are effectively very fancy focal reducers (combined with filter wheels, gratings, focusing mechanisms, etc), and they tend to work around F/2. Going faster is possible (I've seen designs for F/0.9 cameras), but is expensive, complex, and usually only works in very specialized cases. A hyperstar is probably the closest you'll get in the amateur world I guess...
  14. Yeah, Rubbish data is worse than useless, because it costs you time afterwards trying to salvage something... Best just to chuck it. Even with professional telescopes, it takes a few attempts to realise it's pointless trying to take data if the conditions aren't up to it -- it's a better use of time just to sit idle...
  15. I've done a bit of stuff with Maxim scripts (usually scripting maxim from an external programme). I don't have any examples to hand, but I always trip myself up by thinking it's more complex than it is! The interface and calling structure is very simple -- so much so it usually seems to me that it shouldn't work :-\
  16. Simplistically; Hotter == more energy == more likelihood of producing more energetic photons == shorter 'average' emission wavelength (the shorter the wavelength, the more energy the photon has; e.g. ultraviolet vs infrared)
  17. The core temperature is quite a strong function of the object mass. 13Mjup is the lower limit for sustainable Deuterium burning (when you get enough energy from Deuterium fusion to match the object's luminosity), at which point the core will be about a 0.5 million kelvin or so. Unfortunately Deuterium is so rare compared to Hydrogen, even a 13Mjup brown dwarf uses all it's Deuterium up in a few million years -- which is why the 13Mjup 'limit' for brown dwarfs is stupid; but that's just my professional opinion on the matter After that, it will start cooling. It depends what you mean by 'significantly', but the core of a 13Mjup object will only have cooled by about 1/2 (i.e. still >200,000K) over the age of the universe...
  18. Hi Paul, Saving the linked image (I think it's new-image.fits) has worked for me in the past... It has the WCS info added via the standard fits keywords (CRPIX1, CRVAL1, etc). Can't really help why it's not working for you -- sounds like it is solving the image?
  19. Looks pretty clean to me... certainly most professional detectors I've ever used would have at least this number of cosmic hits in 600 seconds. I'd expect even more than this on our ST8 camera -- but the observatory does sit on top a former nuclear physics building... maybe I should be getting the Geiger counters out I wouldn't worry about it at all. As IanL says, it'll clean up with even the most basic stacking algorithms. I've never seen a cosmic damage a CCD, though I'm sure it could happen. It is an issue with electronics in space for sure, but not something to worry much about on the ground.
  20. Remember that you need to get the focus outside the tube, so you need to include the diameter of the tube in your focal length calculations. You don't want the focus ending up on the secondary mirror -- you want it an inch or two beyond the sidewall of the tube. Here is an online calculator; https://stellafane.org/tm/atm/mirror-refs/sag2fl-calc.html But as you've already derived it from first principles, it's not much use The stellafane site is very useful to browse through though. I'm not sure how close in the north-east US you are to stellafane itself... bit of a mecca for telescope builders.
  21. Do remember the difference between detecting single photons (i.e. gain) and detecting every single photon (i.e QE). DSLRs certainly don't do the latter!
  22. One of the key things that goes in favour of splitting the exposures are cosmic ray hits. These can *only* be removed by combining multiple exposures.
  23. Yeah, diminishing returns is very good description (every doubling of SNR needs four times more exposure). When you start pushing towards high signal-to-noise ratios (>50, say), there are a bunch of other effects which start to come in. Unfortunately noise never goes down quite as sqrt(t) because of all sorts of correlated effects. You probably want to avoid getting into that regime though -- it can get very painful trying to understand them all Astrophotography is no different to spectroscopy as far as the statistics goes; except that for making pretty pictures people only worry about whether things look right -- not whether they actually are right... That allows you take a whole bunch of short-cuts and not worry about it...
  24. Have a look in the DIY-Observatories forum; http://stargazerslounge.com/forum/61-diy-observatories/ Lots on information on how to do things there. Lots of ways of doing remote dome opening/control. There are lots of bits of electronics out there which can turn on/off motors via computer control; a simple relay which can turn a motor on/off only costs a few pounds.
  25. One-wire is cheap to extend once you've got it set up, but you do need a USB dongle at ~£25 to start. Only thing to be a bit aware of is that the temperature/humidity at the back of your PC probably isn't representative of the rest of the dome, due to the heating by the PC itself. Close enough to get a rough idea of what's going on though.
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