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Whirlwind

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About Whirlwind

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  1. As a few thoughts. It might be worth mentioning Echelle spectrographs as well - or more importantly that these should be avoided to start with (they are generally expensive) but with "more money than sense" might be risked because of the potential higher resolution. It might be worth mentioning that higher resolution (greater number of lines/mm etc) will give more detailed spectra but at the cost of more imaging time as the light is spread out more. Also flats are needed if you want to completely flux calibrate your spectra. The flat corrects for the response of the optical system. In principle you can do this with the standard but relies on getting the standard spectrum on exactly the same pixel location as the target of object but that is nigh impossible. It's better to flat this out and then apply the standard calibration (strictly speaking there should be both a normal flat and a sky flat but generally you can get away with just the former!). If you are just looking at radial velocity changes etc, just a normalisation is fine. But to get temperatures you should fully flat calibrate the image. Yeah don't touch the grating, breath on it try and wipe it or otherwise anyway misuse it. You can use a filter to block out second order effects
  2. Sorry if I wasn't clear. Yes I know this, but was working on Danon's focal length aspirations of around 400mm to be able to the fit the targets of choice into the object. Hence the suggestion of the RASA 8" which is a similar focal length. I don't like the focal ratio 'myth' as it gives a false impression that you are gaining extra signal for the same aperture which is not what is happening - it's just how much you are spreading the light out by. By simply binning the camera to a similar resolution for a larger aperture telescope (that has a base longer focal length) would give you the same effect as the same aperture but at a shorter focal length at the 1x1 pixel size. In this case an 200mm F2 would capture lots more photons than an 80mm F5 even though they are the same focal length. You could get the same result by getting an 8"Edge F10 and binning by a, albeit, substantial amount. The disadvantage in the latter is that your field of view is small.
  3. Realistically if you want speed then the largest aperture is the way to go as it will collect more photons per pixel (for the same camera). You could get something like the RASA 8" with a ASI1600 which would really help maximise your imaging if you are limited by a few days per month. You would just need a filter drawer (or choose a colour CMOS).
  4. There's a lot of technical bits in all the responses but honestly I wouldn't worry too much about it all. You will having to learn so much to start with that in itself will be a challenge. You can make excellent images from most setups (realistically your mount is the most important part). You really want something that is relatively forgiving at this stage. That means a short focal length and larger pixels as it will mean tracking errors, polar alignment errors etc show up more slowly. Then you need to consider how you are going to guide the telescope. That's another smaller guide camera and either a small finder telescope or an off axis guider. Also with a mono camera you need filters and a filter wheel. The most important thing though is what you want to image. Galaxies/planetary nebulae tend to be small (except a few) which means smaller pixels, longer focal length telescope. Emission nebulae tend to be larger and need shorter focal length, larger pixel camera which would be just fine. The 8300 class cameras are just fine for imaging, they are a good workhorse. Their advantage is that CCDs tend to calibrate easier but are noisier (generally) but the Sony (460 etc) ones are relatively low noise. CMOS needs a bit more consideration on the calibration frames because of amp glow and walking floor noise, plus they tend to have smaller pixels needing better guiding. However CMOS tend to work better with shorter but many frames that puts less onus on guiding compared to CCD which tends to need more and longer images. So each option has its strengths and weaknesses, however from a beginners perspective they are all a challenge whichever way you look at it. Honestly from a beginners perspective I'd go with a CCD simply because there are tens of years of experience on these cameras and you will always find help. CMOS are still relatively new to the field and I still think people are working out how to get the best out of them. Therefore guidance might be a bit less on how to resolve certain issues.
  5. I would suggest cabling as well. The feature looks like something is increasingly resisting motion until it snaps back. That would suggest something is getting caught until such point as it is stretched to far and whatever is resisting it slips over and it releases the pressure. I'm thinking maybe the spiral wrap is snagging on something (e.g. polar scope cover etc) and it stretches until it is pulled so taught that it slips against whatever is holding it back.
  6. If you have a relatively large amount of backfocus to play with a thin helical focuser between the corrector and the imaging set up can work wonders...
  7. A better term for these would "active" optics rather than adaptive optics. They won't correct for seeing in the same way that scientific adaptive optics will do. However, they can be useful for fast guiding where that might be useful such as mirror movement, flexure etc or where you might have a mount that has rapid error changes that don't respond well to corrections (e.g. lots of backlash). This is because you only have small amount of glass to move rather than the whole mount. To get the ultrafast adjustments you do need a very bright guide star though which isn't always practical on longer focal telescopes. The SX AO does have a USB connection though. They tend not to update their images very frequently.
  8. That could probably be tested as you should see less difference at larger apertures and if you pick different comparison stars. To get such a discontinuity you need something to change the relative flux between the objects so that the apertures are no longer suitable to capture equivalent flux between pre and post flip (perhaps because one star is now no longer circular). If you expand the aperture used then this effect should reduce because you start capturing the flux that has been spread out because of 'some' movement. If it is, say, due to tilt then selecting comparison stars closer to the target object should also reduce this effect as all these stars will have similar optical defects. If none of these resolve the issue then it may not be due to the optical train and perhaps to do with the camera. If you don't place the star on exactly the same position then if there are any discontinuities on the camera then these would result in an absolute difference in your photometry. This is why for scientific work most people recommend higher grade cameras (although that's no protection against later discontinuities arising). It's very common even in professional set ups to see discontinuities where there is movement of the object onto another areas of the CCD and using differential photometry.
