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See fig 2 (radius v magnitude) on this page https://ned.ipac.caltech.edu/level5/Sept18/Simon/Simon1.html It's basically all about how concentrated the stars are. NigelM

Infra-red astronomy from the ground is tricky. Not only is the background sky very bright, most of the signal gets absorbed by water vapour in the atmosphere! NIgelM

John The link to the Durham University Grubb Parsons lecture series is now here https://www.durham.ac.uk/departments/academic/physics/major-lecture-series/grubb-parsons/ cheers NigelM

Error 16 is no response from the AZ motor controller I think. If it is under guarantee then I would ask the retailer to sort it out. NigelM

The other way to think of it is that by definition the Sun is at 0h on March 21st - it obviously crosses the meridian at midday, so will anything else with 0 hrs RA. So NGC7822 will be due N at midday on the 21st.

J2000 coords are relative to the position of the equinox at 12:00 1st Jan 2000 i.e. the position at which the equator and the ecliptic cross at that time. This moves continuously due to precession. NigelM

I have used SRB Photographic before now. They have lots of T2 adapters. NIgelM

JPL HORIZONS? https://ssd.jpl.nasa.gov/horizons.cgi NigelM

According to the manual you can set the coordinate system in the drop down menu to the right of the location window. J2000 should be one of the options. NigelM

Hmm - this seems to have stopped working for me with the latest W10 update. Happily had it working with Hildo's instructions in June, but no longer. The driver installs (apparently successfully) from the device manager as usual, but does not appear under sound & video controllers, and if I close and re-open the device manager it goes back to reporting unknown device, no driver installed. Anyone else had this? NigelM

I highly recommend the 12", but mine is on an EQ8! Here are some DSLR images (I only take 45sec subs in very light polluted skies, so don't expect miracles from these!) http://community.dur.ac.uk/nigel.metcalfe/astro/photo_quattro.php I use the Skywatcher Aplanatic f/4 coma corrector. NigelM

Err, well it took me a while to see it, and I am not sure I can help much. With my DSLR I usually point the telescope relatively near the zenith at dusk and wait until 0.5s exposures just saturate the back of the camera histogram (in blue of course, not green or red). Note that on my 1000D 'saturation' in the histogram is actually when the jpeg saturates - there is another two stops (4x) to play with in RAW. I usually aim for about 40 before I get bored (or it gets too dark). Professionally it is easy - you just look up a suitable 'blank' field and then ask the night assistant when to start observing or sneak a look at the logs to see when the previous observers did theirs)!! I have also used night flats professionally - they waste too much time amateur-wise given how little clear sky I get round here. If you targets are small and you dither (or you take lots of different fields) then they can work extremely well. Obviously they do not work well if you are imaging a large nebula! NigelM

Remember that if you are dominated by read noise, which this shot probably is, then lower ISO on a 450D is bad news - it just makes the signal-to-noise worse. The only real answer is to take multiple shots and stack them. If you do use a tracker then I would take a separate static shot for the foreground stuff (with the same total exposure as your tracked shots) and combine the two in software. NigelM

What does the entropy weighted stacking do? It seems this is designed to combine data taken with different exposure times or on different nights. NIgelM

When I started CCD imaging many,many years ago this is exactly what was done. An average value was calculated from the bias frames (or indeed the overscan region) and subtracted from the lights and flats. We never subtracted bias frames to avoid adding noise. It worked perfectly well. NigelM

Both CMOS & CCD manufacturers tend to add a constant value to the image before it is read out, in order to avoid negative values. This needs to be subtracted (both from flats and lights) before flats will work correctly. Easiest way to find out what this value is is to take zero-length exposure (i.e. with no light getting in), known as the bias. For DSLR cameras the best approximation to 'zero-length' is to use the shortest shutter speeds available with the camera in the dark. You could use darks and flat-darks instead, as darks also have the bias signal added, so subtracting a dark gets rid of bias as well. NIgelM

I happily re-use flats, even taken months previously. The only issue is if there are significant dust spots, but my Canon does sensor cleaning whenever you switch it on and off, and there are very rarely any visible dust spots. If there are they can be fixed in post-processing. NigelM

As I read it, the main benefit of harmonic drives is zero backlash. They still have periodic error, and as with most mounts it would seem that the more you pay the better it is. NigelM

This is because stars are essentially single pixels at this resolution - so DSS tends to think they are hot pixels! It is possible to get caught by this on shots at much longer focal length as well, if the seeing is good. I have had the central pixels of stars removed at 1200mm focal length! For this reason I am very wary of the cosmetic cleaning, but if you turn it off all together I find that some real hot pixels get through. NigelM

According to another forum It has large inherent periodic error, so you really need to guide. NIgelM

Note that Skywatcher do two coma correctors at very different prices. The expensive one is the Aplanatic, which is optimised for the Quattro f/4 scopes. I can vouch for the fact that this one works. NIgelM

Because this encourages people to think that it is their camera which is the cause of all the noise, and that by spending lots of money they will do better. In fact the signal from the source really does fluctuate and there is nothing you can do about this. NIgelM

A hairdryer (I am in a mains powered observatory)! Which usually works, although you do have to wait a few mins for the focus to settle back down. NigelM

I can confirm it does!! NigelM

Sorry vlaiv, but this is misleading. Software binning does not change the S/N per sq arcsec on the sky, so the two halves of your images should look identical. It looks like you threw away 3/4 of the data when downsampling the unbinned shot. NigelM