  9. Ah OK, so if you aren't saving the files in a certain format you might never get to see that option until you've entered your reference details. So to return your issue when you create your light curve do you determine the differential magnitudes separately pre- and post- flip or all at the same time. It's actually not that uncommon where a star has not been relocated back onto the same location to get some discontinuity. When you stated that every star has the same discontinuity is it the same difference in differential magnitude in each object?
  10. Sorry, I'm still unclear on how this is done. I can only see the differential magnitudes as an option. I guess this is selective blindness, where do you get to enter the magnitudes specifically? I'm just wondering how this is done. In particular do you state this in two separate files before and after the flip?
  11. On an aside how did you get the magnitudes out of Muniwin. I thought it just gave you relative magntiudes? I've never been able to see a function where you can astrometrically link your stars?
  12. I think the difference in benefit between mono and osc is more pronounced that that. If we assume some relatively arbitrary numbers:- Each pixel produces one photon per minute from non filtered (L) light (i.e. 60 per hour) Each filter (including the OSC coating) reduces the photons received at a pixel by a third The CCD is made up of 4 pixels As such if you were shooting 4 Hrs of OSC then the number of photons you would capture would be:- photons x number of hours x number of pixels x filter reduction factor = 60 x 4 x 4 x 1/3 = 320 photons captured For LRGB you have to do the above for each channel each being 1 hour in length L = 60 x 1 x 4 x 1 (no filter) = 240 photons RGB = 60 x 3 x 4 x 1/3 = 240 photons Total = 480 photons Factor difference = 480/320 = 1.5 So you capture at least 1.5 times more photons in the same time. The more you focus on L of the image the more pronounced this becomes so 2 Hrs Lum and 2 Hrs RGB would be 640 / 320 = double the photons captured. This would hence reinforce your argument that for faint objects mono is the way to go. This uses basic assumptions that camera sensitivity is the same in the same bands. This is generally an incorrect assumption as the coated OSC are substantially less sensitive over the total band colour band compared to the same filters (which usually have a 90 - 98% transmission range). However filters generally are much more specific in the wavelengths they pass (for example looking at the OSC above the colours overlap. So I'm balancing this out even though in reality you have red light in the green osc pixel. It assumes that the cut off of the L filters is similar to that of the osc coating. Generally for refractors this is correct as we cut at about 400nm - 700nm as lenses aren't well corrected after this point. However for reflector system (e.g. RC) where this isn't necessary and you can happily use a clear filter then there the mono can capture a lot more flux from outside this range. As such I'd argue that 1.5x might be the lower end of how much data you might be collecting.
  13. I've always this thought this argument depends on what you want to image and how often. OSC cameras definitely have benefits when you only have the opportunity relatively rarely to image, need to setup and strip down everything all the time and so forth. In these cases OSC helps because you get all the data at once and if clouds appear earlier than forecast then you still have data this is useable (and aren't stuck with the R and G but not the B! If you want to focus on galaxies then OSC can also work well (and works even better in non light polluted areas) On the other hand if you are looking at narrowband targets as your primary dish of the day then mono is the way to go as you can be much more selective as to how much data you want from each band and that some narrowband will overlap (SII Halpha will both trigger the red pixels in an OSC etc).
  14. It comes down to what halos are and what causes them. Baader are correct in this statement to some extent, although the commentary on professional telescopes also having halos isn't really relevant (professional telescope filters are set for maximising throughput not pictures). Halos are in effect defocussed images of the star. To get a halo off a filter you need it to reflect light from a source. This could be, for example light reflecting off the cover slip back on to the filter that then gets re-reflected back towards the CCD. The light path for this is much longer than the focal length and hence by the time the light gets to the CCD it is way out of focus (hence the halo). Alternatively the light may reflect off the filter onto a reducer/flattener glass and then be reflected back towards the CCD. The same effect is seen. A cover slip that transmits almost 100% of the light and has a very effective anti reflective coating hence shouldn't have halo issues with *any* filter as nothing is being reflected back on to the filter. On the other hand a filter with extremely effective anti-reflective coating would not reflect light as well and prevent the halos (e.g. astrodon). If both have anti reflection issues then you will get halos. As such it is not impossible that the some combinations work and some don't with any filter. I'm not sure of the set up here but it is some form of reflector so if there is a flattener/reducer/coma corrector in the way you could try removing this and seeing if you still have the halos. If they are still there then the chances the reflection is between the CCD cover slip and the filter. If it goes away then you may find that a flattener/reducer/coma corrector with excellent AR properties (or possibly longer back focus to spread the light so far that it is not visible) may solve the problem. If it is between the cover slip and the filter then you probably will need new filters or a camera with better anti-reflective characteristics (which might be cheaper given the price of astrodons!)
  15. You should consider the Sitech more as a mount controller than a GOTO system. You are correct in that is doesn't provide a hand controller option where you can slew to 'XX' target and you would need a computer for this (not sure whether it has wifi so you can run from an ipad etc). In fact on the old site they recommended the Argo Navis system (of which I have no knowledge). So if you want to have a local hand pad you need to factor this cost in (it's not excessive especially when you are looking at the 10Micron!). One thing to note is I can't see an autoguide input on the Sitech I just an Aux and RS232 port. Might be worth checking. Realistically the cost of the difference between the Sitech 1 and Sitech II is negligible compared to savings against the 10Micron so you might as ask whether it is an upgrade option?
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